This is geology B with Oliver's trampled.
We all know about the massive earthquake that occurred in
March 2011 off the east coast of Japan.
It had a registered magnitude greater than 9.0 making it the
most powerful earthquake ever recorded in Japan and the fourth
most powerful earthquake in the world.
Since modern record keeping began in 1900, it triggered its
tsunami with waves of up to 40 meters that swept ashore.
Killing nearly 20,000 people and causing the Fukushima.
And Nuclear disaster, the earthquake occurred.
Where the Pacific Plate is subducting under the plate that
Lies Beneath the Japanese island of Honshu.
During the earthquake, the Eastern edge of the overriding
plate. Jumped over 50 meters to the
east over a length of about 500 kilometers.
It was this unexpectedly large slip, that displaced enough
water to set the giant tsunami in motion.
Why was the slip so large? Was it something to do with the
nature of the fault Zone on the boundary between the two plates
as it nears, the Japan trench Patrick Fulton led a team that
probe, the tohoku. Okay, earthquake fault, which is
the Fault along which the main rupture occurred, his team,
overcame enormous engineering challenges to install what he
calls a temperature, Observatory?
Right through the fault Zone, the observations revealed the
temperature. I peek across the fault which in
turn enabled him to estimate the coefficient of dynamic friction
at the plate boundary and it turned out to be extremely low.
Patrick Fulton is an assistant professor in the department of
Earth and atmospheric sciences at Cornell University.
Patrick Fulton, welcome to geology B.
Thank you. And thank you for having me.
Can we start with a bit of tectonic context for the
subduction zone off? The east coast of Japan?
Does Japan's sit on its own plate, or is it on the Eastern
edge of a Eurasian plate? Japan is actually in a
Confluence of a number of different plates.
The northern part of Japan where this earthquake happened
actually sits on the North American Plate.
And some people will see that the plate goes all the way
around, from the United States up through Canada, around from
Alaska. And reaches down into the
northern part of Japan. Some people call that little
part of it. In Japan.
The acoustic plate kind of a micro plate and then when it
hits Mount Fuji, south of Tokyo, that's where it intersects with
the Eurasian plate and how fast of the plates, converging in the
region, where the earthquake occurred somewhere around eight
centimeters per year. I think that must be one of the
faster convergence speeds of plates around the planet.
That's right, it's fast. And some of it's because the
rocks are old and cold roughly, 130 million years and that old
cold rocks are fairly tense. And it's trying to sink down
into these the unisphere. So slab pull is especially
effective for that reason. So what exactly happens when an
earthquake occurs along a subduction zone, so when the
down going oceanic plates are pulled down from their own
weight. Into the steam is for they get
stuck to the overlying plate and they build up elastic energy and
every once in a while, they relieve that elastic energy, and
that's what we observe is. And how deep down does the
earthquake normally start? Does that stress?
Get released. So in subduction zones, about 30
kilometers depth is where we see a lot of earthquake starts.
That's fine. The middle of the seismic Janek
Zone on was the tohoku earthquake at that depth.
It was it was about 30 kilometers deep.
As I said in the introduction there was a 50 meter slip
recorded at the surface and that was unexpectedly large.
What would one normally expect for such an earthquake and and
what can we infer from this enormously large slip?
Yeah this was a very interesting and confusing earthquake.
Generally we thought that most of the slip would happen where
they start. So at 30 kilometers depth you
mapped out on a map where the Contours of sip were you would
see maybe at the beginning of the earthquake you had somewhere
around 5 or 10 meters or sip and then it would Decay out as the
rupture try to expand it would essentially hit the brakes.
The surrounding rocks and died out.
That was not the case in this earthquake.
And this earthquake, most of the slip actually happened at the
very shallowest parts. So where it was roughly 30
meters, a huge amount of slip down where it started at the
shallowest part. At the sea floor, it jumped as
much as 50, 60 meters. That reminds me of the podcast I
did with Roger bilham in which he talked about the earthquake
in 2015, in goloka in the Paul. And in that case, the rupture
did not propagate to the surface, the brakes, if you like
were effective, thus, the stress was built up further along the
fault, and everyone's expecting another big Quake sometime soon.
They're so why do we think this one was so different?
This is really kind of the big question and kind of motivated,
some of our work on land. Like in the Corker earthquake,
the rocks are strong and they are brittle and they break, but
sometimes they don't go all the way to the surface.
It's because they intersect areas where it's a bunch of soft
sediments where the rupture kind of will dissipate into different
areas or may deform more plastically.
Some of the rocks and sediments above it here.
