[inaudible background conversations]
Okay. Let’s kick things off.
And I will introduce Tim Dawson to introduce our speakers.
- All right, people out in the hallway. Get in.
Put the coffee down.
I don’t have many introductory remarks except for I’m pleased to
be back in Menlo Park, as are some of you USGS folks who used to
hang out around here who have moved to Moffett Field.
I’d like to kick off this morning’s mid-morning session on the
Ridgecrest earthquake sequence with the theme of fault complexity –
lessons from Ridgecrest. And our first speaker will be
Ken Hudnut, who was instrumental in the post-earthquake response,
along with a lot of people in this room. I think almost every geologist in this
room somehow participated in the earthquake response.
And the title of his talk is Cross-Fault Interaction in the
July 2019 Ridgecrest Earthquake Sequence, Southern California –
what we did see in the field from imagery and GPS data
prior to the M 7.1. Take it away, Ken.
[silence]
- So this is a story of serendipity and teamwork and being ready.
I won’t read all the names here, but I just want to say how much I
appreciate all of those, especially those who got out into the field right away.
This proves to be an example of how important that can be
when it comes to making observations of perishable data.
You’ve seen this figure many times. What I’d like to do is zoom in
progressively to that yellow square – the intersection of the 6.4 rupture,
shown by the lower-right red star – the 6.4, and the 7.1 epicenter,
shown by the upper-left red star.
So the intersection of those two main ruptures is where we will zoom into.
And we’ll be looking, in many cases, on these figures at the best available
compilation as of now. This is a ongoing effort,
as Tim mentioned, of this large group effort represented by the SCEC poster
by Kendrick et al., the submitted SRL Ponti et al. paper, and the Dawson et al.
to-be-submitted paper, as well as slip distributions from DuRoss et al.
So many, many people, as Tim mentioned, worked on this.
But I will focus my discussion here today on the observations
from prior to the 7.1.
What I wanted to do in sort of setting it up is talk about cross-fault
interaction in the 1987 Superstition Hills earthquake sequence, which,
of course, really got off to a start with the 6.2 Elmore Ranch event.
You see in the A panel upper left, left-lateral on a northeast-southwest-
oriented fault. Followed 11.4 hours later by Panel D, below left, and that is
a right-lateral, northwest-southeast- striking right-lateral fault, 6.6.
And so you hopefully know this case well.
Just one figure from the paper we did in GRL in ‘89 showing our stress
change model. And, in particular, how we explained the delay
between those two events. Well, how can this cross-fault
triggering hypothesis help to understand the 2019 sequence?
I’ve used green to indicate yes – good, and yellow, maybe.
So the delay between foreshock and main shock – I think that our model
is one that works also for Ridgecrest. It’s not required. There are alternative
models that could also explain the 34.3-hour delay that was
seen in Ridgecrest between the 6.4 and 7.1.
The rupture propagation, on the other hand, I give that a maybe in yellow
color here because it’s interesting that, when we do flip Superstition to look at
it as – and try to explain Ridgecrest in a similar way, remember that
Ridgecrest looked like this – a capital L shape with the epicenter of the 6.4 here.
And this is after the 6.4 but before the 7.1, it looked like this.
And so, what we’re going to look at is surface rupture on this leg of the L.
Okay, but, in general, at the time, from seismology, we knew the epicenter
was close to where they intersected. And so we were presuming that
there was right-lateral slip here, left-lateral slip here.
Well, now we flip that. And here we have 87 Superstition Hills,
where the 6.2 was like this on the Elmore Ranch fault, and the 6.6
like this on the Superstition Hill main rupture.
So that’s all I’ve done here is just flip it. And so you see the problem now,
in the lower right, with the three question marks in the yellow oval.
In the Ridgecrest 7.1, the rupture propagated
from its epicenter of the 7.1 toward the southeast.
And then, somehow – I’m not quite sure why – it kept on going beyond
the intersection with the cross-fault. So just simply from the geometry
on Ridgecrest, the 6.4 left-lateral would have unclamped this portion
where the 7.1 occurred. But it would have also clamped this portion.
So the rupture kept going. That’s a surprise.
Now, in terms of the delay, the equation in the lower right
of this slide is straight from our ‘89 paper.
The delay may well be for a similar or same reason it would work –
this mechanism would work to explain it.
Okay. So, in terms of addressing the complexity, as Sarah said,
this session is about trying to get a handle on fault rupture complexity.
Steve gave some examples, too, of these incredibly complex earthquakes
that we’ve seen in recent time. And I would argue that’s partly
because of all these improvements in our capability to observe –
as Steve mentioned – all the new techniques.
Among them, seismicity relocation methods have gotten so much better.
I’ll show some examples from the work of David Shelly, but of course,
you’ve seen the Ross et al. Science paper and Hauksson’s talk at AGU.
Other examples where seismicity relocation is really sharpening
our knowledge of rupture complexity.
So, in this case, what you see in blue is all of the seismicity associated
with the 6.4 and the hours after it prior to the 7.1.
And in red is the 7.1 and after it. And so what you’ll see here as we,
again, zoom in toward that yellow square, in the 6.4 – whoops.
I got ahead of myself there.
I wanted to mention that there is some complexity and some cross-faulting
between the time of the 6.4 and the 7.1 that is important to understand better.
I’m not going to get into that in detail here today.
But there was a 5.4 about 16 hours before the 7.1.
And that appears to have been on a cross-fault.
There was also this 5.0 that – those of us that were in Ridgecrest for the 7.1,
we also were in the earthquake clearinghouse meeting, and we also
felt this 5.0 that was just a few minutes before the 7.1.
It may have been on a cross-fault, but we need to really zoom in
and examine that more closely.
Okay, so the question of whether the 6.4 simply triggered the 7.1 in this
simplified cross-fault triggering mechanism like Superstition, it’s really
more nuanced. It’s more subtle. It’s much more interesting than that.
And there’s actually a series of cross-faults that broke going from that
apex of the L toward the northwest, migrating in a way – not cleanly and
simply, but in a more complex way – toward the eventual epicenter of the 7.1.
All of this, by the way – the Southern California Seismic Network data may
be cited using this DOI number. And I hope you’ll cite these other works
as well as the SRL data mine article by David Shelly that’s just come out.
Okay, so now these dots are moving. That’s because these are showing
you three GPS stations from the GAGE NOTA network.
And nothing’s happening much, and then the earthquake starts.
And notice this left-hand panel. This is P595 at 5 hertz.
That station started moving toward the southeast.
That’s what we would expect in this big earthquake – the 7.1.
That station, which is in the southeast quadrant, would start moving towards
the southeast. But, unexpectedly then, after about just a few seconds,
it takes off towards the northeast. And that’s not explained well by
any of the models I’ve seen yet. Maybe Julian Lozos is going to
explain that with his latest model when he gets up here next.
But I would say that this is indicating some rupture complexity in the 7.1
that may involve cross-faulting during the 7.1 as well.
Okay, so let’s leave that aside for the moment and keep blasting through.
Let’s back up now to the 6.4 and look right here – I’ll bring the cursor up.
Nope. Look at the – what’s in the blue circle there.
On a northwest-southeast-oriented structure, that’s where the seismicity
began prior to the 6.4.
So now, back to this idea that we might be able to apply cross-fault
triggering to the 2019 sequence, well, that indicates that this started on a
northwest-southeast right near the 6.4. And then activity migrated, then,
onto the northeast-southwest fault. So, again, this is happening at multiple
scales, including small scale, different time intervals – this is
over 30 minutes prior to the 6.4 that you had a magnitude 4.
