[silence]
Hey, everyone. Welcome to the Earthquake
Science Center seminar for August 17, 2022.
Please remember to mute your mic and turn off your camera.
A quick announcement.
We’ll be having a series of five special seminars over the next couple of weeks.
You should have received an email about that yesterday.
They’ll be hosted by Dave Lockner.
And we’re going to be sending out email reminders about each of these talks
a couple days in advance and right before the talk.
So you can look out for those and remember to join by clicking the link in each email.
As usual, for today’s talk, you can put questions into the chat box at any time.
And we’ll be taking questions at the end, so at that point, you’ll have
an opportunity to unmute yourself and ask questions directly to the speaker.
Today’s speaker is Matt Herman.
Matt completed his bachelor’s degree in geology and physics at Amherst,
and then he went on to do a master’s and a Ph.D. at Penn State University
under the supervision of Kevin Furlong.
From there, he went to Utrecht where he did a postdoc under the supervision
of Rob Govers. And he is now at Cal State-Bakersfield
where he’s an assistant professor.
Matt will be speaking with us today about earthquakes
in an uncoupled subduction zone. Take it away, Matt.
- Great. Thank you so much for the introduction,
thank you for inviting me, and good morning, everyone.
So today I’m going to talk about some of the work I’ve been doing with these
collaborators on the recent sequence of earthquakes
near the Shumagin Islands in Alaska.
Okay, so – this works. Gotcha.
Okay, I want to take you back and give you a little bit of historical context about the
Shumagin Islands and why they’re important
from a seismotectonic perspective. So we’re going to go back to 1979.
Plate tectonics is brand new at this point, and people are just starting to
think about the implications of plate tectonics for various geological
and geophysical observations, particularly earthquakes.
And so this paper back in 1979 thought about this idea of seismic gaps.
These were places that had not had large earthquakes –
they said for more than 30 years.
We’ll just say for some longish amount of time.
And they said that, hey, this place hasn’t had an earthquake for a long time.
We know the plates are moving relative to each other.
This is telling us there’s a likelihood to be a big earthquake in this region.
So let me take you to a map instead of reading just their abstract.
We can see this colored map of the world.
The colors indicate where the authors said, okay, here’s where we’re likely to have
big earthquakes. Here’s where we’re not likely to have big earthquakes.
So, if we circle the Shumagin Islands region of the Aleutian subduction zone,
they said the historic record is incomplete,
but maybe it has the potential for large earthquakes, particularly because
most of the rest of that margin had experienced large to
extremely large earthquakes, some very recently.
I just want to make the point here – I’m not really going to talk much about
Cascadia, but, you know, in 1979, our knowledge was limited. [chuckles]
They didn’t even color Cascadia. It wasn’t even considered as
a seismic hazard. Now we know things a little bit better there.
So just that little bit of trivia for some of you to note that Cascadia
is relatively new in our understanding of seismic hazards.
Okay, so just a couple years later, this paper came out focusing on
the Shumagin seismic gap – this place where big earthquakes
hadn’t occurred before, but the rest of this margin had basically ruptured in the
20th century. And so the authors identified this region
as having a high probability for a great earthquake, something magnitude 8
or larger, during the next one to two decades.
So we’re looking at this paper, published in 1981.
They’re saying, before the turn of the millennium,
there’s going to be a great earthquake here.
But no one said earthquake prediction was easy.
As we know, there hasn’t been a great earthquake there.
In fact, 10 years after that paper was published, this one came out.
And it was re-evaluating the seismic gap hypothesis.
And, in fact, it argued that the increased earthquake potential after
a long quiet period can be rejected with a large confidence.
And one of the places they looked at, in fact, was the Shumagin seismic gap
in here. And they said that, oh, look, this is a place that
hasn’t had a large earthquake. That actually probably means
it’s not going to be able to have a large earthquake.
As opposed to these other places to the east and to the west of it,
which just seemed to be more likely to have large earthquakes.
So there’s this debate about whether we can even use these ideas of plate tectonics
to say anything about seismic hazard,
specifically in this area, but maybe globally as well.
Okay, so this is kind of the picture today of the Shumagin seismic gap.
It’s a fancy version of the pictures we’ve already looked at.
The red areas represent the aftershock zones of great earthquakes.
You can see most of these are in the 20th century.
And most of this plate boundary has ruptured
in a great megathrust earthquake in the 20th century.
The Shumagin seismic gap is the notable area that hasn’t.
The question is, does it mean it’s more likely or less likely to have
a big earthquake. And, to answer this, we need a little bit more information.
We’re going to start with a geodetic view.
Okay, so when we think about whether an earthquake is going to occur
in a subduction zone, we think of maybe coupled versus
uncoupled subduction zones. I’m going to use this term a lot in here.
And I’m going to try and explain what I mean on this slide.
So we’ve got this cartoon image here of the time period between earthquakes,
the interseismic time period in a subduction zone, and, somewhere along their
plate interface, the two plates are locked together, or coupled together.
As a result of that, the motion of the subducting plate drives the
upper plate landward. Okay, so we can see that, if we go to Japan before
March 11th, 2011, all of Japan is moving to the west because
the subducting Pacific Plate here is driving Japan west, okay?
And we sort of know the elastic rebound model.
We know what’s going to happen.
When the big earthquake occurs after a century or a millennium
or something like that, the upper plate is going to rebound landward.
So we’re going to see all of the motion in the upper plate
back the other direction during the earthquake.
So this is what we would expect to see at a coupled subduction zone.
Okay, what do we see in the Shumagin Islands?
Okay, I’m going to actually start over here on the right.
I know we usually read left to right, but the data are over here on the right
from this paper by Li and Freymueller in 2018.
And they’ve got GPS stations on several of the islands in here,
the peninsulas in here. And each of these arrows
represents the speed that that station is moving with respect to the
North American Plate fixed somewhere back here.
Okay, so what we see is these large landward velocities over in the east.
These indicate there must be pretty high coupling on the plate interface
over here driving – as the Pacific Plate subducts,
driving these islands to the northwest.
As we go over to the circled area – that’s the Shumagin seismic gap –
we see lower landward velocities.
And that implies lower plate interface coupling in this region.
