WEBVTT 00:00:04.000 --> 00:00:13.000 Hello, everyone. We have inadverntly started a tradition of having a large earthquake at the Mendocino Triple Junction the month before this workshop. 00:00:13.000 --> 00:00:18.000 I don't know how he did that. We probably should stop. 00:00:18.000 --> 00:00:21.000 But we have done so, and so we must have late-breaking talks. 00:00:21.000 --> 00:00:31.000 And so let us begin an onslaught of new and late-breaking science Lori Dangler, please get us started. 00:00:31.000 --> 00:00:42.000 Great. Thank you. And here is a quick introduction to the setting of the last earthquake. Hopefully there won't be one during my talk. 00:00:42.000 --> 00:01:03.000 Here we're looking at the earthquake sequence in white, the mainshock and the aftershocks, superimposed on earthquakes historic earthquakes going back to 1980, and you can see this is a highly seismic region. 00:01:03.000 --> 00:01:13.000 This zone in orange, fuzzy orange you can kind of see is the Mendocino Triple Junction where the San Andreas fault, the Mendocino fault. 00:01:13.000 --> 00:01:20.000 and the Cascadia Subduction Zone meet. The December 5th earthquake was on the Mendocino fault. 00:01:20.000 --> 00:01:40.000 The right-lateral transform between the Gorda and Pacific plates. Note that seismicity abruptly ends at the Gorda Ridge You can see the great scar of a former transform fault further west about 3,600 kilometers further west. We call this the fracture zone 00:01:40.000 --> 00:01:47.000 It plays no role in current tectonics. 00:01:47.000 --> 00:01:53.000 Here are the basics of what happened on December 5th, the earthquake at 10.44 a.m. 00:01:53.000 --> 00:02:07.000 I felt it. The wireless emergency alert, my ShakeAlert came out 15 seconds later, I got it. In fact, I got it at about the same time I felt the earthquake. 00:02:07.000 --> 00:02:14.000 The National Tsunami Warning Center issued its first bulletin five minutes later. 00:02:14.000 --> 00:02:23.000 Estimating a magnitude of 7.3 and a tsunami warning, and I'll talk about the area in just a minute. 00:02:23.000 --> 00:02:35.000 Bulletin number two came out 35 minutes later. The only change was revising the magnitude to 7.0. The warning remained in place. 00:02:35.000 --> 00:02:41.000 And finally, at 11:54, the warning was canceled. 00:02:41.000 --> 00:02:48.000 Note that the cancellation and the second bulletin do not go out via WEA. 00:02:48.000 --> 00:02:57.000 So why was there a warning? And no, I realize you can't read this, but you can go to tsunami.gov if you want to see all the details. 00:02:57.000 --> 00:03:03.000 The important aspects of this slide, and it's the template that the Tsunami Warning Center has used. 00:03:03.000 --> 00:03:12.000 Whenever an earthquake occurs anywhere in the Pacific or actually in the Caribbean or East Coast as well. 00:03:12.000 --> 00:03:21.000 The magnitude increases from top to bottom. So you can see where the 7.3 is. 00:03:21.000 --> 00:03:27.000 And the operational areas extend across the top. I only want you to look at West Coast. 00:03:27.000 --> 00:03:42.000 And if you go to where 7.3 and west coast intersect you can read the fine print and then find out that if an earthquake of 7.1 or larger occurs. 00:03:42.000 --> 00:03:55.000 all of the areas within 250 kilometers will get put into a tsunami warning. What does that actually look like? 00:03:55.000 --> 00:04:02.000 Well, here is a 250 kilometer circle around that preliminary epicenter. 00:04:02.000 --> 00:04:13.000 And you notice it extends just north of the California-Oregon border And to about Fort Ross in Sonoma County. 00:04:13.000 --> 00:04:23.000 However, the warning area was much bigger and that's because the National Weather Service uses breakpoints to issue alerts. 00:04:23.000 --> 00:04:35.000 Everything between a breakpoint is going to be, you can't have nuances between breakpoints. Everything has to be constant between breakpoints. 00:04:35.000 --> 00:04:49.000 And I've only shown the important ones here, namely, a break point at the Oregon-California border. The next breakpoint further north is just south of Florence. 00:04:49.000 --> 00:04:57.000 As a result, everything all the way up to Florence has to be placed into a tsunami warning. 00:04:57.000 --> 00:05:05.000 Similarly, because similarly because the 250 kilometer zone extends into Sonoma County. 00:05:05.000 --> 00:05:22.000 The next breakpoint is all the way near Santa Cruz. So as a result, everything from roughly Santa Cruz to just south of Florence, including the entire interior of San Francisco Bay gets put into a warning. 00:05:22.000 --> 00:05:35.000 Now, to make things even a little more complicated. The tsunami warning centers don't use the state tsunami hazard maps to issue their tsunami warning. 