cc >> Brooke Norstead: It's my pleasure tonight to introduce geoscience professor
Harold Tobin. Harold's research focuses on what's going on on the edges of earth's tectonic plates.
These are places that have active earthquakes and volcanoes. And in particular, Harold is the
co-leader of an international team of scientists who are interested in and doing research on the Nankai trough, which is
off the coast of Japan. And what this team is doing is actually drilling into the active fault there, taking out
rock specimens and inserting instruments that can help them understand what makes earthquakes begin.
And tonight, he's going to be talking about the recent earthquake and tsunami that happened in Japan.
With that, I would like to introduce Harold Tobin. ( applause ) >> Harold Tobin: Thanks.
Thanks very much, Brooke. And thanks to both Wednesday Nite at the Lab series and the Geology Museum,
of which Brooke is the assistant director, for sponsoring this talk tonight. She told just you a little about
my research and that is exactly what I'm working on. The region that we're doing the drilling project in
is a different part of the offshore region of Japan than the March 11 earthquake happened on.
So this is going to be an interesting talk in that I'm mostly going to tell you about things that are not my personal
research. In fact, it's going to be kind of a current events talk. But I'll try to bring you as
much as I can of the science that has come out of this earthquake since it happened, and explain it, you know, as
sort of clearly as I can for people who are maybe not geophysicists. I'll be happy to take questions
about it as we go along. But so, I titled it, you know, the March 11 Earthquake and Tsunami, what happened and why.
What I would like to do is really explore the events of that day, and actually the day before, and then a number of the
days after, and the times since then, focusing on the main earthquake of course that happened, what occurred in the
planet to make that earthquake occur, why was it so big. As you have probably heard already, this was the fourth
largest earthquake that's been recorded since we've had seismometers active on the planet.
That's since about the 1890s, late 1880s or so. For 120 years, this is the fourth largest earthquake that's
ever occurred. We'll talk a little about the before shock and aftershock sequence that occurred related
to this earthquake. I can also tell you that it is the best recorded earthquake that's ever occurred of any kind
of magnitude like this. We have an incredible richness of scientific instrumental recordings of a whole variety of
types, and I'll show you a bunch of visualizations of some of the data. We'll talk about actually the
warning system that is active in Japan and how it performed. We'll talk about the tsunami that occurred, which of course
was really the source of the vast majority of the terrible loss of life in this earthquake, and most of the source of most
of the destruction, including the key part of the damage to the nuclear reactor that still has people pretty worried.
So, I'll focus on what happened locally, and then I'll also talk about what you heard about on the news a lot at the time,
which is the travel of the tsunami waves across the entire pacific basin, and how we know about that, and what some of the
forecasting, how it's actually done. We'll take a historical perspective on this earthquake
and tsunami, and say well, you may have heard that scientists were surprised by the magnitude of this earthquake.
Actually, that's true, and I'll explain what that means and why. It doesn't mean we're surprised by earthquakes in northern
Japan, but were surprised by some aspects of this particular one. It's actually causing us to go
back and take a second look at some of the really fundamentals models for how these kind of earthquakes work.
Then I'll sort of put it in some perspective at the end. I may talk a little bit about our research project if I have a
little bit of time, in the Nankai trough region of Japan. So with that, let's dive right in.
Here is a Google Earth image, of course, of the Japanese islands. I cut Hokkaido off a little bit up there.
But you can see a couple of key features. Besides the actual islands of Japan themselves, the most
obvious thing on the map is all these yellow dots everywhere. That is the first week of the main earthquake and the
aftershocks above magnitude 6. That's a map of all the magnitude 6 earthquakes that happened in the region.
Every single dot is a magnitude 6 or larger earthquake. You can see that, the largest one, they're scaled according to
the size of the earthquake. The largest one is the main earthquake. It's epicentral location on the
planet, so the point on the earth's surface above where the earthquake actually occurred, it occurred about 20 kilometers or
24 kilometers, 15-18 miles down in the earth's crust. You can see the whole region of aftershocks, which is actually
illustrating for us the region of the earthquake fault that ruptured. I'll explain what I mean by
that. So, an epicenter is just sort of a point where the earthquake waves began to emanate from.