So, you know, many of you are probably familiar.
You can invert these onshore data for a distribution of coupling.
And we see these red areas over here are highly coupled.
That’s not surprising. There were large earthquakes.
The sort of western end of which – the 1964 was over here.
The 1938 earthquake occurred generally in an area of intermediate
to high coupling. The Shumagin seismic gap over here
is an area of intermediate to low coupling, okay?
So this seems to be telling us that maybe the Shumagin seismic gap
isn’t going to be a place that has large earthquakes.
Now, interestingly – and I’m going to kind of punt on this a couple of times
in this talk – the 1946 earthquake is over here.
That was a big earthquake. Big tsunami. Large aftershock zone.
And you’ll notice it’s in an area that has low coupling.
We can talk about that later if people want, but as I said,
I’m generally going to punt on 1946 throughout this talk.
Okay, so that’s the sort of geodetic horizontal motion view.
We can also take a geologic view.
What do we expect to see at a coupled subduction zone?
Throughout the earthquake cycle, we think we’re going to see the coast
go up and down and up and down throughout the earthquake cycle.
Now, that vertical motion is fairly complicated.
And why it goes up in some places and why it goes down in some places is actually
one of the unsolved mysteries, I would say, of various subduction zones.
We think we know what’s going to happen coseismically.
There’s going to be this red line, this pattern, of subsidence to uplift
as you sort of cross the rupture zone, which is going to be generally
in the coupled zone. But what happens interseismically in this green line.
What’s going to happen postseismically.
These are going to be a little more complicated and depend on things like
the distribution of coupling, the distribution of rheology
in the subduction system. In any case, we’re going to see things going up.
We’re going to see things going down.
And, in a coupled subduction zone, we’re going to look in this region
and generally see sudden vertical motions.
Okay, so what are we going to – what do we see in the Aleutian Islands?
If we look over here, this is the part where
the geodesy tells us is pretty well-coupled.
And this is not my area of expertise, so please don’t ask me too much about it,
but if you look at the geology on Sitkinak Island,
there is evidence that the coastline has gone up rapidly and down rapidly
at various times. We can see this in the lithology.
You can see this in the critters that live on the coast at different water depths.
And so there’s several times over the last few thousand years
where the coast has gone up and down, indicating,
oh, yeah, earthquakes must be happening in and around this region consistent with this
being a coupled part of the subduction zone.
Okay, well, what about in the Shumagin Islands?
Is there a comparable signal in the geologic record of the Shumagin Islands?
Apparently not. Okay, so this paper by Rob Witter and
folks argue that there’s little Holocene strain accumulation and release
on this part of the megathrust. Okay? In fact, they say, if the rupture of the
megathrust produced great earthquakes, then coseismic uplift or subsidence
was too small to perturb the onshore geologic record.
Okay, no stratigraphic or geomorphic evidence for sudden vertical motions. Okay?
So a conclusion there could be that there’s just not big earthquakes.
Or maybe these earthquakes are somehow hidden.
So this is the picture of the Shumagin gap before 2020, that it’s poorly coupled
and aseismic. And I’ve highlighted the Shumagin gap region.
Really what we’re talking about is this white dashed area, okay?
We saw the 1938 region had a big earthquake in 1938.
The geodetic data suggests it’s coupled.
The geologic data also suggests it’s coupled.
Questions remain about 1946.
As I said, I’m just not going to get into that too much right now,
but we can talk about it. We do see some seismicity in these various regions.
I didn’t put the depths on here, but we see seismicity clustering in and
around the asperities and actually at the base
of the seismogenic zone of the Shumagin gap.
Okay, so just kind of give a cartoon image summarizing what we think
the mechanical state of the plate boundary is.
We’ve got an uncoupled Shumagin gap.
We’ve got asperities, perhaps on either side, but certainly over here
on the east side of it. Okay, so I’m reading this.
I’m reading these papers. And I’m saying, okay, sort of makes sense.
I sort of understand it. End of story.
We’re going to get earthquakes maybe around the Shumagin gap,
but it’s just never going to have an earthquake.
I wasn’t thinking too deeply about the implications yet, but I have since then.
Okay, so July 21st, 2020.
Magnitude 7.8 earthquake.
And it occurs right here in this area that may or may not be part of the Shumagin gap.
So the initial stories about this paper – the initial reports
said things like, hey, scientists predicted an earthquake like this
would happen in 1981. We looked at that paper. Okay?
But, you know, news that comes out in the first few days
after an earthquake should be taken with large grains of salt.
Science that comes out in the first few days after an earthquake
should also be taken with a large grain of salt.
So we wanted to ask, did this earthquake actually break the Shumagin gap?
Okay, that’s kind of a fundamental question.
And, in fact, the earthquake epicenter was located over here
in the western corner of the 1938 aftershock region.
Now, we actually think the behavior of this earthquake represents a transition
from rupture occurring in an area where we have an asperity,
things are strongly coupled, and then, pop, you know, it goes off in an earthquake.
And then a rupture crossing into maybe a lower coupling area.
So I want to talk a little bit about this.
If we focus on seismic analyses of this event, they locate slip
right near the epicenter. So if we look at the USGS finite
fault model, the one that only uses seismic waves, they see a concentration of slip
right around the star here. That’s the epicenter.
Okay, so, you know, [chuckles] that would be sort of what
a lot of people would expect for an event like this, okay?
Decent size, 7.8, but not enormous.
So just large slip in and around the epicenter.
But it turns out, when we added local information to the inversion –
so when you did geodetic and seismic analyses, the slip started here
at the epicenter – and there was plenty of slip over here,
but it actually propagated over to the west.
So, in fact, the centroid of this event was west of the epicenter.
Okay, so how do we interpret this?
Well, the fast slip – the stuff that goes – you know, pop, the asperity unlocks and
produces shaking, that is happening over in the coupled zone.
We still have slip over here west of the coupled zone,
but maybe it’s a little slower – less seismically productive.
So we can see it with local displacements. It’s still slip.
But it’s not something that is as easy to see teleseismically.
Okay, so, in fact, that transition might be indicating where we’re going
from coupling to something else.