But this is a – sort of an escalation and a migration of seismicity
from one fault to another. In this case, to start the whole thing,
it actually began on a northwest- southeast-oriented fault.
Rather surprising, but now that we see these details in retrospect, I just find it
fascinating that the cross-faulting was occurring at different scales.
Okay.
I’m going to swing into a quick play-by-play mode here.
On the 4th of July, Janis Hernandez and I went straight up to the field together
and got out there and started taking photographs at about 7:00 p.m.
And here’s what we came across when we got to the field.
A wide zone – about 165 meters wide in total – and measured across
all those strands and summed up, we got about 50 centimeters of
left-lateral slip on a north- northeast-striking zone.
And there were two main zones of rupture across Highway 178 that
we initially mapped. And Janis had with her the spray paint, and she was
able to lay down some straight stripes to check for any afterslip later.
And we were joined in the field by Kelly Blake from the Navy,
Ben Brooks, Todd Ericksen from USGS, who helped us
make all these observations. And you’ll see some photographs
down on the right just indicating that these were our first observations
of surface fault rupture. We could see that it was big and
interesting, and we wanted to get back out there the next day.
So we set everything up for initial aerial reconnaissance that took place
the afternoon of July 5th. This is our flight plan on the left
that we provided to the Navy, and they approved it.
We needed to take this helicopter inside the Navy base, and this is when
we found out how tight security is at Naval Air Weapons Station China Lake.
But we were helped by them tremendously,
and Kelly Blake really opened the doors for us.
Okay, so this is that L-shaped pattern that I was talking about.
So we wanted to go out and recon both legs of that L.
In the end, you can see from the right panel, which is what we
actually got to do, we really had to cut it short on recon on the
northwest-southeast segment. We were able to go across it a couple of
times, but because of the short duration of this California Highway Patrol flight,
we didn’t get to do everything that we really intended to do.
But we were able to get a lot done in a hurry.
And so, on the right, what you’re looking at here is our ground base
reconnaissance from the morning of the 5th.
So, as we were waiting for the CHP helicopter to come in in the afternoon,
we got out there to – on the ground into what we call Skytop Valley.
So, on the right-hand panel, that’s got a dashed, heavy orange line around it.
So that’s up near the epicentral area. And, in that Skytop Valley, Janis,
Kelly, and I and some Navy escorts and unexploded ordnance experts
came with us, and we identified some places where there was minor cracking,
but we really couldn’t map surface rupture on any of those.
But it was really during the aerial reconnaissance in the afternoon,
we flew out to Highway 178, and from there, recon-ed to the northeast and
southwest and got air photos, which turned out later to be very useful.
And I’ll spend some time emphasizing that part of the air recon that we did.
So the actual duration of the aerial reconnaissance – we took off
and landed at the Navy airfield inside the base, and it really lasted
only from 1:40 p.m. to 2:50 p.m. But I want to start with the ground
recon in Skytop Valley in the upper right – that orange dashed line.
So that’s where we were able to first make some observations.
And then – nothing large enough to measure, though, like I said.
We then met the Navy and took off and began – we moved from Highway 178
toward the epicenter, then we went all the way down to the southwest end,
back up, and continued to try to chase down – figure out where
the rupture was within the yellow square there.
So, along the way, I captured stereo overlap photos of most of the fault
rupture that we identified from the 6.4. We also then took single images –
as Janis and I were flying, we were just taking pictures when
we saw cracks on the ground. And so some of those are single,
not stereo overlapped, and I’ll show how some of that is also very
interesting in terms of figuring out exactly what broke in the earthquake.
So, but yeah, it was all said and done before 3:00 p.m.
And then after that is when we went in and conducted the emergency
operations center briefings, first with the Navy and then with the city.
Okay, so one of the things that we were very interested in, and Janis and
I finally had an opportunity to just sit down and sort of go through
what we have from this initial air recon.
Just the other day, we actually did this for the first time, believe it or not.
But things started happening fast, I got to say.
So these are circles of places where, either on the ground or in the air,
on July 5th, we either drove by or flew over, and we think we would have seen
surface rupture if it had been there. And so we’re looking for absence
of rupture observed. And so, in the Skytop Valley –
that’s where we’re zoomed in here, and these are showing you places from
the compilations that I mentioned – Kendrick et al., Ponti et al.,
Dawson et al., DuRoss et al. – those compilations that are shown here in
the purple and orange lines, for example, later, on the basis
of satellite imagery and ground mapping, ruptures were mapped later.
And so these are places circled where I think, and Janis and I agree,
that we would have seen rupture on the 5th if it had been there.
And so we’re trying to be quantitative about where we did not observe rupture.
So I’ll show some more circles like this on some other upcoming figures.
The orange circle there is one that Janis visited again in the field,
not only on the morning of the 5th of July, but then again later.
And, at the time when she visited again later, she was quite certain
that there was more offset on those fractures than on the 5th.
And so that’s interesting. That’s what we’re looking for
is evidence that some of these breaks occurred in the 6.4
and then re-ruptured in the 7.1. And I’ll come to it.
We have some other examples where we know that rupture occurred in both
events and that the slip from the 6.4 was increased by the 7.1.
Okay, so back to the initial plan for our day – the red pattern and
then the yellow intersection area on the left there.
And then we’re zooming into that yellow square at the intersection.
Again, that same overlay from all of the compilation work that’s been done
to include both the 6.4 rupture and 7.1 rupture, but trying to
separate out what happened in the 6.4.
So here you see, zoomed in on it, these are the flight tracks as well as the GPS –
the GPS tracks from the ground recon in the morning.
In the upper center of that right-hand panel is part of
what I’m going to refer to as the Bunker Complex.
This is out in the clip area at the NAWS China Lake.
And so – and then there’s the red pond, which also serves as
reference in the lower left of that. But the salty crust on the ground
surface there, that’s called Salt Wells Valley.
And so the northeast-southwest- oriented main surface rupture
and the intersection with what would become the 7.1 main rupture
is what we’re looking at here. And you can see how we really
flew around that intersection area quite a bit – Janis and I did
on the afternoon of the 5th. And so this is where, as we were
coming toward the northeast, we could see the rupture
very prominently on the ground and take photos of it.
And then it just stopped. And so what’s interesting is how,
in the 6.4, that eventual intersection with the 7.1 was very important,
apparently, in just terminating the rupture.
And yet also there’s all this complexity right there.
So what we’re going to take a look at further as we zoom in is that,
in that same intersection area, there was rupture on the northwest-southeast-
oriented fault, but in the 6.4, as well as re-rupture in the 7.1.
And this may be also the best point to mention, there are a couple of other
single photos up in the Bunker Complex and also northwest of the
main strand of the cross-fault that are single shots and triplets over there.
There is rupture in those, but that’s kind of a sidelight issue.
I’m not even going to dwell on it anymore.
But, in places where, in the main compilation, we still don’t have faults
mapped, there were definitely ruptures in there as well that we need to get
onto the main compilation. But this is all part of the sort of
work-in-progress. One further thing to mention is that we did fly over –
as we were returning to the base, we flew over what became later
in the 7.1 the main strand ruptures farther to the northwest.
And we were higher and moving faster at this time because the CHP needed
to return, and we ended up not being able to refuel and get back out there.
