WEBVTT Kind: captions Language: en-US 00:00:01.280 --> 00:00:04.320 Yeah, thanks for showing up this morning. 00:00:04.320 --> 00:00:06.800 And my name is Graham Kent. I’m director of the Nevada 00:00:06.800 --> 00:00:09.440 Seismological Laboratory at the University of Nevada-Reno. 00:00:09.440 --> 00:00:12.800 And I was asked to give a talk about Lake Tahoe faults and 00:00:12.800 --> 00:00:17.016 earthquake swarms. And this picture right here 00:00:17.040 --> 00:00:19.600 kind of shows some of the tools that we’ve been using in 00:00:19.600 --> 00:00:22.480 the Lake Tahoe Basin. Some multibeam bathymetry 00:00:22.480 --> 00:00:25.736 of Fallen Leaf Lake at the south end of the basin. 00:00:25.760 --> 00:00:31.440 And then surrounding that is a 2011 Lidar survey highlighting 00:00:31.440 --> 00:00:36.045 the West Tahoe Fault cutting in front of Mount Tallac. 00:00:36.960 --> 00:00:41.816 Again, as I just mentioned, in chasing the history of fault rupture in the basin, 00:00:41.840 --> 00:00:46.560 we’ve used a arsenal of tools ranging from CHIRP profiling, Lidar, 00:00:46.560 --> 00:00:52.000 side-scan sonar, piston coring, multibeam sonar, seismic network, 00:00:52.000 --> 00:00:55.576 trenching – really all the various bits and pieces. 00:00:55.600 --> 00:00:59.416 And it should also be said that Tahoe was one of the first places to really 00:00:59.440 --> 00:01:03.840 kind of go full in with underwater paleoseismology, and that was really 00:01:03.840 --> 00:01:09.400 kind of, if not perfected, it was certainly a major step in that direction. 00:01:12.320 --> 00:01:16.560 For many of you, hopefully you’ve seen at least this map the USGS 00:01:16.560 --> 00:01:20.720 and the University of New Hampshire made of Lake Tahoe. 00:01:20.720 --> 00:01:24.080 And just going to use this right now to kind of highlight, there are 00:01:24.080 --> 00:01:27.896 three major fault zones. On the west is West Tahoe Fault. 00:01:27.920 --> 00:01:31.120 In the center lake, stepping over to the right, is the 00:01:31.120 --> 00:01:35.360 Stateline-North Tahoe Fault Zone. Then it steps into the Incline Village 00:01:35.360 --> 00:01:39.920 Fault that cuts through the village of Incline. 00:01:39.920 --> 00:01:43.680 And also there’s two more steps into Little Valley and the 00:01:43.680 --> 00:01:48.216 Mount Rose fan fault system. So, again, it’s really five step-overs. 00:01:48.240 --> 00:01:51.416 Three of them are within the Lake Tahoe Basin proper. 00:01:51.440 --> 00:01:57.440 And one of the things that we first noticed when we all saw this multibeam 00:01:57.440 --> 00:02:05.440 bathymetry map in the winter of 1998, I believe, at AGU when we used to 00:02:05.440 --> 00:02:09.760 go in person, of course we saw some lineations that obviously looked like 00:02:09.760 --> 00:02:13.760 fault zones, but we also noticed this debris avalanche coming out 00:02:13.760 --> 00:02:16.160 of McKinney Bay, really forming McKinney Bay. 00:02:16.160 --> 00:02:20.160 And that was quite stunning, actually. And then, if you look a little more 00:02:20.160 --> 00:02:25.816 carefully, you’ll see a submerged shoreline surrounding the lake. 00:02:25.840 --> 00:02:29.120 And what’s interesting about that is it’s not all at the same level. 00:02:29.120 --> 00:02:35.840 There’s as much as about 15 meters of offset from the, let’s see, 00:02:35.840 --> 00:02:38.240 southwest to the northeast side of the lake. 00:02:38.240 --> 00:02:42.800 So the northeast side sees offsets from three different fault zones 00:02:42.800 --> 00:02:46.320 over by Sand Harbor. Whereas, let’s say Rubicon Lake 00:02:46.320 --> 00:02:49.280 up on the footwall obviously doesn’t see any at all. 00:02:49.280 --> 00:02:56.047 So, again, about 15 meters of offset, and of course we’d like to date that surface. 