Yeah, thanks for showing up this morning.
And my name is Graham Kent. I’m director of the Nevada
Seismological Laboratory at the University of Nevada-Reno.
And I was asked to give a talk about Lake Tahoe faults and
earthquake swarms. And this picture right here
kind of shows some of the tools that we’ve been using in
the Lake Tahoe Basin. Some multibeam bathymetry
of Fallen Leaf Lake at the south end of the basin.
And then surrounding that is a 2011 Lidar survey highlighting
the West Tahoe Fault cutting in front of Mount Tallac.
Again, as I just mentioned, in chasing the history of fault rupture in the basin,
we’ve used a arsenal of tools ranging from CHIRP profiling, Lidar,
side-scan sonar, piston coring, multibeam sonar, seismic network,
trenching – really all the various bits and pieces.
And it should also be said that Tahoe was one of the first places to really
kind of go full in with underwater paleoseismology, and that was really
kind of, if not perfected, it was certainly a major step in that direction.
For many of you, hopefully you’ve seen at least this map the USGS
and the University of New Hampshire made of Lake Tahoe.
And just going to use this right now to kind of highlight, there are
three major fault zones. On the west is West Tahoe Fault.
In the center lake, stepping over to the right, is the
Stateline-North Tahoe Fault Zone. Then it steps into the Incline Village
Fault that cuts through the village of Incline.
And also there’s two more steps into Little Valley and the
Mount Rose fan fault system. So, again, it’s really five step-overs.
Three of them are within the Lake Tahoe Basin proper.
And one of the things that we first noticed when we all saw this multibeam
bathymetry map in the winter of 1998, I believe, at AGU when we used to
go in person, of course we saw some lineations that obviously looked like
fault zones, but we also noticed this debris avalanche coming out
of McKinney Bay, really forming McKinney Bay.
And that was quite stunning, actually. And then, if you look a little more
carefully, you’ll see a submerged shoreline surrounding the lake.
And what’s interesting about that is it’s not all at the same level.
There’s as much as about 15 meters of offset from the, let’s see,
southwest to the northeast side of the lake.
So the northeast side sees offsets from three different fault zones
over by Sand Harbor. Whereas, let’s say Rubicon Lake
up on the footwall obviously doesn’t see any at all.
So, again, about 15 meters of offset, and of course we’d like to date that surface.
Now, I’d be lying to you if I [chuckles] told you this is how we actually did it.
But almost after all of the marine work was done – underwater work was done,
we finally worked with the Tahoe Regional Planning Agency and hired
Watershed Sciences to shoot a Lidar survey of the entire basin.
And this is a little screenshot from the Echo Summit area,
crossing Highway 50. You can see the lineation that is the – really almost
the southern extent of the West Tahoe Fault. And Gordon Seitz actually did
a trench within this screen grab of the West Tahoe Fault at that location.
But we had to actually figure most of it out before we had this, we spent
nearly a decade-plus, before we got the on-land portion.
But, once we went underwater, we started working with sub-bottom
CHIRP profiling, and here’s an example on the top of the screen.
It’s a paleo shoreline, 11 meters off of Rubicon, and you can see
the sediments in the deformed footwall being truncated off.
And also you can see, on an ROV photo, of what that
platform looks like at about 30 feet below the surface of the lake.
And below that, across the lake, we can see some oblique clinoforms
crossing across the surface. And this provides an opportunity,
as you can see, from that inset of doing some vibracoring and actually trying to
age date that surface, which we were able to do at about 19,000 years,
plus or minus. So right there, that gives us a sense of how we
can divide 10 meters of offset across about 20,000 years.
Now, of course, a lot of that offset depends on the fault geometry,
but again, these are just kind of rough numbers.
The great thing about the sub-bottom profiler is our ability
to really see almost at the decimeter scale of resolution.
And this is a picture in Fallen Leaf Lake again, south of Lake Tahoe,
but within the Tahoe Basin, looking at a couple strands of the West Tahoe
Fault and actually being able to see – again, what we’ll find to be really
important are slides and turbidites that help us potentially date the age
of the various large events in the lake.
And, more conventionally, this is a trench that we did
with Gordon Seitz over at CGS. This is the Incline Village Fault Zone.
You can see a beautiful colluvial wedge that’s kind of fallen out
of the actual side of the wall. And we were able to date the
most recent event at about 600 years before present.
