WEBVTT Kind: captions Language: en-US 00:00:01.680 --> 00:00:06.460 All right. I guess we’ll get started as people wander in. 00:00:07.760 --> 00:00:13.639 Hello. Welcome to our joint GMEG/Earthquake seminar. 00:00:13.639 --> 00:00:16.330 I am here representing earthquakes, so I’m going to take this opportunity 00:00:16.330 --> 00:00:20.900 to advertise our seminar next week. Earthquake seminar will be at its normal 00:00:20.900 --> 00:00:23.660 time – Wednesday morning at 10:30. And we’re going to have 00:00:23.660 --> 00:00:27.370 Hongfeng Yang from Chinese University of Hong Kong talking about 00:00:27.370 --> 00:00:32.250 locking of subduction zones and earthquake magnitude. 00:00:32.250 --> 00:00:36.520 So today, I have the pleasure of introducing to you Sam Johnson. 00:00:36.520 --> 00:00:39.550 Many of you probably know Sam, who works for the USGS in the 00:00:39.550 --> 00:00:43.120 Pacific Coastal and Marine Science Center down in Santa Cruz. 00:00:43.120 --> 00:00:46.399 He actually did his undergrad down in Santa Cruz, his Ph.D. at the University 00:00:46.399 --> 00:00:50.269 of Washington, and after that, taught for a while at Washington State. 00:00:50.269 --> 00:00:54.569 He started at the USGS, first in the Denver office working for the 00:00:54.569 --> 00:00:59.520 energies team, and then later in Golden working for the hazards team. 00:00:59.520 --> 00:01:02.969 In 2003, he moved out here to California to become the 00:01:02.969 --> 00:01:06.600 center director for the Pacific Coastal and Marine Science Center. 00:01:06.600 --> 00:01:09.540 And recently, though, he’s been back to doing his research. 00:01:09.550 --> 00:01:14.580 And his research has focused a lot on, first, the Pacific Northwest, 00:01:14.580 --> 00:01:17.230 and now in California, in imaging fault structures. 00:01:17.230 --> 00:01:21.820 And so he’s going to talk to us today about the northern San Andreas Fault. 00:01:22.500 --> 00:01:24.220 - Thanks, Jeanne. 00:01:27.240 --> 00:01:32.040 And thanks for coming out, everyone. Is this what the lights are like? 00:01:32.040 --> 00:01:33.960 Does somebody … - No. They [inaudible]. 00:01:33.960 --> 00:01:36.420 - They’ll go down? Okay. - Yeah. There they are. 00:01:36.420 --> 00:01:41.740 - Oh, great. Okay, so, as everyone here probably knows, the right lateral 00:01:41.740 --> 00:01:45.640 San Andreas Fault is the main structure in the boundary between the Pacific 00:01:45.640 --> 00:01:50.930 Plate and the North American Plate. And the northern San Andreas Fault 00:01:50.930 --> 00:01:56.110 is that part of the structure that extends northward from the San Juan Bautista 00:01:56.110 --> 00:02:03.100 area at the junction with the East Bay Fault system. 00:02:03.100 --> 00:02:06.390 The northern San Andreas extends up through the Santa Cruz Mountains and 00:02:06.390 --> 00:02:11.500 the peninsula just a few kilometers west of where we are here into the offshore – 00:02:11.500 --> 00:02:16.870 offshore of San Francisco, back onshore at Bolinas and through Tomales Bay. 00:02:16.870 --> 00:02:20.070 Offshore mostly up to just south of Fort Ross. 00:02:20.070 --> 00:02:24.160 Onshore again up to Port Arena. And then it’s offshore for about 00:02:24.160 --> 00:02:30.540 120 kilometers up to the south flank of the King Range. 00:02:32.060 --> 00:02:38.460 Slip is about 17 millimeters per year, more or less, in this north peninsular 00:02:38.460 --> 00:02:44.980 section, and then it picks up to about 24 millimeters a year at Bolinas, 00:02:44.980 --> 00:02:50.630 where it picks up slip from the Hosgri-San Gregorio Fault system. 00:02:50.630 --> 00:02:58.860 The entire fault ruptured in 1906 in what – in a magnitude 7.9 earthquake 00:02:58.860 --> 00:03:03.300 considered one of the world’s most important historic earthquakes. 00:03:03.300 --> 00:03:06.680 It destroyed San Francisco. 00:03:07.840 --> 00:03:12.740 And, in the aftermath of that event, geologists fanned out across the on-land 00:03:12.740 --> 00:03:20.660 portions of the fault and documented the earthquake effects and ruptures. 00:03:20.660 --> 00:03:24.640 They were published in 1908 in the Lawson Report. 00:03:24.640 --> 00:03:29.000 And subsequently, lots of geologists have used the northern San Andreas 00:03:29.010 --> 00:03:33.709 Fault as a natural laboratory for documenting things like fault structure 00:03:33.709 --> 00:03:38.440 and dynamics, earthquake effects, paleoseismology – really pushed 00:03:38.440 --> 00:03:43.569 back some frontiers. In constrast, the marine part of the fault, 00:03:43.569 --> 00:03:49.780 which fills about – it’s offshore for about 40% of its length – 00:03:49.780 --> 00:03:55.040 more if you consider – throw in Bolinas Lagoon and Tomales Bay – 00:03:55.040 --> 00:04:00.160 has received quite a bit less emphasis, or attention. 00:04:01.540 --> 00:04:04.800 And that’s basically what I’ll be talking about today. 00:04:04.810 --> 00:04:09.160 I’ll describe the three different offshore sections in varying detail, 00:04:09.160 --> 00:04:12.660 focusing most on what’s new. I’ll talk about the importance of 00:04:12.660 --> 00:04:17.160 fault strike in both regional and local geomorphology. 00:04:17.160 --> 00:04:21.560 Talk about our new data mapping, some elements of tectonic geomorphology, 00:04:21.560 --> 00:04:25.850 some elements of hazard assessment, and then I’ll end up with a discussion 00:04:25.850 --> 00:04:30.380 of how the San Andreas Fault – northern San Andreas Fault ends. 00:04:31.460 --> 00:04:34.970 So just – before we get started, just to review a couple of simple 00:04:34.970 --> 00:04:41.840 concepts about strike-slip faulting. If the – if the fault is parallel to driving 00:04:41.840 --> 00:04:45.660 forces, which, in this case, would be the plate motion, then one should expect 00:04:45.660 --> 00:04:50.020 the two faults – the opposite sides of the fault to slide smoothly by each other. 00:04:50.020 --> 00:04:53.580 Where there’s an element of convergence, where the fault is trending 00:04:53.580 --> 00:04:58.880 more westerly, like in the Santa Cruz Mountains, one would expect 00:04:58.880 --> 00:05:03.810 higher elevations and uplift. And, where the fault is trending 00:05:03.810 --> 00:05:09.240 more northerly, like in this offshore area north of Point Arena, one would 00:05:09.240 --> 00:05:13.760 expect transtension, or lower elevations and extension. 00:05:13.760 --> 00:05:16.419 So, with that in mind, let’s look at this graph. 00:05:16.419 --> 00:05:19.650 And it’s important that you kind of zero in on what I’m showing here 00:05:19.650 --> 00:05:23.680 because you’ll see different versions of this throughout the talk. 00:05:23.680 --> 00:05:28.430 On the X axis is just distance from the San Juan Bautista area on the left, 00:05:28.430 --> 00:05:36.030 over to Point Delgada on the right. And on the left axis, the Y axis, 00:05:36.030 --> 00:05:41.230 is just fault strike. The green line is the angle – 00:05:41.230 --> 00:05:47.340 is the vector of motion between the Pacific and North America Plate. 00:05:47.340 --> 00:05:51.620 So when its farther – when it’s above the line, it’s, in theory, in transpression. 00:05:51.620 --> 00:05:54.580 When it’s below the line, in transtension. 00:05:54.580 --> 00:06:00.990 This curve is a plot where I measured the strike of the fault every 2 kilometers 00:06:00.990 --> 00:06:05.460 and then averaged it over two measurements to the – 00:06:05.460 --> 00:06:09.550 to the – on either side of that measure. So it’s essentially an 8-kilometer-wide 00:06:09.550 --> 00:06:13.320 running average, and it shows the trend of the fault – or, the strike 00:06:13.320 --> 00:06:17.620 of the fault from southeast to northwest. 00:06:19.250 --> 00:06:22.320 So what you should immediately notice is that – and then these 00:06:22.330 --> 00:06:26.700 shaded blue areas are the places where the fault lies offshore. 00:06:26.700 --> 00:06:32.940 So the most obvious thing to note is that these two bigger offshore areas offshore 00:06:32.940 --> 00:06:37.380 of San Francisco, and this very large one offshore of – between Point Arena and 00:06:37.380 --> 00:06:41.840 Point Delgada are the most transtensional points of the fault system. 00:06:41.840 --> 00:06:47.760 They fall below the green line. And the third area here is – 00:06:47.760 --> 00:06:50.640 falls a little bit below the line, but it’s also associated with 00:06:50.640 --> 00:06:55.940 a big transtensional bend right here. 00:06:55.940 --> 00:07:04.300 So that’s sort of an indication of the control of strike on the regional 00:07:04.300 --> 00:07:08.259 geomorphology. So I’ll just move now into a discussion of these 00:07:08.259 --> 00:07:11.430 three different offshore areas. This is the area offshore of 00:07:11.430 --> 00:07:14.780 San Francisco, which has received the most attention. 00:07:14.780 --> 00:07:19.040 Beginning about 25 years ago – before that, Alan Cooper in the 00:07:19.050 --> 00:07:21.990 Coastal and Marine Geology group did some mapping. 00:07:21.990 --> 00:07:26.130 But the real big focus came about about 25 years ago, and here’s 00:07:26.130 --> 00:07:30.060 a whole series of papers that describe the geology in this area. 00:07:30.060 --> 00:07:33.220 A lot of it was summarized in our publications for the California Seafloor 00:07:33.220 --> 00:07:37.560 Mapping Program. This plot on the left just shows the density of seismic 00:07:37.560 --> 00:07:43.640 reflection profiles, almost all of which were collected by the USGS. 00:07:44.160 --> 00:07:46.919 And there’s also high-resolution bathymetry in this area that was 00:07:46.920 --> 00:07:52.100 collected for the California Seafloor Mapping Program in about 2010. 