WEBVTT Kind: captions Language: en 00:00:01.000 --> 00:00:06.500 [Silence] 00:00:06.500 --> 00:00:09.040 Good morning, everyone. So next week’s seminar 00:00:09.050 --> 00:00:11.560 will be Ken Stokoe, who will be talking about 00:00:11.560 --> 00:00:15.700 characterizing sites based off surface wave characteristics. 00:00:15.700 --> 00:00:19.080 But today, Nick will be introducing Tom. 00:00:21.480 --> 00:00:24.520 - Okay, so it’s great to have Tom Mitchell here. 00:00:24.529 --> 00:00:27.600 Hopefully most of you have had a chance to talk to Tom. 00:00:27.600 --> 00:00:31.420 He’s going to be here until, I guess, the end of this week. 00:00:31.420 --> 00:00:35.780 We’ve got the regular lunch after seminar. 00:00:35.780 --> 00:00:37.860 We’re also probably going to go out to dinner tonight if anybody 00:00:37.860 --> 00:00:40.760 is interested in joining us. We don’t know what time yet. 00:00:40.760 --> 00:00:45.000 It depends, I think, on an experiment that 00:00:45.000 --> 00:00:47.920 gets put together this afternoon. [chuckles] 00:00:47.920 --> 00:00:52.110 So Tom is a – well, I don’t know what he is. [laughter] 00:00:52.110 --> 00:00:56.260 He’s a jack-of-all-trades. But primarily, he’s a 00:00:56.260 --> 00:01:00.780 experimental rock mechanics guy. But he’s also a field geologist. 00:01:00.780 --> 00:01:06.640 And he’s sort of – you know, I sort of think of him as one of the two people – 00:01:06.640 --> 00:01:11.980 young people in experimental rock mechanics that are sort of the real thing. 00:01:11.980 --> 00:01:15.990 One is Mitchell, yeah. And the other is his adviser, Dan Faulkner. 00:01:15.990 --> 00:01:18.950 So he was at Liverpool as an undergraduate, 00:01:18.950 --> 00:01:21.100 stayed on, and got his Ph.D. there. 00:01:21.100 --> 00:01:26.090 And if you don’t know about what he did, you need to find out. 00:01:26.090 --> 00:01:30.590 So basically, he’s – the structure of fault zones – the permeability structure, 00:01:30.590 --> 00:01:33.580 the strength structure – you know, how it varies with depth, 00:01:33.580 --> 00:01:37.060 how it varies with time, you know, over the seismic cycle. 00:01:37.060 --> 00:01:42.790 And so sort of one of the classic pieces of work in the last probably 15 years 00:01:42.790 --> 00:01:48.370 in earthquake science, I’d say. At least, that’s my opinion. 00:01:48.370 --> 00:01:50.659 Anyway, so he finished there. He went all over the world. 00:01:50.659 --> 00:01:52.869 He’s been everywhere. He’s been to Japan. 00:01:52.869 --> 00:01:55.649 He was in New Zealand. He was in Germany. 00:01:55.649 --> 00:01:59.720 He was in Rome. He was in Japan. 00:01:59.720 --> 00:02:03.329 He was in the United States. And I met him in Rome. 00:02:03.329 --> 00:02:07.869 I don’t even remember when it was, but for some reason, because he is such 00:02:07.869 --> 00:02:09.921 a great experimentalist, when I was there, 00:02:09.921 --> 00:02:13.340 they wouldn’t let him touch the controls of the machine. 00:02:13.340 --> 00:02:16.400 I don’t know what that was all about. [laughter] 00:02:16.400 --> 00:02:20.550 But anyway, so he’s now at the University College of London. 00:02:20.550 --> 00:02:23.720 This is one of the premier, now, rock mechanics – it’s always been 00:02:23.720 --> 00:02:27.280 one of the premier places in the world. And Tom is building machines. 00:02:27.280 --> 00:02:30.280 He’s got an army of postdocs. He’s well-funded. 00:02:30.280 --> 00:02:34.090 And, you know, he’s kind of the – he’s kind of the guy. 00:02:34.090 --> 00:02:38.209 As you can see from the list of collaborators, he’s doing – working with 00:02:38.209 --> 00:02:44.690 a lot of different people – ancient people like Chris Scholz. Young people. 00:02:44.690 --> 00:02:49.050 So he’s the guy. So this is going to be a talk, I think, 00:02:49.050 --> 00:02:53.690 that spans everything from his Ph.D. to things that he hasn’t 00:02:53.690 --> 00:02:57.120 even started working on yet. And it’s called cumulative coseismic 00:02:57.120 --> 00:02:59.720 fault damage and feedbacks on earthquake rupture. 00:03:00.240 --> 00:03:02.880 - Okay. Everyone hear me? Yeah? 00:03:02.880 --> 00:03:05.620 It’s working. Okay, well, thanks for inviting me. 00:03:05.620 --> 00:03:07.920 And that very nice introduction. 00:03:07.920 --> 00:03:12.200 So, yeah, the title of what I’m going to talk about today is – I put loosely as 00:03:12.209 --> 00:03:16.020 cumulative coseismic fault damage and feedbacks on earthquake rupture. 00:03:16.020 --> 00:03:20.269 So probably what I’m going to end up today doing is showing you that there’s 00:03:20.269 --> 00:03:24.470 a lot of things that I’m working on and a lot of things that I don’t understand. 00:03:24.470 --> 00:03:26.530 And how many questions I actually answer, I don’t know. 00:03:26.530 --> 00:03:29.480 But one thing is, it’s certainly been enjoyable and made me realize how 00:03:29.480 --> 00:03:33.940 complicated damage around fault zones related to earthquakes actually is. 00:03:34.660 --> 00:03:37.580 So, yeah, I had to mention a bunch of collaborators here because this 00:03:37.590 --> 00:03:42.099 particular talk involves a lot of bits and pieces which have built in ideas 00:03:42.099 --> 00:03:44.360 and talks from visits exactly like this one. 00:03:44.360 --> 00:03:47.490 I get a lot out of these sort of talks. Give me ideas and directions. 00:03:47.490 --> 00:03:51.569 And of course, it’s really great to be here at USGS where Dave Lockner’s lab 00:03:51.569 --> 00:03:53.540 and everybody else is – where some of the – you know, 00:03:53.540 --> 00:03:57.209 the fundamental stuff that I first learned as an experimentalist was actually done. 00:03:57.209 --> 00:04:00.989 So I’d say it’s pretty exciting and feel proud to be talking about that today. 00:04:00.989 --> 00:04:05.160 So this is a drone image of a strike-slip fault called the 00:04:05.160 --> 00:04:08.080 Laguna Salada Fault in Baja, California, and Mexico. 00:04:08.080 --> 00:04:11.760 Main reason is, it’s just a fault, and it looks cool from a drone. 00:04:11.760 --> 00:04:15.280 But jumping straight in there, so why do we study fault damage, okay? 00:04:15.280 --> 00:04:18.500 And we have to write this every time we write a NERC or a NSF proposal. 00:04:18.500 --> 00:04:20.620 And I actually think it’s generally quite easy to sell 00:04:20.620 --> 00:04:24.140 earthquake science to some degree. You can play the earthquake card. 00:04:24.140 --> 00:04:28.420 But you can also play the fluid transport and strength of the crust card, okay? 00:04:28.430 --> 00:04:31.050 And that controls crustal permeability, 00:04:31.050 --> 00:04:34.660 the seismic cycle – I’ll come to that in a second. 00:04:34.660 --> 00:04:37.720 Damage and permeability also – studying that allows us to understand 00:04:37.720 --> 00:04:40.770 how earthquakes nucleate, and also how they propagate. 00:04:40.770 --> 00:04:43.280 And both have an effect on rupture dynamics. 00:04:43.280 --> 00:04:47.410 And of course, they have controls and transport and the deposition of economic 00:04:47.410 --> 00:04:51.020 minerals and oil migration as well. But the thing I want to talk about today 00:04:51.020 --> 00:04:55.740 is that most of the action tends to take place during seismic rupture. 00:04:55.740 --> 00:04:57.720 So a lot of the stuff I’ll be talking about today will be 00:04:57.720 --> 00:04:59.330 things that are happening on very quick time scales. 00:04:59.330 --> 00:05:02.810 And I’ll give you an example, or a flavor of a few things that I think 00:05:02.810 --> 00:05:07.370 is going on and that we potentially don’t really know enough about. 00:05:07.370 --> 00:05:09.500 So this I’m sure you all recognize. It’s not far from us. 00:05:09.500 --> 00:05:11.259 What do I mean by the fault seismic cycle, okay? 00:05:11.260 --> 00:05:12.760 And what are the time scales? 00:05:12.760 --> 00:05:17.210 This very simple equation here relating fault strength to a few things. 00:05:17.210 --> 00:05:21.780 In this case, it’s cohesion, friction, stress, and fluid, okay? 00:05:21.780 --> 00:05:23.930 So here – I just put some simple relationships here. 00:05:23.930 --> 00:05:27.690 You’ve got normal stress on the fault, and you’ve got pore fluid pressure. 00:05:27.690 --> 00:05:30.740 And this is kind of modified from a classic Sibson paper. 00:05:30.740 --> 00:05:34.020 But what I – I put this up here really to sort of demonstrate the idea 00:05:34.020 --> 00:05:36.090 that you’ve got different time scales going on on the earthquake cycle. 00:05:36.090 --> 00:05:39.980 You’ve got the interseismic period, okay, where you’ve got rising stress. 00:05:39.980 --> 00:05:43.020 Okay, this might be rising shear stress and normal stress, for example. 00:05:43.020 --> 00:05:45.740 And eventually, that stress is going to reach the strength of the fault, 00:05:45.740 --> 00:05:50.100 and that’s going to – you’re going to have an earthquake, okay? 00:05:50.100 --> 00:05:52.770 The coseismic part, of course, can happen very – where you have a stress 00:05:52.770 --> 00:05:57.970 drop can happen very quickly. Okay, so this is fast, and this is slow, okay? 00:05:57.970 --> 00:06:01.490 Coupled to that, of course, is – so the stress control here on 00:06:01.490 --> 00:06:03.569 whether earthquakes happen or not from tectonic loading. 00:06:03.569 --> 00:06:07.080 There’s also the fluid pressure control. Okay, so if you add to that 00:06:07.080 --> 00:06:09.560 the pore pressure cycle. Okay, so if we imagine, 00:06:09.560 --> 00:06:13.870 directly after an earthquake – here’s a cheesy Photoshop of a fracture sample. 00:06:13.870 --> 00:06:16.710 But just imagine, we suddenly dump in a bunch of fracture damage, 00:06:16.710 --> 00:06:19.790 which we’ll be talking about today. Suddenly, the rock around the 00:06:19.790 --> 00:06:22.130 fault becomes very permeable. So fluids can move around the cracks. 00:06:22.130 --> 00:06:24.590 Okay, the more cracks there are, the more permeable it is. 00:06:24.590 --> 00:06:26.060 This means that fluids can move around. 00:06:26.060 --> 00:06:27.760 Okay, so they can move around quite quickly. 00:06:27.760 --> 00:06:30.360 But of course, for every kilometer we go down through the crust, 00:06:30.360 --> 00:06:32.900 temperature goes up by 25 degrees or even higher – sometimes 00:06:32.910 --> 00:06:37.810 60 or 80 in some settings where you have a high geothermal gradient. 00:06:37.810 --> 00:06:41.010 These things won’t necessarily remain permeable for long, okay? 00:06:41.010 --> 00:06:44.500 So actually, another cheesy Photoshop animation here, but it shows the idea 00:06:44.500 --> 00:06:47.050 that these fractures can actually seal. They can heal, and they can seal. 00:06:47.050 --> 00:06:49.310 This is something I’ll come to at the end of the talk, okay? 00:06:49.310 --> 00:06:52.220 So point is, this damage is dynamic. It can be put in there very quickly. 00:06:52.220 --> 00:06:54.520 It can also put in there very slowly during the quasi-static 00:06:54.520 --> 00:06:57.770 stages of fault damage. And it also seal as a function of time, 00:06:57.770 --> 00:06:59.440 which is pressure- and temperature-controlled. 00:06:59.440 --> 00:07:01.800 This, of course, then allows – if you seal up all your fractures can 00:07:01.810 --> 00:07:05.819 allow pore pressures to build up, and you can get over-pressured. Okay. 00:07:05.819 --> 00:07:09.509 So coming back to this sort of classic Sibson sketch here of stress versus time 00:07:09.509 --> 00:07:12.139 and fluid pressure and permeability directly after our earthquakes, 00:07:12.140 --> 00:07:15.460 we might have a stress drop, we might have a bunch of damage put in there. 00:07:15.460 --> 00:07:18.020 Fluid pressures will drop because they can move around. 00:07:18.020 --> 00:07:22.990 But as that fracturing – fractures heal, okay, the fluid pressures can build up. 00:07:22.990 --> 00:07:26.230 Okay, so while you’ve got rising stress from changes in tectonic loading, 00:07:26.230 --> 00:07:28.910 you’re also going to have reductions in stress going on – 00:07:28.910 --> 00:07:32.319 strength going on due to pore pressure changes too. 00:07:32.319 --> 00:07:34.281 That’s kind of exactly what fracking is. 00:07:34.281 --> 00:07:38.930 You could either initiate an earthquake by increasing the stress, but you could 00:07:38.930 --> 00:07:44.259 also do it by just drilling into a fault and increasing the pore pressure. Okay. 00:07:44.259 --> 00:07:48.449 So just a quick fault zone structure recap. This is – this is a schematic diagram 00:07:48.449 --> 00:07:51.460 of a strike-slip fault zone. And this is a version – something I’ve 00:07:51.460 --> 00:07:54.590 done, but many people worked on this, such as Fred Chester and many others. 00:07:54.590 --> 00:07:57.760 What we tend to have is a fault core. This is where all the fault slip happens. 00:07:57.760 --> 00:07:59.780 Okay, it tends to be very narrow, and it’s very localized. 00:07:59.780 --> 00:08:02.471 And the result of that, we get a lot of fine-grain material and 00:08:02.471 --> 00:08:06.449 high-strain products, such as [inaudible] clay sites, fault gouge, and so on. 00:08:06.449 --> 00:08:10.190 Surrounding that, you have a fracture damage body of rock. 00:08:10.190 --> 00:08:13.789 And this can range up to hundreds of meters, sometimes kilometers, in width. 00:08:13.789 --> 00:08:15.400 Okay, and generally what we see is that, 00:08:15.400 --> 00:08:18.729 as we get closer to the fault core, the fracture density increases. 00:08:18.729 --> 00:08:22.110 So you actually have variations in physical properties too. 00:08:22.110 --> 00:08:25.070 A similar diagram from Fred Chester – a schematic diagram also including 00:08:25.070 --> 00:08:29.530 some roughness in there – not as planar. He has a nice 3D diagram showing that 00:08:29.530 --> 00:08:31.889 because the fracture density increases, the permeability increases. 00:08:31.889 --> 00:08:34.819 So you’ve got changes over a few-hundred-meter 00:08:34.820 --> 00:08:37.620 length scale of permeability. 00:08:37.620 --> 00:08:40.220 Of course, the permeability in the fault core is very low because you’ve got lots 00:08:40.220 --> 00:08:44.500 of fine-grain materials that can slow down the flow, and it’s very impermeable. 00:08:44.500 --> 00:08:45.899 Other than that, if you’ve got more cracks 00:08:45.899 --> 00:08:47.529 in a volume of rock, overall it’s weaker. 00:08:47.529 --> 00:08:49.950 Okay, you’ve got more flaws in there, the strength goes down. 00:08:49.950 --> 00:08:52.830 So the strength modulus and ductility changes as well. 00:08:52.830 --> 00:08:55.160 Okay, so you got – within fault zones – 00:08:55.160 --> 00:08:58.820 the damage zone, you’ve actually got strong variations in various 00:08:58.820 --> 00:09:03.720 physical properties, okay – both hydrological and mechanical. 00:09:04.560 --> 00:09:08.560 So I guess the way I’m framing this talk today is – and some of 00:09:08.560 --> 00:09:11.370 my motivation is looking at your earthquake energy budget, okay? 00:09:11.370 --> 00:09:14.790 And I don’t think I have to explain to any of you, really, what an earthquake is. 00:09:14.790 --> 00:09:18.181 Okay, but a key part of seismology is, you know, 00:09:18.181 --> 00:09:21.360 predicting the radiated energy and ground motion, okay? 00:09:21.360 --> 00:09:24.220 So I want to – so all of this stored energy, it goes somewhere 00:09:24.220 --> 00:09:26.230 during earthquakes. And we know that a lot of that 00:09:26.230 --> 00:09:29.580 goes into – or some of that – a portion of that goes into frictional heating. 00:09:29.580 --> 00:09:33.900 A bunch of this goes into fracture surface generation and also chemical 00:09:33.900 --> 00:09:37.279 processes, at least on the fault plane and around – on the fault zone. 00:09:37.279 --> 00:09:41.500 There’s a relatively simplified equation here from Kanamori and Brodsky, 00:09:41.500 --> 00:09:46.010 and this is just relating that the total elastic energy stored into a few terms. 