But still, we haven’t really ruptured the Shumagin gap here.
So the main shock here appears to have broken across the northwest edge
of the 1938 asperity. We’ve got these aftershocks
and afterslip mainly down-dip of the Shumagin gap.
But the Shumagin gap itself is mainly aseismic – a couple of aftershocks
here and there, but nothing major in here, okay?
So still looks aseismic from a megathrust perspective.
Fast forward a couple of months. Boom. Magnitude 7.6 earthquake
in the Shumagin gap. So, without even looking at any details
of this event, I looked at this and said, okay, well, I guess a lot of people
were wrong about the Shumagin gap. It’s not aseismic after all.
But it turns out this was not the expected event.
So I think most people, when they’re looking at subduction zones,
and they’re looking at the big earthquakes in subduction zones,
they are expecting megathrust earthquakes –
these shallow thrust-faulting earthquakes on the plate interface.
This was not what happened. So, first of all, the mechanism
is not thrust faulting at all. It’s oblique strike-slip.
Also, when we look at the locations of these events, particularly evident
in cross-section here, so going from the Pacific Plate over to North America,
Pacific Plate over to North America, we see the aftershocks are pretty much
exclusively below the plate interface here.
So it appears that this is an intra-slab, right-lateral strike-slip –
oblique strike-slip earthquake.
And new information has come out suggesting this might be a little more complicated
than that, but, you know, all of the analyses show this
dominantly strike-slip character of this event.
So, just to give you a little bit of context to why this is weird,
okay, first of all, how you get it. It’s in the seismic gap.
But the seismic gap, I guess, only refers to megathrust earthquakes.
This is intra-slab strike-slip.
There have been big, you know, oceanic intraplate earthquakes before.
About 10 magnitude greater than 7.5 since 1980.
This is the only one that happened in the subduction zone.
Everything else is kind of outer rise or even further offshore regions.
Okay, so this is an unusual event. This is weird.
And, to answer some of these questions and think about some of these issues,
we asked ourselves, okay, let’s assume the Shumagin gap is, in fact, uncoupled.
What stresses would accumulate within the subducting slab where
this weird oblique strike-slip earthquake happened?
Okay, so just to give you a little bit of sense of the modeling we do
to think about this, remember we’ve got this picture of GPS velocities on
the upper plate. You know, high velocities indicate high coupling.
Low velocities indicate lower coupling.
Got their Shumagin gap here. Okay, well, what is coupling?
Coupling is some kind of mechanical connection between the subducting
Pacific Plate and the overriding North American Plate –
so Alaska and the Aleutian Islands.
So we’ve developed some very simple locking models for subduction zones.
We first published this work back in 2018.
We’ve published a few things on it since then looking at some more details.
But, in any case, we’ve got this picture here of the Pacific Plate subducting.
Something is locked on the plate interface between the Pacific and the Alaska Plates.
And we kept these models very geometrically simple because we don’t want to
deal with all the complications of geometry and rheology and things like that.
We just want to focus on the patterns of locking.
Okay, so this is what our models look like.
And some of the details are a little different from this initial paper and
the ones we did for the Shumagin area, but in general,
all these models have a very similar setup.
We’ve got this triangular wedge of an upper plate.
We’ve got a subducting plate that’s just planar.
We add displacements to the upper plate to move it.
And we put a backstop fixing the back of the upper plate
to have relative motion.
Okay, so, you know, relatively straightforward geometry.
Purely elastic for now.
Now, I guess the most important part is how we’re going to define locking
on the plate interface. And we define areas – rectangular areas
on the plate interface as locked – as saying, you cannot have any slip.
We make the dramatically simplifying assumption that
everything outside of locked areas is just free to slide.
You can think of it like Teflon. It’s totally frictionless.
And the only areas, then, in the context of this model, that can start earthquakes
are these locked zones – these so-called asperities.
Okay, so when we run this model, interseismically, what do we get?
Here is that sort of perspective view of the model.
On the right here, we’re sort of looking perpendicularly down onto
the plate interface. We’ve got the trench over here on the left.
Over here on the right, this is the plate interface down at 185-ish kilometers.
Okay? So, in the areas we [audio cuts out] the colors indicate the amount of slip.
Black indicates no slip. It would be kind of surprising if there were slip
in areas we designated as having no slip.
That would be mean our models weren’t working.
So these are the locked areas.
When we go far from the locked areas, everything is white.
The plates are just happily cruising along at the relative motion.
In this case, it’s 1 meter of relative motion,
but because it’s elastic, we can scale it to plate motions.
Now, there’s these areas in between the locked zones that, rheologically,
mechanically, they could slip at the plate rate.
But, because the locked zones are holding them back,
they actually don’t slip at the full plate rate.
We call this pseudo-coupling because
it looks just like something that’s mechanically coupled, but it’s not.
As I said, if these asperities, if these locked zones were not there,
they would be happy to move at the full plate rate.
Okay? So this produces a pattern
of surface motions that would be seen in geodetic data.
And we can say, hey, we’ve got this pattern of locking.
What would this do to stresses in the plates around it?
We don’t just have to look at the kinematics on the plate interface
at the surface. We can probe this model for stresses as well.
And that’s what we did. We set up this model for the Shumagin gap region,
and we asked what the stresses would be in the [inaudible].
So, just to give you a little context, we’ve got our subduction trench
right here. So we’re – got the Pacific Plate to the south.
We’ve got the North American Plate to the north.
We’re looking at the colors in here represent the stresses along the surface
of the subducting Pacific Plate. We put a locked zone here in the east.
That’s like the 1938 asperity.
And everything else on the plate interface – the Shumagin gap region is uncoupled.
Okay, again, think of it like Teflon. It’s just allowed to go freely.
Okay, the colors in the background represent the stresses resolved
on right-lateral strike-slip faulting perpendicular to the trench –
so of the kind that occurred in October 2020.
And we see that this is about the location of the earthquake in the context
of the model. That’s squarely in the red to dark red area.
So this earthquake is strongly favored by the stresses that accumulate
in the subducting slab. And the reason why is really easy to see.
So, if we’re out here over in the western part of the model, the subducting
Pacific Plate is just happily cruising, you know, down the subduction zone.
