WEBVTT Kind: captions Language: en 00:00:00.080 --> 00:00:03.220 [Music] Good morning. Hello. 00:00:03.220 --> 00:00:07.220 Welcome to today’s Earthquake Science Center seminar. 00:00:07.220 --> 00:00:10.610 Before we get started, just want to remind everyone that next week, 00:00:10.610 --> 00:00:13.820 Ellen Rathje will be here from the University of Texas giving the – 00:00:13.820 --> 00:00:16.760 a preview of the 2018 Joyner Lecture that she’ll later give 00:00:16.760 --> 00:00:21.940 at the EERI National Conference on Earthquake Engineering. 00:00:21.960 --> 00:00:25.560 But today, we have the pleasure of bringing in Scott Bennett 00:00:25.570 --> 00:00:32.050 from across the street over in GMEG, who’s an expert in continental-scale 00:00:32.050 --> 00:00:35.320 deformation and landscape evolution and has focused recently 00:00:35.320 --> 00:00:38.270 and will be sharing some of his work from the Pacific Northwest 00:00:38.270 --> 00:00:41.620 and how it looks the way it does. 00:00:46.340 --> 00:00:50.500 - Thanks, Alex. It’s good to be here. Can everybody hear me okay? Wonderful. 00:00:51.300 --> 00:00:53.640 Well, it’s good to be back in the Earthquake Science Center 00:00:53.640 --> 00:00:56.670 seminar series. It’s been a couple years since I presented here. 00:00:56.670 --> 00:01:00.370 And I’ve been busy doing new and different things since last time 00:01:00.370 --> 00:01:03.421 you heard from me. And so today I’ll summarize 00:01:03.421 --> 00:01:07.390 a few of those things that I’ll be working on in the recent past and 00:01:07.390 --> 00:01:10.750 continue working on in the GMEG Science Center up in the Pacific 00:01:10.750 --> 00:01:16.420 Northwest involving plate tectonics and regional-scale deformation. 00:01:16.420 --> 00:01:19.040 So we’ll get to – I’ll be talking about 00:01:19.050 --> 00:01:20.980 three projects that I’ve been recently working on. 00:01:20.980 --> 00:01:25.000 The last one I’ll be talking about is where this photograph is taken from. 00:01:25.000 --> 00:01:28.230 This is a paleoseismic trench in the southern part of the 00:01:28.230 --> 00:01:30.620 Olympic Mountains in the state of Washington. 00:01:30.620 --> 00:01:34.360 And if you’re not familiar with looking at tectonic deformation, this is a pretty 00:01:34.370 --> 00:01:39.260 good example of some young, brittle deformation near the surface. 00:01:39.260 --> 00:01:43.520 And so we’ll talk about a lot of these projects similar to this this morning. 00:01:44.180 --> 00:01:47.580 Because I will probably run out of time, I’ll go ahead and acknowledge 00:01:47.580 --> 00:01:50.560 several of my collaborators. A lot of these projects involve 00:01:50.560 --> 00:01:53.979 paleoseismic trenching and research cruises, and each one of these – 00:01:53.980 --> 00:01:58.020 all these types of projects involve a lot of man and woman power 00:01:58.020 --> 00:02:00.979 and a lot of bodies in the field and a lot of help. 00:02:00.979 --> 00:02:04.320 And these are just a few, and I probably missed a couple of my collaborators that 00:02:04.320 --> 00:02:08.300 have been intimately involved in these projects I’ll be talking about today. 00:02:09.240 --> 00:02:13.860 So when you look to global tectonic – global tectonic map, 00:02:13.860 --> 00:02:16.660 convergent plate boundaries kind of stand out. 00:02:16.660 --> 00:02:19.300 They are host to some of the highest, 00:02:19.310 --> 00:02:22.220 most elevated seismic hazards around the world. 00:02:22.220 --> 00:02:25.340 Here on this map of global seismic hazards, the warmest colors 00:02:25.340 --> 00:02:29.560 are the highest seismic hazard. And you see that they – most of those 00:02:29.560 --> 00:02:33.390 high seismic hazards coincide with convergent plate boundaries, whether 00:02:33.390 --> 00:02:37.840 they’re ocean-continent subductions or continent-continent collision zones. 00:02:37.840 --> 00:02:41.090 And to no surprise, these convergent plate boundaries 00:02:41.090 --> 00:02:43.840 are also host to some of the highest elevations because 00:02:43.840 --> 00:02:47.680 collisional plate boundaries are great at building topography. 00:02:48.420 --> 00:02:52.220 We’re fortunate enough here in the domestic 48 of the United States 00:02:52.220 --> 00:02:55.310 to have our own convergent plate boundary that we can study, 00:02:55.310 --> 00:02:59.180 and even drive to, here in the Pacific Northwest. 00:02:59.180 --> 00:03:01.010 And so, zooming into the Pacific Northwest, I wanted to 00:03:01.010 --> 00:03:03.380 give a brief overview of the tectonic setting. 00:03:03.380 --> 00:03:06.570 Of course, the plate boundary is dominated by the convergence 00:03:06.570 --> 00:03:08.959 of the Juan de Fuca Plate and the North America Plate along the 00:03:08.960 --> 00:03:13.320 Cascadian subduction zone here. This is the location of the trench. 00:03:14.720 --> 00:03:17.940 And of course, the stuff I’ll be talking about today is focused on 00:03:17.940 --> 00:03:20.740 what’s going on in the upper plate – the North American Plate. 00:03:20.740 --> 00:03:26.240 And it’s a complicated and pretty interesting story. 00:03:26.240 --> 00:03:31.800 The upper plate – North American Plate here involves a lot of motions, 00:03:31.800 --> 00:03:35.620 primarily driven by the Sierra Nevada block moving to the northwest thanks 00:03:35.620 --> 00:03:40.640 to relative motion on the – oops. Sorry, I lost my cursor there. 00:03:41.740 --> 00:03:44.340 Thanks to relative motion on the Walker Lane belt here. 00:03:44.340 --> 00:03:49.060 The drives the forearc blocks in the Oregon Coast belt – 00:03:49.060 --> 00:03:52.620 Oregon Coast blocks, drives them northwards, 00:03:52.620 --> 00:03:57.900 rotating clockwise around some Euler pole here in the backarc. 00:03:57.900 --> 00:04:02.500 This also drives extension in the trailing edge behind these rotating blocks, 00:04:02.510 --> 00:04:06.400 which is intimately related to extension in the Basin and Range. 00:04:06.400 --> 00:04:11.050 This drives – these blocks drive north into Washington state, 00:04:11.050 --> 00:04:16.729 where a long-term compression setting is located here in the northern Cascade 00:04:16.729 --> 00:04:21.949 arc and results in a lot of transpression between the Oregon Coast block 00:04:21.949 --> 00:04:26.469 here and the Olympic Mountains and southern Vancouver Island. 00:04:26.469 --> 00:04:28.550 And of course, closer to the Euler pole, 00:04:28.550 --> 00:04:32.909 there’s additional shortening across the Yakima Fold region here. 00:04:32.909 --> 00:04:37.580 So in general, the rotational deformation increases from east to west, 00:04:37.580 --> 00:04:41.460 in the same way that, as you sit on the outside part of a merry-go-round, 00:04:41.460 --> 00:04:44.099 you’re going faster than you would if you were sitting closer 00:04:44.099 --> 00:04:47.839 to the center of a merry-go-round. So deformation will be increasing 00:04:47.840 --> 00:04:52.620 from this Euler pole rotation from east to west. 00:04:54.190 --> 00:04:58.340 I think this is very well illustrated by this figure from a recent paper by 00:04:58.349 --> 00:05:03.060 Tom Brocher and co-authors showing a slightly different location 00:05:03.060 --> 00:05:08.159 for a rotation pole, here defined by geologic structures, not GPS. 00:05:08.160 --> 00:05:11.000 A little bit farther north than the typical GPS pole. 00:05:11.000 --> 00:05:12.860 And you can see these small circle paths. 00:05:12.860 --> 00:05:19.800 These are paths of similar surface velocities relative to North America. 00:05:21.980 --> 00:05:26.300 And in this paper, in this figure, he – Tom and co-authors attribute 00:05:26.300 --> 00:05:29.949 a lot of the structures and deformation that’s happening across the northern 00:05:29.949 --> 00:05:33.169 Basin and Range and throughout the Pacific Northwest to this rotation 00:05:33.169 --> 00:05:37.710 of the upper plate, from extension in the Basin and Range to – 00:05:37.710 --> 00:05:44.069 transitioning to a collision zone in Washington, and buttressing – colliding 00:05:44.069 --> 00:05:49.300 with a backstop in stable North America in southwestern British Columbia. 00:05:51.690 --> 00:05:55.380 So what does this deformation look like over several million years? 00:05:55.400 --> 00:06:01.560 So we – folks have studied paleomagnetic rotation vectors of 00:06:01.560 --> 00:06:05.980 Miocene flood basalts throughout the region and come up with a great – 00:06:05.990 --> 00:06:07.909 captured a great history of this deformation 00:06:07.909 --> 00:06:10.280 happening since Middle Miocene time. 00:06:10.280 --> 00:06:12.749 And this is an animation that Pat McCrory and Doug Wilson 00:06:12.749 --> 00:06:15.580 presented at GSA this past fall and – 00:06:15.580 --> 00:06:20.260 which I think illustrates this tectonic system pretty well. 00:06:20.710 --> 00:06:26.360 And so, starting around Middle Miocene, this reconstruction takes into account 00:06:26.379 --> 00:06:29.210 several – I’ll play it – I’ll play it a couple times here. 00:06:29.210 --> 00:06:33.089 It takes into account block motions that we know from fault observations 00:06:33.089 --> 00:06:36.580 and paleomagnetic directions and a lot of geophysical data. 00:06:38.420 --> 00:06:40.900 And you can see that there is – a lot of the action is happening 00:06:40.900 --> 00:06:43.529 in Washington state. And this is what I call the 00:06:43.529 --> 00:06:47.969 Washington collision zone – this area broadly outlined in this gray box here. 00:06:47.969 --> 00:06:50.729 And we’ll talk about this a lot today. 