00:09:46.010 --> 00:09:51.880 In this case, it’s the frictional energy, the radiated energy, and the fracture energy. 00:09:51.880 --> 00:09:55.220 And I just put this up because one of the things I want to focus on a bit more 00:09:55.220 --> 00:09:58.390 is this term – the fracture energy. How much energy goes into creating 00:09:58.390 --> 00:10:01.180 fracture damage around faults for individual earthquakes? 00:10:01.180 --> 00:10:05.450 For me, that’s an important question. I’m not sure we fully understand yet. 00:10:05.450 --> 00:10:06.810 I would argue that we actually know quite a lot 00:10:06.810 --> 00:10:08.430 about this term – the frictional energy now. 00:10:08.430 --> 00:10:10.190 And I’ll show you a few slides to explain that now. 00:10:10.190 --> 00:10:14.580 This a nice example – the Nojima Fault – well, the damage after the 00:10:14.580 --> 00:10:19.620 Kobe earthquake by a fault – which was located on the Nojima Fault, okay. 00:10:19.620 --> 00:10:23.960 And this is this famous – the highway that fell over in the early hours. 00:10:23.960 --> 00:10:26.360 Okay, if we actually go there, this is the actual Nojima Fault. 00:10:26.360 --> 00:10:28.330 This is pretty cool. You can see this rice field has been – 00:10:28.330 --> 00:10:31.310 it’s been – had a couple meters of slip offset there. 00:10:31.310 --> 00:10:32.610 You can imagine, standing there, you would – 00:10:32.610 --> 00:10:35.220 probably thrown into the air pretty quickly. 00:10:35.220 --> 00:10:39.410 But this indicates very nicely that this slip – this localized earthquake 00:10:39.410 --> 00:10:42.620 slip is in a very narrow slip zone, okay? And you actually go there now. 00:10:42.620 --> 00:10:44.220 They’ve actually built a museum over the top of it. 00:10:44.220 --> 00:10:46.060 Some of you may well have gone there. It’s pretty cool. 00:10:46.060 --> 00:10:49.540 This actual scarp here is the same scarp, just a bit further down. 00:10:49.540 --> 00:10:51.940 They’ve actually cut down – you can actually look at the fault core 00:10:51.940 --> 00:10:55.610 and see the localization and the damage next to that. 00:10:55.610 --> 00:10:59.110 This was always a great place to take people as a geological tourist. 00:10:59.110 --> 00:11:01.891 However, one time I did – I would note that one time I went down there, 00:11:01.891 --> 00:11:05.519 and they’d actually put a tent over it, and they were painting the surface. 00:11:05.519 --> 00:11:08.160 So maybe – I’m not sure entirely [laughter] how real that is. 00:11:08.160 --> 00:11:11.910 But it’s definitely – it’s very clear that localized slip is happening in a fault core, 00:11:11.910 --> 00:11:14.730 and you can see the damage there as well. 00:11:14.730 --> 00:11:16.519 Another example is the Punchbowl Fault, which I’m sure 00:11:16.519 --> 00:11:19.020 most of you know about. It’s, you know, a 44-kilometer 00:11:19.020 --> 00:11:24.040 displacement fault, now inactive – a once-active strand of the San Andreas. 00:11:24.040 --> 00:11:27.660 There’s a photo of it here. This is probably about 20 or 30 meters high. 00:11:27.660 --> 00:11:30.100 The basement versus the Punchbowl sandstone formation. 00:11:30.100 --> 00:11:33.890 Okay, and if we look in these classic images from Fred and Judi Chester, 00:11:33.890 --> 00:11:38.529 it’s a – we’ve got a narrow cataclastic zone, okay, maybe a foot wide or so. 00:11:38.529 --> 00:11:41.430 And if we zoom in there, some of Fred’s work, 00:11:41.430 --> 00:11:44.160 Dave Goldsby shows this beautiful principal slip surface. 00:11:44.160 --> 00:11:46.149 This principal slip surface where the majority – 00:11:46.149 --> 00:11:47.950 they believe the majority of the earthquake slip happened. 00:11:47.950 --> 00:11:49.990 And this is in a millimeter-wide slip zone. 00:11:49.990 --> 00:11:52.899 So this is pretty cool if you think about the fact that this is, you know, 00:11:52.900 --> 00:11:57.180 44 kilometers of displacement, a substantial part of that is actually being – 00:11:57.180 --> 00:12:00.860 you know, focused in a very, very narrow slip zone, right? Okay. 00:12:00.860 --> 00:12:03.540 After that – I’m going to keep hammering this idea of localized slip 00:12:03.540 --> 00:12:05.710 and give you some examples. This is the Alpine Fault. 00:12:05.710 --> 00:12:10.500 And many of you work here, both in the lab and also on the seismology aspects. 00:12:10.500 --> 00:12:12.670 This is Virginia Toy. You can probably see, 00:12:12.670 --> 00:12:15.010 the fault core is complicated, and it’s got cataclastic zones, 00:12:15.010 --> 00:12:18.139 but the principal slip zone, you can probably see this beautiful planar feature. 00:12:18.139 --> 00:12:22.040 It’s a perfectly blade-like feature going through there – very narrow slip. 00:12:22.040 --> 00:12:26.180 This is hundreds of kilometers of slip happening in a very narrow slip zone. 00:12:26.180 --> 00:12:30.760 Again, one of – probably one of the coolest things that’s happened in – 00:12:30.760 --> 00:12:33.459 since SAFOD, at least, is this JFAST drilling project. 00:12:33.459 --> 00:12:37.070 So directly after the Tohoku earthquake, they went out there – Fred Chester 00:12:37.070 --> 00:12:40.600 and a huge group of people – Emily Brodsky heavily involved – 00:12:40.600 --> 00:12:44.260 and drilled into the sweet spot. So where there’s about 50 meters’ slip, 00:12:44.260 --> 00:12:45.850 they drilled down, tried to get temperature measurements. 00:12:45.850 --> 00:12:50.790 But they also picked up some core. And this is cool because they identified, 00:12:50.790 --> 00:12:55.290 again, shear surfaces where it’s likely the majority of this slip happened. 00:12:55.290 --> 00:12:59.860 So again, this is huge amounts of slip happening in a very localized slip zone. 00:12:59.860 --> 00:13:04.019 More recently, the Kaikoura earthquake in 2016. 00:13:04.019 --> 00:13:07.760 I think you’ve seen some awesome – now in the days of drone footage, 00:13:07.760 --> 00:13:11.180 there’s all sorts of really cool footage here with these amazing scarps, 00:13:11.180 --> 00:13:17.060 again showing that this is very localized slip during earthquakes. 00:13:17.060 --> 00:13:19.630 This is work [inaudible] with Tom Rockwell and others 00:13:19.630 --> 00:13:21.370 on the Borrego Fault in Baja, California. 00:13:21.370 --> 00:13:26.600 Okay, you see this nice rupture trace here from the 2010 rupture. 00:13:26.600 --> 00:13:30.420 Okay, Tom Rockwell, I think, there for scale. This is a drone image again. 00:13:30.420 --> 00:13:35.149 Again showing very localized fault slip during the actual earthquake slip – 00:13:35.149 --> 00:13:37.480 the majority of the slip is taken. 00:13:37.480 --> 00:13:40.590 So I would argue that that’s why people study faults. 00:13:40.590 --> 00:13:43.760 This is why people study the physical properties of faults on the fault plane. 00:13:43.760 --> 00:13:46.470 It’s things like friction and things that are going on there that are going to 00:13:46.470 --> 00:13:49.070 control – because that’s where the slip is happening during earthquakes. Okay. 00:13:49.070 --> 00:13:51.980 And just to quickly recap. I don’t know what the audience – 00:13:51.980 --> 00:13:55.050 how many different – what areas and backgrounds are in the audience, 00:13:55.050 --> 00:13:58.779 but this is – just to go – step over friction, this is what we call Byerlee 00:13:58.779 --> 00:14:01.640 friction, which is a relationship between shear stress and normal stress. 00:14:01.640 --> 00:14:04.990 And these are actually experiments that were conducted here, I guess, 00:14:04.990 --> 00:14:06.930 many years ago. But this is fundamental, and it’s cool. 00:14:06.930 --> 00:14:09.899 Because what it does it is relates the relationship between the 00:14:09.899 --> 00:14:12.410 normal stress and the shear stress to a coefficient, which in this case, 00:14:12.410 --> 00:14:15.540 is between 0.6 to 0.85. So pretty much for all rocks, 00:14:15.540 --> 00:14:19.100 apart from a few clay-bearing rocks, we have this standard coefficient of friction. 00:14:19.100 --> 00:14:23.139 Now, one thing we found in recent years in the advent of new machines and 00:14:23.139 --> 00:14:27.800 high-velocity machines is that, when slip is localized in a narrow slip 00:14:27.800 --> 00:14:32.990 zone, and you have speeds of about a centimeter to 10 centimeters per second, 00:14:32.990 --> 00:14:36.200 friction can get actually significantly reduced for multiple reasons. 00:14:36.200 --> 00:14:38.850 And this is a nice plot from Wibberley et al. – a summary diagram 00:14:38.850 --> 00:14:41.540 of multiple studies on all sorts of experiments in the lab. 00:14:41.540 --> 00:14:43.530 This is showing friction as a function of slip rate. 00:14:43.530 --> 00:14:49.829 This is log slip rate, so here we’ve got, like, microns p-s and subduction rate 00:14:49.829 --> 00:14:53.490 speeds right up to 10 meters per second. And what this nicely shows is that, 00:14:53.490 --> 00:14:58.260 for a huge amount of slip ranges that we do have friction – Byerlee friction 00:14:58.260 --> 00:15:00.290 right up until we get to these high speeds, where actually 00:15:00.290 --> 00:15:03.649 friction drops right down to zero – okay, potentially zero here. 00:15:03.649 --> 00:15:06.510 And what does that mean? If the – if the friction of your car tires 00:15:06.510 --> 00:15:09.330 was zero, I mean, you could just kick your car, and off it would slide, okay? 00:15:09.330 --> 00:15:13.060 It’s – it means – and that’s important in terms of earthquake dynamics. 00:15:13.060 --> 00:15:17.459 So in terms of typical experiments, you know, in a sort of triaxial range here, 00:15:17.459 --> 00:15:19.860 we tend to be in this range. But in the advent of high-velocity 00:15:19.860 --> 00:15:23.110 apparatus range, and this is something that’s been on certainly in the 00:15:23.110 --> 00:15:25.800 last 15 years or so – people have been focusing on a lot of these processes that 00:15:25.800 --> 00:15:29.740 go on coseismically in terms of the frictional heating and the energy there. 00:15:29.740 --> 00:15:33.279 So coming back to this diagram. If we localize the slip – like, you know, 00:15:33.279 --> 00:15:36.339 meters of slip in a few seconds in a very narrow slip zone, and it’s got 00:15:36.339 --> 00:15:41.040 implications for frictional heating, okay? So for less than – if you localize the 00:15:41.040 --> 00:15:44.230 slip to less than a centimeter, okay, for, say, a meter of slip, 00:15:44.230 --> 00:15:47.690 you’re going to get temperature rises of 1,400 degrees Celsius, okay – 00:15:47.690 --> 00:15:50.250 frictional melting. And I think that’s all pretty accepted that happens. 00:15:50.250 --> 00:15:53.540 These bad boys – pseudotachylytes, pretty much everyone now accepts – 00:15:53.540 --> 00:15:58.060 or pretty much most people. There are still some non-believers, 00:15:58.060 --> 00:16:01.440 but pretty much everybody agrees that these are frozen frictional melts, okay? 00:16:01.440 --> 00:16:04.570 Often you have these cool asymmetric injection veins. 00:16:04.570 --> 00:16:05.820 And that’s because, when you melt a rock, 00:16:05.820 --> 00:16:08.820 you get 16 to 18% volume increase in its melt form. 00:16:08.820 --> 00:16:10.770 So it becomes immediately pressurized, 00:16:10.770 --> 00:16:12.940 and it wants to inject off into crack it can. 00:16:12.940 --> 00:16:16.100 And these are – these cracks are arguably related to the 00:16:16.100 --> 00:16:20.480 rupture tip propagation, which I’ll come to in a bit too. 00:16:20.480 --> 00:16:24.100 So these are cool. These suggest that we’ve got earthquakes. 00:16:24.100 --> 00:16:26.149 Now, many people in the beginning didn’t believe that necessarily. 00:16:26.149 --> 00:16:28.300 But now, in the lab, I think we can actually show that. 00:16:28.300 --> 00:16:31.150 This is experiments from Giulio Di Toro’s rig that we working on in the lab. 00:16:31.150 --> 00:16:32.899 This is high-velocity apparatus. 00:16:32.899 --> 00:16:36.990 We’re putting a load in this direction and then spinning it meters per second, okay? 00:16:36.990 --> 00:16:39.000 So what – there’s no external heating here, but what we’re showing 00:16:39.000 --> 00:16:41.940 is just within, you know, a few millimeters of slip, 00:16:41.940 --> 00:16:45.340 we’ve taken it from room temperature to melting temperature in a Gabbro, okay? 00:16:45.340 --> 00:16:48.000 And that’s just from frictional heating alone, okay? 00:16:48.000 --> 00:16:50.040 And then you can see it cools pretty quickly, in fact. 00:16:50.040 --> 00:16:52.699 It actually cools very quickly. And to point out that actually, 00:16:52.699 --> 00:16:54.779 once you get that in, you can actually weld the fault 00:16:54.779 --> 00:16:57.420 and actually regains complete strength. That’s also something we worked on 00:16:57.420 --> 00:17:01.750 and Dave’s also worked on a little bit with Brooks Proctor here. 00:17:01.750 --> 00:17:06.829 So – I mean, I guess – it’s very easy to melt a rock. I’ll just mention this now. 00:17:06.829 --> 00:17:09.520 And so lots of people argue – and if you imagine your typical aftershock sequence 00:17:09.520 --> 00:17:11.370 around a big earthquake with all sorts of events – 00:17:11.370 --> 00:17:13.870 like, tens of thousands of events with all sorts of slips. 00:17:13.870 --> 00:17:16.699 You know, I think at something like 100 MPa or something like that, 00:17:16.699 --> 00:17:18.960 you only need something like 20 or 30 microns for this – 00:17:18.960 --> 00:17:20.760 on normal stress to get to a melting temperature. 00:17:20.760 --> 00:17:25.230 So would literally expect to see, in the rock record, from exhumed active fault, 00:17:25.230 --> 00:17:27.220 pseudotachylytes everywhere. But actually, we don’t. 00:17:27.220 --> 00:17:29.799 Now, people have been fighting about this for a while. 00:17:29.799 --> 00:17:32.470 Maybe they are there and they just – things like glass, gets easy to replace, 00:17:32.470 --> 00:17:36.250 and we just don’t see it. I actually think there’s lots of 00:17:36.250 --> 00:17:38.010 reasons why you would inhibit the temperature rise. 00:17:38.010 --> 00:17:40.690 I’m just going to give you one nice example now of why 00:17:40.690 --> 00:17:43.559 temperature can be inhibited. This is some really nice experiments 00:17:43.559 --> 00:17:46.820 from my colleague Nick Brantut. And this is a high-velocity experiment. 00:17:46.820 --> 00:17:49.380 It’s a vertical machine, in a sense. It’s the same sort of experiment 00:17:49.380 --> 00:17:52.340 where you’re putting a load on, and then you are spinning at high speeds. 00:17:52.340 --> 00:17:54.610 But this is gypsum. Now, gypsum actually is 00:17:54.610 --> 00:17:57.789 very weak rock, and it dehydrates around 100 degrees, okay? 00:17:57.789 --> 00:18:00.830 So these are experiments where – so this is friction versus displacement. 00:18:00.830 --> 00:18:04.260 And this is – this is a zoom-in here of the first 10 meters. 00:18:04.260 --> 00:18:06.350 Okay, so I’m going to show you this video here. 00:18:06.350 --> 00:18:09.370 [loud humming from video] Skip. 00:18:09.370 --> 00:18:12.640 So what happens as soon as we start slipping … 00:18:14.580 --> 00:18:16.560 [loud grinding sounds] That’s not gouge. That’s steam. 00:18:16.560 --> 00:18:20.100 So we instantly hit 100 degrees Celsius, and we see the temperature 00:18:20.100 --> 00:18:26.360 [inaudible] vibration. So the temperature is rocketing past 00:18:26.360 --> 00:18:29.440 100 degrees while this reaction – this endothermic reaction is going on, 00:18:29.440 --> 00:18:32.260 all the energy has been used up for the reaction, okay? 00:18:32.260 --> 00:18:34.040 The steam’s coming out. Now, once this whole thing 00:18:34.040 --> 00:18:36.200 is dehydrated, you can see there’s no more steam coming out. 00:18:36.200 --> 00:18:37.840 You can start to see it glowing, okay? 00:18:37.840 --> 00:18:42.160 So this is where we come out with this [inaudible] here [inaudible], okay? 00:18:42.160 --> 00:18:47.000 So I think it’s kind of cool. And what this is showing is that 00:18:47.000 --> 00:18:50.120 the temperature [inaudible] in this case is buffered due to this 00:18:50.120 --> 00:18:52.960 endothermic dehydration reaction. It’s buffered at 100 degrees 00:18:52.960 --> 00:18:56.169 because that’s the dehydration reaction to the gypsum, okay? 00:18:56.169 --> 00:18:57.920 [grinding sounds stop] 00:18:57.920 --> 00:19:02.300 And so you can – so this is an important energy thing. 