This is Geology Bites with Oliver Strumpel.
We've all heard of the San Andreas Fault that runs along
the coast of California. It represents the boundary
between the North American plate and the Pacific plate.
It's 800 mile long extent is visible from space and it has
caused some powerful earthquakes, one of which
devastated San Francisco in 19 O 6.
Most faults leave little or no trace of their localities on the
Earth's surface, but it is still vital to know where they are for
both hazard assessment and scientific reasons.
Rufus Catchings has been hunting down Californian faults for over
40 years. He is a research geophysicist at
the US Geological Survey. Rufus Catchings, Welcome to
Geology Bites. Thank you, Oliver.
It's a pleasure to join you. In your research, you've used
many different approaches to pin down the locations of faults.
Do these all rely on seismic waves?
Would you use other methods as well?
Most methods I use involve seismology, but I also use other
methods as appropriate. For example, in many places we
use geomorphology or changes in the Earth's landform or surface
to infer that there are likely faults there.
Sometimes we can see them at the surface and it's obvious and we
act accordingly there. For example, using
geomorphology, sometimes there's a long linear valley within a
mountain chain, and we suspect that those are likely faults.
There are geologists who specialize in geomorphology and
they also help me in my efforts. OK, let's talk about the seismic
methods that you use and how they work, and then we'll talk
about the results. There are two kinds of seismic
wave, primary or pressure waves called P waves, which are akin
to sound waves in air, and secondary or shear waves called
S waves. Both types of waves are affected
by the presence of faults. Can you tell us how they're
affected? Yes, within the subsurface, S
waves slow down once it encounters a fault zone.
So relative to the rocks on either side, the velocity of the
S waves are much slower in the faulted rock body than the
outside rocks. So faulting essentially damages
the rock, and this changes the rigidity of the rock, which
makes the velocity slower. It's the same way for P waves,
but one complicating factor is in the near surface.
Groundwater can saturate these materials, the rocks as well as
the faulted material, and that actually causes an increase in
velocity, which makes it a little more complicated when we
use just P waves alone. OK, so how does this change in
velocity of the S waves and the P waves enable you to pin down
the location of faults? So what we typically do is put
out an array of seismometers across the fault and then we
import some type of source into the ground so that we can
measure these waves. And from that we measure the
travel times, usually to within milliseconds, and then we can
locate the fault themselves. Using methods like tomography,
we can actually see or get an image of where these low
velocities are located. We can do the same thing for
both P&S waves, but one of the most powerful techniques is to
use the ratio of the 2P waves to S waves.
Because S waves are slower, P waves are faster.
Even if it's saturated, we get VPVS ratios that are very high
in fault zones, so that shows us very nicely where the fault is
in the subsurface. Also, typically a fault is a
barrier to groundwater flow, and we use these seismic waves to
map out the top of groundwater. The top of groundwater's
velocity contour is typically 1500 meters per second.
In sediments, a fault is often a barrier to the flow of
groundwater. So one side of the fault
groundwater will be lower than the other.
And using the tomographic methods, we can actually see
this step and that's another indicator of faulting.
So there are multiple methods that we use.
We use the low P wave velocities, the low S wave
velocities, the high VPVS ratios, the sudden change in
depth of the 1500m per second velocity contour.
All of these are strong evidence of faulting, and when you have
them all in one place, you're quite certain of the location of
the fault. And all the waves that you're
picking up, typically waves that have been refracted by the fault
zone or reflected at the edges of the fault zone.
For the tomographic methods, we're using waves that have been
refracted as they move through the subsurface, but we do use
the reflection method as well. Tell us a bit about the
reflection method as well. The reflection method is a very
powerful method and it's very precise.
The oil industry have used this method for exploration for oil
for many, many decades. But if we do it at very high
resolution, what we can see is the seismic waves go down.
They reflect off surfaces. Faults typically also cause the
layers of the earth on either side of the fault to be offset
somewhat. And reflection methods very good
at showing a visual image of this offset.
But unfortunately you can't use it everywhere.
Many places you don't have this layered subsurface.
What kind of spatial resolutions can you get for these fault
locations? Well, it really depends on the
myth that we're using. And as I mentioned, reflection
seismology is one of the most accurate, particularly at
greater depths. And if we used very dense
spacing on our seismometers, very high frequency seismometers
and the energy that we input into the ground is high
frequency, we can typically image quite deep to within a
meter or so where the fault is. But as we get deeper, we lose
frequency and so the resolution becomes less and we can probably
go to several meters to several 10s of meters at great depth in
our accuracy. But there are other methods as
well that we can use to get high accuracy, particularly in the
shallow subsurface. How deep can you track a fault?
