WEBVTT Kind: captions Language: en-US 00:00:00.969 --> 00:00:03.969 Hi. I’m John Baldwin. I’ll be presenting the preliminary 00:00:03.969 --> 00:00:07.859 findings of a paleoseismic investigation on the North Coast 00:00:07.859 --> 00:00:11.601 segment of the San Andreas Fault near Gualala, California. 00:00:11.601 --> 00:00:15.047 This is a collaborative research project between Lettis Consultants 00:00:15.047 --> 00:00:21.788 International, California Geological Survey, and the U.S. Geological Survey. 00:00:22.918 --> 00:00:26.380 The Fish Rock Road site is located southeast of Point Arena 00:00:26.380 --> 00:00:30.098 and northwest of Fort Ross near the town of Gualala. 00:00:30.098 --> 00:00:33.890 It was originally identified by Koehler and Baldwin as part of 00:00:33.890 --> 00:00:38.800 a strip mapping exercise along the San Andreas Fault to identify 00:00:38.800 --> 00:00:42.510 potential paleoseismic research sites. 00:00:42.510 --> 00:00:45.625 The resultant map is shown here on the left as well as the region 00:00:45.625 --> 00:00:52.160 of the study site. On the far right is a schematic geologic map showing 00:00:52.160 --> 00:00:58.121 the fault zone defined by a bedrock high on the east, shown in gray, 00:00:58.121 --> 00:01:02.960 that impounds Quaternary alluvium being deposited by creeks 00:01:02.960 --> 00:01:07.463 that flow from west to east across the fault zone. 00:01:07.463 --> 00:01:12.140 The map also shows two trenches, T1 and T2, in black that are part of 00:01:12.140 --> 00:01:17.713 the Zachariasen and Baldwin pilot study that were the first trenches to 00:01:17.713 --> 00:01:22.471 evaluate the stratigraphy at the site. Those trenches confirm the location 00:01:22.471 --> 00:01:27.270 of an older fault located at the base of the bedrock high, 00:01:27.270 --> 00:01:33.660 as well as the 1906 rupture located to the west of the bedrock high, 00:01:33.660 --> 00:01:39.566 as well as multiple secondary fault strands shown in purple. 00:01:42.388 --> 00:01:46.654 As part of the 2021 study, we also developed a ground-based Lidar map, 00:01:46.654 --> 00:01:51.614 shown here on the left. We’ll use it to finish the tectonic 00:01:51.639 --> 00:01:55.349 geomorphic mapping and geomapping of the site. 00:01:55.349 --> 00:01:58.661 What it does show is that it shows the bedrock high. 00:01:58.661 --> 00:02:03.482 It shows the impounded alluvium behind the bedrock high in the area. 00:02:03.507 --> 00:02:09.306 In the lower right, it shows where we excavated the trenches from the 2021 00:02:09.306 --> 00:02:14.517 study, which included six trenches, two of which were part of the pilot 00:02:14.517 --> 00:02:20.990 study in 2016 in which we re-excavated portions of those trenches. 00:02:20.990 --> 00:02:24.720 The site stratigraphy that was identified consisted of fine- to coarse-grain 00:02:24.720 --> 00:02:28.200 alluvium with lesser amounts of colluvium. 00:02:28.200 --> 00:02:35.609 We confirmed that the western strand is the 1906 strand and has multiple 00:02:35.609 --> 00:02:39.836 potential events along it. And we confirmed the eastern strand 00:02:39.836 --> 00:02:44.813 shown in T1 as being discontinuous, and if it does continue to the north, 00:02:44.813 --> 00:02:49.770 maybe as a warp. And also recording older events that do not project up 00:02:49.770 --> 00:02:53.211 into the younger more modern surface. 00:02:53.211 --> 00:02:59.457 We also identified anomalous secondary soft-sediment deformation features, 00:02:59.457 --> 00:03:04.691 primarily in T3, that may help with the event chronology. 00:03:04.691 --> 00:03:08.450 The upper part of this slide shows a photomosaic developed as part 00:03:08.450 --> 00:03:14.470 of the pilot study performed in 2016 by Zachariasen and Baldwin, 00:03:14.470 --> 00:03:18.780 which we were to confirm that the site had the proper stratigraphic 00:03:18.780 --> 00:03:22.508 and structural relations to collect event timing information. 00:03:22.549 --> 00:03:27.070 So you can see we identified a very prominent zone of primary 00:03:27.070 --> 00:03:32.554 faulting offsetting bedded alluvial deposits as well as a zone of 00:03:32.554 --> 00:03:37.297 shallow secondary faulting. This trench was limited in depth due to 00:03:37.297 --> 00:03:42.234 a buried log as well as to a shallow groundwater table west of the fault. 00:03:42.234 --> 00:03:47.967 2021 we re-excavated part of Trench T2 to get a longer and closer look. 00:03:47.967 --> 00:03:53.030 And the preliminary interpretation of that re-excavation is shown below, 00:03:53.030 --> 00:03:58.180 in which we interpret the 1906 trace and several earlier events. 00:03:58.180 --> 00:04:02.900 We also excavated a new trench, Trench T3, south of T2, 00:04:02.900 --> 00:04:06.430 in which that trench also confirms the presence 00:04:06.430 --> 00:04:12.551 of the 1906 rupture as well as multiple earlier events. 00:04:13.918 --> 00:04:17.410 In summary, the Fish Rock Road site along the western strand, 00:04:17.410 --> 00:04:21.467 we interpret multiple events from the trenches. 