00:19:02.300 --> 00:19:05.539 So if you’ve got anything in your localized fault core that dehydrates, 00:19:05.539 --> 00:19:07.920 that could potentially buffer the temperature, right? 00:19:07.920 --> 00:19:10.049 Now, clays is a common thing in fault cores, right? 00:19:10.049 --> 00:19:12.800 And clays dehydrate in the range of 300 to 600 degrees, okay? 00:19:12.800 --> 00:19:16.320 So to me, I’m not worried about the fact that we don’t 00:19:16.330 --> 00:19:17.840 see pseudotachylytes everywhere. 00:19:17.840 --> 00:19:20.980 Because they can easily not – temperature can easily not go up. 00:19:20.980 --> 00:19:23.520 Also, if you’ve got water in there, the water can pressurize. 00:19:23.520 --> 00:19:26.940 If that water can’t escape because the wall rock is impermeable, 00:19:26.940 --> 00:19:28.960 that can actually thermally pressurize, 00:19:28.960 --> 00:19:31.260 reduce the normal stress, and buffer the temperature too. 00:19:31.260 --> 00:19:34.130 So there’s lots of reasons why you wouldn’t get heating, which is also 00:19:34.130 --> 00:19:36.490 based on, I would argue, the damage zone around that – 00:19:36.490 --> 00:19:38.190 what that’s doing in terms of the physical properties 00:19:38.190 --> 00:19:42.470 and what’s in the fault core. Okay, so I just wanted to explain that. 00:19:42.470 --> 00:19:45.809 I would argue that we actually understand this term a lot better now, 00:19:45.809 --> 00:19:48.940 and certainly with these high-speed experiments in the lab, okay? 00:19:48.940 --> 00:19:50.679 And certainly because we know the width of the zone. 00:19:50.679 --> 00:19:54.350 What we don’t really understand is how much energy is going into 00:19:54.350 --> 00:19:56.660 this off-fault fracturing process. Largely because we don’t really 00:19:56.660 --> 00:19:59.830 understand the – how much, for a single earthquake, 00:19:59.830 --> 00:20:02.160 what sort of area is the damage. Yes, we know how much 00:20:02.160 --> 00:20:04.930 damage is around a fault. We can go count it, right? 00:20:04.930 --> 00:20:07.870 But we don’t know how much of that damage went in for an individual event 00:20:07.870 --> 00:20:11.820 and how that overprinted over previous events, okay? 00:20:11.820 --> 00:20:15.780 So this is a cool paper – I think, at least – called “G,” and they put it in Journal of 00:20:15.780 --> 00:20:18.820 Seismology for a reason – to wave a flag by Stephen Nielsen and the guys there. 00:20:18.820 --> 00:20:20.640 And what they did was effectively compared 00:20:20.640 --> 00:20:22.950 fracture energy versus slip here. 00:20:22.950 --> 00:20:25.580 They compared it from a bunch of seismological studies, 00:20:25.580 --> 00:20:28.070 and then everything in red is high-velocity experiments. 00:20:28.070 --> 00:20:32.020 So what a lot of seismologists do when we’ve shown them friction 00:20:32.020 --> 00:20:35.059 experiments like this, like, well, this is really cool, but how does this help me? 00:20:35.059 --> 00:20:36.420 And that’s a good question. 00:20:36.420 --> 00:20:39.110 You know, all right, how does this help me as a seismologist? 00:20:39.110 --> 00:20:43.440 Well, what Stephen shows in this study is that – it’s quite cool, is that, 00:20:43.440 --> 00:20:48.270 in this sort of range of – in terms of the breakdown energy in the range of 00:20:48.270 --> 00:20:54.220 magnitude 3 to 7 earthquakes, okay, what he suggested, the G from 00:20:54.220 --> 00:20:58.000 lab experiments in red compare quite well with seismological estimates. 00:20:58.000 --> 00:21:01.620 But when you’re above sort of slips of, say, 10 centimeters, 00:21:01.620 --> 00:21:05.040 then these things start diverging. And what they argue in that paper is, 00:21:05.040 --> 00:21:08.220 maybe when you’re getting big earthquakes above magnitude 5 or 00:21:08.220 --> 00:21:10.870 slips above 10 centimeters, then maybe actually you’ve got 00:21:10.870 --> 00:21:16.130 a significant amount of energy which is going into inelastic yielding 00:21:16.130 --> 00:21:17.820 of the wall rocks, okay, they argue. 00:21:17.820 --> 00:21:19.580 Whether you agree with that or not, I like that idea. 00:21:19.580 --> 00:21:22.550 It’s cool that big earthquakes, there might significantly more amount of 00:21:22.550 --> 00:21:29.200 energy going into [static sounds] fractures. Is that me? Sorry. 00:21:29.200 --> 00:21:30.750 So just coming back to this fault structure. 00:21:30.750 --> 00:21:34.800 Okay, so now [static] … Is this me? I’m sorry. 00:21:36.180 --> 00:21:39.200 All right. Coming back to fault structure. Let’s recap. I mentioned this before. 00:21:39.200 --> 00:21:42.330 So what do these look like in the field, these fault damage zones, okay? 00:21:42.330 --> 00:21:44.460 This is a fault core in Chile. In this case, it’s green because it’s 00:21:44.460 --> 00:21:46.860 had a lot of hydrothermal alteration from fluids going through. 00:21:46.860 --> 00:21:49.559 And there’s a damage zone there. What we – what we typically do 00:21:49.559 --> 00:21:52.500 as structural geologists when looking at damage zones, we sort of plot – 00:21:52.500 --> 00:21:54.240 we go and count fracture density. It could be in the – 00:21:54.240 --> 00:21:57.800 it could be macroscopically or it could be in the microscope. 00:21:57.800 --> 00:21:59.080 In this case, it’s microscopic. 00:21:59.090 --> 00:22:02.120 We’ve got samples that we collected, you know, every 5 meters or so. 00:22:02.120 --> 00:22:04.419 And you can see a nice relationship between the microfracture density 00:22:04.419 --> 00:22:07.500 with distance from the fault, okay, and where this relationship hits 00:22:07.500 --> 00:22:11.530 the sort of background levels, we can sort of infer a zone of rock 00:22:11.530 --> 00:22:14.110 that’s related to the fault, or a damage zone. Okay. 00:22:14.110 --> 00:22:17.420 Now, actually, if you look at thin sections, you can see qualitatively that 00:22:17.420 --> 00:22:20.770 the microfracture density does decrease. I would point out these probably 00:22:20.770 --> 00:22:22.890 were permeable, but now they’re all fluid inclusion planes. 00:22:22.890 --> 00:22:25.570 In this study right now, they’re not permeable at all. 00:22:25.570 --> 00:22:27.380 They have completely healed up. 00:22:27.380 --> 00:22:29.960 Okay, and we’ll talk about that again later. 00:22:29.970 --> 00:22:33.760 And there’s also lots of nice geophysical evidence for fault zone damage. 00:22:33.760 --> 00:22:37.309 Okay, some great stuff coming out from Allam and Ben-Zion’s group. 00:22:37.309 --> 00:22:40.520 I’m showing low-velocity zones. You know, the more cracks, 00:22:40.520 --> 00:22:42.990 the lower the velocity, okay – seismic velocity. 00:22:42.990 --> 00:22:45.660 On the San Jacinto and around, there’s a huge amount of studies 00:22:45.660 --> 00:22:47.740 I probably don’t need to tell you all about. 00:22:47.740 --> 00:22:51.730 Also work by Elizabeth Cochran et al. also showing, again, nice examples 00:22:51.730 --> 00:22:53.550 where actually, in this case, they argue that these things 00:22:53.550 --> 00:22:55.790 are long-lived damage zones. 00:22:55.790 --> 00:23:01.280 And I will talk about longevity of damage zones a little bit later. 00:23:01.280 --> 00:23:04.880 There’s also evidence for healing of fault zones. 00:23:04.880 --> 00:23:06.820 Okay, so let’s imagine this is a schematic diagram 00:23:06.820 --> 00:23:09.180 of crustal velocity with time, okay? 00:23:09.180 --> 00:23:11.530 There’s some intact crustal rock. 00:23:11.530 --> 00:23:13.900 We have an earthquake, and we put a bunch of fractures in there. 00:23:13.900 --> 00:23:17.140 Okay, the seismic velocity goes down, cracks full of air, okay. 00:23:17.140 --> 00:23:22.450 Then what we often see after these several big earthquakes is this sort of 00:23:22.450 --> 00:23:26.539 time-dependent increase in Vp. Okay, so and this suggests that 00:23:26.539 --> 00:23:30.260 there’s something going on that’s giving us apparent healing of velocity. 00:23:30.260 --> 00:23:35.250 Now, I would argue we’ve got no idea what that – what the mechanics behind 00:23:35.250 --> 00:23:38.740 that are, on the small scale, at least. Because if you look at the majority 00:23:38.740 --> 00:23:40.830 of papers that talk about this, they actually end up referring to 00:23:40.830 --> 00:23:44.190 rate-and-state friction experiments, which has got nothing to do with the 00:23:44.190 --> 00:23:48.270 velocity of the bulk crust, I would argue. But we can argue that later. 00:23:48.270 --> 00:23:50.600 So that’s something that we’re certainly working on. I’ll come on later. 00:23:50.600 --> 00:23:56.650 This is a nice paper by these guys in Nature several years back showing 00:23:56.650 --> 00:24:00.380 this velocity drop after the Landers earthquake and this 00:24:00.380 --> 00:24:03.770 time-dependent increase, potentially, arguably interrupted 00:24:03.770 --> 00:24:06.660 by the Hector Mine earthquake. 00:24:08.190 --> 00:24:11.380 One thing I will add is that these sort of ray path velocity studies 00:24:11.390 --> 00:24:14.159 is in the top, you know, 4 – 3 or 4 kilometers. 00:24:14.159 --> 00:24:16.279 Okay, so these are where temperatures are probably 00:24:16.279 --> 00:24:17.800 not much higher than 100 degrees. 00:24:17.800 --> 00:24:19.850 So that’s important in terms of when we think about what these 00:24:19.850 --> 00:24:24.640 healing processes are, what are the processes controlling it. 00:24:24.640 --> 00:24:27.179 Another really nice example here in Science. 00:24:27.179 --> 00:24:30.910 This is [inaudible] velocity with time. Okay, after the Parkfield earthquake. 00:24:30.910 --> 00:24:35.110 This is 1 over the velocity, so it’s the inverse, so this is 00:24:35.110 --> 00:24:37.700 a velocity drop and then this really nice time-dependent healing. 00:24:37.700 --> 00:24:41.420 And this is something that we’ve been thinking about recently. 00:24:41.420 --> 00:24:43.850 What is controlling that? And I’ll come back again. 00:24:43.850 --> 00:24:46.441 But so I think the point is is that we get damage 00:24:46.441 --> 00:24:49.190 from earthquakes, but it also heals. And relatively fast time scales. 00:24:49.190 --> 00:24:50.480 We’re not talking about millions of years. 00:24:50.480 --> 00:24:53.150 There’s something that’s apparently healing, okay? 00:24:53.150 --> 00:24:58.270 What the actual healing means in terms of mechanisms, we don’t really know. 00:24:58.270 --> 00:25:01.590 So again, in summary, the processes for this are 00:25:01.590 --> 00:25:04.520 not well-understood, or at least are not well-understood to me. 00:25:04.520 --> 00:25:07.340 Now, to give you an idea of the scale of fault damage, okay, this is – 00:25:07.340 --> 00:25:10.159 this is a photo of Chuquicamata, which at one point, 00:25:10.159 --> 00:25:12.779 was one of the largest copper mines in the world. 00:25:12.779 --> 00:25:16.429 Okay, it’s about 1,200 meters deep. It’s a huge hole. Okay. 00:25:16.429 --> 00:25:18.430 The reason that’s there is because of that bad boy. 00:25:18.430 --> 00:25:21.080 That is an enormous strike-slip fault – the West Fissure Fault. 00:25:21.080 --> 00:25:23.400 Okay, if you look at a map of South America, every big mine 00:25:23.400 --> 00:25:27.640 is along that lineation because all these copper and gold and everything else 00:25:27.640 --> 00:25:30.490 is always related to faults with fluids coming up from depth. 00:25:30.490 --> 00:25:33.080 These fault systems really do control mass fluid movement, okay? 00:25:33.080 --> 00:25:36.150 Now, if I overlay the map there, okay, you can see this enormous – 00:25:36.150 --> 00:25:39.480 in purple here, this is a huge [inaudible] deposit 00:25:39.480 --> 00:25:42.070 controlled by fractures related to this fault, okay? 00:25:42.070 --> 00:25:44.549 And actually, people are still arguing – I’ve got to be careful because 00:25:44.549 --> 00:25:46.520 people around the world might be watching this – people still arguing 00:25:46.520 --> 00:25:48.730 about exactly what sort of fault type this is. 00:25:48.730 --> 00:25:51.179 But some people believe that there’s been something like, you know, 00:25:51.180 --> 00:25:53.980 30 kilometers of sinistral slip and then another 30 kilometers 00:25:53.980 --> 00:25:56.740 of dextral slip, at least. So currently, the other side 00:25:56.740 --> 00:25:58.990 of Chuquicamata is somewhere along the [inaudible] for Chile. 00:25:58.990 --> 00:26:00.929 Or, if you believe some people, it might be under the ground. 00:26:00.929 --> 00:26:02.570 Okay, so they’ll find it one day. 00:26:02.570 --> 00:26:05.270 But I wanted to show you, if we zoom into a spot here – 00:26:05.270 --> 00:26:08.429 this is a student of mine for scale – a supposedly inactive fault. 00:26:08.429 --> 00:26:11.720 You can see some recent movement there, relatively. 00:26:11.720 --> 00:26:14.200 This is a very narrow, localized fault core. 00:26:14.200 --> 00:26:17.179 But just to give you an idea of scale of these fracture systems, 00:26:17.179 --> 00:26:19.630 this little dot here – this is the trucks in there, okay? 00:26:19.630 --> 00:26:22.940 So these are absolutely gigantic fault fracture systems related to 00:26:22.940 --> 00:26:27.240 big previously active faults. And there are pseudotachylytes 00:26:27.240 --> 00:26:29.500 on this fault, so it certainly had earthquakes, okay? 00:26:29.500 --> 00:26:31.460 So we had masses of fluid movement through. 00:26:31.460 --> 00:26:33.510 And actually, I was – as a side story, this is – 00:26:33.510 --> 00:26:35.950 they actually did – give you the scale as well – 00:26:35.950 --> 00:26:39.060 they had the opera [inaudible] there on one of these banks. 00:26:39.060 --> 00:26:43.190 I’m not sure I’d want to spend too much time under a cliff face like this. 00:26:43.190 --> 00:26:46.529 But apparently, the president was happy, and he was in there. 00:26:46.529 --> 00:26:50.580 Another example here is a thrust fault in northern Iraq, okay, 00:26:50.580 --> 00:26:53.679 I was lucky enough to see. There’s a bodyguard for scale here. 00:26:53.679 --> 00:26:55.980 This is – the fault goes through here, but this is the good stuff. 00:26:55.980 --> 00:26:59.680 This is oil, showing very clearly that it’s going to come out of the fractures. 00:26:59.680 --> 00:27:04.800 It’s fractures controlling the geo-fluids, and in this case, it’s oil. Okay. 00:27:04.800 --> 00:27:06.990 Another example – again, and I showed this picture before. 00:27:06.990 --> 00:27:10.279 The Borrego Fault, okay, and it’s ruptured in 2010. 00:27:10.279 --> 00:27:12.640 I showed this image before, but this is something we’re working on 00:27:12.640 --> 00:27:14.850 in terms of the fracture patterns. In this case, we’re actually doing 00:27:14.850 --> 00:27:18.460 a very high-resolution fracture mapping. So here I’ve got a grad student 00:27:18.460 --> 00:27:22.270 who’s tracing out 15,000 fractures at multiple scales. 00:27:22.270 --> 00:27:24.169 And now we’re looking at the subsidiary faults. 00:27:24.169 --> 00:27:27.830 So if we look at the damage intensity maps, you can actually see 00:27:27.830 --> 00:27:30.419 that damage on earthquake faults is really complicated. 00:27:30.419 --> 00:27:34.790 It’s really heterogeneous, okay? And so simplifying that in a way 00:27:34.790 --> 00:27:37.669 that’s useful for us in terms of understanding how much energy 00:27:37.669 --> 00:27:42.220 goes into individual earthquakes is a complicated thing to do, okay? 00:27:42.220 --> 00:27:45.070 So damage is complex and heterogeneous. 