00:07:52.800 --> 00:07:56.340 This is an area, again, that’s received intense attention. 00:07:56.340 --> 00:08:00.260 This is just an example of a multi-channel airgun profile on the 00:08:00.260 --> 00:08:04.940 bottom that was collected by the USGS. And then, above it, it’s a 00:08:04.940 --> 00:08:09.380 higher-resolution minisparker profile that was collected in 2006. 00:08:09.380 --> 00:08:13.889 And all of the other profiles – this profile extends down to about a kilometer 00:08:13.889 --> 00:08:19.800 and a half depth. This one extends down to about 160 meters’ depth. 00:08:19.800 --> 00:08:22.150 And all of the other profiles that you’re going to see in this talk 00:08:22.150 --> 00:08:28.800 will be of this high-resolution minisparker – were collected by 00:08:28.800 --> 00:08:31.780 this high-resolution minisparker system. 00:08:31.780 --> 00:08:35.260 Vertical exaggeration on all these profiles will be about – somewhere 00:08:35.260 --> 00:08:40.260 between 8 to 12 to 1, except for one exception, which I’ll point out. 00:08:40.260 --> 00:08:43.580 Faults are shown in red lines. These are recognized by deformed 00:08:43.589 --> 00:08:47.680 or distorted reflections or juxtaposition of different panels 00:08:47.680 --> 00:08:50.190 with different reflection properties. 00:08:50.190 --> 00:08:55.380 The green lines are just basically showing continuous reflections. 00:08:55.380 --> 00:08:58.770 They’re just designed to sort of train your eye to see the structure. 00:08:58.770 --> 00:09:05.890 The yellow line is the echo of the water bottom. It’s called a multiple. 00:09:05.890 --> 00:09:11.470 And then the blue is sediments deposited in the last 21,000 years after 00:09:11.470 --> 00:09:15.810 the last sea level low stand. And I’ll tell you how we know that in a minute. 00:09:15.810 --> 00:09:18.440 But you’ll just take – have to take my word on that for now. 00:09:18.440 --> 00:09:21.870 But anyway, as you can see, that this offshore area is cut by 00:09:21.870 --> 00:09:26.900 a number of significant faults, as shown on this map right here. 00:09:26.900 --> 00:09:30.780 The star, by the way, is the epicenter of the 1906 earthquake. 00:09:30.780 --> 00:09:36.320 And it is an extensional area, so one would expect to see extensional features 00:09:36.320 --> 00:09:40.630 like the San Andreas graben right here, which is shown 00:09:40.630 --> 00:09:45.330 here on this isopach map – includes as much as 57 meters 00:09:45.330 --> 00:09:51.570 of sediment deposited in about the last 10,000 or 12,000 years. 00:09:51.570 --> 00:09:55.640 So that’s as much as I’m going to say about this. It’s mostly older work. 00:09:55.640 --> 00:09:59.860 I’m going to focus now on the offshore area between Tomales Point and 00:09:59.860 --> 00:10:05.660 Salt Point, which is this box shown on the map here on the left. 00:10:06.880 --> 00:10:10.460 And then these two maps on the right have been rotated so that 00:10:10.470 --> 00:10:15.740 north is up and to the right to fit more into this panel. 00:10:15.740 --> 00:10:19.640 We collected seismic profiles here crossing the San Andreas Fault system 00:10:19.640 --> 00:10:23.970 38 times between where it goes onshore at Timber Gulch 00:10:23.970 --> 00:10:27.040 and where it goes into Tomales Bay. 00:10:27.040 --> 00:10:32.080 500-meter line spacing in Bodega Bay, which is the name of a town – oops – 00:10:32.080 --> 00:10:36.440 the name of a town and the name of a – oops, sorry – the name of 00:10:36.440 --> 00:10:39.620 a town and the name of a water body. So when I refer to Bodega Bay, 00:10:39.620 --> 00:10:42.860 I’ll be talking about the water body that’s right here. 00:10:43.480 --> 00:10:50.240 And then, at 500-meter line spacing in Bodega Bay and kilometer line 00:10:50.240 --> 00:10:56.500 spacing north of Bodega Head. The map on the right is a geologic map 00:10:56.500 --> 00:11:00.340 simplified from the maps we published in our Seafloor Mapping Program 00:11:00.340 --> 00:11:04.520 publications. The pink is cretaceous granite. Many of you have probably 00:11:04.520 --> 00:11:09.839 seen the outcrops of this at Tomales Point or at Bodega Head. 00:11:09.840 --> 00:11:14.200 And the green – I think it looks green from where I’m standing – 00:11:14.200 --> 00:11:19.300 that’s the Franciscan complex. The brown is a different block. 00:11:19.310 --> 00:11:25.310 It’s the Gualala block that I won’t be discussing today, but it’s north of where 00:11:25.310 --> 00:11:31.190 the San Andreas Fault goes onshore. These yellow blobs here are 00:11:31.190 --> 00:11:35.170 sand dune complexes. I’ll talk a little bit about those in a minute. 00:11:35.170 --> 00:11:42.279 And then these funny-shaped blocks out here in the offshore, those are 00:11:42.279 --> 00:11:45.790 slope failures that I’ll also be discussing a little bit – a little bit more. 00:11:45.790 --> 00:11:50.540 So the next slide is going to show you a profile that comes from the offshore 00:11:50.540 --> 00:11:56.770 and crosses the tip of this submerged granitic outcrop right there. 00:11:56.770 --> 00:12:00.200 And basically – I should have pointed out – in fact, I’ll go back 00:12:00.200 --> 00:12:04.720 and point it out, that 21,000 years ago, during the last low stand, 00:12:04.720 --> 00:12:08.860 the shoreline was out here. And, as the shoreline migrated 00:12:08.860 --> 00:12:14.330 back toward the present coast with sea level rise, it generated a 00:12:14.330 --> 00:12:19.350 wave-cut platform transgressive surface that gets progressively 00:12:19.350 --> 00:12:25.050 younger to the – toward the shoreline. So the value of that – and this is 00:12:25.050 --> 00:12:26.769 the unconformity that you can see right here. 00:12:26.769 --> 00:12:31.970 It’s the bottom of the yellow unit. And one of the cool things about it is 00:12:31.970 --> 00:12:36.950 that – because it’s a transgressive surface of erosion that tracks sea level, 00:12:36.950 --> 00:12:42.070 at any depth that you find of that, you can predict – it gives you 00:12:42.070 --> 00:12:46.920 a general age indication. So that, if it’s at 80 meters’ depth, that’s 00:12:46.920 --> 00:12:54.300 when that wave-cut platform was cut, so that tells you approximately its age. 00:12:54.300 --> 00:12:57.640 I hope everybody follows that. Basically, you can look at the surface 00:12:57.640 --> 00:13:01.209 and – at its depth, because it’s a transgressive surface getting younger 00:13:01.209 --> 00:13:06.680 toward the coast, you can use that to – as a rough age indication – 00:13:06.680 --> 00:13:10.510 rough age indicator. And you’ll see how I use that in a minute. 00:13:10.510 --> 00:13:17.000 Okay, so if you look at Bodega Bay, this is a – sort of a raised granitic 00:13:17.040 --> 00:13:21.840 bedrock platform. It’s at a depth of about 20 meters to the left. 00:13:21.840 --> 00:13:25.300 And then there’s these rugged Franciscan outcrops to the right. 00:13:25.300 --> 00:13:29.600 And, when you – and then they’re separating this smooth, 00:13:29.600 --> 00:13:31.779 flat area between them. So when you just look at the 00:13:31.779 --> 00:13:35.300 geomorphology, you think, oh, great. This is going to be a cool pull-apart 00:13:35.300 --> 00:13:39.730 basin. But then, when you actually map it, it’s a different story. 00:13:39.730 --> 00:13:44.670 You realize it’s essentially a distributed fault zone. And that the depression must 00:13:44.670 --> 00:13:52.460 be related to sort of fault-generated weakening of bedrock and then erosion. 00:13:52.460 --> 00:13:57.890 So – and it’s only got a very thin sediment fill that the yellow stuff 00:13:57.890 --> 00:14:00.400 is what was deposited in the last 21,000 years. 00:14:00.400 --> 00:14:05.020 And then the pink stuff is probably older Pleistocene stuff, much of which 00:14:05.020 --> 00:14:11.560 is non-marine origin – in origin. And I think that because, basically, 00:14:11.560 --> 00:14:18.529 if you – for about 85% of the last 70,000 years, when sea level was 00:14:18.529 --> 00:14:22.610 lower than 30 meters, this is what the geomorphology looked like. 00:14:22.610 --> 00:14:26.310 It was essentially the Tomales Bay and up through Bodega Head and 00:14:26.310 --> 00:14:30.711 north of Bodega Bay was essentially a continuation of the Tomales Bay 00:14:30.711 --> 00:14:35.070 straight-as-an-arrow linear trough, which extended for about 00:14:35.070 --> 00:14:40.170 50 kilometers from north to south. And this thin, little finger of granite 00:14:40.170 --> 00:14:44.240 that extends now to Tomales Bay actually extended – that same little 00:14:44.240 --> 00:14:48.260 finger was a little bit wider but extended it about 15 kilometers farther to the 00:14:48.260 --> 00:14:54.350 north. So very extraordinary – very extraordinary geomorphology. 00:14:54.350 --> 00:14:58.710 Matched only along the northern San Andreas Fault by this area, 00:14:58.710 --> 00:15:03.070 which is along the Garcia-Gualala River to the north, 00:15:03.070 --> 00:15:08.040 where you can again see one of these straight-as-an-arrow valleys. 