00:04:21.467 --> 00:04:26.545 And along the eastern strand, we infer a single older event. 00:04:26.545 --> 00:04:32.020 The soft-sediment deformation that’s exposed, we intend to evaluate that to 00:04:32.020 --> 00:04:36.750 see whether or not it can provide any additional event timing information. 00:04:36.750 --> 00:04:41.680 Our next steps, we’re awaiting the C-14 age results, and we plan to 00:04:41.680 --> 00:04:45.387 finalize the final technical report. Thank you. 00:04:46.105 --> 00:04:49.220 - Hello. This is Chris Bloszies presenting the preliminary findings 00:04:49.220 --> 00:04:52.900 of the strip mapping study of the Quién Sabe Fault conducted by 00:04:52.900 --> 00:04:57.160 myself and John Baldwin of Lettis Consultants International. 00:04:57.160 --> 00:05:00.230 The Quién Sabe Fault is a 25-kilometer-long, 00:05:00.230 --> 00:05:03.880 1- to 1.7-kilometer-wide, zone of discontinuous fault strands 00:05:03.880 --> 00:05:06.680 located northeast of the San Andreas Fault and 00:05:06.680 --> 00:05:11.680 Southern Calaveras Fault junction. Right-lateral strike-slip motion is 00:05:11.680 --> 00:05:15.914 inferred for the fault zone with a small component of up-to-the-northeast 00:05:15.947 --> 00:05:22.773 vertical, as evidenced by low southwest-facing fault scarps. 00:05:22.773 --> 00:05:26.800 The fault’s also associated with a concentrated area of seismicity 00:05:26.800 --> 00:05:31.100 and is historically active with two moderate magnitude events 00:05:31.100 --> 00:05:34.290 occurring on the fault in the mid-’80s. 00:05:34.290 --> 00:05:39.754 Both of these events showed dominant dextral strike-slip motion. 00:05:39.754 --> 00:05:43.210 Paleoseismic information for the fault is poorly constrained. 00:05:43.210 --> 00:05:46.894 Net slip estimates of 0.22 to 0.67 millimeters a year 00:05:46.894 --> 00:05:53.467 by Bryant 1985 are based on estimates of vertical displacement. 00:05:53.467 --> 00:05:57.460 UCERF incorporates this uncertainty conservatively by estimating 00:05:57.460 --> 00:06:02.680 a 1-millimeter-a-year slip rate and assumes a fault creep rate 00:06:02.680 --> 00:06:06.789 of 0.1 millimeters a year. The mapping of the Quién Sabe Fault 00:06:06.789 --> 00:06:09.930 by Bryant in the mid-’80s to date represents the most comprehensive 00:06:09.930 --> 00:06:13.489 evaluation of the fault-related geomorphology and activity. 00:06:13.489 --> 00:06:17.570 Even so, the FER is limited based on coarse-resolution topographic mapping 00:06:17.570 --> 00:06:20.596 and largely inferred geochronologic estimates. 00:06:20.596 --> 00:06:24.550 More recent mapping efforts have either adopted the Bryant depiction 00:06:24.550 --> 00:06:28.135 or locate fault strands based on bedrock relationships. 00:06:28.135 --> 00:06:30.810 Thus the understanding of the location and connectivity of 00:06:30.810 --> 00:06:33.630 individual strands within the fault zone and the overall 00:06:33.630 --> 00:06:39.020 paleoseismic history for the fault not being updated in the last 35 years. 00:06:39.020 --> 00:06:42.667 Our study utilizes a newly acquired Lidar data set to revisit the Quién Sabe 00:06:42.667 --> 00:06:46.150 Fault and produce two maps, one showing the tectonic 00:06:46.150 --> 00:06:49.389 geomorphology and another depicting the surficial geology 00:06:49.389 --> 00:06:53.460 within a 155-square-kilometer mapping area around the fault zone. 00:06:53.460 --> 00:06:56.360 Together, these maps represent the key parameters used 00:06:56.360 --> 00:06:59.130 to produce an updated map of the Quién Sabe Fault. 00:06:59.130 --> 00:07:02.370 The methodology employed for this study is similar to other nearby 00:07:02.370 --> 00:07:05.767 studies such as those for the Central Calaveras Fault. 00:07:05.767 --> 00:07:09.340 The following slides show some of our preliminary interpretations 00:07:09.340 --> 00:07:13.460 and initial findings to date. We start at the southernmost portion 00:07:13.460 --> 00:07:17.173 of the original Bryant trace mapped through the Santa Anita Valley based on 00:07:17.173 --> 00:07:21.330 aligned troughs and scarps developed in an order alluvial surface. 00:07:21.330 --> 00:07:24.360 Our field observations show little to no discernable fault scarp, 00:07:24.360 --> 00:07:27.728 and the older alluvial surface is relatively unbroken. 00:07:27.728 --> 00:07:32.556 It may be mid- to late Pleistocene in age or older. 00:07:32.556 --> 00:07:35.690 To the north and southern Santa Ana Valley, the fault is located 00:07:35.690 --> 00:07:38.520 along the eastern valley margin where a series of southwest-facing 00:07:38.520 --> 00:07:42.730 scarps are developed in small alluvial fan deposits of varying age. 00:07:42.730 --> 00:07:47.210 Here, as shown in this photo, deposits are interpreted to be relatively young, 00:07:47.210 --> 00:07:51.130 maybe latest Pleistocene or even early Holocene, based on our observations 00:07:51.130 --> 00:07:55.410 of limited argillic soil development or yellowish soil color and well-defined 00:07:55.410 --> 00:07:58.403 geomorphic expression in Lidar hillshades. 