And then, prior to that, it’s many tens of thousands of years before the last event.
And, again, I alluded to it, but we were also going out and collecting cores
within Lake Tahoe to try to map out these debris flows or turbidites.
And this is an example of just one core that we collected in the middle
of the lake by the Stateline Fault. And, again, if we piece them all
together – and this was work done with Bob Karlin and his student,
we see quite a rich history of turbidites and debris flows that may have been
triggered by shaking events. Doesn’t mean they have to be shaken
events in the Lake Tahoe Basin, but certainly enough ground motion to
actually cause these events to occur. And not all events are the same size.
There were some very large events, and then there’s other ones
that are not continuous across the entire lake.
When you finally piece it together – and we’ll go through this in a little more
detail, but what you basically find out is that the West Tahoe Fault,
or West Tahoe-Dollar Point Fault, you can see there in orange, has a
slip rate of somewhere between about 0.4 and 0.8 millimeters per year.
And we can look at the most recent events, depending on where we are
along the fault line, somewhere about 4,000 to 5,000 years ago.
The Stateline-North Tahoe Fault is very interesting because we
don’t really have a good way of dating the most recent events given its location
in the lake and how it doesn’t really come on shore in any major sense.
What we do know, though, is that it is providing offset that we see on the
submerged shoreline, and also in the sense of, if there are large events like
you’d expect on the West Tahoe-Dollar Point Fault, it’s not like we’re seeing
a separate set of turbidites, necessarily, that are related to both different events.
So there’s some question as to synchronicity and whether we’re really
burying two events in one slide, or at least there’s not enough period of time
between the two events where that – it would look like one continuous slide.
But that’s where we are right now. And, again, the slowest slipping of
the three would be the Incline Village Fault.
And, again, it’s had a recent event at about 660 years, plus or minus.
This is an example of why the sub-bottom profiler is really useful.
And this is over at Fallen Leaf Lake again.
And we’re looking at these various debris flows or turbidites that we are
able to not only see seismically, but then we can put a core in and
actually age-date and bound those events.
So, again, this has been done, obviously, in many areas around the
world, but most notably in the Pacific Northwest along Cascadia.
And Chris has done a great job doing that, and it seems to work here as well.
We all started about the same time.
Other things like measuring the offset of the McKinney Bay debris complex,
right? So, again, we’ve not penetrated the entire sediment stack,
so any extrapolation might be dangerous, but if one were to
extrapolate, the McKinney Bay debris complex is probably on order
60,000 years ago. And so at least we have some constraints.
So, in this next set, we’re going to go through and essentially look at various
lines of evidence based on turbidites or seismic CHIRP-related constraints.
And what we want to do here is kind of fill in for all these various segments.
And first looking at Lake Tahoe with all the turbidites, for example,
and then filling in with Fallen Leaf Lake and now with Emerald Bay.
And, last but not least, Cascade Lake.
And the last one there is assuming the sedimentation rate that’s similar
to Fallen Leaf Lake, which is probably a pretty good bet.
And, again, what you start to see are some events start to line up very nicely,
especially the relatively large event around – in terms of its expression
in turbidites or debris flows, around 7,800 years.
And so now we’re going to go over here next, and ...
And we’re going to look at, again, some of the evidence
as we look at the fault trace.
So let’s move forward here.
And, again, we’re looking at the color-coded events over on the
left-hand side and the fault trace that they’re associated with.
Obviously the green is the West Tahoe-Dollar Point Fault here.
And looking at it in cyan for the cyan event, showing it
appears to be across the entire basin.
And then some of these other ones more recently may be constricted to
the center and central segments in terms of their expression
in the kind of turbidite record or in offsets of faulted sediments
within the CHIRP data. And just an aggregate.
If you look at it, again, the most recent event is sitting around 4,000, 4,500
years ago for the southernmost segment. And then potentially a little bit later
for the central segment. One also would notice that,
no matter how you calculate it, the kind of recurrence interval
across three events seems to be a smaller number than actually
what we see in terms of the last event. So one might say that this system
is reaching the end of its period, however long that is.
Now, one of [audio cuts out] …
So if we want to skip up north or kind of to the northwest and head over
towards Donner Lake just outside of Truckee, California, and you’ve had
a CHIRP profiler, here’s an example of a longitudinal dip line across what
appears to be two faults that comprise a system at the west end of the lake.