But the actual portion of the fault that slipped was so large that it extended for about 400 kilometers along the coast of
Japan and about 200 kilometers in the perpendicular direction defining a whole area of the fault that slipped.
And aftershocks actually end up mapping that out pretty well. The other thing that you can see in this image is the sea bed
bathymetry, right, the topography of the ocean floor. You maybe have heard of the oceanic trenches, we'll get into
them in them a bit when we talk about some of the basic plate tectonic questions. This region is called the Japan
trench. This is Honshu Island. Some refer to it as Northern Honshu.
In Japanese, the region is called Tohoku. It's been called the great Tohoku earthquake.
That's sort of the official seismological name of this earthquake of 2011. You can also see there's
actually a second trench down here. This is the region called the Nankai trough.
The studies that I'm doing with the offshore drilling project are actually right down in here. We're looking at a likely future
magnitude 8 earthquake, but we're in a different region from where that earthquake occurred. Okay.
So, I'm going to start with some real fundamentals. And bear with me for a second, but let's just make sure
everybody is kind of on the same page. The first question is just fundamentally, you know, what is
an earthquake? Well, it's really two different things. One is that the earthquake waves
that we feel, the thing that shakes the ground if you standing in an area near an earthquake.
How many people have felt an earthquake before in their lives? Oh, most of you.
That's pretty good. When I talk to undergraduates, there's not nearly as many who've felt an earthquake around
here. It shows that some life experience is valuable, right? ( laughter )
The waves that you feel are the vibrations of the earth, but they're not the actual earthquake itself.
Because the earthquake itself is really the rupture or the break in the ground that occurs during slip on a fault.
So we say, what is a fault? Well, a fault is sort of just simply a flaw, or why we call it a fault, a break in the earth's
crust where two very large-scale, as large as the entire depth of the crust, in these kinds of earthquakes,
are pressed against each other with a lot of stress, because they're down in the earth, there's a lot of pressure,
but tectonic forces that I'll explain in a moment cause those ultimately to break and fail.
So they can stick together and slide past each other when they're stressed.
Now if we look at a little animation of just a little chunk of the earth here, and I have to remember not to push that laser
button, because some bug in Power Point means it stops the animations from going. You can see is that the rupture
is somewhere down in the subsurface and the waves that travel away from it, right, are the seismic waves that you feel.
That can be felt at great distances, hundreds, even thousands of kilometers away from the earthquake.
And of course, the seismograph is just an instrument that measures the displacement of the ground, or sometimes it actually
measures the velocity or the acceleration of the ground, which are just related to the displacement, either up and
down, or side to side. We usually measure north-south or east-west, so we get three components of motion for a
3-dimensional motion of the ground. We talk about earthquake as an elastic rebound process.
I want to explain what I mean just a little bit when I say that. Rocks might not seem like they
can bend, right? A rock is sort of a brittle thing. It just sits there, and if you
stress it hard enough, maybe you can break it. But when we are talking about miles into the earth, we're
talking about the thickness of the tectonic plate with all the stress that's down there, and then thousands of miles
perhaps area, rocks are quite bendable. And in fact they're elastic, in the sense this is elastic.
I can take the rigid stick, but I can bend it. Elastic means that I can bend it, but if I let go, it pops
back to the original shape, right? >> When you say east to west, or north to south,
is that how it actually--? >> No, that's just how we measure three components of the motion to rebuild
what the actual motion was, which could be could be in any arbitrary direction. We measure up and down,
north and south, and east and west. >> It could be northeast, southeast?
Absolutely. Yes, any direction, that's right. So, if we think then about
the elasticity of the crust, basically we know that there are these tectonic plates that form the surface of the earth.
There's a whole series of tectonic plates, and they're in motion relative to one another. So, if for a second,
the floor is one tectonic plate, and this thing is another tectonic plate, and it's trying to force its way in that
direction along the floor, if I push on it, it slowly bends, right? I will try hard not to
break this thing, actually. But tectonic forces move very slowly. We measure plate motions
in a few centimeters per year, maybe up to 10-15cm per year. It's about the rate your fingernails grow.