I mean, if a fault's going all the way down to the Moho, for
example, could you see it all the way down?
Yes, you can. The resolution of course once
you get down, you know 3040 kilometers is off.
But yes, you can see these faults quite deep.
I'm reminded of a recent episode of Geology Bytes with Bob White
in which he described using a dense array of seismometers to
pinpoint the locations of thousands of micro quakes and
track the movement of magma within the crusts of Iceland.
Do you also rely on naturally occurring earthquakes as the
source of the seismic waves? Yes, we do.
We use both the naturally occurring earthquakes or ambient
noise, and we also use artificial sources, man made
sources. How did the man made sources
work? In the shallow subsurface, it's
as simple as, say, striking the ground with a hammer.
We just input energy into the ground and that generates
seismic waves. But there's also another way we
use what's called accelerated weight drops.
It's essentially a heavyweight. We just drop on the ground.
It's imparting energy into the ground.
We use shotguns and rifles, sometimes shooting blanks into
the ground. That generates a lot of energy.
We can use small vibrators, essentially anything that is
imparting energy into the ground.
But for the deeper imaging, say kilometers to 10s of kilometers,
we typically use repeated heavyweight drops or some type
of truck mounted huge vibrator or explosions.
Any of those can generate the kind of energy that we need to
map out the faults. I suppose a big explosion is
fine if you're in the middle of the desert, but that wouldn't go
down too well in the middle of Los Angeles.
That is correct. So we tend to use other methods
when we're in urban areas. Apart from the effect of faults
on seismic wave velocities, you also use another property of
fault zones, namely they can act as wave guides.
Can you explain how that works? Yes, as I mentioned earlier,
fault zones, due to the shearing of the fault, the movement on
either side, it really damages the rock.
And so these are low velocity zones relative to the
surrounding rock. And so that's effectively like a
waveguide. Once you get energy into that
fault zone, it acts like a waveguide and energy moves about
in the fault zone reflecting off the internal surfaces of the
fault as it moves forward. You can think of this like an
electromagnetic waveguide or think of a fault as very similar
to a coaxial cable or fiber optic cable where this energy
just once it gets in there, it stays in there and it travels
discreetly in there. So if we impart that energy, it
travels at very high amplitudes and it's typically much stronger
than either the P or the S wave and we can take advantage of
that because it only travels within default zone.
Then if we can map out that high amplitude area, that shows us
exactly where the fault is. And is that another way that's
useful for finding the location of the faults, or do you use it
for other purposes as well? Yeah, it's very powerful
technique for getting the location of the faults.
If we place those seismometers across, and if we want very
precise location, we space our seismometers at a meter spacing
as they go across the fault, then that's usually the
resolution we can get. If it's submeter, we can get
submeter. So just by looking at those high
amplitudes or what I call the peak ground velocity of the wave
as it comes through and I look at it after the S wave has
passed, this gives us a lot of information about absolutely
precisely where that fault is coming to the surface.
And the nice thing that we find is that it's often that there's
more than just one strand. And we can see that using this
technique. So it's a very powerful
technique. It tells us precisely where it
is near the surface, but it can also tell us, say, if you have
two different fault strands, if they're connected to each other.
And we use it both for determining connectivity as well
as the individual location of the fault near the surface.
So how do you use this method to determine connectivity?
Is it that like if there's a break in a fault, the energy
leaks out in between and you wouldn't see the continuity from
one fault zone to the other? Absolutely.
Just like if you were to take a fiber optic cable and you break
that cable and it's separated, the energy no longer travels.
So if we see energy, say from an earthquake input into one fault
and there's another fault, say to the South of there and we
have an array across there and we don't see that same guided
wave energy traveling onto the second fault, we know they're
not connected. But if we see that same energy
traveling, we generally know the speed at which it travels.
We know that they are connected. So are there some instances then
when you've seen two faults which initially look like they
might be two separate faults, you put the energy into one of
them and you find it's travelled to the other one, and then you
learn for the first time that actually they're part of the
same fault system? Has that happened to you?
Yes, yes, that has. And we've done that in multiple
places. You might want to say, why does
it matter if there are two faults that are connected or
not? And it makes a big difference,
because a short fault, for example a fault that's only say
10 or 12 kilometers long, can only generate about a magnitude
6. But if you have one long
continuous fault, it can generate magnitude 7-8 and
above. So the length of the fault is
very important. You study faults in dense urban
areas where humans generate lots of vibrations, not least from 18
Wheeler trucks that might be thundering along the freeways.