00:15:08.980 --> 00:15:11.840 It’s sort of the Tomales Bay waiting for sea level to rise 00:15:11.850 --> 00:15:15.070 another 30 meters before it becomes the next one. 00:15:15.070 --> 00:15:18.700 But what’s truly extraordinary, at least to me, is that both of these 00:15:18.700 --> 00:15:28.500 linear valleys have the exact same strike and the same dip of the strike line. 00:15:29.420 --> 00:15:33.860 So, again, I guess it’s – to some degree, it’s predictable, but it’s also astonishing 00:15:33.860 --> 00:15:37.670 to me that it would – you know, that it happens that way. 00:15:37.670 --> 00:15:42.310 So, again, these two straight-as-an-arrow areas, 00:15:42.310 --> 00:15:45.670 geomorphologically, are characterized by fault 00:15:45.670 --> 00:15:51.360 with the exact same strike and then the slope of the strike line. 00:15:52.120 --> 00:15:56.880 Okay, getting back to Bodega Bay. This is where the 1906 offset occurred. 00:15:56.889 --> 00:16:00.850 The yellow triangle, I should point out, was where a nuclear power plant 00:16:00.850 --> 00:16:04.910 was proposed in the late 1960s. There was a lot of work done 00:16:04.910 --> 00:16:09.630 on land surrounding that siting issue, 00:16:09.630 --> 00:16:12.670 but essentially nothing offshore that I could find. 00:16:12.670 --> 00:16:18.160 And this is what – this is how the 1906 rupture was described. 00:16:18.160 --> 00:16:21.050 The main earthquake fissure – in the Lawson Report – 00:16:21.050 --> 00:16:23.990 the main earthquake fissure was found passing about 50 yards west 00:16:23.990 --> 00:16:28.040 of a house occupied by Mr. Johnson. It could be traced as a multitude 00:16:28.040 --> 00:16:31.880 of small cracks in the swampy land from the bay to the road, then as a 00:16:31.880 --> 00:16:34.820 well-defined fissure up the small depression west of the house 00:16:34.820 --> 00:16:38.300 for 200 yards to where it disappeared in the sand dunes. 00:16:38.300 --> 00:16:40.500 No trace of it could be detected in the sand dunes, 00:16:40.500 --> 00:16:43.440 which reach from this point entirely across the peninsula. 00:16:43.440 --> 00:16:49.180 So, again, it’s a very good description of a rupture associated with 1906. 00:16:49.180 --> 00:16:57.160 Okay, I’m going to now show you details of this box, which is rotated here, 00:16:57.160 --> 00:17:03.910 from the – from the floor of Bodega Bay – you’ll see it 00:17:03.910 --> 00:17:08.559 in greater detail here – where it appears from this – again, look at this rough 00:17:08.559 --> 00:17:16.329 texture, that the back of this sandbar – this feature is offset by about 75 meters. 00:17:16.329 --> 00:17:20.980 The bar itself is about 4 kilometers long. It’s 300 to 600 meters wide. 00:17:20.980 --> 00:17:22.880 It dips a degree. 00:17:22.880 --> 00:17:28.460 And, again, the north flank is pretty clearly offset about 75 meters. 00:17:32.600 --> 00:17:36.220 For this to be useful in terms of sort of evaluating the earthquake hazard, 00:17:36.220 --> 00:17:39.529 it’s important to figure out what it is and what its age is, which is what this 00:17:39.529 --> 00:17:43.760 slide is all about. So that’s, again, the offset we’re talking about. 00:17:43.760 --> 00:17:48.799 And the interpretation is that this bar here was this bar here. 00:17:48.799 --> 00:17:52.690 So it’s essentially the sand spit connecting the granites to the 00:17:52.690 --> 00:17:58.940 Franciscan complex when sea level was about 18 meters lower. 00:17:58.940 --> 00:18:04.629 And these bars typically form sort of where the angle of the 00:18:04.629 --> 00:18:08.760 coastline changes dramatically. Sand moves down, and then it 00:18:08.760 --> 00:18:13.200 basically accretes on the edge of the bar, and then – along-shore drift, 00:18:13.200 --> 00:18:17.340 and then gradually builds out across the bar to form the spit. 00:18:17.340 --> 00:18:20.510 Another great example of this spit complex is the one at the 00:18:20.510 --> 00:18:25.919 mouth of Bolinas Lagoon – Stinson Beach sand spit. 00:18:25.919 --> 00:18:31.570 So basically, if you look at this diagram, then, what we’re looking at here is 00:18:31.570 --> 00:18:36.149 this is the surface that’s right there. And, as sea level rises, it goes back. 00:18:36.149 --> 00:18:40.809 This is another possible sand spit and shore face, and then this is 00:18:40.809 --> 00:18:43.940 the modern one right here where I’ve extended this line. 00:18:43.940 --> 00:18:49.149 And I should point out that this profile is exaggerated about 26 to 1. 00:18:49.149 --> 00:18:55.269 So this is the one exception to that exaggeration that I was telling you 00:18:55.269 --> 00:19:00.940 is valid for all the other profiles. So – and just in case this isn’t clear – 00:19:00.940 --> 00:19:05.180 perfectly clear, I’m projecting this from the north. 00:19:05.190 --> 00:19:10.229 And the point that I’m trying to make is that, at lower sea level, this sand spit 00:19:10.229 --> 00:19:15.809 was out here, which would be out here on this projection with the map flipped. 00:19:15.809 --> 00:19:18.570 And then, as sea level rose, it just basically migrated back 00:19:18.570 --> 00:19:21.409 to the position right here. And, again, these are the three 00:19:21.409 --> 00:19:28.070 different angles of – or, the three different paleo shore faces. 00:19:28.070 --> 00:19:32.269 For this surface to have this angle, one would have to grow it 00:19:32.269 --> 00:19:34.809 about 3 to 5 meters. So there’s possible erosion 00:19:34.809 --> 00:19:37.789 of 3 to 5 meters off the top of this feature. 00:19:37.789 --> 00:19:42.029 So we can – we know that it’s younger than this surface, 00:19:42.029 --> 00:19:44.120 which is the transgressive surface of erosion. 00:19:44.120 --> 00:19:48.809 You know, based on its depth, it’s about 9,500 years old. 00:19:48.809 --> 00:19:53.759 And then, by inferring the sea level that would be responsible for 00:19:53.759 --> 00:19:59.220 the development of this feature relative to sea level, this would have 00:19:59.220 --> 00:20:03.669 formed at sea levels about 13 to 18 meters below present. 00:20:03.669 --> 00:20:08.520 So that age is – let’s see. 00:20:09.140 --> 00:20:11.580 Okay, yeah, okay. 00:20:11.590 --> 00:20:15.720 So that age – that sand bar that we’re looking at, then, formed about – 00:20:15.720 --> 00:20:22.059 and it’s – wave energy basically removed the offset on the side 00:20:22.060 --> 00:20:24.660 of the bar. This is the more protected side of the bar where it 00:20:24.660 --> 00:20:31.140 was actually preserved. That sandbar formed about 7,500, 8,700 years ago, 00:20:31.140 --> 00:20:38.010 and it yields a slip rate on this strand of about 8-1/2 to 10 millimeters per year. 00:20:39.500 --> 00:20:46.520 Okay, but – let’s see. So that’s the slip rate for that fault strand 1. 00:20:46.529 --> 00:20:50.429 But the total rate for the San Andreas system is actually about 24 millimeters 00:20:50.429 --> 00:20:54.659 per year in this area. And we know that from GPS data and paleoseismology. 00:20:54.659 --> 00:21:00.399 So Holocene slip, then, must be distributed on more than one strand. 00:21:00.399 --> 00:21:05.830 This is the obvious other strand. It’s the one where the 1906 rupture 00:21:05.830 --> 00:21:13.080 was monitored. It’s possible that this strand also ruptured in 1906, but one – 00:21:13.080 --> 00:21:17.070 but because it goes through the sand dune complex on this – on this isthmus, 00:21:17.070 --> 00:21:20.929 you wouldn’t be able to see it. So future earthquakes – I guess the 00:21:20.929 --> 00:21:26.919 point is that future earthquakes could rupture on strands 1 or strands 2 00:21:26.920 --> 00:21:32.920 or on both. And we just don’t know exactly how that would happen. 00:21:33.870 --> 00:21:38.440 It – I guess the important thing – another important thing about this 00:21:38.450 --> 00:21:42.400 is that it stresses the importance of fault zone mapping prior to 00:21:42.400 --> 00:21:45.440 any paleoseismology work. If you just showed up and found 00:21:45.440 --> 00:21:49.739 a great scarp, for example, on one of these two strands and just 00:21:49.739 --> 00:21:53.360 did a study there, you’d probably miss some slip or possibly some events. 00:21:53.360 --> 00:21:58.360 So, again, it just points out the importance of good geologic mapping. 00:21:59.020 --> 00:22:02.200 Now we’re going to move a little bit to the north of Bodega Head. 00:22:02.210 --> 00:22:05.210 We were down here in Bodega Bay before. 00:22:05.210 --> 00:22:09.360 This is where we have profiles of 1-kilometer line spacing. 00:22:10.800 --> 00:22:13.799 The next slide you’ll see will be these 00:22:13.800 --> 00:22:20.040 six profiles that you see right here, starting north of Bodega Head. 00:22:22.520 --> 00:22:26.519 Where the fault zone is about 500 meters to 1,000 meters wide. 00:22:26.519 --> 00:22:33.739 It cuts this Russian River delta clinoform, and we know that – 00:22:33.739 --> 00:22:39.159 based on the depth of the transgressive surface of erosion, we can calculate 00:22:39.159 --> 00:22:45.580 post-20,000-year sedimentation rates. And they’re on the order of 00:22:45.580 --> 00:22:48.480 2 to 3 meters per 1,000 years. So this is the big delta from the 00:22:48.480 --> 00:22:53.919 Russian River building out, and this is the thickness of about 30 to 40 meters 00:22:53.919 --> 00:22:57.149 that was deposited at a rate of 2 to 3 meters per 1,000 years, which is – 00:22:57.149 --> 00:23:00.200 will be important, as I’ll show you in a second. 