WEBVTT
Kind: captions
Language: en-US

00:00:02.480 --> 00:00:09.401
Okay. Walker Lane
in 15 minutes or less.

00:00:11.520 --> 00:00:15.920
The fault’s in blue on the left portion
of the slide that strike along the

00:00:15.920 --> 00:00:23.576
east flank of the Sierra Nevada
comprise the area of the Walker Lane.

00:00:23.600 --> 00:00:27.920
As the system progresses to the
southeast into the Mojave,

00:00:27.920 --> 00:00:32.936
it’s often referred to as the
Eastern California Shear Zone.

00:00:32.960 --> 00:00:41.840
The zone itself has been assessed to
accommodate 15 to 25% of the relative

00:00:41.840 --> 00:00:45.440
Pacific North America Plate
motion, depending where you are

00:00:45.440 --> 00:00:50.640
along strike of that.
And that deformation is well manifest

00:00:50.640 --> 00:00:57.520
in the geodetic field here with the
[inaudible] zone of

00:00:57.520 --> 00:01:03.280
elevated shear
[inaudible] displacement

00:01:03.280 --> 00:01:07.347
occurring along the east
flank of the Sierra Nevada.

00:01:08.160 --> 00:01:15.649
First, the seismicity within
the zone, here in the black box.

00:01:16.880 --> 00:01:20.240
Seismicity in a depth
distribution shows that

00:01:20.240 --> 00:01:24.936
the seismicity is limited
to the upper 15 kilometers

00:01:24.960 --> 00:01:30.880
of the crust, which we expect.
Ran from southeast to northwest.

00:01:30.880 --> 00:01:36.480
And the depth distribution of seismicity
shows that the real number of

00:01:36.480 --> 00:01:42.216
earthquakes approach 20 kilometers
north of about the 1872 rupture.

00:01:42.240 --> 00:01:48.295
The catalog begins reporting
depths as 30, 40, 50 kilometers.

00:01:49.360 --> 00:01:53.840
But yet, most of the seismicity
is above 15 kilometers.

00:01:58.000 --> 00:02:02.880
The fault system is, may I say,
more complex than the

00:02:02.880 --> 00:02:12.056
San Andreas system in
its manifestation of deformation.

00:02:12.080 --> 00:02:15.600
San Andreas is dominated by the
San Andreas Fault itself

00:02:15.600 --> 00:02:19.736
and a [inaudible] sub-parallel
systems that connect.

00:02:19.760 --> 00:02:23.760
Whereas, the faults – the strike-slip
faults of the Walker Lane shown here

00:02:23.760 --> 00:02:28.640
in blue, red, and purple,
are discontinuously broken

00:02:28.640 --> 00:02:33.360
by zones of left-lateral faults,
shown here in black, as well as

00:02:33.360 --> 00:02:40.298
a zone of normal fault-bounded
basins and ranges.

00:02:41.440 --> 00:02:46.880
The difference between the San Andreas
and Walker Lane is generally

00:02:46.880 --> 00:02:52.000
attributed to cumulative offset in the –
across the Walker Lane is on the

00:02:52.000 --> 00:02:57.360
order of 20, 30 kilometers in the north,
50 to 100 kilometers or so in the south,

00:02:57.360 --> 00:03:02.056
as compared to hundreds of
kilometers along the San Andreas.

00:03:02.080 --> 00:03:07.680
But equally, if not more important,
that the deformation field across it

00:03:07.680 --> 00:03:13.840
is transtensional. In the middle figure,
I show the arrows of the geodetic field

00:03:13.840 --> 00:03:15.760
with respect to
North America.

00:03:15.760 --> 00:03:20.456
They’re all pointed
sub-parallel to the Walker Lane.

00:03:20.480 --> 00:03:26.296
They increase as you go from northeast
to southwest across the Walker Lane,

00:03:26.320 --> 00:03:29.816
and that’s [inaudible]
right-lateral shear.

00:03:29.840 --> 00:03:34.716
But looking at it in a little bit
more detail, the arrows generally line

00:03:34.716 --> 00:03:39.920
counter-clockwise to the major faults.
This supports intentional transtensional

00:03:39.920 --> 00:03:42.320
component to the strike-slip that’s

00:03:42.320 --> 00:03:47.360
accommodated on these faults.
And, likewise, if one looks at the

00:03:47.360 --> 00:03:51.200
arrows in a little bit more detail,
we see that they grow along the

00:03:51.200 --> 00:03:56.720
length of the Walker Lane
indicating stretching.

00:03:56.720 --> 00:04:00.696
So then we’ve got
transtensional deformation.