00:07:58.403 --> 00:08:01.819 In the northern Santa Ana Valley, the Quién Sabe Fault is best expressed 00:08:01.819 --> 00:08:05.910 as two to three semi-parallel anastomosing fault strands. 00:08:05.910 --> 00:08:09.440 Here at the Samarkand Ranch, the younger western strand is expressed 00:08:09.440 --> 00:08:13.410 as a 5-meter-high scarp developed in young alluvial fan deposits, 00:08:13.410 --> 00:08:17.970 whereas the older eastern strand is defined primarily by bedrock features, 00:08:17.970 --> 00:08:21.289 a series of sidehole benches, aligned troughs, notches, and saddle. 00:08:21.289 --> 00:08:24.789 The deposits displaced by the western strand are inferred to be of similar age 00:08:24.789 --> 00:08:28.599 that is displaced on the previous slide in the southern Santa Ana Valley, 00:08:28.599 --> 00:08:31.910 supporting an interpretation of increasing vertical displacement 00:08:31.910 --> 00:08:34.209 as the fault goes north. 00:08:34.209 --> 00:08:37.464 Our last example is in the southern Hollister Valley, where the Quién Sabe 00:08:37.464 --> 00:08:41.449 Fault is best expressed as a single strand consistent with a prominent 00:08:41.449 --> 00:08:45.879 southwest-facing scarp. Here, where Lone Pine Road intersects 00:08:45.879 --> 00:08:50.290 the primary fault strand, left-stepping en echelon cracks were observed in 00:08:50.290 --> 00:08:54.480 the road surface, consistent with right-lateral strike-slip displacement. 00:08:54.480 --> 00:08:58.959 It is possible that these are the result of aseismic creep along the fault. 00:08:58.959 --> 00:09:01.329 Cursory reconnaissance of other similar fault-normal roads 00:09:01.329 --> 00:09:05.433 have thus far yielded no other such examples. 00:09:05.433 --> 00:09:08.660 Our study is ongoing, but we can reach several preliminary conclusions. 00:09:08.660 --> 00:09:12.310 First, in the southern portion of our study area, fault strands 00:09:12.310 --> 00:09:15.863 are interpreted to be, at the very least, Holocene-inactive. 00:09:15.863 --> 00:09:18.749 In the Santa Ana Valley to the north, the Quién Sabe Fault may be 00:09:18.749 --> 00:09:23.420 Holocene-active, and slip is documented to increase to the north. 00:09:23.420 --> 00:09:26.449 To the north in the southern Hollister Valley, the Quién Sabe Fault 00:09:26.449 --> 00:09:29.600 is best expressed and may be creeping. 00:09:29.600 --> 00:09:33.499 North of Pacheco Creek, there is very limited evidence for recent faulting. 00:09:33.499 --> 00:09:38.050 Our remaining tasks include finalizing our field evaluation and submitting 00:09:38.050 --> 00:09:42.980 the final technical report, which will include GIS files for inclusion in the 00:09:42.980 --> 00:09:46.269 Quaternary Fault and Fold Database. Thank you. 00:09:46.730 --> 00:09:50.050 - Today I’d like to tell you about some preliminary results from a high- 00:09:50.050 --> 00:09:55.790 resolution seismic study examining the Maacama swarm that occurred in 2020. 00:09:55.790 --> 00:09:59.361 So this was a swarm within the seismicity of the Maacama Fault 00:09:59.361 --> 00:10:04.019 Zone in northern California. This is work in collaboration with 00:10:04.019 --> 00:10:08.670 Rob Skoumal and Jeanne Hardebeck. And, as part of this work, I’ll briefly 00:10:08.670 --> 00:10:11.860 discuss a new approach to S-to-P amplitude ratios, 00:10:11.860 --> 00:10:16.579 which may help us to characterize the complexity of this swam. 00:10:17.071 --> 00:10:22.779 Okay, so takeaway number one. This swarm is incredibly geometrically complex. 00:10:22.779 --> 00:10:27.829 We’ve applied high-resolution detection and integrated precise location. 00:10:27.829 --> 00:10:32.989 We’ve used 388 routinely cataloged events as waveform templates. 00:10:32.989 --> 00:10:38.760 And this has enabled us to detect more than 3,500 events and precisely locate 00:10:38.760 --> 00:10:44.230 them using correlation-derived differential times with HypoDD. 00:10:44.230 --> 00:10:48.360 So this gives us about nine times as many events, and this enables us 00:10:48.360 --> 00:10:53.850 to get a lot clearer view of the faulting geometry of this swarm. 00:10:53.850 --> 00:10:58.199 So this is a map view and a 3D perspective view from the northwest 00:10:58.199 --> 00:11:03.362 with events color-coded by time over the nearly four-month analysis period. 00:11:03.362 --> 00:11:06.929 So we see a great deal of complexity with widespread conjugate faults 00:11:06.929 --> 00:11:12.089 striking north-northwest and northeast and perhaps pervasive 00:11:12.089 --> 00:11:16.239 en echelon faulting in the northern part of the swarm. 00:11:16.956 --> 00:11:19.329 So understanding these structures will help us 00:11:19.329 --> 00:11:23.449 to understand the underlying source physics. 