Okay. Walker Lane in 15 minutes or less.
The fault’s in blue on the left portion of the slide that strike along the
east flank of the Sierra Nevada comprise the area of the Walker Lane.
As the system progresses to the southeast into the Mojave,
it’s often referred to as the Eastern California Shear Zone.
The zone itself has been assessed to accommodate 15 to 25% of the relative
Pacific North America Plate motion, depending where you are
along strike of that. And that deformation is well manifest
in the geodetic field here with the [inaudible] zone of
elevated shear [inaudible] displacement
occurring along the east flank of the Sierra Nevada.
First, the seismicity within the zone, here in the black box.
Seismicity in a depth distribution shows that
the seismicity is limited to the upper 15 kilometers
of the crust, which we expect. Ran from southeast to northwest.
And the depth distribution of seismicity shows that the real number of
earthquakes approach 20 kilometers north of about the 1872 rupture.
The catalog begins reporting depths as 30, 40, 50 kilometers.
But yet, most of the seismicity is above 15 kilometers.
The fault system is, may I say, more complex than the
San Andreas system in its manifestation of deformation.
San Andreas is dominated by the San Andreas Fault itself
and a [inaudible] sub-parallel systems that connect.
Whereas, the faults – the strike-slip faults of the Walker Lane shown here
in blue, red, and purple, are discontinuously broken
by zones of left-lateral faults, shown here in black, as well as
a zone of normal fault-bounded basins and ranges.
The difference between the San Andreas and Walker Lane is generally
attributed to cumulative offset in the – across the Walker Lane is on the
order of 20, 30 kilometers in the north, 50 to 100 kilometers or so in the south,
as compared to hundreds of kilometers along the San Andreas.
But equally, if not more important, that the deformation field across it
is transtensional. In the middle figure, I show the arrows of the geodetic field
with respect to North America.
They’re all pointed sub-parallel to the Walker Lane.
They increase as you go from northeast to southwest across the Walker Lane,
and that’s [inaudible] right-lateral shear.
But looking at it in a little bit more detail, the arrows generally line
counter-clockwise to the major faults. This supports intentional transtensional
component to the strike-slip that’s
accommodated on these faults. And, likewise, if one looks at the
arrows in a little bit more detail, we see that they grow along the
length of the Walker Lane indicating stretching.
So then we’ve got transtensional deformation.
And that transtension allows for accommodation of space with
deformation and manifest at large scale many of the features
we expect in the development of a strike-slip fault system.
Here in the north, transtension is accommodated by – primarily by this
development of en echelon range basins. We know where these thrust cross-faults
are, we’re looking at block rotations. In Owens Valley – the Owens Valley
1872 rupture in Sierra Nevada range front, a beautiful little striation
of slip partition along sub-parallel faults.
And, of course, some major pull-apart basins in Death Valley,
Saline and Panamint Valleys down here.
I’m going to move my attention up here
to the north of the area in the gold – the dotted lines, the box.
I’ve replotted the geodesy here with respect to the Sierra Nevada.
Here’s Reno. Carson’s actually down here. [chuckles] Funny.
The cumulative right-lateral displacement across this zone,
I just want to reiterate it’s about 7 or 8 millimeters per year.
A significant amount of geologic effort has taken place on the faults
that are known – the major faults that are known in trying to assess
the history of earthquakes and paleoearthquake studies
and how fast they’re moving in slip rates studies.
Summarize the locations of these studies on this slide on the right
and then divided them up between whether they’re in peer-reviewed
journals in green, or in gray, which are unreviewed technical reports and such.
The gold box on the right is the subject of the next slide, and I’ll start here,
then I’ll move up to the basins here, and then I’ll move up here.
So these dots in this area are a reflection of Steve Angster’s efforts during his
recent thesis where he calculated, or he observed, offset fan deposits
and used cosmogenic profiles to assess the ages of those offset deposits and
calculated slip rates on each of these major faults through here to get slip
rates on the order of 0.7, 0.8, 1.5 – all around 1 millimeter per year and
added that to another strike-slip rate on a portion of the Wassuk Range
to conclude that he’s captured about 4 millimeters a year of slip across
this portion of the range – half of the total amount across this region.
Where is that other half, and how is it occurring, is somewhat
illustrated in this slide, I think. Physiography, here’s the faults that
Steve studied. These would be left-lateral faults through here,
and each one of these faults shows geomorphic offsets of left-lateral
displacement and also basins on alternate sides of the fault, which are
consistent with left-lateral displacement. This should be a left-lateral symbol
showing the basin [inaudible] opposite sides of these.
But, in general, the location is in a broad right step in the major
strike-slip faults in the Walker Lane at this location, so in essence, there’s
a torque rotation occurring in here. And so these left-lateral faults can
be attributed to block rotations within that.
And I think that probably extends all the way over into
Mono Lake Basin and Long Valley.