But this shows you those two red circles in the right panel that we did
fly over areas that later then had large surface rupture, and we did not observe
surface rupture at those places on the afternoon of the 5th.
And this would have been about 2:45 on the afternoon of the 5th of July.
Now, I’m going to go through several slides quickly here.
This is back at the intersection, and you see here those features
we’ve described – the Bunker Complex in the upper right,
Salt Wells Valley in the lower left, and the red pond for reference.
So this is the full compilation of the 6.4 and the 7.1.
And now we’re going to look at the observations that are in
the latest version of the compilation from Ponti et al.
And this is places where people have been to on the ground since
then to make observations. So you can see that, even though
a lot of observations have been made, many of the key areas, there has
not been a – we haven’t yet been able to get back in there and
do a concerted effort to really do detailed mapping to look for,
for example, cross-cutting relationships of the surface rupture in here.
Again, here are the photos that we got. And the yellow square zoomed in.
Just to get you that reference again. And so this is an area of great interest,
of course, for understanding fault interaction.
So here, again, and with a COSI-Corr overlay,
this is including the 6.4 and 7.1 ruptures and the photo centers from the
7.5 air recon and the flight tracks from that air recon at the intersection area.
You see the stereo overlap photos along the main rupture.
You see where the main rupture stopped at the eventual 7.1 main rupture.
And then those single photos that I mentioned – 0073, practically in the
center of the photo, is one that I’ll show in more detail,
and we’ll zoom in on that. The other ones on the left-hand side
that I’m not going to show today that do have rupture in them, we haven’t
made sense out of that yet, but it’s probably some additional rupture on
northwest-southeast and northeast- southwest-oriented faults.
But over off the main ruptures – these were not captured in any
of the imagery by Milliner or Donnellan or anybody.
All right. So back again and zooming in.
Oops. I was going backwards instead of forwards there. Forgive me.
So this is now with some interpreted line work
from Alex Morelan from CGS.
And now that same view but without the COSI-Corr just to show you the
background imagery [phone ringing] – sorry. Time is up already.
Okay. I’ll keep moving. So here we go. [chuckles]
Yeah. Sorry. 0073 that I mentioned shows very interestingly that we had
northwest-southeast-oriented rupture associated with the 6.4, very clearly.
And here we zoom in onto that frame of imagery.
And the little red arrows in this left panel show you that. Okay?
So, as we zoom in, we see that, yep, northwest-southeast-oriented,
right-lateral rupture – because we see that stepping en echelon left-lateral.
And then here it is after the 7.1. This is from a shot that Gordon Seitz
took looking back toward the southeast along the main 7.1 rupture
but after the 7.1. So this is the same place shown
with the yellow bracket here where we clearly have rupture after the
6.4 northwest of the intersection of the cross-fault.
So that’s a pretty amazing observation. And here are more photos of that same
intersection, just differently oriented. And this is new material.
A lot of this since AGU, so even those of you that saw that talk.
Again, also this is new from Alex Morelan who took photos that I took
on the 5th and that Gordon and I took on the 12th of the intersection area.
We used COSI-Corr to show that the – not only did the northwest-southeast-
oriented fault re-rupture in the 7.1, so did the northeast-southwest cross-fault.
It also re-ruptured in the 7.1 at this location.
Now, Ben Brooks has already shown from his data that,
down at Highway 178, first of all, they re-measured those afterslip lines,
and those did not show slip. And the GPS didn’t either.
But up near the intersection, in the 7.1, there was additional left-lateral slip.
Again, this a hot new result. And here, just as we’re trying to
summarize where we did not see slip, there are a bunch of places where
Janis and I believe we would have seen slip on the 5th if it had been there.
So, again, we’re trying to be quantitative about the
absence of observations. And those blue circles show you
the off-main ruptures that we talked about earlier.
Sorry to have taken too much time, but I will just go briefly through the –
in terms of cross-fault interaction, it’s reminiscent of ‘87
Superstition Hills. That fluid diffusion is a good explanation,
we think, also, for Ridgecrest. There are issues with the fault
interaction in general and rupture in the 7.1 of Ridgecrest going through
the intersection area and continuing southeast.
Not well understood. Very interesting.
And ‘87 appears simpler. Maybe that’s because the data now
are so much better and also because the events were bigger, so more to study.
And so also we had no surface rupture observations in ‘87 after the –
after the Elmore Ranch but before the Superstition main shock.
So here we have these great observations. That’s really helpful.
And we’re just working our way through continuing to study this
example of fault interaction. We find it very significant
that we have this definitive evidence now of re-rupture
on both the cross-fault and main rupture.
Both of them broke in the 6.4 and again in the 7.1.
And it really underscores the importance of doing rapid-response
aerial reconnaissance and getting out there quickly.
And I know I don’t have any time to get into last point.
So I’ll do it very fast.
[laughter]
Sorry about this, so yeah, this is – I didn’t have a slide when
I talked about this at AGU, so I created a slide just for you.
We looked at the Gulf of California, Salton Trough, Eastern California
Shear Zone, Walker Lane, and the possible future of that.
It’s been written about in Wired and so forth.
But you know the story. A lot of this has come from GPS.
But really, basically, the upper-right panel shows how, if you orient the
whole system to show the plate boundary structure, it’s really kind of
weird that you’ve got the big bend, and it really would be simpler to just
take a straight shot all the way through, not only to Cape Blanco, but maybe
even eventually to the Haida Gwaii Fairweather System.
Furthermore, where we see cross-fault interaction in the northwest end of
El Mayor-Cucapah and Superstition and Landers-Big Bear and in Ridgecrest –
also Chalfant Valley and 9-Mile Ranch examples, it’s really along
this alignment where we’re between the right-lateral shear of the
San Andreas and the extension of the Basin and Range.
So these red circles in the center upper panel are where I think we may have a
tendency to have cross-fault interaction, and that may well be where we have
detachments and metamorphic core complex formation.
Sorry to take too much time. Thank you.
[Applause]
- We have some time built in for discussion at the end,
so I’m going to hold off the questions and introduce Julian,
who will be – oh, yeah. Please pull yours up – who will
be talking about dynamic rupture simulations of the July 2019 Ridgecrest
earthquakes and implications for other cross-faults.
[Silence]
All right. So thank you all for inviting me. Thank you all for coming.
Hopefully, because this started as an AGU talk that I added
a few more slides to, I will make up for some of this time.
But anyway, so I’m here from SoCal to talk about some SoCal earthquakes, but
I think they do have, as we’ve already heard, larger-scale implementations.
And so the title I was given was Dynamic Rupture Simulations of
the 2019 Ridgecrest Earthquakes and Implications for Other Cross-Faults.
And this is work I’m doing with Ruth here – Ruth Harris, and this
photo really has very little to do with the actual talk.
It’s just, as a dynamic rupture modeler, I almost never get to put a cool
field photo in a talk. So here’s a right-laterally offset bush.
But anyway [laughs] – which was – I thought was pretty cool.
But anyway, so I guess I don’t need to give a lot of introduction to Ridgecrest
at this point, both from the last talk and because I know that there’s been
a lot of energy focused across many organizations, especially
the USGS, on these earthquakes. I also know this is an older figure
of the geometry by now, so please be patient with me.
But the super-duper basics. So it started with a 6.4 on a –
predominantly on a northeast-striking left-lateral fault at 10:33 a.m.
on July 4th. Many of us dynamic rupture modelers were flying back
from a dynamic rupture meeting in Europe at the time and landed
and were, like, oh, no. But anyway, predominantly on –
pointer went away. Okay. Here it is.