00:06:50.729 --> 00:06:54.389 And so this tectonic setting drives a lot of the seismic hazard in the 00:06:54.389 --> 00:06:58.800 Pacific Northwest, where not only do we have the large megathrust earthquakes 00:06:58.800 --> 00:07:01.110 on the subduction interface here and intraslab earthquakes 00:07:01.110 --> 00:07:07.979 like the 2001 Nisqually earthquake, but we also have a lot of sources 00:07:07.980 --> 00:07:12.840 of shallow earthquakes in this upper plate as it deforms. 00:07:14.340 --> 00:07:17.550 So the motivation for the work I’m going to be discussing today 00:07:17.550 --> 00:07:21.740 is to help characterize the seismic hazard of these upper-plate faults. 00:07:22.780 --> 00:07:25.979 And to better understand how this strain, how this clockwise 00:07:25.980 --> 00:07:31.820 rotation of the North American Plate is manifested at the surface. 00:07:33.229 --> 00:07:35.720 And ultimately, we want to understand how these upper-plate 00:07:35.720 --> 00:07:38.899 faults interact and communicate with the underlying subduction zone. 00:07:38.899 --> 00:07:42.899 So the subduction zone, of course, is popping off every 300 or 500 years, 00:07:42.899 --> 00:07:46.050 and these upper-plate faults are rupturing every several centuries 00:07:46.050 --> 00:07:49.150 or several thousand years. And we want to get a better 00:07:49.150 --> 00:07:53.550 understanding of how these interacting – the stress fields – 00:07:53.550 --> 00:07:56.080 the evolving stress fields of these two different systems, 00:07:56.080 --> 00:08:01.040 these two different processes, are interacting over the recent past. 00:08:01.040 --> 00:08:04.770 And of course, to do this, you need to characterize faults and 00:08:04.770 --> 00:08:08.140 get a better understanding of the timing and recurrence of earthquakes. 00:08:09.620 --> 00:08:12.360 So we’ll zoom into Washington state here. 00:08:12.360 --> 00:08:15.020 And this is just SRTM topography with the 00:08:15.020 --> 00:08:19.340 Quaternary Fault and Folds database – the white lines. 00:08:19.340 --> 00:08:24.059 And broadly outlined, this is the zone of – 00:08:24.059 --> 00:08:26.610 the collision zone that I alluded to earlier. 00:08:26.610 --> 00:08:29.770 And the – so the rotation poles, whether you’re thinking about GPS 00:08:29.770 --> 00:08:33.510 or geologic features, are over here in eastern Washington. 00:08:33.510 --> 00:08:36.880 And so this fanning pattern of deformation related to the 00:08:36.880 --> 00:08:39.500 collision zone is what I’ve outlined in this gray polygon. 00:08:40.360 --> 00:08:46.380 And today I’ll be talking about three studies that kind of transect this region. 00:08:47.160 --> 00:08:49.760 First we’ll zoom into the Yakima Folds region. 00:08:49.760 --> 00:08:54.800 Here is some 10-meter topographic data of the Yakima Folds. 00:08:54.800 --> 00:08:59.580 What you can see here are a lot of east-west trending folds – 00:08:59.580 --> 00:09:02.180 anticlines and synclines. And the great thing about the 00:09:02.180 --> 00:09:05.570 Yakima Folds is that the landscape is basically reset in Middle Miocene 00:09:05.570 --> 00:09:10.800 time by the eruption of the Columbia River basalt group. 00:09:10.800 --> 00:09:15.540 And since then, deformation mimics – 00:09:15.540 --> 00:09:18.620 sorry, the landscape here mimics the structures. 00:09:18.620 --> 00:09:23.780 And what I mean by that is, the topography you see here 00:09:23.780 --> 00:09:30.240 is very telling of the – of the structures that are deforming the upper crust. 00:09:31.840 --> 00:09:33.700 And so the shortening builds topography. 00:09:33.700 --> 00:09:37.960 And I’ll show you a schematic cross-section from north to south here 00:09:37.960 --> 00:09:41.570 where several of the – this series of anticlines and synclines – 00:09:41.570 --> 00:09:45.650 and this is about 15 or 20 times vertically exaggerated here. 00:09:45.650 --> 00:09:48.870 But a series of synclines and anticlines you can see that are cored typically 00:09:48.870 --> 00:09:53.660 by thrust faults all – kind of mostly north-vergent structures. 00:09:54.520 --> 00:09:59.560 It’s what’s driving the creation of this topography in the Yakima Folds region. 00:10:00.820 --> 00:10:03.830 And we know a lot of these structures are Quaternary-active. 00:10:03.830 --> 00:10:07.220 The orange lines here on this map are traces from the Quaternary Fault 00:10:07.220 --> 00:10:10.070 and Fold database showing traces of thrust faults 00:10:10.070 --> 00:10:13.260 and also the fold axes at the center of the folds. 00:10:14.260 --> 00:10:18.920 So this is the 10-meter data, and most of this – extent of this map 00:10:18.920 --> 00:10:24.920 has 2-meter resolution Lidar or better. And I’ve spent the last couple years 00:10:24.920 --> 00:10:28.960 doing a lot of office- and field-based mapping on that Lidar. 00:10:28.960 --> 00:10:30.290 And that’s what these lines are here. 00:10:30.290 --> 00:10:32.290 And I’ll turn of the Quaternary faults here. 00:10:32.290 --> 00:10:37.380 And so some of the scarps – these red lines – coincide pretty well 00:10:37.380 --> 00:10:41.780 with some of the kind of large-scale landscape-scale folds, 00:10:41.780 --> 00:10:43.260 but not all of them. 00:10:43.260 --> 00:10:47.520 Some of these scarps cut obliquely across some of these structures. 00:10:49.270 --> 00:10:53.620 And when you throw on those small circles from that Brocher figure 00:10:53.620 --> 00:10:58.360 that I showed earlier into this kind of more of a postage-stamp study area – 00:10:58.360 --> 00:11:02.870 I should say that these small circles were, in part, derived by structures like – 00:11:02.870 --> 00:11:06.540 this is the Wallula Fault here, these arcuate features. 00:11:06.540 --> 00:11:09.870 And so these structures here, you would predict would have 00:11:09.870 --> 00:11:14.340 some strike-slip motion because they’re parallel to these flow paths. 00:11:14.900 --> 00:11:18.700 And when structures deviate from this – say they take a less step across here – 00:11:18.700 --> 00:11:23.010 you’d expect some compression. And in fact, we see a nice youthful- 00:11:23.010 --> 00:11:28.480 looking fold across this left step, or this compressional step, in the fault system. 00:11:30.560 --> 00:11:34.270 And so it’s possible – one thing that we – that we speculate 00:11:34.270 --> 00:11:39.310 is that the Yakima Folds – the folds you see – these large-scale folds you see 00:11:39.310 --> 00:11:44.540 in the landscape, they’re not oriented perpendicular to these small circles. 00:11:44.540 --> 00:11:47.500 So it’s possible that they’ve actually been rotated clockwise 00:11:47.500 --> 00:11:50.440 over the last 10, 15 million years as well, as they’ve been shortening. 00:11:50.440 --> 00:11:52.360 And that’s why they don’t line up perfectly 00:11:52.360 --> 00:11:56.480 with kind of a orthogonal trace to these small circles. 00:11:57.640 --> 00:12:00.480 There’s been numerous paleoseismic studies, 00:12:00.490 --> 00:12:05.260 indicated by these big white circles. And I’ll be zooming in on one of 00:12:05.260 --> 00:12:08.530 these structures – the Umtanum Ridge and 00:12:08.530 --> 00:12:14.340 Gable Mountain anticline system here, where we conducted some 00:12:14.340 --> 00:12:17.700 paleoseismic trenching. That’s site number 11 there. 00:12:17.700 --> 00:12:21.470 So north has been rotated slightly to the left here just to get the 00:12:21.470 --> 00:12:23.380 fold parallel to the edges of the map. 00:12:23.380 --> 00:12:27.180 And you can see the crest of the Umtanum Ridge is almost coincident 00:12:27.180 --> 00:12:32.200 with the trace of – the actual trace of the fold – the purple line there. 00:12:33.400 --> 00:12:36.860 The primary thrust fault actually daylights in a few places 00:12:36.860 --> 00:12:39.510 along the front side of the range. 00:12:39.510 --> 00:12:42.480 We also have a fault that’s mapped – the Burbank Fault – 00:12:42.480 --> 00:12:45.610 along the backside of the range. And the interpretation of this fault 00:12:45.610 --> 00:12:55.040 system on this cross-section from north, up here, to south, down here, 00:12:55.040 --> 00:12:58.460 is that we have a north-vergent structure with a backthrust 00:12:58.460 --> 00:13:01.400 on the backside, or southern side, of the fold. 00:13:01.400 --> 00:13:05.850 And together, collectively, the compression across 00:13:05.850 --> 00:13:08.980 these structures is building the topography of the fold. 00:13:10.360 --> 00:13:12.920 I’m going to show you a photograph standing on the edge of the river here 00:13:12.920 --> 00:13:21.940 looking at this peak called Mount Baldy, where it’s hopefully terribly obvious the 00:13:21.940 --> 00:13:27.700 landscape-scale anticline in this region here on a nice, snowy day in the field. 00:13:32.100 --> 00:13:36.560 And this is basically schematically that same structure 00:13:36.560 --> 00:13:40.120 where we’ve a northeast-vergent thrust fault. 00:13:40.120 --> 00:13:45.130 And then the hanging wall of that structure, we’ve got a backthrust. 