WEBVTT Kind: captions Language: en 00:00:00.680 --> 00:00:03.900 [ Music ] 00:00:04.240 --> 00:00:08.040 Good morning, everyone. Welcome to today’s Earthquake Science 00:00:08.040 --> 00:00:10.750 seminar on a Monday instead of Wednesday. 00:00:10.750 --> 00:00:12.520 Thanks for coming. 00:00:12.520 --> 00:00:15.140 I’m Tom Brocher, and with Jack Norbeck, 00:00:15.150 --> 00:00:21.930 we’re the incoming seminar chairs for the Earthquake Science Center series. 00:00:21.930 --> 00:00:29.070 And we’re very pleased to acknowledge our former seminar co-chairs, 00:00:29.070 --> 00:00:36.470 Belle and Ezer, for holding our last six months’ worth of seminars 00:00:36.470 --> 00:00:40.640 and want to congratulate them and thank them for doing a great job. 00:00:41.000 --> 00:00:45.680 [ Applause ] 00:00:47.300 --> 00:00:50.520 We’re doing pretty well with finding speakers for the next six months, 00:00:50.530 --> 00:00:53.059 but if you have anyone who’s coming to visit, 00:00:53.059 --> 00:00:57.530 or if you want to give us a seminar, please feel free to do so. 00:00:57.530 --> 00:01:02.559 We do have openings, especially beginning in October. 00:01:02.559 --> 00:01:05.170 And we have an opening on August 16th. 00:01:05.170 --> 00:01:10.060 So if you have a talk all ready to go, we’d love to hear it. 00:01:10.060 --> 00:01:16.560 We have a presentation next week by Christos Kyriakopoulos 00:01:16.560 --> 00:01:21.869 from UC-Riverside, and he’ll be talking about the possible connectivity 00:01:21.869 --> 00:01:27.490 between the San Andreas and the Imperial Faults in southern California. 00:01:27.490 --> 00:01:34.860 And next Wednesday, we’ll be having a talk by Richard Styron of GEM. 00:01:35.900 --> 00:01:40.460 And he’ll be talking about paleoearthquakes in the Puget Lowland. 00:01:41.320 --> 00:01:46.000 Joan’s busy this afternoon, but she has some – she might have some time on 00:01:46.009 --> 00:01:51.759 Wednesday afternoon to meet with you. So if you’re interested in meeting 00:01:51.759 --> 00:01:56.240 with Joan, you can contact me, or you can contact Joan. 00:01:56.240 --> 00:01:59.219 And we will be taking Joan out for lunch afterwards 00:01:59.219 --> 00:02:02.039 if you’re interested in joining us for that. 00:02:02.040 --> 00:02:05.760 - [inaudible] - Yeah. 00:02:05.760 --> 00:02:08.179 We’re just going to eat at the Tectonic Grill. 00:02:08.179 --> 00:02:12.910 So you can meet us there, or we’ll – yeah, just meet us there. 00:02:12.910 --> 00:02:17.030 But let us know if you’re planning to come. 00:02:17.030 --> 00:02:20.650 Just say a few a words about Joan. Most of you know Joan. 00:02:20.650 --> 00:02:27.010 Joan’s undergraduate from MIT, and she did her Ph.D. from UC-San Diego. 00:02:27.010 --> 00:02:32.570 She did a postdoc at University of Nevada in Reno. 00:02:32.570 --> 00:02:39.250 And then she worked since about 1988 for the USGS, 00:02:39.250 --> 00:02:45.860 starting at Golden, and then she worked for a long time in Memphis. 00:02:45.860 --> 00:02:50.560 But for about the past 10 years or so, she’s been in the office in Seattle. 00:02:51.420 --> 00:02:58.280 She’s an AGU fellow, and she was awarded a 00:02:58.290 --> 00:03:05.060 Professional Excellence Award from the Association of Women Geoscientists. 00:03:05.060 --> 00:03:10.040 She has served in many capacities, as an officer and as a board member, 00:03:10.040 --> 00:03:13.590 with the Seismological Society of America. 00:03:13.590 --> 00:03:18.790 And she’s lately been heading the effort by the USGS 00:03:18.790 --> 00:03:23.230 to further our studies of hazard along subduction zones. 00:03:23.230 --> 00:03:28.940 But today, she’s going to be focusing on the Cascadia subduction zone. 00:03:28.940 --> 00:03:31.140 So, Joan. 00:03:36.720 --> 00:03:39.850 - Can everybody hear me? Okay, great. 00:03:39.850 --> 00:03:44.460 Well, first, thank you for taking this extra time. 00:03:44.460 --> 00:03:47.820 I know this is not a normally scheduled time. 00:03:47.820 --> 00:03:50.820 And I’m going to talk about some work that is usually – 00:03:50.820 --> 00:03:55.770 seems to be the case when I come here is something that’s very new to me, 00:03:55.770 --> 00:04:00.460 and I’m really here to tap your brains and get feedback from you. 00:04:00.460 --> 00:04:03.610 So please interrupt me. Feel free to interrupt me. 00:04:03.610 --> 00:04:08.710 Just raise your hand or something as I talk, 00:04:08.710 --> 00:04:12.820 and certainly afterwards, I’d be happy to meet with anybody. 00:04:13.420 --> 00:04:18.630 Well, there’s a sort of subtitle to this. I won’t re-read this long title. 00:04:18.630 --> 00:04:22.370 But as Tom mentioned, I’ve been thinking about subduction zones for 00:04:22.370 --> 00:04:27.500 a long – for quite a while now, but never really thought about the offshore. 00:04:27.500 --> 00:04:33.420 And most of – a big fraction of subduction zones are offshore. 00:04:33.430 --> 00:04:37.180 And these – this is – what I’m going to talk about today is really 00:04:37.180 --> 00:04:43.090 one of two studies that made use of an offshore data set called the 00:04:43.090 --> 00:04:50.700 Cascadia Initiative Data Set, which came from a big experiment. 00:04:50.700 --> 00:04:55.580 And it’s sort of serendipity. There are two studies that we’ve 00:04:55.580 --> 00:05:00.540 made use of these data for complete – very different purposes. 00:05:00.540 --> 00:05:07.080 One was just published, and I talked about this the last time I was here, about 00:05:07.080 --> 00:05:13.960 using temperature data from offshore to look at turbidity currents in turbidites. 00:05:13.960 --> 00:05:20.300 And today is a little bit related, but it’s – as the title says, it’s about site response. 00:05:20.300 --> 00:05:24.259 This is, as you’ll see in a minute, related to – we’ve been charged 00:05:24.259 --> 00:05:28.020 in the earthquake program in the Pac Northwest to look at 00:05:28.020 --> 00:05:31.389 the question of Cascadia recurrence intervals. 00:05:31.389 --> 00:05:37.090 And I’ll illustrate the motivation for that in the next couple slides. 00:05:37.090 --> 00:05:42.039 So to just motivate why this – why we’re doing this work, or why – 00:05:42.039 --> 00:05:51.469 and this is largely a solo effort, I guess, with a lot of questioning of many people. 00:05:51.469 --> 00:06:00.360 This is – and it’s focused on offshore sediment transport, in part, by slope 00:06:00.360 --> 00:06:06.240 failures. One of the motivations – the reasons this is very – this is hazardous. 00:06:06.240 --> 00:06:09.880 This is a figure from a paper by Tappin and others. 00:06:09.880 --> 00:06:18.030 Shown on this is some run-up records from Japan following 00:06:18.030 --> 00:06:21.569 the Tohoku earthquake. The dots are the data. 00:06:21.569 --> 00:06:27.230 The blue – they’ve attempted to model this run-up using just a model 00:06:27.230 --> 00:06:29.910 of the deformation from the earthquake. That’s the blue line. 00:06:29.910 --> 00:06:32.930 And what they found was there’s this – a lot of uplift they couldn’t – 00:06:32.930 --> 00:06:37.800 of run-up that they couldn’t explain – tens of meters. 00:06:37.800 --> 00:06:41.460 And what they’ve shown – what they showed was that this was likely due to 00:06:41.460 --> 00:06:47.190 a submarine landslide triggered by the shaking from the earthquake. 00:06:47.190 --> 00:06:51.099 So this is sort of a cascade of hazards. 00:06:51.100 --> 00:06:55.540 The shaking triggered the landslide, which triggered a tsunami. 