00:23:00.200 --> 00:23:05.070 This is what these – there are several depressions within the fault zone. 00:23:05.070 --> 00:23:10.289 This is what they look like in higher resolution and zooming in. 00:23:10.289 --> 00:23:13.299 Again they’re sort of like little keystone pieces. 00:23:13.300 --> 00:23:24.000 And they’re all – they actually map out in map view – oops – in map view, 00:23:24.000 --> 00:23:28.800 they map out as sort of three different discrete little sag depressions. 00:23:28.800 --> 00:23:33.360 And may be similar to some of the things that, for example, are in the 00:23:33.360 --> 00:23:40.040 mountains just west of here. The – so that’s right in here. 00:23:40.040 --> 00:23:46.020 Now I want to turn your attention to – oops – to – okay. 00:23:46.020 --> 00:23:48.280 I’m going to go back a slide. 00:23:50.250 --> 00:23:53.500 And point these features out to you, which I’ll be talking about next. 00:23:53.520 --> 00:23:57.060 These are – these very interesting chutes and lobes. 00:23:58.980 --> 00:24:03.280 They extend from water depths of about 35 meters down to about 70 meters. 00:24:03.280 --> 00:24:08.460 The lobes themselves have as much as 4 meters of relief on them, although 00:24:08.460 --> 00:24:12.559 it’s quite flat. Again, this is vertically exaggerated 12-1/2 to 1, so this is 00:24:12.559 --> 00:24:21.080 sort of a trick of visualization that makes them look this prominent. 00:24:21.080 --> 00:24:26.070 But nevertheless, they’re very impressive and important features, 00:24:26.070 --> 00:24:30.440 and I’ve interpreted them as liquefaction-induced slope failures. 00:24:30.440 --> 00:24:34.619 And I’ll explain that in a second, but the grain size of the material that’s 00:24:34.619 --> 00:24:39.889 failing is sand and then it’s – these lobes are prograding out over muddy sand. 00:24:39.889 --> 00:24:43.729 These are samples from 44 meters in the source areas and then 70 meters 00:24:43.729 --> 00:24:46.649 where the ends of the lobes are deposited. 00:24:46.649 --> 00:24:50.090 Again, here’s the – a cross-section across the lobes. 00:24:50.090 --> 00:24:54.849 And then here’s a view of the clinoform where we think the liquefaction is 00:24:54.849 --> 00:24:59.060 occurring. There’s a little indication of maybe a breakaway right there. 00:25:00.780 --> 00:25:06.009 So this liquefaction hypothesis is supported by observations that 00:25:06.009 --> 00:25:11.669 were made after a 1980 earthquake offshore of the Klamath River. 00:25:11.669 --> 00:25:16.519 And this was documented by Mike Field in our – in our science center 00:25:16.519 --> 00:25:22.109 and his colleagues. This is the site of a magnitude 7.2 earthquake. 00:25:22.109 --> 00:25:24.320 This was the site in Mike’s report, 00:25:24.320 --> 00:25:29.120 but it’s subsequently been relocated to a point closer to shore. 00:25:29.120 --> 00:25:35.440 And Mike and his colleagues basically went out and mapped, in 1977 and 1979, 00:25:35.440 --> 00:25:39.299 as part of a regional study of sort of deltaic sedimentation. 00:25:39.299 --> 00:25:44.460 Then the earthquake occurred in 1980, and a – and a bottom fisherman 00:25:44.460 --> 00:25:46.479 came and told them, look, I think there’s something crazy 00:25:46.479 --> 00:25:49.740 going on on the seafloor there. So they went out and re-surveyed 00:25:49.740 --> 00:25:53.859 in 1980 and ’81 and collected some side-scan data 00:25:53.859 --> 00:25:56.769 and some seismic reflection profiles. 00:25:56.769 --> 00:26:00.830 And they documented this 10- to 15-kilometer-long and 00:26:00.830 --> 00:26:06.429 1-to-4-kilometer-wide failure zone. It occurred on a slope of 00:26:06.429 --> 00:26:09.859 just a quarter of a degree. Water depths of about 60 meters. 00:26:09.859 --> 00:26:14.710 And a 20- to 30-meter-thick section of sediments deposited in the 00:26:14.710 --> 00:26:18.219 last 20,000 years at the sand-mud transition. 00:26:18.219 --> 00:26:20.789 There’s no multibeam mapping of this area, so unfortunately, 00:26:20.789 --> 00:26:24.090 there’s no sort of 1-to-1 correspondence that one can make. 00:26:24.090 --> 00:26:28.609 But this is – these are some side- scan images that they generated. 00:26:28.609 --> 00:26:34.529 And this is about a kilometer from – it’s about a kilometer-wide image 00:26:34.529 --> 00:26:39.909 looking at the toe ridge and the pressure ridges and the lobes 00:26:39.909 --> 00:26:45.660 forming at the – at the lower end of this failure surface. 00:26:45.660 --> 00:26:49.580 They envision this as, again, forming from liquefaction. 00:26:49.580 --> 00:26:52.840 This is – would be the liquefied zone right here. 00:26:52.840 --> 00:26:57.500 Basically, sand injecting, coming to the surface, and then 00:26:57.500 --> 00:27:02.780 flowing down the slope to form those lobes and those pressure ridges. 00:27:04.000 --> 00:27:11.399 So this is sort of more of what I’m envisioning, that, again, this – and 00:27:11.399 --> 00:27:14.799 these are images from the magnitude 7.1 earthquake in Christchurch. 00:27:14.799 --> 00:27:19.489 And interestingly, this surface here is about 30 kilometers from 00:27:19.489 --> 00:27:24.359 the earthquake rupture. And it’s a magnitude 7.1 earthquake. 00:27:24.359 --> 00:27:27.539 So I’m envisioning that, with a magnitude 7.9 earthquake 00:27:27.539 --> 00:27:32.849 2 to 3 kilometers from the – with rupture 2 to 3 kilometers 00:27:32.849 --> 00:27:39.929 from this site, that this whole slope looks something like this. 00:27:39.929 --> 00:27:43.169 And with the duration of shaking and the steeper slopes, that sediment 00:27:43.169 --> 00:27:49.549 basically flowed down to form those distinctive – it was liquefied, came to 00:27:49.549 --> 00:27:53.409 the surface, and then flowed downslope to form those distinctive lobes. 00:27:53.409 --> 00:27:54.830 This is just a comparison between 00:27:54.830 --> 00:27:58.180 the Russian River and the Klamath River sites. 00:27:58.760 --> 00:28:00.700 The width of failure zone is about the same. 00:28:00.710 --> 00:28:03.429 The length of downslope runout is about the same. 00:28:03.429 --> 00:28:05.799 The water depth is about the same. 00:28:05.799 --> 00:28:08.690 The sediment thickness is greater in the Russian River. 00:28:08.690 --> 00:28:11.889 And, again, we know that that sediment was rapidly deposited sand 00:28:11.889 --> 00:28:15.299 that would be easily liquefied. The grain size is actually – 00:28:15.299 --> 00:28:19.830 and then what I’ve shown in yellow are all the factors that actually favor 00:28:19.830 --> 00:28:23.259 the Russian River site, making it much more favorable for this kind of 00:28:23.259 --> 00:28:26.929 liquefaction-induced slope failure. It’s coarser sediment. 00:28:26.929 --> 00:28:31.059 It’s fine sand. It’s a steeper slope – significantly steeper – 0.8 relative to 00:28:31.059 --> 00:28:36.169 a quarter of a degree. The earthquake magnitude was about 8 times stronger. 00:28:36.169 --> 00:28:41.219 It’s, you know, 2 to 3 kilometers from the slope site as opposed to 50 00:28:41.219 --> 00:28:46.539 kilometers away. You know, the ground motions were clearly much stronger. 00:28:46.539 --> 00:28:51.299 But the preservation issue – you could still see these and map these fairly 00:28:51.299 --> 00:28:55.259 clearly on the Klamath River because it was – it was mapped one to two years 00:28:55.259 --> 00:29:02.210 after the event, whereas, the image from the Russian River was collected 00:29:02.210 --> 00:29:07.089 104 years after – in 2010, 104 years after the earthquake. 00:29:07.089 --> 00:29:13.520 So there’s a real issue in terms of when and where these things form. 00:29:13.520 --> 00:29:16.570 Do they form in every 1906-like earthquake with 00:29:16.570 --> 00:29:21.340 a 250- to 300-year recurrence? Yeah. I think they do. They should. 00:29:21.340 --> 00:29:24.809 Because, again, the threshold from the Klamath River example 00:29:24.809 --> 00:29:28.880 is quite a bit lower than what we see here. 00:29:29.649 --> 00:29:31.759 But we don’t recognize older chutes and lobes. 00:29:31.759 --> 00:29:34.729 And so the real question to ask is, what’s their preservation potential? 00:29:34.729 --> 00:29:37.360 And, you know, this is kind of a nerdy question of whether 00:29:37.360 --> 00:29:40.440 we’d be able to see more if we had even higher-resolution data. 00:29:40.440 --> 00:29:44.200 But just looking at this image, you can see that this field on the left, 00:29:44.200 --> 00:29:48.440 to the northwest, is actually a lot more subdued than the field on the right. 00:29:48.440 --> 00:29:51.520 So there’s a real sort of preservation issue. 00:29:52.840 --> 00:29:56.560 They’re very shallow. They’re being covered by deltaic sediment. 00:29:56.570 --> 00:30:00.599 They’re being covered by sediment that’s remobilized on the shelf by 00:30:00.600 --> 00:30:03.440 shelf currents induced by waves and tides. 00:30:03.440 --> 00:30:07.710 And then there’s also bioturbation- related smoothing of the seafloor. 00:30:07.710 --> 00:30:12.719 So I guess the point here is that these things are likely to form 00:30:12.719 --> 00:30:17.179 in all of these large earthquakes, but they have very limited preservation 00:30:17.179 --> 00:30:23.190 potential that’s smaller than the time increment that’s between earthquakes. 00:30:23.190 --> 00:30:28.799 So, because of that factor, we’re just not seeing them on the surface. 