00:27:45.070 --> 00:27:48.330 So if we think about the origins of fault damage, okay, this is a very 00:27:48.330 --> 00:27:52.090 simplified damage map here. This is – this is of the early stages 00:27:52.090 --> 00:27:56.640 of fault formation, okay, where we have maybe a fault growing, okay? 00:27:56.640 --> 00:27:59.950 As that fault grows, we get – so we get damage-related initiation. 00:27:59.950 --> 00:28:02.740 Of course, all faults end up, as we know in California, 00:28:02.740 --> 00:28:05.290 there’s lots of parallel strands. They start interacting with their 00:28:05.290 --> 00:28:08.530 neighbors, getting even more complicated in these linkage zones. 00:28:08.530 --> 00:28:11.440 Then you’ve got things like stress tip concentrations. 00:28:11.440 --> 00:28:14.240 Linear elastic fracture mechanics suggest the longer the crack – 00:28:14.240 --> 00:28:16.580 active crack length, the bigger the stress concentration. 00:28:16.580 --> 00:28:18.260 So you get process zone effects at the tip, 00:28:18.260 --> 00:28:20.080 and that gets bigger with the crack length. 00:28:20.080 --> 00:28:22.920 Of course, all faults are not really planar. They’re wavy and rough. 00:28:22.920 --> 00:28:24.541 So you get all sorts of scales of roughnesses. 00:28:24.541 --> 00:28:26.169 Emily Brodsky and people have worked a lot on this 00:28:26.169 --> 00:28:28.880 where you’ve got all sorts of variations – damage. 00:28:28.880 --> 00:28:30.560 Then, on top of that, you’ve got earthquakes 00:28:30.570 --> 00:28:32.440 putting all sorts of wacky stuff. And I’m going to show you some 00:28:32.440 --> 00:28:34.840 examples of wacky damage from earthquakes on top of that. 00:28:34.840 --> 00:28:39.470 So when you actually go to a fault that’s got – shear fault, okay, 00:28:39.470 --> 00:28:42.480 and I want to go and look at damage from earthquakes, not quasi-static stuff, 00:28:42.480 --> 00:28:45.320 it’s really hard to do, okay? I’m not trying to sort of 00:28:45.320 --> 00:28:47.049 give up here, but I’m just letting you know, 00:28:47.049 --> 00:28:49.011 as you probably all do know, it is difficult to do. 00:28:49.011 --> 00:28:52.750 And I think – I show this to students sometimes, this guy who I definitely 00:28:52.750 --> 00:28:57.419 don’t need to introduce famously once got up and said, it’s not a simple job. 00:28:57.419 --> 00:29:00.330 Fault zones are complicated. But it doesn’t have to put us off. 00:29:00.330 --> 00:29:02.560 There are ways of simplifying and understanding this. 00:29:02.560 --> 00:29:06.020 And certainly, I’m going to try and show you more in this study. 00:29:06.020 --> 00:29:11.020 This is some stuff – [inaudible] and so I thought, to explain this diagram, 00:29:11.020 --> 00:29:13.029 this might be a good place to show this in the talk. 00:29:13.029 --> 00:29:15.929 This is experiments where we actually have been trying to do imaging damage 00:29:15.929 --> 00:29:18.110 zones and slow rupture experiments in the lab. 00:29:18.110 --> 00:29:23.520 So this is triaxial experiments similar to what Dave et al. and the lab do here. 00:29:24.640 --> 00:29:27.720 Off-fault damage is important, though, because it’s a sink for rupture energy 00:29:27.730 --> 00:29:29.710 if you’ve got damage related to rupture tip propagation. 00:29:29.710 --> 00:29:33.400 And it will potentially affect the rupture directivity, the rupture velocity, 00:29:33.400 --> 00:29:35.919 and also the slip behavior. If you put more damage in, then it’s 00:29:35.919 --> 00:29:39.120 going to affect directly things like thermal pressurization, for example. 00:29:39.120 --> 00:29:42.590 So what we’re going to do here, so an example of characterizing seismic 00:29:42.590 --> 00:29:45.220 structure of a dynamically induced damage zone during slow rupture. 00:29:45.220 --> 00:29:48.570 And by slow rupture, I mean, what we’re going to do is use some clever 00:29:48.570 --> 00:29:52.770 techniques where we control the rupture propagation in the lab quite cleverly. 00:29:52.770 --> 00:29:55.010 I didn’t come up with this. 00:29:55.010 --> 00:29:56.919 Dave Lockner did, which I’ll summarize in a second. 00:29:56.920 --> 00:30:00.260 We’re going to do some time-resolved tomography on that using the AEs 00:30:00.260 --> 00:30:03.160 and the – and the active-source sensing, okay? 00:30:03.169 --> 00:30:06.020 So in our samples, okay, which I’ll come to in a second, 00:30:06.020 --> 00:30:09.169 we’re actually – we’re pulsing across the sample in all sorts of directions. 00:30:09.169 --> 00:30:11.770 We can actually measure the velocity across the sample. 00:30:11.770 --> 00:30:14.730 But we’re also collecting, you know, megahertz frequencies, 00:30:14.730 --> 00:30:17.620 the microseismicity – so the acoustic emissions, or AEs, 00:30:17.620 --> 00:30:18.960 and their arrival times. 00:30:18.970 --> 00:30:22.010 And we’re going to try and control the rupture and make it – 00:30:22.010 --> 00:30:24.789 make it propagate and measure what the velocity’s doing as we do that. 00:30:24.789 --> 00:30:26.700 And this is using a very cool methodology my colleague 00:30:26.700 --> 00:30:29.620 Nick Brantut – in GJI, just came out – hot off the press. 00:30:29.620 --> 00:30:32.840 And all of his codes to do this are online, so you can download this from GitHub, 00:30:32.840 --> 00:30:36.800 so I would – I suggest any of you interested to have a look at that. 00:30:36.800 --> 00:30:38.740 Postdoc Frans – so some of these are 00:30:38.740 --> 00:30:40.720 based on these controlled rupture experiments. 00:30:40.720 --> 00:30:43.340 And this is – and I don’t mind stroking Dave’s ego because it deserves to be. 00:30:43.340 --> 00:30:47.649 This is something that motivated me as an experimentalist many years ago. 00:30:47.649 --> 00:30:50.610 This graph showing stress versus strain, okay, it’s a typical one. 00:30:50.610 --> 00:30:53.500 But what Dave did in these experiments is he used his acoustic emissions. 00:30:53.500 --> 00:30:55.970 This part here normally happens incredibly quickly, okay? 00:30:55.970 --> 00:30:57.900 Where you fracture the rock, all the fractures localize, 00:30:57.900 --> 00:31:00.270 and the fault plane goes through, right? 00:31:00.270 --> 00:31:02.630 So what Dave did is actually manage to slow this down and actually 00:31:02.630 --> 00:31:08.120 propagate this fracture very slowly and look at how it propagated, okay? 00:31:08.120 --> 00:31:12.720 So if this – so this – so you manage to look at the – during the stress drop – 00:31:12.720 --> 00:31:15.299 actually, this is – you see the fractures coalescing and then 00:31:15.299 --> 00:31:17.620 propagating across the sample until you have stress drop. 00:31:17.620 --> 00:31:20.130 Now, this is the high-speed camera of a uniaxial experiment – 00:31:20.130 --> 00:31:24.909 similar thing, okay? That’s something like 30,000 frames per second, okay? 00:31:24.909 --> 00:31:29.260 I can’t do it here, but pretty much this rupture suddenly appears 00:31:29.260 --> 00:31:32.230 between two frames, okay? And I’m recording at 20,000 per second. 00:31:32.230 --> 00:31:35.690 So that ruptures goes across the sample by at least 800 meters per second. 00:31:35.690 --> 00:31:39.929 I had a student trying to do this, and he got it up to about 3 kilometers 00:31:39.929 --> 00:31:43.010 per second with tiny images. But they go really fast. 00:31:43.010 --> 00:31:45.700 And what Dave did was actually use the acoustic emissions as 00:31:45.700 --> 00:31:48.530 a feedback for his loading system. So as soon as this thing was, like, ooh, 00:31:48.530 --> 00:31:51.019 I’m going to start cracking loads and run away, it backed off 00:31:51.019 --> 00:31:52.519 the loading system to slow it down. 00:31:52.519 --> 00:31:55.070 So you could slow that down to sort of any time scale you want. 00:31:55.070 --> 00:31:56.559 So what we’re doing is a similar technique, 00:31:56.559 --> 00:31:59.039 but we’re adding tomography on top of that, and there’s an example. 00:31:59.039 --> 00:32:05.000 So this is similar setup to Dave’s here with six – we’ve got 16 Piezo-electric 00:32:05.000 --> 00:32:08.200 transducers, strain gauges in the sample, and we’re doing active acoustics. 00:32:08.200 --> 00:32:11.460 We’re pulsing every five minutes, and we’re getting a passive recording 00:32:11.460 --> 00:32:13.710 of AEs as well during the experiment. 00:32:13.710 --> 00:32:17.399 We’re not doing as automated as Dave. I’ve – my control system 00:32:17.399 --> 00:32:20.210 is a postdoc, Frans Aben. He’s sitting there actually waiting for 00:32:20.210 --> 00:32:22.399 the AEs, and he actually is backing it off manually, okay? 00:32:22.399 --> 00:32:25.620 I was too excited to try it. So watch this space. 00:32:25.620 --> 00:32:27.049 But still, we got some really cool results. 00:32:27.049 --> 00:32:29.299 Okay, so this is a video of that experiment. 00:32:29.299 --> 00:32:32.679 Okay, the AEs – so this is loading up. Okay, this is time in hours. 00:32:32.679 --> 00:32:36.060 So we can see we’ve managed to slow this bit down here as the rupture 00:32:36.060 --> 00:32:39.430 is propagating in terms of the AEs from bottom to top here. 00:32:39.430 --> 00:32:42.650 I’ll come back to this in more detail, but this just gives you an overview. 00:32:42.650 --> 00:32:48.580 So just run that again. You can see the – this bit that 00:32:48.580 --> 00:32:50.260 would normally happen very quickly, 00:32:50.260 --> 00:32:54.140 we’ve slowed that down over a few hours and got the AEs from that. 00:32:54.149 --> 00:32:59.370 So we’ve got 25,000 recorded AEs. 20,000 AEs after filtering. 00:32:59.370 --> 00:33:05.080 So 12,000 AEs, 16 channels, so we’ve got 192 arrival times manually checked. 00:33:05.080 --> 00:33:06.810 To say that – it was – Frans and Nick did that. 00:33:06.810 --> 00:33:11.120 I had no part in that. So thanks, guys. I’m not going to take credit for that. 00:33:11.120 --> 00:33:13.880 And it’s split into 38 bins. 00:33:13.880 --> 00:33:16.539 So this is a – I think a really cool video showing 00:33:16.539 --> 00:33:19.210 the same stuff, but with this – with this time result tomography on. 00:33:19.210 --> 00:33:21.529 So what you can see is the – so a fracture is propagating. 00:33:21.529 --> 00:33:23.470 You start developing a low-velocity zone. 00:33:23.470 --> 00:33:27.210 And you can see it’s propagating across the sample, which is kind of cool. 00:33:27.210 --> 00:33:29.210 And I think, with the interpolation, you’ve got something like 00:33:29.210 --> 00:33:33.299 a millimeter resolution here. So I’ll just show you that again. 00:33:33.300 --> 00:33:35.180 Check my time. 00:33:36.340 --> 00:33:39.120 You can see this thing propagating ahead of the microseismicity. 00:33:39.120 --> 00:33:42.399 You can see, actually, the velocity drops as well, which is kind of cool. 00:33:42.399 --> 00:33:44.630 And then once we get to here, we’re actually stable sliding, 00:33:44.630 --> 00:33:47.710 so then it’s just frictional processes going on. 00:33:47.710 --> 00:33:52.260 So I’ll show you some static images. This is – this is – these three stages 00:33:52.260 --> 00:33:55.450 A, B, and C. So A, B, and C is during the propagation. 00:33:55.450 --> 00:33:58.970 You can see this – you can see this cool low-velocity zone forming here. 00:33:58.970 --> 00:34:01.130 And then once you get to D, you’re actually sliding. 00:34:01.130 --> 00:34:03.750 So then, once you’re sliding, you see lots of AEs happening all over the fault, 00:34:03.750 --> 00:34:06.610 which is probably due to the roughness, which is kind of cool. 00:34:06.610 --> 00:34:08.720 So this actually shows quite nicely the – you know, 00:34:08.720 --> 00:34:11.350 the fault zone stands out with the low-velocity zone. 00:34:11.350 --> 00:34:13.870 We get P wave velocity drops by 1-1/2 kilometers per second. 00:34:13.870 --> 00:34:17.430 Okay, and a spacing of centimeters. And the low-velocity zone 00:34:17.430 --> 00:34:20.679 migrates with the rupture tip, okay? Interesting, actually, the P wave 00:34:20.679 --> 00:34:22.521 velocity recovers at the boundary of the damage zone. 00:34:22.521 --> 00:34:24.669 I can show – I think I can show you better in this plot. 00:34:24.669 --> 00:34:31.010 Okay, so this is the same thing again, but this is – oh – three positions. 00:34:31.010 --> 00:34:35.390 So these three lines represent the velocity at these three points. 00:34:35.390 --> 00:34:39.120 Okay, so you can see here, as the rupture passes this point, 00:34:39.120 --> 00:34:41.300 you see the velocity permanently reduces. 00:34:41.300 --> 00:34:44.379 But here the velocity drops further away from the fault zone, 00:34:44.380 --> 00:34:47.060 but then actually recovers a little bit, which is kind of cool. 00:34:47.920 --> 00:34:50.179 And this is actually kind of qualitatively similar to linear elastic 00:34:50.179 --> 00:34:55.089 fracture mechanics in terms of stress tips once the fracture is propagated. 00:34:55.089 --> 00:34:57.900 And then you’re in a sort of frictionally controlled environment. 00:34:57.900 --> 00:35:00.050 So watch this space. That’s a cool paper. 00:35:00.050 --> 00:35:04.230 So I think – to me, that really shows just, you know, the importance of damage 00:35:04.230 --> 00:35:07.100 related to – off-fault damage related to rupture propagation, 00:35:07.100 --> 00:35:10.600 at least at these scales in the lab. There’s always a scaling issue. Okay. 00:35:10.600 --> 00:35:12.520 And this is a bit of a texty slide, but I’ll whiz through it. 00:35:12.520 --> 00:35:15.970 To me, it’s really important, off-fault damage, because, you know, 00:35:15.970 --> 00:35:19.670 this reduced eleastic moduli, cohesion, and yield strength can affect – 00:35:19.670 --> 00:35:22.440 can cause attenuation and potentially nonlinear 00:35:22.440 --> 00:35:24.380 wave propagation effects during ruptures. 00:35:24.380 --> 00:35:27.030 Damaged rocks are more permeable. I’ve talked about that. 00:35:27.030 --> 00:35:29.270 And that can affect – over the seismic cycle. 00:35:29.270 --> 00:35:31.910 But this damage generation as the earthquake propagates itself 00:35:31.910 --> 00:35:34.090 can influence the dynamics of rupture propagation. 00:35:34.090 --> 00:35:36.820 So what’s happening at the tip can affect actually what’s happening 00:35:36.820 --> 00:35:40.020 at the slip behind the tip, okay? Can increase the amount 00:35:40.020 --> 00:35:42.490 of energy dissipation. It can decrease the rupture velocity, 00:35:42.490 --> 00:35:44.700 depending on what’s going. And it can modify the – potentially, 00:35:44.700 --> 00:35:48.290 the size of the earthquake and change also the efficiency of 00:35:48.290 --> 00:35:51.280 weakening mechanisms, such as thermal pressurization. 00:35:51.280 --> 00:35:52.620 There was a question. 00:35:52.620 --> 00:35:56.680 - And that’s what the [inaudible] experiment, [inaudible]? 00:35:57.480 --> 00:36:03.140 - They are – it’s a mode two shake rack. Yep. 00:36:03.140 --> 00:36:08.329 So we can come back and talk about that later. 00:36:08.329 --> 00:36:11.080 But I think the point is is that, if you put this fracture damage in, 00:36:11.080 --> 00:36:13.310 that’s going to affect things like thermal pressurization, 00:36:13.310 --> 00:36:15.600 which rely on the importance of the the permeability off-fault. 00:36:15.600 --> 00:36:19.170 Okay, so to me, all of these effects imply there’s a strong feedback 00:36:19.170 --> 00:36:21.310 between the damage imparted immediately after the rupture 00:36:21.310 --> 00:36:24.410 propagation but prior to the early stage of fault slip, okay? 00:36:24.410 --> 00:36:27.460 And that’s going to affect subsequent rupture dynamics. 00:36:27.460 --> 00:36:29.840 So in terms of field evidence of earthquake faulting, 00:36:29.840 --> 00:36:31.810 certainly in the fault core, in terms of frictional melting, 00:36:31.810 --> 00:36:34.920 we definitely got this one thing that we – pseudotachylytes. 