00:06:57.380 --> 00:07:02.660 It turns out that shaking-induced sediment transport, or remobilization, 00:07:02.660 --> 00:07:07.640 is a big way that sediment is redistributed on the Earth 00:07:07.659 --> 00:07:13.330 from offshore to – from onshore to offshore and in the oceans. 00:07:13.330 --> 00:07:16.229 The basic idea is illustrated in this cartoon. 00:07:16.229 --> 00:07:20.710 The ground shakes. And these are submarine slopes, 00:07:20.710 --> 00:07:26.111 loosely consolidated in some cases, in a variety of different ways, either through 00:07:26.120 --> 00:07:31.540 failure or processes like liquefaction. I’ll show a slide in a second. 00:07:31.540 --> 00:07:38.300 Sediment gets entrained in the water column. It’s denser, so it slides downhill. 00:07:38.300 --> 00:07:42.749 If it gets going fast enough, it came become turbulent and be – 00:07:42.749 --> 00:07:45.410 get going really fast, which can be damaging. 00:07:45.410 --> 00:07:50.839 This is known to take out communications cables, for example. 00:07:50.839 --> 00:07:54.539 And when it runs out of gas, it dumps the sediments, and those 00:07:54.539 --> 00:08:00.430 form deposits called turbidites. Well, turbidites can be useful. 00:08:00.430 --> 00:08:02.759 And that’s one of the principal ways that 00:08:02.759 --> 00:08:07.839 we know about earthquake recurrence in Cascadia. 00:08:07.840 --> 00:08:15.900 So this is a map – a topographic and bathymetry map on the – shown here. 00:08:16.740 --> 00:08:24.369 This is – basically spans all of Cascadia. You can see, I think, the coastline. 00:08:24.369 --> 00:08:25.939 And what – these numbers here 00:08:25.940 --> 00:08:31.120 are the recurrence intervals determined from the turbidite record. 00:08:31.120 --> 00:08:36.360 And basically, based on the fact that they see many more 00:08:36.370 --> 00:08:41.430 frequent turbidites in the geologic offshore record, 00:08:41.430 --> 00:08:44.300 even for the recurrence interval for great – 00:08:44.300 --> 00:08:50.720 magnitude 8-1/2 and greater earthquakes decreases as you go southward. 00:08:52.120 --> 00:08:57.740 Well, one of the questions I had – and there are many things that affect 00:08:57.740 --> 00:09:01.670 these deposition of turbidites. But one of the questions that 00:09:01.670 --> 00:09:06.459 hasn’t really been asked is, does the ground, for some reason – 00:09:06.459 --> 00:09:11.170 and site response is the question I’m going to address – does the ground 00:09:11.170 --> 00:09:17.240 inherently shake harder in the south? Having nothing to do with the 00:09:17.240 --> 00:09:21.320 frequency of earthquakes, but just, do we see more turbidites in the south 00:09:21.320 --> 00:09:26.350 because the ground shakes harder so we get more failures? 00:09:26.350 --> 00:09:32.600 Just to make – put some context here, what I’ve labeled on this – on the 00:09:32.600 --> 00:09:36.540 topography here are some of the things that I’m going to refer to. 00:09:36.540 --> 00:09:41.899 In the top is the physiography of the coastline and the offshore. 00:09:41.899 --> 00:09:46.290 So we have the coastline, and everything to the east of the coast is land. 00:09:46.290 --> 00:09:51.899 I’m not really focusing on the variability onshore. 00:09:51.899 --> 00:09:56.680 Then we have the shelf, which, as you can see, is relatively flat. 00:09:56.680 --> 00:10:01.100 The slopes – and there’s – it’s hard to see, but there’s – 00:10:01.100 --> 00:10:04.740 I’ve identified the slopes here as – there’s a sort of green – 00:10:04.740 --> 00:10:10.900 a gray transparent zone over it labeled “slopes.” 00:10:10.900 --> 00:10:15.740 And it’s basically the depth between 200 and 1,000 meters’ depth. 00:10:15.740 --> 00:10:18.420 And that’s where the slopes are the steepest. 00:10:18.420 --> 00:10:20.820 And the white line is just meant to guide your eyes. 00:10:20.820 --> 00:10:23.310 That’s sort of – that’s the 1,000-meter contour. 00:10:23.310 --> 00:10:26.610 That’s basically the base of the steep slopes – 00:10:26.610 --> 00:10:29.600 the edge of the continental shelf. 00:10:29.600 --> 00:10:34.900 Then we have the deformation front. There is no well-defined trench in 00:10:34.900 --> 00:10:41.259 Cascadia. This is just chock full of sediments, and so there’s no deep trench. 00:10:41.259 --> 00:10:47.139 But the line of where the Pacific Plate begins its descent is called – 00:10:47.139 --> 00:10:52.180 is called the Deformation Front. And that’s – line there as well. 00:10:52.180 --> 00:10:57.740 As I mentioned, the whole margin is very heavily sedimented. 00:10:57.740 --> 00:11:02.281 Those sediments even cross over the deformation front, and there are these 00:11:02.281 --> 00:11:09.340 big alluvial fans on the seafloor. So that’s just a bit of context. 00:11:09.340 --> 00:11:13.420 Well, what could cause variability in shaking? 00:11:13.430 --> 00:11:16.230 This is a model – we haven’t had a great earthquake there, 00:11:16.230 --> 00:11:21.399 but this is a model from Olson of the peak ground velocity, 00:11:21.400 --> 00:11:26.500 which is one measure of shaking, from a model of a great Cascadia earthquake. 00:11:26.500 --> 00:11:29.960 Red is greater shaking. And what you can see is this shaking 00:11:29.960 --> 00:11:36.940 is indeed very spatially variable. Red is really strong shaking. 00:11:37.460 --> 00:11:43.100 Well, this is the slip model that went into generating this shaking. 00:11:43.110 --> 00:11:46.029 And you can see it, too, is very variable. 00:11:46.029 --> 00:11:49.920 And there’s clearly a correlation between the two. 00:11:49.920 --> 00:11:55.449 But this is a difficult thing to call upon at a variability in turbidites because this 00:11:55.449 --> 00:12:01.480 slip pattern is going to change from earthquake to earthquake, presumably. 00:12:01.480 --> 00:12:07.230 So appealing to the slip distribution to explaining the turbidite record, 00:12:07.230 --> 00:12:13.940 or the variability in the turbidites, is not necessarily a sensible thing to do. 00:12:13.940 --> 00:12:19.500 However, we know from many, many – and people in this room are far more 00:12:19.500 --> 00:12:24.750 expert in this – from something called site response, this is a 00:12:24.750 --> 00:12:31.949 permanent modifier of the shaking. And it’s basically – we know that 00:12:31.949 --> 00:12:36.959 the local geologic structure around a particular location can have 00:12:36.959 --> 00:12:41.360 a very significant effect on how the ground shakes there. 00:12:41.360 --> 00:12:45.129 This is a model of sediment thickness from Stephenson. 00:12:45.129 --> 00:12:48.480 And I’m going to make reference to this a number of times. 00:12:49.140 --> 00:12:53.500 Sediments can have a big effect on the shaking. 00:12:53.509 --> 00:12:56.360 And this is a – the sediments have been, 00:12:56.360 --> 00:13:00.459 more or less, in this configuration for a very long time. 00:13:00.460 --> 00:13:05.540 So we expect this is going to permanently modify, in a – 00:13:05.540 --> 00:13:08.120 in a consistent way, the shaking. 00:13:08.129 --> 00:13:11.519 And that’s the motivation for looking at site response. 00:13:11.519 --> 00:13:17.080 Is there an overprinting on the shaking due to this – what we call site response. 00:13:17.080 --> 00:13:22.779 It’s the inherent pattern of variable shaking. 00:13:23.400 --> 00:13:26.940 Well, another reason for looking at this is, it turns out that – 00:13:26.949 --> 00:13:32.080 and the details of this are – this is a variety of mechanisms 00:13:32.080 --> 00:13:37.319 that have been proposed as to how shaking puts sediment into the water 00:13:37.319 --> 00:13:43.760 column and entrained – and creates these turbulent flows and turbidites. 