There’s no coupling, so it’s just, you know, going down without any problem.
These arrows represent the motion of the subducting Pacific Plate.
They’re large over here.
In the locked zone, the upper plate acts as a buttress preventing the subducting
Pacific slab from going down the subduction zone.
So there’s no arrows in here because it’s being – it’s locked.
It’s coupled to the overriding plate.
So there’s a huge right-lateral shear as you cross from the locked zone
into the unlocked zone.
Okay, this was a right-lateral strike-slip fault strongly favor in the transition from
locked to unlocked. So exactly where the October earthquake occurred.
So our conclusion is that an uncoupled Shumagin gap
strongly favors the type of faulting that occurred.
Now, remember, there was a series of earthquakes and aftershocks and afterslip
that also occurred. So we can ask how they affected
the stress conditions at the October epicenter.
We do this by calculating the Coulomb static stress change, thinking about the
shear stresses on the fault, thinking about the normal stresses on the fault.
We just do this in an elastic half-space, which is pretty consistent with
the more complicated interseismic model I just showed you.
And the conclusion – the take-home here is that the stress changes from
the magnitude 7.8, particularly when you add this down-dip afterslip,
strongly promote this type of faulting, okay, this oblique trench-perpendicular
right-lateral strike-slip faulting, okay?
So you can see the location is in the orange dot here.
That is nice and red, particularly when you add the afterslip.
Even without the afterslip, it’s generally red.
But, when you add the afterslip, it goes squarely in the dark red.
So that means high stresses promoting this type of faulting.
Okay, so we’ve got two things going on here.
We’ve got the interseismic stresses, which are strongly favoring this
October type of mechanism.
We’ve got the coseismic stress changes from the megathrust plus afterslip.
So this is triggered faulting occurring in the Pacific slab.
All right. So we’ve got this picture now of a normal – I should say typical –
normal has other meanings – megathrust event here
with its afterslip and aftershocks.
We’ve got this unusual event that now seems to make a little bit of sense in
an uncoupled to coupled transition.
And we can put this in our sort of cartoon of what’s going on.
We’ve got the locked asperity area.
Okay, that’s what produces the megathrust event.
This zone of pseudo coupling – so kinematic coupling here.
That’s why it kind of looks like it has intermediate coupling even though
the whole area is mechanically unlocked.
But, in that transition is where the October 2020 strike-slip
earthquake occurred. Okay.
So there’s a little bit of an elephant in the room.
Some of you may be thinking about it already.
The question is, a 7.6 is not a small earthquake.
It’s not like this is just first breaking the slab in a 7.6 earthquake.
So where did the strike-slip fault come from?
The answer is not totally clear.
We have some hypotheses, but we’re not entirely sure.
There do seem to be some magnetic anomalies over here that might indicate there are
fracture zones oriented roughly north-south in this region.
So it could be activating a fracture zone.
It’s possible, I guess, that it could also be some kind of outer rise, but those would
be nearly perpendicular to the actual strike-slip faulting that occurred.
So it would be a little surprising to be an outer rise fault.
The slab undergoes some curvature in this region along strike,
so there could be stresses associated with that as well.
But our guess at the moment, and really is a guess,
is that it’s related to subducting fracture zones that are being re-activated
in the stress field associated with the transition and coupling.
Okay, so we’ve got some conclusions. Through June 2020,
these earthquakes actually paradoxically, maybe, provide evidence for low coupling
on the plate interface in the Shumagin gap.
But low coupling doesn’t mean aseismic, okay?
We’ve got strike-slip faulting favored in this transition
from coupled to uncoupled.
We see that, if that can be triggered, as you have this sort of typical event
triggering the atypical event. And, in fact, the atypical event
occurred in a region thought to be aseismic but really should not be
thought of as aseismic. In fact, the tsunami from the
October 19th event was substantially larger than the tsunami from
the July 21st event, which I think surprised me and a lot of people.
I’m not a tsunami scientist, but I would have certainly thought a thrust-faulting
event would have produced a bigger tsunami, but it didn’t.
I’m not going to get too much into the tsunami,
but I’m happy to talk about it if people want to.
Okay, so that could be the end of the talk, but I got a little more time.
And you may be thinking, hey, wait, wasn’t there another earthquake there?
And you’d be right. Here’s my title slide again.
And we didn’t talk at all about the green earthquakes.
And we probably should talk about the green earthquakes, because, in fact,
the earthquake in July 2021 was the largest event in the sequence.
Magnitude 8.2.
It occurred on the megathrust in the 1938 asperity region along its down-dip edge.
So it didn’t seem to actually break the entire 1938 asperity.
The earthquake epicenter is here, so it started right next to the July 2020 event,
but most of the slip, instead of propagating to the west,
propagated to the east of the epicenter.
Okay, so the first thing we ask when this sort of thing happens –
or, I ask, at least, when this sort of thing happens,
is did the previous earthquakes perhaps trigger the big one.
And those of you who have thought about stress changes are probably, like,
well, yeah, this is going to be an easy one to model.
We see that the previous events strongly load the epicenter of the magnitude 8.2.
And this is expected behavior for the region right off the edge
of a previous earthquake on the same fault. Okay, so we see,
here is the rough rupture area of the 8.2. Here’s its afterslip zone.
And here is the – sorry, this is the 7.8. This is then the 7.6.
And we see that those strongly favor the magnitude 8.2 thrust faulting there.
Okay, so the triggering story is relatively straightforward, but we actually think that
this earthquake tells us something about the coupling
in the Shumagin gap.
And we had to think about this in a little more nuanced way to make this argument.
So let’s start with the earthquake slip.
Models of the earthquake showed that the peak slip was
somewhere in the 4 to 5-ish meters range.
Okay, so that’s fine. That’s pretty typical for a magnitude 8.
The slip deficit that accumulated since 1938 was roughly 5 to 5-1/2 meters.
So, if you take this 65 millimeters a year, you multiply it by 2022 minus 1938,
[chuckles] so, you know, you do that subtraction in your head,
and you get about 5.4 meters. Okay, so this may be normal to everybody.
Everybody may look at this and say, well, what’s surprising about that?