00:02:56.720 --> 00:03:01.656 Now, I’d be lying to you if I [chuckles] told you this is how we actually did it. 00:03:01.680 --> 00:03:07.440 But almost after all of the marine work was done – underwater work was done, 00:03:07.440 --> 00:03:11.440 we finally worked with the Tahoe Regional Planning Agency and hired 00:03:11.440 --> 00:03:17.280 Watershed Sciences to shoot a Lidar survey of the entire basin. 00:03:17.280 --> 00:03:22.936 And this is a little screenshot from the Echo Summit area, 00:03:22.960 --> 00:03:26.800 crossing Highway 50. You can see the lineation that is the – really almost 00:03:26.800 --> 00:03:33.040 the southern extent of the West Tahoe Fault. And Gordon Seitz actually did 00:03:33.040 --> 00:03:39.748 a trench within this screen grab of the West Tahoe Fault at that location. 00:03:40.400 --> 00:03:44.880 But we had to actually figure most of it out before we had this, we spent 00:03:44.880 --> 00:03:49.440 nearly a decade-plus, before we got the on-land portion. 00:03:50.400 --> 00:03:56.160 But, once we went underwater, we started working with sub-bottom 00:03:56.160 --> 00:04:01.200 CHIRP profiling, and here’s an example on the top of the screen. 00:04:01.200 --> 00:04:05.280 It’s a paleo shoreline, 11 meters off of Rubicon, and you can see 00:04:05.280 --> 00:04:10.160 the sediments in the deformed footwall being truncated off. 00:04:10.160 --> 00:04:13.520 And also you can see, on an ROV photo, of what that 00:04:13.520 --> 00:04:18.240 platform looks like at about 30 feet below the surface of the lake. 00:04:18.240 --> 00:04:24.296 And below that, across the lake, we can see some oblique clinoforms 00:04:24.320 --> 00:04:28.800 crossing across the surface. And this provides an opportunity, 00:04:28.800 --> 00:04:35.336 as you can see, from that inset of doing some vibracoring and actually trying to 00:04:35.360 --> 00:04:40.800 age date that surface, which we were able to do at about 19,000 years, 00:04:40.800 --> 00:04:44.560 plus or minus. So right there, that gives us a sense of how we 00:04:44.560 --> 00:04:50.536 can divide 10 meters of offset across about 20,000 years. 00:04:50.560 --> 00:04:54.320 Now, of course, a lot of that offset depends on the fault geometry, 00:04:54.320 --> 00:04:57.976 but again, these are just kind of rough numbers. 00:04:58.000 --> 00:05:02.880 The great thing about the sub-bottom profiler is our ability 00:05:02.880 --> 00:05:06.480 to really see almost at the decimeter scale of resolution. 00:05:06.480 --> 00:05:10.160 And this is a picture in Fallen Leaf Lake again, south of Lake Tahoe, 00:05:10.160 --> 00:05:14.240 but within the Tahoe Basin, looking at a couple strands of the West Tahoe 00:05:14.240 --> 00:05:18.880 Fault and actually being able to see – again, what we’ll find to be really 00:05:18.880 --> 00:05:24.320 important are slides and turbidites that help us potentially date the age 00:05:24.320 --> 00:05:28.056 of the various large events in the lake. 00:05:28.080 --> 00:05:31.840 And, more conventionally, this is a trench that we did 00:05:31.840 --> 00:05:37.280 with Gordon Seitz over at CGS. This is the Incline Village Fault Zone. 00:05:37.280 --> 00:05:40.240 You can see a beautiful colluvial wedge that’s kind of fallen out 00:05:40.240 --> 00:05:47.280 of the actual side of the wall. And we were able to date the 00:05:47.280 --> 00:05:50.960 most recent event at about 600 years before present. 00:05:50.960 --> 00:05:56.216 And then, prior to that, it’s many tens of thousands of years before the last event. 