How do you manage to subtract out what you rather charitably
refer to as cultural noise? With some difficulty, often in
some cases we can actually use the noise as an exploration tool
itself, particularly if it's coming from many different
directions, and we sometimes do that.
However, in most cases, the cultural noise is a major
headache and it's something that we want to try to mitigate.
So for active source imaging, we're actually inputting energy
into the ground ourselves with a weight drop, with vibrators,
etcetera. We've found that if you
continually input that source into the ground over and over
again, say we're using a weight drop and we just keep dropping
that same weight, it's the same signal over and over again.
Where is the noise from? All the cultural activity is
kind of random. And so if we stack that seismic
wave over and over again, then the energy stacks in, all that
noise stacks out and we can get a beautiful signal through even
very noisy areas. One example was where we did
this in Hollywood and Los Angeles area.
We just continuously dropped the weight twice a minute for an
hour and a half. And there was a freeway nearby,
there were local traffic, there were tall buildings, there was
all kinds of noise going on. So by doing this stack, all of
that freeway traffic just disappeared, all that other
noise just disappeared, and we got this huge signal.
So it really works well in certain environments.
If you're in a dense urban area with this thumping, is that
something that somebody living in the neighborhood would be
able to feel? They probably wouldn't feel it,
they may hear some of it and we're not talking about dropping
huge weights, maybe we're talking about a £500 weight
that's dropping a couple feet. That's not very much compared to
vehicular traffic. OK, so let's turn to what these
various methods have revealed. Which faults have you
principally studied? I have studied a very large
number of faults. It would take me a long time to
name them all. I guess I've never really
counted the number of faults that we've studied.
Just say here in in California, these would include the San
Andreas Fault, the Hayward Fault, the Rogers Creek Fault, W
Napa, Silver Creek, Calaveras, Monte Vista, Garlock Fault.
There is a very, very long list, but we've studied a lot of these
faults and we see a lot of similarities no matter where we
go. Yeah, California's got no lack
of faults. There is no shortage of faults
and I've also studied faults outside of California and
Oregon, Nevada, Washington, Arizona, Alaska, Hawaii,
Virginia, Delaware, Tennessee, Arkansas, Mississippi, Missouri
and internationally too. Let's focus on the San Andreas
Fault for a moment, then. Naively, one might think that as
a strike slip fault, the fault plane would be roughly vertical.
Is that what you found? Well, generally it's vertical to
sub vertical. And we've looked at the San
Andreas pretty much along its entire length, Southern
California all the way up to Northern California where it
goes off to the Mendocino triple junction.
And yes, there are places where the fault is sub vertical.
It has a depth to it and there are places where it's vertical.
But what we also find is that frequently there are sub
parallel faults to it. There may be a main trace and
then there are many sub parallel faults and those often dip into
the main fault or sometimes they're just their own fault
that goes spare a little to it. Any of these can move during a
major earthquake and even if they don't move, they can carry
that guided wave energy that I explained and they can be quite
dangerous. California has something called
the Alquispio Act, which prohibits building on a known
fault within a certain distance of a known active fault.
And if you have all these subparallel faults to it, you
often don't know if you're on an active trace of a main fault.
So that's one of the reasons why we do so much of this imaging is
to try to find every fault so that we don't put something like
a huge apartment building or a hospital or something like that
on top of these faults because it can be very dangerous.
So you said earlier that you can actually measure the fault
location to some considerable depth.
Does the subsurface geometry of a fault affect how violent the
shaking is on the fault when it moves?
Yes it does. I should point out that there
are multiple types of faults. The San Andreas is a strike slip
fault where the two pieces are moving kind of parallel to each
other. But there are also normal faults
where one fault goes down relative to the other, reverse
faults where one fault goes up relative to the other.
And that's often at an angle. And there are many combinations
of those. And generally among the most
violently shaking for the same size earthquake are the reverse
faults. And sometimes there are places
even along the San Andreas where there's reverse faulting going
on, or strands of it are reverse.
And so we really want to find that and know what type of
fault, what type of movement on it, because again, the reverse
faults tend to give us the strongest shaking.
If you're not on the foot wall, if you're on the hanging wall,
that's where you're going to get the most violent.
Shaking. In 2014, I think you used guided
waves resulting from an earthquake near San Francisco to
learn that certain faults that were previously thought to be
distinct from each other were actually connected.
Can you tell us about that instance?
Yes. In 2014, there was an earthquake
in Napa Valley, which is a region famous for wine.
And it was a magnitude 6 earthquake and it occurred on a
fault that ruptured the surface for about 12 kilometers.