Predominantly on this northeast-southwest-striking
left-lateral feature. And then, about 34 hours later
on the following night, a 7.1 predominantly on this mostly
orthogonal – like, this orthogonal mostly right-lateral strike-slip structure.
A long, complex rupture. As we’ve just heard, this is a complex
surface-rupturing sequence with many cross-faults at different scales.
And I’m going to apologize right now that, you know, in dynamic
rupture land, meshing all the cross-faults can be complicated.
But we also started working on this, like, less than a week after
the earthquake. So, for my purposes today, I’m going to be focusing
on two primary strands. So the primary northeast- and
southwest-striking left-lateral strand, the primary northwest-southeast-
striking right-lateral strand and finding primary where the
most consolidated surface rupture with the largest amount of slip.
So I’m not going to be able to get to Ken’s question about
some of the smaller interactions yet.
That is something we’re hoping to do in the future.
But, right now this is a two-fault model.
But it still, I think, is useful for addressing some questions about what
happened in Ridgecrest and also some just general questions about
cross-fault interactions. So we had a couple of big questions.
We had four – well, five, but I’m not going to talk about
the Garlock Fault today. But our four questions about these
cross-fault interactions in particular is, first of all, what prevented the first
earthquake from just rupturing through all of this at once?
Why didn’t we just get one larger earthquake on two faults?
Or on, I guess, however many faults you want to quantify it to in our model?
And, in that case, then, because they didn’t rupture together, did the 6.4
actually create the conditions necessary for the 7.1 to even happen on that fault?
Like, was the right-lateral fault, like, not too close to being ready,
and then the 6.4 really made it ready? Or was it the kind of thing where the
right-lateral fault was already pretty close, and the 6.4 just
accelerated the clock on that event and made something that would have
probably happened fairly soon anyway happen even sooner?
And so those are sort of related questions.
And then our other related questions are, was the right-lateral fault
significantly involved in the 6.4 rupture?
So I’m going to – I’m going to verbally add the word “significantly”
because, as we’ve seen with time and further understanding of this rupture
trace, everything is super complex, and there’s a lot of structures and
different orientations involved. So, in this case, we’re really going to
talk about the two main strands. And then, as an extension of that,
if the right-lateral fault was substantially involved,
or even a little bit involved, what allowed that to be re-ruptured?
Why was this able to rupture twice? And I guess, now we can get at,
why was the left-lateral fault able to rupture or at least slip twice?
And I guess, before I go on, I’m going to distinguish now,
when I say “rupture,” I really mean, like, a propagating rupture front causing
substantial stress drop and actually really slipping at depth
and at the surface. Whereas, when I’m going to
say “triggered slip,” I mean much more of just a surface opening of
small response – usually just centimeter scale – in response to the stress changes
and movement on nearby faults. Because, as we’ll – as I’ll show you
soon, we see both of those things in our models, and I think that’s a point to
look at in terms of interactions in cross- faults in general, not just Ridgecrest.
So anyway, sort of the biggest – I guess the biggest debate on this was,
was the 6.4 a conjugate or orthogonal or two faults,
or whatever you want to call it, or not?
And so, again, Ken’s talk was a really great introduction to this question.
So the aftershocks suggest an L-shaped rupture.
And that’ – so, on this left panel here is the – it’s from the Southern
California Seismic Network. It’s all the aftershocks between –
it’s from one minute after the 6.4 to one minute before the 7.1.
This is very definitely an L.
But, as Ken was talking about, there’s not – I mean, there’s a little bit
of right-lateral displacement, but there’s not really evidence
for major right-lateral surface structures slipping in a – in a large way.
And similarly, the satellite imagery – I should have made this larger.
So this is from Milliner and Donnellan’s SRL paper
that just came out. And so these – the blue lines –
actually, I think the green are the ones where they’re really definitely –
I’m forgetting which is green and which is blue.
There’s no scale on this figure. Basically, the stuff that is marked on
this figure are the things that they see – so there’s one satellite image from
between the 6.4 and the 7.1, and so this is differenced between that
and prior to the whole sequence. And so, in the 6.4, they see a whole
bunch of these northeast-southwest- striking structures – maybe, like,
a teeny-weeny bit in a couple of places northwest-southeast.
But, for the most part, this is actually a pretty complex network of
mostly parallel faults that are not parallel to the 7.1 main shock.
So, on the one hand, the seismicity said, yeah, this was a conjugate rupture.
Surface rupture pattern says it probably wasn’t.
And there are definitely a bunch of different inversions that
see a bunch of different things. So, for one thing, you have the
Ross et al. inversion that says that this was absolutely an L-shaped
rupture and that this sort of branch of the right-lateral fault was involved.
On the other hand, Chen Ji’s group has a seismic inversion that says
that it wasn’t involved. There were geodetic inversions.
I looked the longest at Gareth Funning’s poster because he did some stuff where,
when you told the right-lateral fault to rupture, it did.
Or it showed – it worked with the geodetic inversion.
But, when you didn’t require it to, it didn’t.
So correct me if that’s changed, Gareth, but – so this is a – this is a question.
Like, was this involved? And so that’s one of the things
we really want to get at with dynamic rupture modeling.
Because we don’t have to assume it was or wasn’t.
We just put the fault there and see what happens.
And so, in my case, what I’m trying to match when I’m trying to see
what happens is, you can make a model do anything.
I want to make it look like what we actually saw.
So, in this case, I’m trying to make two earthquakes – one that is a 6.4
and one that is a 7.1 – that match – at this point, when we started
working on this, pretty much what we had was surface slip data.
And so trying to match the surface – the overall surface slip distribution
and the magnitude. So I’m not necessarily trying to do,
like, every single site. Because, so first of all, the resolution
of our model might not capture the exact sites.
And also because each site is so specific.
So we’re trying to match the overall shape of where there’s
more or less slip and the overall magnitude of slip in that area
as well as the magnitude of the earthquake.
So – and, again, we have two faults. So, if you see, in the figure with
the aftershocks, the things that are lined up in white – although I guess they’re
hard to see under the aftershocks, those are our primary fault geometries.
And so, to do this, it’s a fully dynamic 3D rupture model.
I’m using FaultMod – Michael Barall’s software.
And I generated the mesh for this using Trellis, which is a commercial meshing
software that allows you to mesh pretty much anything if you are patient.
For now, I’m using slip-weakening friction, which, in past studies,
has shown to do well for full coseismic rupture speeds just as comparable to
friction laws with more complex formulations. So we went with that for now.
And, in this case, I’m starting the earthquakes by basically
kicking the fault really hard. Basically, I’m raising the shear
stress higher than the yield stress over an area large enough to
make sure that the earthquake goes.
And so I did that both – so this type of model doesn’t allow nucleation
to grow organically, so I have to kickstart the 7.1 as well.
But, in both of these cases, I kickstart them at the – at the USGS hypocenter.
So I have this big old slide of parameters.
So I’m using a regional stress orientation from seismicity –
the average for southern California.
Using the SCEC community velocity model.
Pretty standard slip-weakening friction coefficients.
And I do want to call some attention to the principal stresses.
So this might look like a very, very large difference.
But I did need to use different stresses on – both on the right-lateral fault
and the left-lateral fault to get behaviors that match what we saw.
And so – and a way to think about that, though, is – so the overall stress on
the right-lateral fault had to be higher than on the left-lateral fault.