00:13:43.760 --> 00:13:51.480 And the bottom line is that, at least as I read the literature, anything is possible. 00:13:51.480 --> 00:13:57.019 Not only, you know, what metric we use to measure the strength of shaking, 00:13:57.019 --> 00:14:03.660 whether they – what frequency pass band has a big effect, as you’ll see, 00:14:03.660 --> 00:14:09.750 but what is relevant to the mechanisms by which – and there’s – by which 00:14:09.750 --> 00:14:16.629 shaking causes sediments to remobilize is really a wide-open question. 00:14:16.629 --> 00:14:21.470 So maybe we can at least shed some observational light on this. 00:14:21.470 --> 00:14:24.959 There’s even models that say, as you shake the ground, 00:14:24.959 --> 00:14:27.800 you strengthen the sediments. 00:14:27.800 --> 00:14:32.740 And there’s some observational evidence to suggest that that could be the case. 00:14:32.740 --> 00:14:38.209 So this is a – we really – very little is known, as I understand it, about the 00:14:38.209 --> 00:14:42.900 connection between – we know it happens, but we don’t really know why. 00:14:42.900 --> 00:14:47.750 And I am particularly focused on the question of the frequencies 00:14:47.750 --> 00:14:53.660 of shaking and how they affect the variability of shaking. 00:14:53.660 --> 00:14:57.910 Well, what we want to have some idea about, what is a 00:14:57.910 --> 00:15:03.660 significant variation in shaking? This is a paper by Scholz and others. 00:15:03.660 --> 00:15:07.879 And what’s shown here are what’s called attenuation curves. 00:15:07.879 --> 00:15:10.720 It’s distance versus peak ground acceleration – 00:15:10.720 --> 00:15:16.420 another measure of shaking – for magnitude 8 to 9 earthquakes. 00:15:16.420 --> 00:15:20.920 And I think you – and they did a stability analysis. 00:15:20.920 --> 00:15:26.139 And the bottom line is that gray – the green dashed line – this is for Cascadia – 00:15:26.140 --> 00:15:30.700 is roughly the distance to where they expect slope failures. 00:15:32.980 --> 00:15:36.740 What they conclude is that even a magnitude 9 00:15:36.749 --> 00:15:41.949 can’t shake hard enough to destabilize the slopes. 00:15:41.949 --> 00:15:46.470 Unless some other things come into play. 00:15:46.470 --> 00:15:52.009 But what you can see is a change of a factor of 2 would put it into 00:15:52.009 --> 00:15:58.769 the failure – the failure regime. So this line – any shaking stronger 00:15:58.769 --> 00:16:02.779 in this gray area would be enough to cause failure. 00:16:02.779 --> 00:16:06.300 So if we could elevate the shaking by a factor of 2, 00:16:06.300 --> 00:16:08.959 we could destabilize the slopes. 00:16:08.959 --> 00:16:14.081 So the point is, is that a factor of 2 makes a big difference in whether or not 00:16:14.081 --> 00:16:18.399 you generate – so that’s the kind of order of magnitude 00:16:18.400 --> 00:16:22.160 we’re going to be looking for. That’s a significant variability. 00:16:22.940 --> 00:16:26.800 Well, let’s look at just the data that we have to work with. 00:16:26.810 --> 00:16:32.810 So on the map shown here, it shows the seismic stations, 00:16:32.810 --> 00:16:39.139 both onshore and offshore, that we have data from to look at. 00:16:39.139 --> 00:16:41.769 Onshore – those are permanent stations. 00:16:41.769 --> 00:16:46.319 They have been operating for a long, long time, provide lots of great data. 00:16:46.319 --> 00:16:52.430 Offshore is the experiment that I referenced called the Cascadia Initiative. 00:16:52.430 --> 00:16:58.080 These were instruments placed on the seafloor for over a period of five years. 00:16:58.080 --> 00:17:01.120 And I’ll point out, they weren’t all deployed concurrently. 00:17:01.120 --> 00:17:06.680 So unfortunately, we don’t have recordings of the same signals 00:17:06.680 --> 00:17:11.720 on this uniformly over the entire margin at the same time. 00:17:11.720 --> 00:17:14.400 But anyway, there’s a lot to work with. 00:17:14.400 --> 00:17:20.570 These are recordings of two earthquakes – two distant earthquakes. 00:17:20.570 --> 00:17:27.240 And they’re ordered in – basically in longitudinal order 00:17:27.240 --> 00:17:31.770 and with distance from the deformation front. 00:17:31.770 --> 00:17:35.700 So the deformation front is significant basically because we have this 00:17:35.700 --> 00:17:42.210 big sedimentary wedge between the deformation front and the coastline. 00:17:42.210 --> 00:17:45.179 So going – this red line is the deformation front. 00:17:45.179 --> 00:17:48.020 This is going landward. 00:17:48.020 --> 00:17:51.340 Two different earthquakes from different azimuths. 00:17:51.340 --> 00:17:55.690 What you can see – even anyone can see that, 00:17:55.690 --> 00:17:59.539 when you pass – and the coastline is about here. 00:17:59.539 --> 00:18:02.429 When you cross the – onto the accretionary prism, 00:18:02.429 --> 00:18:06.330 the ground shakes harder and longer. 00:18:06.330 --> 00:18:11.029 So it’s – you don’t need to do any analysis to see that the sediments 00:18:11.029 --> 00:18:15.440 of the accretionary prism have a really big effect on the shaking. 00:18:15.440 --> 00:18:19.780 Well, we have about 27 earthquakes that were recorded 00:18:19.780 --> 00:18:24.840 on at least three or more seismic stations on all three components. 00:18:26.140 --> 00:18:29.340 And these, because they traveled from a long distance, 00:18:29.340 --> 00:18:33.970 record largely what I call low-frequency ground shaking. 00:18:33.970 --> 00:18:38.150 Dominant periods in these is between 10 and 20 seconds. 00:18:38.150 --> 00:18:41.220 The high frequencies have been attenuated out. 00:18:41.220 --> 00:18:44.450 So we’re going to use these observations to look at 00:18:44.450 --> 00:18:49.190 low-frequency shaking – what I’m calling low-frequency shaking. 00:18:49.190 --> 00:18:54.809 We also have a data set of local and regional earthquakes. 00:18:54.809 --> 00:18:58.140 Two examples shown here in the same format. 00:18:58.140 --> 00:19:01.279 The pattern now is not so clear. 00:19:01.279 --> 00:19:04.010 And that’s two – because of two things. 00:19:04.010 --> 00:19:08.779 One is you’re much closer to the earthquake, so the distance effects 00:19:08.779 --> 00:19:14.730 and the radiation pattern and so on is much more dominant. 00:19:14.730 --> 00:19:18.799 And these are much higher frequencies. So these are going to provide 00:19:18.799 --> 00:19:26.320 constraints on high-frequency shaking – basically 1 to 10 hertz pass band. 00:19:26.320 --> 00:19:30.570 We have, again, 18 earthquakes. And we’re going to try to 00:19:30.570 --> 00:19:35.990 put all these together to try to look at, what is the variability in these records 00:19:35.990 --> 00:19:42.570 that’s just due to the site – the recording station around the site? 00:19:42.570 --> 00:19:48.950 So we need to get rid of what’s due to the source and the propagation. 00:19:48.950 --> 00:19:52.680 Well, I just took a very simple-minded approach, 00:19:52.680 --> 00:19:55.620 because that’s about all I could do. 00:19:56.680 --> 00:19:58.520 But we’re going to look in two pass bands. 00:19:58.530 --> 00:20:02.450 We have a pass band from the distant earthquakes and a high-frequency – 00:20:02.450 --> 00:20:05.480 a low-frequency pass band from the distant events, 00:20:05.480 --> 00:20:10.559 a high-frequency pass band from the local and regional earthquakes. 00:20:10.559 --> 00:20:16.900 And we’re going to just parameterize the shaking as in a – this is a very 00:20:16.900 --> 00:20:21.649 standard approach to people who look – to ways of looking at site response. 