We think in subduction zones, the rocks are often softer,
especially when they get to the shadow parts that we almost
never have really seen a surface break like this or at least of
this magnitude. And so that led to a lot of
questions about why the shallow part Slip so much was it because
the shallow part was really strong and built up.
A lot of elastic energy or was it really weak?
And the rupture came through. And it just essentially
hydroplaned didn't have any brakes on the show.
Part that actually takes us to your work which was indeed to
probe the fault Zone to see if you could gather data that would
help us figure out what the friction walls along the
surface. So how did you go about this one
way that we can figure out how much She resistance or how much
brakes were on the float when it slipped is by measuring, how
much heat is generated during the earthquake.
So, just like when you rub your hands together, it generates
heat. And it's a function of how much
you move them pass each other to displacement, but also how much
force and friction is between your hands as you're sharing it.
So if we could measure how much heat was generated on the fault,
we could figure out this big question and to do that meant
that we would have to drill down at depth where the earthquake
happened and measured any temperature anomaly measure the
excess heat across the fault after an earthquake, which is
really challenging because that heat could generate really hot
temperatures. May be up to 1000 degrees on the
fault in a very thin Zone. Maybe it's CM thick during the
earthquake, but then it diffuses into the surroundings.
And so, we figured that we would have to be there within about a
year or two. Afterwards to be able to do
that. And this earthquake with a lot
of slip will give it a very big signature, and because so much
of that soup was that shallow depth?
It meant that perhaps, we could actually drill into it and put
sensors down there, to actually measure the temperature anomaly
across the fall. How's anyone successfully
measured temperature normally across the fault during all
right. After an earthquake, not quite
there has been a few attempts particularly After the chi chi
earthquake in Taiwan large earthquake.
But there it took many years before they could drill across
the fault and I could only get one profile of temperature
before the hole collapsed. So if you see something across
the fault, you don't know whether that little temperature
blip is really due to frictional heat does it diffuse over time
as you would expect, or is it due to water flowing through a
fracture? Or is it maybe just due to
differences in rock type as you go?
So we learned a lot from that experience.
We learned that you really need to monitor the temperature over
time. And you really need to get a
good understanding of the Rocks themselves.
Take core samples, and rock samples.
There's also been a long-term study of trying to look for
frictional, heat, not from a single earthquake, but over
millions of years of shearing, along the San Andreas fault.
And there you would expect a large increase over many
kilometers of heat, coming out around the The fault Zone and
there it was not observed, despite a lot of attempts,
including some of my own during my PhD, which has led us to
think that maybe rocks, aren't as strong as we think they are
at least how they behave in the lab.
Maybe there is something different during an earthquake
or something different about mature plate.
Boundary fault zones. That makes it all the more
interesting that you actually were successful in determining a
temperature anomaly. So, as I mentioned earlier, you
went about this bye. Installing what you call a
temperature Observatory, tell us about what that is and how you
did that. Exactly.
So one of the big challenges with this project is that
perhaps we could drill Across The Fault in a place where it
had a lot of slip and perhaps measure the temperature across
it. But to do that, we were just at
the limits of where technologically, we could
actually drill. We're working in a water depth
of 7 km and then the fault Zone. The Beneath that was another
eight hundred and twenty meters below the sea floor.
So, no one had ever drilled that deep within the ocean before let
alone trying to put instruments under there.
So to do that, we first had to characterize the hole, we
drilled and found out exactly where the fault was first.
We took core samples out. We did you physical lugging but
now we drilled underneath the sea floor and we put a
protective casing at the very shallow part just so that we
Reenter the whole, then we built the observatories.
So we built a 855 M, long steel pipe inside of it.
We built our temperature Observatory, which was
essentially a static climbing rope, or a rope.
That does not stretch much where we had woven in temperature
sensors, each of them with their own Data Logger and each of them
encased in titanium. So all the electronics would not
implode, do the huge pressures at those steps?
And then the temperature sensors themselves, Elves have an
accuracy of a thousandth of a degree celsius.
We want to be able to see just the smallest little details in
the smallest little increases of what the temperature is.
So we then lower that down to the sea floor, and then
re-entered the hole and install that bit of casing, all the way
down. And then left it there, and went
away for several months and then came back to collect the data,
those temperature sensors, 55 of them, at different depths were
measuring the You're at those steps over time and once the
whole rig equilibrated, we could see if we saw a temperature
anomaly Crossville just before we get to your results.
I'm completely boggled by the idea that you could return to a
hole that I don't know what is it about a meter across or
something at the depth of 7 kilometers of water from a ship
that's like floating on the waves above.