But, if you think about it, we are in a right-lateral plate boundary,
even though the setting in the Ridgecrest area is very complex.
You still are in a predominantly right-lateral plate boundary, and this is
a right-lateral fault, so that you might expect, for a given right-lateral stress
field, to be accumulating more stress on a right-lateral structure and perhaps
less on a left-lateral structure. And so, in the sense of sort of
how stress builds on faults in different orientations,
we felt comfortable about setting it up this way.
And, as it turned out, when you assigned the exact same principal
stresses on both faults in that orientation, it just did not
match the earthquake. You either got something
that was much too large or much too small.
Or didn’t follow the pattern that we actually saw.
So our procedure – sorry for two all-text slides in a row – so first,
we just implemented a regional stress field on both faults.
North-7-east just resolved on the complex fault geometry and said –
and then we started a 6.4. Well, we started a earthquake
at the 6.4 nucleation point. And basically, we ran a bunch of
those until we got a 6.4 that had an appropriate amount
of slip for what was observed.
And then, once we had that, we then used the final stresses from the 6.4 model
as the initial stresses for the 7.1 model. So this is close – 34 hours in time is
not going to have a substantial stress change from tectonics,
so we figured that was safe to do. And then we nucleated at the
7.1 hypocenter to see if we got something to match the 7.1.
And so then it was kind of a give-and-take.
Like, maybe we got something that matched the 6.4 but not the 7.1.
So we had to go back and do another 6.4 and try another 7.1.
So there was a lot of back-and-forth. We ran over 100 models.
And what I’m going to show here are the one or two that actually, we think,
match best. To illustrate a little more of what I was talking about with
the initial stresses – I just got louder. Have you been able to
hear me this entire time? Okay. [laughs] Cool.
So here are two panels of a lot of things. I will mention, the Garlock Fault is
in here because we were looking at stress changes on it, but I’m not talking
about that today. You’re welcome to ask me about that later.
But basically, on the left is just the regional stresses.
And so – by the way, these faults go down 12 kilometers,
which is the base of the seismicity.
And, in this case, both normal and shear – so normal stress and
shear stress we show on a similar color scale, but note that
the scales have different top. So the reason it’s kind of stripey is,
again, because the fault is bent, and you’re resolving a homogeneous
stress direction on a bent structure. So you get different accumulation of
stress or different resolution of stress. And the reason that it’s tapered is that
we tapered it because stress gets less as you get towards the
surface of the Earth. And so, for our homogeneous one,
you can see it’s – you know, you see a lot of complexity
just due to the fault geometry. These are rotated to the show
the left-lateral fault and the right-lateral fault.
But then, on the right, we have the stress field post-6.4, pre-7.1.
So, in this case – so the 7.1 hypocenter is around here, and there’s very little –
there’s no obvious difference between the two cases at the nucleation point.
But, when you look closer to the intersection, it’s especially noticeable
on the left-lateral fault. You see where the shear stress has dropped.
You see some places where normal has increased at bends.
And then also I realize that the color scale on this figure is not great,
and I’m revising it for the paper.
But – oh, I just lost my pointer.
Makes it – oh, here we go. So around here – and actually, I guess
I should have put in – I have a different figure that shows this better. My bad.
But there is some increased normal stress around the junction point.
So just think about the sense of slip for – so you would –
there’s increased normal stress on the side of the – on the right-lateral
fault because – just thinking about the sense of slip on the left-lateral fault,
it’s moving, and it’s pushing it. It’s clamping it.
And similarly, there’s a reduction in normal stress on this side.
This is – I definitely put, actually, the wrong figure in here.
I do have a better figure of this I can show you later.
But also there’s a reduction of shear stress around this area just because the
slip on the left-lateral fault actually produces something of a stress shadow.
So, again, I apologize for not putting in the best figure of this.
So this is our initial stress from the – or the – before the 6.4 initial stress –
after the 6.4, before the 7.1.
And so I’m going to jump straight to our preferred model.
So our preferred model of the 6.4. So here, this is the same earthquake.
I just rotated it to show the right-lateral fault and the left-lateral fault.
Notice that these are different color scales.
So I only go up to 10 centimeters on the right-lateral fault and 1.5 meters
on the right-lateral fault to show what we’re – excuse me –
on the left-lateral fault, and this is the right-lateral fault.
So, in this case, we see, you know, 3/4 of a meter to just over a meter
of slip along the surface of the documented left-lateral rupture.
Which is consistent with the observations we saw at the time.
Or, that we got at the time from people. I did not measure any of this. [laughs]
And then, interestingly, looking at the right-lateral fault, there actually is
a little bit of – we’re going to call this triggered slip on the surface of the
right-lateral fault near the intersection. This is a place where there was,
you know, just some unclamping that allowed this to slip a little.
And so, in this case, we’re talking on the scale of, like, 5 to 7 centimeters.
It’s not a lot, but it’s the kind of thing that, because it is
on the surface, you might – it might show up – might see.
So – and that doesn’t extend further north into the whole aftershock zone.
So anyway, there’s our 6.4. Then going on to the 7.1,
when we nucleated the 7.1 hypocenter under those initial stresses from the 6.4,
we get a 7.1 that ruptures through the whole right-lateral fault.
And, again, this colored bar is another thing I’m fixing for the paper.
But south – so, north of here, we do, in fact, get the highest slip around
the hypocentral area, which is what was observed.
Here we get slip – we were trying to match the magnitudes of 4-ish meters
of slip, which is what we do have in this area.
I will be adding some contours in the paper.
And then, in the real earthquake, there was less slip south of
the junction than there was north of it, and we do see that here.
And there’s still – there’s still more than in the real thing, but the real
earthquake also splayed into three right-lateral strands down there.
So that’s something that, when you sum across those strands,
it’s consistent with what we have.
And also notice that here – again, this scale bar goes up to 3 meters.
For the left-lateral fault, we go up to 50 centimeters, and do,
in fact, see some – I hesitate to call this re-rupture because
it’s just at the surface. But I would say this is a, you know,
triggered slip of, you know, 20 to 30 centimeters on the left-lateral fault.
So that’s the kind of thing that could account for those observations of
re-rupture or more consolidated fractures along the left-lateral fault,
even if this isn’t necessarily a propagating rupture front.
Okay, here’s the other slide that I was talking about that’s better.
So dynamic clamping and stress shadowing, we think are what
are prevented the rupture front – both faults from going at once.
So now this is just normal stress and shear – normal stress and
shear stress change rather than amounts.
So this is ignoring pre-stress. It’s just difference.
So, again, in the – after the 6.4 – so this is between the two earthquakes –
after the 6.4, you have a reduction of normal stress here because, you know,
this side – this left-lateral fault is going that way.
It’s pulling away and reducing normal stress here.
Where, meanwhile, this side is going this way and is increasing
normal stress here. And so, in the case of the 6.4, this clamping
would have very much just said, no, rupture, don’t go south.
But, in this case, the fact that there’s a little bit of reduction to the north,
this is exactly where we see the triggered slip in our model.
Meanwhile, with the shear stress, you see this blue means a reduction
on both sides of the junction point. And so, in this case, increased normal
stress and decreased shear stress is a super, like, rupture, don’t go this way.
Which could have both stopped the rupture from going south in the 6.4
and also actually led to some of that decrease in slip – that lower slip
south of the junction in the 7.1. Meanwhile, even though the decreased
normal stress kind of started letting the rupture go north a little, this decreased
shear stress might not have given it enough fuel to actually take off.