00:05:56.240 --> 00:06:03.840 And, again, I alluded to it, but we were also going out and collecting cores 00:06:03.840 --> 00:06:08.616 within Lake Tahoe to try to map out these debris flows or turbidites. 00:06:08.640 --> 00:06:13.600 And this is an example of just one core that we collected in the middle 00:06:13.600 --> 00:06:17.680 of the lake by the Stateline Fault. And, again, if we piece them all 00:06:17.680 --> 00:06:22.720 together – and this was work done with Bob Karlin and his student, 00:06:22.720 --> 00:06:29.816 we see quite a rich history of turbidites and debris flows that may have been 00:06:29.840 --> 00:06:33.200 triggered by shaking events. Doesn’t mean they have to be shaken 00:06:33.200 --> 00:06:39.440 events in the Lake Tahoe Basin, but certainly enough ground motion to 00:06:39.440 --> 00:06:43.920 actually cause these events to occur. And not all events are the same size. 00:06:43.920 --> 00:06:47.760 There were some very large events, and then there’s other ones 00:06:47.760 --> 00:06:51.888 that are not continuous across the entire lake. 00:06:52.880 --> 00:06:56.080 When you finally piece it together – and we’ll go through this in a little more 00:06:56.080 --> 00:07:01.600 detail, but what you basically find out is that the West Tahoe Fault, 00:07:01.600 --> 00:07:07.280 or West Tahoe-Dollar Point Fault, you can see there in orange, has a 00:07:07.280 --> 00:07:12.536 slip rate of somewhere between about 0.4 and 0.8 millimeters per year. 00:07:12.560 --> 00:07:16.720 And we can look at the most recent events, depending on where we are 00:07:16.720 --> 00:07:21.096 along the fault line, somewhere about 4,000 to 5,000 years ago. 00:07:21.120 --> 00:07:23.680 The Stateline-North Tahoe Fault is very interesting because we 00:07:23.680 --> 00:07:30.800 don’t really have a good way of dating the most recent events given its location 00:07:30.800 --> 00:07:36.456 in the lake and how it doesn’t really come on shore in any major sense. 00:07:36.480 --> 00:07:43.760 What we do know, though, is that it is providing offset that we see on the 00:07:43.760 --> 00:07:49.280 submerged shoreline, and also in the sense of, if there are large events like 00:07:49.280 --> 00:07:53.760 you’d expect on the West Tahoe-Dollar Point Fault, it’s not like we’re seeing 00:07:53.760 --> 00:07:58.720 a separate set of turbidites, necessarily, that are related to both different events. 00:07:58.720 --> 00:08:03.816 So there’s some question as to synchronicity and whether we’re really 00:08:03.840 --> 00:08:10.240 burying two events in one slide, or at least there’s not enough period of time 00:08:10.240 --> 00:08:15.416 between the two events where that – it would look like one continuous slide. 00:08:15.440 --> 00:08:19.440 But that’s where we are right now. And, again, the slowest slipping of 00:08:19.440 --> 00:08:22.560 the three would be the Incline Village Fault. 00:08:22.560 --> 00:08:31.983 And, again, it’s had a recent event at about 660 years, plus or minus. 00:08:32.720 --> 00:08:37.736 This is an example of why the sub-bottom profiler is really useful. 00:08:37.760 --> 00:08:41.920 And this is over at Fallen Leaf Lake again. 00:08:41.920 --> 00:08:47.920 And we’re looking at these various debris flows or turbidites that we are 00:08:47.920 --> 00:08:53.600 able to not only see seismically, but then we can put a core in and 00:08:53.600 --> 00:08:56.880 actually age-date and bound those events. 00:08:56.880 --> 00:09:00.480 So, again, this has been done, obviously, in many areas around the 00:09:00.480 --> 00:09:07.576 world, but most notably in the Pacific Northwest along Cascadia. 