00:11:24.081 --> 00:11:27.209 Just to give you a little bit better sense of this complexity, I’m going to 00:11:27.209 --> 00:11:30.689 play a depth slice animation. This is going to show – start at 00:11:30.689 --> 00:11:34.290 the shallow end of the swarm and show a 100-meter-thick zone 00:11:34.290 --> 00:11:38.309 moving down through the swarm depth from 6 to 8 kilometers 00:11:38.309 --> 00:11:42.929 with events color-coded by time. So here at the shallow end of the swarm, 00:11:42.929 --> 00:11:45.994 most events are striking toward the northeast. 00:11:45.994 --> 00:11:51.579 Up here in the north, you can start to see what may be more en echelon faults 00:11:51.579 --> 00:11:54.660 striking toward the northeast, but there are also faults striking toward 00:11:54.660 --> 00:11:59.171 the north and the north-northwest. I’ll quickly play this one more time 00:11:59.171 --> 00:12:02.259 so you can get a little bit better sense of the complexity here 00:12:02.259 --> 00:12:04.857 at different depth levels. 00:12:06.476 --> 00:12:13.359 [silence] 00:12:13.359 --> 00:12:17.350 So seeing this complexity in the locations motivated us to consider 00:12:17.350 --> 00:12:21.489 what tools we could use to help constrain focal mechanisms. 00:12:21.489 --> 00:12:24.779 In the past, we’ve used correlation measurements to constrain relative 00:12:24.779 --> 00:12:28.569 polarities, and we’re also using – also measuring amplitude ratios 00:12:28.569 --> 00:12:33.660 for correlated event pairs. And we do this separately for P and S. 00:12:33.660 --> 00:12:38.009 And we’ve averaged these amplitude ratios to constrain magnitudes of newly 00:12:38.009 --> 00:12:41.299 detected events, but we’re actually throwing away a lot of useful 00:12:41.299 --> 00:12:44.552 information when we just average these values. 00:12:44.552 --> 00:12:47.889 So we realize that we can set up an inverse problem with all of these 00:12:47.889 --> 00:12:52.899 amplitude ratios, plus a few traditional S-to-P amplitude ratios, and solve 00:12:52.899 --> 00:12:59.410 for the S-to-P amplitude ratios on each individual seismic channel. 00:12:59.410 --> 00:13:02.119 So here is an example from one seismic channel. 00:13:02.119 --> 00:13:06.168 You can see that some of the variations in S-to-P amplitude ratios 00:13:06.168 --> 00:13:09.309 correspond with some of the location structures. 00:13:09.309 --> 00:13:11.759 And finally, we can incorporate this information 00:13:11.759 --> 00:13:16.699 to get improved constraints on focal mechanisms. 00:13:17.863 --> 00:13:21.980 So our combined analysis, including S-to-P amplitude ratios, 00:13:21.980 --> 00:13:25.759 can constrain focal mechanisms for a large population of events. 00:13:25.759 --> 00:13:29.680 This is still a work-in-progress, but this plot is showing focal 00:13:29.680 --> 00:13:34.600 mechanisms for about 1,600 events, which is about four times the number 00:13:34.600 --> 00:13:37.300 of events included in the routine catalog. 00:13:37.300 --> 00:13:42.519 It’s difficult to show so many mechanisms on one plot, so this plot 00:13:42.519 --> 00:13:47.869 is showing – these are dominantly strike-slip mechanisms. 00:13:47.869 --> 00:13:55.209 And each symbol here – the cross symbol is showing these two line 00:13:55.209 --> 00:13:59.100 directions. So these are showing the two possible strike orientations 00:13:59.100 --> 00:14:02.945 with the line length approximating the physical rupture dimension. 00:14:02.945 --> 00:14:06.579 And the color scale is redundantly showing the strike direction, but it helps 00:14:06.579 --> 00:14:11.219 highlight variations in strike direction for spatially clustered events. 00:14:11.219 --> 00:14:14.679 So having mechanisms for so many events is a really powerful complement 00:14:14.679 --> 00:14:19.309 to the detailed hypocenter interpretations – hypocenter patterns 00:14:19.309 --> 00:14:23.949 to help us make – start to make fault structure interpretations. 00:14:23.949 --> 00:14:28.399 Such as this visual interpretation, where we can start to resolve faults 00:14:28.399 --> 00:14:32.559 on the scale of tens of meters. So this is a plausible – an example 00:14:32.559 --> 00:14:35.829 of a plausible fault interpretation, but you can see the potential 00:14:35.829 --> 00:14:40.846 to illuminate faulting complexity at increasingly small scales. 00:14:40.846 --> 00:14:44.509 I think the combination of earthquake detection, precise location, and focal 00:14:44.509 --> 00:14:48.319 mechanisms is a really powerful – toward our goal of understanding 00:14:48.319 --> 00:14:54.275 the faulting physics underlying this and other swarms. Thank you very much. 00:14:55.627 --> 00:14:58.730 - Hi. I’m Litong Huang from UC-Santa Cruz. 00:14:58.730 --> 00:15:02.050 And I’m glad to present my ongoing study on the migration of 00:15:02.050 --> 00:15:08.989 microseismicity on the Calaveras Fault between March to August 2021. 00:15:08.989 --> 00:15:15.579 In Bilham’s 2021 SCEC meeting poster, he proposed a propagating pulse of slip 00:15:15.579 --> 00:15:22.391 based on the migration of seismicity, surface creep, and radon and CO2 00:15:22.440 --> 00:15:25.493 flows increases. I’m trying to increase the 00:15:25.493 --> 00:15:30.470 microearthquake catalog in this area during the proposed slip episode 00:15:30.470 --> 00:15:35.399 using template matching to provide more details to the previously identified 00:15:35.399 --> 00:15:40.849 earthquake migration and the proposed slip event. 