00:05:35.000 --> 00:05:49.000 They use the National Weather Service weather advisories. And as a result, it looks a whole lot larger than what it actually is. 00:05:49.000 --> 00:06:03.000 Here's the WEA alerts that I got in orange. We got the ShakeAlerts. In red, it shows everyone in the counties got the WEA alert that looked like this. 00:06:03.000 --> 00:06:10.000 You are in danger. Even if you were 50 miles away from the coast. 00:06:10.000 --> 00:06:32.000 This is what you saw. And there was, in fact, a tsunami it was about nine centimeters high as recorded at arena cove and I will Stop right there. 00:06:32.000 --> 00:06:39.000 Great. Hi, everyone. My name is Volkan Seven, I'm co-founder and chief technology officer of Temblor. 00:06:39.000 --> 00:06:59.000 At Temblor, we published a peer-reviewed article about this earthquake and what you are seeing here is the red circles are the earthquakes after the magnitude 7 mainshock, and the question that we had was whether or not this magnitude 7 earthquake 00:06:59.000 --> 00:07:06.000 triggered aftershocks on the San Andreas fault which is seen here as blue line. 00:07:06.000 --> 00:07:27.000 This area is very well known for having many earthquakes and one of the place perhaps for the rupture to occur would be the gap between 1983 magnitude 6.1 earthquake and 1994 magnitude 7 00:07:27.000 --> 00:07:48.000 earthquake. We looked at the seven days preceding of this earthquake and we've seen essentially 2 foreshocks, 7 hours before the mainshock. Besides those 2 foreshocks 00:07:48.000 --> 00:08:09.000 the Mendocino fault was almost completely clear of earthquakes. Seven days after the mainshock what you are basically seeing the lower panel, and what you are seeing is the Mendocino fault is completely illuminated by the aftershocks and some of the aftershocks are leaking towards the 00:08:09.000 --> 00:08:21.000 tip of San Andreas or faults parallel to San Andreas as well as some some activity in the inland. 00:08:21.000 --> 00:08:31.000 In 2021, there was a couple magnitude six earthquakes within 11 seconds from each other. 00:08:31.000 --> 00:08:55.000 And here on panel A, what you are basically seeing the seven days preceding the magnitude 6 doublets, and interestingly enough, there was a magnitude 4 earthquake 5.2 hours before the magnitude 6.0 earthquake and that seems to be the 00:08:55.000 --> 00:09:15.000 Only seismicity occurring around Mendocino fault during that time. And if you look at panel B is the seven days period after the 2021 magnitude 6.0 doublets what you are seeing here is two distinct clusters of earthquakes 00:09:15.000 --> 00:09:33.000 each associated with the magnitude 6.0 shock. The Mendocino fault seems to be producing quite extensive aftershocks for a magnitude 6.0 and that was kind of striking to my eye. 00:09:33.000 --> 00:10:01.000 In 2021, magnitude 6.4 Ferndale earthquake occurred. And here in panel C, what we are basically seeing is the area now a year after the 2021 doublets mostly clear of aftershocks and basically what we are seeing 7 days after the 2021-22 earthquake 00:10:01.000 --> 00:10:07.000 the aftershock pattern goes inland. 00:10:07.000 --> 00:10:29.000 We calculated Coulomb stress changes imparted by the magnitude 7 earthquake and here on the left panel is the receiver fault is the tip of the San Andreas fault and what we are calculating the stress increase for the tip of the San Andreas. 00:10:29.000 --> 00:10:44.000 The panel on the right shows the stress result on the Cascadia megathrust and we do see some stress increases on the Cascadia megathrust as well. 00:10:44.000 --> 00:11:00.000 Another way of calculating column stresses we can resolve them on focal mechanisms here plot on the top left the magnitude 7 earthquake stresses. 00:11:00.000 --> 00:11:19.000 And what you are basically seeing the magnitude 7 earthquake increased stress throughout the Mendocino fault. If you look at the Ferndale earthquake stress changes imparted by the Ferndale quake what we are seeing is the tip of the San Andreas 00:11:19.000 --> 00:11:40.000 see some stress increase, and if you combine these two earthquakes as a source fault then you are getting the Mendocino fault and perhaps false oriented close to San Andreas fault orientation also has some stress increases. 00:11:40.000 --> 00:11:49.000 Thank you. 00:11:49.000 --> 00:11:55.000 [Can I control? I cannot see my, yeah, great. Thank you.] 00:11:55.000 --> 00:12:03.000 Hi, everyone. My name is Bo from UC Berkeley, and today I'm happy to share our findings in Mendocino Triple junction. 00:12:03.000 --> 00:12:19.000 Mendocino Triple Junction is one of the most seismically active region and a complex area in California. And there are several large earthquakes happened in the past few years. 