How on Earth is that possible? Well it's just barely possible.
So essentially we have 850 M long Observatory of steel pipe
connected to another seven kilometers of drill pipe that
bring it down to the sea floor and we're trying to re-enter
that into the hole. There's no motor on the bottom
of this. All we have is the ship, which
can use Dynamic positioning to stay in one position on the
surface of the water. But down below, we have a TV
camera that can see only a few metres ahead of it at the
bottom. Of the whole assembly.
And so with the light on it we could see where we are.
We tell the captain. Well we think it might be over
to the north and he/she will move the ship over there.
And then the whole thing, all that steel pipe below us it's
like a wet. Spaghetti noodle.
It's just hanging there in it. Slowly starts to wiggle over in
that direction in response. And we did that for many hours
until we see the observatory or the whole rather on the seafloor
and little bit of Tubing that sits above the sea floor.
And once we're above it, someone says now.
Now now and then the Drillers drop everything into it.
And luckily, it was a hole-in-one.
We got straight in and then continue to lower it down.
That's up to incredible. I want to off.
So how you selected the precise location for the whole because
it seems like you are 850 meters of Observatory that you could
feed in below the level of the sea floor, the earthquake itself
was at 30 kilometers depth. So, obviously, The fault zone of
the fault plane makes it all the way up.
Gradually to the surface, at some point.
So how did you pick a point? Where the fault plane was at it
about it said, 820 M down. We needed to find a place where
we could drill across the fault at somewhere less than a
kilometer deep. If it's too shallow, maybe we
start to get to less weight acting on things.
And so, we wanted to find very clear position of the fault.
Less than a kilometer deep underneath the ground.
We also Want to find a place where we knew the amount of sip
fairly well and that it was large.
Here is a position where there was clear evidence that it was
roughly 50 meters of slip. The other kind of constraining
thing was we needed to find a place where the water depth was
less than 7 km. And the reason for that is we're
going to put our temperature sensors down there and they're
going to record but we need to get them back at some point in
time. There is only one remotely
operated Essentially an unmanned submarine that could go to those
depths and pull the sensors out, get our data back out and its
maximum operating depth with seven kilometers.
So inside this pipe and you have your senses, I think you said 55
of them. How far apart are they down
inside that bowl? We want to make sure that we can
really see the profile of an anomaly.
And so we made our sensors one and a half.
Meters apart from each other anticipating that the full
anomaly, maybe over some tens of meters wide.
And so most of them are about one and a half meters apart and
then X shallower depths we have some other ones that have
further spacing across that allow us to get some information
about the background temperature gradient through the Earth.
So you then seal the hole and left these senses to their own
devices for about nine months. And then you return.
So how did you pick that time interval and what were you?
During that time with the senses, when you first drill the
hole, you get all those broken up pieces of rock, out of the
hole by circulating, cold, ocean water, so that drilling process
actually cooled off the whole right around the edge of the
bore hole for a while and it would take some time usually a
few weeks to months to re-equilibrate with the
formation temperatures. So then we went away we were
anxious want to know the answer but this is still a very active.
Of plate. Boundary fault Zone with lots of
huge aftershocks, including a magnitude, 7.3 Aftershock,
pretty much right at this site, on a deeper fault below our
Observatory. And so, we were afraid that with
all of these sensors hanging on a rope, if we left it too long,
maybe the fault was moving and it would share through the
observatory and grab hold of the sensors and no matter how much
we tugged on them, we wouldn't get them back.
So, when we first went back, we couldn't find the observatory.
Some of that is just due to the challenges of navigating in
seven kilometers of water depth, you can use GPS in the air on
the ship but communicating to the ROV underneath.
You just, the smallest little bits of imprecision in your
communication can lead to you being several tens of meters
away from where you think you are.
But luckily some of our Japanese colleagues offered by us to use
some of their time with the our review few weeks later.
We went back and we were able Able to find it.
And I grabbed hold of it with the ROV and we pulled it all the
way back up to the seafloor and we recovered all of it.
The fault and not moved. It had not shared through
everything which was actually an important observation in itself.
It meant that a lot of the observations on land where GPS
sensors were showing that Japan was still moving to the east,
maybe as much as a MM per day, it wasn't because of slip on the
fault, it's probably due to movements within the man.
The mantle recovering relaxing after having such a big
earthquake. Well, that's fascinating.
So you're saying the actual land.