00:20:21.649 --> 00:20:25.760 We have some measure of the recorded shaking, 00:20:25.760 --> 00:20:28.530 and I’ll show those metrics in a minute. 00:20:28.530 --> 00:20:35.600 We have a source term – some amplitude that gets radiated from the source. 00:20:35.600 --> 00:20:40.360 A propagation term – it’s some exponential attenuation. 00:20:40.360 --> 00:20:44.200 And a site term. And this is what’s of interest to us. 00:20:44.200 --> 00:20:47.360 But we need to account for all these things. 00:20:47.360 --> 00:20:49.179 This is for the distant earthquakes. 00:20:49.180 --> 00:20:54.549 For a local earthquake, we also include a spreading – a geometric spreading term. 00:20:55.440 --> 00:21:00.460 Well, we can take – for the two different data sets, we can take all our data. 00:21:00.460 --> 00:21:05.970 We want to solve for these site terms, largely. 00:21:05.970 --> 00:21:08.059 We take all our data together. 00:21:08.059 --> 00:21:11.450 We make some measurements of the ground shaking. 00:21:11.450 --> 00:21:15.660 And we solve for these unknowns, which are in red. 00:21:15.660 --> 00:21:18.420 And we take all our earthquakes. We put them – we make our 00:21:18.429 --> 00:21:25.900 measurements. We take the log – so this is a nice linear equation. 00:21:25.900 --> 00:21:30.340 We have a big matrix equation, but it’s mostly ones and zeros. 00:21:30.340 --> 00:21:34.660 And we can solve this to find these site response terms. 00:21:34.660 --> 00:21:39.020 So it’s basically measuring just the variability – how much of the signal is 00:21:39.020 --> 00:21:46.680 due to variability in the sites – the local structure around the recording site. 00:21:48.060 --> 00:21:53.700 Well, this is a result, and you’re going to see a lot of these maps. 00:21:55.920 --> 00:21:59.800 So I’ve – what I’ve measured – I’ve measured three different 00:21:59.800 --> 00:22:06.830 metrics of shaking. And these are standardly used for hazard studies. 00:22:06.830 --> 00:22:10.760 One is peak ground acceleration, shown on the right here. 00:22:10.760 --> 00:22:16.210 It’s just the largest amplitude of the acceleration. 00:22:16.210 --> 00:22:20.649 Peak ground velocity in the middle. And the energy, which is basically just 00:22:20.649 --> 00:22:24.371 an integrated measure of the velocity. So it accounts for 00:22:24.371 --> 00:22:29.419 if something shakes for a long time. It includes the duration of shaking. 00:22:29.419 --> 00:22:35.990 At each site, I’ve – these are the results from the analyses of – 00:22:35.990 --> 00:22:40.610 this is the local and regional earthquake data set. 00:22:40.610 --> 00:22:44.860 Each triangle is a site where I have a measurement. 00:22:44.860 --> 00:22:49.779 And they’re color-coded by the – and these are just relative. 00:22:49.779 --> 00:22:54.840 So the color – the range of colors shows you the variability in shaking 00:22:54.840 --> 00:22:59.899 due to the site – the local site effects. 00:22:59.899 --> 00:23:05.350 And what you can see is that – and this is plotted on a log scale. 00:23:05.350 --> 00:23:11.480 So there’s a variability throughout Cascadia of a factor of almost 100. 00:23:11.480 --> 00:23:14.679 This is a lot of variability in the shaking. 00:23:14.679 --> 00:23:19.340 As you can see, it’s very geographically systematic. 00:23:19.340 --> 00:23:25.549 And the good thing is, it doesn’t really – you get, to first order, the same pattern 00:23:25.549 --> 00:23:30.820 of shaking variability regardless of what metric you use. 00:23:30.820 --> 00:23:33.440 So going forward, we’re just going to look at 00:23:33.440 --> 00:23:37.640 the peak ground velocity, or PGV. 00:23:37.640 --> 00:23:43.420 But know that the other metrics basically are going to tell you the same thing. 00:23:44.420 --> 00:23:50.520 If we look at low – the low-frequency pass band from the distance earthquake, 00:23:50.520 --> 00:23:56.620 again, you get the same basic pattern regardless of the metric that you use. 00:23:56.620 --> 00:23:58.929 So that’s good. 00:23:58.929 --> 00:24:01.760 We can just focus on one of them. 00:24:02.580 --> 00:24:06.820 Well, now if we look at some of the features of this, and I hope – 00:24:06.820 --> 00:24:11.520 I’ve stared at these little colored dots for a long time. 00:24:12.380 --> 00:24:15.920 So I hope some of the things are as clear – are as clear to you. 00:24:15.920 --> 00:24:19.590 But one of the things you’ll notice – and so now what I – 00:24:19.590 --> 00:24:24.140 the map on the right is the high-frequency shaking. 00:24:24.140 --> 00:24:27.900 The map on the left is the low-frequency shaking. 00:24:27.900 --> 00:24:33.480 And what you may notice, I hope, is that, particularly offshore, 00:24:33.480 --> 00:24:37.299 they’re almost – they’re almost mirror images of each other. 00:24:37.300 --> 00:24:40.480 They shake in a opposite way. 00:24:42.269 --> 00:24:48.220 So that’s one feature that is pretty striking, I thought. 00:24:48.220 --> 00:24:53.049 The other thing to note on this is that most of the variability is longitudinal. 00:24:53.049 --> 00:25:00.360 It runs east-west. There’s very little resolvable variability going north-south. 00:25:00.360 --> 00:25:04.850 So that’s relevant to our initial question about the turbidites, 00:25:04.850 --> 00:25:07.440 which vary from north to south. 00:25:13.480 --> 00:25:18.539 In both pass bands, the ground motion, the shaking, the site response 00:25:18.539 --> 00:25:23.380 decreases as you cross the coastline. I went into this expecting that 00:25:23.380 --> 00:25:27.659 the biggest change would occur at the coastline. 00:25:27.659 --> 00:25:31.160 In fact, you’ll see that – you can see that that’s not the case. 00:25:31.160 --> 00:25:35.510 But in both cases – and the reason this is important is that in some of these 00:25:35.510 --> 00:25:41.130 site stability studies, they take recorded ground motion from onshore and 00:25:41.130 --> 00:25:45.450 just say, let’s assume that’s what we’re going to see offshore. 00:25:45.450 --> 00:25:51.380 That is not – this would suggest that that’s not necessarily a good assumption. 00:25:51.380 --> 00:25:56.710 It’s largely going to be greater as you go – as you cross the coastline. 00:25:56.710 --> 00:26:02.090 Nevertheless, it varies in – where we have measurements close by, 00:26:02.090 --> 00:26:05.060 it varies relatively smoothly. 00:26:05.780 --> 00:26:13.360 Well, the other striking feature of these maps, at least to me, was that the real – 00:26:13.360 --> 00:26:16.870 that there’s an abrupt change in the character of the shaking, 00:26:16.870 --> 00:26:23.559 or the site response, as you – as you cross this magic line at the 00:26:23.559 --> 00:26:29.130 base of the slope – the steep slopes – the edge of the continental margin. 00:26:29.130 --> 00:26:33.340 You can see that it – in low frequencies, it goes from 00:26:33.340 --> 00:26:39.710 very strong shaking to weak shaking. And it’s very abrupt. 00:26:39.710 --> 00:26:44.380 The opposite is true in the high-frequency pass band. 00:26:44.380 --> 00:26:49.970 So there’s some significant – something significant is changing the way that the 00:26:49.970 --> 00:26:57.190 ground shakes when you cross onto the steep slopes and the continental shelf. 00:26:57.190 --> 00:27:00.039 So that’s – they have – and this was surprising. 00:27:00.040 --> 00:27:06.230 It’s not the deformation front or the coastline, but offshore. 00:27:08.020 --> 00:27:11.059 Well, we’re going to try to look at some of the ways that – 00:27:11.059 --> 00:27:16.299 some of the things that could be causing this variability in shaking. 