Well, good morning. Want to take this opportunity,
as we’re talking about the faults of eastern California, to talk about one that
roared to life in July 2021 and produced the magnitude 6.0 Antelope Valley
earthquake that was, you know, widely felt across a lot of northern California.
So, to review generally the setting, the map on the – on the right here shows
one year of seismicity in this area leading up to the day of the main shock.
And you can see that there is no appreciable, you know,
seismic activity going on in the area of the main shock.
In fact, most of the – or, most of the earthquake activity seems to be focused
up near the Walker Lake area, which is along the California-Nevada border.
And so this event specifically had no appreciable foreshock sequence.
There were, that day, only two magnitude 2 earthquakes leading up to
the main shock at 2:49. And immediately, the main shock was
followed by the largest aftershock, which is a 5.0, just one minute later.
So, to the locals, it may have even felt like
one long earthquake because they were so close together.
The big question a lot of geologists and a lot of us will ask when we, you know,
get moderate quakes of this size – moderate to strong, you know, 6.0s,
what are the surface effects of a quake like this.
And so it’s well-documented that there were earthquake-induced
rockfalls on Highway 395 from this event.
You can see here a screen capture clip from a video taken, and you can see that
the rockfalls here on Highway 395 created a nice slalom course and
an opportunity for people to get out of their cars and test their strength.
Usually that doesn’t go well with boulders which are
much heavier than people anticipate.
Another thing we look about was, was there any surface rupture from this
earthquake, and no, there wasn’t any documented surface rupture.
And that’s maybe not too surprising because magnitude 6.0 is kind of
on the – on the verge of the edge of where we might expect surface rupture.
There was no documented liquefaction from this event, and that is, more than
likely, to be attributed to the – probably the depth of groundwater
in the Antelope Valley and not necessarily to be attributed to
the not-strong-enough shaking.
Surface effects we saw here include ground subsidence about
10 centimeters, from – based on InSAR data from the Sentinel satellites.
So we do know that this – it did subside. We did get some ground deformation
here, but nothing on a more specific, you know, surface rupture
or liquefaction, much more – more on a regional scale.
The earthquake itself was felt quite widely across northern California
and northwestern Nevada. You can see the yellow coloring here –
yellowish coloring from the ShakeMap showing intensities 5 up to 6 in the
Walker area, but this area is, you know, fairly sparsely populated, although
it was felt quite strongly in area cities, like Carson City, South Lake Tahoe,
and Reno. The shaking did travel all the way across the Sierra into
Sacramento and into the Bay Area, which got a lot of people’s attention.
You could see it also followed a fairly predictable decay pattern,
or drop-off pattern, with density over – or, density compared –
distance compared to intensity for this quake.
Now, a lot of us that may, you know, live closer to more active fault regions
in the state may think that an intensity, you know, 2 or 3 may not be that big
a deal, but to maybe a state senator in Sacramento feeling an intensity 3, well,
that, you know, might qualify as one heck of an earthquake to you.
So it got a lot of people’s attention for being in such a remote area
of California for this reason. As we’ve been talking, this area this
earthquake occurred in is within the Basin and Range province of western
North America, a large zone of tectonic extension initiating
in the early Miocene. And specifically, it’s a part of
the Basin and Range that is a transition zone on the western end
of the Basin and Range, a transition zone between the Sierra Nevada and the
Basin and Range province, which is characterized –
is an interestingly complex area of strike-slip and normal faulting together.
It’s woven in and out. It’s pretty complicated.
And you’ll see earthquakes with both focal mechanisms, faulting styles.
And this area accommodates 20% – I’ve seen up to 25% cited for the
relative plate motion between the Pacific and North American Plates.
And that comes out to on the order of about 10 millimeters per year.
Looking at the historic earthquakes that have happened in this area,
and this is just since instruments have been recording, you can see
it’s not tremendously robust. There are two 5s-ish earthquakes
down – in 1962 and 1979, down west of Bridgeport.
And then you see the Double Spring Flat earthquake, a magnitude 6.1,
in 1994 just north of Walker Lake on the state border there.
And then last year’s magnitude 6 2021 quake.
