WEBVTT
Kind: captions
Language: en-US

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Well, good morning.
Want to take this opportunity,

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as we’re talking about the faults of
eastern California, to talk about one that

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roared to life in July 2021 and produced
the magnitude 6.0 Antelope Valley

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earthquake that was, you know, widely
felt across a lot of northern California.

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So, to review generally the setting, the
map on the – on the right here shows

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one year of seismicity in this area
leading up to the day of the main shock.

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And you can see that there
is no appreciable, you know,

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seismic activity going on
in the area of the main shock.

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In fact, most of the – or, most of the
earthquake activity seems to be focused

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up near the Walker Lake area, which is
along the California-Nevada border.

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And so this event specifically had
no appreciable foreshock sequence.

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There were, that day, only two
magnitude 2 earthquakes leading up to

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the main shock at 2:49.
And immediately, the main shock was

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followed by the largest aftershock,
which is a 5.0, just one minute later.

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So, to the locals,
it may have even felt like

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one long earthquake because
they were so close together.

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The big question a lot of geologists and
a lot of us will ask when we, you know,

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get moderate quakes of this size –
moderate to strong, you know, 6.0s,

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what are the surface
effects of a quake like this.

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And so it’s well-documented
that there were earthquake-induced

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rockfalls on Highway 395
from this event.

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You can see here a screen capture clip
from a video taken, and you can see that

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the rockfalls here on Highway 395
created a nice slalom course and

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an opportunity for people to get out
of their cars and test their strength.

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Usually that doesn’t go well
with boulders which are

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much heavier than
people anticipate.

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Another thing we look about was,
was there any surface rupture from this

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earthquake, and no, there wasn’t
any documented surface rupture.

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And that’s maybe not too surprising
because magnitude 6.0 is kind of

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on the – on the verge of the edge of
where we might expect surface rupture.

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There was no documented liquefaction
from this event, and that is, more than

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likely, to be attributed to the –
probably the depth of groundwater

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in the Antelope Valley and
not necessarily to be attributed to

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the not-strong-enough
shaking.

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Surface effects we saw here include
ground subsidence about

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10 centimeters, from – based on InSAR
data from the Sentinel satellites.

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So we do know that this – it did subside.
We did get some ground deformation

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here, but nothing on a more specific,
you know, surface rupture

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or liquefaction, much more –
more on a regional scale.

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The earthquake itself was felt quite
widely across northern California

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and northwestern Nevada.
You can see the yellow coloring here –

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yellowish coloring from the ShakeMap
showing intensities 5 up to 6 in the

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Walker area, but this area is, you know,
fairly sparsely populated, although

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it was felt quite strongly in area cities,
like Carson City, South Lake Tahoe,

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and Reno. The shaking did travel
all the way across the Sierra into

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Sacramento and into the Bay Area,
which got a lot of people’s attention.

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You could see it also followed
a fairly predictable decay pattern,

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or drop-off pattern, with density
over – or, density compared –

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distance compared to
intensity for this quake.

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Now, a lot of us that may, you know,
live closer to more active fault regions

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in the state may think that an intensity,
you know, 2 or 3 may not be that big

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a deal, but to maybe a state senator in
Sacramento feeling an intensity 3, well,

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that, you know, might qualify as
one heck of an earthquake to you.

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So it got a lot of people’s attention
for being in such a remote area

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of California for this reason.
As we’ve been talking, this area this

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earthquake occurred in is within the
Basin and Range province of western

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North America, a large zone of
tectonic extension initiating

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in the early Miocene.
And specifically, it’s a part of

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the Basin and Range that is
a transition zone on the western end

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of the Basin and Range, a transition zone
between the Sierra Nevada and the

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Basin and Range province,
which is characterized –

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is an interestingly complex area of
strike-slip and normal faulting together.

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It’s woven in and out.
It’s pretty complicated.

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And you’ll see earthquakes with both
focal mechanisms, faulting styles.

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And this area accommodates 20% –
I’ve seen up to 25% cited for the

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relative plate motion between the
Pacific and North American Plates.

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And that comes out to on the order
of about 10 millimeters per year.

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Looking at the historic earthquakes
that have happened in this area,

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and this is just since instruments
have been recording, you can see

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it’s not tremendously robust.
There are two 5s-ish earthquakes

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down – in 1962 and 1979,
down west of Bridgeport.

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And then you see the Double Spring
Flat earthquake, a magnitude 6.1,

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in 1994 just north of Walker Lake
on the state border there.

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And then last year’s magnitude 6
2021 quake.