00:09:07.600 --> 00:09:12.320 And Chris has done a great job doing that, and it seems to work here as well. 00:09:12.320 --> 00:09:15.651 We all started about the same time. 00:09:16.800 --> 00:09:22.560 Other things like measuring the offset of the McKinney Bay debris complex, 00:09:22.560 --> 00:09:27.200 right? So, again, we’ve not penetrated the entire sediment stack, 00:09:27.200 --> 00:09:32.240 so any extrapolation might be dangerous, but if one were to 00:09:32.240 --> 00:09:39.907 extrapolate, the McKinney Bay debris complex is probably on order 00:09:39.947 --> 00:09:45.976 60,000 years ago. And so at least we have some constraints. 00:09:46.000 --> 00:09:51.920 So, in this next set, we’re going to go through and essentially look at various 00:09:51.920 --> 00:09:59.040 lines of evidence based on turbidites or seismic CHIRP-related constraints. 00:09:59.040 --> 00:10:03.736 And what we want to do here is kind of fill in for all these various segments. 00:10:03.760 --> 00:10:08.800 And first looking at Lake Tahoe with all the turbidites, for example, 00:10:08.800 --> 00:10:14.056 and then filling in with Fallen Leaf Lake and now with Emerald Bay. 00:10:14.080 --> 00:10:16.880 And, last but not least, Cascade Lake. 00:10:16.880 --> 00:10:20.240 And the last one there is assuming the sedimentation rate that’s similar 00:10:20.240 --> 00:10:22.856 to Fallen Leaf Lake, which is probably a pretty good bet. 00:10:22.880 --> 00:10:26.880 And, again, what you start to see are some events start to line up very nicely, 00:10:26.880 --> 00:10:33.040 especially the relatively large event around – in terms of its expression 00:10:33.040 --> 00:10:37.416 in turbidites or debris flows, around 7,800 years. 00:10:37.440 --> 00:10:45.410 And so now we’re going to go over here next, and ... 00:10:47.990 --> 00:10:51.520 And we’re going to look at, again, some of the evidence 00:10:51.520 --> 00:10:53.520 as we look at the fault trace. 00:10:53.520 --> 00:10:55.656 So let’s move forward here. 00:10:55.680 --> 00:11:00.000 And, again, we’re looking at the color-coded events over on the 00:11:00.000 --> 00:11:03.680 left-hand side and the fault trace that they’re associated with. 00:11:03.680 --> 00:11:07.360 Obviously the green is the West Tahoe-Dollar Point Fault here. 00:11:07.360 --> 00:11:11.760 And looking at it in cyan for the cyan event, showing it 00:11:11.760 --> 00:11:14.456 appears to be across the entire basin. 00:11:14.480 --> 00:11:19.176 And then some of these other ones more recently may be constricted to 00:11:19.200 --> 00:11:23.336 the center and central segments in terms of their expression 00:11:23.360 --> 00:11:29.840 in the kind of turbidite record or in offsets of faulted sediments 00:11:29.840 --> 00:11:33.120 within the CHIRP data. And just an aggregate. 00:11:33.120 --> 00:11:39.336 If you look at it, again, the most recent event is sitting around 4,000, 4,500 00:11:39.360 --> 00:11:44.296 years ago for the southernmost segment. And then potentially a little bit later 00:11:44.320 --> 00:11:50.560 for the central segment. One also would notice that, 00:11:50.560 --> 00:11:54.960 no matter how you calculate it, the kind of recurrence interval 00:11:54.960 --> 00:12:00.616 across three events seems to be a smaller number than actually 00:12:00.640 --> 00:12:06.056 what we see in terms of the last event. So one might say that this system 00:12:06.080 --> 00:12:10.696 is reaching the end of its period, however long that is. 00:12:10.720 --> 00:12:12.834 Now, one of [audio cuts out] … 00:12:13.280 --> 00:12:18.480 So if we want to skip up north or kind of to the northwest and head over 00:12:18.480 --> 00:12:23.120 towards Donner Lake just outside of Truckee, California, and you’ve had 00:12:23.120 --> 00:12:31.280 a CHIRP profiler, here’s an example of a longitudinal dip line across what 00:12:31.280 --> 00:12:38.800 appears to be two faults that comprise a system at the west end of the lake. 00:12:38.800 --> 00:12:43.016 And you see the offset sediments, and they’re denoted in the CHIRP profile. 