00:12:19.000 --> 00:12:28.000 However, our understanding of the fault structures at depth still remains unclear because of the limited observations. 00:12:28.000 --> 00:12:36.000 To solve this problem, we built a high-resolution machine learning catalog from 2000 to 2023. 00:12:36.000 --> 00:12:45.000 And I will enhance the catalog showing more detailed structure compared with the NCEDC catalog. 00:12:45.000 --> 00:12:53.000 So based on the tectonic settings and the slab geometry, we divide the seismicity into several groups. 00:12:53.000 --> 00:13:05.000 And here we will mainly focus on the seismicity in the lower Gorda plate, the pink group and near the subduction interface, the green groups. 00:13:05.000 --> 00:13:09.000 So let's first look at seismicity in the lower Gorda plate. 00:13:09.000 --> 00:13:23.000 So here we show the fabric features in the gutter plate and from layer to all the maximum principal stress in this region is horizontally, a compression of space from north to the south. 00:13:23.000 --> 00:13:44.000 And we might assume this kind of conjugate fault formed under the stress condition in Mendocino Triple Junction, just like the rock shear failure. However, these angles did not negative friction coefficient, which makes our assumption like almost impossible. So how to answer this question? 00:13:44.000 --> 00:13:50.000 Here we may need to have a broader view of the deformation history of the Gorda plate. 00:13:50.000 --> 00:13:58.000 So from the sea floor bathymetry map, there are several reactivated strike-slip fault inside the Gorda plate. 00:13:58.000 --> 00:14:04.000 And we assume this kind of thought reactivation continue to happen in the Mendocino Triple Junction. 00:14:04.000 --> 00:14:17.000 However, the complex interactions between the Gorda plate and North America plate and also Pacific plate make the deformation process in this region more complex 00:14:17.000 --> 00:14:25.000 and could possibly result in these kind of conjugate faults. Which also requires our further analysis. 00:14:25.000 --> 00:14:37.000 To alter the conjugate faults and then let's move to the seismicity near the subduction interface. It looks quite different with the seismicity in the lower Gorda plate. 00:14:37.000 --> 00:14:46.000 And here we will mainly focus on several earthquake clusters, which might denote the fluid and migration near the subduction interface. 00:14:46.000 --> 00:14:55.000 Here, let's first look at the Fortuna cluster and if the detailed structure of the Fortuna cluster colored by time. 00:14:55.000 --> 00:15:21.000 And at the starting time of this cluster is around the time when the coupling ratio of the subducting interface increased and we also calculate the distance from the earthquake to the initial center and it shows some issues logarithmically-like migration pattern, which possibly indicate the fluid diffusion in these fault channels. 00:15:21.000 --> 00:15:41.000 So beside the Fortuna cluster, we also find several more clusters with also the migration patterns and this kind of clusters could possibly denote that the migration of seismicity is a common phenomenon for the earthquake near the subduction interface. 00:15:41.000 --> 00:15:52.000 And further, this kind of observation could indicate the rich content of fluid in the subduction interface. 00:15:52.000 --> 00:15:58.000 Beside that, we also look at the timing of these clusters with large earthquakes. 00:15:58.000 --> 00:16:03.000 And here from A to D, these are the four clusters shown in the right panel. 00:16:03.000 --> 00:16:15.000 And the fifth clusters first when the large earthquakes happen closeby, which could indicate the stress perturbation caused by the large earthquake. 00:16:15.000 --> 00:16:23.000 And possibly or indirectly denote some possibility of the fluid migration near the subduction interface. 00:16:23.000 --> 00:16:32.000 So in conclusion, here we build a high resolution earthquake catalog in Mendocino Triple Junction over the past two decades. 00:16:32.000 --> 00:16:43.000 And the seismicity in the lower Gorda plate and near the subduction interface could possibly denote a very deformed and fluid-rich subducting plate. 00:16:43.000 --> 00:17:05.000 So that's it. And I'm happy to take any questions. Thank you. 00:17:05.000 --> 00:17:21.000 Hi, everyone. I'm Claire Yoon from USGS and I'll be talking about a rapid enhanced relocated aftershock catalog I developed after the magnitude 7 Mendocino earthquake last month and here "rapid" means the catalog is available soon after the mainshock. 00:17:21.000 --> 00:17:34.000 "Enhance" means the catalog is more complete to lower magnitudes and "relocated" means it's got precise relative locations and depth so that you can see the faults. 