If you let them grow for a whole year, that's how far the pacific plate has moved over that time. But it's slowly and inexorably
over centuries building up the stress and it's stuck, right? The rug is holding the stick. And that's the same as the
friction along that fault line between the two plates. So eventually that builds up and builds up, and the strain,
just like I'm an archer pulling a bow back, is building up more and more stress and eventually it pops, right?
I overcome the friction with the amount of elastic strength. That's an earthquake. That pop is the earthquake.
Okay. So, let's go on from there a little bit. Earthquakes, as we know,
can happen anywhere. Some of those earthquakes you guys felt might have been ones Illinois, right, or one--
There's very few in Wisconsin. We have not had a magnitude larger than 2 or 2.5 in
recorded history in Wisconsin. I might be wrong about that. But certainly not bigger than 3. Anyway, earthquakes can happen
anywhere. But the fact is the vast majority happen, not in the middles of tectonic plates,
but the edges where two of the Earth's plates meet. That's why we call them plates. They tend to be rigid,
and translating or moving relative to one another. So this is a map of all the earthquakes greater
than magnitude 6 again, but for an entire decade. Okay? On the whole planet.
We won't worry about the depth for the moment, but every little dot on that image is a single earthquake
that's mapped over that decade. You can see that they don't occur randomly everywhere, but they occur at specific places.
And in fact, this is part of how the story of plate tectonics was found out, because they observed that these earthquakes were
associated with the mid-Atlantic ridge. That's where the plates are spreading apart as you've heard,
opening up the Atlantic Ocean. But most of the earthquakes, in fact, 90% of the earthquakes actually happen in these belts
where you see much thicker numbers of individual dots. And those are right next to those oceanic trenches.
In fact, those are these regions we call subduction zones. A subduction zone, a big word. All it means is a place where
one plate is converging on another plate. They can meet and they can sort of buckle and rumple, but when
one of them is an oceanic plate and it's low, it's actually very dense, compared to the continents.
So what it does is, it tends to, one plate rides over the other, and the other one descends descends back into the mantel
of the earth, ultimately to be remelted and reincorporated into the interior of the earth. So all these earthquakes all
around what's called the Pacific Rim of Fire are subduction zone earthquakes. That's where most of the
earthquakes occur. You can see them along Central America and South America, let me get
my geography right. The Aleutian Islands, Japan, and so on, around the Pacific. So, subduction zones
are the big player in terms of plate tectonics on the surface of the earth. Here's a cross section that kind
of illustrates what those are like. We call the crust, plus the upper part of the mantle
below it, but the rigid part of the earth's upper region, lithosphere, below that, it's sort of got some
partially molten material. It flows like taffy, slower than taffy, but it's very viscous.
This rigid plate goes down beneath the overriding plate. In the case of Japan, this would be the Japanese Islands,
the Pacific plate out here in the region. The trench occurs at the point where the plate literally bends
over and dives back down into the interior of the earth. Eventually, that plate gets to a depth of about 100 kilometers
or so, and that actually is a region where it gets hot and high pressure enough to melt material, and that fuels
the volcanoes. That fuels the volcanoes of Japan. That fuels the Cascades.
Mt. St. Helens and Mt. Ranier are exactly the same situation in the Pacific Northwest. But down there, it's actually
too hot for big earthquakes. The region shallower than that. this upper portion is where the conditions are right for
that frictional behavior, like I showed you on the rug, so the plates stick together. What we see is that this region
that locks up and builds up that stress. The plates just keep moving and moving, and moving,
but their boundary is stuck, and then it breaks. That's when the earthquake occurs, and that earthquake can
spread and propagate along that region, in fact along the length of it, but also up towards the surface,
and ultimately move the sea bed to generate the tsunami. I'll get into that in a minute. >> This formation is kind of the
in northwestern United States, but we don't see that type of earthquake activity you see in the pacific.