00:13:45.130 --> 00:13:48.420 And so this is the Burbank Fault here, and we dug a paleoseismic trench 00:13:48.420 --> 00:13:52.090 on the backside here, mainly because the scarps on the 00:13:52.090 --> 00:13:58.210 main fault here are very discontinuous and difficult to get access to. 00:13:58.210 --> 00:14:03.520 So I’ll zoom into this map here. North is back up – up again. 00:14:03.520 --> 00:14:08.600 And one thing you’ll see is some scarps that we’ve mapped in this Lidar. 00:14:08.600 --> 00:14:14.040 This is illuminated from the east, so not your traditional hill shade direction. 00:14:14.040 --> 00:14:18.410 Trying to highlight these west-facing scarps right through here. 00:14:18.410 --> 00:14:24.070 The scarps – these thin red lines here – you’ll see deviate from the map trace – 00:14:24.070 --> 00:14:27.760 mapped bedrock fault trace – this thin black line here, 00:14:27.760 --> 00:14:32.440 indicating that maybe the fault system is more distributed, and recent 00:14:32.440 --> 00:14:35.840 deformation isn’t coincident with the bedrock mapped fault. 00:14:36.690 --> 00:14:42.360 This scarp truncates some very obvious steep bedrock with lithologic contacts 00:14:42.360 --> 00:14:46.180 in the landscape here that do not appear on the other side of this fault. 00:14:47.140 --> 00:14:50.700 And the active trace continues to the southeast and goes right through an 00:14:50.700 --> 00:14:54.940 active freshwater spring called Gus Spring, which is the name of the site. 00:14:55.990 --> 00:15:00.060 So this next photograph is looking from this dirt road here up at the – 00:15:00.060 --> 00:15:06.261 at the scarp, and you can see, in this nice mid-morning sunlight, the scarp 00:15:06.261 --> 00:15:09.800 is well-illuminated. The white arrows pointing to the top of the scarp. 00:15:09.800 --> 00:15:12.540 And we dug two paleoseismic trenches. Here’s Trench 1 and Trench 2. 00:15:12.540 --> 00:15:16.800 And I’ll be showing a lot of – I’ll be highlighting some findings 00:15:16.800 --> 00:15:19.830 from Trench 1 today. And here’s the trees that are 00:15:19.830 --> 00:15:23.120 happily growing at the spring right along the fault trace. 00:15:24.040 --> 00:15:27.060 We took a topographic profile from the Lidar data, 00:15:27.060 --> 00:15:30.220 and we found that there is about a 4- to 5-meter-high scarp with about 00:15:30.220 --> 00:15:35.560 2 to 3 meters of vertical separation of that inclined bedrock surface. 00:15:37.900 --> 00:15:40.340 And we dug a trench along that profile right there. 00:15:40.350 --> 00:15:43.560 So this is looking east at the scarp. 00:15:43.560 --> 00:15:46.480 This is the top of the scarp, and this is the bottom of the scarp down here. 00:15:46.480 --> 00:15:49.800 And the fault zone is right behind Tabor’s back right here. 00:15:50.660 --> 00:15:52.860 Standing on the edge of the trench, basically where that tripod is, 00:15:52.860 --> 00:15:56.350 and looking at the trench wall, you can see the fault contact 00:15:56.350 --> 00:15:59.840 between Miocene sandstone and Miocene basalt. 00:15:59.840 --> 00:16:03.920 It’s host to sheared and truncated sandstone and shale beds. 00:16:03.930 --> 00:16:06.430 So these beds are actually truncated and cut off, 00:16:06.430 --> 00:16:09.780 and there’s been a lot of deformation and shear along this zone. 00:16:12.300 --> 00:16:16.560 Standing in the trench site here and looking to the south, 00:16:16.560 --> 00:16:19.120 you can get a feel for the scale of these folds. 00:16:19.130 --> 00:16:25.370 We’re sitting on the flank – the backside of a fold here at the trench site. 00:16:25.370 --> 00:16:30.350 And then this dip slope goes down into the core of a syncline, which comes back 00:16:30.350 --> 00:16:34.580 up into the next anticline to the south – Selah Butte anticline here. 00:16:36.360 --> 00:16:40.060 Here’s our trench log and photomosaic of this 00:16:40.060 --> 00:16:43.620 Trench number 1 at Gus – the Gus Spring site. 00:16:43.630 --> 00:16:48.520 Some of the first order observations that I’ll summarize is that we’ve got basalt 00:16:48.520 --> 00:16:55.690 thrust over the sandstone, and thrust in fault contact with Late Quaternary 00:16:55.690 --> 00:17:00.500 colluvial wedge deposits right here. And I’ll zoom in on that zone there. 00:17:01.640 --> 00:17:06.080 And so we’ve identified three wedges of colluvium that are truncated by 00:17:06.080 --> 00:17:11.340 different generations of faults that we’ve collected radiocarbon samples and 00:17:11.350 --> 00:17:15.780 dated them from this trench, and we – and we get ages from about 7000 near 00:17:15.780 --> 00:17:19.850 the bottom of this colluvial wedge stack to about 2000 to 3000 in the top. 00:17:21.120 --> 00:17:23.340 Here’s a photograph of the fault zone here. 00:17:23.340 --> 00:17:25.740 This is what it looks like after you carefully sample 00:17:25.760 --> 00:17:29.060 different colluvial wedge horizons. 00:17:29.060 --> 00:17:33.669 And – [coughs] excuse me – an important observation here 00:17:33.669 --> 00:17:38.600 is this is the fault contact where we have basalt faulted against 00:17:38.600 --> 00:17:42.210 Quaternary deposits here, providing good support 00:17:42.210 --> 00:17:45.040 that this is a Quaternary-active structure. 00:17:47.720 --> 00:17:52.300 We also have fault slickenline data from several fault surfaces. 00:17:57.100 --> 00:18:00.159 They weren’t amazing, but they are there. 00:18:00.159 --> 00:18:03.330 And we measured a half-dozen or so of these. 00:18:03.330 --> 00:18:09.429 Here’s some pictures of these polished and slicken-sided surfaces. 00:18:09.429 --> 00:18:13.370 And when you plot these up, the ones with actual kinematic 00:18:13.370 --> 00:18:16.919 information, it suggests some – it suggests that here, locally, 00:18:16.919 --> 00:18:20.269 along the fault we have evidence for strike-slip motion 00:18:20.269 --> 00:18:27.540 with a lot of shallow to horizontal-oriented slickenlines. 00:18:29.420 --> 00:18:33.020 So zooming back out and talking about the seismic hazard that we 00:18:33.020 --> 00:18:36.780 can deduce from these observations, here’s the trench site here. 00:18:36.780 --> 00:18:39.280 And there are many other earthquake sources. 00:18:39.289 --> 00:18:42.909 All these red lines are places where we’ve had surface rupture 00:18:42.909 --> 00:18:47.470 in earthquakes in the recent past. And some of – and these are all very 00:18:47.470 --> 00:18:51.190 proximal to several critical pieces of infrastructure including several large 00:18:51.190 --> 00:18:56.499 dams on the Columbia River and the Hanford nuclear waste facility here. 00:18:56.499 --> 00:18:58.899 The trench is only about 50 to 90 kilometers away 00:18:58.899 --> 00:19:03.380 from Hanford and about 35 to 40 kilometers from these dams. 00:19:03.380 --> 00:19:05.230 So understanding the timing information and the 00:19:05.230 --> 00:19:08.080 recurrence of these earthquakes is terribly important. 00:19:10.200 --> 00:19:13.840 Okay, so zooming back to the Washington scale map, 00:19:13.850 --> 00:19:16.240 here are the Yakima Folds. There’s that trench site 00:19:16.240 --> 00:19:20.720 that I just talked about. But where does this – where did the strain go? 00:19:20.720 --> 00:19:23.919 Remember the rotation pole is off here somewhere to the east, 00:19:23.919 --> 00:19:27.960 and if deformation is increasing to the west, we should continue to 00:19:27.960 --> 00:19:33.980 see contractional deformation farther west from the Yakima Folds. 00:19:35.260 --> 00:19:41.039 And, as with many stories in the Pacific Northwest, geophysics provides 00:19:41.039 --> 00:19:46.750 a plethora of information that you cannot deduce from surface geology. 00:19:46.750 --> 00:19:52.490 And so here’s a great figure from Blakely et al. 2011 where Rick and 00:19:52.490 --> 00:19:56.549 co-authors used – and I’ll turn off the transparency there – 00:19:56.549 --> 00:19:59.929 used high-resolution aeromagnetic data and other – and other geophysical 00:19:59.929 --> 00:20:04.960 techniques to speculate about structures that connect some of these 00:20:04.960 --> 00:20:10.139 Yakima Folds through the high-standing Cascade Range and into the Puget 00:20:10.140 --> 00:20:14.289 Lowland, connecting the structures like the Tacoma and Seattle Faults. 00:20:16.500 --> 00:20:21.220 So we’ll zoom into this western Washington setting here. 00:20:21.230 --> 00:20:23.409 Like I mentioned before, GPS tells us that 00:20:23.409 --> 00:20:26.580 surface velocities are increasing to the west. 00:20:26.580 --> 00:20:28.680 And there are several Quaternary faults surrounding the 00:20:28.680 --> 00:20:30.690 Olympic Mountains and the Puget Lowland. 00:20:30.690 --> 00:20:33.440 So here’s the Yakima Folds, structures cutting through the 00:20:33.440 --> 00:20:36.740 Cascade arc and linking up to these structures in the Puget Lowland. 00:20:38.020 --> 00:20:41.460 The Canyon River Fault, which we’ll talk about shortly 00:20:41.470 --> 00:20:45.539 likely links to the Seattle Fault here and possibly links to offshore structures 00:20:45.540 --> 00:20:50.160 that Pat McCrory and other folks have studied in the offshore setting. 