But it turns out, in many of our models we looked at, for an earthquake
that’s smaller than about 8.5, we actually would have expected
a little lower peak slip.
So this was actually surprisingly large peak slip accounting for most
of the slip deficit accumulated since 1938.
And this is only from doing those models I showed you earlier when we were
looking at the kinematics of coupling and what happens when you unlock them.
So we decided to dig in this a little bit deeper.
We took basically the same types of coupling models as before.
So we’ve got our Shumagin gap region in here that’s uncoupled.
We’ve got the 1938 asperity region in here – the red area that is coupled.
And all we’re going to do here is run this model interseismically,
then we’re going to unlock the western edge of the 1938 asperity region.
And, when we unlock it, it’s going to slip in response to all these stresses that have
accumulated in the plates around it. Okay, so it’s going to rebound elastically.
And we’re going to run two models.
We’re going to assume, first, that there is a coupled asperity in this Shumagin gap.
Okay, and we’re going to run another model where there is no coupling
in the Shumagin gap. Okay, so let’s take a look at this.
Now we’re looking straight down again on the megathrust.
We’ve got our trench down here at the bottom now.
We’ve got the deep part of the megathrust up here.
So we see this is probably at about 125 kilometers’ depth over here.
So, if we unlock this region of the 1938 – this is the western part
of the 1938 asperity, we have a Shumagin gap region in here that in fact is
coupled – okay, so if we lock it, that actually reduces the maximum
potential slip magnitude in the 2021 earthquake to about 2-1/2 meters.
Okay, why does this happen? It’s the same reason we get that pseudo coupling effect.
Locked zones restrict slip on the plate interface around them from occurring.
So, if you have a locked zone on the east side and a locked zone on the west side.
They both hold back the plate interface, reducing the amount of slip that occurs.
Okay, so now what happens if the Shumagin gap is uncoupled?
Well, perhaps unsurprisingly, when this area is unlocked, you can get
a lot more slip, particularly on the side of the rupture
closest to the unlocked zone. Because that’s free.
It’s free to go and slip however much it wants.
Okay, but max slip now is 4-1/2 meters.
If we, in fact, add up all this moment here, it’s equivalent to about
a magnitude 8.1, okay, for the amount of relative motion since 1938.
So this is looking fairly similar to the observations.
Just to remind you, since 1938, there were 5.4 meters of convergence.
We ran this model with 5.4 meters of convergence.
And the earthquake recovered most of this motion as we see in this model here
with an unlocked Shumagin gap. Okay, so this seems like a nice story.
Is there maybe any other evidence that we’ve got close to
full strain release from this event?
We looked a little bit at the aftershocks here.
And maybe they can tell us something.
So the aftershocks do show this pretty typical decay over time.
Not really surprising.
But, when we looked at the magnitudes of the aftershocks, or particularly
the cumulative moment over time, we noticed that the aftershocks
had kind of low magnitudes compared to the main shock.
Not, you know, sort of egregiously low, but somewhat low.
So we decided to quantify this a little more precisely.
We found that the aftershock sequences of the megathrust events, in this case,
both the 7.8 and the 8.2, actually, only released
about 1% of the main shock’s seismic moment.
Now, this is lower than is typically seen for thrust-faulting aftershock sequences,
which generally release about 5% of the main shock moment.
So what is this telling us?
This seems like we are getting [audio cuts out] …
[silence]
… relatively larger aftershocks occurring.
Okay, so the last thing we want to do is probably the hardest part of
subduction zone science in general, and that’s to answer the question,
what the heck is happening up-dip?
Okay, this is a notoriously difficult place to observe because it’s underwater.
Our observations are typically onshore.
So we often have 100 or more kilometers between the nearest observation
and the action going on up here.
There are a lot of great marine geophysics, marine geology with cores
and things like that, but it’s hard to get sort of a time span of observations
in the shallow part of the subduction zone.
There are some new observation techniques that are a little expensive,
but it sounds like some of them are just getting funded,
even in the last few days, to do GPS acoustic and things like that.
So those data sets are going to be really, really valuable
for answering these kinds of questions.
But, in any case, when I did this study, and when I put this talk together,
there really isn’t a lot of information about the shallow part
of the plate interface. And this is really important.
This is the part where tsunamis that are generated are going to be the largest.
If we have slip in the shallow part, that’s going to move the seafloor the most.
So we want to know what’s happening up here in terms of its coupling state,
potential for large earthquakes, potential for slip, etc.
Okay, so, in the 1938 asperity region, we’ve actually seen
relatively little slip occurring in 2020 and 2021.
Why do we think that’s the case?
Well, we’ve got these slip models here.
These are based on geodetic data but also on seismic data,
which are showing relatively little slip up here.
The tsunami is consistent with the earthquake being buried
relatively deep on the plate interface.
So also very few aftershocks in this shallow region too.
So very low coseismic slip occurring here.
As I said, it’s just really hard to resolve the coupling state here on
the shallow megathrust, whether it’s locked and building up to the next earthquake,
whether it’s unlocked, but maybe still has slip deficit because of
our pseudo coupling story, or something else entirely.
So we’re actually going to go back to our locking models.
And this is kind of the best we can do right now is sort of model
what’s going on here and say, hey, is this consistent with other observations?
Okay, so we took the locking model again.
We started with an unlocked Shumagin gap.
Because, again, this seems to be most consistent with
the slip occurring in 2021 when we unlocked this region.
And now we said, okay, let’s lock the shallow plate interface.
So this is the model I showed you before with 4-1/2 meters of slip.
Now we’re going to lock this part of the plate boundary.
And we see this has the exact same effect as locking the Shumagin gap.
It changes the spatial pattern, but the general effect is
reducing the magnitude of coseismic slip, okay?
So the magnitude equivalent of this slip distribution is about magnitude 7.7.
Okay? So it seems like locking the shallow plate interface mechanically completely
is not consistent with what was observed in the 2021 earthquake.
We would have seen a smaller earthquake occur then.
Unless, of course, it broke that part.
Okay, so now, you know, what happens if this area is unlocked?
Now, in the original model, we actually said, well,
we should slip this shallow part, okay? But what if it’s unlocked interseismically,
but it’s velocity-strengthening?