So, we think this is what’s going on here.
But, as for the second question, was the right-lateral fault, you know,
actually brought to failure by the 6.4? Well, again, looking here, you know,
maybe – it’s a little pink – it’s maybe a slight increase in both normal stress
and shear stress, but it’s a very insubstantial change at this nucleation
point compared to at the junction. And so, as it turns out, when we ran
a model with just the regional stress field – no effects of the 6.4 –
and started at the 7.1 nucleation point, we actually still got the 7.1.
And, again, I’ll be putting contours on this in the paper.
There is – the slip north of the junction is about the same as in the model
with the effects of the 6.4. South of the junction, this is redder.
It’s a lot most saturated. The slip down here is higher.
But we still actually, either way, see some of that triggered slip
on the left-lateral fault, but it’s less, perhaps because the fault hasn’t
already weakened and isn’t just sort of open and ready to go.
So what this says is, yeah, the right-lateral fault was
probably already really, really close.
And so it might be just that the 6.4 advanced the nucleation clock.
Oh, I lied about time. Anyway, I’ll try to finish.
so we think the main right-lateral fault probably wasn’t involved –
not in a substantial way. It might have triggered slip.
There’s definitely aftershocks showing that there’s a stress change there.
And, as we showed, there was definitely a stress change there.
But we were able to reproduce the magnitude and observe slip distribution
of the 6.4 without involving a major right-lateral strand.
And I should change it to not, no observed surface rupture,
but minor observed surface rupture. And so we wonder if these aftershocks
are actually the early stages of the nucleation process of that part of the
right-lateral fault growing towards instability and actually – rather than
being aftershocks of the 6.4, being foreshocks of the 7.1, which I know
is a little semantic, but I think interesting from a physics standpoint.
And, again, I will note we haven’t modeled the right-lateral splays yet.
Our next step is, we’re going to add more fault strands.
We’re going to see what made this delay.
We’re going to use rate-state friction because that’s useful
for doing the delay. But that’s coming later.
But what is – but, you know, this is Ridgecrest.
Not all cross-fault ruptures behave like Ridgecrest.
So we just heard a lot from Ken about Elmore Ranch and Superstition Hills,
which were pretty similar to Ridgecrest. You had one fault go and then, 11 hours
later, a cross-fault, separated in time. But, on the other hand, you have
something like Off Sumatra in 2012, which was an 8.5 strike-slip rupture
that went through this network – the actual title of Meng et al.’s paper
is Earthquake in a Maze. Went through this really
complex network of cross-faults in one event.
So that’s kind of the opposite end is this really big sucker.
And then there’s other examples. Thing like – I just learned about
this one – the Kita Tango earthquake of 1927 in Japan, where you had
the first rupture on this left-lateral strike-slip fault, which kind of stops,
and then jumped a maybe 5- or 6-kilometer step-over onto a cross-fault.
And so that’s the kind of thing that is – that’s the distance the Garlock is
from Ridgecrest, but also this is the kind of thing – if that fault is
really ready, it might go, even at more of a distance like that.
And then, on the right, another thing you can have.
So the Kumamoto earthquakes in Japan – again, predominantly
on these right-lateral structures. But there were a lot of cross-faults
that were activated with triggered slip. They show no indication of coseismic
rupture, but still, a triggered slip fault in your house is still a fault in your house.
This is something you still need to think about from a hazard standpoint.
So there are a lot of things cross-faults can do. Ridgecrest is just one thing.
And so I think some things that boil down to is, it’s not just geometry.
Pre-stress is really key.
Cross-faults need to be close to failure or – in order to go.
Dynamic clamping, in and of itself, doesn’t do it.
And sometimes the stress orientation leads to things like triggered slip
even if there isn’t major rupture. So this example here is one of
our models that didn’t work. It’s a 6.8 where I started at the
6.4 nucleation point, and it actually did rupture through the junction,
just because the stresses weren’t right for Ridgecrest, but, you know,
the stresses for Ridgecrest aren’t the stresses everywhere.
You can still get a situation like this. So, last slide, for reals.
Ideas for moving forward. It’s multi-disciplinary, for sure.
So I think we need to identify these things.
I know that Tim has said it was, what, 70-some percent of the Ridgecrest
surface rupture traces were something you could have identified in the past.
So find them. And maybe if you can find a way to assess that the
cross-faults and the main faults go at the same time, that’s useful.
Definitely get a sense of the stress field and see how it –
how stress over time accumulates on faults of different orientations.
And definitely I think we need some dynamic models of just basic
interactions of cross-faults. And so hopefully my SCEC
proposal for that will be funded. But this is an inter-disciplinary
question. And so I think these are some things that can be not just for
Ridgecrest, but – not just for northern California, but for anywhere.
So, with that, I’d like to thank a lot of people.
And sorry [laughs] for going so long.
[Applause]
- Thank you, Julian. Again, I think we can take questions
at the end and I’ll introduce David Schwartz back from Japan
to talk about conjugate faulting – potential occurrence and implications
for the Bay Area and northern California.
- All right. Let’s see if we can get me up here.
All right.
Great. Well, thank you, Tim. This is an instance where my wife says
to me, David, why are you doing all of this work without pay? You’re retired.
[laughs]
You’ve heard two really, really detailed presentations
on what went on in Ridgecrest. And I’d like to say something else
about cross-faults and the potential implications for the
Bay Area and northern California.
And, to put this – and why am I giving this talk to begin with?
When the organizing committee was sitting around, and Ken’s talk
and Julian’s talk were sort of put on the agenda, somebody said, well,
that’s great, but that’s southern California.
What are the implications for the Bay Area?
And I think I said, yeah, what are the implications for the Bay Area?
So, Sarah – Sarah Minson looked at me. And then I looked for the door,
but the door was closed. And she said, well, David,
you’re going to tell us what the implications are.
So here I am today.
Conjugate faults, cross-faults, are nothing new.
They’ve been mapped ever since geologists have
been doing geologic mapping.
And really, though, it’s been in the last several decades where there’s
been an appreciation for coseismic cross-faulting – two different faults,
or a series of faults, rupturing together or very close in time.
And we saw some examples of that. Cross-faults and conjugate faults come
in a range of magnitudes, lengths, and faulting types, and I’ll run through
a number of just quick examples. You have the Elmore Ranch/
Superstition Hills, which we’ve heard about.
In the Brawley Seismic Zone, you have cross-faults sort of
connecting the Imperial Fault with the San Andreas Fault.
And these are really well-defined in the seismicity.
You can go to a place like Lake Tahoe, and just to the north, there’s the
Polaris Fault and the cross-cutting Dog Valley Fault.
These are Holocene and Quaternary faults that haven’t ruptured yet.
They will at some time in the future. You could ask the question,
will they go together, or will they be independent?
In a really interesting paper by Fukuyama, he brought up a number
of points that Julian mentioned, and he kind of summarized a range
of conjugate and cross-faults, and these are all earthquakes in Japan.
And, in the top, you’re looking at strike-slip faults, the lower part of
the diagram, thrust faults, and he grouped these into what he called
conjugate rupture which occurred simultaneously with the main rupture.
So this is a set of faults. They all went at the same time.
And then he had a conjugate rupture that occurred after the main rupture.
Superstition Hills we’ve already seen, and then two large earthquakes in the
ocean – the Wharton Basin 2000 – a 7.8 followed by another 7.8.