00:20:52.010 --> 00:20:54.880 And I won’t be talking about this today, but we also – I was collaborating 00:20:54.890 --> 00:20:58.490 with some academic colleagues working on the Leech River Fault here, 00:20:58.490 --> 00:21:02.769 which appears to – which is Holocene-active and appears to 00:21:02.769 --> 00:21:07.040 link to the Darrington-Devils Mountain Fault across the Salish Sea. 00:21:09.309 --> 00:21:13.179 So we know that – so here’s a figure from Wells and McCaffrey 00:21:13.179 --> 00:21:18.789 showing the GPS velocity field model after removing the signal 00:21:18.789 --> 00:21:22.539 of the shortening from the Cascadia subduction zone. 00:21:22.539 --> 00:21:27.159 And we see that gradients – sharp gradients in this velocity field, 00:21:27.159 --> 00:21:31.759 which are identified by these bold gray lines, are places – these are – 00:21:31.759 --> 00:21:34.120 these are clues, or places where you should look, 00:21:34.120 --> 00:21:37.620 for structures that accommodate some of this shortening. 00:21:38.520 --> 00:21:44.420 If you take two transects across from north to south through the forearc, 00:21:44.429 --> 00:21:49.200 we can see that the northward velocities basically decrease 00:21:49.200 --> 00:21:54.080 across kind of 46 to 48 degrees north latitude. 00:21:54.080 --> 00:21:57.340 And that’s where they identify these structures here. 00:21:57.340 --> 00:22:02.499 And so going from the Puget Lowland out towards the coast, we – 00:22:02.499 --> 00:22:08.179 like I’ve been alluding several times, the strain rates increase from about 00:22:08.180 --> 00:22:12.740 5 millimeters in the Puget Lowland to almost 7 millimeters towards the coast. 00:22:14.460 --> 00:22:18.240 Okay, the next map I’ll show is zooming into the Puget Lowland area here. 00:22:19.020 --> 00:22:25.300 And this is a compilation that Megan Anderson presented at GSA this past fall 00:22:25.300 --> 00:22:31.460 summarizing some of the fault network that cross-cuts the Puget Lowland. 00:22:31.460 --> 00:22:36.220 And let me get you located here. The blue lines here are the shoreline. 00:22:36.220 --> 00:22:39.380 And here’s the Seattle Fault right here. 00:22:39.380 --> 00:22:41.800 And downtown Seattle is right here. 00:22:41.800 --> 00:22:44.440 And here’s the east edge of the Olympic Mountains. 00:22:46.300 --> 00:22:49.879 I’ll show you several interpretations of the Seattle Fault Zone. 00:22:49.879 --> 00:22:52.740 So taking a north-south cross-section through the Seattle Fault zone. 00:22:52.740 --> 00:22:57.580 These are just four of several published interpretations of the Seattle Fault zone. 00:22:57.580 --> 00:23:02.789 But the upshot is basically the fault zone is – consists of a primary north-vergent 00:23:02.789 --> 00:23:07.980 thrust fault, and all the interpretations believe that it’s a blind fault. 00:23:07.980 --> 00:23:12.169 The primary thrust is a blind fault. It does not actually reach the surface. 00:23:12.169 --> 00:23:15.739 But involves a lot of complicated deformation of its hanging wall, 00:23:15.739 --> 00:23:20.450 including backthrust, ramps, and south-vergent structures. 00:23:20.450 --> 00:23:24.039 These south-vergent structures are mostly – the most common structures 00:23:24.040 --> 00:23:27.520 we actually see breaking into the post-glacial landscape. 00:23:29.840 --> 00:23:32.820 So we’ll zoom in here to the Seattle Fault zone just to show an example 00:23:32.820 --> 00:23:36.080 of what this looks like in Lidar and bathymetry. 00:23:36.080 --> 00:23:39.269 Here is the southern edge of Bainbridge Island. 00:23:39.269 --> 00:23:41.529 Downtown Seattle is just off to the right of this figure. 00:23:41.529 --> 00:23:45.260 And you can see some very obvious topographic scarps that cut through 00:23:45.260 --> 00:23:50.509 the striated glacial landscape. Here’s another less subtle one through here. 00:23:50.509 --> 00:23:53.359 And they continue offshore in the bathymetry as well. 00:23:54.020 --> 00:23:57.720 Even where this glaciated landscape is preserved underwater. 00:23:58.640 --> 00:24:02.860 And so working with – and so I’m going to zoom out here to a larger 00:24:02.860 --> 00:24:06.300 picture of the Seattle Fault zone. That last figure is this red box right here. 00:24:06.300 --> 00:24:08.160 We’ll get rid of that. 00:24:08.169 --> 00:24:10.570 So working with folks from the Costal and Marine Science Center 00:24:10.570 --> 00:24:15.230 in Santa Cruz, we – and also Emily Roland at University of Washington, 00:24:15.230 --> 00:24:21.970 we devised a cruise of high-resolution seismic and CHIRP profiles, which are 00:24:21.970 --> 00:24:27.259 the white lines on this map here, to help characterize the Seattle Fault zone. 00:24:27.259 --> 00:24:31.230 So the red lines are the Quaternary Fault and Fold database – 00:24:31.230 --> 00:24:34.859 traces of the Seattle Fault zone. And so we’re basically going across 00:24:34.859 --> 00:24:38.419 the map – the map traces of the fault and also these very obvious scarps 00:24:38.420 --> 00:24:41.740 in the landscape to better characterize these features. 00:24:42.920 --> 00:24:49.210 So we used a combination of co-located multi-channel seismic and CHIRP 00:24:49.210 --> 00:24:54.369 high-resolution seismic techniques. Here are the streamers we used here 00:24:54.369 --> 00:24:59.380 on the RV Barnes. Here we are passing through the Chittenden Locks. 00:24:59.380 --> 00:25:04.820 And with a large team on a small research ship, we were out there for 00:25:04.820 --> 00:25:10.020 about eight days in February of last year. It was not always sunny and 00:25:10.020 --> 00:25:13.900 beautiful like this. Trust me. We actually had snow thunder and 00:25:13.909 --> 00:25:18.200 snow lightning one day on Lake Washington, was which really exciting. 00:25:18.200 --> 00:25:20.999 But calmly, we’re out there deploying the equipment 00:25:21.000 --> 00:25:23.820 before sunrise and staying until dinnertime. 00:25:24.980 --> 00:25:27.519 So I’ll highlight two profiles from this survey. 00:25:27.519 --> 00:25:29.809 Here’s one off of Bainbridge Island. 00:25:29.809 --> 00:25:34.190 And again, here’s that scarp that I was – that I pointed out earlier. 00:25:34.190 --> 00:25:39.090 And on the CHIRP data, you can see the scarp on the seafloor right here. 00:25:39.090 --> 00:25:42.600 And you can see what we’re interpreting are Holocene kind of post-glacial 00:25:42.600 --> 00:25:50.279 sediments that are deformed less than these glacial sediments below the 00:25:50.279 --> 00:25:55.300 red line. And they thin across this north-side-up structure here. 00:25:55.300 --> 00:25:59.600 So, again, this structure here is probably one of those faults in the 00:25:59.609 --> 00:26:04.690 hanging wall of the Seattle Fault, which is north-side-up, kind of south-vergent. 00:26:04.690 --> 00:26:08.399 A little counterintuitive for a north-vergent structure, but these 00:26:08.400 --> 00:26:12.280 are the – these are the structures that actually make it to the surface. 00:26:13.100 --> 00:26:15.889 One more example from this cruise. This is Lake Washington. 00:26:15.889 --> 00:26:18.200 So here’s Seattle over here. Here’s Lake Washington. 00:26:18.200 --> 00:26:20.380 Here’s Mercer Island in the middle of Lake Washington. 00:26:20.399 --> 00:26:25.869 And the red traces, again, are the Seattle Fault zone from the QFF database. 00:26:25.869 --> 00:26:30.260 The orange lines are some of our cruise lines. 00:26:30.260 --> 00:26:33.370 And the red line here is shown in these two profiles. 00:26:33.370 --> 00:26:36.320 Here’s our CHIRP and multi-channel seismic. 00:26:36.320 --> 00:26:39.649 And here, faulting is a little bit less clear, but we definitely see evidence for 00:26:39.649 --> 00:26:46.680 south-side-up deformation. And we see a subtle scarp on the lake floor here. 00:26:46.680 --> 00:26:52.300 So it’s possible that this south-side-up faulting is one of the rare places 00:26:52.309 --> 00:26:56.960 where we actually see the north-vergent structure with the south-side-up motion 00:26:56.960 --> 00:27:00.880 reaching the shallow surface – the shallow subsurface. 00:27:00.880 --> 00:27:03.039 So this might be evidence of the deformation front 00:27:03.040 --> 00:27:05.200 of the Seattle Fault zone. 00:27:06.960 --> 00:27:11.220 Another thing that we targeted during this cruise were the abundant landslides 00:27:11.220 --> 00:27:12.920 that are present in Lake Washington. 00:27:12.920 --> 00:27:17.210 Here’s a map by Karlin et al. in 2004 showing the locations of several 00:27:17.210 --> 00:27:21.109 landslides that were mapped a couple decades ago. 00:27:21.109 --> 00:27:25.159 And this red box shows the topography and bathymetry of this part of 00:27:25.159 --> 00:27:28.870 Lake Washington where you can see evidence of these large landslide 00:27:28.870 --> 00:27:31.279 deposits that are sitting in the middle of the lake. 00:27:31.279 --> 00:27:35.049 And so we targeted these with several strike lines and dip lines 00:27:35.049 --> 00:27:37.580 with our multi-channel seismic and CHIRP. 