00:15:40.879 --> 00:15:47.027 I’m using all the earthquakes in the NCEDC catalog in 2021 as template 00:15:47.027 --> 00:15:51.850 events and relocate them according to Real-Time Double Difference catalog. 00:15:51.850 --> 00:15:56.869 And I used the EQcorrscan package to generate templates and detect 00:15:56.869 --> 00:16:02.589 matches in this area. After removing repeated detections and earthquakes 00:16:02.589 --> 00:16:08.029 which are already in the catalog, I got 44 new detections using templates 00:16:08.029 --> 00:16:15.814 in 2021. And here is a example of one of the matches in 2021. 00:16:15.814 --> 00:16:22.679 I also got 61 cases that one template matches the other template. 00:16:22.679 --> 00:16:28.829 By analyzing the distances between this cross-match pulse, we can see that 00:16:28.829 --> 00:16:35.375 most of the matches have distance smaller than 0.5 kilograms, which allows us 00:16:35.375 --> 00:16:42.220 to locate the new detect events by the location of their corresponding templates. 00:16:42.220 --> 00:16:45.879 On the right side is a map of all the template events 00:16:45.879 --> 00:16:51.474 in 2021 and all the new detected events. 00:16:51.474 --> 00:16:56.439 And the stars represent the template events, 00:16:56.439 --> 00:17:01.089 and circles represents a newly detected event. 00:17:01.089 --> 00:17:06.880 We got a similar result to Bilham’s poster that seismicity migrates 00:17:06.880 --> 00:17:11.972 from northwest to southeast. However, those migration of these 00:17:11.972 --> 00:17:20.110 three clusters are apparent. There’s still not very much detail in the gap area. 00:17:20.110 --> 00:17:24.100 That’s an intrinsic problem of our template matching because all 00:17:24.100 --> 00:17:28.030 the matches are clustered near the templates. 00:17:28.030 --> 00:17:33.830 So one possible way to provide more details in this gap area is to use more 00:17:33.830 --> 00:17:41.964 template in this area from past few years, as in the left figure. 00:17:41.986 --> 00:17:45.772 There’s many events occurring in the above-mentioned gap area. 00:17:45.797 --> 00:17:53.630 I already processed the template event in 2020 and found 11 new matches. 00:17:53.630 --> 00:17:58.269 And, in the future, I’m trying to do the same for template events 00:17:58.269 --> 00:18:03.900 from 2012 to 2020 and see if we will get more details. 00:18:03.900 --> 00:18:09.339 Thank you and welcome to leave any comments or any suggestions. 00:18:10.965 --> 00:18:14.033 [silence] 00:18:14.058 --> 00:18:17.519 - Today I’d like to give you a brief overview of work that was recently 00:18:17.519 --> 00:18:21.669 published in G-cubed describing marine paleoseismic evidence 00:18:21.669 --> 00:18:25.130 for seismic and aseismic slip along the Hayward-Rodgers Creek 00:18:25.130 --> 00:18:29.103 Fault system in Northern San Pablo Bay. 00:18:29.103 --> 00:18:32.178 Distinguishing between seismic and aseismic fault slip in the 00:18:32.178 --> 00:18:36.090 geologic record is difficult yet fundamental to estimating the 00:18:36.090 --> 00:18:40.890 seismic potential of faults and the likelihood of multi-fault rupture. 00:18:40.890 --> 00:18:44.724 Currently, the maximum earthquake magnitude, or M-max, along the 00:18:44.724 --> 00:18:50.171 Hayward-Rodgers Creek Fault system is estimated to be between 7 and 7.4. 00:18:50.171 --> 00:18:53.601 And this includes a scaling factor for creep. 00:18:53.601 --> 00:18:58.710 Now, the presence of creep essentially lowers the M-max for this fault system. 00:18:58.710 --> 00:19:01.930 In addition, prior to this study, the extent of creep beneath 00:19:01.930 --> 00:19:06.100 San Pablo Bay was largely unknown but suggested by the presence of 00:19:06.100 --> 00:19:11.797 repeating earthquakes denoted in the blue circles on the map on the left. 00:19:11.797 --> 00:19:15.720 The previous paleoseismic work, combined with the recent work 00:19:15.720 --> 00:19:19.751 documenting fault connectivity between the Hayward and Rodgers Creek Faults 00:19:19.751 --> 00:19:24.263 suggest that mutli-section fault rupture is possible. 00:19:27.102 --> 00:19:32.430 So we integrated chirp sub-bottom imaging with targeted cross-fault coring 00:19:32.430 --> 00:19:37.630 and core analyses of sedimentary proxy data to characterize vertical deformation 00:19:37.630 --> 00:19:42.409 and slip behavior within an extensional fault bend along the Hayward-Rodgers 00:19:42.409 --> 00:19:48.184 Creek Fault system in northern San Pablo Bay, as shown on the map. 00:19:48.184 --> 00:19:53.059 We identified and traced four key seismic horizons across the fault, 00:19:53.059 --> 00:19:57.551 all younger than approximately 1400 CE. 00:19:57.551 --> 00:20:00.169 We developed a stratigraphic age model using detailed 00:20:00.169 --> 00:20:04.080 down-core radiocarbon and radioisotope dating combined 00:20:04.080 --> 00:20:09.325 with measurements of anthropogenic metal concentrations down-core. 00:20:09.325 --> 00:20:13.769 Now, the onset of hydraulic mining within the Sierra Nevada in 1852 left 00:20:13.769 --> 00:20:18.520 a clear geochemical and magnetic signature within core samples. 