Earthquake Briefing — The Antelope Valley, CA Earthquake, July 8, 2021

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Earthquake Briefing with Austin Elliot, USGS on the Antelope Valley, CA earthquake that occurred on July 8, 2021.

Details

Date Taken:

Length: 00:19:41

Location Taken: Markleeville, CA, US

Transcript

strong that's enough to knock things 
over really give you a rattle you'd  

hear lots of sounds and get 
pretty uh may get alarmed by that.   

Shaking was felt quite widely from san 
Francisco to east of Carson City in Nevada  

and as far south as Visalia California. Um shaking 
related effects that we've heard of so far  

include rockfall along highway 395 near Coalville 
that's right near the epicenter of the earthquake.

The earthquake is the result of normal faulting 
uh it's a pretty well understood and expected  

mechanism for an earthquake at the eastern 
boundary of the Sierra Nevada mountains

in the region that it in in which it happened. The 
uh our Earthquake Early Warning system ShakeAlert

detected the earthquake 25.6 seconds after 
its origin uh the system underestimated the  

original magnitude of that event and an 
estimated a location that was slightly  

off from the true location. That meant that 
as the waves propagated across the state  

other stations misregistered the event so 
several different shake alerts were sent out  

for earthquakes in different areas and 
people who joined and visited our site upon  

feeling it really early will have seen multiple 
different events show up sort of simultaneously.  

This is a fairly common thing that happens 
at the beginning as our instruments are just  

detecting and automatically sending signals 
from these earthquakes and so it's something  

that's somewhat to be expected. Uh it's worth 
noting that this earthquake just like the large  

one that happened near Lake Tahoe a few weeks 
ago happened at the border of the ShakeAlert

region and so as we build out the system 
further and develop install more instruments  

in the remote locations at the margins of 
our of our state and the state that covers it  

that's covered by ShakeAlert these sorts 
of errors will become less and less common.  

Um we have produced an aftershock forecast 
for this event it shows that there's a six  

percent chance of one or more aftershocks larger 
than the initial earthquake. I'll emphasize that  

a six percent chance is far less likely it's 
it's uh the particularly unlikely scenario. Um  

more likely and as we've seen so far there have 
been about 40 or 40 or more aftershocks so far  

smaller ones than the main event. Uh 
the largest one so far was a magnitude  

4.5 and i believe there was just another 4.4 
maybe 10 minutes ago. All this information you  

can sort of keep track along with the incoming 
information on our website as many of us are doing.

So this earthquake is not particularly uncommon 
for the region it's a region with known active  

faults this happened uh along the trace of a 
mapped fault and um it is the largest in the  

region since the magnitude 6.1 Double Spring Flat 
earthquake which was about 36 kilometers to the  

northwest in 1994, but there have been other 
earthquakes over the decades in this region.  

There have been about a couple dozen earthquakes 
larger than magnitude 5 in the last 50 years here.  

The instrumentally recorded ground motions 
across uh in our recorded by our sensors across  

the state are typical for this size of earthquake, 
so reports of shaking that we've had at large  

distances in San Francisco, in Las Vegas, are what 
we would expect for this event and there was  

likely some amplification in places that 
are built on basin sediments and flat soils.

That's the overview of the 
earthquake that we've all um  

that we just experienced here 
and the aftershocks that continue,  

as i said it's within the realm of possibility 
that a larger earthquake will follow this one  

as is the case with any earthquake and is is 
ultimately the case with even when earthquakes  

don't occur as we live in earthquake countries 
so take this event especially if it whether it  

shook you hard or not take it as a reminder to 
get yourself prepared for future earthquakes  

and visit the USGS website for up-to-date 
information. I'm happy to take any questions that  

you have at the moment on any of this information 
or other information you'd like to know.  

Okay thank you very much Austin this is Paul 
Laustsen, again, um in the chat um there was a  

question from Tim Sheehan uh does the fault 
have a name?

[Austin] Yeah this is the Antelope Valley fault um and that's tentatively the name that we've given the earthquake the Antelope Valley  

earthquake although we recognize that the Antelope 
Valley that's on the California/Nevada border is  

a very sparsely populated area and there is an 
Antelope Valley in southern California that's much  

more populous nonetheless this is the fault 
is the Antelope Valley fault um and so for now  

that's the relevant information. [Paul] Great thank 
you very much Austin i'm going to unmute  