00:27:39.840 --> 00:27:44.740 Here’s a zoom-in on the bathymetry for – this is the Denny Park landslide, 00:27:44.749 --> 00:27:48.730 which is a gorgeous landslide. The headwall is probably 00:27:48.730 --> 00:27:53.440 onshore here up in Denny Park. You can see the compressional toe 00:27:53.440 --> 00:27:57.350 of this landslide and some lateral slide margins with some kind of detachment 00:27:57.350 --> 00:28:03.399 features and a little – a little basin forming behind the – on the – 00:28:03.399 --> 00:28:06.080 on the landslide block up here and the chaotic kind of 00:28:06.080 --> 00:28:09.560 block complex in the landslide mass down here. 00:28:09.560 --> 00:28:12.879 And I’ll show you a quick glimpse of one strike line 00:28:12.879 --> 00:28:17.739 across this landslide where this high topography here 00:28:17.739 --> 00:28:21.749 is the kind of undeformed lake deposits off to the north. 00:28:21.749 --> 00:28:25.519 And then you step across this landslide – the lateral margin of the landslide, 00:28:25.519 --> 00:28:30.509 and you can see some of the deformed and truncated layers 00:28:30.509 --> 00:28:33.450 on the right side of this profile. 00:28:33.450 --> 00:28:37.590 The landslide in this perspective is moving towards the viewer. 00:28:37.590 --> 00:28:44.639 And we have evidence for thin well-laminated post-landslide sediments 00:28:44.639 --> 00:28:50.470 that might be good for coring targets to help date the timing of this landslide. 00:28:50.470 --> 00:28:54.149 And of course, one research question that people always want to – 00:28:54.149 --> 00:28:56.889 are interested in for landslides near active faults 00:28:56.889 --> 00:28:58.759 is whether they’re coseismic or not. 00:28:58.759 --> 00:29:02.700 So if we’re able to date these landslides, we might be able to answer the question, 00:29:02.700 --> 00:29:06.970 is this landslide coincident with large earthquakes on the Seattle Fault zone. 00:29:07.980 --> 00:29:10.840 Here’s the Juanita Bay landslide. 00:29:10.850 --> 00:29:14.460 Here’s – Juanita Bay comes out here. There’s a creek that feeds a little delta. 00:29:14.460 --> 00:29:17.720 And basically the delta has failed and collapsed. 00:29:17.720 --> 00:29:22.880 And you can see the deposits of that landslide here. 00:29:22.880 --> 00:29:29.460 Here is the dip line going from the head scarp down to the toe of the deposit. 00:29:30.080 --> 00:29:31.700 Right here. 00:29:31.700 --> 00:29:34.659 And here is the strike line kind of showing 00:29:34.660 --> 00:29:36.640 the cross-section of that landslide. 00:29:36.640 --> 00:29:40.100 And we’ve got some potential coring targets for this slide as well. 00:29:41.120 --> 00:29:45.620 So, as I alluded to, the Seattle Fault zone has had several large earthquakes. 00:29:45.620 --> 00:29:49.230 It’s actually been active for several million years. 00:29:49.230 --> 00:29:50.989 But the most recent major earthquake 00:29:50.989 --> 00:29:55.880 has been dated pretty well to about A.D. 900, so – A.D. 900 to 930. 00:29:55.880 --> 00:29:59.140 Some of the earliest evidence that folks documented for this 00:29:59.149 --> 00:30:04.169 earthquake is – this is on the edge of Bainbridge Island where we have 00:30:04.169 --> 00:30:08.909 this wave-cut platform – the modern platform exposed here. 00:30:08.909 --> 00:30:15.740 But there’s a higher, uplifted relicked ancient platform that’s raised up here. 00:30:15.740 --> 00:30:19.909 And this was interpreted to be due to folding of the Seattle Fault zone 00:30:19.909 --> 00:30:24.019 and uplift in the hanging wall of that – of that structure. 00:30:24.019 --> 00:30:28.879 And so coseismic uplift of the – of the shoreline angle is a – serves as 00:30:28.880 --> 00:30:34.480 a great strain marker to quantify and characterize deformation at the surface. 00:30:35.600 --> 00:30:40.000 Here’s a schematic cartoon of what this – what I’m talking about. 00:30:40.010 --> 00:30:43.129 So here’s the modern-day – in the schematic, the water – 00:30:43.129 --> 00:30:47.799 modern-day water level carving a shoreline angle – this feature here. 00:30:47.800 --> 00:30:51.320 It’s the angle between the modern sea cliff and the wave-cut platform. 00:30:51.320 --> 00:30:55.700 And if there has been tectonic uplift of a region, you may preserve 00:30:55.710 --> 00:31:00.460 a higher shoreline angle feature. And in order to actually obtain 00:31:00.460 --> 00:31:03.759 the information – the elevation information from this, 00:31:03.759 --> 00:31:08.419 you need to basically see through typically a few meters of 00:31:08.419 --> 00:31:13.250 hillslope-derived colluvium to actually obtain the information. 00:31:13.250 --> 00:31:17.830 And so we use Lidar data to basically project the surface of this – 00:31:17.830 --> 00:31:23.849 of this paleo sea cliff and the wave-cut platform to determine the elevation 00:31:23.849 --> 00:31:28.680 of this point here when it’s buried beneath several meters of colluvium. 00:31:29.840 --> 00:31:32.360 So we had a NAGT up in Seattle that – 00:31:32.360 --> 00:31:36.080 when she wasn’t out in the field, Mattie Reid, she did a great job. 00:31:36.080 --> 00:31:40.340 She did this 850 times around Puget Lowland. 00:31:40.340 --> 00:31:43.250 So here’s the – the white lines are the Seattle Fault zone here. 00:31:43.250 --> 00:31:46.019 She did a lot of fault trace – fault trace mapping – 00:31:46.020 --> 00:31:51.520 these colorful lines through here. And the important part to take away 00:31:51.520 --> 00:31:53.960 from this figure are the colored dots along the shoreline. 00:31:53.960 --> 00:31:57.289 So each one of these dots are places where Mattie calculated 00:31:57.289 --> 00:32:00.220 the elevation of the shoreline angle by projecting the Lidar surfaces 00:32:00.220 --> 00:32:04.239 into the subsurface beneath that wedge of colluvium. 00:32:04.239 --> 00:32:07.820 And they’re colored by the elevation of that point relative 00:32:07.820 --> 00:32:13.539 to modern-day sea level. And you might see – 00:32:13.539 --> 00:32:17.440 towards the north, they’re only uplifted, oh, say a meter or so. 00:32:17.440 --> 00:32:19.409 And there’s actually a couple places where they’ve actually 00:32:19.409 --> 00:32:22.629 been down-dropped and are – and are submerged. 00:32:22.629 --> 00:32:28.139 You can see there’s basically no sharp topography up here along the shoreline. 00:32:28.139 --> 00:32:31.450 And then there’s pockets of warmer colors in here in the core 00:32:31.450 --> 00:32:35.509 of the fault zone, indicating places where the shoreline’s 00:32:35.509 --> 00:32:38.100 been elevated higher than other locations. 00:32:39.680 --> 00:32:44.479 And when you contour this data – these contours here, the warm colors 00:32:44.479 --> 00:32:51.419 are kind of in the 9- to 10-meter range – you see these peaks of high uplifted 00:32:51.419 --> 00:32:55.740 shoreline angle features along the center of the – of the Seattle Fault zone. 00:32:55.740 --> 00:32:58.559 And you see – you can kind of get a sense of the asymmetric nature 00:32:58.559 --> 00:33:00.869 of this, where we have tight contours to the north 00:33:00.869 --> 00:33:03.279 and more broad contours to the south. 00:33:03.279 --> 00:33:06.289 And when you look at this data in cross-section along this cross-section – 00:33:06.289 --> 00:33:09.879 this blue line here, you see, from north to south, we can actually – 00:33:09.879 --> 00:33:12.700 using the shoreline angle – these shoreline angle features, 00:33:12.700 --> 00:33:18.539 you can actually kind of characterize the asymmetric nature of this 00:33:18.540 --> 00:33:22.400 north-vergent deformation as the – as the fault ruptured in a 00:33:22.400 --> 00:33:26.260 north-vergent nature, where the elevation of these 00:33:26.260 --> 00:33:30.340 shoreline angle features kind of taper off more gradually to the south. 00:33:32.369 --> 00:33:35.320 This is very similar to some work published by 00:33:35.330 --> 00:33:38.009 Uri ten Brink and co-authors on the Seattle Fault zone 00:33:38.009 --> 00:33:41.519 where they used about 16 or 20 data points. 00:33:41.519 --> 00:33:43.590 And so we find similar results, 00:33:43.590 --> 00:33:46.240 that there’s about a 10-, maybe 12-kilometer-wide zone 00:33:46.240 --> 00:33:50.039 of deformation related to the most recent earthquake on the – 00:33:50.040 --> 00:33:52.300 on the Seattle Fault zone. 00:33:53.800 --> 00:33:55.500 Skip that. 00:33:55.500 --> 00:34:00.140 And so the last study I want to talk about today is some work we did 00:34:00.149 --> 00:34:03.940 on the Canyon River Fault, which is a fault that links to 00:34:03.940 --> 00:34:07.389 the Seattle Fault here, Saddle Mountains Fault zone system, 00:34:07.389 --> 00:34:11.400 and continues and projects towards offshore. 00:34:12.700 --> 00:34:15.120 Ooh, that didn’t turn out too great. 00:34:15.120 --> 00:34:17.260 Here’s some 10-meter topographic data of the region. 00:34:17.270 --> 00:34:21.030 And the red lines on here are pre-existing faults that have been mapped, including 00:34:21.030 --> 00:34:25.080 the Saddle Mountain Fault system and the Frigid Creek Fault system here. 00:34:26.020 --> 00:34:30.280 But I’ll highlight some topographic lineaments that are evident in the 00:34:30.290 --> 00:34:35.300 10-meter data that cross-cut the landscape through here. 00:34:35.300 --> 00:34:38.060 And so we’ve mapped a lot of these lineaments, and not all 00:34:38.060 --> 00:34:41.960 these are in Quaternary deposits. So some of these are bedrock lineaments. 00:34:43.020 --> 00:34:47.320 And they – collectively, between the Canyon River Fault 00:34:47.330 --> 00:34:49.560 and the Saddle Mountain Fault system here, 00:34:49.560 --> 00:34:52.950 we have a fault system that’s at least 60 kilometers in length. 