00:20:18.520 --> 00:20:23.679 And this key time horizon was used to calculate a local reservoir correction 00:20:23.679 --> 00:20:30.763 and reduce the uncertainty in our radiocarbon age calibration and models. 00:20:30.763 --> 00:20:35.260 Vertical fault offset of strata younger than the most recent surface rupturing 00:20:35.260 --> 00:20:41.559 earthquake on the Hayward Fault in 1868 suggests that near-surface 00:20:41.559 --> 00:20:44.158 vertical creep is occurring along the fault in northern 00:20:44.185 --> 00:20:49.733 San Pablo Bay at a rate of approximately 0.4 millimeters per year. 00:20:49.733 --> 00:20:54.330 In addition, we present evidence of at least one, and possibly two, 00:20:54.330 --> 00:21:00.010 coseismic events associated with growth strata above horizons R1 and R2 00:21:00.010 --> 00:21:07.700 with median event ages estimated to be 1400 CE and 1800 CE, respectively. 00:21:09.584 --> 00:21:16.010 Now, the overlap in timing of E2 and events on adjacent fault sections, 00:21:16.010 --> 00:21:21.910 plus the amount of coseismic vertical offset of 0.5 meters, and the off-fault 00:21:21.910 --> 00:21:25.920 deformation that we recognize in the chirp data strongly suggest that 00:21:25.920 --> 00:21:31.410 multi-fault section rupture occurred around 1400 CE. 00:21:32.147 --> 00:21:36.460 Also, based on the recent dynamic rupture modeling work of Harris et al., 00:21:36.460 --> 00:21:40.690 multi-section ruptures are possible, however, extending the area 00:21:40.690 --> 00:21:46.200 of significant creep northward along the Hayward Fault reduces 00:21:46.200 --> 00:21:50.659 the ability of ruptures to propagate through San Pablo Bay. 00:21:51.026 --> 00:21:53.030 Thank you for listening. 00:21:55.460 --> 00:21:58.951 - Hello. I’m Jenna Hill with the USGS PCMSC speaking today 00:21:58.951 --> 00:22:01.613 about the relationship between submarine landslides and 00:22:01.613 --> 00:22:04.823 earthquake recurrence records in southern Cascadia. 00:22:04.848 --> 00:22:07.049 I’m showing here a rotated bathymetry map for offshore 00:22:07.049 --> 00:22:10.910 southern Cascadia from Cape Blanco to Cape Mendocino. 00:22:10.910 --> 00:22:13.955 In Cascadia, the primary offshore record of earthquake recurrence, 00:22:13.955 --> 00:22:17.809 which is thought to be about 500 years for full-margin ruptures, 00:22:17.809 --> 00:22:20.930 comes from correlation of abyssal turbidites interpreted to be 00:22:20.930 --> 00:22:24.471 synchronously deposited at multiple sites along the margin. 00:22:24.471 --> 00:22:28.151 Additional mud turbidites have been found at sites in central and southern 00:22:28.151 --> 00:22:31.880 Cascadia and used to suggest more frequent shorter-length ruptures, 00:22:31.880 --> 00:22:35.533 but these interpretations have been subject of much debate. 00:22:35.533 --> 00:22:38.110 As a result, we have been doing extensive work in this region to 00:22:38.135 --> 00:22:41.549 examine the sources and pathways of these abyssal turbidites to 00:22:41.549 --> 00:22:45.446 better understand both the timing and spatial extent of shaking. 00:22:45.488 --> 00:22:49.134 We have noted that there appears to be very little transfer of sediment 00:22:49.134 --> 00:22:52.899 from the other shelf and upper slope to the abyssal plane in this region. 00:22:52.899 --> 00:22:57.000 So here I’m showing several examples of sediment core CT scans where the 00:22:57.000 --> 00:23:02.460 purple is low-density mud and the pinks to yellows are coarser sand and gravels. 00:23:02.460 --> 00:23:05.340 In the mid- to upper portions of both Rogue and Trinidad Canyons, 00:23:05.340 --> 00:23:09.450 we see mostly mud with no depositional or erosional events. 00:23:09.488 --> 00:23:13.639 In contrast, we see evidence of debris flows in the lower slope 00:23:13.639 --> 00:23:16.490 and sandy turbidites on the abyssal plane, which suggest 00:23:16.490 --> 00:23:20.210 the primary contributions here are from the lower slope. 00:23:20.210 --> 00:23:23.629 In between, in the Eel Basin, we see no evidence of deep-sea connected 00:23:23.629 --> 00:23:27.759 canyons, where we have a robust Pleistocene turbidite record and 00:23:27.759 --> 00:23:32.863 thick Holocene deposits that suggest the basin is a sediment sink. 00:23:32.895 --> 00:23:37.139 Since it appears that most of southern Cascadia canyons are not delivering 00:23:37.139 --> 00:23:40.490 sediment to the deep sea in the Holocene, we’ve hypothesized 00:23:40.490 --> 00:23:44.049 that the abyssal turbidite record is instead derived from disintegration 00:23:44.049 --> 00:23:47.899 of the outer wedge. To test this, I began mapping seafloor failure scarps, 00:23:47.899 --> 00:23:51.753 which are shown here as the thin black lines along the entire length of the 00:23:51.778 --> 00:23:56.149 margin, and a major trend emerged. There appear to be far more failures 00:23:56.149 --> 00:23:58.973 along the lower slope than anywhere else. 00:23:58.973 --> 00:24:02.210 So I’m showing a few examples of this where you can see extensive 00:24:02.210 --> 00:24:06.210 failures in the lower portions of both Rogue and Trinidad Canyons 00:24:06.210 --> 00:24:08.200 and a nearly continuous band of slope failures 00:24:08.200 --> 00:24:11.799 along the lower slope outside of these canyons as well. 00:24:11.799 --> 00:24:16.190 I’m going to zoom in here to this region in this little tiny red box at the bottom 00:24:16.190 --> 00:24:19.679 and show some of our AUV and ROV data that was recently acquired 00:24:19.679 --> 00:24:22.480 in collaboration with our partners at MBARI. 00:24:22.480 --> 00:24:25.840 So the background map here is 1 meter AUV bathymetry from 00:24:25.840 --> 00:24:28.979 the toe of the deformation front. We can see a well-defined slope 00:24:28.979 --> 00:24:33.320 failure zone with a 6- to 8-meter-high headwall scarp and associated 00:24:33.320 --> 00:24:36.491 mass transport deposits at the base of the slope. 00:24:36.491 --> 00:24:40.200 The yellow line indicates the location of the AUV chirp sub-bottom profile, 00:24:40.200 --> 00:24:44.610 which is shown here on the bottom, that shows multiple buried MTDs 00:24:44.610 --> 00:24:47.597 overlain by a few meters of layered strata. 00:24:47.597 --> 00:24:51.649 We collected two transects of precisely located ROV vibracores 00:24:51.649 --> 00:24:54.809 whose locations are shown by the colored stars. 00:24:54.809 --> 00:24:58.120 The cores within the failure zone show three to four well-defined 00:24:58.120 --> 00:25:03.070 turbidites atop the larger buried MTDs, while the adjacent core transect shows 00:25:03.070 --> 00:25:07.179 much less distinct event deposits just outside of the failure zone. 00:25:07.179 --> 00:25:10.009 To me, this is a nice example of how multiple earthquake-triggered 00:25:10.009 --> 00:25:13.441 turbidites may be sourced from the same failure zone. 00:25:13.441 --> 00:25:16.409 While not every large earthquake produces a large slope failure, 00:25:16.409 --> 00:25:20.029 since there are a lot of factors required to precondition slopes for failure, 00:25:20.029 --> 00:25:24.100 when these large failures do occur, they can debuttress the slope such that 00:25:24.100 --> 00:25:27.620 subsequent earthquakes may shake loose additional material and smaller 00:25:27.620 --> 00:25:30.165 failures that are recorded as recurring turbidites. 00:25:30.165 --> 00:25:34.329 And I think that’s what we see along much of southern Cascadia. 00:25:34.329 --> 00:25:37.680 Now, to take a step back, I’m showing here the full distribution of seafloor 00:25:37.680 --> 00:25:41.889 failure scarps for southern Cascadia, represented by a density of features 00:25:41.889 --> 00:25:45.080 where the hotter colors, the higher density of scarps. 00:25:45.080 --> 00:25:48.850 There’s a strong clustering of failures along the outer wedge and in areas 00:25:48.850 --> 00:25:52.280 around Rogue and Eel Canyons where the seafloor gradient is steeper due to 00:25:52.280 --> 00:25:56.289 the plate boundary interactions. Notably, though, there’s a significant 00:25:56.289 --> 00:26:00.175 lack of failures across the upper slope despite the fact that this region has 00:26:00.175 --> 00:26:04.960 thick, continuous, and relatively rapidly emplaced Quaternary strata, 00:26:04.960 --> 00:26:08.910 all of which are hallmarks of a slope preconditioned for failure. 00:26:08.910 --> 00:26:11.649 I find this absence of slope failure across such a large portion of the 00:26:11.649 --> 00:26:16.289 margin particularly interesting in comparison with the latest M9 scenario 00:26:16.289 --> 00:26:21.280 predictions shown here from Wirth et al. in 2021 that indicate this entire region 00:26:21.280 --> 00:26:24.549 is likely to experience violent, severe shaking. 00:26:24.549 --> 00:26:28.030 The striking absence of failures further up the slope leads to the question 00:26:28.030 --> 00:26:32.250 of whether the most intense shaking is actually more restricted to a narrower 00:26:32.250 --> 00:26:36.320 zone along the trench, as represented by the submarine landslide distribution, 00:26:36.320 --> 00:26:38.980 or whether there are other geomechanical factors at play here. 00:26:38.980 --> 00:26:43.368 Although our initial analyses suggest that might not be the case. 00:26:44.720 --> 00:26:47.009 So here is my summary. For next steps, we’re working through 00:26:47.009 --> 00:26:50.639 our vast array of core transects and geophysical data to characterize the 00:26:50.639 --> 00:26:54.832 variability and reduce age uncertainties. We’re also looking to do geotechnical 00:26:54.832 --> 00:26:58.080 analyses of slope failure susceptibility and site response 00:26:58.080 --> 00:27:01.160 and welcome any potential collaborators on this front. 00:27:02.020 --> 00:27:05.833 - Hi. My name is Vicki Langenheim, and I’ll be talking about new 00:27:05.833 --> 00:27:09.590 aeromagnetic data in northeastern California. 