The idea of these block rotations is emphasized by the recent
Monte Cristo earthquake that showed left-lateral displacement and surface
ruptures along the extension of this Candelaria Fault Zone right in here.
Moving to the northwest, in the next slide, it’s going to take a little bit
to digest, so let’s take a second. Over on the right, we’ve got the normal
fault down the basins, Tahoe all the way down to number 7, Walker Lake.
And these east-west striking faults [inaudible].
So this little box is [inaudible] projection, but here we’re looking at
the east-west striking left-lateral faults and lineaments and Lake Tahoe and
Walker Lake. And, in black, are the major basin-bounding faults.
Within the boxes are my synthesis of what geologists have estimated are
the slip rates of the individual faults. And, in particular, extension rates
so that they can be compared to geodesy.
Only along one of the ranges do we have a clear right-lateral offset
of shorelines in this case that give us 1 millimeter per year
of right-lateral strike-slip. In all the other ranges, we have
no observations, really, of clear right-lateral offsets.
And these have been used for strike-slip.
Geodetic rates of extension are presented by black arrows,
here 0.5 to 1.7, for example. And here, 0.7 to 1.7, for example.
All these values, you can see, they’re very small, and they’re oriented in
a fashion such that neither the orientation nor the magnitude of those
displacement rates is sufficient to account for the 6 to 8 millimeters
of right-lateral shear that’s occurring across this boundary.
Indeed, you can walk from A to B and really not even intersect a nice fault
where you might want to get a slip rate. So that’s a little conundrum.
So, investigation of this conundrum, working with Jayne Bormann,
Jayne took the geodetic data and gridded it out and projected the
extensional strain and shear strain rates we expect are looking at the strain
accumulating in the 15-kilometer volume along each of these faults.
And, in this case, two observations. If you look at the purple/magenta
arrows, they generally overlap with the geologist’s, and vice versa.
So there’s no real eye-openers.
We’re probably on the right track with the estimates of vertical offsets and ages.
And so you could say there’s a general agreement.
With strike-slip, we don’t have anything to compare it to, and some of these
values are quite high millimeters per year of possible geodetic strike-slip.
So what gives? So there’s a couple of options.
First option is, again, we come back to block rotations.
I’ve re-oriented these basins from Tahoe to Walker Lake
in the geodetic strain field. This is a block of wax heated
by Jim Brune and subjected to transtensional shear.
And the same block of wax below, heated again, but a block of ice put
on it for some amount of time to create a brittle layer on the top
and subjected to the same shear. And look at the pattern of offsets
one observes. That’s kind of an exact analog to the Earth that’s
very suggestive that, in this case, for example, I can walk across this
entire zone without crossing a specific fault, yet I know there’s that much
displacement that’s occurred. Clearly, block rotations
are occurring. And even the basins that form, in some cases, are quite triangular,
much like the basins in Antelope Valley, Bridgeport Valley.
Even Tahoe looks [inaudible] enough to [inaudible].
So block rotations are playing a role, but also there’s probably missing
strike-slip, and [inaudible], in his thesis said,
looked at a number of features along these and suggests, and rightly so,
probably just being dominated by the dip-slip and not really
seeing any cumulative offset. But nonetheless, if there were
tens of kilometers or hundreds of kilometers of right-lateral offset through
this zone, we would expect to see a right-lateral fault, and we don’t.
So we’re probably missing some of the strike-slip, and we can expect oblique slip
on large earthquakes that occur on these basement object-bounding faults.
And we’re probably missing some in the sense because of rotations and
an inability to quantify how that shear is taking place on the active fault.
Now moving to the north up here to the Mohawk Valley and Honey Lake
Faults and Pyramid Lake Faults, we run into a similar problem.
In this case, the deformation field seems simpler.
It’s primarily strike-slip faults. Investigators have gone to the
individual faults and come up with estimates of strike-slip rates.
And, again, when you summarize the rates, it’s very difficult to come up with
the total 7 millimeters per year of right- lateral displacement you might expect.
So, in this case, it’s been surmised that a lot of this is missing in smaller faults
in the forest or deformation that we aren’t able to see and document and
hasn’t manifested itself well enough geologically for us to document it.
So interesting in all of this, I just – because I have a minute left, I think,
I’ll just turn to the observation that, in all of this, one of the most populated
areas is Reno, and we don’t have any quantification of slip rates
in the immediate vicinity of Reno. And I think, with some recent work
that I’ve been doing correlating terraces and documenting them along the
Verdi-Reno corridor, I should be able to ultimately take these correlations of
these terraces along the range front to the north and south and even here,
be able to quantify the needed amounts and rates of displacement,
add some more information to that. That said, I’m hitting 15 minutes,
and I think the main message in the zone of the Walker Lane
is this problem of meshing geologic and geodetic data sets that’s going to
present itself when trying to reduce the seismic hazard model.
So thanks.
[silence]