Ron Lin here in just a second i just wanted to 
make sure you guys are looking at the chat um in  

the chat you will find uh a link to the earthquake 
event page that has tons of information on it  

um including that aftershock forecast that Austin 
uh spoke about so without further ado let me on  

you up Ron you're unmuted so go for it. [Ron] Hey this 
is Ron Lin with the LA Times thanks for taking  

our questions. Two questions for you today, can 
you talk about what you felt and where were you  

and assuming you're in the Bay Area can you 
explain you know why some parts of the Bay Area  

felt it and other parts didn't like me in millbrae 
and uh can you also talk more about the seismicity  

of the Walkerland System how often does it have 
big quakes what's the maximum magnitude a quake  

can can hit there and is it a threat to places 
like Mammoth Mountain and Lake Tahoe thanks so  

much. [Austin] Absolutely thanks for the questions Ron um 
yeah i uh i felt the earthquake fairly strongly  

i'm in downtown San Francisco essentially 
on a relatively high floor and i think that  

that may have um that certainly amplified 
the experience compared to neighbors and  

other friends around me who did not feel the 
earthquake. So there are a lot of reports of  

the earthquake being felt around the Bay 
Area but it's probably not as widely felt as  

even one of the more local earthquakes that we 
had for example last week on the Hayward Fault um  

which will be you know sharper and stronger 
to people living near the epicenter. So this  

one because of its distance was probably more 
perceptible in places that really amplify the  

the sort of slow distant waves and 
so like the high building that i'm in.  

Um as with any earthquake there's also 
amplification in soft soils, basin sediments,  

so if um if you're living on a bedrock hillside 
or a ridge somewhere you're less likely to feel  

this same strength of shaking as people who are 
uh living or working on in the flatlands basically.  

Um you're uh so that was the experience in the Bay 
Area um certainly if you're in Reno, Carson City,  

uh some of the bigger places in Nevada, this 
was a much stronger jolt um for you there  

and it but more than a jolt it's probably 
a fairly good shake it was pretty alarming  

understandably so. Uh your second question was 
basically how frequently how commonly earthquakes  

happen along here, and you know just in the last 
year we've seen a fair number of earthquakes  

around this region and I think it's not 
particularly uncommon, they're pretty  

representative, this is a classic place that 
geologists go to study uh the active faults  

that we know formed the pretty dramatic topography 
when you go east of the Sierra Nevada mountain  

range and through Nevada. So this is just 
the latest among those there was another one  

last year you may remember the 5.9 Lone Pine 
earthquake which happened farther to the south  

um in may last year there was amenities 
6.5 a fair bit farther east in Nevada um  

there are quite a few examples spanning the 
decades of these kind of magnitude five to six  

earthquakes uh happening in the eastern Sierra. 
Certainly um so the fault that this happened  

on is essentially a small portion of the Sierra 
Nevada frontal bounding fault system so it's what  

constitutes that massive majestic scarp 
that you see if you drive up Highway 395  

um visiting places like Owens Valley, Bishop, 
Mammoth Lakes, June Lake, all the way to Reno um and  

so this fault this was a small earthquake along 
essentially that whole fault system and that's a  

very active structure it forms one of the highest 
relief mountain fronts in the United States.

[Paul] great thank you for the crescent question 
Ron and just a reminder if you guys have  

had your questions answered then 
just go ahead and lower your  

hand appropriately next uh Dale and 
Dale can you please also let us know  

of course. hi uh Dale Castler Sacramento Bee 
and sorry about the video I just like to  

call attention to myself i guess um and you 
sort of touched on this a few minutes ago but  

I mean we're 200 miles away here in Sacramento we 
usually don't feel earthquakes at all around here  

and everybody it's as if everyone around here felt 
it. Was there something about this earthquake that  

for some reason it just 
spread and spread and spread?  

[Austin] Yeah based on the measure well based on the 
measurements that we have this is not out of  

the ordinary earthquake in terms of the shaking 
intensities uh either instrumentally recorded  

or reported to our "Did You Feel It?" system based 
on people's eyewitness shaking experiences.  

Um i think some things that may make this notable 
it was a a fairly large earthquake and it's worth  

remembering that the magnitude scale is 
logarithmic and every unit increase in magnitude  

is a 10 times an increase in the size of the 
earthquake, um which ultimately means that a  

magnitude 5.9 earthquake shakes much more strongly 
and has a much wider reach than many others.  

So other comparable examples to this one would 
be things like the Monte Cristo earthquake last  

May um so a year and a couple months ago and I 
think that was pretty widely felt although it  

was in the middle of the night so another factor 
here maybe that it was at 3:30 in the afternoon  

a time when probably people are sitting around 
awake attentive um a lot of people just at work  

basically um and were awake to perceive the 
the way the waves from this large earthquake.

[Dale] Okay thank you uh and if i could follow up in the 
the Farmington quake that wasn't that was because  

in the initial ripple or the original initial 
waves it was just misregistered as its own quake?  