00:17:34.000 --> 00:17:50.000 So why would we want such an aftershock catalog? Well, it could help situational awareness of earthquake activity to help answer questions like what faults ruptured? Are there new faults becoming active? Are aftershocks moving towards where people live? 00:17:50.000 --> 00:17:57.000 And can help guide post-event instrument deployment decisions, like where to put temporary seismic sensors. 00:17:57.000 --> 00:18:03.000 And such catalogs should be available in real-time, hopefully sometime in the near future. 00:18:03.000 --> 00:18:16.000 But until then, I'm here to fill the gap and so after the magnitude 7.0 earthquake, I developed an aftershock catalog the first version was two days after the mainshock. Second one was 6 days after. 00:18:16.000 --> 00:18:32.000 The third one was 13 days after, which I'll cover today. And each of these catalog updates was done 24 hours behind real- time, but I can speed this up or update it more often if there's a need in the future. 00:18:32.000 --> 00:18:44.000 So the method I used to create this detailed catalog is pretty much the same that is the same method from Yoon and Shelley 2024 in the seismic record, 00:18:44.000 --> 00:19:09.000 which was done following the 2022 magnitude 6.4 Ferndale earthquake. Please see my talk in tomorrow's session on Ferndale for more info on that. But the input to this method is continuous seismic data from a total of 67 nearby stations, and that includes 10 GM network code stations 00:19:09.000 --> 00:19:15.000 that the Northern California Seismic Network field team quickly deployed soon after the mainshock. 00:19:15.000 --> 00:19:23.000 That was a really huge effort and so I want to thank everybody involved in that and I tried to use your news station data as soon as it came in. 00:19:23.000 --> 00:19:32.000 The first step in the workflow was automated event detection and face picking using the EQ transformer deep learning model. 00:19:32.000 --> 00:19:49.000 Then followed by an event association with a PyOcto grid search associator, and this part is actually new because it works a lot faster. The next step was absolute event location using a probabilistic event location method called HypoSVI. 00:19:49.000 --> 00:20:04.000 Then followed by a local magnitude calculation and a final round of false event removal and then the final step was event relocation of the GrowClust relative event location method, 00:20:04.000 --> 00:20:13.000 and all the location stops were done using a 1D velocity model the same one that's used in the Northern California network. 00:20:13.000 --> 00:20:23.000 So just to orient you, here I'm showing the ComCat aftershock catalog and so just to show you on the bottom left, 00:20:23.000 --> 00:20:33.000 the magnitude 7 hypocenters shown by this mechanism offshore, which is partially hidden. And then there's many aftershocks, mostly extending to the east. 00:20:33.000 --> 00:20:51.000 They're colored by depth and you can see they're mostly offshore as well. The triangles are the stations that were used and the pink ones are the new stations that were deployed following the mainshock. And then there's a west-east cross section shown on the top right. 00:20:51.000 --> 00:21:07.000 Here the aftershocks are colored by time where earlier aftershocks are darker and I also plotted the 2021 magnitude 6.0 Petrolia double aftershocks and red, kind of for reference. 00:21:07.000 --> 00:21:28.000 So here ComCat is complete to about magnitude 2.7. So in comparison, here's the enhanced catalog, it's developed before relocation and so, you know, compared to ComCat, you can see there's a lot more earthquakes. It's complete down to about magnitude 1.4. 00:21:28.000 --> 00:21:37.000 And although the locations are not very accurate here, you do see these kind of two zones of aftershocks at different depths. 00:21:37.000 --> 00:22:07.000 There's the kind of the shallower zone, including the magnitude 7.0 hypocenter, and then there's a second deeper zone of aftershocks that about 25 kilometers depth that includes the largest amount to 5.3 aftershock and then following a relocation, this is what the aftershocks look like. And so compared to before relocation, you can see it's easier to see the kind of the two zones at different depths and the aftershocks appear to line up on the 00:22:13.000 --> 00:22:22.000 Mendocino Triple transform zone, but we do end up missing the locations of the larger aftershocks here. 00:22:22.000 --> 00:22:36.000 So yeah, that's all I have for now. There's a lot of things I want to do in the future to improve the catalog. I want to update it to a more recent date and relocate earthquakes with other velocity models and 00:22:36.000 --> 00:22:48.000 interpret what's going on in the context of