And there was a lot of questions about, well, is this just a
single small fault? And if it's only 12 kilometers
long, that's about the maximum magnitude earthquake it can
generate magnitude 6. So we wanted to know, well, is
it longer? So we did this guided wave
technique where we put a rays across faults to the South,
faults to the north. And we looked to see if the
guided waves were continuously coming through onto those other
faults, and we found that they were.
In fact, instead of being just 12 to 14 kilometers long, we
found that it was connected to a fault to the South.
And that made the entire fault zone at least 128 kilometers
long, which is about 10 times what we suspected originally.
And because we know that some of those faults are also connected
to some of the larger faults, for example to Calaveras Fault,
it's possible and likely that there's a continuous connection
of faults that as much as 300 kilometers long.
And to put that in perspective, the 19 O 6 earthquake that
destroyed San Francisco occurred along the San Andreas along an
area about 300 kilometers long. So that's saying that this other
fault that we thought was relatively minor might be quite
a major fault that could generate a very large magnitude
earthquake if it did a continuous rupture as the San
Andreas did in 19 O 6. Wow, that finding really could
have pretty serious implications.
I'm wondering if as a result of this kind of work, there are new
regulations or building codes that are put in place in the
danger zone that might not have been there before.
All of these faults are within close proximity and they are
part of the San Andreas Fault system that's largely in place.
But for people say you're within 10 kilometers, you may think
that oh, I'm quite a ways I'm 30 kilometers from the San Andreas.
It's going to generate a large magnitude earthquake, can be
very damaging, but at least it may not rupture the ground
underneath me. Well, I think people have to
start thinking a little differently when you have these
long faults that are subparallel to the San Andreas and they go
for a long way. So people have considered it.
The USGS does this probability map or where earthquakes are
likely to occur over the next 20 years, 30 years, and it
certainly takes those kind of things into consideration.
I've seen some of your results in which you overlay fault lines
onto an aerial shot of an urban neighborhood, and I must say I'd
be very disturbed if the red line delineating a fault ran
right through my home. Do you recommend that people
take some action if they live that close to or right on top of
one of the fault splays that you've located?
Absolutely, if you live in close proximity to an active fault,
you should take every precaution such as making sure you have
water, food, shelter, the medicines that you need stocked
up and that you can have access to them.
Be prepared to leave. You may not be able to live in
your home. Have a plan.
You and your family, where are you going to meet?
Are you going to contact each other?
And maybe you contacted a relative across the country and
that relative across the country contacts your other significant
person. Also, if you live in one of
these areas, retrofit your home. Even if the fault doesn't run
directly beneath your home, the strong shaking from the fault,
even if you are 10s of kilometers away, can still
destroy your home. If you live right on top of the
fault itself. You may rethink where you're
living, or I would. As you mentioned earlier, you've
worked on faults outside California, both in the US and
abroad. One of the areas that you
studied, surprisingly, is located right in the middle of
the North American plate. Can you tell us about that?
Yes, for a very long time it's been known this area called New
Madrid Seismic Zone, or the locals call it New Madrid
Seismic Zone. It's perhaps the most
seismically active area in the US east of the Rocky Mountains.
In the winter of 18111812, it generated 3 large magnitude
earthquakes and they were so strong that it reportedly rang
church bells in Boston and in places it's reported that the
Mississippi River actually ran backward for a while.
It's mainly located along the Mississippi River in Missouri,
Arkansas, Tennessee and it's believed to be the third leg or
the failed rift of a rift system that goes down to the Gulf and
the faults are believed to be reactivated due to slight
changes in stress from the North American plate reorienting.
But the true 'cause is not well known, but there are some
speculation that there is a very dense lower crust in this area
and the pull down effect could also be adding a stress to the
system to generate these earthquakes.
So it's important because if we have a repeat of the 18111812
sequence, it could be devastating to the whole mid
continent area for two reasons. One, seismic energy propagates
much more efficiently east of the Rocky Mountains then say in
California. So the area of damage and strong
shaking would be much wider for the same size earthquake there
than say in California. And in 18111812, there were very
little population there. So there wasn't a lot of deaths
related to this. But given the large population,
the millions of people who live in this area now, it could be
devastating. So this is a very important
zone. We at USGS have been studying it
for some time. The seismic surveys I did in the
area, we can see that very dense high velocity lower crust and it
actually wells up, looks like a bell shaped feature.
And we can also see that the fault zone is very wide.
There are multiple, multiple faults in the surface.
So you tend to think of earthquakes in California
primarily or Alaska or some other places, but you don't tend
to think of it that much in the center of the continent.
But this is one very, very active area.
Yeah, it's surprising. You mentioned that it may be the
remnants of a formal rifting system or failed rift.