At the high strain rates of an earthquake, it actually kind of seizes up.
It doesn’t slip coseismically.
This is maybe not surprising to some people here.
There are materials that do this – that get stronger as slip rates increase.
Okay, so this area is unlocked interseismically but locks up seismically.
When that occurs, actually we get significant slip occurring
still in the down-dip part of the rupture, the part that went off in 2021.
Okay, in fact, the peak slip gets even higher.
I have to probe this model a little bit more to see why that’s the case,
but it actually seems to focus slip down at the western end of the rupture here.
Okay, so that’s kind of an interesting thing.
But then we sort of push the question a little bit further.
Okay, there’s no coseismic slip, but this area is unlocked interseismically.
So, at some point, it seems like this area is going to slip.
Does it do it shortly thereafter? Is it aseismic?
Is it going to be rapid enough to produce a tsunami?
These are some really important questions that I don’t actually have the answers to
right now but is part of the thing that I’m working on
moving forward with these kinds of subduction zones.
Okay, so just to kind of wrap up a little bit, and here’s the sort of picture.
You can see the July 2021 earthquake has sort of been just plastered on top
of this picture. We’ve got the subduction zone here.
We’ve got coupling in the area that produced the 1938 earthquake but also
now the 2020 and 2021 megathrust earthquakes.
There’s a transition to an uncoupled Shumagin gap here, okay?
This produces all sorts of things that we’ve seen in these recent sequences.
It produced this intraplate trench-perpendicular strike-slip fault.
Okay, which had a pretty significant [audio cuts out] …
[silence]
… aseismic. We see that, in the 2020 and 2021 megathrust earthquakes,
there was near-complete strain release.
Okay? We see this from the amount of slip relative to the slip deficit.
We also see this in the aftershocks.
So our models and our observations of these events are suggesting this poorly coupled
Shumagin gap doesn’t slip by itself as a megathrust earthquake,
but can cause all other kinds of seismicity.
And then, of course, the shallow part,
our models are suggesting that this area is not coupled interseismically,
but also isn’t slipping coseismically.
So a few questions we have.
As I said, I totally just said 1946, I’m not going to deal with that.
But there was a large earthquake with a very large tsunami in 1946.
So is it an asperity that we can’t resolve because it’s shallow?
Or is something else going on there?
Is it some different kind of plate interface rheology?
When does slip occur near the trench, right?
I said, you know, there’s this zone up here that should be unlocked at this point.
Is it slowly, gradually releasing its slip deficit?
Or is it holding on for some reason and will get maybe a tsunami earthquake
or a big, shallow earthquake, or something like that?
I don’t know. I don’t have the answer to that right now.
And then we might be looking at the seismic signals,
or maybe the geologic records of this coupling transition.
Now that we’ve seen some of the complexity that can occur here,
maybe we can start looking in a little more detail
at some of the islands over here – the Shumagin Islands over here.
Just to remind you, you know, there’s this sequence of islands over here
that might have records of this transition from coupled to uncoupled.
Okay, so that’s about 45 minutes. Thank you so much for having me.
I’m going to leave my conclusion slide up,
and I am happy to take any questions you might have.
[silence]
- Thanks so much, Matt. That was really awesome.
We’re going to take questions.
So, as usual, you can put your question in the chat or you can raise your hand
and we’ll call on you.
And we already have a bunch of questions in the chat and a bunch of raised hands.
I’m going to start off with the one
that was put in the chat during your talk from Natalie Culhane.
It says, what type of information about the plates did you need to put into the model
to output the resultant slip magnitude, lithology, material strength, etc.?
- Yeah. Good question.
It turns out – I’m just going to go back to just this model for now.
So this model here is actually purely elastic.
So, if we look at sort of the view here, the upper plate and
the subducting plate are identical, purely elastic materials.
Now, we gave them a sort of realistic shear modulus.
So the stresses here would be scaled appropriately,
but these stresses would scale linearly to the shear modulus.
We have tried models where we contrast the slab in the upper plate.
And again, it sort of scales the stresses, but not the patterns of what’s going on.
This is a pretty robust result irrespective of the specific properties we use.
Now, where it gets interesting is when you put in a little more rheological realism.
So, if you have an upper plate that’s relatively thin over a viscoelastic mantle,
again, this is sort of shallow enough that it’s just elastic interactions.
But there’s some changes to the details of what’s going on.
So, because this is in mostly the elastic part of the system,
treating it as a purely elastic problem has been pretty robust.
- Thanks. Awesome. I’m going to take a raised hand.
Let’s go with Ben Brooks since you’re already here.
- Hi. Can you hear me?
- Yes, I can.
- Great. Thanks for a great talk, Matt.
And I’ve enjoyed very much reading your papers on the topic
over the past couple years. - Thanks.
- Kind of a comment and a – and a related question.
With respect to the Chignik earthquake, at least, some of the work that
we’ve been doing recently – well, it’s important in terms of
the coseismic slip models, at least the ones that are published, all share
the exact same modeling methodology – the Chen Xi modeling methodology.
And they all result in high peak slip values.
That’s very much not required by the data. [chuckles]
So there can absolutely be – if you – if you take a model that’s
a little bit more harmonious with the data, these models tend to have
16-kilometer- by-16-kilometer patch sizes. - Right.
- Which is not necessarily justified by the very, very sporadic data.
It leads to, I think, 1,000 unknowns and 45 knowns. [laughs]
And there’s lots of smoothing, as you’re aware.
So that’s kind of a comment.
And then, related is that some of the modeling that we’ve been doing
shows that you could have 2 to 3 meters’ peak slip, no problem,
and satisfy the data just as well.
So that leads into the question, then, how would you think about Chignik
if it were a 3-meter peak slip event?
- Yeah. I think that’s a really great question,
and it’s something I’m talked with Gavin Hayes about as well.
He said, oh, peak slip is just sub-fault size-dependent.
And so, you know, he’s, like, never believe peak slip.
Like, okay, well, I got to use it somehow or – you know, I don’t have to,
but I like to use it because it’s there in the model.
Otherwise, don’t do these models.