And you see the offset sediments, and they’re denoted in the CHIRP profile.
And one of the more interesting things that we saw – again, if we now shoot
a strike line, and then blow it up, we’ll notice that we see, again, three slides.
So why that’s interesting is that we tend to see three slides in the
Lake Tahoe area. And are we getting some level of synchronicity of faulting?
Because clearly these faults up in the Mohawk Valley fault system are not,
you know, connected, per se, to, let’s say the West Tahoe Fault nearby.
But what’s also interesting, although we don’t have actually a sediment core,
but we kind of know where the bottom of the Holocene is.
And, if you were to go and do the math, we’re seeing about a millimeter per year
in all these smaller lakes. So, if you give me that,
what we’ll find out is that the – not the most recent event,
but the penultimate and the event prior to that seem to line up with the
events around 7,800 and around 11,000 years that we see in Lake Tahoe.
But the more recent event is quite a bit younger, and in that regard,
it may line up more with an event that we see here that Tom Sawyer
has for the Mohawk Valley. So, again, this seems to be a potential
viable way of measuring at least large shaking events.
It might be hard to put it on a particular fault, but I think our experience in
Lake Tahoe and the small lakes of Cascade and Fallen Leaf south of that,
you tend to see at least a footprint.
So it doesn’t look like every big large shaking event causes slides everywhere.
And, again, more work could be done on that, but at least for now,
it does show the utility of doing this. And we actually have some CHIRP data
from up at Lake Independence – or, Independence Lake, so, again,
we can extend this to the north. And now, just changing topics
just a wee bit, I was asked to talk about some of the swarm events.
And probably the most famous swarm event was in 2004.
And the reason why it was really interesting is that the events were
located almost at 30 kilometers’ depth. And so that’s definitely like the
transition between the lower crust and the Moho lid, or the top of the mantle,
and if that wasn’t curious enough, about seven years later, we had
a nearly identical event beneath Sierra Valley, and again,
a little bit shallower, but not much. And this paper, written by Ken Smith
and others, kind of highlights these two swarms.
And, if you just look at the 3D character of where these earthquakes
are, they’re literally on a plane that fits right through both of them [laughs],
on strike, you don’t have to change the dip very much,
and they just line on a perfect line. And so we’re wondering if this –
or these events have something to do with, like, kind of breaking
the microplate and the most strongest part of that is right at that
lower crust-mantle transition zone. And, again, these smaller events
ultimately trigger larger events in the upper crust on order, in this case,
you know, high magnitude 4s, like 4.7 in the Sierra Valley case.
So, again, they’re very curious and rare type events. But we had two of them,
and they really connect very well, even though they’re quite a distance apart.
Last but not least, we’ve been getting some events of recent – certainly during
the pandemic period, so when everything’s even a bit more interesting,
but yeah, we were getting events out in Lake Tahoe.
And that’s always of concern because there’s the potential of a tsunami if
we trigger a large normal event. These happened to be left-lateral
strike-slip events that are probably interconnecting two of the fault zones
at depth. But still, if you’re playing around at the ends of faults,
that always makes you nervous because what a way to finish off
the pandemic then to have a tsunami in Lake Tahoe due to a normal faulting
earthquake that gets triggered by a strike-slip event.
So, again, this caused a lot of stir. It was in the New York Times.
Slow news day, but nonetheless. So we’re always keeping an eye
on these events in Lake Tahoe because of the consequences of water.
And, again, one of the things that Steve Ward showed is, if you have to
worry about the 10-meter event, in case of a large normal event,
the McKinney slide tsunami certainly had wave heights in the several
hundreds of feet and runup heights that were quite impressive.
And, again, this may be a singular event, or you may have these
every few hundred thousand years or maybe more often now that
the basin’s deeper and with the walls more perched.
But certainly, when you’re having a large normal event, you have to worry
about the contribution of a landslide. So I thought I’d end the talk this way.
I think there’s a lot of extra work to do here.
And potentially this marine record could be used for looking at things like
synchronicity because we can get some pretty good dates bracketing
the actual turbidite or debris flows. And so I think there’s a lot of work
to be done in the future. It may not be by me, but I think there’s
quite a rich tapestry of things to do. So thanks a lot. Bye.