00:27:09.590 --> 00:27:13.679 Why this region? Well, it lies at the intersection of the Cascade arc, 00:27:13.679 --> 00:27:18.457 the Basin and Range, the clockwise- rotating Oregon Coast block, 00:27:18.457 --> 00:27:21.152 and the Walker Lane. 00:27:21.152 --> 00:27:24.399 And one of the questions we want to address is how much 00:27:24.399 --> 00:27:28.822 right-lateral displacement occurs along the Walker Lane 00:27:28.822 --> 00:27:32.080 Fault Zone and how far it encroaches into this area. 00:27:32.080 --> 00:27:35.340 Especially since previous estimates of right-lateral displacement 00:27:35.340 --> 00:27:38.072 range from zero to 30 kilometers in this region. 00:27:38.072 --> 00:27:43.300 And we also want to address where faults extend beneath cover. 00:27:43.300 --> 00:27:46.298 So we’re going to now zoom into our study area. 00:27:46.298 --> 00:27:50.629 This is a simplified geologic map. Indicates that aeromagnetic data 00:27:50.629 --> 00:27:55.100 should provide a great way to address these questions as the area is blanketed 00:27:55.100 --> 00:27:59.720 by geologically young volcanic rocks. Volcanic rocks are very magnetic, 00:27:59.720 --> 00:28:03.759 and offsets of these rocks produce characteristic magnetic anomalies. 00:28:03.759 --> 00:28:07.029 And, as you can see, there’s a lot of faulting in this area, 00:28:07.029 --> 00:28:11.420 so we should see a lot of characteristic anomalies. 00:28:11.420 --> 00:28:16.940 I’ll point out that the region also encompasses Medicine Lake Volcano, 00:28:16.940 --> 00:28:22.750 which is the largest volcano in the Cascade arc, and the Pit River, 00:28:22.750 --> 00:28:25.730 which is the largest tributary to the Sacramento River, 00:28:25.730 --> 00:28:29.279 and it’s fed by some of the largest freshwater springs 00:28:29.279 --> 00:28:32.820 in the western United States here in Fall River Valley. 00:28:32.820 --> 00:28:37.820 It also hosts a number of dams and power houses along the stretch. 00:28:37.820 --> 00:28:42.750 So this what the aeromagnetic data looked like before our new data 00:28:42.782 --> 00:28:47.249 were flown. They were flown decades ago along flight lines 00:28:47.249 --> 00:28:51.720 that were 1- to 2-mile line spacing and at a constant elevation. 00:28:51.720 --> 00:28:56.009 This was all before the advent of GPS. 00:28:56.009 --> 00:28:59.990 We used these data in our 2016 Geosphere paper to examine the 00:28:59.990 --> 00:29:03.809 geometry of the Hat Creek Fault, which is one of the more prominent 00:29:03.809 --> 00:29:09.179 normal faults in the region, and to propose little discrete 00:29:09.179 --> 00:29:14.309 cumulative right-lateral offset on faults that cross this 00:29:14.309 --> 00:29:17.299 prominent northeast-striking magnetic gradient. 00:29:17.299 --> 00:29:23.889 Let’s now look at what the new data show, which is a huge improvement 00:29:23.889 --> 00:29:27.679 in resolution, much richer data set. And I’m just going to overlay 00:29:27.679 --> 00:29:34.470 the faults here. These new data confirm little cumulative displacement 00:29:34.470 --> 00:29:41.629 along that northeast-striking gradient as well as little discrete right-lateral 00:29:41.629 --> 00:29:46.600 displacement across these east-west- striking bands of magnetic highs. 00:29:46.600 --> 00:29:52.742 The likely fault, which is an enigmatic fault with a funny name, 00:29:52.742 --> 00:29:58.190 has likely little discrete offset. But filtered versions of the magnetic 00:29:58.190 --> 00:30:01.809 data suggest as much as 5 to 10 kilometers of right-lateral 00:30:01.809 --> 00:30:06.869 shearing across the fault zone. We could also use the magnetic data 00:30:06.869 --> 00:30:10.440 to extend faults south of Medicine Lake Volcano, 00:30:10.440 --> 00:30:14.220 to connect to the springs here in Fall River Valley, as well as 00:30:14.220 --> 00:30:21.990 extend faults across the Tule Lake and Lower Klamath Basins. 00:30:21.990 --> 00:30:26.249 So stay tuned, and new aeromagnetic data are coming online in the coming 00:30:26.249 --> 00:30:29.899 months and years in California and Nevada, in large part funded 00:30:29.899 --> 00:30:34.940 for assessment of mineral and geothermal resources. 00:30:34.940 --> 00:30:38.940 Jon Glen is leading the efforts in central Nevada and in the 00:30:38.940 --> 00:30:43.882 Salton Trough, and I am leading the efforts elsewhere. 00:30:43.882 --> 00:30:47.299 These data should aid in the characterization of faulting in these 00:30:47.299 --> 00:30:51.241 regions in terms of location and offset. And I just point out that there are 00:30:51.241 --> 00:30:55.779 even more surveys in the pipeline due to a huge influx of funding. 00:30:55.779 --> 00:30:59.281 So potential for incredible data sets for students, particularly in the 00:30:59.281 --> 00:31:05.009 Mojave Desert and in the Great Basin. So talk to me or Jonathan Glen for 00:31:05.009 --> 00:31:09.671 additional details. And thank you so much for your attention and time.