That's right. And you know what? You've hit the nail on the head for something that's actually
really important in our understanding. You see, you know, lots of earthquakes there,
lots of earthquakes there. If you look really closely, the Pacific Northwest in this region right here,
it's kind of a gap. For a long time, we thought, or scientists I should say thought, it was a little
before my time. After plate tectonics came around in the late '60s, early '70s, it was thought something
was special about the Cascadia subduction zone, and it slipped down without having earthquakes.
There's been virtually no earthquakes recorded on the boundary. There are earthquakes in Oregon
and Washington, but they are shallower ones. It turns out that it's virtually certain that there was
a magnitude 9 earthquake in the year 1700. The story of that is really interesting one, in that it
was discovered through some really clever work that the coastline had some unusual subsidence to it, where a whole
bunch of trees along the coast were all drown. They could do the tree ring dating and get approximately
the year out of that, all up and down the coastline of northern California, all the way to Vancouver Island.
Then there was a tsunami in the historical records in Japan that had no known earthquake associated with it.
It was called an orphan tsunami in the historical records in Japan. So even though there were no
people with written records, there were Native Americans in the region, but no Europeans in the Northwest, on January 29,
1700, at about 9:30am, we know there was a magnitude 9 earthquake. And looks like from other kinds
of really interesting sort of, almost like archaeological work, we call it paleoseismology, digging holes in the ground,
looking for the characteristic telltale deposits, we know that that's probably happened about every 500 years over
the last few thousand years. It's been 310 years since then, so, plus/minus error, we're right within in the range of
time for that to happen again. That's why the Northwest should be worrying about tsunamis and earthquakes, and in fact
is worrying about it, and taking some steps. Okay, that was a great question. I have some slides about that
later, but it was a good time to answer it now. Anyway, so let's move on. The uplift of the sea bed during
the earthquake is the cause of the tsunami, fundamentally. I'm going to show you how that works in a few minutes.
But first I want to show you a little bit about the earthquake itself. We'll take the story through
one step at a time. We are back to northern Japan here. Again, here's a map of the early
aftershock sequence. There's the main earthquake, once again, in this location and all those aftershocks.
Here's the same thing, but it's depicted as a function of time. So these are dates in March, and then just the latitude,
north and south. The main earthquake is here, and you can see what happens. That earthquake occurred and
then all these aftershocks just started lighting up that zone. And what happens is the earthquake slips, and it slips
a big patch of earth's crust along the boundary between the two plates. Then once that happens,
it's sort of like the fault is in its new configuration, and needs to settle in a little bit. It's kind of like you sit down
in a chair, then you've got to get yourself comfy. So the fault has suddenly slipped a whole lot.
It's got this sort of fresh scar to it, and lots of little stresses pop off lots and lots more earthquakes.
That's what aftershocks are. Aftershocks are a completely normal occurrence they happen in a kind of an exponentially
decaying time series after an earthquake. Rule of thumb is you can always expect, you can virtually always
expect at least one aftershock of about one magnitude number lower on the earthquake magnitude scale.
So for a magnitude 9 earthquake, we expect at least one about magnitude 8 aftershock and then kind of 10 times more for
magnitudes 7s and so on, 100 times more for magnitude 6s. It turns out this has followed that pattern pretty well.
There was a magnitude 7.9 aftershock a few minutes after the earthquake actually, in fact, so close in time that
in Tokyo they felt the waves just kind of run all together from those two different earthquakes.
But more interestingly for this earthquake, there was what we call a foreshock. Foreshock, aftershock,
they're just terms defined on the basis if you have a big earthquake, that a littler ones that come before it on the same
fault are foreshocks, and littler ones that come after it are aftershocks. Of course, no one knew it was
a foreshock when this magnitude 7.3 earthquake happened a day or so, two days beforehand. But it turns out it did happen
in just the same region as the main earthquake did, and it was probably that sense that you know, if I took my stick
and I took it all the way to breaking it, we'd probably hear it creak and pop a little right before it snapped.
I'm not going to break my stick. But that creaking and popping just before it goes, the initial giving way of the fault zone
is a foreshock. So we should be able to see those foreshocks and predict earthquakes on the basis of
them, right? We should be able to see that happening and issue a warning that this was a foreshock for a
big earthquake that was coming. The problem is that statistically, we never know when an earthquake happens
whether it is a foreshock for a bigger event or not. When that earthquake happened, people knew there was a long
history of magnitude 7 earthquakes in this region, and just thought it was another one in the series.