00:04:00.720 --> 00:04:05.600
And that transtension allows for
accommodation of space with

00:04:05.600 --> 00:04:12.160
deformation and manifest at
large scale many of the features

00:04:12.160 --> 00:04:16.056
we expect in the development
of a strike-slip fault system.

00:04:16.080 --> 00:04:22.240
Here in the north, transtension is
accommodated by – primarily by this

00:04:22.240 --> 00:04:28.480
development of en echelon range basins.
We know where these thrust cross-faults

00:04:28.480 --> 00:04:35.630
are, we’re looking at block rotations.
In Owens Valley – the Owens Valley

00:04:35.655 --> 00:04:40.535
1872 rupture in Sierra Nevada range
front, a beautiful little striation

00:04:40.560 --> 00:04:43.760
of slip partition
along sub-parallel faults.

00:04:43.760 --> 00:04:46.880
And, of course, some major
pull-apart basins in Death Valley,

00:04:46.880 --> 00:04:49.736
Saline and
Panamint Valleys down here.

00:04:49.760 --> 00:04:51.520
I’m going to move
my attention up here

00:04:51.520 --> 00:04:58.176
to the north of the area in the gold –
the dotted lines, the box.

00:04:59.040 --> 00:05:05.736
I’ve replotted the geodesy here
with respect to the Sierra Nevada.

00:05:05.760 --> 00:05:11.675
Here’s Reno. Carson’s actually
down here. [chuckles] Funny.

00:05:13.200 --> 00:05:16.960
The cumulative right-lateral
displacement across this zone,

00:05:16.960 --> 00:05:21.056
I just want to reiterate it’s about
7 or 8 millimeters per year.

00:05:21.080 --> 00:05:25.760
A significant amount of geologic
effort has taken place on the faults

00:05:25.760 --> 00:05:29.520
that are known – the major faults
that are known in trying to assess

00:05:29.520 --> 00:05:32.800
the history of earthquakes
and paleoearthquake studies

00:05:32.800 --> 00:05:39.096
and how fast they’re
moving in slip rates studies.

00:05:39.120 --> 00:05:45.016
Summarize the locations of these
studies on this slide on the right

00:05:45.040 --> 00:05:48.640
and then divided them up between
whether they’re in peer-reviewed

00:05:48.640 --> 00:06:00.553
journals in green, or in gray, which are
unreviewed technical reports and such.

00:06:01.120 --> 00:06:05.040
The gold box on the right is the subject
of the next slide, and I’ll start here,

00:06:05.040 --> 00:06:09.816
then I’ll move up to the basins
here, and then I’ll move up here.

00:06:09.840 --> 00:06:19.520
So these dots in this area are a reflection
of Steve Angster’s efforts during his

00:06:19.520 --> 00:06:30.936
recent thesis where he calculated,
or he observed, offset fan deposits

00:06:30.960 --> 00:06:37.280
and used cosmogenic profiles to assess
the ages of those offset deposits and

00:06:37.280 --> 00:06:43.760
calculated slip rates on each of these
major faults through here to get slip

00:06:43.760 --> 00:06:49.840
rates on the order of 0.7, 0.8, 1.5 –
all around 1 millimeter per year and

00:06:49.840 --> 00:06:57.016
added that to another strike-slip rate
on a portion of the Wassuk Range

00:06:57.040 --> 00:07:01.920
to conclude that he’s captured
about 4 millimeters a year of slip across

00:07:01.920 --> 00:07:08.809
this portion of the range – half of
the total amount across this region.

00:07:12.640 --> 00:07:17.816
Where is that other half,
and how is it occurring, is somewhat

00:07:17.840 --> 00:07:23.520
illustrated in this slide, I think.
Physiography, here’s the faults that

00:07:23.520 --> 00:07:29.920
Steve studied. These would be
left-lateral faults through here,

00:07:29.920 --> 00:07:35.360
and each one of these faults shows
geomorphic offsets of left-lateral

00:07:35.360 --> 00:07:40.880
displacement and also basins on
alternate sides of the fault, which are

00:07:40.880 --> 00:07:46.160
consistent with left-lateral displacement.
This should be a left-lateral symbol

00:07:46.160 --> 00:07:52.936
showing the basin [inaudible]
opposite sides of these.

00:07:52.960 --> 00:07:57.600
But, in general, the location is
in a broad right step in the major

00:07:57.600 --> 00:08:02.400
strike-slip faults in the Walker Lane
at this location, so in essence, there’s

00:08:02.400 --> 00:08:07.920
a torque rotation occurring in here.
And so these left-lateral faults can

00:08:07.920 --> 00:08:11.300
be attributed to block
rotations within that.

00:08:11.300 --> 00:08:14.400
And I think that probably
extends all the way over into

00:08:14.400 --> 00:08:19.501
Mono Lake Basin
and Long Valley.