00:30:28.799 --> 00:30:32.190 And yet they’re almost certainly occurring with every 00:30:32.190 --> 00:30:34.809 large significant earthquake. So I guess the point is that these are – 00:30:34.809 --> 00:30:38.429 it’s an underappreciated earthquake hazard that’s obviously important 00:30:38.429 --> 00:30:43.680 for offshore infrastructure and marine spatial planning. 00:30:45.160 --> 00:30:51.480 A couple more things on this area before I move farther to the north. 00:30:51.480 --> 00:30:56.840 Terrace uplift rate in this area is about half a millimeter per year. 00:30:56.850 --> 00:31:00.690 Down here, it’s warped down to the south, and uplift of 00:31:00.690 --> 00:31:04.680 coastal terraces here is about 1/10 of a millimeter per year. 00:31:05.640 --> 00:31:09.089 Those terraces – it’s kind of like the coastline north of Santa Cruz. 00:31:09.089 --> 00:31:12.919 They get up to almost 200 meters. 00:31:12.920 --> 00:31:16.400 I think there actually may be even higher ones in this area. 00:31:16.400 --> 00:31:25.599 They’ve been correlated back to 330 – to stage 6, I guess – or, stage 7. 00:31:25.599 --> 00:31:30.149 And then it correlates with a transpressive bend in the 00:31:30.149 --> 00:31:33.889 San Andreas Fault. And just to sort of, again, clue you in a little bit more 00:31:33.889 --> 00:31:37.719 on the topography, this is the area. Farther to the north, you can see 00:31:37.719 --> 00:31:42.751 it’s hillier. It’s – elevations within a couple kilometers of the coast 00:31:42.751 --> 00:31:45.999 get up to about 500 meters here. Down here to the south, 00:31:45.999 --> 00:31:49.969 they’re about 130 meters within the same distance from the coast. 00:31:49.969 --> 00:31:52.000 So there’s clearly more relief to the north. 00:31:52.000 --> 00:31:55.620 Again, the terrace surfaces dip down to the south. 00:31:55.620 --> 00:32:02.740 And the obvious – or, not the obvious, but the control seems to be 00:32:02.749 --> 00:32:05.210 this transpressive bend on the San Andreas Fault. 00:32:05.210 --> 00:32:08.440 There’s no onshore faults that would influence uplift. 00:32:08.440 --> 00:32:12.289 There’s no growing folds. It’s not an issue of proximity to the 00:32:12.289 --> 00:32:16.960 fault because it’s actually farther away from the fault zone here and here. 00:32:16.960 --> 00:32:21.020 And, again, it’s – I don’t think one can attribute this to sort of 00:32:21.039 --> 00:32:24.490 high heat flow associated with the Mendocino Triple Junction, 00:32:24.490 --> 00:32:27.330 which is way, way to the north. 00:32:27.330 --> 00:32:30.249 So what’s almost certainly happening is, with that little bend, 00:32:30.249 --> 00:32:35.919 there’s a change in the dip of the fault. And so that change in the dip is 00:32:35.920 --> 00:32:39.640 bringing this area up relative to the area down here to the south. 00:32:40.160 --> 00:32:45.060 The other thing I want to talk about is how the geomorphology of the – 00:32:45.070 --> 00:32:50.519 of the fault zone is controlling littoral sediment transport. 00:32:50.519 --> 00:32:54.879 In this what we call littoral cell, the Russian River littoral cell sand is 00:32:54.880 --> 00:33:01.339 being driven by predominant northwest- trending swells down to the south. 00:33:01.900 --> 00:33:07.080 It’s not getting around – only a little bit of it is getting around Bodega Head. 00:33:07.080 --> 00:33:12.299 Instead, it’s being trapped here. It’s a tectonic sand trap so that there’s 00:33:12.299 --> 00:33:16.349 a 4-kilometer – 4-square-kilometer complex of sand dunes, 00:33:16.349 --> 00:33:20.209 elevations up to 30 meters in the sand. And the beach is accreting at this 00:33:20.209 --> 00:33:25.609 very high rate, when most beaches in California are actually eroding. 00:33:25.609 --> 00:33:30.700 If you come further south, you see the same thing, actually, at the southern end 00:33:30.700 --> 00:33:34.389 of the Bodega Bay littoral cell, where there’s this massive sand dune 00:33:34.389 --> 00:33:39.750 complex – 4.8 square kilometers – at offshore of Dillon Beach, 00:33:39.750 --> 00:33:42.599 with 2.7 meters per year of beach accretion. 00:33:42.599 --> 00:33:46.169 So, again, it’s a big collection site for offshore sand. 00:33:46.169 --> 00:33:53.019 So one doesn’t normally think of tectonics as a control on the coastal sand 00:33:53.019 --> 00:33:58.659 dunes and a tectonic geomorphological element, but there you have it. 00:33:58.659 --> 00:34:01.999 In fact, if you just look at where coastal sand dunes are along the 00:34:01.999 --> 00:34:07.039 coast of California – and this is from this Cooper 1967 reference, 00:34:07.039 --> 00:34:11.250 where the sand dunes are is in red. And all of them occur either 00:34:11.250 --> 00:34:16.379 immediately adjacent to river mouths or in tectonic traps, like I just described. 00:34:16.379 --> 00:34:21.119 There’s another actually at Point Arena. So, essentially, for, you know, 00:34:21.119 --> 00:34:25.212 a couple hundred kilometers of the California coast, all of the 00:34:25.212 --> 00:34:28.839 sand that’s being, you know, carried by littoral drift is being 00:34:28.840 --> 00:34:33.280 trapped by uplifts along the San Andreas Fault system. 00:34:34.260 --> 00:34:39.120 Okay, now I’m going to jump to the north and talk about this northern area 00:34:39.129 --> 00:34:46.149 between Point Arena and Point Delgada. Again, it’s this big transtensional area 00:34:46.149 --> 00:34:54.460 where the strike of the fault gets as much as, I think, 27 degrees off 00:34:54.460 --> 00:34:58.750 of the driving force, which would be the plate motion vector. 00:34:58.750 --> 00:35:04.800 So it’s – again, it’s falling – and this is the area where the fault is the farthest 00:35:04.800 --> 00:35:08.820 offshore. It’s as much as 20 kilometers offshore, and it’s at the greatest depth. 00:35:08.829 --> 00:35:12.769 It’s down to depths of 200 to 250 meters. 00:35:12.769 --> 00:35:17.760 A lot of what I’m going to be telling you has been published in this paper. 00:35:17.760 --> 00:35:22.160 In 2017 – Jeff Beeson is the first author. He’s a Ph.D. student – 00:35:22.160 --> 00:35:25.519 we collected all the data I’m telling you about on three USGS cruises – 00:35:25.519 --> 00:35:28.420 two in cooperation with Oregon State University. 00:35:28.420 --> 00:35:34.869 And, again, the work – most of the work I’ll tell you about is in this paper. 00:35:34.869 --> 00:35:39.299 Prior to this work, the only – it’s kind of hard to believe, but the only real 00:35:39.299 --> 00:35:43.941 geophysical work that was published for this entire 120-kilometer section 00:35:43.941 --> 00:35:49.890 of the fault was – came out in 1967 in a short note – a five-page short 00:35:49.890 --> 00:35:53.910 note published in GSA Bulletin. It included two profiles crossing 00:35:53.910 --> 00:35:57.550 the northern San Andreas Fault – seismic profiles of very poor quality. 00:35:57.550 --> 00:36:03.039 So we went back out there and mapped the fault at 1-kilometer line spacing. 00:36:03.039 --> 00:36:05.620 So we have a 120 crossings of the structure between 00:36:05.620 --> 00:36:08.360 Point Arena and Point Delgada. 00:36:08.360 --> 00:36:11.220 A few of them from the southern area are shown right here. 00:36:11.230 --> 00:36:14.160 And I just picked – these are – there are 28 of these profiles 00:36:14.160 --> 00:36:17.119 that are actually published in that paper. 00:36:17.119 --> 00:36:20.440 And I picked these just basically to show that this is another area 00:36:20.440 --> 00:36:23.910 where there’s a multi-strand offset, where two strands of the fault 00:36:23.910 --> 00:36:26.960 are actually offsetting this upper Holocene unit. 00:36:26.960 --> 00:36:30.020 So, again, it’s emphasizing the importance of mapping 00:36:30.029 --> 00:36:32.180 a whole fault zone. Because if you just studied 00:36:32.180 --> 00:36:36.069 one of these strands, you might not get the whole story of its history – 00:36:36.069 --> 00:36:39.880 of the fault history and its slip. 00:36:39.880 --> 00:36:44.920 Again, we relied on both a combination of high-resolution bathymetry – 00:36:44.920 --> 00:36:48.400 or, high-resolution seismic reflection data and bathymetry. 00:36:48.400 --> 00:36:53.710 This is just one cool example from that paper where we’re showing two – 00:36:53.710 --> 00:37:00.140 what we think is basically a migration of the fault from this trace here to 00:37:00.140 --> 00:37:03.980 this more efficient trace – the straighter trace right here. 00:37:03.980 --> 00:37:07.860 And in the process, you’re actually moving material that was originally 00:37:07.869 --> 00:37:11.109 on the Pacific Plate over to the North America Plate. 00:37:11.109 --> 00:37:13.519 So that’s what this diagram shows here. 00:37:13.519 --> 00:37:18.490 And if you’re more interested in this issue, you can, again, consult the paper. 00:37:18.490 --> 00:37:24.820 The fault’s most northerly trend, or strike, occurs up to the north 00:37:24.820 --> 00:37:31.770 of Fort Bragg, which is right here. It’s – again, this is where the fault is at 00:37:31.770 --> 00:37:35.549 its deepest, where it’s farthest offshore. It’s 20 kilometers offshore right here. 00:37:35.549 --> 00:37:41.059 And there’s this 11-kilometer-wide and 37-kilometer-long lazy Z basin. 