Mass of Honshu was moving, but the sea flow was not what part
of the sea floor was, but that movement was not between the
overlying plate in a downgoing plate, it was between both of
those things and the mantle underneath was turning a little
bit after the earthquake. So what's the GPS location of
the ball? Actually slightly different when
you came back after 9 months. Well, we don't have a very good
GPS location of it. The main issue was that the
position was correct. But our communication, for where
we thought we were was slightly Miss calibrated, because of the
wavelengths that you use to communicate from the ship to the
ROV. It's just a very long path.
Any little bit of imprecision just adds up and if your 1020
meters away from where you think you are down there, You can't
see very far ahead of you with an ROV light.
Doesn't travel that well, through the water and at these
steps, this is the deepest darkest trenches, it is very
dark down there. Okay.
Well, let's talk about what the temperature sensors had logged.
During those nine months in the Bohol, what you found out as is
the case, pretty much everywhere on Earth the temperature
increases as you go deeper, it's roughly about three degrees on
the sea floor, and then the temperature, it seems to
increase down to about 25 degrees Celsius, kind of I'm
temperature down at the bottom of the observatory and if we
remove that background gradient, we see the anomalous
temperatures at first. It was cold because of that
drilling disturbance. But then we started to see a
relatively big temperature, anomaly of about Point 3,
degrees Celsius centered right on the fault Zone.
The same position where we had logged in, have all these
geophysical images of the fault, we've actually taken.
Accor sample out from the Vault itself.
Right there is where we see a very large temperature.
Anomaly, that we interpret to be the frictional heat signature
from the magnitude 9 earthquake. How wide is the Fault Zone down
there? Very interesting.
It's very thin compared to a lot of other fault zones.
So, fault Zone has its main core.
It's roughly a couple of meters or less than 4 meters, thick and
filled with dense, clay and then around it.
There are other subsidiary faults and fractures that really
like a knife edge on these scales that were talking about.
So you have your temperature signal and you already said you
had to subtract out the thermal gradient associated with the
steady state heat flow, that's coming up from the mantle to the
surface. But whether any other effects
that you had to account for an order to really understand the
contribution of the earthquake to the temperature.
Normally we didn't know if that anomaly was due to fluid flow or
to determine Properties. We did lots of measurements on
those core samples that we got to make sure that that anomaly
that we saw could not be due to differences in thermal
conductivity or other Rock property.
We also looked at the data very closely to see is it very
smooth? And does it look like the
temperature is diffusing, as we would expect or are there?
Lots of signatures of water moving through, open faults and
fractures and in the plate, boundary fault itself.
We did not see any Indications of that.
But we did see water flowing through fractures away from the
main fault and other faults and fractures.
And that gave us a whole other insight into earthquake
mechanics. During the nine months that you
are logging the temperature down at that bull.
Whole, did you see this temperature?
Normally start to diffuse away through thermal conductivity?
Did it Peter out. It was hard in that time,
because we were still seeing a lot of the effects of the
drilling disturbance, but generally the shape of it, The
relative stable behavior of It Through Time, suggested to us
that it was a very conductive signature just a moment ago.
You were saying it was actually quite significant that you
managed to pull your whole sense of string out of the whole
intact without any of its snagging or getting caught.
I assumed you meant the possibility that the whole was
deformed during those nine months.
The whole could have been shared across the fault, could have
still been moving and we didn't see that some of the other
things. Is that we measured while we
were down there before we install the observatory was take
the resistivity image 3D 360 degree image of the walls and we
could see how the tectonic stresses were deforming that
hole after we drilled it. So when we drilled it was a
cylinder but we could see from those images that in some
places, the tectonic stresses were forcing it a little bit to
collapse in certain places. Where the hole was becoming more
oval shaped. And that told us about the
magnitude of the stress, has down near the fault Beyond.
Just what we see in the temperature, that told us what
the stresses were during the earthquake.
So after the earthquake, we could see that the amount of
shear stress on the fault, was essentially zero.
So, not only did this fall Sip and have no brakes on it, and
that the stress on the fault, went to a value that was very
small. That after the earthquake at
least in the year afterwards had not built up a lot of stress,
that's interesting. So the coefficient of friction
was very low and at the same time all the stress was relieved
perhaps because of that in part. So did you manage to get some
quantitative results from this that you were able to put in
perspective with the parameters that we've observed another
false around the world? Yeah, I guess I jumped ahead of
myself there and so I told you that we saw this point three
degrees Celsius I'm sure know me but I didn't tell you what it
meant and so we could use that heat anomaly and the width of it
to constrain and with the thermal properties of the Rocks
down there to constrain how much heat must have been deposited on
the fault during the earthquake. And I had said that the fault
Zone was about two meters thick but they're in that two meters.