[laughs] But, in any case, yeah, if it’s lower peak slip, you know,
that’s something we have to think about a little bit.
But I would say it’s still probably been – there’s other, I think, better evidence
for the Shumagin gap being uncoupled, but then we have to start considering
the fact that maybe the shallow part has an asperity, too,
reducing the amount of slip on the plate interface.
There actually are some details here that I didn’t get into involved with multiple
earthquake cycles at the base of the seismogenic zone.
And you see these in some of the more, I would say, dynamic earthquake cycle
models that are dealing with sort of, you know, rupture processes
and sort of frictional properties in a way we’re not doing.
I’ve got the interfaces locked or unlocked.
There’s some things that can sort of change particularly the down-dip magnitude
of slip that I just – I can’t resolve in these
relatively simple models.
But, if I were being sort of naive about it, [chuckles] in a simplistic way,
I would say, if it were lower slip,
that would imply there has to be something locked elsewhere holding it back.
Where can you put that locked zone? Probably up-dip of the rupture.
- Cool. Thanks. - Yeah.
- We can go to another question in the chat.
And I think you already touched on this in your answer to Natalie’s question,
but Ruth Harris wrote, hi, Matt, I’m enjoying your really good talk.
I realize that the models you’re using are simple – my favorite,
but I’m wondering if pore pressure variations or other unmodeled geophysical
or geological parameters might be affecting the earthquake rupture locations.
- Yeah, absolutely. I mean, that’s totally the case.
So, as I’ve just mentioned in my response to Ben,
we treat the plate interface as being super simple.
We treat these stresses in the plates as being, you know, purely elastic.
So there are all sorts of complexities here that could
change the details of what’s going on.
I have not played with poroelasticity at all, like, in my entire career.
So I have no intuition for how that would change things here.
My speculation is that it would change things more in detail than in essence.
But that is pure speculation on my part.
I honestly, as I said, don’t have good insight there.
So I know that, you know, as – there is – you know, there’s processes by which
the plate interface can strengthen and plate interface can weaken.
That can change things, but it seems like these locked/unlocked models
are doing a pretty good job of capturing, as I said, the essence of what’s going on.
But, yeah, unfortunately I don’t have a much better answer for you than that.
- That sounds good.
Ruth says thanks.
- You’re welcome.
- So Alex Hatem, you want to ask a question?
- Hey, Matt. Really awesome talk. Thank you.
So I have a question about the strike-slip faulting that you talked about
with the magnitude 7.6 event.
And this is probably an unanswerable question, but it got me thinking
during your talk. So you pointed out really clearly with your Coulomb modeling
that the right-lateral strike-slip faulting is totally encouraged by
having that locked patch and unlocked patch.
And I guess I’m wondering, what is the chicken and the egg here?
Is the strike-slip fault causing that locked fault – or, locked patch boundary?
Or is the strike-slip fault forming because of that locked patch?
And how are you thinking about that?
- Yeah.
So I am of the opinion – although I don’t have a great –
you know, I don’t have great evidence for it – I’m of the opinion that
the transition in coupling is kind of what exists and is activating these faults.
That it’s not something specifically about the fault that is causing the
transition in coupling. And one of the reasons –
actually, no, maybe there is a little bit of evidence for this.
I don’t have the mechanism on here unfortunately, but it turns out this event
is actually the same mechanism as this event.
It is – this is not a thrust-faulting event.
This is an inter-slab, nearly trench-perpendicular, strike-slip event.
So you’re looking at this. You’re, like, wait, wait, wait, wait.
Why is it over here in the coupled zone?
But it kind of turns out, if you look at our – if you go back here,
and you look at these models, this zone over here is strongly favored
with right-lateral strike-slip, down-dip – at the down-dip edge of the locked zone.
Now, we haven’t looked at Coulomb stress changes from the magnitude 8.2 main shock
or anything like that to see if it, in fact, is favored.
But we think that the coupling is more fundamental,
and it’s these faults probably – as I said, I think it’s on fracture zones,
but it could be something else, are being activated in that
sort of stress regime setup by the coupling transition.
- So a follow-on, then.
Does that, then, help you determine that these patches are locked in the long-term?
Like, these are always been locked.
If it’s an – if it’s locked enough to cause a strike-slip fault to grow
and to have these really big earthquakes, it’s …
- Yeah.
- It means that this patch has been locked for a really long time in our history.
- Right. Well, and then that’s the question, right?
Is this activating a big structure that exists already?
That’s my opinion.
If it were producing these big structures, yeah, then I’d expect
to see more surface signal of it. Now, [chuckles] unfortunately,
these earthquakes are happening in the slab.
If these were happening in the upper plate, it’d be, like, oh, yeah,
probably could see them really easily.
We might see a complementary signal in the upper plate.
Yeah. I think, you know, there’s no reason to expect why it –
so, if we go – well, we can just look at this.
If we drew the arrows for the upper plate relative to the subducting plate,
they’d be exactly the opposite. We’d see left-lateral shearing, okay?
So maybe there is, you know, a periodic left-lateral
big earthquake going on in here.
Would it be big enough to produce a geomorphic signal?
Would it be big enough to exceed the signal that we get from
subduction zone coupling – vertical motions and things like that there?
I don’t know.
It’s very likely, in my mind, that the type of earthquake we had in
October, that strike-slip earthquake, doesn’t always happen
throughout the megathrust earthquake cycle.
That’s why I think the triggering story is kind of important.
It was this right set of circumstances that allowed this big event to happen.
But, you know, again, this is – this is all in the realm of
sort of [laughs] speculation at this point. I don’t have a good argument.
Remember, all these are also happening offshore too.
So, as we get more onshore, where we might be able to see these things,
the likelihood, I think, of getting a trench-perpendicular strike-slip earthquake
in the coupling transition, it’s just less likely, for the same reason
we can’t observe the offshore stuff. That’s where the coupling action is.
- Cool. Thank you. - Yeah. Thanks for the question.
- Tom, you want to go ahead?
- Yes. Could you go to the slide where you show the aftershocks and,
in particular, the yellow earthquake and its aftershocks?
- Yeah. - Or your title slide, I think,
shows it as well.