00:36:34.920 --> 00:36:37.360 Something that I’ve also been worked on is pulverized fault rocks. 00:36:37.369 --> 00:36:40.530 Okay, this is something that Tom Rockwell’s worked a lot on, 00:36:40.530 --> 00:36:43.089 and Yehuda Ben-Zion and so on. I’m not going to go too much 00:36:43.089 --> 00:36:45.810 into the mechanisms of how you generate the stresses. 00:36:45.810 --> 00:36:49.240 But I’m certainly convinced in terms of the relationship 00:36:49.240 --> 00:36:52.589 between pulverization and earthquake rupture. 00:36:52.589 --> 00:36:54.170 For those of you who haven’t heard about pulverized rocks, 00:36:54.170 --> 00:36:56.130 they’ve been described a lot in the San Andreas Fault. 00:36:56.130 --> 00:36:58.290 On many other faults too – several papers. 00:36:58.290 --> 00:37:02.599 And this is cool stuff in that it’s got – it looks like an intact granite. 00:37:02.599 --> 00:37:04.349 But if you pick it up, it just crumbles to bits. 00:37:04.349 --> 00:37:06.210 It’s highly fractured on the [inaudible] scale, 00:37:06.210 --> 00:37:07.859 but it doesn’t have any apparent shear. 00:37:07.859 --> 00:37:12.510 Okay, so it’s a bit texty, but it’s shattered to a very fine grain size. 00:37:12.510 --> 00:37:15.590 You’ve got a lot of dilational microfractures. 00:37:15.590 --> 00:37:18.620 No significant shear strain. If you think about a classic fault rock, 00:37:18.620 --> 00:37:20.420 when you – that has a lot of shear, you’re going to have 00:37:20.420 --> 00:37:24.369 cataclastic behavior, grain size reduction, and so on. 00:37:24.369 --> 00:37:26.109 But it’s spatially related to fault, okay? 00:37:26.109 --> 00:37:28.010 It decreases as you go away from fault zones. 00:37:28.010 --> 00:37:30.839 And some people also suggest that perhaps it’s asymmetric. 00:37:30.839 --> 00:37:33.150 Here’s a nice example here from Wechsler et al. comparing 00:37:33.150 --> 00:37:36.630 pulverization to cataclastic. This is a standard fault rock with low strain. 00:37:36.630 --> 00:37:40.080 This is strained rock, but it doesn’t have a lot of offset, 00:37:40.080 --> 00:37:43.060 so you can see whole grains here in these very unique angular rocks. 00:37:43.070 --> 00:37:47.970 So this stuff looks like it’s potentially explode in situ, okay? 00:37:47.970 --> 00:37:50.980 Right down to the fine grain size. You’ve got individual grains here. 00:37:50.980 --> 00:37:53.390 For those of you who know anything about microscopes and cross-polars, 00:37:53.390 --> 00:37:55.859 none of this stuff shows any rotation. 00:37:55.859 --> 00:38:00.089 These things almost been – sort of imploded or exploded in situ, okay? 00:38:00.089 --> 00:38:03.310 And again – and completely shattered, right down to the micron scale. 00:38:03.310 --> 00:38:07.970 So quite a lot of fracture energy has gone in to do – a lot of the damage. 00:38:07.970 --> 00:38:11.280 This is a – this is a nice paper where we’ve been trying to, previously, 00:38:11.280 --> 00:38:14.930 on the San Andreas, combine seismological observation tomography 00:38:14.930 --> 00:38:17.480 and relate that to small scale, to varying degrees of success. 00:38:17.480 --> 00:38:21.060 Probably not going to go into that, but if you’re interested in knowing 00:38:21.060 --> 00:38:23.390 the sort of 3D structure of this sort of damage, then I would 00:38:23.390 --> 00:38:26.560 point you to this Rempe et al. paper in JGR 2013. 00:38:26.560 --> 00:38:31.099 But the thing I want to go to is – for the last part of the talk, is – 00:38:31.099 --> 00:38:35.329 what I’m interested in is these – this fast loading in terms of compression 00:38:35.329 --> 00:38:37.740 and tensional damage, okay? And what do I mean by that? 00:38:37.740 --> 00:38:41.460 If we have a fault, and the rupture is zipping on in one direction, very simply, 00:38:41.460 --> 00:38:44.450 okay, we’re going to have a tensional side and we’re going to have 00:38:44.450 --> 00:38:47.230 a compressional side, right? So often you get fractures like this 00:38:47.230 --> 00:38:49.130 because it’s – rocks are 10 times weaker in tension, 00:38:49.130 --> 00:38:52.119 so you’re going to get – easier to get damage there. 00:38:52.119 --> 00:38:55.260 But the point is that these ruptures move very quick. 00:38:55.260 --> 00:38:57.800 And in fact, the reality is, it’s – behind the rupture tip, you’ve actually 00:38:57.800 --> 00:39:01.510 got the opposite of that, okay? So you’ve got the inverse of that and 00:39:01.510 --> 00:39:03.510 something that looks more like that – your compression and tension. 00:39:03.510 --> 00:39:06.310 So if you imagine you’re a bit of rock standing there, as this rupture zips past, 00:39:06.310 --> 00:39:10.380 you’re going to have a very high strain rate loading, okay, and, you know, 00:39:10.380 --> 00:39:12.890 strong compressional tension as that rupture goes past, okay? 00:39:12.890 --> 00:39:14.990 And that’s going to happen multiple times. 00:39:14.990 --> 00:39:17.240 So I’m going to just show you – quickly show you two examples 00:39:17.240 --> 00:39:19.300 of experiments we’re doing to explore this sort of damage. 00:39:19.310 --> 00:39:21.460 I’m trying to figure out how you make pulverized rocks. 00:39:21.460 --> 00:39:24.230 I’m not sure I’ve answered that yet, but we’ve got some stuff that looks like 00:39:24.230 --> 00:39:26.440 the real deal. So I’m going to do two experiments here. 00:39:26.440 --> 00:39:29.390 We do high strain rate compressive loading and also explosive dilation 00:39:29.390 --> 00:39:31.980 and tension. Okay, and that’s what I’m going to show you next. 00:39:31.980 --> 00:39:34.579 Just to show you a nice example of that, this is work by Di Toro et al. and 00:39:34.579 --> 00:39:36.980 Stephen Nielsen where they’ve showed modeling, you know, on a standard 00:39:36.980 --> 00:39:42.390 sub-shear rupture propagating crack. This is just a – the principal stresses 00:39:42.390 --> 00:39:44.790 of tensile field. And often, you know, 00:39:44.790 --> 00:39:48.050 in the tensile field, it predicts that you get these high-angle fractures. 00:39:48.050 --> 00:39:49.740 And that’s often what we see on pseudotachylytes. 00:39:49.740 --> 00:39:51.640 Okay, you can actually infer the rupture direction from the 00:39:51.650 --> 00:39:57.089 pseudotachylytes – the injections. As the rupture propagated, you got the – 00:39:57.089 --> 00:40:01.339 at first, you got the high-angle mode one cracks opening, 00:40:01.339 --> 00:40:04.290 and then the frictional melt was related to the slip is behind that. 00:40:04.290 --> 00:40:06.050 And then it’s injected into those cracks. 00:40:06.050 --> 00:40:10.470 Those cracks are then also propagated by the pressurization of the melt. 00:40:10.470 --> 00:40:12.480 So let’s imagine that we’re on the San Andreas Fault. 00:40:12.480 --> 00:40:13.510 Let’s flip him around. 00:40:13.510 --> 00:40:18.100 This is an animation that sometimes works and didn’t work, so there we go. 00:40:18.100 --> 00:40:20.000 Let’s see if we can give it a kickstart, 00:40:20.000 --> 00:40:21.869 and then maybe it’ll start to work in a bit. 00:40:21.869 --> 00:40:25.280 Okay, what that would normally show would be, we’re at point P, 00:40:25.280 --> 00:40:27.290 and a rupture would be zipping past here, okay? 00:40:27.290 --> 00:40:29.150 So just imagine our rupture is shooting along here, 00:40:29.150 --> 00:40:32.069 and I’m a bit of rock there, okay? It means that we’re going to have 00:40:32.069 --> 00:40:36.380 some sort of high strain rate loading related to that rupture, okay? 00:40:36.380 --> 00:40:38.800 Moving past pretty quickly, right? 00:40:38.800 --> 00:40:42.290 So if we look at, say, the particle velocity as the rupture goes past. 00:40:42.290 --> 00:40:45.900 It means we’re going to have very – we’re going to get stress waves. 00:40:45.900 --> 00:40:48.589 So it means a loading. But for very short durations, okay? 00:40:48.589 --> 00:40:52.990 So they’re going to have high-stress amplitudes, but short loading durations. 00:40:52.990 --> 00:40:59.250 Which is why we get high strain rate loading, okay? Off-fault. Okay. 00:40:59.250 --> 00:41:03.240 So this is just a very simple schematic diagram showing a 00:41:03.240 --> 00:41:07.260 schematic fault and then the same sort of rupture tip migrating along. 00:41:07.260 --> 00:41:09.800 The sort of strain rates you get here is just a very simple 00:41:09.800 --> 00:41:13.680 calculation from Rice formulations of the strain rates that you would get. 00:41:13.680 --> 00:41:16.440 You can get strain rates of, you know, 100 per second in sub-shear ruptures 00:41:16.440 --> 00:41:19.380 very close to the fault quite easily in terms of – and bear in mind, 00:41:19.380 --> 00:41:22.119 we’re talking, in normal experiments in the lab. 00:41:22.119 --> 00:41:26.050 Experiments of 10 to the minus 5, 4 – these are strain rates of, like, 00:41:26.050 --> 00:41:29.790 100 per second to sometimes 1,000 per second. So very fast strain rates. 00:41:29.790 --> 00:41:33.359 So we can do – we can reproduce the damage we see off-fault and strain – 00:41:33.359 --> 00:41:36.220 you know, in standard triaxials, but the sort of pulverization we see 00:41:36.220 --> 00:41:39.780 close to the fault requires you to have strain rate – much higher strain rates. 00:41:39.780 --> 00:41:41.220 And that’s what we’re going to show you today, 00:41:41.220 --> 00:41:45.560 some experiments of pulverization at high speed and also some – 00:41:45.560 --> 00:41:49.099 so, on the other side of the fault, you also have very high strain rate 00:41:49.099 --> 00:41:52.250 unloading, so tensional damage as you go past. 00:41:52.250 --> 00:41:56.589 So just going quickly to summarize, if we squash a rock slowly, okay – 00:41:56.589 --> 00:41:59.560 so that’s our rock diagram, okay? As I showed you before with the 00:41:59.560 --> 00:42:02.730 Lockner stuff and the stuff we had done, you’re going to have – 00:42:02.730 --> 00:42:07.760 all the cracks are going to coalesce. And – sorry, I jumped the gun there. 00:42:07.760 --> 00:42:09.640 All the cracks are going to coalesce, and you’re going to form a single 00:42:09.640 --> 00:42:13.180 fracture, okay, at low strain rates. Yeah? We know that. 00:42:13.180 --> 00:42:16.520 But if you load a rock above a critical strain rate, very, very quickly, 00:42:16.520 --> 00:42:19.730 you’re actually squashing the rock faster than it can elastically expand. 00:42:19.730 --> 00:42:21.990 So what you actually get is a – rather than one big fracture, 00:42:21.990 --> 00:42:24.380 you get a lot of small cracks propagating. It’s called multi-fracturation. 00:42:24.380 --> 00:42:30.300 It’s quite well-known in the – in the ceramics and impact ballistics industry. 00:42:30.300 --> 00:42:32.680 And this is something that several people – Mai-Linh Doan 00:42:32.690 --> 00:42:35.880 and Prakash and all sorts of people – and Terry Tullis have been working 00:42:35.880 --> 00:42:39.300 over the years as perhaps how we get this pulverization. 00:42:39.300 --> 00:42:43.569 And the pulverized rocks don’t have macroscopic damages. 00:42:43.569 --> 00:42:45.500 Mostly intense micro-scale damage. 00:42:45.500 --> 00:42:47.630 So again, I showed you this video before. 00:42:47.630 --> 00:42:51.920 This is where we have a localization of a single fracture from multiple 00:42:51.920 --> 00:42:54.910 microfractures coalescing. And then, in these experiments here – 00:42:54.910 --> 00:42:58.990 this is from – animated from Yuan et al. is within microseconds. 00:42:58.990 --> 00:43:04.010 So the same peak stress, you’re getting, like, massive, pervasive pulverization. 00:43:04.010 --> 00:43:07.170 So these are experiments – I’ve actually – it’s a bit of a messy diagram, but this is 00:43:07.170 --> 00:43:11.609 some experiments we did in Paris there. This is virtually a gas gun. 00:43:11.609 --> 00:43:13.580 So all it is – you’ve got two metal bars where the sample is. 00:43:13.580 --> 00:43:15.780 This is a simple version of this. Two metal bars with 00:43:15.780 --> 00:43:18.090 the sample in the middle. We fire a bullet, effectively, into that. 00:43:18.090 --> 00:43:20.950 It sets a stress wave through that. 00:43:20.950 --> 00:43:23.850 It goes through the sample, goes through the output bar and comes back. 00:43:23.850 --> 00:43:26.930 And you actually use wave propagation theory to figure out what 00:43:26.930 --> 00:43:30.550 your stress in the sample is indirectly. But that allows you to, say, 00:43:30.550 --> 00:43:34.619 load your sample to a couple hundred MPa extremely fast – you know, 00:43:34.619 --> 00:43:38.380 milliseconds, giving you strain rates of 1,000 per second or something like that. 00:43:38.380 --> 00:43:41.790 So if you just increase the – this is some of Mai-Linh’s experiments – 00:43:41.790 --> 00:43:44.470 just increase the strain rate – this is a section-through sample – 00:43:44.470 --> 00:43:47.530 for the same peak stress, you can see that you’re going 00:43:47.530 --> 00:43:51.829 towards this pulverization, or multi- fracturation, which is kind of cool. 00:43:51.829 --> 00:43:54.300 So we’ve been kind of looking at this damage, and it’s cool stuff, 00:43:54.300 --> 00:43:57.220 I would argue. This is a graph of stress versus strain – 00:43:57.220 --> 00:44:02.300 a very common diagram that we have in the lab on some sort of tonalitic rocks. 00:44:02.300 --> 00:44:06.880 Okay, in this case – but these are strain rates of 25 per second, okay? 00:44:06.890 --> 00:44:10.480 Again, just remind you that, in standard triaxial experiments, 00:44:10.480 --> 00:44:12.440 we’re talking of something on the order of 10 to the minus 5, 00:44:12.440 --> 00:44:15.920 something like this – really, really fast – orders of magnitude faster. 00:44:17.420 --> 00:44:19.900 So if we actually look at the – so the permeability. 00:44:19.900 --> 00:44:21.830 So what we did is, we impacted these rocks 00:44:21.830 --> 00:44:24.230 and then put them in a permeameter. 00:44:24.230 --> 00:44:27.200 This is permeability as a function of strain rate. Again, very fast. 00:44:27.200 --> 00:44:31.410 Okay, and this – so each one of these is a sample, and the different colors is 00:44:31.410 --> 00:44:33.900 effective pressure. So I measured the permeability, different effective 00:44:33.900 --> 00:44:37.050 pressure. So you just ignore all these and maybe just focus on the red one. 00:44:37.050 --> 00:44:41.280 What we can see with a single impact – say, for example, nearly 20 per second, 00:44:41.280 --> 00:44:45.090 I can increase single shockwave loading. I can increase the permeability of 00:44:45.090 --> 00:44:49.839 tonalite from 10 to the minus 20 to nearly 10 to the minus 15 instantaneous. 00:44:49.839 --> 00:44:54.020 Huge increase in permeability just by putting a bunch of damage in there. 00:44:54.020 --> 00:44:57.140 And this is kind of cool. If you look at the thin – the samples 00:44:57.140 --> 00:44:59.600 of these, okay, you’re actually – this is increasing strain rate. 00:44:59.600 --> 00:45:01.579 You can see we’re actually getting more and more pervasive damage. 00:45:01.580 --> 00:45:04.420 This is really starting to look like this pulverization that we see in the field, 00:45:04.420 --> 00:45:05.800 at least – at least qualitatively. 00:45:05.800 --> 00:45:08.060 And we’re working on this quantitatively. 00:45:08.060 --> 00:45:11.320 If I look – the same sample. This is a 3-by-3-centimeter sample 00:45:11.320 --> 00:45:13.819 chopped in half. You can actually see there’s still a complete lack 00:45:13.819 --> 00:45:17.980 of macroscopic fractures, okay? It’s all pervasive microfractures. 00:45:17.980 --> 00:45:20.220 Now, interesting – for those of you that know anything about 00:45:20.220 --> 00:45:23.570 permeability-porosity relationships, the permeability-porosity relationship 00:45:23.570 --> 00:45:26.440 with this – I mean, these samples, when I took them out of the rig, were boring. 00:45:26.440 --> 00:45:29.920 I was, like, oh, god, nothing’s happened. But the permeability is gigantic. 