00:27:16.300 --> 00:27:19.980 And we have some tools that we can make use of. 00:27:21.040 --> 00:27:24.059 Site response is often attributed to the sediments 00:27:24.059 --> 00:27:29.160 that sit beneath the location of interest. 00:27:29.160 --> 00:27:32.850 This is a image from – reflect at the top. 00:27:32.850 --> 00:27:36.220 A high-res – as you can see, this is quite high-resolution. 00:27:36.220 --> 00:27:42.380 This is from marine reflection and refraction lines with lots of details. 00:27:42.380 --> 00:27:46.240 There’s lots of low velocity sediments. 00:27:47.450 --> 00:27:55.020 This is the same – roughly at the same latitude from a regional model 00:27:55.029 --> 00:27:57.200 that extends throughout Cascadia. 00:27:57.200 --> 00:28:01.019 And as you can see, it’s – and they’re almost color-coded 00:28:01.019 --> 00:28:09.490 the same way – that it lacks the resolution that the top one does. 00:28:09.490 --> 00:28:16.539 But nevertheless, this kind of resolution exists in only a very few profiles. 00:28:16.539 --> 00:28:20.000 So I used this model because we can extend it 00:28:20.000 --> 00:28:22.889 throughout the entire study region. 00:28:22.889 --> 00:28:27.519 So we’re going to use this velocity model to try to see, can – 00:28:27.519 --> 00:28:31.419 do we see some correlation between the velocity structure 00:28:31.420 --> 00:28:34.460 and the site – the shaking variability? 00:28:35.540 --> 00:28:40.220 Just a quick digression. One of the explanations that’s often 00:28:40.220 --> 00:28:46.840 given for site response is resonance. And that’s illustrated here. 00:28:46.840 --> 00:28:51.799 Many people here – this is basic elementary, but basically, 00:28:51.799 --> 00:28:58.580 for those of you who aren’t familiar, if you have a low-velocity sediment layer, 00:28:58.580 --> 00:29:06.240 it can resonate at a fundamental frequency that’s roughly – 00:29:06.240 --> 00:29:12.880 that the frequency of that resonance is roughly a quarter wavelength by which 00:29:12.880 --> 00:29:17.120 the thickness of the layer is a quarter wavelength of the frequency. 00:29:17.120 --> 00:29:21.860 And it comes from, basically you consider that the sediments 00:29:21.860 --> 00:29:26.809 are acting like an open pipe, and they vibrate at a frequency 00:29:26.809 --> 00:29:31.330 given by the length of the pipe or the depth of the sediments. 00:29:31.330 --> 00:29:34.909 So if we know the depth of the sediments, we can predict 00:29:34.909 --> 00:29:38.540 the resonant frequency. And if it’s resonating, 00:29:38.540 --> 00:29:44.850 the ground is going to shake longer and harder at that frequency. Okay. 00:29:44.850 --> 00:29:48.840 So we can try to look at these measurements statistically 00:29:48.840 --> 00:29:52.149 and see if they correlate with something meaningful 00:29:52.149 --> 00:29:56.059 that we could then maybe use in a predictive way. 00:29:56.059 --> 00:30:00.960 The things that we have regional measure – or models of – and 00:30:00.960 --> 00:30:05.990 these are all correlated with each other. We know, to first order, the depth 00:30:05.990 --> 00:30:10.870 of the plate interface, which controls where we have sediments. 00:30:10.870 --> 00:30:16.610 We know – there’s some relatively good bathymetry and elevation model – 00:30:16.610 --> 00:30:22.180 elevation – we know that throughout all of Cascadia. 00:30:22.180 --> 00:30:26.270 We have a crude model of the sediment thickness. 00:30:26.270 --> 00:30:30.990 And we can estimate from our model the resonant frequency. 00:30:30.990 --> 00:30:33.149 And we can just correlate our measurements 00:30:33.149 --> 00:30:36.830 of ground shaking with these various things at each location. 00:30:36.830 --> 00:30:42.899 And what we find is that, for the high-frequency motions, as – 00:30:42.899 --> 00:30:46.080 and again, all these things are really correlated with one another. 00:30:46.080 --> 00:30:49.360 So they’re not independent measures. 00:30:49.360 --> 00:30:52.660 What we find, to first order, is that there is some – 00:30:52.660 --> 00:30:57.480 there seems to be some correlation between weaker site response, 00:30:57.490 --> 00:31:02.630 or decreasing shaking, with increasing sediment thickness. 00:31:02.630 --> 00:31:07.830 And all these measures that are basically measures of the sediment thickness. 00:31:07.830 --> 00:31:13.070 The opposite is basically true at low frequencies. 00:31:13.070 --> 00:31:22.529 So where the sediments get thicker, the ground shakes harder at low frequencies. 00:31:22.529 --> 00:31:28.390 And the sort of simple – the interpretation of this could be that, 00:31:28.390 --> 00:31:32.259 at low frequencies, the sediments are resonating 00:31:32.259 --> 00:31:35.090 and acting to amplify the motion. 00:31:35.090 --> 00:31:39.470 And at high frequencies, the sediments attenuate the motion. 00:31:39.470 --> 00:31:44.710 So in one pass band, attenuation dominates at high frequencies. 00:31:44.710 --> 00:31:48.570 And at low frequencies, they resonate. 00:31:48.570 --> 00:31:54.049 Well, again – and I would argue that we get – 00:31:54.049 --> 00:31:56.909 the correlation is not overwhelming. 00:31:56.909 --> 00:32:02.140 You have to accept a relatively low correlation coefficient. 00:32:02.140 --> 00:32:04.330 But a lot of that is due to the fact that we have 00:32:04.330 --> 00:32:09.180 really poor models of what these parameters are. 00:32:10.320 --> 00:32:12.340 We can also learn something from looking at 00:32:12.340 --> 00:32:16.860 how these things vary in space, not just statistically. 00:32:16.860 --> 00:32:20.659 So this is, again, our map of high-frequency shaking. 00:32:20.659 --> 00:32:27.390 And the sediment thickness model from the Stephenson et al. velocity model. 00:32:27.390 --> 00:32:30.279 So I’ve color-coded them so they look similar. 00:32:30.279 --> 00:32:35.600 So this is, again, our PGV – our peak ground velocity measurements. 00:32:35.600 --> 00:32:41.630 And these are measurements from the model of the sediment thickness. 00:32:41.630 --> 00:32:46.679 And what you can see is there is a kind of first – in the triangles. 00:32:46.679 --> 00:32:52.200 In the abyssal plain of the seafloor, we have another study that made 00:32:52.200 --> 00:32:56.330 more precise measurements of the sediment thickness, and those are 00:32:56.330 --> 00:33:00.470 shown on the upside-down triangles to give you a sense of how similar 00:33:00.470 --> 00:33:04.059 they are – how much you might trust one or the other. 00:33:04.060 --> 00:33:09.700 And they’re not too vastly different in – overall. 00:33:09.700 --> 00:33:12.860 But the thing to notice in the sediment model – again, 00:33:12.870 --> 00:33:19.620 there’s some similarity between sediment thickness and the – 00:33:19.620 --> 00:33:24.059 where we see high and low shaking at high frequency. 00:33:24.059 --> 00:33:29.909 But what the sediment model does not predict is the abrupt change 00:33:29.909 --> 00:33:35.870 in shaking as you cross off of the continental shelf. 00:33:35.870 --> 00:33:41.090 The sediments vary quite smoothly across this transition. 00:33:41.090 --> 00:33:46.710 So this is not a perfect explanation by a long shot. 00:33:46.710 --> 00:33:49.409 The other thing that’s noteworthy in the sediment model 00:33:49.409 --> 00:33:55.080 is that the sediments decrease in thickness going southward. 00:33:55.080 --> 00:33:59.539 We don’t see as profound a change in the shaking, 00:33:59.539 --> 00:34:05.330 but it’s also very poorly sampled. We had very few sites as you go southward. 