00:34:53.500 --> 00:34:57.980 There’s been a few – handful of paleoseismic studies on this fault system 00:34:57.980 --> 00:35:03.340 here in the north – one in the north, one here in the – in these low-relief hills. 00:35:03.340 --> 00:35:10.220 And we’ll be focusing on some of the trenches we’ve excavated last summer. 00:35:11.760 --> 00:35:15.600 Here are some Lidar data where the fault system cuts across these – 00:35:15.600 --> 00:35:18.740 several generations of fluvial terraces. 00:35:18.740 --> 00:35:22.980 These are kind of post-glacial fluvial terraces draining the Wynoochee 00:35:22.980 --> 00:35:27.900 River Valley. So the river is flowing from north to south here. 00:35:27.900 --> 00:35:33.080 And here’s the fault trace map here, where I’ve highlighted two places – 00:35:33.080 --> 00:35:37.560 two trenches that we dug – Zebra Trench and the Mosquito Trench. 00:35:40.220 --> 00:35:44.980 And these are the topographic profiles across each of those trench locations. 00:35:44.980 --> 00:35:49.080 And Zebra Trench has about a 6- to 8-meter-high scarp. 00:35:49.080 --> 00:35:52.040 And the Mosquito Trench, which is located on a lower 00:35:52.040 --> 00:35:58.680 and probably younger terrace, has about 2-1/2 meters of surface offset. 00:35:59.320 --> 00:36:02.760 I’ll be focusing entirely on Zebra Trench because Mosquito Trench was less 00:36:02.770 --> 00:36:07.200 exciting and less useful and kind of – would take too much time. 00:36:07.200 --> 00:36:10.380 So the Zebra Trench is a 3-1/2-meter-deep bench trench 00:36:10.380 --> 00:36:12.720 with shallow groundwater. 00:36:13.590 --> 00:36:18.040 This is the top of the scarp up here. See that nice A horizon exposed. 00:36:18.040 --> 00:36:21.380 And I’m basically standing on the bottom of the scarp down here. 00:36:22.100 --> 00:36:25.980 So this is the west wall of the trench that I’ll be showing you. 00:36:25.990 --> 00:36:28.860 And one thing to keep in mind – when you’re trenching in a 00:36:28.860 --> 00:36:32.530 temperate rainforest, I recommend bringing two things. 00:36:32.530 --> 00:36:36.090 One is a lot of patience. And two is Kate Scharer because 00:36:36.090 --> 00:36:40.710 she is a great sport, even if you’re scraping, logging, 00:36:40.710 --> 00:36:43.900 and hauling buckets of mud in the pouring rain. 00:36:43.900 --> 00:36:47.820 So those are two great components to trenching out in this part of the world. 00:36:47.820 --> 00:36:50.480 And be careful because if you leave your trench open too long, 00:36:50.480 --> 00:36:54.830 you’ll actually start growing a lot of material. 00:36:54.830 --> 00:36:59.220 And the rainforest will take your trench back. [laughter] 00:37:00.500 --> 00:37:03.400 So here’s looking at the west wall of that – of that trench. 00:37:03.400 --> 00:37:05.220 This is the photomosaic. 00:37:05.220 --> 00:37:08.760 This is basically a combination – a mosaic of about 800 photographs. 00:37:08.760 --> 00:37:13.840 And this is what we used as a base map to do our – conduct our trench logs on. 00:37:13.840 --> 00:37:17.960 Here are the geologic contacts. And I will – so this is – basically, 00:37:17.960 --> 00:37:24.760 this is the title slide here showing this fault zone graben on the west wall. 00:37:25.700 --> 00:37:29.680 And here’s Kate pointing out a very obvious fault to me. 00:37:30.860 --> 00:37:35.800 And so here – I’ll turn off the clasts on the trench log, and the 00:37:35.810 --> 00:37:44.900 important units here are, we have a series of lake lacustrine deposits in brown. 00:37:44.900 --> 00:37:50.640 The blue lines are these finely laminated layers in the – in the lacustrine deposits. 00:37:50.640 --> 00:37:53.500 That’s overlaid by an upper terrace – 00:37:53.500 --> 00:37:57.790 this fluvial terrace that’s – that I was talking about on the Lidar map. 00:37:57.790 --> 00:38:00.340 We have a lower terrace down here in the yellows. 00:38:00.340 --> 00:38:03.320 And then a series of colluvial wedges in gray. 00:38:04.060 --> 00:38:06.580 And that was a very complicated trench log. 00:38:06.590 --> 00:38:08.630 And that was the most complicated trench I’ve actually ever logged. 00:38:08.630 --> 00:38:10.760 So I’m going to simplify it into a cartoon here. 00:38:10.760 --> 00:38:16.000 So basically, we have siltstone that is cut by the fault. 00:38:16.000 --> 00:38:18.910 And a upper terrace, which is also cut by the fault, and a lower terrace 00:38:18.910 --> 00:38:21.400 that does not appear cut by the fault but involved in a landslide. 00:38:21.400 --> 00:38:23.000 And then, of course, the colluvial wedges 00:38:23.000 --> 00:38:26.000 that kind of track across the top of this whole system. 00:38:26.280 --> 00:38:33.180 About 1 meter of motion on the landslide restores this strath – 00:38:33.180 --> 00:38:39.580 original surface at the base of this terrace deposit to the region here. 00:38:39.980 --> 00:38:43.640 So I’ll zoom into the fault zone here. So we’ve got a distributed zone of 00:38:43.640 --> 00:38:48.700 brittle faults, which looks like a graben. We actually have the base – gravel on 00:38:48.700 --> 00:38:53.660 siltstone contact faulted down here, which is the same contact as up here. 00:38:53.660 --> 00:38:56.050 And that same contact is faulted back up on this 00:38:56.050 --> 00:39:00.240 horse block right underneath this big cobble. 00:39:00.240 --> 00:39:03.220 We’re really fortunate to have this well-laminated siltstone. 00:39:03.220 --> 00:39:06.430 It serves as a nice strain marker. And the analogy I like to 00:39:06.430 --> 00:39:10.960 draw is to a faulted barcode. If you have a nice barcode of 00:39:10.960 --> 00:39:14.850 stratigraphy, you hopefully can be able to match that up across a fault zone. 00:39:14.850 --> 00:39:20.490 And so I argue that we have a faulted – a faulted barcode across the 00:39:20.490 --> 00:39:23.660 Canyon River Fault here. And this is a close-up of some of 00:39:23.660 --> 00:39:26.180 this nice laminated stratigraphy, and it’s very unique. 00:39:26.180 --> 00:39:29.810 It’s not – it’s not kind of a cyclical deposition. 00:39:29.810 --> 00:39:32.360 So we can use that to our advantage. 00:39:32.360 --> 00:39:33.530 And so I’ll show you two blow-ups of these 00:39:33.530 --> 00:39:36.760 two red boxes here on either side of the fault zone. 00:39:37.380 --> 00:39:45.000 And I’ve mapped out several laminae in the lacustrine deposits. 00:39:45.010 --> 00:39:49.110 And with a little bit of wiggle-matching, hopefully I can convince you that 00:39:49.110 --> 00:39:51.810 we found a pretty good correlation across the fault zone. 00:39:51.810 --> 00:39:55.980 And because these layers are basically centimeter-or-smaller scale, we got pretty 00:39:55.980 --> 00:40:02.380 good precision – about 1.78 meters of vertical separation across the fault zone. 00:40:03.380 --> 00:40:06.490 We also found slickenlines in this fault exposure here. 00:40:06.490 --> 00:40:10.670 Here’s some of those horizontal silt bed – parting surfaces – 00:40:10.670 --> 00:40:14.450 the bedding surfaces. And here are fault slickenlines – 00:40:14.450 --> 00:40:17.670 these subtle lineations on this fault surface here. 00:40:17.670 --> 00:40:21.440 This is kind of a photo location of where it is in the trench. 00:40:22.120 --> 00:40:24.420 So the average rake of these slickenlines is 00:40:24.430 --> 00:40:31.490 about 29, 30 degrees, suggesting left-lateral down-on-the-north faulting. 00:40:31.490 --> 00:40:34.300 And this left-lateral oblique fault motion is consistent with 00:40:34.300 --> 00:40:40.340 the two previous trench studies along-strike to the northeast. 00:40:42.420 --> 00:40:47.020 We collected several – this is radiocarbon paradise 00:40:47.020 --> 00:40:50.210 out here in the rainforest. You basically have an abundant 00:40:50.210 --> 00:40:53.830 supply of radiocarbon in your – in your deposits. 00:40:53.830 --> 00:40:57.090 We’ve dated 28 samples so far. 00:40:57.090 --> 00:41:00.860 Most yield ages in kind of mid-to-late Holocene. 00:41:00.860 --> 00:41:04.211 And we use these ages to model the timing of the earthquake 00:41:04.211 --> 00:41:10.340 using a Bayesian statistical approach called OxCal modeling, where we 00:41:10.340 --> 00:41:14.150 order the number – the samples that predate the earthquake, 00:41:14.150 --> 00:41:17.270 and then samples that post-date the earthquake, to model the age. 00:41:17.270 --> 00:41:19.290 This is a probability – this is the PDF – 00:41:19.290 --> 00:41:24.510 probability distribution function of the age of the earthquake. 00:41:24.510 --> 00:41:26.350 And so we come up with a model age of 00:41:26.350 --> 00:41:32.660 about 6100 plus or minus 800 years, 2 sigma, for this earthquake. 00:41:33.460 --> 00:41:39.830 I will say that there is a possible two- earthquake interpretation to this trench. 00:41:39.830 --> 00:41:42.240 And we’re currently dating additional samples to 00:41:42.240 --> 00:41:45.360 distinguish between a one-earthquake and two-earthquake model. 00:41:46.320 --> 00:41:48.200 But in a two-earthquake model, we would have an earthquake 00:41:48.210 --> 00:41:52.670 at 8700 and 4900. Although the one-earthquake 00:41:52.670 --> 00:41:56.020 model is currently our preferred – the preferred interpretation. 