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And here’s an interesting case
I just wanted to point out that,

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here’s where the complexity
of Walker Lane is really visible

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and can be seen quite well.
In the 1994 Double Spring Flat

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earthquake, focal mechanism shows
predominantly strike-slip faulting

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in this area of the Walker Lane.
And if you look at the magnitude 6

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Antelope Valley earthquake, we can
see the focal mechanism gives us

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kind of the opposite, and we have
almost exclusively normal faulting

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going on here for this earthquake.
So it just gives you an idea of the

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varied and different kinds of
earthquakes that we can see in Walker

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Lane and the Antelope Valley most
recently giving us an example of that.

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Regionally, we can take a zoom-out,
kind of get an idea of where this –

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where this quake specifically –
where the magnitude 6 from

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July 2021 falls in the bigger picture.
And you can see this, and there’s

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north-south trending normal faults,
generally, that are sub-parallel.

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And, if we zoom in a little bit more,
we see that this earthquake likely

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occurred on the – what is known as
the Slinkard Valley Fault.

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It’s the blue fault trace here in
the center of the – of the diagram.

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And the lighter gray fault there is the –
is the Antelope Valley Fault Zone,

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which is kind of the more prominent
in the area, but, based on mapping

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and looking at things in the subsurface
it’s pretty clear the Slinkard Valley

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Fault is the causative fault here.
And this Slinkard Valley Fault is

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an east-dipping normal fault that
forms a half-graben in the area.

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And that half-graben, as you may have
guessed, hosts the Slinkard Valley itself.

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Interestingly, if you look at the
seismicity here, and this is

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a west-to-east cross-section
looking north – simple cross-section

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of the seismicity that’s happened
since July 8th – all the aftershocks.

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You can see the dark cluster there in
the center that kind of highlights,

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and you can really – pops out the
east-dipping fault zone that is

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the Slinkard Valley Fault and
highlights the – there the star

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for the main shock location.
But there seems to be some shallower

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features off to the – to the east here
that were – that were triggered,

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and they represent secondary structures.
And so I’ve kind of highlighted those

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here to, you know, give us a possible
idea, just to kind of give us a picture

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of what might be happening.
And these might be shallower

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secondary structures that were
triggered by the 6.0 main shock

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and may have had some triggered
seismicity on it. I don’t know.

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But I think it’s an interesting piece
of the puzzle here and might be

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an interesting bit of data to look at
and to see what might be going on here,

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and is this the Antelope
Valley Fault Zone that’s triggered

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in the shallow subsurface?
Could be.

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As far as the faults in this area in the
Slinkard Valley and Antelope Valley

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Fault Zone, CGS, as part of its mandate
with the state, has a mandate to map

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all active faults – active surface faults
in the state which pose a surface

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rupture hazard. And so that’s under
the Alquist-Priolo Earthquake Fault

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Zone Mapping Act of 1972.
And this specific area was evaluated

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in 1983 by Bill Bryant with CGS.
And, back in these days, you know,

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before planes flew through the air
shooting the ground with lasers,

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we had paper topographic maps and
stereo-paired aerial photographs.

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And that’s all that was really available
for the geologists to evaluate faults.

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And so Bill took a look at this area,
and you can see a lot of detailed notes

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here going over the Slinkard Valley
Fault on the west, the Antelope Valley

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Fault Zone in the central part,
and then specifically the eastern

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Antelope Valley Fault Zone,
which is highlighted and noted on the –

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on the east side
there, sorry.

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And, for a fault to be zoned under the
AP Act, it has to be sufficiently active,

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which is any fault that has had
surficial movement in the last

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about 11,000 years.
And it has to be well-defined.

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Can be found at the surface,
traced, and investigated.

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So we see, you know, detailed
notes here – things like possible scarp

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in talus deposits.
That was on an aerial photo.

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You’ll see that there was field
reconnaissance done,

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and we’ve noted here, alluvial
fan surface offset with an angle

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of the scarp of about 32 degrees
and a height of 15 feet.

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And things over here like,
subtle scarps in older alluvium.

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And so all of this is looked at.
All of it is evaluated and weighed

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and to determine, is there a fault that
is sufficiently active, well-defined,

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that should be zoned.
And ultimately, after all this was

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done in 1983, the final map that came
out showed that really only the main

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Antelope Valley Fault Zone was the
one that was found to be zone-able

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as an AP fault, not any fault in
the Slinkard Valley Fault Zone.

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And nothing in the eastern
Antelope Valley Fault Zone.

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And that just means it has no surface
rupture potential that was documented

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or determinable at that time
with the resources available.

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Now, in the days of Lidar, and now,
in the days of SAR imagery and

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things like that, could this
be different? We don’t know.

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It could be something that
can be looked at again.

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But it’s interesting that the
Slinkard Valley Fault was

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clearly the active fault here.
It is the causative fault.

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It’s the seismogenic fault.
It just doesn’t seem to have ruptured

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the surface in this earthquake
or in Holocene time, at least as

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was available in 1983 with the
resolution that we had then.

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So thank you, everyone,
and good to be here.