00:12:43.040 --> 00:12:49.360 And one of the more interesting things that we saw – again, if we now shoot 00:12:49.360 --> 00:12:56.456 a strike line, and then blow it up, we’ll notice that we see, again, three slides. 00:12:56.480 --> 00:13:01.920 So why that’s interesting is that we tend to see three slides in the 00:13:01.920 --> 00:13:07.120 Lake Tahoe area. And are we getting some level of synchronicity of faulting? 00:13:07.120 --> 00:13:13.840 Because clearly these faults up in the Mohawk Valley fault system are not, 00:13:13.840 --> 00:13:20.536 you know, connected, per se, to, let’s say the West Tahoe Fault nearby. 00:13:20.560 --> 00:13:25.896 But what’s also interesting, although we don’t have actually a sediment core, 00:13:25.920 --> 00:13:29.920 but we kind of know where the bottom of the Holocene is. 00:13:29.920 --> 00:13:33.840 And, if you were to go and do the math, we’re seeing about a millimeter per year 00:13:33.840 --> 00:13:37.280 in all these smaller lakes. So, if you give me that, 00:13:37.280 --> 00:13:42.880 what we’ll find out is that the – not the most recent event, 00:13:42.880 --> 00:13:50.216 but the penultimate and the event prior to that seem to line up with the 00:13:50.240 --> 00:13:57.736 events around 7,800 and around 11,000 years that we see in Lake Tahoe. 00:13:57.760 --> 00:14:03.040 But the more recent event is quite a bit younger, and in that regard, 00:14:03.040 --> 00:14:08.960 it may line up more with an event that we see here that Tom Sawyer 00:14:08.960 --> 00:14:14.560 has for the Mohawk Valley. So, again, this seems to be a potential 00:14:14.560 --> 00:14:18.480 viable way of measuring at least large shaking events. 00:14:18.480 --> 00:14:22.480 It might be hard to put it on a particular fault, but I think our experience in 00:14:22.480 --> 00:14:28.136 Lake Tahoe and the small lakes of Cascade and Fallen Leaf south of that, 00:14:28.160 --> 00:14:32.560 you tend to see at least a footprint. 00:14:32.560 --> 00:14:37.520 So it doesn’t look like every big large shaking event causes slides everywhere. 00:14:37.520 --> 00:14:40.800 And, again, more work could be done on that, but at least for now, 00:14:40.800 --> 00:14:46.080 it does show the utility of doing this. And we actually have some CHIRP data 00:14:46.080 --> 00:14:51.360 from up at Lake Independence – or, Independence Lake, so, again, 00:14:51.360 --> 00:14:56.640 we can extend this to the north. And now, just changing topics 00:14:56.640 --> 00:15:01.256 just a wee bit, I was asked to talk about some of the swarm events. 00:15:01.280 --> 00:15:05.120 And probably the most famous swarm event was in 2004. 00:15:05.120 --> 00:15:09.920 And the reason why it was really interesting is that the events were 00:15:09.920 --> 00:15:16.320 located almost at 30 kilometers’ depth. And so that’s definitely like the 00:15:16.320 --> 00:15:22.296 transition between the lower crust and the Moho lid, or the top of the mantle, 00:15:22.320 --> 00:15:27.520 and if that wasn’t curious enough, about seven years later, we had 00:15:27.520 --> 00:15:34.296 a nearly identical event beneath Sierra Valley, and again, 00:15:34.320 --> 00:15:39.760 a little bit shallower, but not much. And this paper, written by Ken Smith 00:15:39.760 --> 00:15:43.360 and others, kind of highlights these two swarms. 