00:37:41.059 --> 00:37:43.740 So it’s not sort of a pull-apart basin that’s forming between 00:37:43.740 --> 00:37:49.390 sub-parallel strands. It’s forming in the inside crook of 00:37:49.390 --> 00:37:51.810 this 9-degree fault bend that’s right here. 00:37:51.810 --> 00:37:56.579 And it’s giving you this asymmetric basin fill that’s right here. 00:37:56.580 --> 00:38:02.060 It’s called a lazy Z basin because of the shape of the – of the bend. 00:38:07.160 --> 00:38:10.960 Okay, so now I just want to sort of finish the talk with a discussion of 00:38:10.970 --> 00:38:14.319 how the San Andreas Fault ends. And I’m going to present a lot of data 00:38:14.319 --> 00:38:18.900 to you, and then you can sort of decide on your own what you think. 00:38:18.900 --> 00:38:22.420 This is a – this, by the way, is where the fault goes onshore at Point Delgada. 00:38:22.420 --> 00:38:27.020 There’s – it’s sort of starting to bend back to the northwest, and there’s 00:38:27.020 --> 00:38:31.380 an uplifted block here at Tolo Bank. Okay, this is from Bob McLaughlin 00:38:31.380 --> 00:38:37.440 et al.’s map in 2000, in which they postulated that there was a strand – 00:38:37.440 --> 00:38:44.609 that the San Andreas actually broke off of this trend south of Point Delgada 00:38:44.609 --> 00:38:50.240 and came across the heads of Delgada and Spanish Canyon … 00:38:50.240 --> 00:38:53.820 - That actually comes from earlier work. - Yeah, okay. 00:38:53.820 --> 00:38:57.580 - You’ll notice that that’s all theory. - Well, that was the point that I was 00:38:57.580 --> 00:39:00.280 going to make is, I’ve never seen a fault on a map that was more 00:39:00.280 --> 00:39:04.280 queried than this one. [laughter] This is basically two dashes, 00:39:04.280 --> 00:39:08.700 a query, two dashes, a query, so … - We had to put that on there, Sam. 00:39:08.700 --> 00:39:10.880 That map probably wouldn’t have been published. 00:39:10.890 --> 00:39:14.132 - Okay. It’s a hypothesis. It was clearly – it’s … 00:39:14.132 --> 00:39:17.069 - That was conventional wisdom also. - Okay. Conventional wisdom. 00:39:17.069 --> 00:39:18.920 It was a hypothesis. Okay, so … - [inaudible] 00:39:18.920 --> 00:39:23.660 - So then we – basically, to test that hypothesis, we went out and collected 00:39:23.660 --> 00:39:26.789 this grid of data on the south flank of the King Range. 00:39:26.789 --> 00:39:30.329 And all the profiles look sort of like these where there’s good 00:39:30.329 --> 00:39:37.329 sort of continuous young reflections above a basement surface. 00:39:37.329 --> 00:39:42.410 And so we were certain that actually no fault cut through 00:39:42.410 --> 00:39:44.910 our grid of seismic reflection profiles. 00:39:44.910 --> 00:39:48.260 These were, again, collected at 1-kilometer line spacing. 00:39:48.260 --> 00:39:52.980 And these two profiles you see below are the ones that you show right here. 00:39:52.980 --> 00:40:00.530 So, after that survey, we were pretty certain that the fault wasn’t offshore 00:40:00.530 --> 00:40:05.220 and that it was probably on land. And then, it’s only been sort of recently 00:40:05.220 --> 00:40:10.089 when we’ve been re-thinking this area as part of a couple of other publications 00:40:10.089 --> 00:40:13.339 we’re working on that this other hypothesis, which I know 00:40:13.339 --> 00:40:17.930 Bob McLaughlin likes, is that this fault actually curves 00:40:17.930 --> 00:40:23.039 around and essentially is a range-front surf zone fault 00:40:23.040 --> 00:40:29.380 on the south flank in the near shore just offshore of the Kings Range. 00:40:29.380 --> 00:40:36.180 So, to evaluate that hypothesis, we’re going to look at these kinds of data – 00:40:36.180 --> 00:40:40.480 fault strike, how the fault might connect, potential fields data, structural relief, 00:40:40.480 --> 00:40:45.650 and the history of uplift and subsidence. So, initially, I didn’t sort of like 00:40:45.650 --> 00:40:52.000 the area – like this hypothesis because it would involve essentially about 00:40:52.000 --> 00:40:58.830 a 45- or 50-degree bend in the San Andreas Fault from its trend south of 00:40:58.830 --> 00:41:03.700 Point Delgada to this trend right here. But if – when you look at this diagram, 00:41:03.700 --> 00:41:09.279 which is now extended to the right, you realize actually that the larger part 00:41:09.279 --> 00:41:15.089 of the bend comes from the anomalous northerly trend south of Point Delgada 00:41:15.089 --> 00:41:20.980 that the part north of the – you know, north of Point Delgada essentially 00:41:20.980 --> 00:41:25.180 is kind of a strike that’s similar to what you see in the San Juan – or, in the 00:41:25.180 --> 00:41:31.260 Santa Cruz Mountains, for example. So it’s a huge bend, but this part is not – 00:41:31.260 --> 00:41:33.360 you know, is no more anomalous than this part. 00:41:33.360 --> 00:41:36.500 In fact, this part is probably more anomalous in terms of a fault strike. 00:41:36.500 --> 00:41:40.859 It does become more westerly out to the west if you connect it with the 00:41:40.859 --> 00:41:43.529 Mattole Canyon Fault, which we did map right out here, 00:41:43.529 --> 00:41:51.490 which I’ll talk about in a second. So if – but it has to connect. 00:41:51.490 --> 00:41:53.980 So you can’t just have a big fault out there that doesn’t connect 00:41:53.980 --> 00:41:57.880 both to the southeast and the northwest with other structures. 00:41:57.880 --> 00:42:02.369 So this is just a suite of published maps that I’m showing from the area. 00:42:02.369 --> 00:42:06.769 Again, this is from the McLaughlin et al. map down here. 00:42:06.769 --> 00:42:12.000 And several of these maps show sort of faults coming into 00:42:12.000 --> 00:42:14.820 the King Range and then dying out. 00:42:17.160 --> 00:42:20.480 This map here is the easiest for you to look at because it’s colored, but it’s – 00:42:20.480 --> 00:42:25.020 again, it shows sort of a main San Andreas strand coming into 00:42:25.029 --> 00:42:26.910 the King Range and then dying out. 00:42:26.910 --> 00:42:31.769 But there is this – another queried fault that comes – goes through the 00:42:31.769 --> 00:42:36.220 Quaternary landslide deposits that come out at the mouth of Telegraph Creek. 00:42:36.220 --> 00:42:38.980 And I haven’t been on the ground up here, but I have looked at lots of 00:42:38.980 --> 00:42:45.599 photos and images, and it’s hard to make this fault go any farther north 00:42:45.599 --> 00:42:48.280 than this landslide complex because there’s fairly continuous 00:42:48.280 --> 00:42:53.079 coastal outcrop right in here. So the most likely bet seems to me 00:42:53.079 --> 00:42:56.690 is that, if one wants to connect – or, run the San Andreas Fault 00:42:56.690 --> 00:43:01.480 back offshore, then it’s – the wisest place to put it would be 00:43:01.480 --> 00:43:05.520 at the mouth of this big landslide complex as far north as you can 00:43:05.520 --> 00:43:07.830 within it at the mouth of Telegraph Creek. 00:43:07.830 --> 00:43:13.130 Okay, off to the northwest, you don’t have the same problem where you can 00:43:13.130 --> 00:43:20.630 sort of easily connect this nearshore range front postulated fault to the 00:43:20.630 --> 00:43:24.400 Mattole Canyon Fault, which we see in our seismic reflection profiles 00:43:24.400 --> 00:43:29.020 to the end of our grid. And this profile here on the bottom 00:43:29.020 --> 00:43:32.059 is what you’re seeing right here – right there. 00:43:32.059 --> 00:43:34.210 So the potential fields data don’t really help. 00:43:34.210 --> 00:43:37.289 This is the high-resolution marine mag data that we collected 00:43:37.289 --> 00:43:41.369 that’s reported in that paper. But, again, we didn’t get into 00:43:41.369 --> 00:43:46.700 the nearshore area, so we don’t have a contrast across it. 00:43:46.700 --> 00:43:50.980 This is the lower resolution, but still considered by some high-resolution, 00:43:50.980 --> 00:43:55.950 aeromag data that Vicki Langenheim published back in 2011. 00:43:55.950 --> 00:44:00.150 This is – I think there’s five different maps showing different things. 00:44:00.150 --> 00:44:04.619 This area is not rotated, so that’s Punta Gorda. That’s Point Delgada 00:44:04.619 --> 00:44:06.799 right here, so these maps have different orientations. 00:44:06.799 --> 00:44:11.849 I included a larger area because the maps don’t really show much of a 00:44:11.849 --> 00:44:18.170 gradient across this nearshore area. But that doesn’t necessarily argue 00:44:18.170 --> 00:44:21.109 against the San Andreas Fault being here because there are other places 00:44:21.109 --> 00:44:24.210 down here where, again, you’re crossing the fault zone, 00:44:24.210 --> 00:44:27.509 and there’s no steep gradient there, either. 00:44:27.509 --> 00:44:31.950 So the potential fields data that I’ve seen really don’t help much. 00:44:31.950 --> 00:44:37.309 This is, I think, maybe the most important data set, at least to me. 00:44:37.309 --> 00:44:40.269 The basement – this is a very steep range front. 00:44:40.269 --> 00:44:44.490 And so you go about, like, 1 to 1-1/2 kilometers inland, 00:44:44.490 --> 00:44:49.800 and you’re up at elevations at 850 or 917 meters. 00:44:51.300 --> 00:44:55.280 So – and then, down here, in the offshore, on this profile, 00:44:55.299 --> 00:44:59.289 you can see that the basement surface is down about 100 meters. 00:44:59.289 --> 00:45:03.099 So there’s about a kilometer of relief on the basement surface 00:45:03.099 --> 00:45:06.470 over 3 to 4 kilometers going from onshore to offshore. 