past seismicity. But if you want to use the enhanced relocated catalog I have so far, please get in touch. I'm happy to work with you. 00:22:48.000 --> 00:23:02.000 Thank you. 00:23:02.000 --> 00:23:21.000 Hey everybody. I'm David Shelley from the USGS and today I want to tell you about some tiny low frequency earthquakes that we've identified that both can illuminate some of what's happening at the southern edge of Cascadia and can show us some of the far-reaching effects of the recent magnitude 7.0 Mendocino earthquake. In fact, they shut off 00:23:21.000 --> 00:23:34.000 for a brief period of time following the earthquake. So many of you are probably familiar with the extensive episodic tremor and slip activity along Cascadia margin. 00:23:34.000 --> 00:23:54.000 However, a few years ago, Aaron Weck published a paper identifying an anomalous zone of tremor that is offset more than 50 kilometers west of the main zone of tremor. And this is the zone I'm going to be talking about today. We've worked to pull out low frequency earthquakes and to locate these 00:23:54.000 --> 00:24:08.000 events more precisely. So these low frequency earthquakes, this is showing a map view and cross-section on the left. The seismicity is color coded by depth and the low frequency earthquake sources are shown as the white outline 00:24:08.000 --> 00:24:26.000 diamonds on the right at the two different 3D views showing some of the geometry. So these low frequency earthquake sources locate between 22 and 29 kilometers depth is a similar depth to subduction zone at the Gorda plate seismicity. 00:24:26.000 --> 00:24:37.000 They're adjacent to a long-lived persistent zone of microseismicity and both the microseismicity and the low frequency earthquakes, if you look at the upper right plot. 00:24:37.000 --> 00:24:43.000 They align on this dipping arcuate structure that's dipping about 50 degrees to the northeast. 00:24:43.000 --> 00:24:58.000 Intriguingly, there's about a 15 kilometer gap if you look at the map view between the southern limit of Gorda slab seismicity and the low frequency earthquakes and microseismicity. 00:24:58.000 --> 00:25:09.000 It turns out this gap is a region of low P-wave velocity. This is a slice through at 27 kilometers depth through the velocity model recently published by Furlong et al., 00:25:09.000 --> 00:25:18.000 and it shows this pronounced low velocity zone separating the high velocity Gorda slab to the north and another high velocity feature to the south 00:25:18.000 --> 00:25:33.000 that may be the pioneer fragment. The pioneer fragment has been proposed to be this partially subducted fragment that is translating northward. It's an old Farallon fragment that has been captured by the Pacific plate 00:25:33.000 --> 00:25:40.000 and maybe translating northward beneath the North American plate. So this is our working hypothesis of the geometry. 00:25:40.000 --> 00:25:43.000 Like many things to the Mendocino Triple Junction is quite complicated, 00:25:43.000 --> 00:26:01.000 but our hypothesis is that this pioneer fragment may essentially be bulldozing subducted sediments subducted on top of the Gorda slab and that these tremor and low frequency earthquakes may be denoting the southern edge of subduction there. 00:26:01.000 --> 00:26:18.000 Okay, so how did the magnitude 7.0 Mendocino earthquake affect low frequency earthquake activity. Now, this zone is quite far from the epicenter of the earthquake about 125 kilometers away. So with these low frequency earthquake sources we can really see some of the subtle 00:26:18.000 --> 00:26:23.000 far-reaching effects of this earthquake that we wouldn't be able to see otherwise. 00:26:23.000 --> 00:26:40.000 So here I'm plotting a cumulative events. This is combined from all 27 families of low frequency earthquakes. On the left is a plot of showing this since the beginning of 2024 on the right is a plot zooming in on the shorter time period since November of 2024. 00:26:40.000 --> 00:26:47.000 So things are going. Activity is relatively continuous leading up to the magnitude 7.0 earthquake, 00:26:47.000 --> 00:26:58.000 and then the earthquake happens and the low frequency earthquakes shut off almost entirely for a period of a couple of weeks before resuming their activity. 