Thick, there are lots of thousands, tens of thousands of
little sip surfaces. From probably tens of thousands
of past earthquakes across that fault.
And so each of them are, as thin as even a few millimeters.
And so, most of the slip can be in a very thin zone, or it could
be distributed across those two meters, it's unknown, and it's
not really that relevant for the calculation, we do.
But what we found by figuring out how much heat energy was on
the fault. Now, he energy per unit area
related to the Stress or the shear resistance times the
displacement, we take that defied up by 50 meters of slip.
We get the average Shear resistance on the fault.
We know that the average Shear resistance is related to the
friction coefficient times, the normal stress, which is related
to the weight of the Rocks. Calculate the weight of the
rides based on their density and we end up with a friction
coefficient. That is point zero eight.
Which is outrageously small, most rocks.
Have a friction coefficient of point 6.
It's two point, eight. So it is told us that the fault
had no brakes for very little resistance to slip.
Once the rupture got into this shallow part, I might we explain
such a very low coefficient of friction.
Those sediments in The Fault Zone that are really slippery.
They already have a low friction coefficient lot smaller than
most rocks in general, and perhaps a combination of the
rupture, going through them and other effects associated with
the generic A tree of the fault being very shallow, or very
horizontally dipping together, may have allowed the rupture to
find a week playing in a set of diffusing out or rupturing into
Hard Rock's. It affects found this easy
Pathway to just continue to slip all the way to the sea floor.
So, you mentioned clay, I guess Clay is a sort of lubricant as
compared to say other materials like more, silica Rich,
materials that many. Any sediments are formed.
Is that right? Yeah.
In the lab granular, materials Sands or crystalline materials,
have very high friction coefficients of REM point six
point eight, when you put them under stress, but Clays are
interesting. Their material structure is
different in their play D minerals.
They tend to be a bit weaker in general and these particular
Clays are sure smectite clay, which is largely Ash, not a lot
of sediments from River. As I said, these are some of the
oldest Oceanic rocks, 130, million years old or older, they
are probably deposit out in the middle of the ocean, far away
from any land at such great. Depths that all the animal life
has dissolved when they get to these great depths where their
deposit. So, pretty much all this
material that sediment is ash from volcanoes that have gone
through the atmosphere and found their way to the bottom of the
ocean. So, ash from volcanoes, Is not
in itself, necessarily clay-rich right is what happens.
Once it goes down at depth, and you're below the depth at which
carbon will read is also no carbonates are intact down.
There, is that what happens? That's right, these soft
particles and Ash when they go down and they're exposed to
water and they can break down into these clay minerals.
So obviously enough Quake, like this one has enormous
destructive potential. Do these Studies have practical
implications? For earthquake preparedness or
maybe even for prospecting for economically important minerals
that are deposited by the kinds of transient hydrothermal.
Fluid flows, that you saw further up in the old borehole
Observatory certainly. So we saw that other faults and
fractures would respond to after shots pulses of fluid flow
through them. They were opening up.
And that's a very similar process to how we think a lot of
ore minerals. Are precipitated.
That's also another process that can change.
The fluid pressure. Change the stress State as the
fluids move through. They can redistribute fluid
pressures, which affects stress State, and those changes in,
stresses can trigger subsequent earthquakes.
So, that tells us a lot about the processes that can affect
aftershocks. But just in general,
understanding something about our initial question was the
shallow part locked in building up a lot of stress really
strong. Or it was it really weak and
just sit along for the ride that can have direct and important
implications for earthquake and tsunami Hazard preparedness.
Perhaps one of the more analogous parts of the world
would be the Pacific northwest coast of the United States and
Cascadia where people talk about a huge earthquake with tsunami
potential waiting to happen. Is that a similar situation?
There it is a similar situation. This is a place that also like
the northern part of Japan. Japan has not had a lot of
earthquakes in modern era. People had thought that Cascadia
doesn't have big earthquakes, it's just quiet.
Maybe it's just always gradually sliding, not building up any
stress but Native American stories and also now, paleo
seismological records evidence in the Rock record of past
tsunamis that it does have big earthquakes, including a
magnitude 9 earthquake in January 1700 there.
The geology is a little bit different.
But instead of having lots of ash and clay, there are a lot of
sediments. It's not as easy to drill into
the fall. Although we could perhaps do
some follow-up survey shins to learn more about the seismic
potential there as well. Patrick Fulton.
Thank you very much. Thank you for more about geology
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