00:45:29.930 --> 00:45:32.670 And actually, the permeability-porosity relationship is something that’s got an 00:45:32.670 --> 00:45:35.630 exponent of something like 2, which is what you have for tubular sandstones. 00:45:35.630 --> 00:45:37.720 It’s got these really weird hydraulic properties 00:45:37.720 --> 00:45:40.310 I have not seen for any other granitic rock. 00:45:40.310 --> 00:45:43.250 So we’ve been looking at the damage of this quantitatively now at this scale. 00:45:43.250 --> 00:45:46.140 So this is the same plot again. In this case, the same samples, 00:45:46.140 --> 00:45:48.099 but you can see I’ve actually traced the fractures here. 00:45:48.099 --> 00:45:50.349 Don’t know if you can see that. You can certainly see that the 00:45:50.349 --> 00:45:52.480 fracture density increases with increasing strain rate. 00:45:52.480 --> 00:45:55.950 The cool thing, actually, I didn’t mention is that the – actually, the fracture density 00:45:55.950 --> 00:45:59.150 dropped – sorry, the permeability drops at the highest strain rates. 00:45:59.150 --> 00:46:02.530 And that’s because it’s becoming a gouge. It still looks like an intact rock. 00:46:02.530 --> 00:46:05.300 And the fracture density is increasing, but can you actually see the – 00:46:05.300 --> 00:46:09.109 actually the average fracture length is decreasing as we go up. 00:46:09.109 --> 00:46:13.431 Okay, and permeability – so this is quantitative analysis of this. 00:46:13.431 --> 00:46:15.460 This is fracture density in red. You can see with increasing 00:46:15.460 --> 00:46:18.400 strain rate, fracture density goes up. You can see that quite easily. 00:46:18.400 --> 00:46:23.160 But interestingly, the highest strain rates, the average fracture length decreases. 00:46:23.160 --> 00:46:26.680 So we got tons of fractures, but really short ones, okay? 00:46:26.680 --> 00:46:29.070 And they’re so short, they’re not actually connecting with each other. 00:46:29.070 --> 00:46:31.940 So actually the permeability is lower, even though there’s 00:46:31.940 --> 00:46:33.880 higher fracture density. They’re just so short, they’re not 00:46:33.880 --> 00:46:36.500 connecting, and there’s no fluid pathways, which I think is kind of cool. 00:46:36.500 --> 00:46:39.140 And you can model that with a classic fluid flow equations of permeability 00:46:39.140 --> 00:46:42.640 relating aperture, crack density, and length, okay? 00:46:42.640 --> 00:46:46.960 And just putting in these actual values of crack length and density, we can actually 00:46:46.960 --> 00:46:51.530 get this flip. So it’s strongly controlled by this transition to small cracks. 00:46:51.530 --> 00:46:54.970 And these small – this transition to small cracks is unique to this high strain 00:46:54.970 --> 00:46:59.750 rate damage – okay, this is pervasive. Again, this is the same data in pink 00:46:59.750 --> 00:47:03.589 that I just showed you before, okay? But I’ve also added a stronger rock 00:47:03.589 --> 00:47:07.470 of a Chinese version of Westerly granite – pandang granite. 00:47:07.470 --> 00:47:10.070 You can see that, for impacts of, say, 30 per second, 00:47:10.070 --> 00:47:12.930 I’ve got permeabilities of 10 to the minus 13. 00:47:12.930 --> 00:47:15.240 Again, these things aren’t broken macroscopically. 00:47:15.240 --> 00:47:17.609 These are enormous permeabilities. I didn’t even believe them in 00:47:17.609 --> 00:47:19.410 the beginning, they were so high, which is kind of cool. 00:47:19.410 --> 00:47:23.060 So this is really, really weird hydraulic properties here. 00:47:23.060 --> 00:47:26.800 Then we’re actually going on to the onset of pulverization. Okay. 00:47:27.600 --> 00:47:29.800 Now, one of the things we have been looking at – and I guess the title of 00:47:29.800 --> 00:47:35.320 my talk – is this – is this idea of, okay, if you think about faults – if you think 00:47:35.320 --> 00:47:40.020 about off-fault damage, you’re going to have multiple earthquakes, with time, 00:47:40.020 --> 00:47:42.430 going past them, right? You’re going to have lots of overprinting damage. 00:47:42.430 --> 00:47:45.510 Okay, certainly a mature fault has had tens of thousands of earthquakes. 00:47:45.510 --> 00:47:47.630 So we’ve been trying to look at the effects of – I’m not going to 00:47:47.630 --> 00:47:50.790 go into this too much – of the effects of multiple impacts. 00:47:50.790 --> 00:47:54.790 Okay, so yes, you can go very, very fast and pulverize this thing and lose all the 00:47:54.790 --> 00:47:59.000 strength, but what about if you maybe go to 60% of the – you know, 00:47:59.000 --> 00:48:03.059 the pulverization threshold multiple times and bang it a few times? 00:48:03.059 --> 00:48:04.911 Does that overall weaken it? Well, what we actually show is, 00:48:04.911 --> 00:48:08.982 this is the – so this – I haven’t mentioned this yet – the pulverization threshold. 00:48:08.982 --> 00:48:11.640 The relationship between strain rate and peak strength 00:48:11.640 --> 00:48:14.829 is actually linear up until you get to this pulverization threshold. 00:48:14.829 --> 00:48:17.250 Then the thing just explodes, and you lose it. 00:48:17.250 --> 00:48:20.790 If I impact it – so, once you hit it once, the actual threshold drops down 00:48:20.790 --> 00:48:23.420 for the next time. So if you hit it once below 00:48:23.420 --> 00:48:26.730 the threshold, then the next time you do it, it’s much easier to pulverize it. 00:48:26.730 --> 00:48:29.200 So if you hit it lots of times, you can actually pulverize it as well. 00:48:29.200 --> 00:48:33.980 So it’s clearly – I guess one of the questions to me, these huge – 00:48:33.980 --> 00:48:38.000 is this just some enormous supershear damage? I don’t know. 00:48:38.000 --> 00:48:40.360 You know, I could look at the fault. Is that what it’s – probably not. 00:48:40.360 --> 00:48:44.720 It’s very likely related to multiple incremental damage overprinted. 00:48:44.720 --> 00:48:46.400 What this also means, without going into it too much, 00:48:46.400 --> 00:48:49.300 and it’s what we focused on in this paper is that actually, you then got feedback 00:48:49.300 --> 00:48:51.750 saying – in terms of the elasticity. So you can – feedback, and actually 00:48:51.750 --> 00:48:54.119 you’re just going to start – the more – the more damage there is, 00:48:54.119 --> 00:48:56.640 the easier it is to pulverize. So we start localizing the damage. 00:48:56.640 --> 00:48:58.430 And that’s exactly what we see in the San Andreas. 00:48:58.430 --> 00:49:02.500 We see localization of damage very close to the fault core. 00:49:02.500 --> 00:49:05.370 So some implications of that. Okay, one thing that interests me 00:49:05.370 --> 00:49:08.250 was that, like, the highest density of fractures close to the fault 00:49:08.250 --> 00:49:11.030 doesn’t necessarily mean the highest permeability, which is something 00:49:11.030 --> 00:49:13.740 I’ve always been working on the opposite on myself, at least in terms of 00:49:13.740 --> 00:49:15.970 dynamic damage, which is kind of cool. And the implications of that – 00:49:15.970 --> 00:49:19.260 and this is a classic Swanson diagram here, which is just 00:49:19.260 --> 00:49:23.440 a diagram of a fracture tip with time. This is the rupture propagation. 00:49:23.440 --> 00:49:26.870 So let’s imagine the rupture zipping along at 3 kilometers per second. 00:49:26.870 --> 00:49:29.829 I’m the rupture. I’m running that way at 3 kilometers per second. 00:49:29.829 --> 00:49:32.079 Behind me, the slip starts, and that’s a few meters per second. 00:49:32.079 --> 00:49:34.890 If me, as the rupture, has imparted a huge amount of damage, 00:49:34.890 --> 00:49:38.150 depending on what speed I went through, the slip is going to see 00:49:38.150 --> 00:49:40.690 variable physical properties depending on what the rupture did 00:49:40.690 --> 00:49:43.740 in terms of hydraulics, permeability. And that’s going to potentially act as 00:49:43.740 --> 00:49:49.839 a toggle switch to promote potentially a fault arrest or fault slip in terms of, say, 00:49:49.839 --> 00:49:51.760 dynamic weakening mechanisms such as thermal pressurization. 00:49:51.760 --> 00:49:55.330 If I put a huge amount of damage in there, and it’s super permeable, 00:49:55.330 --> 00:49:57.530 the fluid’s going to escape. It won’t pressurize. 00:49:57.530 --> 00:50:01.710 If it’s so pulverized that it’s – the permeability is actually quite low, 00:50:01.710 --> 00:50:04.500 then maybe it’d very efficient, for example. If you see what I’m getting at. 00:50:04.500 --> 00:50:07.530 The other mechanism I’m going to quickly whip through is just this – 00:50:07.530 --> 00:50:09.180 I mentioned that, when you have a fault, 00:50:09.180 --> 00:50:11.760 you’ve got a compressional side and a tension side. 00:50:11.760 --> 00:50:14.540 One of the things we try to do is also look at very fast decompression. 00:50:14.540 --> 00:50:18.260 So the idea is that – so you might suddenly unload your fault dynamically, 00:50:18.260 --> 00:50:20.859 take off all the stress, okay? One way – I’m not going to get 00:50:20.859 --> 00:50:23.530 into the mechanism that might do that. But let’s imagine we have sudden stress 00:50:23.530 --> 00:50:27.900 changes as the earthquake propagates, and you have tensional – 00:50:27.900 --> 00:50:30.059 so the idea I’m working on is, imagine this is my rock. 00:50:30.059 --> 00:50:31.809 And suddenly I take off all the confining pressure. 00:50:31.809 --> 00:50:34.970 Again, a little cheesy Photoshop to illustrate a point. 00:50:34.970 --> 00:50:38.520 But we might – if there’s fluids in there, okay, we take off all the load, 00:50:38.520 --> 00:50:41.810 then those fluids might actually hydro-fracture and blow the rock apart. 00:50:41.810 --> 00:50:43.630 Okay, and that’s an idea. 00:50:43.630 --> 00:50:45.740 And this is something I talked about a few years back – pulverization. 00:50:45.740 --> 00:50:48.940 And I think Chris Scholz heckled me at the back of the [inaudible] and go, 00:50:48.940 --> 00:50:52.380 nah, bullshit – something like that, and saying, I don’t believe this. 00:50:52.380 --> 00:50:54.650 It’s all tensional damage. So I said, okay, cool heckle. 00:50:54.650 --> 00:50:57.099 Because of that heckle, we’ve been working on this. 00:50:57.099 --> 00:51:00.390 He used to work with a company called SRI International, 00:51:00.390 --> 00:51:02.670 which I believe are your neighbors next-door. 00:51:02.670 --> 00:51:05.270 And Steve Miller, who I work with, actually used to work with him. 00:51:05.270 --> 00:51:07.990 Believe it or not, they used to allow – I’m not sure if I’m supposed to say this, 00:51:07.990 --> 00:51:10.440 but he did something with dynamite. I would never let Steve Miller 00:51:10.440 --> 00:51:15.440 go anywhere near dynamite. But nevertheless, he went to these guys 00:51:15.440 --> 00:51:17.559 and said, these guys have got these cool decompression vessels. 00:51:17.559 --> 00:51:18.859 You could go and do these experiments there. 00:51:18.859 --> 00:51:21.730 So we went over and had a talk, and they were – they were interested. 00:51:21.730 --> 00:51:26.550 And so what it is is a very simple vessel. It’s a big thing. It’s about this big. 00:51:26.550 --> 00:51:28.690 And it’s effectively got a blow-out diaphragm at the top. 00:51:28.690 --> 00:51:32.430 Okay, and the idea is, if I can remove all of the confining pressure quickly – 00:51:32.430 --> 00:51:36.599 very quickly, let’s see what happens to the samples. 00:51:36.599 --> 00:51:40.200 So on the basis that they would let me do one experiment and then never come 00:51:40.200 --> 00:51:43.809 back, probably, once they saw the crazy things we’re trying to do, I pretty much 00:51:43.809 --> 00:51:48.530 measured – got as many rocks as I could, from pumice to obsidian, measured the 00:51:48.530 --> 00:51:52.700 physical properties of everything on the basis that we’d have one decompression. 00:51:52.700 --> 00:51:56.670 They pumped it up to 14 MPa, okay? So the gas went into the samples, 00:51:56.670 --> 00:51:59.609 and we left it overnight, and then they – pretty much what they had is a – 00:51:59.609 --> 00:52:02.890 in here, this is a diaphragm, so this is a view looking down the hole. 00:52:02.890 --> 00:52:06.610 They initially pumped to 14 MPa, kind of like a rupture disc, 00:52:06.610 --> 00:52:08.580 which might be rated to, say, 20 MPa, and then they pumped it up 00:52:08.580 --> 00:52:10.869 a bit more and blew the diaphragm out. All the gas came out. 00:52:10.869 --> 00:52:13.359 It was opposite of what you actually want to have 00:52:13.359 --> 00:52:16.030 when you’ve got a gas rig, okay? 00:52:16.030 --> 00:52:18.880 But they’ve – they specialize in explosions, and they actually – 00:52:18.880 --> 00:52:21.579 so there’s all my samples in there at just multiple levels 00:52:21.579 --> 00:52:24.260 with beads in there – very porous beads. 00:52:24.260 --> 00:52:26.880 And they actually – I think they wheeled it out to there. 00:52:26.880 --> 00:52:28.820 That’s not my experiment, but I like to show that. 00:52:28.820 --> 00:52:31.230 That’s the – they wheeled out to their testing site outside 00:52:31.230 --> 00:52:33.660 where you can explode gas. So this is a very simple experiment. 00:52:33.660 --> 00:52:35.410 This is permeability and effective pressure. 00:52:35.410 --> 00:52:39.440 So what I did was decompress my samples and then measured all the same 00:52:39.440 --> 00:52:42.410 physical properties afterwards to see – simply see the difference. 00:52:42.410 --> 00:52:45.180 And so this is permeability with effective pressure. 00:52:45.180 --> 00:52:49.450 This is intact rock – pre-decompression. And this is post-decompression. 00:52:49.450 --> 00:52:51.819 Sudden instantaneous decompression. You can see there’s enormous 00:52:51.819 --> 00:52:53.470 increase in the permeability. And if we look at the samples – 00:52:53.470 --> 00:52:56.040 this is the intact sample – again, a 3-by-3-centimeter sample, 00:52:56.040 --> 00:52:58.910 right, nice and intact. And then we look at the 00:52:58.910 --> 00:53:00.950 decompressed samples. And this thing is blasted apart. 00:53:00.950 --> 00:53:02.800 Okay, and these are not enormous stresses. 00:53:02.800 --> 00:53:05.480 This is only 14 MPa gas pressure, and I’m just suddenly taking off 00:53:05.480 --> 00:53:07.849 the confining pressure. Now, interestingly enough, 00:53:07.849 --> 00:53:10.369 if we start looking – and I’m still not finished going through all the rocks. 00:53:10.369 --> 00:53:12.420 I’ve talked about this for the last year or two. 00:53:12.420 --> 00:53:14.260 If you put a Darley Dale sandstone in there, 00:53:14.260 --> 00:53:15.580 before and after is exactly the same. 00:53:15.580 --> 00:53:18.410 And that’s probably because all the gas can just escape, okay? 00:53:18.410 --> 00:53:20.760 Interestingly, in pulverized rocks on the San Andreas, 00:53:20.760 --> 00:53:22.630 we actually see pulverization in the granites. 00:53:22.630 --> 00:53:25.880 Then you go along strike a little bit, and the sandstones are not polarized at all. 00:53:25.880 --> 00:53:28.600 So there you go. Maybe there’s a reason there. 00:53:29.680 --> 00:53:31.960 Westerly granite, on the other hand – I was surprised at the time. 00:53:31.960 --> 00:53:35.000 That didn’t pulverize at all. It had no damage, just sat there. 00:53:35.000 --> 00:53:37.250 And that is because, if you look at the tensile strength – 00:53:37.250 --> 00:53:40.460 the tensile strength of Westerly is about 18 MPa. 00:53:40.460 --> 00:53:43.730 The pore pressure was 14 MPa, so Mr. Westerly just sat there going, 00:53:43.730 --> 00:53:48.869 I’m keeping this gas in. Not letting – probably still letting out even now. 00:53:48.869 --> 00:53:52.540 So I had this variety of cool damage, actually. 00:53:52.540 --> 00:53:57.079 And so actually, these guys probably thought we were mad, but it’s also – 00:53:57.079 --> 00:54:00.270 suggests to me that – it’s also potentially suggesting negative feedbacks, right? 