00:26:58.000 --> 00:27:23.000 Now, intriguingly this shutoff is not spatially homogeneous. This is a plot of low frequency earthquake activity, time increasing upward versus longitude and we can see that the time the shutoff is relatively short it served as 8 days on the western end and relatively long up to 24 days on the eastern end. 00:27:23.000 --> 00:27:43.000 So why might this be happening? What are we learning here? Well, the closest analog we have is the Static stress shadow from the San Simeon earthquake 2003 magnitude 6.5 San Simeon earthquake causes stress shadow that shut off low frequency earthquake activity on the lower part. 00:27:43.000 --> 00:27:50.000 The lower extension of the San Andreas Fault for a period of about 30 days and that shutoff region corresponded very well 00:27:50.000 --> 00:28:08.000 with the zone of decreased Coulomb stress. So a similar thing may be happening in this case. I've shown the zone of low frequency earthquakes superimposed on the stress, the Coulomb stress modeling plots by Sevilgen et al. 00:28:08.000 --> 00:28:25.000 That was shown just a couple of talks ago. And so the Coulomb stress transfer depends on the fault orientation. We're still working to model the effects on fault location and orientation that actually accurately represent the low frequency earthquake fault, so 00:28:25.000 --> 00:28:37.000 stay tuned on that. But this really does show the far-reaching effects of this earthquake, likely we're seeing a brief cessation of subduction related creep that we really can't see any other way. 00:28:37.000 --> 00:28:48.000 Thanks very much. 00:28:48.000 --> 00:29:14.000 Hello, everyone. So my name is Hao Guo from University of Wisconsin-Madison. So this talk has knowledge about magnitude 7.0 Mendocino earthquake by [indiscernible] earthquake two years ago in the 2022 December magnitude of 6.4 Ferndale earthquake sequence which occurred within the crust of the subjected Gorda plate. So this talk is about some evidence 00:29:14.000 --> 00:29:24.000 from this earthquake sequence for understanding the stress level near the plate interface. 00:29:24.000 --> 00:29:44.000 So this map shows the megathrust stress, the locking stage in southern Cascadia from Materna et al., 2023. So the red color means high locking, so you can see the southern Cascadia megathrust is highly locked at less than 20 kilometer depth. 00:29:44.000 --> 00:30:01.000 So here's the locked zone. So one question to ask is what's the stress date in locked zone? So this is important because if this region can generate high stress events, then this ground motion will be stronger. 00:30:01.000 --> 00:30:11.000 So some previous studies have already investigated this issue so this is on my low-course grade tomography study 00:30:11.000 --> 00:30:30.000 published a couple of years ago so the the image shows the vertical cross-section of the Vp/Vs ratio model along the deep direction of the conducting plates. So the red color means high Vp/Vs ratio, blue color means low BPVS ratio. 00:30:30.000 --> 00:30:49.000 So, and the two solid lines black lines, outlines approximate location of the slab crust. So you can see the model shows the high Vp/Vs ratio values are near the interface in the slab crust. 00:30:49.000 --> 00:31:06.000 So those values are about 1.85, which are higher than the typical values at these steps, so we think those high Vp/Vs ratio values are indicative of high foot pressure in a slab crust. 00:31:06.000 --> 00:31:17.000 So also shown in this figure are the principal stress orientations or within a step and within the overlying crust. 00:31:17.000 --> 00:31:30.000 So you can see they are very different. So the decoupled stress fields of the slab and the overlying continental crust means that the megathrust faults in between must be very weak. 00:31:30.000 --> 00:31:50.000 So also shown in this figure are the histograms of the stress shock values for the earthquakes in the mantle and for the earthquakes in the slab crust so we can see the earthquakes in the slab crust have generally 00:31:50.000 --> 00:32:11.000 lower stress slab values than earthquakes in the slab mantle. So these studies are combined to suggest a low effective stress level in a slab crust which is probably likely due to the high fluid pressure. 00:32:11.000 --> 00:32:17.000 So the 2022 Ferndale earthquake occurred on the strike-slip fault at the top of the Gorda slab 00:32:17.000 --> 00:32:29.000 in the lower portion of the locked zone, so providing a very good opportunity to understand the stress level in this critical region. 00:32:29.000 --> 00:32:46.000 So the figure on the left shows the vertical cross-section of the Vp/Vs ratio model and also the aftershock locations with the 3D velocity model along the faults and the figure on the right shows the vertical cross-section of the coseismic slip model 00:32:46.000 --> 00:33:07.000 from Guo and Zhang et al. So the high-precision event locations and the coseismic model shows that the mainshock and also the high coseismic slip are content using the slab crust that has high Vp/Vs ratio values. 