00:41:57.860 --> 00:42:01.280 But regardless, we do have evidence for at least one, maybe two, 00:42:01.280 --> 00:42:07.400 Holocene earthquakes of pretty significant size in Holocene time. 00:42:07.400 --> 00:42:10.880 So to kind of walk you through the evolution of – 00:42:10.880 --> 00:42:15.600 our interpreted evolution of this scarp, we’ve got these lacustrine beds 00:42:15.600 --> 00:42:18.880 that were deposited, probably at some proglacial lake. 00:42:18.880 --> 00:42:23.540 The river system cut a strath surface across the top of those lacustrine beds. 00:42:23.540 --> 00:42:27.070 Uplift – regional uplift, probably due to the Cascadia subduction zone, 00:42:27.070 --> 00:42:31.130 lifted the area and helped incise the river system and – 00:42:31.130 --> 00:42:38.680 forming a fluvial riser and a lower strath surface and younger fluvial deposits. 00:42:38.680 --> 00:42:40.960 Then our one earthquake happened, 00:42:40.960 --> 00:42:46.100 dropped down this region about 1.8 meters of throw across the fault zone. 00:42:46.100 --> 00:42:49.840 And we don’t know the relative timing terribly well, 00:42:49.850 --> 00:42:52.110 but we speculate that the landslide we observed in the trench is 00:42:52.110 --> 00:42:56.540 potentially coseismic, being that it’s only 2 meters from a surface rupture. 00:42:58.000 --> 00:43:01.300 And then, of course, the colluvial wedge systems here shown with their calibrated 00:43:01.300 --> 00:43:05.740 carbon-14 ages kind of track across and fill in this hummocky topography. 00:43:06.430 --> 00:43:11.020 So not – so this scarp was 6 to 8 meters high. 00:43:11.030 --> 00:43:15.530 But maybe only 2 meters of that 6 to 8 meters is actually tectonic. 00:43:15.530 --> 00:43:17.830 The rest of it is due to down-cutting and incision 00:43:17.830 --> 00:43:21.460 on the north side of the fault and landsliding processes. 00:43:22.860 --> 00:43:28.860 So we can use our slickenlines and our – of about 30 degrees and our 00:43:28.860 --> 00:43:32.890 vertical separation of about 2 meters. Using some really complicated 00:43:32.890 --> 00:43:38.800 trigonometry here, we can calculate that we have about 3.73 meters 00:43:38.800 --> 00:43:42.560 of displacement. So it’s possible that this – 00:43:42.560 --> 00:43:46.260 in the one-earthquake interpretation, the earthquake produced about – 00:43:46.260 --> 00:43:49.560 almost 4 meters of displacement in one earthquake. 00:43:49.560 --> 00:43:51.830 If it was two earthquakes, you might have to divvy that up 00:43:51.830 --> 00:43:54.140 across two different – separate events. 00:43:54.900 --> 00:44:00.260 Depending on what age – bracketing ages you use, we calculate slip rates – 00:44:00.260 --> 00:44:04.300 maximum slip rates in the range of 0.3 to 0.6 millimeters per year and 00:44:04.300 --> 00:44:06.840 minimum slip rate – we don’t know the age of the silt bed, 00:44:06.840 --> 00:44:09.840 so the minimum slip rate might be less than 0.3 millimeters a year. 00:44:09.840 --> 00:44:13.050 So we’re talking about a – not a fast-moving structure, 00:44:13.050 --> 00:44:16.940 but definitely one that accommodates active shortening in western Washington. 00:44:18.060 --> 00:44:20.200 We do find evidence for older earthquakes that 00:44:20.210 --> 00:44:24.830 predate formation of those fluvial straths and those fluvial deposits. 00:44:24.830 --> 00:44:28.220 Specifically, we see upward termination of faults, where faults 00:44:28.220 --> 00:44:32.560 are actually capped by younger siltstone beds, here and here. 00:44:33.460 --> 00:44:35.680 And evidence for soft sediment deformation where we have highly 00:44:35.680 --> 00:44:41.930 contorted beds that are basically capped by perfectly laminated strata as well, 00:44:41.930 --> 00:44:44.650 suggesting there was an earthquake when this – 00:44:44.650 --> 00:44:48.230 these beds were near the floor of the lake. 00:44:48.230 --> 00:44:52.600 And then subsequent sedimentation basically capped those contorted beds. 00:44:52.600 --> 00:44:56.320 And that might be possibly due to seismic shaking nearby. 00:44:58.740 --> 00:45:03.440 So we interpret the Canyon River Fault system as left-lateral faults with a little 00:45:03.440 --> 00:45:07.760 bit of down-on-the-north motion, at least locally here at our trench site. 00:45:07.760 --> 00:45:11.280 In our one-earthquake model, we have an earthquake at 6100. 00:45:11.280 --> 00:45:14.900 The trenches nearby had different-age earthquakes – 00:45:14.900 --> 00:45:18.540 1900 and about 1000 to 1300. 00:45:18.540 --> 00:45:24.200 This one possibly is related to the 900 A.D. event on the Seattle Fault. 00:45:24.200 --> 00:45:27.920 So it appears that we might have identified a rupture boundary at the 00:45:27.920 --> 00:45:32.580 restraining bed where the fault system – the entire fault system probably does not 00:45:32.580 --> 00:45:38.420 rupture collectively all in one rupture. It may rupture in shorter segments. 00:45:39.640 --> 00:45:44.180 So the earthquake hazard for the Canyon River Fault – this is a scenario 00:45:44.180 --> 00:45:49.920 of the shaking predicted from a 7.2 earthquake on the Canyon River Fault, 00:45:49.920 --> 00:45:53.880 which affects – produces strong shaking throughout the Olympic Peninsula, 00:45:53.880 --> 00:45:58.200 including Olympia, Tacoma, and Seattle. 00:45:59.030 --> 00:46:01.800 The Wynoochee River – the dam at the Wynoochee Lake is only 00:46:01.800 --> 00:46:06.010 about 5 kilometers from the fault trace. And this is upstream of towns 00:46:06.010 --> 00:46:10.570 like Montesano and Aberdeen, which may be – could be affected 00:46:10.570 --> 00:46:15.020 by a rupture on this – on this – on the Canyon River Fault. 00:46:17.260 --> 00:46:19.040 So the regional tectonic implications for this – 00:46:19.040 --> 00:46:21.280 the Canyon River Fault likely is linked through the 00:46:21.280 --> 00:46:24.670 Saddle Mountain Fault system to the Seattle Fault, 00:46:24.670 --> 00:46:29.740 as several other folks have speculated in the recent past. 00:46:29.740 --> 00:46:33.340 And it could also be linked offshore to faults and folds 00:46:33.340 --> 00:46:37.340 that have been identified in the shallow offshore setting. 00:46:38.960 --> 00:46:42.020 And we – and it’s – the fault is not well-mapped along strike 00:46:42.030 --> 00:46:45.410 to the southwest, so there’s an unknown and possible interaction 00:46:45.410 --> 00:46:48.650 with inferred strike-slip motion that may be coming off of the 00:46:48.650 --> 00:46:53.920 Doty Fault system and Mount St. Helens seismic zone here. 00:46:56.000 --> 00:47:00.020 So left-lateral slip on the Canyon River Fault that we’ve identified here in 00:47:00.020 --> 00:47:07.220 our trenches also corroborates recent interpretations for this tectonic escape 00:47:07.220 --> 00:47:11.180 model – Alan Nelson and co-authors have recently published, where basically, 00:47:11.180 --> 00:47:16.700 the Olympic Mountains are being squeezed north-south. 00:47:16.710 --> 00:47:19.540 And structures on the southern flank of the Olympic Mountains are 00:47:19.540 --> 00:47:22.560 accommodating left-lateral motion, and structures on the northern side 00:47:22.560 --> 00:47:25.480 of the Olympic Mountains are accommodating right-lateral motion, 00:47:25.480 --> 00:47:29.220 collectively accommodating this tectonic escape – 00:47:29.220 --> 00:47:33.820 westward escape of – and uplift of the Olympic Mountains. 00:47:36.360 --> 00:47:43.390 And also, Ray Wells and others have speculated that these upper-plate faults 00:47:43.390 --> 00:47:46.360 along the entire length of the Cascadia subduction zone, 00:47:46.360 --> 00:47:52.980 highlighted as these white lines here, may have an intimate role in regulating 00:47:52.980 --> 00:47:58.380 processes of the subduction zone itself, whether that’s limiting the 00:47:58.390 --> 00:48:02.260 lateral extent of ruptures on the megathrust or 00:48:02.260 --> 00:48:08.120 modulating the density of tremor that we can detect. 00:48:12.220 --> 00:48:16.400 So in summary, hopefully today I’ve helped explain a little bit of 00:48:16.400 --> 00:48:20.840 where the strain goes in this Washington collision zone, 00:48:20.840 --> 00:48:24.420 which includes this large area of transpression that’s 00:48:24.420 --> 00:48:29.860 accommodating this northward translation of the Oregon Coast block 00:48:29.860 --> 00:48:33.280 as it rotates around rotation poles off to the east here. 00:48:34.100 --> 00:48:40.000 And I think this figure from Rick’s paper is probably the best illustration 00:48:40.010 --> 00:48:43.060 to kind of leave this at, where these structures are not kind of 00:48:43.060 --> 00:48:48.480 these distinct phenomena happening. That the Yakima Folds deformation 00:48:48.480 --> 00:48:50.900 happening in the Cascade Range, the deformation happening in the 00:48:50.900 --> 00:48:55.070 Puget Lowland, is all intimately linked in this system and network 00:48:55.070 --> 00:48:58.860 of faults that is accommodating this transgression across Washington. 00:48:58.860 --> 00:49:00.300 Thank you. 00:49:00.300 --> 00:49:07.320 [Applause] 00:49:09.600 --> 00:49:11.380 - Questions for Scott? 