00:34:05.330 --> 00:34:08.950 So I think this is still somewhat of an open question as to 00:34:08.950 --> 00:34:13.859 what happens as you go southward. We have only a couple sites here. 00:34:13.860 --> 00:34:19.120 And down in Mendocino, it gets really complicated – probably not surprisingly. 00:34:20.260 --> 00:34:25.600 Well, if we look at also the resonance – and again, we’re hypothesizing that the 00:34:25.609 --> 00:34:29.570 low-frequency shaking is dominated by resonance effects. 00:34:29.570 --> 00:34:31.820 We can predict the resonant frequencies. 00:34:31.820 --> 00:34:37.740 And again, I’ve color-coded them to look similar. Same thing is true. 00:34:37.740 --> 00:34:42.399 There’s kind of a first-order correlation, spatially. 00:34:42.399 --> 00:34:45.599 But again, there’s no abrupt transition at the base 00:34:45.599 --> 00:34:48.869 of the continental – at the base of the slopes. 00:34:48.869 --> 00:34:52.659 In addition, if you look at the difference between these two measurements, 00:34:52.659 --> 00:34:59.030 or the two models of the resonance, they’re really different. 00:34:59.030 --> 00:35:04.280 And you can see the different colors here. I think most people 00:35:04.280 --> 00:35:13.010 would argue that the upside-down triangles are way more precise. 00:35:13.010 --> 00:35:16.280 And largely, it’s because the Stephenson model 00:35:16.280 --> 00:35:20.170 lacks the really low-velocity sediments. 00:35:20.170 --> 00:35:23.380 So part of the mismatch between the measurements 00:35:23.380 --> 00:35:28.950 and the predictions could be that the model is just really crude. 00:35:28.950 --> 00:35:34.780 But again, there’s a first-order similarity, but it does not reproduce 00:35:34.780 --> 00:35:39.260 this abrupt change that we see at the base of the slopes. 00:35:40.660 --> 00:35:46.470 So we can also look at this question of resonance observationally. 00:35:46.470 --> 00:35:50.119 And a standard way that this is done in terrestrial studies 00:35:50.119 --> 00:35:55.050 of site response is to look at – you can measure resonance by looking at 00:35:55.050 --> 00:36:02.160 the spectral ratio between the horizontal and the vertical component. 00:36:02.160 --> 00:36:07.839 When you do that – this is an example at three different – three different sites. 00:36:07.839 --> 00:36:12.260 So the top – we have two horizontal components and one vertical. 00:36:12.260 --> 00:36:14.930 And I’ve measured them separately, those ratios, 00:36:14.930 --> 00:36:22.300 so this is a spectrum. Frequency versus ratio here. 00:36:22.300 --> 00:36:25.660 The two different components, I made the ratios separately 00:36:25.660 --> 00:36:30.040 to see how similar they are. It gives you an idea how robust it is. 00:36:30.040 --> 00:36:33.280 And, at each location, I made this measurement 00:36:33.280 --> 00:36:40.670 for all the earthquakes that I had as well as noise – just ambient noise. 00:36:40.670 --> 00:36:44.700 The noise measurements – the ratios are shown in red. 00:36:44.700 --> 00:36:48.040 The earthquakes are in black. 00:36:48.040 --> 00:36:52.060 The individual earthquakes are in the light lines, and the median values – 00:36:52.060 --> 00:36:58.980 mean values are the solid – the darker lines. At three different locations. 00:36:58.980 --> 00:37:02.910 This really surprised me. What you can see is that there’s 00:37:02.910 --> 00:37:07.550 a huge amount of variability, but it doesn’t really matter which component 00:37:07.550 --> 00:37:14.520 you use or whether you use a signal or the noise. So you don’t need any signals. 00:37:14.520 --> 00:37:18.840 You can still get a measure of what these spectral ratios look like. 00:37:18.840 --> 00:37:22.700 Which, to me, tells me this is really measuring something about 00:37:22.700 --> 00:37:29.320 the local site and how it amplifies and modifies the shaking. 00:37:31.700 --> 00:37:35.320 The hypothesis, you know, that you would see some correlation 00:37:35.329 --> 00:37:41.569 between this and where you see large motions if resonance is in effect. 00:37:41.569 --> 00:37:45.681 So I know this is probably way too complicated a figure, but what 00:37:45.681 --> 00:37:51.339 I’ve shown here are these spectral ratios. This is at high frequencies. 00:37:51.339 --> 00:37:54.050 At a bunch of different sites. 00:37:54.050 --> 00:37:59.860 Where it’s pink and tan is where we see large site response. 00:38:00.640 --> 00:38:03.920 Basically, the bottom line is, there’s not a strong correlation 00:38:03.920 --> 00:38:12.060 between strong shaking and resonance in these ratios. 00:38:12.060 --> 00:38:17.359 And that’s perhaps consistent with the notion that, what dominates 00:38:17.359 --> 00:38:22.970 the high-frequency shaking is attenuation, not resonance. 00:38:22.970 --> 00:38:26.310 If we look at low frequencies, the same kind of plot – 00:38:26.310 --> 00:38:33.060 these are all from the shelf where we have large peak ground velocities. 00:38:33.060 --> 00:38:38.180 Again, there’s not – there’s some cases where there’s clear resonance peaks, 00:38:38.180 --> 00:38:42.810 others where there’s not. But what you see onshore, 00:38:42.810 --> 00:38:50.310 where you have very low site response in the PGV measurements, 00:38:50.310 --> 00:38:54.369 is that there’s almost no resonance at low frequencies, 00:38:54.369 --> 00:38:58.740 except for this one site, which is above a big sedimentary basin. 00:38:58.740 --> 00:39:02.860 So this gives us some confidence that this is, again – 00:39:02.860 --> 00:39:07.710 it’s suggestive that, yes, resonance is important at low frequencies, 00:39:07.710 --> 00:39:11.630 not so important at high frequencies. 00:39:11.630 --> 00:39:17.860 But it’s certainly not knock-your-socks-off evidence. 00:39:19.320 --> 00:39:26.180 So broadly speaking, again, the message is, is that, to first order, 00:39:26.180 --> 00:39:31.680 the site response correlates with the distribution of sediments and sediment 00:39:31.680 --> 00:39:37.770 thickness if you appeal to sediment resonance and attenuation. 00:39:37.770 --> 00:39:44.920 But that does not explain this abrupt change at the base of the slopes. 00:39:46.640 --> 00:39:49.630 What I suggest is that there’s got to be a – 00:39:49.630 --> 00:39:54.030 another thing that affects site response is topography. 00:39:54.030 --> 00:39:58.609 And that this is, again, where we have the most significant topographic 00:39:58.609 --> 00:40:04.860 gradients. And certainly topography has been correlated with site response. 00:40:04.860 --> 00:40:09.520 I would also suggest that this is some major structural boundary that, 00:40:09.520 --> 00:40:15.170 in some way – and maybe someone else can suggest why – a big structural 00:40:15.170 --> 00:40:20.260 change that affects the shaking in a very profound way. 00:40:20.260 --> 00:40:25.050 One suggestion that this is reasonable comes from 00:40:25.050 --> 00:40:29.040 a very different set of observations. 00:40:29.040 --> 00:40:33.730 And this is from my colleagues who I’ve worked on the other studies with. 00:40:33.730 --> 00:40:37.609 Their interest in the offshore is looking at methane – at hydrates 00:40:37.609 --> 00:40:45.369 and methane emissions offshore. And this is highly speculative, I realize. 00:40:45.369 --> 00:40:51.170 But they, too, suggested that this – the base – these continental slopes here 00:40:51.170 --> 00:40:58.960 are a highly – they’re highly fractured. The dots here are methane plumes. 00:41:01.260 --> 00:41:07.080 And what you can see is that those methane plumes concentrate along 00:41:07.089 --> 00:41:12.680 this boundary where we see this profound change in the site response. 