Yeah. You know, that’s – it’s one thing, you know,
where you talked about mainly the – on the onshore,
where you have these strike-slip mechanisms.
I guess you could say there’s a – there’s a linear feature there.
But then, if you propagate it to the – towards the trench and beyond,
it’s more of an arc than a linear feature.
And then the mechanisms change … - Yep.
- … as well.
And then the question is,
so how are they associated with each other, and is there anything that you
might be able to see in the topography of the ocean floor offshore
of the trench that might indicate a fault or sequence of fault zones?
- I think that’s a great question. We have been thinking about it.
We didn’t come up with a convincing answer, even though these sort of normal faulting
seemingly outer rise events were occurring when we were writing our paper on this.
That’s why it didn’t make it into the paper.
But we’ve been looking at these and thinking, okay, well,
this is not just coincidence that these events are happening here.
Probably not coincidence that they’re spatially related to what’s going on
with the strike-slip events.
Now, I think this gap here is probably about 40 kilometers or so.
I suspect that these are actually different structures.
I do not think – although it does appear like it’s a curved structure going
all the way to the trench, I suspect that there actually is a gap here.
But apparently whatever has happened with the megathrust, the afterslip,
and now the oblique strike-slip is also sufficient to trigger activity,
which I would – I guess I would probably consider outer rise activity.
And whether that is a consistent relationship or not,
I don’t have good insight for, but I think it’s a really important observation that,
you know, these are really only active after the recent events.
So I don’t think the structures are connected.
But, you know, it’s possible they are and we just can’t see it.
- Okay. Thank you. - Yeah. Thank you.
- We’re just about at the end of the hour, but we have a couple more questions
in the chat, so I’ll try to read maybe one of them for a start.
Morgan Page asked, how do we know that the 1938 and 1946 events
had shallow slip approaching the trench?
- Ah, yes. Because both of them had substantially larger tsunamis
than anything we’ve seen recently.
So, first of all, they’re larger earthquakes, as we can tell
by their magnitudes and their aftershock areas and things like that.
But they have substantially larger tsunamis.
And at places where we can make repeat measurements.
So we know 1946 – I’m not – again, I’m not a tsunami expert,
but 1946 is a really anomalous event.
I think it’s considered one of these slow tsunami earthquakes.
And then 1938 is just a big earthquake that appears to have ruptured close to
the trench, again, because of its big tsunami, much larger than 2021.
- Well, I like that answer. - [laughs]
- So Nicholas Cunetta wrote, great talk.
What was the depth of the magnitude 7.6 right-lateral event?
I’m curious how this coupling transition, slash, oblique earthquake relate to
subduction accretion, most of which occurs at about 30 kilometers’ depth.
- Ah, okay. Yeah. So let me show this cross-section here.
So, yeah, the earthquake occurred just below the megathrust.
And so I’d put that roughly at 30 kilometers.
I don’t – I don’t have the depth off the top of my head, but you can see
in this cross-section here, you know, roughly 30 kilometers.
The plate interface here is probably at about 25 kilometers.
And you can see all of these events are happening right below the plate interface.
So, yeah, how that’s related to subduction accretion, ooh,
that’s not something I’ve thought about in a long time. [laughs]
And certainly not in the context of this sequence.
So I think that is an interesting question.
- That’s fair.
You got a lot of thank-yous in the chat.
Thanks for the enthusiastic talk.
Clara Yoon has a question in the chat. Thanks for the great talk, Matt.
What’s your opinion about the earthquake hazard in the Shumagin gap?
Since you found it to be aseismic, slash, uncoupled,
how concerned should we be? For example, might a future
large rupture on either side of the gap extend all the way across the gap?
Or would the uncoupling boundary reliably stop it from getting
through the gap? And thinking about dynamic weakening effects.
- Yeah. I think that’s a great question, and I don’t have
a great answer for it, unfortunately.
Our argument is that, if we go back to – oop, wrong direction, sorry.
Let me maybe scroll this way a little more.
Find my mouse. There’s my mouse.
If we go to, say, these models, and we say, how much can any of this slip?
In our – in the context of our models, anything with slip deficit can slip.
So, you know, this is not a representation of the Shumagin gap,
but in these relatively simple models, from the edge of an asperity
at 600 kilometers along the Y axis, all the way to 700 kilometers on the Y axis
could have slip. Now, the slip over here would be much less.
But, you know, I think that we’ve seen some earthquakes with sort of
very long tails [chuckles] of relatively low slip.
And maybe, you know, my argument might be those aren’t rupturing asperities.
Those are rupturing the edges of the pseudo coupled zones.
You know, I don’t have great reasoning for that, but – other than it seems to
be consistent with the magnitudes of slip deficit we’re getting in these models.
But whether and earthquake can specifically cross the Shumagin gap, you know,
I think there we just have to look at the history
of earthquakes there – history of the geological evidence.
And we just don’t see that happen very often.
So perhaps it can occur.
Perhaps there are earthquakes that can extend into the Shumagin gap partially.
But it just doesn’t seem to cross the gap all that often.
Otherwise we’d probably see evidence of significant tsunamis and vertical motions
and things like that. So it appears to be a place where,
although, you know, I completely agree dynamic weakening can be invoked,
we’re not – it doesn’t seem to be happening much there.
It seems to be a place that sort of impedes megathrust rupture, generally.
Now, as far as just seismic hazard there, what we’re seeing from this is
seismic hazard doesn’t equal megathrust earthquake hazard.
And I think that’s important to recognize in subduction zones that,
you know, we tend to – I tend to get this tunnel vision
where it’s just megathrust earthquake, megathrust tsunamis, but, you know,
any earthquake that displaces the ocean floor produces a tsunami.
Any big earthquake produces strong shaking.
And so those don’t have to be plate interface events, necessarily.
So I think that’s a really good question with a lot of different answers. [laughs]
- Okay. Well, thanks so much for the awesome talk.
Let’s thank Matt one more time and also for giving very thorough awesome answers
to all of our questions. - Thanks so much for having me.
This was a lot of fun.
- Okay. We’re glad you enjoyed.
We’re going to end the recording since we’re a little over the end of the hour.
I don’t know if Matt can stick around for a minute or two.