00:54:00.270 --> 00:54:03.810 Because if you go something from this to this, then the next time 00:54:03.810 --> 00:54:05.900 you decompress that, it’s [inaudible] enough to let all the gas out. 00:54:05.900 --> 00:54:08.660 Maybe it’s got some sort of shut-off mechanism there. Okay. 00:54:09.600 --> 00:54:13.099 So you look at the thin sections again. This is 100 microns of scale here. 00:54:13.099 --> 00:54:16.460 Again, you can see huge amounts of – huge amounts of pervasive damage 00:54:16.460 --> 00:54:19.910 that’s really starting to look – in a single event, very much like pulverization. 00:54:19.910 --> 00:54:24.490 Huge amounts of fracture energy there. Okay, and this comparison to Tom 00:54:24.490 --> 00:54:28.500 Rockwell’s sort of field samples. So these guys actually got quite excited, 00:54:28.500 --> 00:54:30.280 so they started doing some more tests, 00:54:30.280 --> 00:54:31.780 taking some more stuff off of [inaudible]. 00:54:31.780 --> 00:54:35.100 This is like a foot-long bar of limestone. That was before and after. 00:54:35.100 --> 00:54:37.170 You can see this things has completely been exploded. 00:54:37.170 --> 00:54:38.890 This is a piece of concrete. You can see there’s all sorts of 00:54:38.890 --> 00:54:42.680 different types of damage going on. In this case, they’re following 00:54:42.680 --> 00:54:45.910 porosity in this case. So it’s very easy to pulverize a rock 00:54:45.910 --> 00:54:49.059 with this particular mechanism by just removing all the confining pressure. 00:54:49.059 --> 00:54:52.300 And if you think about, you know, whatever rupture mechanisms are 00:54:52.300 --> 00:54:54.880 going on – I’m not going to mention wrinkle pulses or anything like – 00:54:54.880 --> 00:54:58.690 but any situation around a fault where you have sudden changes in stress, 00:54:58.690 --> 00:55:01.680 tension, or compression for whatever reason, you can potentially 00:55:01.680 --> 00:55:05.720 generate damage like this. So one of the things I’m just 00:55:05.720 --> 00:55:09.600 blatantly selling – we’ve spent the last three years building a rig now. 00:55:09.600 --> 00:55:12.160 So all these experiments we aim to be able to do – control this is – 00:55:12.160 --> 00:55:16.730 they said I was mad, and I probably am. But this is – we spent two years 00:55:16.730 --> 00:55:19.820 designing this dynamically. This is a huge triaxial, but it’s got a – 00:55:19.820 --> 00:55:24.160 I can move a 500-kilogram piston about 70 centimeters 00:55:24.160 --> 00:55:27.940 at about 4 meters per second. So we can do high-speed impacts 00:55:27.940 --> 00:55:31.079 in a triaxial vessel, but I can also move it out very – oops – 00:55:31.079 --> 00:55:34.030 move it out very quickly and also do extremely fast decompressions. 00:55:34.030 --> 00:55:36.880 But while we’re doing that, we can also measure the permeability, 00:55:36.880 --> 00:55:39.710 the physical seismic properties during that and directly after 00:55:39.710 --> 00:55:42.390 we do that, which should be fun. So watch this space. 00:55:42.390 --> 00:55:44.820 This is something that we hope to be working on in the future. 00:55:44.820 --> 00:55:50.200 And I’ll just end on this just quickly, that I mentioned before – I’m not sure 00:55:50.200 --> 00:55:53.280 what I’ve answered here because the reality is is I’ve created 00:55:53.280 --> 00:55:56.790 pulverized rocks in single impacts in both compression and tension. 00:55:56.790 --> 00:55:59.809 And the damage both looks the same. So in terms of explaining what I see 00:55:59.809 --> 00:56:02.890 in the field, the mechanisms, it’s – I’m still not only the wiser, 00:56:02.890 --> 00:56:05.500 but one thing I’m definitely convinced of is that pulverization is probably 00:56:05.500 --> 00:56:08.299 related to earthquakes in some way, whether it’s a single huge event 00:56:08.299 --> 00:56:10.150 or multiple – you know, tens of thousands of events. 00:56:10.150 --> 00:56:11.880 That’s what we’re going to be looking at in the future. 00:56:11.880 --> 00:56:15.160 Fractures actually heal, remember [inaudible] and the funny – 00:56:15.160 --> 00:56:18.339 and so this is a – this is a fault core here. This is full of [inaudible]. 00:56:18.339 --> 00:56:20.220 Fluids have gone in there and have sealed it up in a sense. 00:56:20.220 --> 00:56:24.170 I’m going to just skip through this slide, and this is a classic Sibson idea that, 00:56:24.170 --> 00:56:26.190 if you suddenly create damage, fluids will move in. 00:56:26.190 --> 00:56:29.040 They will migrate in the suction pump effect, or the fault valve effect, 00:56:29.040 --> 00:56:33.720 if you suddenly break a seal, and the fluids come out from depth quickly. 00:56:33.720 --> 00:56:36.359 If you look at this in a thin section, it’s just something you see all the time 00:56:36.359 --> 00:56:38.520 in cores. These are fractures, fluid inclusion planes. 00:56:38.520 --> 00:56:43.210 And the reality is that these fractures – and remember, pulverization is a 00:56:43.210 --> 00:56:47.340 micro-scale phenomenon, okay? And so microfractures actually heal very quickly. 00:56:47.340 --> 00:56:52.340 Smith and Evans did some really nice work in the ’80s, and many people since. 00:56:52.340 --> 00:56:54.200 And it’s pressure- and temperature-controlled. 00:56:54.200 --> 00:56:57.210 In this paper, they have this really nice diagram showing how these fractures 00:56:57.210 --> 00:57:00.609 heal from the tip inwards. And this is how you form a fluid inclusion plane. 00:57:00.609 --> 00:57:03.109 And these are actually nice experiments on the [inaudible] lab. 00:57:03.109 --> 00:57:07.320 What we don’t understand is, you know, sort of what – maybe we do understand, 00:57:07.320 --> 00:57:10.030 at least in the lab, what time scales this healing occurs over. 00:57:10.030 --> 00:57:13.410 These are some – so Brantley and people – and people at MIT, 00:57:13.410 --> 00:57:16.780 they would suggest, you know, fractures – you know, a fracture 00:57:16.780 --> 00:57:19.420 100 microns in length and 10 microns wide would heal 00:57:19.420 --> 00:57:22.300 in about four hours at 600 degrees, so super fast. 00:57:22.300 --> 00:57:27.460 So how long does this sort of damage stay around, okay? 00:57:28.580 --> 00:57:31.680 And the smaller the crack, the faster it heals. 00:57:31.690 --> 00:57:33.450 So there’s also a scale dependence to healing 00:57:33.450 --> 00:57:37.220 between macrofractures and microfractures, okay? 00:57:37.220 --> 00:57:40.319 This is really nice work by Mareike Messar and Brian Evans 00:57:40.319 --> 00:57:42.570 and Jörg Renner. And this is actually – 00:57:42.570 --> 00:57:50.440 this is some synthetically cracked [inaudible] granite. 00:57:50.440 --> 00:57:52.490 In this case, you can see – and this has been cooked 00:57:52.490 --> 00:57:54.319 at hydrothermal conditions for several hours. 00:57:54.319 --> 00:57:56.259 You can see here, it’s – it looks like it’s been welded. 00:57:56.259 --> 00:58:00.230 You see all this mass transfer going on – this seal – this fracture is sealing up. 00:58:00.230 --> 00:58:02.020 And these are the things that are happening very quickly, 00:58:02.020 --> 00:58:05.930 you know, in a few hours. And if you look at the permeability 00:58:05.930 --> 00:58:08.090 during these experiments, okay, this is a log scale. 00:58:08.090 --> 00:58:10.580 You know, these – it’s faster with higher temperature, but you can 00:58:10.580 --> 00:58:14.339 actually drop the permeability because of the healing of these cracks. 00:58:14.340 --> 00:58:16.620 In less than a day at these sort of conditions. 00:58:16.620 --> 00:58:21.619 So before and after examples there. So you can heal these sort of 00:58:21.619 --> 00:58:25.869 fractures in hours to days. And guys here – Morrow and 00:58:25.869 --> 00:58:28.460 Lockner and Diane, they did some cool experiments that’s just showing 00:58:28.460 --> 00:58:30.829 that even larger-scale fractures, even those heal quite quickly 00:58:30.829 --> 00:58:33.650 in terms of human time scales. You can heal macroscopic fractures 00:58:33.650 --> 00:58:36.549 on the order of a month, at least in terms of the permeability as well, 00:58:36.549 --> 00:58:39.980 showing quite nicely the scale dependence with that – days to months. 00:58:39.980 --> 00:58:44.160 So I’m going to end it there with the takeaway messages – just say that – 00:58:44.160 --> 00:58:47.530 I’m not sure what I’ve answered, but I’ve certainly convinced myself 00:58:47.530 --> 00:58:49.630 it’s complicated, which you guys probably 00:58:49.630 --> 00:58:50.910 didn’t need telling because you know that. 00:58:50.910 --> 00:58:55.480 But I think, at least from the seismic cycle point of view, it’s dynamic. 00:58:55.480 --> 00:58:58.250 It’s a competition between earthquake rupture, damage generation, 00:58:58.250 --> 00:59:03.600 and healing and fluid – and controlling fluid flow and fault strength. Okay? 00:59:03.600 --> 00:59:06.770 Fault slip and damage generation and fluid – subsequent fluid flow and 00:59:06.770 --> 00:59:09.130 healing can happen at very fast rates. And I’m talking fast – 00:59:09.130 --> 00:59:11.630 hours in some cases. Dynamic damage has a unique 00:59:11.630 --> 00:59:14.369 physical and transport properties, and we’re only starting to get to know that. 00:59:14.369 --> 00:59:16.290 Okay, it just seemed very unusual in the lab and how that 00:59:16.290 --> 00:59:20.890 compares with the field will be something of a future work. 00:59:20.890 --> 00:59:22.920 Dynamic feedbacks happen on two time scales. 00:59:22.920 --> 00:59:26.309 Okay, your feedback during the same rupture – so as a rupture propagates, 00:59:26.309 --> 00:59:30.740 it will – it will affect the damage that that rupture tip propagation puts 00:59:30.740 --> 00:59:34.980 into the wall rock will directly affect what the slip is doing behind it. 00:59:34.980 --> 00:59:38.800 But also on a scale, also, of aftershocks. If you’ve got aftershocks occurring 00:59:38.800 --> 00:59:42.839 on the same fault or around, the stiffness of the rock has completely changed. 00:59:42.839 --> 00:59:44.640 The permeability of the rock has changed. 00:59:44.640 --> 00:59:48.340 So I would argue that dynamic rupture models need to take into account this 00:59:48.340 --> 00:59:51.740 sort of evolution of fault damage and changing properties – seismic velocity, 00:59:51.740 --> 00:59:54.839 Young’s modulus, Poission’s ratio, permeability, porosity. 00:59:54.839 --> 00:59:57.480 These all evolve significantly with the number of ruptures. 00:59:57.480 --> 00:59:59.770 The quantity of the damage either side of the rupture tip – 00:59:59.770 --> 01:00:02.670 tensional/compression – can be very different, okay? 01:00:02.670 --> 01:00:05.520 The thing is, with the compression, you need really large peak stresses 01:00:05.520 --> 01:00:08.610 for compression damage. So if you actually go below about 01:00:08.610 --> 01:00:11.430 4 or 5 kilometers, then to pulverize it, you would actually need to 01:00:11.430 --> 01:00:14.099 get supershear, which you could argue is fairly rare. 01:00:14.099 --> 01:00:16.710 Unless, potentially, locally, it’s not. 01:00:16.710 --> 01:00:19.860 But so it’s much, actually, easier to do the damage in terms of 01:00:19.860 --> 01:00:24.220 tension – this type of damage. Again, but these are single events too. 01:00:24.220 --> 01:00:26.910 Perhaps when you’re deeper and you have multiple events 01:00:26.910 --> 01:00:30.160 weakening the rock, then that’s something to think about. 01:00:30.160 --> 01:00:32.400 Okay. I’m going to leave it there. Thanks a lot. 01:00:32.560 --> 01:00:38.860 [Applause] 01:00:41.500 --> 01:00:47.960 [Silence] 01:00:48.660 --> 01:00:52.780 - Thanks. I enjoyed your talk. A whirlwind chore. 01:00:52.790 --> 01:00:56.650 So my question is, can you talk a little bit more about the depth 01:00:56.650 --> 01:01:02.089 dependence of damage? You touch on it maybe on this last slide a little bit. 01:01:02.089 --> 01:01:07.150 So we have examples of exhumed faults, but those are pretty specialized cases. 01:01:07.150 --> 01:01:11.060 So some of the studies you showed were in the lab, and maybe you could 01:01:11.060 --> 01:01:13.220 get to pretty high normal stresses. 01:01:13.220 --> 01:01:16.850 But then most of the field observations, of course, are at the Earth’s surface. 01:01:16.850 --> 01:01:20.700 So what can you say in terms of that? And then also, the implications for 01:01:20.700 --> 01:01:24.690 static and dynamic stress drops, which tie into the amount of energy 01:01:24.690 --> 01:01:27.280 that might be radiated in earthquake. 01:01:27.280 --> 01:01:31.310 And static, of course, what you might see in a geodetic model. Thank you. 01:01:31.310 --> 01:01:36.070 - Okay, so your first question, I guess, in terms of the depth extent. 01:01:36.070 --> 01:01:38.340 So this is the thing. 01:01:39.300 --> 01:01:42.780 The people that have worked on sort of looking at the depth extent, 01:01:42.780 --> 01:01:46.710 like Yehuda Ben-Zion and so on, of pulverized rocks, it’s, you know, 01:01:46.710 --> 01:01:50.890 showing these low-velocity zones that go 4 or 5 kilometers, right? 01:01:50.890 --> 01:01:52.870 Which I entirely believe, okay? 01:01:52.870 --> 01:01:58.930 But then I would say is that, who’s to say that that damage wasn’t there 01:01:58.930 --> 01:02:03.300 from 6 to 10 kilometers, it’s just healed very quickly, I could also argue. 01:02:03.300 --> 01:02:08.589 So what would be nice to see would maybe be some sort of full crustal noise 01:02:08.589 --> 01:02:11.799 studies where it’s computed every month, the same sort of 01:02:11.799 --> 01:02:13.300 time-dependent velocities or something like that. 01:02:13.300 --> 01:02:16.230 And that would be a really interesting question. 01:02:16.230 --> 01:02:19.799 In terms of the – so whether you’re talking about the tensional or 01:02:19.799 --> 01:02:23.150 compressional damage, again, below about 5 kilometers, 01:02:23.150 --> 01:02:26.569 you’d need to get really, really fast ruptures to get those 01:02:26.569 --> 01:02:30.830 compressive stresses needed, okay, which is probably unlikely. 01:02:30.830 --> 01:02:34.160 Again, for the – for it to get an absolute tension below about 01:02:34.160 --> 01:02:37.740 5 kilometers is just as hard too, right? Removing it all. 01:02:37.740 --> 01:02:44.480 So I suspect it is a shallow phenomenon, in terms of damage generation. 01:02:45.440 --> 01:02:49.020 What was your – your last question? - What are the implications for, say, 01:02:49.020 --> 01:02:52.880 depth dependence on static or dynamic stress drops? 01:02:56.440 --> 01:02:57.760 - That is a good question. 01:02:57.760 --> 01:03:01.260 I probably – I’m not even going to attempt to answer here. 01:03:02.040 --> 01:03:04.720 Because I don’t know. Have to give it a bit of thought. 01:03:04.720 --> 01:03:07.680 Maybe have to talk about it afterwards. 01:03:07.680 --> 01:03:11.440 You said something about geodetic. I mean, this is – the geodetic community 01:03:11.440 --> 01:03:14.760 is something I’ve wanted to long talk to about this sort of thing. 01:03:14.760 --> 01:03:17.120 So maybe we can talk about this afterwards. 01:03:18.580 --> 01:03:24.660 - Thanks, Tom, for the breathtaking tour. You’ve given us a lot to think about. 01:03:24.670 --> 01:03:27.930 One thing that you only mentioned in passing in your 01:03:27.930 --> 01:03:33.120 last slide – aftershocks of shallow earthquakes. 01:03:33.120 --> 01:03:41.359 And I think that there’s a lot to be said for the damage science that 01:03:41.359 --> 01:03:47.260 you talked about and the time dependence of aftershock activity. 01:03:47.260 --> 01:03:51.799 And one simple model, which has been around for a long time, 01:03:51.799 --> 01:03:58.170 is the idea of dilatancy hardening in the fault core, where you get 01:03:58.170 --> 01:04:07.000 increased fracture density and reduced pore pressure. 