[Austin] yeah essentially yes the um the ShakeAlert system 
our instrumentation is sparse away from the large  

population centers and as you go farther into 
Nevada and the ShakeAlert Early Warning system

is structured to function for California so 
this is on the mar this earthquake really  

occurred right at the margin of the operational 
earthquake warning system uh and in an area with  

relatively sparse seismic instrumentation. So 
what that means is that the initial magnitude  

takes a little bit longer to calculate may have 
some greater uncertainty to it because there just  

aren't as many instruments there recording 
it. What that means is that if an earthquake  

it's always a balance of speed if we're 
trying to get early warnings out we want  

to determine the magnitude and location 
of an earthquake as quickly as possible,  

but if you only have one or two stations uh 
reporting you're going to have some on some  

rather large uncertainty in where exactly that 
earthquake happened. So in this case the initial  

estimate of the location of the earthquake was a 
few tens of kilometers off and that meant that as  

other stations registered the seismic 
waves they actually registered them as  

a separate earthquake because they arrived 
at an unexpected time compared to what our  

systems forecast the wave arrivals to how they 
forecast the wave arrivals to reach the stations.  

[Dale] Thank you thanks very much. [Paul] Great thanks 
Dale thanks Austin okay um next we have  

um Tim Sheehan uh and you please state your 
affiliation uh let me make sure you're unmuted.  

[Tim] can you hear me okay?
[Paul] Yep.
[Tim].okay great yeah i'm with  the Fresno Bee uh one of Dale's colleagues uh  

with the company what I wanted to find 
out is geologically is this area prone to  

you mentioned that this area is 
not uncommon to see earthquakes  

I think in in way past history we're looking 
at volcanic activity is there any reason for  

anybody to get worked up about you know the the 
say the Long Valley Caldera as a result of this uh  

quake or is this just uh circumstance? [Austin] It's a great 
question and we're talking about an area that in  

um the eastern Sierra the eastern California 
is full of tectonic and volcanic features  

and activity. Uh the Long Valley Caldera, 
Mammoth Mountain and the Ineo craters  

are a fair way south of where this earthquake 
happened and so they're likely unrelated um  

to the to the event that we had this afternoon. 
Furthermore what we saw this afternoon has a

source mechanism that is consistent with motion 
on a tectonic fault rather than having a volcanic  

source and so um in in that regard we have no 
reason to expect that it has anything to do with  

the volcanoes that are a fair bit farther south of 
it. Um and i i yeah i don't believe that there are  

any uh volcanoes or any volcanic activity that's 
commonly registered anywhere north of that so this  

is quite reliably a tectonic earthquake and a fair 
bit away from the active volcanoes that do exist.  

[Tim] Alright thank you very much. [Paul] Great thank 
you very much Tim, um Amanda del Castillo  

could you please state your organization's 
name and feel free to ask your question  

[Amanda] okay hello yeah i'm Amanda del Castillo with ABC 
7 news here in the Bay Area wanting to know  

historically does the USGS in California report 
more earthquakes during drought years? Thank you.

[Austin] Thanks for the question Amanda that's 
uh an interesting question and obviously  

one that will be on people's minds 
considering the the confluence of  

the dramatic natural events that are going 
on around us. I think it's unlikely that  

drought has any impact on the production of 
tectonic earthquakes and we have not detected  

any substantial changes in the overall rates 
of earthquakes uh in in recent years uh or  

any fluctuations over the decades so it's unlikely 
that there's an observable effect at this point.

[Paul] Okay thank you very much uh Austin. Are there 
any other questions from the group here?

Okay we will have a USGS feature story that will 
go up on the main USGS website here pretty soon  

and we will keep our information 
information updated there.  

I will try to post a recording of this call as 
well for uh reference for your uh for stories  

um unless there's a larger unless there's a larger 
event um we will not plan on taking more media  

inquiries uh this evening but you can go ahead 
and send those to myself or Elizabeth Goldbaum,  

we can get those emails posted in the chat but if 
you're looking for updates on the story we will do  

that in um on the USGS main page. Okay this is 
your last chance to ask a question. [Austin] I may jump  

in and just make one little addendum which 
i noticed just as i was beginning the press  

conference here the magnitude has of the main shock 
at 3:30 this afternoon has been upgraded to a 6.0.  

Um that's nothing unusual nothing 
to be worried about this happens  

as information comes in in the first minutes 
and hours following an earthquake, magnitudes  

get refined and recalculated as more and more 
seismometers report um and data is processed so  

uh in reporting for the stories that's the latest 
magnitude you can always keep track on the USGS  

website um i just want to add that. [Paul] Yeah 
that's huge, uh thank you very much Austin.  

Okay well thank you for everyone's um time and uh 
we hope we won't be speaking to you anytime soon.  

Um well i'm, just in jest, you're always welcome 
to call and reach out to us if you need  

something thank you very much for your time this 
afternoon and thank you Austin.

[Austin] Thanks everyone.