00:08:23.680 --> 00:08:28.640
The idea of these block rotations is
emphasized by the recent

00:08:28.640 --> 00:08:32.400
Monte Cristo earthquake that showed
left-lateral displacement and surface

00:08:32.400 --> 00:08:38.696
ruptures along the extension of this
Candelaria Fault Zone right in here.

00:08:38.720 --> 00:08:47.680
Moving to the northwest, in the
next slide, it’s going to take a little bit

00:08:47.680 --> 00:08:53.040
to digest, so let’s take a second.
Over on the right, we’ve got the normal

00:08:53.040 --> 00:08:57.896
fault down the basins, Tahoe all the way
down to number 7, Walker Lake.

00:08:57.920 --> 00:09:04.376
And these east-west striking 
faults [inaudible].

00:09:04.400 --> 00:09:09.200
So this little box is [inaudible]
projection, but here we’re looking at

00:09:09.200 --> 00:09:18.536
the east-west striking left-lateral faults
and lineaments and Lake Tahoe and

00:09:18.560 --> 00:09:26.364
Walker Lake. And, in black, are the
major basin-bounding faults.

00:09:27.360 --> 00:09:33.040
Within the boxes are my synthesis
of what geologists have estimated are

00:09:33.040 --> 00:09:39.816
the slip rates of the individual faults.
And, in particular, extension rates

00:09:39.840 --> 00:09:43.096
so that they can be
compared to geodesy.

00:09:43.120 --> 00:09:48.640
Only along one of the ranges do
we have a clear right-lateral offset

00:09:48.640 --> 00:09:52.320
of shorelines in this case that
give us 1 millimeter per year

00:09:52.320 --> 00:09:56.960
of right-lateral strike-slip.
In all the other ranges, we have

00:09:56.960 --> 00:10:05.052
no observations, really,
of clear right-lateral offsets.

00:10:05.052 --> 00:10:09.091
And these have been
used for strike-slip.

00:10:11.040 --> 00:10:17.336
Geodetic rates of extension are
presented by black arrows,

00:10:17.360 --> 00:10:24.696
here 0.5 to 1.7, for example.
And here, 0.7 to 1.7, for example.

00:10:24.720 --> 00:10:29.120
All these values, you can see, they’re
very small, and they’re oriented in

00:10:29.120 --> 00:10:34.160
a fashion such that neither the
orientation nor the magnitude of those

00:10:34.160 --> 00:10:38.960
displacement rates is sufficient to
account for the 6 to 8 millimeters

00:10:38.960 --> 00:10:42.856
of right-lateral shear that’s
occurring across this boundary.

00:10:42.880 --> 00:10:50.000
Indeed, you can walk from A to B and
really not even intersect a nice fault

00:10:50.000 --> 00:10:55.176
where you might want to get a slip rate.
So that’s a little conundrum.

00:10:55.200 --> 00:10:58.960
So, investigation of this conundrum,
working with Jayne Bormann,

00:10:58.960 --> 00:11:08.856
Jayne took the geodetic data and
gridded it out and projected the

00:11:08.880 --> 00:11:15.760
extensional strain and shear strain rates
we expect are looking at the strain

00:11:15.760 --> 00:11:20.696
accumulating in the 15-kilometer
volume along each of these faults.

00:11:20.720 --> 00:11:26.696
And, in this case, two observations.
If you look at the purple/magenta

00:11:26.720 --> 00:11:31.816
arrows, they generally overlap
with the geologist’s, and vice versa.

00:11:31.840 --> 00:11:33.600
So there’s no
real eye-openers.

00:11:33.600 --> 00:11:38.456
We’re probably on the right track with
the estimates of vertical offsets and ages.

00:11:38.480 --> 00:11:41.656
And so you could say
there’s a general agreement.

00:11:41.680 --> 00:11:46.000
With strike-slip, we don’t have anything
to compare it to, and some of these

00:11:46.000 --> 00:11:52.456
values are quite high millimeters per
year of possible geodetic strike-slip.

00:11:52.480 --> 00:11:58.577
So what gives?
So there’s a couple of options.

00:12:00.880 --> 00:12:04.320
First option is, again,
we come back to block rotations.

00:12:04.320 --> 00:12:09.256
I’ve re-oriented these basins
from Tahoe to Walker Lake

00:12:09.280 --> 00:12:13.000
in the geodetic strain field.
This is a block of wax heated

00:12:13.000 --> 00:12:17.176
by Jim Brune and subjected
to transtensional shear.