00:15:43.360 --> 00:15:48.720 And, if you just look at the 3D character of where these earthquakes 00:15:48.720 --> 00:15:55.200 are, they’re literally on a plane that fits right through both of them [laughs], 00:15:55.200 --> 00:15:58.640 on strike, you don’t have to change the dip very much, 00:15:58.640 --> 00:16:02.800 and they just line on a perfect line. And so we’re wondering if this – 00:16:02.800 --> 00:16:05.360 or these events have something to do with, like, kind of breaking 00:16:05.360 --> 00:16:12.776 the microplate and the most strongest part of that is right at that 00:16:12.800 --> 00:16:19.440 lower crust-mantle transition zone. And, again, these smaller events 00:16:19.440 --> 00:16:25.120 ultimately trigger larger events in the upper crust on order, in this case, 00:16:25.120 --> 00:16:29.680 you know, high magnitude 4s, like 4.7 in the Sierra Valley case. 00:16:29.680 --> 00:16:35.840 So, again, they’re very curious and rare type events. But we had two of them, 00:16:35.840 --> 00:16:40.824 and they really connect very well, even though they’re quite a distance apart. 00:16:42.000 --> 00:16:47.200 Last but not least, we’ve been getting some events of recent – certainly during 00:16:47.200 --> 00:16:50.960 the pandemic period, so when everything’s even a bit more interesting, 00:16:50.960 --> 00:16:54.240 but yeah, we were getting events out in Lake Tahoe. 00:16:54.240 --> 00:17:02.480 And that’s always of concern because there’s the potential of a tsunami if 00:17:02.480 --> 00:17:06.320 we trigger a large normal event. These happened to be left-lateral 00:17:06.320 --> 00:17:10.880 strike-slip events that are probably interconnecting two of the fault zones 00:17:10.880 --> 00:17:14.560 at depth. But still, if you’re playing around at the ends of faults, 00:17:14.560 --> 00:17:18.960 that always makes you nervous because what a way to finish off 00:17:18.960 --> 00:17:23.680 the pandemic then to have a tsunami in Lake Tahoe due to a normal faulting 00:17:23.680 --> 00:17:26.536 earthquake that gets triggered by a strike-slip event. 00:17:26.560 --> 00:17:30.560 So, again, this caused a lot of stir. It was in the New York Times. 00:17:30.560 --> 00:17:35.176 Slow news day, but nonetheless. So we’re always keeping an eye 00:17:35.200 --> 00:17:40.456 on these events in Lake Tahoe because of the consequences of water. 00:17:40.480 --> 00:17:46.480 And, again, one of the things that Steve Ward showed is, if you have to 00:17:46.480 --> 00:17:51.120 worry about the 10-meter event, in case of a large normal event, 00:17:51.120 --> 00:17:58.720 the McKinney slide tsunami certainly had wave heights in the several 00:17:58.720 --> 00:18:03.656 hundreds of feet and runup heights that were quite impressive. 00:18:03.680 --> 00:18:09.120 And, again, this may be a singular event, or you may have these 00:18:09.120 --> 00:18:12.720 every few hundred thousand years or maybe more often now that 00:18:12.720 --> 00:18:16.240 the basin’s deeper and with the walls more perched. 00:18:16.240 --> 00:18:21.120 But certainly, when you’re having a large normal event, you have to worry 00:18:21.120 --> 00:18:26.640 about the contribution of a landslide. So I thought I’d end the talk this way. 00:18:26.640 --> 00:18:28.960 I think there’s a lot of extra work to do here. 00:18:28.960 --> 00:18:34.080 And potentially this marine record could be used for looking at things like 00:18:34.080 --> 00:18:37.600 synchronicity because we can get some pretty good dates bracketing 00:18:37.600 --> 00:18:43.200 the actual turbidite or debris flows. And so I think there’s a lot of work 00:18:43.200 --> 00:18:48.160 to be done in the future. It may not be by me, but I think there’s 00:18:48.160 --> 00:18:53.280 quite a rich tapestry of things to do. So thanks a lot. Bye.