00:45:06.470 --> 00:45:11.180 And then, again, this area is coming up at incredibly high rates if you 00:45:11.180 --> 00:45:15.059 believe this Merritts and Bull paper. But even if it’s coming up, you know, 00:45:15.059 --> 00:45:20.069 at half this rate – these are 3 millimeters per year, 4 millimeters per year, 00:45:20.069 --> 00:45:23.690 1 millimeter per year over here, published in this – in this paper. 00:45:23.690 --> 00:45:27.950 Even if this rate is, you know, half or a quarter of this rate, 00:45:27.950 --> 00:45:33.109 it’s still going up like gangbusters. And yet, we know from the seismic 00:45:33.109 --> 00:45:41.160 stratigraphy that you wouldn’t get these sort of progrational sequences 00:45:41.160 --> 00:45:43.680 that we see on the data, and you wouldn’t get the shelf break 00:45:43.680 --> 00:45:49.390 way out at, like, 180 meters if the offshore wasn’t subsiding. 00:45:49.390 --> 00:45:52.599 So it’s subsiding at a slower rate, but it is, in fact, subsiding. 00:45:52.599 --> 00:45:55.990 So rapid uplift here. Slow subsidence here. 00:45:55.990 --> 00:46:00.619 There’s a gradient somewhere there, and it’s possibly associated 00:46:00.619 --> 00:46:04.900 with a tectonic break. So if you want just to summarize, 00:46:04.900 --> 00:46:09.280 why is there a nearshore surf zone range-front fault on the south flank 00:46:09.280 --> 00:46:11.450 on the King Range? That would be this thing here. 00:46:11.450 --> 00:46:14.190 The faults – and these are the data that I’ve talked about. 00:46:14.190 --> 00:46:17.619 The fault strike is not anomalous. Again, it’s – that huge bend is 00:46:17.619 --> 00:46:21.960 at least more attributed to how far north it is to the south 00:46:21.960 --> 00:46:26.200 than it is how far sort of northwest it is to the north. 00:46:26.200 --> 00:46:31.230 There’s an okay hookup with the structure coming out of Delgada. 00:46:31.230 --> 00:46:34.340 Again, I’d want to run it through that landslide complex. 00:46:34.340 --> 00:46:38.190 There’s a good link to the west with the Mattole Canyon Fault. 00:46:38.190 --> 00:46:41.609 There’s significant structural relief between the King Range flank 00:46:41.609 --> 00:46:44.310 and the seismic profiles – again, about a kilometer 00:46:44.310 --> 00:46:47.809 of relief in 3 to 4 kilometers of length. 00:46:47.809 --> 00:46:52.300 The onshore is rapidly uplifting, and the offshore is slowly subsiding. 00:46:52.300 --> 00:46:56.440 And the – you know, the fault isn’t found on land, 00:46:56.450 --> 00:47:00.299 and it’s not found farther offshore. So, again, where are you going to put it? 00:47:00.299 --> 00:47:02.720 Okay, so why not? Why isn’t it there? 00:47:02.720 --> 00:47:08.680 Well, it needs to fit into this narrow curved nearshore lane. 00:47:08.680 --> 00:47:12.140 And you can’t document it with any geophysical data that you 00:47:12.140 --> 00:47:15.829 can collect in the offshore. So it’s kind of circumstantial. 00:47:15.829 --> 00:47:19.800 It’s convenient. It’s definitely ad hoc. 00:47:19.800 --> 00:47:24.380 And, you know, that’s what it is. [laughter] 00:47:24.380 --> 00:47:28.230 You know, it’s kind of one of these frustrating problems that you’ve got 00:47:28.230 --> 00:47:32.249 a lot of, you know, convenient evidence and circumstantial evidence, 00:47:32.249 --> 00:47:35.390 but you can’t go out and get some direct observations of it. 00:47:35.390 --> 00:47:38.440 Because it’s – again, it’s – this area is just too close to shore. 00:47:38.440 --> 00:47:41.599 You can’t get a boat in that shallow. It’s not safe. 00:47:41.599 --> 00:47:45.480 Okay, so that was my last slide. I just want to sort of leave you 00:47:45.480 --> 00:47:50.020 with a few points. The most important take-homes or that would be, I guess, 00:47:50.020 --> 00:47:56.660 just to emphasize both the regional to local control of fault strike 00:47:56.660 --> 00:48:01.440 on geomorphology in these different geomorphologic elements. 00:48:02.580 --> 00:48:06.880 The importance of sort of detailed fault zone mapping, particularly 00:48:06.890 --> 00:48:11.980 in the concept of – context of multi-strand offsets. 00:48:11.980 --> 00:48:17.089 And then that these massive nearshore slope failures are probably common, 00:48:17.089 --> 00:48:21.190 but they’re ephemeral in terms of their surface morphology. 00:48:21.190 --> 00:48:27.569 And I think they’re probably an underappreciated earthquake hazard. 00:48:27.569 --> 00:48:31.510 So there’s still a lot to learn in the marine realm, but we’re – we have 00:48:31.510 --> 00:48:34.970 made a lot of progress lately, and that’s what I’ve been telling you about today. 00:48:34.970 --> 00:48:36.560 Thanks for your attention. 00:48:36.560 --> 00:48:42.400 [Applause] 00:48:42.800 --> 00:48:47.340 - Great. Thank you, Sam, for a really great talk with a lot of stuff in it. 00:48:47.340 --> 00:48:50.260 Do we have questions from the audience? 00:48:53.900 --> 00:49:00.020 [Silence] 00:49:00.700 --> 00:49:02.420 - I have a question about that Bodega Bay. 00:49:02.420 --> 00:49:05.809 You have those two strands – 1 and 2. 00:49:05.809 --> 00:49:09.970 Could it be that Strand 1 was abandoned about 4,000 years ago? 00:49:09.970 --> 00:49:13.320 - Oh, sure. I mean, all – yeah, all we know is that it’s slipped 00:49:13.320 --> 00:49:18.480 75 meters in the last 9,000 years or so – something like that. 00:49:18.480 --> 00:49:23.470 - Right. And there’s no slip indicators on the strand that broke in 1906. 00:49:23.470 --> 00:49:26.989 For that time period – like, latest Holocene. 00:49:26.989 --> 00:49:30.800 - No. It’s impossible to study. It’s basically, again, kind of 00:49:30.800 --> 00:49:33.450 an ephemeral rupture there where it’s kind of in lagoonal 00:49:33.450 --> 00:49:37.019 sediments if you read that description. - Right. Right. 00:49:37.019 --> 00:49:39.920 - So I don’t know that – no one’s ever studied that site. 00:49:39.920 --> 00:49:43.780 - Okay. And on the northern – on that northern-most portion, 00:49:43.780 --> 00:49:47.380 could you get – could you image that with that Kelpfly? 00:49:50.060 --> 00:49:52.620 - You could probably image it. 00:49:53.100 --> 00:49:55.760 The Kelpfly, for people who don’t know, is basically 00:49:55.769 --> 00:50:00.749 a multibeam bathymetric mapping tool that’s mounted on a Jet Ski. 00:50:00.749 --> 00:50:05.040 So, yeah, you could – you know, you’d have to – you have to basically – 00:50:05.040 --> 00:50:08.300 because the water is so shallow, and the cone of measurement is 00:50:08.309 --> 00:50:14.020 so narrow, you have to basically run transects at, like, 10 meters 00:50:14.020 --> 00:50:15.340 line spacing or something like that. 00:50:15.340 --> 00:50:20.529 The shallower you get, the closer – the more tight the profiles need to be. 00:50:20.529 --> 00:50:25.580 But that’s a tool for measuring high-resolution bathymetry, 00:50:25.580 --> 00:50:28.660 but not seismic reflection profiles. - You got to put a trip on it. 00:50:28.660 --> 00:50:32.300 - You’d have to put a trip on it, yeah. And the other issue there is that it’s 00:50:32.300 --> 00:50:36.519 probably a wave-cut rocky platform where there aren’t sediments that 00:50:36.519 --> 00:50:39.920 would be measurable anyway. But there might be. 00:50:39.920 --> 00:50:41.270 Actually, there are some big bars there. 00:50:41.270 --> 00:50:43.369 So you probably could see some stuff, yeah. 00:50:43.369 --> 00:50:44.369 - Cool. 00:50:44.369 --> 00:50:46.650 - So that’s the kind of approach you’d want to have. 00:50:46.650 --> 00:50:49.260 The other issue with that area is it’s really remote. 00:50:49.260 --> 00:50:52.150 I mean, you have to – well, you could probably take – 00:50:52.150 --> 00:50:55.280 go to Shelter Cove and launch out of there. We launched out of Noyo Harbor. 00:50:55.280 --> 00:50:58.980 So just to get out there took, like, a day, almost. 00:51:03.260 --> 00:51:05.940 - There’s a question in the front. - Bob. 00:51:05.940 --> 00:51:11.019 - Okay. Could you go back to your last slide? [chuckles] 00:51:11.780 --> 00:51:15.680 - I can try. There you go. - Okay. Yeah. Okay. 00:51:15.680 --> 00:51:20.400 A couple of insights there. See, there was a slide there 00:51:20.400 --> 00:51:25.360 that had the – your uplift – the uplift rates on it. 00:51:26.160 --> 00:51:28.320 It was – yeah, there we go. 00:51:29.640 --> 00:51:37.940 So I do definitely believe there has to be some kind of a fault offshore, or either 00:51:37.940 --> 00:51:44.080 on the beach along the range front there. I mean, just from the scalloped ridges. 00:51:44.080 --> 00:51:50.700 And, under the 4 is Big Flat. And that’s an alluvial fan that 00:51:50.700 --> 00:51:55.940 comes right out to the coast. And on either lobe to that fan, there are 00:51:55.940 --> 00:52:04.640 uplifted beach ridges – cobble berms. And so the uplift rates – 00:52:04.640 --> 00:52:11.519 that high uplift rate in there comes from dating [inaudible] in – 00:52:11.519 --> 00:52:15.730 from holes in the cobbles in those berms. 00:52:15.730 --> 00:52:25.630 So those beach ridges say that that range has to be just – has to be 00:52:25.630 --> 00:52:28.900 going up like gangbusters. So … 00:52:28.900 --> 00:52:32.500 - That’s not a sustainable rate, though. I mean, it’d be the Himalayas there, 00:52:32.510 --> 00:52:34.720 you know … - Yeah. And it’s also been suggested 00:52:34.720 --> 00:52:39.950 that those berms could be related to storm waves and that sort of thing. 