00:41:12.680 --> 00:41:17.360 Their interpretation – and much of this methane originates at the base of the 00:41:17.360 --> 00:41:22.940 sediments, which means there has to be conduits coming up from the base of the 00:41:22.940 --> 00:41:30.280 sediments, and the methane travels up these vents, which are probably faults. 00:41:30.290 --> 00:41:36.780 So the base of the slopes – the slopes are highly fractured, which allows the 00:41:36.780 --> 00:41:42.260 methane to travel up them, and represent some major structural boundary. 00:41:42.260 --> 00:41:49.980 Now, I – probably some offshore geologist can pose some – 00:41:49.980 --> 00:41:56.160 say whether this is – this makes sense or not, but anyway, both this – I would – 00:41:56.160 --> 00:42:01.520 these observations all suggest that there’s – this is a critical boundary – 00:42:01.520 --> 00:42:04.319 not the coastline or the deformation front – 00:42:04.319 --> 00:42:08.590 that really affects how the ground shakes. 00:42:08.590 --> 00:42:11.590 So in summary – I’ll just read the implications 00:42:11.590 --> 00:42:14.620 because we’ve just heard all this other stuff. 00:42:15.620 --> 00:42:20.520 We really – if you’re going to look at slope stability and 00:42:20.520 --> 00:42:26.420 sediment remobilization due to shaking, you can’t just extrapolate from 00:42:26.420 --> 00:42:30.820 onshore measurements without accounting for the site response. 00:42:30.820 --> 00:42:32.660 The effects are big. 00:42:32.660 --> 00:42:37.660 They vary by orders of magnitude, or at least an order of magnitude. 00:42:39.960 --> 00:42:44.260 We don’t – they don’t really provide great insight into 00:42:44.270 --> 00:42:49.430 which measure affects – is it high frequencies or low frequencies 00:42:49.430 --> 00:42:53.160 that mobilize the sediments, or both, but it does say 00:42:53.160 --> 00:42:55.900 that the mechanism does matter. 00:42:55.900 --> 00:43:01.790 If it’s a mechanism – if low frequencies are the most effective at mobilizing 00:43:01.790 --> 00:43:08.730 sediments and causing slope failures, then there’s a sort of feedback. 00:43:08.730 --> 00:43:11.050 The low frequencies are going to be amplified 00:43:11.050 --> 00:43:14.480 on the slopes where you expect them to fail. 00:43:14.480 --> 00:43:23.490 If high frequencies are most effective, then they may act to stabilize the slopes. 00:43:23.490 --> 00:43:32.640 And finally, our original hypothesis was, does the site response explain the 00:43:32.640 --> 00:43:39.460 turbidite distribution? Does the ground inherently shake harder to the south? 00:43:42.200 --> 00:43:47.720 My preliminary conclusion is that certainly the greatest variability 00:43:47.720 --> 00:43:51.630 is east-to-west, not north-to-south. 00:43:51.630 --> 00:43:56.180 So that suggests that that’s probably not going to explain it. 00:43:56.180 --> 00:44:00.800 Although I would add that there are very few observations, and certainly 00:44:00.800 --> 00:44:07.190 these are not high-resolution observations, in southern Cascadia. 00:44:07.190 --> 00:44:11.859 So I think there’s a little bit more work to be done to rule this out completely, 00:44:11.860 --> 00:44:17.920 but certainly this first preliminary look suggests that that’s not the case. 00:44:18.960 --> 00:44:20.980 And I’ll leave it at that. 00:44:21.920 --> 00:44:27.220 [ Applause ] 00:44:27.220 --> 00:44:29.660 - Thank you, Joan, for a really interesting presentation. 00:44:29.660 --> 00:44:32.700 Do we have any questions for Joan? 00:44:36.580 --> 00:44:38.460 - Jonathan? 00:44:43.040 --> 00:44:45.100 - Thank you. Let me start off by saying 00:44:45.110 --> 00:44:48.490 I know nothing about earthquake site response. 00:44:48.490 --> 00:44:52.069 But in a lot of the Cascadia Initiative active-source seismic lines that 00:44:52.069 --> 00:44:56.211 were shot, there’s pretty pervasive bottom-simulating reflectors from a 00:44:56.211 --> 00:45:00.870 gas hydrate zone that’s in between the basement and the sediment. 00:45:00.870 --> 00:45:04.180 So I’m just curious whether you think it would make a difference 00:45:04.180 --> 00:45:07.120 in your resonance for the site response if, 00:45:07.120 --> 00:45:11.599 instead of using the sediment depth, you used a scaled version of that. 00:45:11.599 --> 00:45:14.510 Since it would follow sediment topography, 00:45:14.510 --> 00:45:17.540 but it would be between the basement and the sediment. 00:45:17.540 --> 00:45:22.720 - Sure. And the figure I showed you with the methane plumes 00:45:22.720 --> 00:45:28.400 is from the group that’s working on the BSR and that issue. 00:45:28.400 --> 00:45:32.839 So it’s certainly something to look at. I haven’t yet, but yeah. 00:45:33.680 --> 00:45:37.099 - Hi, Joan. Great talk. I was – so the other question I had – 00:45:37.099 --> 00:45:43.050 I guess if we ran with your hypothesis and said there was nothing about the 00:45:43.050 --> 00:45:48.500 wedge characteristics that changed the sensitivity to slope failures, 00:45:48.500 --> 00:45:52.619 could an explanation for having more turbidites to the south 00:45:52.619 --> 00:45:57.950 be that it’s also more proximal to, like, the Triple Junction? 00:45:57.950 --> 00:46:01.369 Is there anything – is there a dependence latitudinally on, like, 00:46:01.369 --> 00:46:05.750 susceptibility to remote triggering by earthquakes? 00:46:05.750 --> 00:46:09.960 Because you have this crustal – the San Andreas is right there. 00:46:09.960 --> 00:46:14.670 - Well, certainly there are a lot more seismic sources as you go south. 00:46:14.670 --> 00:46:18.579 Because Mendocino is certainly the most active. 00:46:18.579 --> 00:46:22.849 And then the Blanco Fracture Zone is also very active. 00:46:22.849 --> 00:46:25.830 So that certainly – there’s certainly 00:46:25.830 --> 00:46:30.920 a lot more earthquake sources, and that’s stronger shaking. 00:46:33.320 --> 00:46:37.700 But the susceptibility is kind of a separate question. 00:46:37.700 --> 00:46:40.750 And that, I think, again, it’s really poorly sampled. 00:46:40.750 --> 00:46:47.550 So I think, again, this preliminary look suggests it’s not – it doesn’t, like, 00:46:47.550 --> 00:46:56.020 jump right out at you, but the sampling is really not so great at this point. 00:46:57.620 --> 00:47:00.360 - Joan, I had a question about what you meant by sediments. 00:47:00.369 --> 00:47:04.640 Did you have some sort of definition? - So, yeah, and that’s another question. 00:47:04.640 --> 00:47:09.160 And certainly resonance is also – if I understand it, you need an 00:47:09.160 --> 00:47:14.230 impedance contrast. So just having – how you define things and so on. 00:47:14.230 --> 00:47:18.920 In the way that it’s been defined here is – and this is the way 00:47:18.920 --> 00:47:25.200 that Bill Stephenson defined it in his model – is a shear – when the 00:47:25.200 --> 00:47:30.800 P wave velocity goes – is at 4-1/2 kilometers per second. 00:47:30.800 --> 00:47:32.580 It’s arbitrary. 00:47:32.580 --> 00:47:36.800 And the other study that I compared with has a different definition. 00:47:36.800 --> 00:47:41.450 So that’s another challenge. [chuckles] 00:47:41.450 --> 00:47:45.000 And you really need – again, it’s the impedance contrast, 00:47:45.000 --> 00:47:48.520 not an absolute velocity criteria. 00:47:48.520 --> 00:47:50.660 - Okay. - Yeah. 00:47:51.180 --> 00:47:53.240 Annemarie? - Hey, Joan. Thanks for a really nice talk. 00:47:53.240 --> 00:47:55.820 I think it’s a really interesting question to consider the 00:47:55.829 --> 00:47:59.970 offshore site response, and that’s not really maybe been studied. 