And Julian showed a nice slide of the 2012 Sumatra earthquake –
an 8.6 where these all went together, followed by an 8.2.
So this is just some of the variability in the way these conjugate faults work.
And, coming closer to home, we could look at some smaller events.
The 1984 Round Valley, California, earthquake.
And that started on a northeast-trending left-lateral strike-slip fault,
which is here. And then, an hour later, there was a 5.2, and within 24 hours,
seismicity had propagated onto a northwest-trending right-lateral
fault with a – with a different dip. So this is a small rupture sequence,
but just an example of cross-faulting.
And the 1994 Double Spring Flat, Nevada, earthquake.
It started on a vertical northeast- trending left-lateral fault.
And then, within eight days, seismicity had migrated down along
a northwest-trending right-lateral fault. This went on, actually, for a significant
period of time, and the seismicity extended further.
One more small event – a recent 5-1/2 in Osaka, Japan.
The earthquake nucleated on a northwest-trending thrust fault
and then seismicity developed along the northeast-trending
vertical strike-slip fault. And then, with time,
additional strike-slip faults lit up, extending out of the vertical.
So these are – these are examples of kind of small-ish events
that occur everywhere. And the last example I want to
give is a favorite of mine. This is the 1976 magnitude 7-1/2
Motagua Fault rupture. The fault ruptured for 230, or 240, kilometers.
My Ph.D. dissertation area was right here.
So I have a lot of good feelings about the fault.
Not about the earthquake and its results.
And then, south of the Motagua are a series of extensional
north-south-northeast-trending graben. And, six hours after the main Motagua,
the Mixco Fault, just west of Guatemala City, had a 5.9 or a 6,
and it produced 16 kilometers of surface rupture, spread over about 8 kilometers.
So this was sort of a surprise to everybody.
And it was 20 kilometers from the main Motagua.
And I think this is a nice example of triggering of ruptures at faults that
are distant from the main fault and do not have direct rupture
propagation involved. All right?
So, with that sort of little feeling for the – a range of some of the types
of conjugate ruptures you can get, let’s look at northern California.
And the Bay Area.
I put this figure in. This is an old seismicity map.
You’ll see the seismicity is 1984 to 2002, but I think it really shows
some things that I want to point out. This is really focused on the East Bay.
The red dots are historical events of 5 and larger.
There aren’t many of those out there. And Livermore, Pleasanton,
Dublin, Alamo, Walnut Creek – I live here in Danville –
Hayward, Fremont, to sort of orient you.
And I want to talk about three locations. One is this,
which is the magnitude 5.7 Mount Lewis rupture in 1986.
I want to talk about this, which is the Greenville-Las Positas rupture in 1980.
And then we’re going to talk about the San Ramon Valley
and earthquake swarms.
So, the 1986 Mount Lewis earthquake is really, really fascinating.
I went back and I read the paper by Kilb and Rubin, and I would
recommend everybody go and do that. It really was – it was a small event.
It was a vertical strike-slip fault right in here.
It was about – estimated to be about – it’s a right-lateral,
about 2, 2-1/2 kilometers long. And then, with time, this very, very
complex aftershock sequence grew to the north, so it extended
out to about 20 kilometers. It grew east to west.
And this is in an area that really had very little seismicity.
The fault itself is at a very high angle to all of the major plate boundary faults.
And, if you look closely, you see this whole series of
east-west-trending seismicity distributions.
These are all left-lateral, vertical, strike-slip faults.
And they called these – they called these sliced bread faults because it
was like having – taking a loaf of bread and slicing it up.
And so this is – this is a young fault. There was no surface rupture.
There’s no expression, as far as I know, in the geology itself of this area.
And I think this is part of a zone of similar-oriented seismicity
that is helping transfer slip from the Calaveras Fault eventually
over to the Greenville Fault. So this is one type of cross-faulting
in this part of the East Bay. In 1980, there was a 5.8 followed
by a 5.5 a couple days later on the Greenville Fault –
one of the biggies in the East Bay.
And then it was noted that the Las Positas Fault – so this is right-lateral.
The Las Positas is left-lateral. And there was rupture.
There was rupture along the Greenville anywhere between
4-1/2 and 6-1/2 kilometers. And then rupture was found
on the Las Positas Fault. And the rupture was small.
Another view of this conjugate, but this is – this is one of the few
conjugates in the Bay Area where something actually happened.
And Lawrence Livermore Lab is right here.
So the amount – the amount of slip was small.
Only up to about 3 centimeters on the Greenville and about a
centimeter and a half on the Las Positas. And a lot of it was afterslip.
But this is a really nice conjugate relationship in this part of the region.
And then, here’s the Greenville-Las Positas.
This is Mount Diablo with a blind thrust underneath.
This is the San Roman Valley. The Calaveras Fault.
So the Calaveras is right-lateral, and these arrows represent the
direction of compression associated with the Mount Diablo blind thrust.
So this area in the San Ramon Valley where I live is really being –
it’s being torn apart. It’s being squeezed. It’s being compressed.
It’s breaking in all sorts of different complex ways.
And this is the area where the San Ramon swarms occurred.
And, if you want to just look at it in 3D,
this is from the northern California ...
... fold and fault model.
This is the Las Positas Fault. We’re looking north.
This is the Mount Diablo blind thrust. This is the Calaveras Fault.
And, actually, I think the Calaveras extends – it has branches that –
or, splays that extend into the Napa Valley, driving the system there.
But this area is just really being compressed and broken
and pulled apart.
And there have been a series of earthquake swarms in this region
between 1970 and the most recent one was 2018.
The most recent one was actually about a kilometer
from my house, so I took a lot of interest in that.
And what I just want to show is, this is a map trace of the Calaveras.
And it’s shown as a – as an active structure ending here just
a little bit north of Danville. And you can see this.
This is the swarm. This is the swarm from 1990.
The largest swarm in this area. There were 351 earthquakes
over 42 days. There were four earthquakes larger than magnitude 4.
And one – and it’s left-lateral. The slip is left-lateral.
Right-lateral on the Calaveras. And there was concern at the time
that slip along this Alamo swarm sequence might unclamp the
Calaveras and allow it to rupture. And maybe when the Calaveras –
the northern Calaveras does rupture, this will be
the mechanism that promotes it.
These other swarms have different orientations.
Some of them are normal to the Calaveras.
Some are at high angle and are made up of multiple fault patches.
But, again, this is an area of observable conjugate,
and cross-faulting in the region.
All right.
As I’m losing my voice.
When you step back and take a look at the fault map of the Bay Area –
and this is from Graymer et al. It was put together for the
2006 100th anniversary of the 1906 earthquake.
And you look very closely, and the colors are hard to see.
You’re hard-pressed, except for what’s going on out in the East Bay
where I just showed you, to see any faults that have been
mapped conjugate to the main northwest-trending structures.
So I challenge you to go do it.
And, when we go – here’s San Pablo Bay.
When we go north of San Pablo Bay – and San Pablo Bay is right here,
this is the Rodgers Creek Fault, the Maacama Fault, The Geysers.
Santa Rosa is located here. The San Andreas.
There’s one – in this entire area, there’s one – well, there are
actually two, but there’s one fault called the Wight Way Fault.
I was completely unaware of that, and I tried to find some information
on it, and I couldn’t.
But it’s conjugate with the Maacama, and then there’s a little unnamed fault
out here that also has this sort of a conjugate geometric orientation.