Do we understand that tectonic history that led to the
existence of this fault zone? Yeah, there are many papers on
it. A rifting system typically has
three arms. The two successful arms form an
ocean. So you can think of the gulf
like that. And the third arm, you can't
have all three be successful. So one that does not quite make
it to a rift. We've seen that many places in
the world, and that's likely what's going on there.
Is that like a geometric constraint?
Yes, yes. That split apart the pace at one
point, there's no room for it to do so in the third location.
Correct. Yes.
You've also worked on zones sort of seismically active because of
human activity, and in particular a zone in India's
Deccan Traps where a damaging earthquake of magnitude 6.3 was
induced. Fracking has received the most
publicity, but is that the human activity involved there?
There are several types of human activity that are known to cause
earthquakes. That includes hydraulic
fracking, wastewater disposal at high pressures, water reservoir
impoundment, mining, geothermal extraction, among others.
So basically these activities altered the existing stresses in
the earth and the induced earthquakes are generally rather
small. However, in Special Situations,
significant damaging earthquakes are known to have occurred.
And for the India area, the induced seismicity was caused by
the impoundment of this reservoir.
It started in the 60s. We refer to this type of
activity as reservoir induced seismicity.
Shortly after the dam was put up in the 1960s, earthquake started
to occur. It didn't take very long at all,
maybe six months or so after they first did it, and then they
got progressively more and more. And in 1967 a magnitude 6.3
earthquake occurred, which was quite destructive and killed
quite a few people. The earthquake even continued to
this day, particularly following large monsoons.
So our data indicate that the reservoir was impounded at a
step over between two fault systems, strike slip type
faults. And when that happens, you have
many perpendicular faults to the two main fault systems.
And when you put a lot of water on that, just the weight of the
water put stress on it. The water going down also gets
into the pores of the rocks. That changes the pore pressure.
And so that can generate these earthquakes.
And we see that again after a very large monsoon.
You will see say six months later, a flurry of activity.
We've seen that over and over again.
There are other areas in the US where we've seen similar things.
If you have these step over in the faults, you can get some
induced seismicity there. Nothing the size that we've seen
from India, but they can occur. So you have to pay special
attention to the geometry of the faults if you're doing certain
activities. We have reservoirs all over the
world, and in fact, hydroelectric powers, if
anything, is becoming more important as we're going through
this energy transition. So does this kind of work give
us more precise pointers to dam engineers and surveyors and so
on, as to where and where not to place dams?
I would hope so if the study is done ahead of time.
That's not always the case, but I think we're learning a lot
more about fault systems and how they're oriented, how they're
shaped and relative to the stresses that are there.
I should point out that most of the induced sites, Misty, in the
US, particularly in Oklahoma and Kansas, Ohio, Texas, Colorado,
they're caused by the injection of wastewater related to
petroleum production. So when you inject this
wastewater at high pressures, it tends to change the stresses on
the faults and you can generate these earthquakes.
And for example, in 2011, there was a magnitude 5.7 in Oklahoma
that was quite a strong earthquake that was felt all
throughout much of the Midwest. And that was definitely an
induced earthquake. And there were two other
magnitude 5 earthquakes in that sequence that occurred and they
all occurred very close to where they were injecting wastewater
at very high pressures. What are you working on at the
moment? I've been working on data that I
acquired in Turkey in February of last year 2023.
There was a magnitude 7.8 earthquake and that earthquake
was followed by a magnitude 7.6 in the same province, same
general area in Turkey and the shaking was devastating.
It resulted in more than 60,000 deaths in eastern Turkey, in
northern Syria. And Turkey has two major fault
systems and they're both very similar to the San Andreas one's
called the North Anatolian fault near the East Anatolian.
This earthquake occurred on the East Anatolian.
And so we need to understand what happened there.
It will help the Turks. It will also help us to
understand the San Andreas, and it will help people worldwide if
we understand these things better, because such large
terrestrial earthquakes, no matter where they occur, they're
going to be devastating. So I LED a group of scientists.
We went over and we deployed a lot of seismometers there to try
just see in three dimensions what's going on in the
subsurface, understand how this energy propagated so far away
and killed so many people. And we're working, of course,
with the Turkish government and with our colleagues nationally
and internationally. We have, I think, an incredible
data set and we hope to get a lot of information from that.
I'm reminded of an early episode of the podcast that I did with
Roman Jolivet fairly soon after the earthquake.
And although there were some preliminary conclusions, he was
very much telling us about the need to do some serious follow
up studies on that very complex fault system there.
So it's great to see that you're doing just that.
Rufus Catchings, thank you very much.
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