And people have tried very hard to use foreshock information to predict earthquakes, and it turns out it just doesn't work.
You can't make a statistically useful prediction based on any particular earthquake, until the bigger one happens, and then you
go back and say ah-ha, that was a foreshock, right? ( laughter ) Scientists are smart, you know.
The old adage about prediction is, it's a very thing to do, especially about the future. So, we try to predict the past
instead. Okay, so let's look at what actually happened in the earth's crust on the fault zone.
It turns out that when the waves propagating away from the earthquake, they are recorded literally all over the world,
as you know, right? The seismic waves travel all around the planet. We'll look at that in a second.
We can use computer models to basically use those wave arrivals, the measurements made of seismographs all over the
planet, to backtrack to the source of the earthquake was. It arrives everywhere, and you can imagine the wave spreading
out, so you can basically calculate backwards in time to get back to the source of the earthquake, kind of triangulate
back in its position, essentially. Well, there's more to it than that.
They fit the wave forms. There's some very sophisticated modeling that goes on. But what can come out, is
actually a pattern of the slip on the fault. This is one of several earthquake slip models, but it
basically suggests the star is where the epicenter was. And the sort of big blue rectangle is actually a plane
that they fit in the model that you have to imagine going down into the earth at an angle. So it's like it comes up to near
the surface over here. It's at about 40 kilometers depth under the coastline, and so it's a plane dipping down,
sloping down into the earth's crust. It's actually the boundary between the Pacific plate over
here, which is going underneath Japan at 8.3 centimeters per year. Anyway, what it looks like
happened was, the slip started there, but it spread out over about 400 kilometers, or 350 kilometers.
The remarkable thing about this earthquake was that it generated very large seismic waves. That's how we can calculate the
initial magnitude of the earthquake, but we actually estimate the magnitude in total of an earthquake nowadays by the
area that the fault slipped, and then the amount it slipped, and then the frictional strength, the frictional resistance to
slip. It's an energy calculation, how much work is done by the earthquake.
That energy calculation suggests the moment was something times 10 to the 22nd joules. It's a big number.
But in particular, it shows that the slip over this large area between the two plates in that 1-1/2 minutes or so,
2-1/2 minutes or so, slipped somewhere between 30 and 50 meters, okay? That's 100 to--
what, 160 or 170 feet that those two chunks of rock slipped relative to one another, built up over hundreds of years
slipped in a few minutes. That's remarkable, and that's actually what causes the tsunami.
If we look at this, this is an interesting animation. It's one of the back projections of the earthquake
energy recorded at seismographs all over the USA. I should actually restart it, so you can see what's going on.
But seconds going by here. The slip starts off here, maybe you can see the coastline just there.
It basically shows the different patches of the fault, we had started at one place, and that kicked off another
part of the fault, and that kicked off another part of the fault and it's spread out over this whole region.
Actually, there were times nothing much happened, and then another piece of the fault kicked off.
It was a very dynamic event that took a lot of time. We're up to 500 seconds, or so, having taken place down here.
The slip, if it holds up of 30 to 50 meters, is the largest slip recorded in any earthquake.
There's been some larger earthquakes, the Sumatra earthquake in 2004, the one that caused the Indian Ocean tsunami,
that was actually an earthquake that had a larger area. It slipped over something like 1500 kilometers, all the way up
to the Bay of Bengal, but it only slipped somewhere between 15 and 20 meters, not shabby, actually.
But it was a slightly larger earthquake, but with slightly less slip. Here's another visualization.
Japan has seismographs everywhere, all over the country. They have thousands of recording
seismographs. It's the best instrumented place in the world for earthquakes. What I'm going to show you is a
little visualization, where in an very exaggerated way, they'll show us the ground motion, up and down, from the earthquake
waves as time went by after the initiation. Here are the seconds down here. Let's just watch this run for a
bit here.
The earthquake starts and the waves start. Oh, I forgot, I pushed the wrong button there.