00:33:07.000 --> 00:33:22.000 So given the stress regime in the slab, so we can infer the ideal orientations of the strike-slip on conjugated faults, which are shown by the two white lines on the map. 00:33:22.000 --> 00:33:38.000 So the 2022 Ferndale mainshock fault is about 40 degree off from the ideal orientation, which means the mainshock fault is misoriented. 00:33:38.000 --> 00:33:48.000 So we also calculated the stressor values for the magnitude 4.0 to 5.0 events, including some abstracts in the slab crust and in the slab mantle. 00:33:48.000 --> 00:34:01.000 So as you can see in this histogram the events in a slab crust have generally lower stressor values than in the experimental. 00:34:01.000 --> 00:34:13.000 So, combining these results support a low effect of stress level near the plate interface in the lower portion of the locked zone in southern Cascadia, which is likely due to elevated flow pressure. 00:34:13.000 --> 00:34:41.000 So this study has been studying published last year in SRL. So if you're interested you can check it out. Thank you. 00:34:41.000 --> 00:34:49.000 Yeah, good afternoon, everyone. My name is Bob McPherson. Thanks, Sarah, for holding this workshop. 00:34:49.000 --> 00:35:13.000 Today I'd like to talk about the magnitude 7.0 2024. And it's the last part of a sequence of three earthquakes that more or less formed a triangular within the Gorda plate triangular piece and that piece moved under North America. So that's the story. 00:35:13.000 --> 00:35:19.000 And so my talk outline is I'm going to put that into a tectonic setting. 00:35:19.000 --> 00:35:30.000 Show how the piece was formed the strangler piece and how the movement of it under North America help define the megathrust. 00:35:30.000 --> 00:35:42.000 Before the earthquake I was looking out this window here, shown in this photo that there was a king tide, a very high tide. 00:35:42.000 --> 00:35:51.000 And it took me an hour to look back out the window after the earthquake and I noticed that the Bay had retreated 00:35:51.000 --> 00:35:57.000 way off in the distance exposing this convex 00:35:57.000 --> 00:36:07.000 sediment pattern from Jacoby Creek, this deltaic pattern and then within 15 minutes 00:36:07.000 --> 00:36:14.000 the water started coming back and when it came back as shown in this little box. 00:36:14.000 --> 00:36:25.000 You can see that there were ripples and waves. So, and this continued going in and out for hours and I have pictures of that. 00:36:25.000 --> 00:36:39.000 And so the question is, what caused that? And I think the rupture offshore and maybe slumping is probably the culprit, but that's to be determined. 00:36:39.000 --> 00:36:53.000 Alright, the tectonic setting for the magnitude 7.0 earthquake is here. You have the Mendocino fault, a vertical barrier between Gorda to the north, Pacific to the south, very aseismic strong plate to the south. 00:36:53.000 --> 00:37:04.000 And then it intersects with the San Andreas, which is followed very clearly offshore, coming onshore at Shelter Cove, forming a corner. 00:37:04.000 --> 00:37:09.000 This corner is going to be very important in understanding what goes on. 00:37:09.000 --> 00:37:15.000 So if we put the epicenter at which occurred on the Mendocino fault, again, a vertical barrier. 00:37:15.000 --> 00:37:29.000 Unyielding barrier and we put some slip information 40 kilometers of slip, maybe as much as 5 meters of slip coming to the surface. 00:37:29.000 --> 00:37:47.000 And that's Doug Dreger's fault study. And we actually compare that with a very interesting talk given by Jimmy Adderholt using the Gordas optic experiment 00:37:47.000 --> 00:38:03.000 as a beam. And what he saw using the optic fiber as a beam was the source of energy moved a little bit to the west and then moved 40 kilometers to the east, which is very consistent with what we see. 00:38:03.000 --> 00:38:10.000 A very good use of the optic fiber I'm excited about the future of that experiment. 00:38:10.000 --> 00:38:29.000 Now the corner is important. Again, the intersection of Mendocino fault and the San Andreas fault in that it divides the Gorda plate into two different regions as shown on this hill-shaded fault map provided by Jay Patton. 00:38:29.000 --> 00:38:37.000 So over in the west here, you're being pinched by the convergence of the Juan de Fuca 00:38:37.000 --> 00:38:48.000 pinching the Gorda plate against the Mendocino fault and this has been going on a long time and it's dominated by north-south compression with these kinds of mechanisms. 00:38:48.000 --> 00:39:04.000 And these size earthquakes in response to that north-south compression contrasting to what's occurring over here, again, Intraslab sigma 1 is vertical in this case. This is sigma 3 00:39:04.000 --> 00:39:19.000 showing east-west tension or lease compression and these are the resultant mechanisms and the earthquakes are smaller, never getting larger than magnitude 5. 