00:12:17.200 --> 00:12:22.160
And the same block of wax below,
heated again, but a block of ice put

00:12:22.160 --> 00:12:25.280
on it for some amount of time to
create a brittle layer on the top

00:12:25.280 --> 00:12:31.120
and subjected to the same shear.
And look at the pattern of offsets

00:12:31.120 --> 00:12:35.040
one observes. That’s kind of
an exact analog to the Earth that’s

00:12:35.040 --> 00:12:40.160
very suggestive that, in this case,
for example, I can walk across this

00:12:40.160 --> 00:12:45.760
entire zone without crossing a specific
fault, yet I know there’s that much

00:12:45.760 --> 00:12:48.880
displacement that’s occurred.
Clearly, block rotations

00:12:48.880 --> 00:12:54.480
are occurring. And even the basins that
form, in some cases, are quite triangular,

00:12:54.480 --> 00:12:58.640
much like the basins in Antelope
Valley, Bridgeport Valley.

00:12:58.640 --> 00:13:01.816
Even Tahoe looks [inaudible]
enough to [inaudible].

00:13:01.840 --> 00:13:08.880
So block rotations are playing a role,
but also there’s probably missing

00:13:08.880 --> 00:13:13.736
strike-slip, and [inaudible],
in his thesis said,

00:13:13.760 --> 00:13:20.160
looked at a number of features along
these and suggests, and rightly so,

00:13:20.160 --> 00:13:24.880
probably just being dominated
by the dip-slip and not really

00:13:24.880 --> 00:13:28.800
seeing any cumulative offset.
But nonetheless, if there were

00:13:28.800 --> 00:13:32.560
tens of kilometers or hundreds of
kilometers of right-lateral offset through

00:13:32.560 --> 00:13:37.336
this zone, we would expect to see
a right-lateral fault, and we don’t.

00:13:37.360 --> 00:13:42.560
So we’re probably missing some of the
strike-slip, and we can expect oblique slip

00:13:42.560 --> 00:13:47.925
on large earthquakes that occur on
these basement object-bounding faults.

00:13:49.360 --> 00:13:54.480
And we’re probably missing some
in the sense because of rotations and

00:13:54.480 --> 00:14:00.303
an inability to quantify how that shear
is taking place on the active fault.

00:14:01.200 --> 00:14:05.830
Now moving to the north up here to
the Mohawk Valley and Honey Lake

00:14:05.830 --> 00:14:12.776
Faults and Pyramid Lake Faults,
we run into a similar problem.

00:14:12.800 --> 00:14:15.840
In this case, the deformation
field seems simpler.

00:14:15.840 --> 00:14:22.400
It’s primarily strike-slip faults.
Investigators have gone to the

00:14:22.400 --> 00:14:27.096
individual faults and come up
with estimates of strike-slip rates.

00:14:27.120 --> 00:14:39.360
And, again, when you summarize the
rates, it’s very difficult to come up with

00:14:39.360 --> 00:14:47.016
the total 7 millimeters per year of right-
lateral displacement you might expect.

00:14:47.040 --> 00:14:53.600
So, in this case, it’s been surmised that
a lot of this is missing in smaller faults

00:14:53.600 --> 00:14:58.320
in the forest or deformation that we
aren’t able to see and document and

00:14:58.320 --> 00:15:08.229
hasn’t manifested itself well enough
geologically for us to document it.

00:15:09.120 --> 00:15:18.136
So interesting in all of this, I just –
because I have a minute left, I think,

00:15:18.160 --> 00:15:25.280
I’ll just turn to the observation that,
in all of this, one of the most populated

00:15:25.280 --> 00:15:30.616
areas is Reno, and we don’t have
any quantification of slip rates

00:15:30.640 --> 00:15:35.920
in the immediate vicinity of Reno.
And I think, with some recent work

00:15:35.920 --> 00:15:41.440
that I’ve been doing correlating terraces
and documenting them along the

00:15:41.440 --> 00:15:46.480
Verdi-Reno corridor, I should be able
to ultimately take these correlations of

00:15:46.480 --> 00:15:53.360
these terraces along the range front to
the north and south and even here,

00:15:53.360 --> 00:15:59.096
be able to quantify the needed
amounts and rates of displacement,

00:15:59.120 --> 00:16:05.280
add some more information to that.
That said, I’m hitting 15 minutes,

00:16:05.280 --> 00:16:10.856
and I think the main message
in the zone of the Walker Lane

00:16:10.880 --> 00:16:16.960
is this problem of meshing geologic
and geodetic data sets that’s going to

00:16:16.960 --> 00:16:22.981
present itself when trying to
reduce the seismic hazard model.

00:16:24.080 --> 00:16:26.440
So thanks.

00:16:27.602 --> 00:16:35.086
[silence]