01:04:07.000 --> 01:04:15.480 Delay – and so delayed seismicity. And then, in your off-fault damage, 01:04:15.480 --> 01:04:23.200 you’re not only increasing fracture density, but presumably in the 01:04:23.200 --> 01:04:30.849 interseismic era – part of the cycle, that you are at least partially 01:04:30.849 --> 01:04:36.349 saturating this porosity. So you have a source of water 01:04:36.349 --> 01:04:43.060 outside the core, and a possible time dependence, which is related to 01:04:43.060 --> 01:04:51.800 the permeability of that water source re-saturating the core. 01:04:51.800 --> 01:04:54.980 What are your thoughts about this? And I think this is attractive to 01:04:54.980 --> 01:05:05.799 think about building on Ruth’s comment as you go to greater depths, 01:05:05.799 --> 01:05:16.650 the temperature and normal stress increase lithostatic pressure. 01:05:16.650 --> 01:05:27.190 And so one could think about this off-fault damage decreasing as you 01:05:27.190 --> 01:05:33.690 go to greater depths, and the permeability decreasing as well. 01:05:33.690 --> 01:05:41.349 Maybe explaining why aftershocks of deeper earthquakes – say at intermediate 01:05:41.349 --> 01:05:51.180 depths are very rare and represent very little in the way of post-main shock slip. 01:05:51.180 --> 01:05:54.050 Sorry for taking so much time, but anyway. 01:05:54.050 --> 01:05:56.960 - I think you’ve made multiple good points there that I’ll agree with. 01:05:56.960 --> 01:05:59.540 And so I would – in terms of the depth extent. 01:05:59.540 --> 01:06:04.900 So I think the time-dependent increase in velocity seismic studies actually 01:06:04.900 --> 01:06:09.720 tell me a lot. I mean, if – I made this figure for this sort of question. 01:06:09.730 --> 01:06:11.510 If we need to get from that to that, and we want to – 01:06:11.510 --> 01:06:13.970 but we want to heal those cracks to get that increase in velocity, 01:06:13.970 --> 01:06:17.080 I can see doing in one of four ways, right? 01:06:17.080 --> 01:06:20.460 Yes, usually you dilate the sample, and it drops the velocity. 01:06:20.460 --> 01:06:23.460 I could increase the velocity of the bulk just by closing those cracks 01:06:23.460 --> 01:06:28.819 by target reloading, okay? I could simply fill the cracks with fluid, 01:06:28.819 --> 01:06:31.770 and Dave Lockner showed that quite nicely in a BSSA paper years ago. 01:06:31.770 --> 01:06:34.200 You know, [inaudible] depths, and the velocity dropped. 01:06:34.200 --> 01:06:36.880 And then just the fluids percolate. If you think about suddenly 01:06:36.880 --> 01:06:40.660 inducing damage in a saturated rock, the fluids is in the – is in the 01:06:40.660 --> 01:06:42.920 grain boundaries and so on. If you suddenly induce damage 01:06:42.920 --> 01:06:46.140 in a grain, it’s going to be dry when it opens. The velocity will drop. 01:06:46.140 --> 01:06:49.030 So fluids don’t actually necessarily need to move far. 01:06:49.030 --> 01:06:53.089 They just need to sort of percolate and diffuse sideways, in a way. 01:06:53.089 --> 01:06:55.820 I’m not saying it doesn’t come from deep. It probably does as well, okay? 01:06:55.820 --> 01:06:59.170 But – so I’m coming around to the idea that – and that is not 01:06:59.170 --> 01:07:04.880 pressure- or temperature-controlled, so that, to me, the fluids redistributing 01:07:04.880 --> 01:07:07.710 around – and I think Greg Beroza did a paper some years ago 01:07:07.710 --> 01:07:10.160 suggesting that from S wave velocity stuff. 01:07:10.160 --> 01:07:15.200 I’m convinced that this dilation and then re- sort of homogenization of 01:07:15.200 --> 01:07:18.360 fluid pressures could cause that effect. And I would also get around this 01:07:18.360 --> 01:07:22.110 pressure and temperature issue. We can heal stuff in the lab quick. 01:07:22.110 --> 01:07:24.010 But that’s at, you know, 10 kilometers’ depth quick. 01:07:24.010 --> 01:07:27.220 You know, I mean, shallow is much – it’s much, much slower, okay? 01:07:27.220 --> 01:07:30.000 So I think it’s probably related to fluids, if I … 01:07:30.000 --> 01:07:34.230 - But those fluids in the crack healing, they don’t go away. 01:07:34.230 --> 01:07:35.549 - No. - They’re still there. 01:07:35.549 --> 01:07:36.969 - They’re still there. Absolutely. - And – yeah, and … 01:07:36.969 --> 01:07:39.099 - But I guess you could argue that, therefore is – but that’s reversible, 01:07:39.099 --> 01:07:41.500 right? It’s not a healed crack, and it’s not a sealed crack. 01:07:41.500 --> 01:07:46.210 It’s just a fluid-filled crack that’s giving you that velocity increase. Yeah. 01:07:46.210 --> 01:07:49.190 I mean, so I could do – I could do four experiments and show you all of 01:07:49.190 --> 01:07:53.079 these things happening, giving you the same macroscopic velocity effect. 01:07:53.080 --> 01:07:55.660 Okay, so yeah. 01:07:56.960 --> 01:08:00.960 But, again, I think one thing – when you’re shallow in terms of the – 01:08:00.960 --> 01:08:04.329 you’re going to have – the heterogeneity is over a bigger scale. 01:08:04.329 --> 01:08:07.420 Everything localizes as you get deeper. So the strain rate projections for 01:08:07.420 --> 01:08:10.660 a rupture actually gets – the strain rate with distance from 01:08:10.660 --> 01:08:14.490 the fault gets steeper with distance. So you would expect more localization. 01:08:14.490 --> 01:08:18.109 That’s kind of what you see from the velocity plots as well, again. 01:08:18.109 --> 01:08:21.339 But it’s hard – it’s hard to interpret those low-velocity zones things because, again, 01:08:21.339 --> 01:08:24.569 below 5 kilometers, you don’t know what was there and whether it’s healed 01:08:24.569 --> 01:08:27.359 and whether it’s just closed very quickly, for example, 01:08:27.360 --> 01:08:31.060 or much higher fluid pressure is re-precipitating. 01:08:34.840 --> 01:08:37.719 - Here. I’ve got a question in the – in the meantime. 01:08:37.719 --> 01:08:42.049 So going back to the pulverized rocks. So I know you know a lot more 01:08:42.049 --> 01:08:48.259 about this than I do, but I don’t necessarily agree that, 01:08:48.259 --> 01:08:52.609 if you do a decompression and a compression, 01:08:52.609 --> 01:08:56.190 and you generate damage, that it will necessarily be the same. 01:08:56.190 --> 01:09:01.170 So one possibility is the wave speeds might be different. 01:09:01.170 --> 01:09:03.310 And so the question is, have you – have you done 01:09:03.310 --> 01:09:07.440 measurements of wave speeds on those two different cases? 01:09:07.440 --> 01:09:11.310 The other is, just thinking about the decompression one, 01:09:11.310 --> 01:09:15.759 where the fluid is in – it’s basically in the porosity. 01:09:15.759 --> 01:09:18.199 So it’s in the grain boundaries and the pores and stuff. 01:09:18.199 --> 01:09:21.170 And it seems to me that that case is a case where you 01:09:21.170 --> 01:09:27.400 might have trouble generating cross-grain fractures, for example. 01:09:27.400 --> 01:09:30.460 Because it’s trying to get out. It’s getting out the fastest way it can. 01:09:30.460 --> 01:09:34.900 Whereas, in a compression test, you know, you’re going to shock – 01:09:34.900 --> 01:09:39.089 you’re basically going to load grains by the elastic contrast at the 01:09:39.089 --> 01:09:43.159 grain boundary so that it’s maybe more easy to generate 01:09:43.159 --> 01:09:47.170 a trans-granular fracture in that case. So I wonder if you thought about that. 01:09:47.170 --> 01:09:52.210 - Yeah, no – they’re all good points. And I guess what I said is that, 01:09:52.210 --> 01:09:54.890 qualitatively, right now, what I’m showing you is very similar damage. 01:09:54.890 --> 01:09:57.679 I absolutely don’t think it is the same type of damage. 01:09:57.679 --> 01:10:00.560 I think we need to do things – [inaudible] like particle size distribution 01:10:00.560 --> 01:10:04.790 and stuff like that. That will show us, again, this is a work-in-progress. 01:10:04.790 --> 01:10:09.180 But I think it’s – I mean, often people nowadays are looking 01:10:09.180 --> 01:10:11.460 for smoking guns straight off the bat. You know, they want to 01:10:11.460 --> 01:10:14.360 go in and say, that’s cool. I want to – you know, professional 01:10:14.360 --> 01:10:16.440 [inaudible], blah, blah, blah, boom. Get in Nature. 01:10:16.440 --> 01:10:19.640 And the reality is, this is – I would argue that this is complicated. 01:10:19.640 --> 01:10:22.760 It’s subtle annoying, you know? So it’s going to take me a 01:10:22.760 --> 01:10:25.219 while to figure out if I do it all. I probably won’t. 01:10:25.219 --> 01:10:28.780 Some clever kid in my lab will figure it out. But we’ll have a go. 01:10:28.780 --> 01:10:31.780 So yeah, I think – it’s definitely different. 01:10:31.780 --> 01:10:33.850 And, again, the elements, certainly from the wave velocities, 01:10:33.850 --> 01:10:36.770 for sure Vp/Vs ratios and so on will be different. 01:10:36.770 --> 01:10:39.230 But right now, you know, I mean, I’ve go to actually compress it 01:10:39.230 --> 01:10:41.989 in one rig and take it out and put it in another rig. 01:10:41.989 --> 01:10:44.400 You know, I mean, to measure these properties, so what’s real? 01:10:44.400 --> 01:10:47.040 The point of the new all-singing-all-dancing rig is that 01:10:47.040 --> 01:10:49.440 we can do that all in, you know, without decompressing it, 01:10:49.440 --> 01:10:52.739 and overprint the damage and look at what the feedbacks are. 01:10:52.739 --> 01:10:56.380 And it gives you an idea of what – you know, how the velocities evolved. 01:10:56.380 --> 01:10:57.960 Give me a clue to better understand the field. 01:10:57.960 --> 01:11:00.940 So it’s a whole motivation about building that rig while we’ve been doing 01:11:00.940 --> 01:11:04.570 the field work here is to really create something – I don’t think you could 01:11:04.570 --> 01:11:07.020 produce this with any other machine. That’s why we built it. 01:11:07.020 --> 01:11:10.370 And hopefully be able to address and actually answer some of the 01:11:10.370 --> 01:11:11.731 questions we’ve got in the field. Because the problem with 01:11:11.731 --> 01:11:14.489 pulverization is it’s got no shear offset. And as structural geologists, 01:11:14.489 --> 01:11:17.440 we use shear offset to get timing and stuff, right? 01:11:17.440 --> 01:11:21.020 And this stuff doesn’t have any offset. So it’s not easy at all to get timing. 01:11:21.020 --> 01:11:25.090 And the only way I can see to do that – in the moment, at least, is go to the lab 01:11:25.090 --> 01:11:29.120 and try and reproduce the conditions to give us some sort of clues. 01:11:29.120 --> 01:11:31.280 I mean, from the high-resolution stuff from the Borrego Fault, 01:11:31.280 --> 01:11:35.110 that’s kind of the field version of this. We are starting to get, you know, like, 01:11:35.110 --> 01:11:37.970 where there’s recent slip, dating some things that we can 01:11:37.970 --> 01:11:41.150 pick out actually fractures of subsidiary faults that have slipped recently, 01:11:41.150 --> 01:11:44.540 and maybe we can, like, figure out parts of the damage zone that related 01:11:44.540 --> 01:11:48.830 to an individual earthquake. How that maps to all scales, I don’t know. 01:11:48.830 --> 01:11:51.650 And coming back to your point, I guess, 01:11:51.650 --> 01:11:54.650 on the tensional damage in terms of fluids, yeah. 01:11:54.650 --> 01:11:57.560 I mean, these are one set of tests on a bunch of different rocks. Absolutely. 01:11:57.560 --> 01:12:00.780 I mean, I think – I think that’s an extreme example. 01:12:00.780 --> 01:12:04.450 That’s where we pretty much take off all the load and don’t put it back on, right? 01:12:04.450 --> 01:12:07.340 The reality is, that rupture goes across and produces, say, tension damage. 01:12:07.340 --> 01:12:10.940 It’s going to put the load back on, you know, milliseconds later. 01:12:10.940 --> 01:12:14.810 So then it’s a question of poroelasticity and so on 01:12:14.810 --> 01:12:17.620 and depth and these sort of things. Also that’s with gas. 01:12:17.620 --> 01:12:20.060 In reality, it would be water in there, which is stiffer. 01:12:20.080 --> 01:12:22.840 We’re going to do those experiments too. Will that vary? 01:12:22.840 --> 01:12:27.260 So, yeah, there’s a lot of things – a lot of other things to try. 01:12:27.264 --> 01:12:29.630 I’ve got end members now. I can know that I can pulverize. 01:12:29.630 --> 01:12:33.140 And I think everything else is going to take little bit more work. 01:12:33.140 --> 01:12:35.620 That answers your question, I hope. 01:12:35.620 --> 01:12:38.710 - Yeah, Tom, could we go to the middle of your talk? 01:12:38.710 --> 01:12:44.020 You had the work in preparation at – it was looking at the location of the 01:12:44.020 --> 01:12:46.520 acoustic emissions and then … - Oh, yeah. Mm-hmm. 01:12:46.520 --> 01:12:50.160 - … the seismicity – the … - Yeah. This stuff. 01:12:50.160 --> 01:12:54.680 - Basically, you’ve got a good start, Tom, but you got – you got to do more work. 01:12:54.680 --> 01:12:56.080 - Yeah. [laughter] 01:12:56.080 --> 01:12:58.810 Sorry, Walter. I’ll try harder. 01:12:58.810 --> 01:13:04.420 - Okay. Let’s go back one more – well, we can – I wanted to see the cylinder 01:13:04.420 --> 01:13:08.770 and the – you had a video of the … - Oh, the – okay, sorry. 01:13:08.770 --> 01:13:14.360 - Yeah. Here’s my basic question. You know, your experiments are 01:13:14.360 --> 01:13:20.190 on a cylindrical sample, unconfined. And you’re concluding certain things … 01:13:20.190 --> 01:13:23.249 - These are – these are confined. - Okay. They are confined, but … 01:13:23.249 --> 01:13:26.340 - I confused you by showing you a video of a uniaxial experiment 01:13:26.340 --> 01:13:28.960 just to illustrate that it is fast. - Oh, I see. 01:13:28.960 --> 01:13:34.340 I was wondering whether the conclusions – for example, 01:13:34.340 --> 01:13:42.150 the seismic velocity reduction were adequately representing the real Earth. 01:13:42.150 --> 01:13:45.500 Because you were dealing with an idealized … 01:13:45.500 --> 01:13:47.680 - Yeah, okay. So we’re 100 MPa confining pressure. 01:13:47.699 --> 01:13:51.239 So that’s not – that’s not too shallow. That’s pretty deep. 01:13:51.239 --> 01:13:54.140 And, you know, this is sort of the first – hot off the press. 01:13:54.140 --> 01:13:57.110 And, as I said, there’s a lot of Frans Aben 01:13:57.110 --> 01:14:01.310 postdoc slave time in processing here. Absolutely want to look at – 01:14:01.310 --> 01:14:05.130 with this now, we can look at different normal stresses, confining pressures, 01:14:05.130 --> 01:14:07.670 see how that velocity zone changes with rupture propagation. 01:14:07.670 --> 01:14:11.390 We can maybe play with the rupture speed. We can change rock type. 01:14:11.390 --> 01:14:13.320 There’s so many things that we can tweak now that we’ve got 01:14:13.320 --> 01:14:17.020 that technique to look at it, okay? So, yeah, you know, I can think of, 01:14:17.020 --> 01:14:20.400 like, 10 – a decade’s worth of postdoc work now with this. 01:14:20.400 --> 01:14:24.260 You know, funding dependent. And Ph.D. work. 01:14:24.260 --> 01:14:28.570 But, yeah, I definitely think – yeah, the depth extent of that, 01:14:28.570 --> 01:14:30.740 we’ve got – we’ve got to look at. 01:14:36.100 --> 01:14:39.300 - Okay. I think we should end it there, in the interest of time. 01:14:39.300 --> 01:14:41.200 But I’m sure Tom will be happy to … - Sorry for talking to late. 01:14:41.200 --> 01:14:44.180 - Yep, okay. So please thank me – or, join me in thanking Tom. 01:14:44.180 --> 01:14:45.500 Don’t thank me. [laughter] 01:14:45.500 --> 01:14:46.920 Join Tom – yeah, thank you. 01:14:46.920 --> 01:14:52.140 [Applause] 01:14:52.440 --> 01:14:54.720 - All right. - You got it on mute? 01:14:56.460 --> 01:14:57.740 - Oh, good point. 01:15:00.560 --> 01:15:02.780 Did I swear? I think I said “bullshit.” 01:15:02.780 --> 01:15:04.080 I might have said “bullshit.” 01:15:05.240 --> 01:15:09.960 [Silence]