And here’s an interesting case I just wanted to point out that,
here’s where the complexity of Walker Lane is really visible
and can be seen quite well. In the 1994 Double Spring Flat
earthquake, focal mechanism shows predominantly strike-slip faulting
in this area of the Walker Lane. And if you look at the magnitude 6
Antelope Valley earthquake, we can see the focal mechanism gives us
kind of the opposite, and we have almost exclusively normal faulting
going on here for this earthquake. So it just gives you an idea of the
varied and different kinds of earthquakes that we can see in Walker
Lane and the Antelope Valley most recently giving us an example of that.
Regionally, we can take a zoom-out, kind of get an idea of where this –
where this quake specifically – where the magnitude 6 from
July 2021 falls in the bigger picture. And you can see this, and there’s
north-south trending normal faults, generally, that are sub-parallel.
And, if we zoom in a little bit more, we see that this earthquake likely
occurred on the – what is known as the Slinkard Valley Fault.
It’s the blue fault trace here in the center of the – of the diagram.
And the lighter gray fault there is the – is the Antelope Valley Fault Zone,
which is kind of the more prominent in the area, but, based on mapping
and looking at things in the subsurface it’s pretty clear the Slinkard Valley
Fault is the causative fault here. And this Slinkard Valley Fault is
an east-dipping normal fault that forms a half-graben in the area.
And that half-graben, as you may have guessed, hosts the Slinkard Valley itself.
Interestingly, if you look at the seismicity here, and this is
a west-to-east cross-section looking north – simple cross-section
of the seismicity that’s happened since July 8th – all the aftershocks.
You can see the dark cluster there in the center that kind of highlights,
and you can really – pops out the east-dipping fault zone that is
the Slinkard Valley Fault and highlights the – there the star
for the main shock location. But there seems to be some shallower
features off to the – to the east here that were – that were triggered,
and they represent secondary structures. And so I’ve kind of highlighted those
here to, you know, give us a possible idea, just to kind of give us a picture
of what might be happening. And these might be shallower
secondary structures that were triggered by the 6.0 main shock
and may have had some triggered seismicity on it. I don’t know.
But I think it’s an interesting piece of the puzzle here and might be
an interesting bit of data to look at and to see what might be going on here,
and is this the Antelope Valley Fault Zone that’s triggered
in the shallow subsurface? Could be.
As far as the faults in this area in the Slinkard Valley and Antelope Valley
Fault Zone, CGS, as part of its mandate with the state, has a mandate to map
all active faults – active surface faults in the state which pose a surface
rupture hazard. And so that’s under the Alquist-Priolo Earthquake Fault
Zone Mapping Act of 1972. And this specific area was evaluated
in 1983 by Bill Bryant with CGS. And, back in these days, you know,
before planes flew through the air shooting the ground with lasers,
we had paper topographic maps and stereo-paired aerial photographs.
And that’s all that was really available for the geologists to evaluate faults.
And so Bill took a look at this area, and you can see a lot of detailed notes
here going over the Slinkard Valley Fault on the west, the Antelope Valley
Fault Zone in the central part, and then specifically the eastern
Antelope Valley Fault Zone, which is highlighted and noted on the –
on the east side there, sorry.
And, for a fault to be zoned under the AP Act, it has to be sufficiently active,
which is any fault that has had surficial movement in the last
about 11,000 years. And it has to be well-defined.
Can be found at the surface, traced, and investigated.
So we see, you know, detailed notes here – things like possible scarp
in talus deposits. That was on an aerial photo.
You’ll see that there was field reconnaissance done,
and we’ve noted here, alluvial fan surface offset with an angle
of the scarp of about 32 degrees and a height of 15 feet.
And things over here like, subtle scarps in older alluvium.
And so all of this is looked at. All of it is evaluated and weighed
and to determine, is there a fault that is sufficiently active, well-defined,
that should be zoned. And ultimately, after all this was
done in 1983, the final map that came out showed that really only the main
Antelope Valley Fault Zone was the one that was found to be zone-able
as an AP fault, not any fault in the Slinkard Valley Fault Zone.
And nothing in the eastern Antelope Valley Fault Zone.
And that just means it has no surface rupture potential that was documented
or determinable at that time with the resources available.
Now, in the days of Lidar, and now, in the days of SAR imagery and
things like that, could this be different? We don’t know.
It could be something that can be looked at again.
But it’s interesting that the Slinkard Valley Fault was
clearly the active fault here. It is the causative fault.
It’s the seismogenic fault. It just doesn’t seem to have ruptured
the surface in this earthquake or in Holocene time, at least as
was available in 1983 with the resolution that we had then.
So thank you, everyone, and good to be here.