00:52:39.950 --> 00:52:42.980 But … - What's the elevation? 00:52:42.980 --> 00:52:50.860 - They’re – they go right out to the – inward to the back side of the – 00:52:50.860 --> 00:52:55.180 of the various alluvial fan – or, the two fan lobes. 00:52:55.180 --> 00:53:00.680 So they’re preserved on the two lobes of the fan right down to the mountain front. 00:53:01.280 --> 00:53:08.400 So, anyway, I – so that suggests to me, yeah, there has to be something out 00:53:08.410 --> 00:53:13.019 there that accounts for the uplift of the range as opposed so the subsidence 00:53:13.019 --> 00:53:18.550 that you’re pointing out there. The one other insight that I would throw 00:53:18.550 --> 00:53:27.259 out here is that I would point out that the possible connection with the landsliding 00:53:27.260 --> 00:53:33.660 feature at Point Delgada, I think we kind of blew that out of the water when 00:53:33.660 --> 00:53:39.680 we were working in the King Range. Because there are conjugate 00:53:39.680 --> 00:53:46.800 northeast-trending fractures that cross the feature that Bob Brown 00:53:46.800 --> 00:53:56.569 and Wolfe mapped going through the slide area earlier that had adularia veins 00:53:56.569 --> 00:54:02.940 that we dated that give a Miocene age. So it requires that that – that that 00:54:02.940 --> 00:54:09.420 block at Point Delgada – down the lower left there, the strand that’s 00:54:09.420 --> 00:54:15.869 going offshore, the – not the red one. 00:54:15.869 --> 00:54:21.520 The bedrock strand that goes through Point Delgada and then offshore. 00:54:22.600 --> 00:54:25.060 - Not the red one? Which … - Not the – the black one 00:54:25.060 --> 00:54:27.109 that intersects the red one. - Right there? 00:54:27.109 --> 00:54:29.640 - Queried. - Yeah. And then, of to the north … 00:54:29.640 --> 00:54:31.800 - Oh, right there. - Right at the north end there, 00:54:31.800 --> 00:54:36.840 there are – we published a paper on the age of the – 00:54:36.860 --> 00:54:41.940 of the adularia veins that cross that fault strand. 00:54:41.940 --> 00:54:47.520 And we have a 13.8 million-year-old Miocene age. 00:54:47.520 --> 00:54:49.960 - Okay. But I’m not talking about running it through there. 00:54:49.960 --> 00:54:52.860 I’m talking about running it underneath the landslides. 00:54:54.480 --> 00:54:57.380 I’m not talking about that strand is … - Well, that strand is – it runs 00:54:57.380 --> 00:55:01.039 underneath the landslide, so … - Okay, then running it in a different 00:55:01.039 --> 00:55:04.119 place underneath the landslides. All I’m saying is that I kind of agree 00:55:04.119 --> 00:55:07.749 with you that, when you get farther north, it’s impossible to [inaudible] 00:55:07.749 --> 00:55:11.359 the San Andreas Fault there. If you want that fault in the range front, 00:55:11.359 --> 00:55:13.319 you have to bring it back offshore somewhere. 00:55:13.319 --> 00:55:14.389 - Yes. - That’s the point. 00:55:14.389 --> 00:55:19.140 - Exactly. So all I’m saying is you – is it’s probably not – that’s not 00:55:19.140 --> 00:55:21.369 a good place to connect it. [laughs] 00:55:21.369 --> 00:55:28.599 Where – at where it comes back onshore of – southwest of the 1906 trace. 00:55:28.600 --> 00:55:36.500 But I think Jeff Beeson’s suggestion that the 1906 trace actually goes off to the 00:55:36.500 --> 00:55:42.280 north to intersect the east side of the King Range and more or less 00:55:42.280 --> 00:55:50.839 be coincident with it along a good stretch off to the northeast 00:55:50.839 --> 00:55:56.529 could easily account for where the main San Andreas goes. 00:55:56.529 --> 00:56:03.950 So what that means is, the San Andreas has to be onshore in that area. 00:56:03.950 --> 00:56:10.250 But if you accept Jeff’s interpretation, that would – I think you could put about 00:56:10.250 --> 00:56:17.859 8 to 9 kilometers of right-lateral slip along that hypothesized continuation 00:56:17.859 --> 00:56:22.259 of the 1906 strand along the King Range boundary. 00:56:22.259 --> 00:56:31.660 Whatever you do, you have to also account for the long-term displacement 00:56:31.660 --> 00:56:36.640 of the King Range with the – with the rocks – Franciscan rocks to the 00:56:36.640 --> 00:56:42.940 east because those rocks are anomalous to the Franciscan and to any other rocks. 00:56:42.940 --> 00:56:53.780 So they’re – it’s hard to rationalize a lot of slip on the – on any fault 00:56:53.780 --> 00:56:57.040 that cuts through the middle of the King Range. 00:56:58.240 --> 00:56:59.780 - Okay. 00:57:00.320 --> 00:57:02.120 - Question here. 00:57:03.440 --> 00:57:05.819 - I have two very different questions. - Okay. [laughs] 00:57:05.819 --> 00:57:13.279 - And I’m going to just ask one. Sam, is there a causal element in the – 00:57:13.279 --> 00:57:18.279 in the depth of those liquefaction failures you’re describing? 00:57:18.280 --> 00:57:19.880 - Is there a causal element? 00:57:19.880 --> 00:57:23.340 - I mean, you described them as happening about 60-meter depth. 00:57:23.340 --> 00:57:26.400 - 60 meters water depth. - That’s right. 00:57:26.400 --> 00:57:29.560 - Yeah. No, I don’t think so. I think basically the key thing there 00:57:29.560 --> 00:57:34.900 was that they’re in rapidly deposited sand and on the slope. 00:57:35.480 --> 00:57:39.739 So that’s – those are – you know, and then obviously a 7.9 earthquake 00:57:39.740 --> 00:57:42.820 with a long duration of shaking, et cetera, et cetera. 00:57:42.820 --> 00:57:44.720 - You don’t think water depth had any role? 00:57:44.720 --> 00:57:46.320 - I don’t. 00:57:47.160 --> 00:57:48.160 Okay. 00:57:50.200 --> 00:57:54.140 - Hey, Sam. Excellent talk. I think you convinced us pretty well 00:57:54.140 --> 00:57:58.210 that north of Point Delgada, the – or, in that whole stretch, 00:57:58.210 --> 00:58:03.999 the fault is more kind of transpressional at its most northern end. 00:58:03.999 --> 00:58:07.859 Is it possible that the structure that you’re hunting for to get that – 00:58:07.859 --> 00:58:12.269 you know, to explain these extremely rapid uplift rates could, 00:58:12.269 --> 00:58:16.819 at least in part, be a fold and not a fault? And so folding could be a larger 00:58:16.820 --> 00:58:20.360 component of the vertical deformation in this area? 00:58:22.520 --> 00:58:26.560 [Silence] 00:58:27.120 --> 00:58:32.320 - Well, we don’t see a big dip in the reflections offshore. 00:58:32.329 --> 00:58:36.989 So if you’re thinking that there should be a – you know, a rollover fold, 00:58:36.989 --> 00:58:41.099 basically, above a – like, a blind thrust, then you – in the basement, where you 00:58:41.099 --> 00:58:44.160 do see reflections, and even in the young strata, you’d expect to see, 00:58:44.160 --> 00:58:47.160 you know, a hanging wall that’s coming down and dipping like that. 00:58:47.160 --> 00:58:50.700 We don’t see that. So I guess the answer is – 00:58:50.700 --> 00:58:54.940 without thinking anymore about it, I’d say the answer is no. 00:58:58.900 --> 00:59:01.420 - All right. I think we have one more question, and then 00:59:01.420 --> 00:59:03.540 we should probably wrap up. 00:59:06.800 --> 00:59:12.140 - I was just curious if there’s any geodetic data out in the Kings Range? 00:59:12.140 --> 00:59:17.569 Because it seems like you guys – from Point Delgada to kind of the 00:59:17.569 --> 00:59:22.600 triple junction, it’s not quite clear exactly where the San Andreas is going. 00:59:22.600 --> 00:59:30.400 And is it possible that, you know, the strike of the San Andreas, you know, 00:59:30.400 --> 00:59:32.119 might change there? I don’t know if you – if there’s 00:59:32.120 --> 00:59:34.360 any geodetic data that you’d be able to look at that kind of 00:59:34.360 --> 00:59:37.640 shows what’s been going on up in that area. 00:59:39.060 --> 00:59:41.540 - I don’t know. I can’t tell you that. 00:59:41.540 --> 00:59:44.440 I can’t – does anybody else know the answer to that? 00:59:44.700 --> 00:59:47.740 - I don’t know of any recent data. 00:59:50.900 --> 00:59:56.000 - Then I guess – so the strike that you have from Point Delgada to – 00:59:56.000 --> 01:00:03.180 that you show on your graph of the San Andreas, you basically just connected – 01:00:03.180 --> 01:00:04.960 yeah, you had it on that other … - Yeah. 01:00:04.970 --> 01:00:06.470 - Yeah, right there. - Yeah. Right there, I connected it 01:00:06.470 --> 01:00:08.589 with the Mattole Canyon Fault. - Okay. 01:00:08.589 --> 01:00:11.690 - So that’s – yeah, so that’s – from that point – from essentially 01:00:11.690 --> 01:00:14.259 that point up to there is the Mattole Canyon Fault. 01:00:14.259 --> 01:00:16.770 And you can see it’s sort of – Mattole Canyon Fault is sort of 01:00:16.770 --> 01:00:22.980 bending off right there, getting a more northwesterly trend right there. 01:00:26.480 --> 01:00:29.440 - Okay, great. Anybody who wants to continue 01:00:29.450 --> 01:00:32.940 any of these discussions with Sam, we’re going to take him to lunch 01:00:32.940 --> 01:00:37.690 at the on-campus café, and there’s still some slots available 01:00:37.690 --> 01:00:39.519 in his schedule for this afternoon. 01:00:39.519 --> 01:00:43.630 If you’d like to come up and ask me, I can get you on his schedule. 01:00:43.630 --> 01:00:46.030 And let’s give Sam another round of applause for a great talk. 01:00:46.030 --> 01:00:51.820 [Applause]