But the Bay Area as a whole is pretty much without mapped cross-faults.
And this is a figure that we had in a fact sheet that we released
with the probabilities from UCERF3 for the region.
And this is a list of 33 additional faults which, if you look closely, are
shown in yellow – very hard to see – that were incorporated into the analysis.
And there’s one cross-fault out of all of these minor faults.
And that’s up here. That’s the Wight Way Fault.
So I think I’m going to end it here. Am I on time? Good.
I think ruptures such as Ridgecrest provide critical insights into fault
rupture processes and their controls. This is invaluable.
You can take that understanding and build models of rupture in other places.
And so I love what’s being done at Ridgecrest.
I may have gone down there, but our granddaughter was
born that day, and that was – that was it.
For the hazard perspective, for the Bay Area …
Or, just in general, complex and unexpected surface faulting associated
with conjugate ruptures could locally increase damage from surface ruptures.
So we want to – if we want to think about the hazards in general associated
with these faults, I think the potential for surface rupture and what it
might do is number one. But, in the Bay Area and northern
California, the potential for Ridgecrest-type conjugate
faulting to occur is low. If these events do occasionally occur,
there’s little basis to change established shaking hazard estimates for the region.
And I think an important area of research dealing with stress changes
and complexity is to think about large triggered events associated
with ruptures on some of these large faults.
These are events that are on faults that are not involved in
direct propagation of rupture. And certainly, in other parts of the
world, in intraplate settings, we see tremendously large faults
triggered by earthquakes on other large faults.
So this is just another area of concern. So, in the end, I would say that,
let’s keep this great work going on Ridgecrest, but when you’re
driving home tonight, don’t think you have to worry about conjugate faulting,
at least tonight, in the Bay Area. Thank you very much.
[Applause]
- So we’ve got some time for some questions.
If you have a question for David – I’ll keep him up at the microphone.
If you’re going to ask a question, use the microphone.
- I left my hearing aids home today, so you better talk loud.
- [inaudible]
- David, this is more of a comment to add to
your Bay Area conjugate fault database.
In addition to the Wight Way Fault, if you look on the west – to the west
of the Maacama Fault, the south side of Ukiah Valley, there is a zone of
faulting that goes through Anderson Valley in the Boonville area.
You might want to take a look at my poster [laughs] that’s hanging up
that that fault is shown. And the other fault is in the Clear Lake
area, and it would be conjugate to the Bartlett Springs, I believe.
And that would be the Cross Springs Fault, which bounds
the Cache Formation by a Pleistocene basin there.
Those are two to add to your database. [laughs]
- Great. I’ll definitely come to your poster.
And everybody else should also.
- Any other questions? Questions for Ken or Julian?
- So this is a question for Julian. Thanks for the – thanks for showing
the modeling, and that actually I guess those conjugate ruptures are
sort of not as intimately related as they might appear, right?
The stress changes are more subtle and nuanced.
The question is, you talk about the different stresses resolved on these
faults, right? You required these different shear and normal stresses.
The first part is, are those different stress states on the faults compatible
with a single – or, relative uniform regional stress field?
And then the next is more speculative or hypothetical.
What if the faults were not at sort of 90-degree angles
but at a more conventional 60- or 30-degree – you know,
at a more conventional conjugating?
- So, for the first question – so we didn’t change the orientation
of the stress field. So, even though we had different
amounts of stress, it was still at that north-7-east.
So it’s – I mean, it makes more sense to assume just different stress
on different faults than the same amount of stress everywhere anyway.
We kept the orientation the same. It was just the amplitude.
And that’s what we needed to match. But, as for the second question –
so that’s – I haven’t tested that yet because it was specifically
a Ridgecrest model, but it’s very much something I’m interested in.
because I anticipate you would have some of the same general behaviors
because you still have faults with a different sense of motion
intersecting each other. You would still have that clamping
and unclamping, and you would still have the stress shadowing associated
with one fault dropping its stress. But, in terms of how those affects
would matter based on angle, that’s – I can’t tell you yet.
I want to model it.
Sorry that’s a non-answer.
- Any other questions?
- Maybe I just haven’t heard anything, but do your models give you any idea
of what these earthquakes may mean for a potential earthquake
on the Garlock Fault? - So that is something
we did look at in our paper. I figured I didn’t want to talk about that
here today because I was asked to talk about cross-fault interactions.
But we did do, actually, some models where we took the stress changes from
our dynamic models of the Ridgecrest sequence and resolved them under
the Garlock Fault. And so you see the similar pattern of, where you would
see some stress increase at the end of the right-lateral, but also you see
some decrease from the sense of slip. And so basically, long story short,
and I can show anyone figures later if they want, we saw that, basically,
in order to get an increased chance of rupture, you have to have both
increased shear stress and decreased normal stress.
And the area of the Garlock Fault that had that in our models
was actually pretty small. And it lines up with where the
triggered slip and aftershocks were. So that matches nicely.
But, for the most part, we hesitate to say anything because we don’t know
what the pre-stress was on the Garlock. And the nice thing about doing this
with a dynamic model is we didn’t have to assume the pre-stress.
We didn’t have to assume the coefficient of friction.
So there’s a couple of spots that look like they might be closer
just from that – a couple that look like they’re less close.
I think ultimately, it’s going to be one of those, how ready
was the Garlock already?
I think definitely a lot of people have been talking about things like
the Blackwater Fault to the south, which is a right-lateral structure,
being more likely. And it just had a 4.5 last week, but who knows.
And I’m not going to say anything about that, either. [chuckles]
- Any other questions? Ah.
- Julian, you spoke about having to kick off the 6.4 with a certain stress pop
that you imposed upon one place. And that gave you the 6.4, and then
you go through the dynamic rupture calculations.
And then you get to the 7.1, and you have to kick that off too.
To what extent – well, what are those numbers, and to what extent did the
dynamic rupture help or hinder the 7.1, in terms of
what you needed to do there? - So we started the rupture the
same way in both cases, where we’re just raising the shear stress
to 10% about the yield stress over a patch that was – we did a –
like, a 2.5-kilometer radius for the patch.
So it equals about a – like, a 5.6 or 5.4 – I forget exactly.
It’s a mid-5s earthquake nucleation zone.
So that’s kind of – with this method, it is a little bit brute force, but because
this doesn’t run interseismic or nucleation, it’s kind of what
you’d have to do to get it started. Certainly, our patch for the 6.4 was
on the end of the fault and didn’t – you know, once the rupture gets started,
it is running on its own, and we – and similarly, with the 7.1, actually,
the documented hypocenter is pretty far from the junction point.
So, by that point, the rupture was propagating on its own.
So nucleation is always an issue in dynamic models.
And there’s different approaches to it, but in this case,
a rupture was going for a while before it hit the intersection.
Hope that answers your question.
- Okay. Any other burning questions? Otherwise, I’d like to thank all of our
speakers again for an awesome session. [Applause]
And I believe it’s time for lunch.
- Okie dokie. A couple of quick announcements.
There is a fire engine demonstration downstairs.
And let’s call lunch, which is morphed on the agenda as lunch plus
poster session as, instead, lunch plus fire engine session.
And, if you have a chance, go down and explore that.
If you have paid for lunch, lunch is ready for you outside.
If you have not paid for lunch, please don’t eat the lunch. [chuckles]
And come on back, and we will pick up again at 1:15 with lightning talks.
[inaudible background conversations]