00:49:12.700 --> 00:49:16.580 [Silence] 00:49:17.140 --> 00:49:19.460 - Thanks, Scott. That was a great talk. 00:49:19.460 --> 00:49:23.940 I have two sort of small questions. The first one was about the 00:49:23.940 --> 00:49:29.450 terraces in the Puget Lowlands. And it’s probably not so relevant 00:49:29.450 --> 00:49:32.980 because you were looking at a pretty fine spatial scale, or at least on the order of, 00:49:32.980 --> 00:49:35.500 like, kilometers, tens of kilometers. But how much do you have to 00:49:35.510 --> 00:49:38.450 worry about, like, glacial isostatic rebound of the 00:49:38.450 --> 00:49:42.920 Puget Lowlands contributing to the total uplift of those structures? 00:49:42.920 --> 00:49:46.090 - That’s a fair question. And there is – there has been 00:49:46.090 --> 00:49:51.230 isostatic rebound on the scale of – I want to say a couple meters. 00:49:51.230 --> 00:49:54.780 But most of that rebound happened shortly after deglaciation in 00:49:54.780 --> 00:49:58.140 latest Pleistocene time – maybe Early Holocene time. 00:49:58.140 --> 00:50:02.220 So these features – those shoreline angles were carved – were basically 00:50:02.220 --> 00:50:06.810 being carved up until the day before the 900 A.D. earthquake. 00:50:06.810 --> 00:50:11.740 And so those – the magnitude of isostatic rebound is probably done 00:50:11.740 --> 00:50:14.220 and over with by the time those features were formed. 00:50:14.220 --> 00:50:16.320 So it might be negligible. 00:50:16.320 --> 00:50:18.520 - Okay. And the second quick 00:50:18.520 --> 00:50:24.280 question was about the Yakima Folds. And you were talking about rotation 00:50:24.280 --> 00:50:27.230 and when they become unfavorable to accommodate the strain. 00:50:27.230 --> 00:50:30.820 Are there any – are there any other data sets you can pull on to help 00:50:30.820 --> 00:50:33.210 tell that story, like PaleoMag or anything like that? 00:50:33.210 --> 00:50:36.770 - That’s exactly what I would refer to. And there’s been a fair amount of 00:50:36.770 --> 00:50:41.170 PaleoMag data in the Yakima Folds on this great data – you know, 00:50:41.170 --> 00:50:44.240 this great strain marker – the Columbia River flood basalt group. 00:50:44.240 --> 00:50:49.320 And I don’t know if anyone’s – there’s definitely clockwise rotations out there. 00:50:49.320 --> 00:50:53.620 They’re relatively small, especially compared to rotations farther west. 00:50:53.620 --> 00:50:56.830 And that’s a function of – because you’re closer to the rotation pole. 00:50:56.830 --> 00:51:01.700 But that’s – yeah, that – it is pretty speculative, and it’s something that’s – 00:51:01.700 --> 00:51:04.840 that I’d like to test. And, like you said, PaleoMag drilling, 00:51:04.850 --> 00:51:07.550 I think, might be – some fine-scale PaleoMag drilling 00:51:07.550 --> 00:51:11.800 might be – help get to that – those kind of answers. 00:51:14.280 --> 00:51:20.560 [Silence] 00:51:21.380 --> 00:51:23.440 - Hey, Scott. 00:51:24.180 --> 00:51:27.380 What I’m wondering – you probably don’t have enough data to answer this, 00:51:27.390 --> 00:51:33.290 but do you have any idea of the repetitive – the rate through – 00:51:33.290 --> 00:51:38.840 the repeat rate of earthquakes? And do they increase as you go to the west? 00:51:39.480 --> 00:51:41.240 - Ooh. - Frequency is what I’m trying to say. 00:51:41.240 --> 00:51:44.760 - Sure. That’s a good question. 00:51:44.760 --> 00:51:51.280 I have never kind of looked at the spatial pattern of earthquake recurrence. 00:51:51.280 --> 00:51:53.970 I can say that the one trench that I dug in the Yakima Folds 00:51:53.970 --> 00:51:58.000 had two or three earthquakes in 8,000 years. 00:51:58.000 --> 00:52:01.690 And the trench in the – on the Canyon River Fault had one earthquake. 00:52:01.690 --> 00:52:05.920 So at that comparison, there doesn’t seem to be a pattern. 00:52:05.920 --> 00:52:08.090 But it’d be interesting to see, if you looked at a collective data set, 00:52:08.090 --> 00:52:11.530 and I know folks like Brian Sherrod and Richard Styron have been doing this, 00:52:11.530 --> 00:52:17.480 looking at kind of the collective data sets of earthquakes in the Puget Lowland. 00:52:19.040 --> 00:52:22.370 And I – you know, so I can’t say. But I think that’d be an interesting 00:52:22.370 --> 00:52:26.580 thing to pursue, and relatively easy to do. So that’d be – yeah. 00:52:26.580 --> 00:52:31.080 We do know – I can say that the shortening rate does change from 00:52:31.080 --> 00:52:34.210 east to west – about 2 millimeters of shortening – north-south shortening 00:52:34.210 --> 00:52:37.652 in the Yakima Folds and some of those plots I showed of the GPS data, 00:52:37.652 --> 00:52:43.100 it increased to about 4 or 5 millimeters at Puget Lowland, and then about 6 or 7. 00:52:43.100 --> 00:52:45.250 So you would guess that, you know, you would have more 00:52:45.250 --> 00:52:49.700 earthquakes potentially because of that increase in the shortening rate. 00:52:51.680 --> 00:52:53.080 - Thanks. 00:52:54.740 --> 00:52:58.440 [Silence] 00:52:59.200 --> 00:53:02.800 - Hey, Scott. Great talk. You mentioned the westward 00:53:02.810 --> 00:53:05.350 extension of the Canyon River Fault is not well-mapped. 00:53:05.350 --> 00:53:10.420 Is that because we need Lidar? People haven’t looked very carefully? 00:53:10.420 --> 00:53:15.040 We could map it if we tried hard? What needs to be done? 00:53:15.040 --> 00:53:20.010 - They’re – all the above. There’s – I think farther to the 00:53:20.010 --> 00:53:24.150 southwest, the best mapping is probably 100,000 scale. 00:53:24.150 --> 00:53:28.650 And that was done pre-Lidar. There is Lidar for the strip of the fault – 00:53:28.650 --> 00:53:32.490 basically along the fault for maybe 10 kilometers or so. 00:53:32.490 --> 00:53:38.700 But it’s a very localized and kind of fault-centric footprint. 00:53:38.700 --> 00:53:44.400 So, yeah – so more topography, more mapping – maybe 24,000 scale. 00:53:44.400 --> 00:53:47.320 Because there are – you know, it’s not the best exposure out there. 00:53:47.320 --> 00:53:51.040 I mean, you do have a lot of dense vegetation and a lot of 00:53:51.040 --> 00:53:53.520 access issues with logging companies and whatnot. 00:53:53.520 --> 00:53:56.800 But it’s – I think, with the right tools, it’d be possible. 00:53:59.030 --> 00:54:02.340 And I’ll always say that, you know, geophysics can always help. 00:54:02.350 --> 00:54:03.350 [laughter] 00:54:03.350 --> 00:54:06.320 I’m not a geophysicist, but I have [laughter] quickly learned 00:54:06.320 --> 00:54:09.300 that, in this part of the world, you have to rely on it. 00:54:09.300 --> 00:54:12.960 Oh, Rick, perfect. [laughter] 00:54:12.960 --> 00:54:15.780 - Thanks for the plug. [laughter] 00:54:15.790 --> 00:54:20.370 So you started your talk by using Tom Brocher’s concentric circles as sort 00:54:20.370 --> 00:54:26.280 of the regional-scale driver for all this. And then you ended your talk with the 00:54:26.280 --> 00:54:31.630 Canyon River Fault, which is a strike-slip fault with a northeast strike to it. 00:54:31.630 --> 00:54:35.440 Would you say that the Canyon River Fault argues against Tom’s model? 00:54:35.440 --> 00:54:38.650 Or can you fit that into it in some way? - I think I can fit it into it. 00:54:38.650 --> 00:54:42.120 Let’s see if I can find that slide real quick here. 00:54:43.360 --> 00:54:52.600 [Silence] 00:54:52.600 --> 00:54:54.420 There it is. 00:54:58.380 --> 00:55:01.920 Let’s see. And actually – this black – I think – let’s see. Is that right? 00:55:01.920 --> 00:55:06.600 Yeah, that black line there is the Canyon River Fault. 00:55:06.600 --> 00:55:11.770 So actually, in this – in this interpretation, the Canyon River Fault, 00:55:11.770 --> 00:55:15.350 because it’s basically orthogonal to these small circles, 00:55:15.350 --> 00:55:19.700 the prediction is that it’s pure thrust. I’d actually argue that the orientation 00:55:19.700 --> 00:55:23.960 of that black line is – it’s obviously oversimplified, but it actually should be 00:55:23.960 --> 00:55:29.360 more counter-clockwise – more northerly than it’s drawn. 00:55:29.680 --> 00:55:32.160 And then you would actually expect left-lateral motion 00:55:32.160 --> 00:55:33.920 across that – up across that structure. 00:55:33.920 --> 00:55:39.300 Because it’s counter-clockwise of a feature that’s perfectly orthogonal. 00:55:40.600 --> 00:55:46.140 And of course, there’s – you know, there’s – this pole here is a – they – 00:55:46.140 --> 00:55:49.240 Tom and co-authors coined a geologic pole. 00:55:49.240 --> 00:55:53.600 So it’s representative of more long-term processes and structures that have 00:55:53.600 --> 00:55:59.180 formed over several millions of years. And so the GPS-derived pole, 00:55:59.180 --> 00:56:02.900 if you’re looking for more instantaneous kind of contemporary 00:56:02.900 --> 00:56:06.310 signal of deformation, the GPS-derived poles, 00:56:06.310 --> 00:56:09.140 which are typically located a little farther south – 00:56:09.140 --> 00:56:12.880 and that’s going to shift the orientation of these small circles 00:56:12.880 --> 00:56:16.050 around a GPS-derived pole everywhere. 00:56:16.050 --> 00:56:18.310 Those might be more appropriate, at least for talking about, 00:56:18.310 --> 00:56:21.940 you know, Late Holocene kinematics. 00:56:26.940 --> 00:56:29.780 - Any other questions before we let Scott free? 00:56:31.260 --> 00:56:33.300 All right. Well, thank you very much. 00:56:33.320 --> 00:56:38.540 [Applause] 00:56:38.540 --> 00:56:44.620 [Silence]