00:39:19.000 --> 00:39:29.000 So very different regions of the Gorda plate and so what happened in the sequences all occurred in this region here. 00:39:29.000 --> 00:39:53.000 And again, Jay provided me with this slide where he superimposed Doug Wilson's magnetic anomaly information on top of the the fabric shown by the hillshade and it's pretty apparent that the fabric and the magnetic anomalies are the same 00:39:53.000 --> 00:40:03.000 and they're distorted because of the compression and the shearing going on by the convergence of Juan de Luca with the Mendocino fault. 00:40:03.000 --> 00:40:10.000 And all of this embedded fabric was inherited when it formed at the ridge. 00:40:10.000 --> 00:40:25.000 And so I drew these white lines showing that these earthquakes here have been occurring along these embedded weaknesses as the 2022 and 1975 repeating earthquake pair. 00:40:25.000 --> 00:40:32.000 And so just to again reiterate that information here's the magnitude 7.2 00:40:32.000 --> 00:40:40.000 1980 earthquake ruptured all the way to Mendocino fault the response was a magnitude 7.0 along the Mendocino fault. 00:40:40.000 --> 00:40:57.000 And it's very similar to, again, the repeating pair here of the 2022-75 and they point directly to the magnitude 7.0 which is the topic of this session. And so if I zoom into that box , 00:40:57.000 --> 00:41:04.000 here's a cartoon Lori and I made for her article in the Times Standard. 00:41:04.000 --> 00:41:10.000 And again, the story in the limited time, it starts offshore in the Mendocino fault 00:41:10.000 --> 00:41:16.000 11 seconds later, within the Gorda plate down dip, there was a response 00:41:16.000 --> 00:41:42.000 and this sequence, a year later, its response was the 6.4 that ruptured through the transition zone into the down slab tension region as evidenced by some mechanisms north-south mechanisms published by Dave Shelley et al. in his 2024 paper, and Clara Yoon and Shelley in another paper. 00:41:42.000 --> 00:41:52.000 And then that earthquake its response, as I showed earlier, pointing to the Mendocino fault was the 7.0 00:41:52.000 --> 00:42:03.000 that ruptured unilaterally more or less to the east. And if we look at relocations by Anthony Lomax 00:42:03.000 --> 00:42:18.000 using this algorithm here. You can see here's offshore and you can see that it very clearly was along the Mendocino fault as it goes just south of the [indiscernible] under North America. 00:42:18.000 --> 00:42:41.000 And out here, the depths are questionable, but he gets 20 kilometers depth for the mainshock and this pattern. And the importance of this pattern is that the aftershocks go all the way pass what we think is the top of the Gorda plate up to this 00:42:41.000 --> 00:42:49.000 surface that the 1992 earthquake ruptured on, which is important and that projects to the trench. 00:42:49.000 --> 00:43:03.000 The other point that I want to make in this slide is this is Doug Dreger's preliminary fault model and notice that there's 5 meters of slip potential. 00:43:03.000 --> 00:43:13.000 That ruptures to the surface in his estimation. And so if I take Anthony's aftershocks and Doug's slip pattern 00:43:13.000 --> 00:43:30.000 and put them into my cartoony I can make the point from that data that the aftershocks go all the way up to this slip plane. By the way, this was a reverse fault that lifted the coast 00:43:30.000 --> 00:43:48.000 and this block went out to the ocean. And the projection of that plane goes to the trench and so if you start at the trench and follow a reverse fault under North America, it seems to me that that's the definition 00:43:48.000 --> 00:44:06.000 Of a mega thread. So what I think we're seeing now is we're seeing now the megathrust is this old slip plane and a lot of the details are yet to be determined what's going on. 00:44:06.000 --> 00:44:18.000 So in summary, without reading all of these points. I believe this magnitude 7.0 earthquake has shed light on where the megathrust is 00:44:18.000 --> 00:44:24.000 and the megathrust seems to be the 1992 slip. 00:44:24.000 --> 00:44:34.000 Thank you very much. 00:44:34.000 --> 00:44:39.000 Okay, that was wonderful. That was half of Jay and Silent Bob. Thank you. 00:44:39.000 --> 00:44:48.000 So we all, as always, running way behind on time. Are there any really fast questions ready for or take. 00:44:48.000 --> 00:44:55.000 One question. Who wants to ask a question? 00:44:55.000 --> 00:45:01.000 No one. Anyone? 00:45:01.000 --> 00:45:06.000 [indiscernible]. 00:45:06.000 --> 00:45:18.000 Okay, then. Then let's get ready to go on to our next session because there was so much, no wait, hold on. We have another minute before we all break time. 00:45:18.000 --> 00:45:26.000 We do have a break so that people can reset.