00:47:59.970 --> 00:48:03.930 Just curious if you’ve made any connections to the ground motion 00:48:03.930 --> 00:48:09.059 prediction equation community or – that’s basically what you’re doing, 00:48:09.059 --> 00:48:11.670 and then trying to look at what the site response is 00:48:11.670 --> 00:48:13.839 after you’ve corrected for that attenuation. 00:48:13.839 --> 00:48:17.940 So in particular, ground motion prediction equations typically have 00:48:17.940 --> 00:48:21.700 different attenuation relations whether the event is crustal 00:48:21.700 --> 00:48:24.990 or in the slab or below the slab or something. 00:48:24.990 --> 00:48:29.770 And I’m just curious, given your sort of data set where you had 00:48:29.770 --> 00:48:31.970 the local earthquakes in a higher frequency range 00:48:31.970 --> 00:48:35.040 and the more distant earthquakes in the lower frequency range, 00:48:35.040 --> 00:48:39.780 can you do some separation to make sure you’re not sort of crossing those terms, 00:48:39.780 --> 00:48:43.589 or biasing your results? - Yeah. 00:48:43.589 --> 00:48:45.470 - You know, that you’re seeing these very different responses. 00:48:45.470 --> 00:48:49.369 But I will also point out that there’s – in site response studies, there are 00:48:49.369 --> 00:48:51.800 very strong frequency dependencies … - Sure. 00:48:51.800 --> 00:48:55.400 - … of the site response, you know, of a single location to different frequencies. 00:48:55.400 --> 00:48:59.090 - Sure, sure. - But there’s been a lot of work on that. 00:48:59.090 --> 00:49:02.839 - Oh, yeah, no. I definitely am – I mean, I’m not certainly not aware 00:49:02.839 --> 00:49:07.340 of all the work, and I know there’s volumes of it. 00:49:07.340 --> 00:49:11.980 And I have played around – and I didn’t want to bore people with – 00:49:11.980 --> 00:49:16.220 have I not adequately accounted for, say, 00:49:16.220 --> 00:49:22.000 the attenuation given the huge spacing in this and so forth. 00:49:22.000 --> 00:49:25.340 So there’s – it’s possible that some of what’s modeled here 00:49:25.349 --> 00:49:30.660 as site response is really not site response, but regional – 00:49:30.660 --> 00:49:33.460 differences in regional attenuation, for example. 00:49:33.460 --> 00:49:39.599 That’s certainly some – there’s some – probably some in that – particularly in 00:49:39.599 --> 00:49:43.890 the high-frequency pass band where all the events are offshore. 00:49:43.890 --> 00:49:49.830 And there just aren’t very many onshore. So, yeah, that’s certainly a concern. 00:49:49.830 --> 00:49:54.190 And that’s why I’ve really tried not to over-interpret this 00:49:54.190 --> 00:49:59.940 in any real sort of quantitative way, but just kind of wave my arms a lot. 00:49:59.940 --> 00:50:03.330 And show colors – pretty colors and not – yeah. 00:50:05.440 --> 00:50:07.100 - Keith? - Hi, Joan. 00:50:07.100 --> 00:50:12.460 That was a really interesting talk, thanks. So part of your analysis depends on the 00:50:12.460 --> 00:50:19.190 Stephenson et al. model of properties. And I’m wondering how 00:50:19.190 --> 00:50:25.160 well-constrained the offshore is in his model. What data did he use to … 00:50:25.160 --> 00:50:28.740 - Yeah, it’s not – and he – I’ve talked with Bill about it. 00:50:28.750 --> 00:50:33.450 It’s not – and his focus was really – the purpose of this model was 00:50:33.450 --> 00:50:38.890 really to constrain hazard studies where you’re worried about the onshore. 00:50:38.890 --> 00:50:42.380 And it’s probably really good in places like the Puget Lowlands 00:50:42.380 --> 00:50:46.080 and the basins and so forth. That was the purpose. 00:50:46.080 --> 00:50:50.120 He would acknowledge that, offshore, it’s probably not – 00:50:50.120 --> 00:50:56.690 it’s a low-resolution version. He’s used, you know, some of the – 00:50:56.690 --> 00:51:03.609 there are profiles from offshore, but again, it’s – you know, 00:51:03.609 --> 00:51:06.059 it’s just that simple comparison showed. 00:51:06.059 --> 00:51:09.900 It’s a very low-resolution version offshore. 00:51:09.900 --> 00:51:15.780 But it’s the only model we have that is sort of uniform everywhere. 00:51:15.780 --> 00:51:18.960 And so I wanted to use something that I could apply – because I – 00:51:18.960 --> 00:51:23.230 again, where sediment has the same definition everywhere. 00:51:23.230 --> 00:51:26.530 So, yeah. 00:51:26.530 --> 00:51:29.140 It’s a shortcoming. [laughs] 00:51:33.140 --> 00:51:36.530 - This was already touched on, I think, but how do we distinguish in the 00:51:36.530 --> 00:51:41.369 southern part between turbidites that might be triggered by a subduction 00:51:41.369 --> 00:51:43.120 interface event and one that might be triggered by 00:51:43.120 --> 00:51:47.030 a large Mendocino Triple Junction event? 00:51:47.030 --> 00:51:49.480 - This is a good question. [chuckles] 00:51:49.480 --> 00:51:52.750 Which I don’t really have a … - I realize this wasn’t the 00:51:52.750 --> 00:51:56.240 focus of your talk, but … - Yeah, no. I think one of the evidence – 00:51:56.240 --> 00:52:03.930 and there’s a little bit of work being done already and currently, is to look at, 00:52:03.930 --> 00:52:12.930 if you see a turbidite, and you see some tsunami deposit that goes with it, 00:52:12.930 --> 00:52:17.670 then that gives you – or then you have a lot more confidence 00:52:17.670 --> 00:52:24.420 that it’s not a San Andreas event or some onshore event. 00:52:24.420 --> 00:52:28.320 But beyond that, I don’t really know the answer. [chuckles] 00:52:28.320 --> 00:52:35.660 Again, that’s not – I gave this talk in – at the U-dub where there’s a lot of 00:52:35.660 --> 00:52:38.480 doubt about this turbidite stuff. 00:52:38.480 --> 00:52:41.960 And everybody asked me, well, why am I even bothering with this. 00:52:41.960 --> 00:52:46.710 So there’s a lot of question about the – that’s exactly – that’s a good question. 00:52:46.710 --> 00:52:48.900 I don’t know the answer. - Thanks. 00:52:50.240 --> 00:52:52.300 - Yeah? - Hi, Joan. 00:52:53.840 --> 00:52:58.300 So I guess I see the site response is also a convolution of how the 00:52:58.300 --> 00:53:04.020 instrument is coupled to the site. So is there any – can you rule out that 00:53:04.020 --> 00:53:08.660 abrupt change is not just measuring with OBS and not measuring with OBS? 00:53:08.660 --> 00:53:14.549 - Well, the big change occurs offshore where you have OBS’s on both sides. 00:53:14.549 --> 00:53:19.020 - Oh, okay. I guess I missed that. - So it’s at the base of the continental 00:53:19.020 --> 00:53:26.089 margin, which is – you know, there’s a big swath still on the seafloor. 00:53:26.089 --> 00:53:33.839 So unless it’s – you know, they put them – you know, being on steep slopes. 00:53:33.839 --> 00:53:36.330 And they just dump these things over the side. 00:53:36.330 --> 00:53:39.540 But they had to do it – it has to be very systematic because 00:53:39.540 --> 00:53:44.030 you see it along the entire margin. And so I worried about the 00:53:44.030 --> 00:53:49.010 instrumental things, but I don’t see – again, they’re all – the change occurs 00:53:49.010 --> 00:53:52.480 with OBS’s on both sides. - Okay, good. 00:53:56.540 --> 00:53:59.780 - Well, if there are no further questions, let’s thank Joan again. 00:53:59.780 --> 00:54:00.860 - Thank you. 00:54:00.860 --> 00:54:04.940 [ Applause ] 00:54:04.940 --> 00:54:08.880 - And feel free to join us for lunch. 00:54:10.860 --> 00:54:17.420 [ quiet background conversations ]