Let's back up and start again. The earthquake starts, waves start spreading across Japan, right?
The Tokyo region is shaking hard. 200 seconds have gone by. The network went out.
The power went out to the network in that region, so you don't see that anymore. It's still going.
It's shaking the whole country up and down by many centimeters, 500 seconds after the earthquake occurred.
What's that, eight minutes. Is that right? Thinking on my feet, yeah. Anyway, the ground shook all
over the country for a very long period of time. It also moved the ground very hard.
In fact, set a record for the strongest accelerations of the ground that have ever been recorded from an earthquake.
These are a bunch of stations of what we call strong motion instruments. They measure the largest motions
from the earthquake. And really, the key thing to look at is down here. This is a measure of
acceleration, if you can see the vertical axis, in units of g. One gravity, right? We're being held down by 1g.
A 1g vertical acceleration would be enough to throw you to where you are weightless, not quite leaving the ground.
The peak acceleration recorded in the earthquake at a station just up there close to the epicenter was 2.7g.
So it would throw all objects that weren't bolted to the ground thrown into the air, literally quite high.
That's never been recorded before, 2.2g was sort of the previous record. It did some remarkable things
with motion. It moved things so much, actually, that we could record it with instruments that aren't
even seismometers. GPS stations that are permanently affixed to the ground are now in routine use by
geophysicists to measure these very slow plate tectonic motions. This is an example of one of the
GPS stations in Japan. It's got a GPS antenna. It's not really using any different GPS than the one in
your car. It's just a little more sophisticated antenna, and then a lot more sophisticated
processing that goes into it, and it's fixed to a pier that's sunk way into the ground so it remains well fixed to the rock.
We can measure the position with an accuracy of just a couple millimeters and update that every 30 seconds, or in fact,
every second. There are more GPS stations like this in Japan than anywhere in the world, an order of magnitude
more than there are in California, probably. Thousands of stations like this all over Japan.
When we look at what happened in the earthquake, I'll show you the displacement of those GPS monuments.
All these dots on here, and this is going to be a movie in just a second, are the locations of GPS stations.
Let me call your attention to this arrow here. That's a vector. That's a scale of one meter.
So we're going to show you these GPS stations moving, but not in the real map coordinates, but scaled to one meter.
And this is half a meter. This one over here looks like north/south, but it's really showing a vertical displacement.
Did the ground go up or did it go down. This one over here will show you horizontal displacement.
So let's get this guy going, if I can.
They're going. A few will show you a noise blip for a second, that's not anything very real.
And here comes the earthquake.
So you saw two things, right? You actually saw a wave go through, and then you also see a bunch of those vectors are still
there. What they show you, then, is that the wave was actually those same waves that we saw in the
seismograph records going through the country, spreading across the country. But the vectors that are still
there, I think the vectors still there are the permanent displacement. They are how much the ground
moved and didn't go back to where it was in the first place. In fact, the maximum displacement was along the
coastline of about four meters, right, 14 feet, or so in the south eastward direction. The vertical displacements were
up to 70 centimeters, or so. So a couple feet. >> Is the vertical all in one direction?
The vertical is not quite all in one direction, but they're mostly. If you look at the view, the
coastline all went down along here, close to the epicenter. If you actually look around, there are some places around the
edges that actually went up. I'll talk about that. That's actually the elastic change in the crust.
You can also see that it sort of sweeps from small displacements back here, much less than a meter, to these very large ones
at the coast. And they literally look like they are all pointing. It's like the crust of the earth
just suddenly went "whoosh" toward the epicenter, toward the region that actually slipped in the earthquake, and relaxed that
spring loading that had been stored up for actually it looks like over 1,000 years in that time.
That's the displacement of the earth measured by the GPS stations. Here is a version of that, that
uses all of that to do another calculation for how much the fault slipped offshore. The independent data
basically shows us the same thing as the seismic records. Go ahead. >> In a previous slide, you
showed how close Japan is to that trench. Does this mean that the Japanese Islands are closer to that
trench? >> This region, this coastline, moved four meters closer to the trench.
We'll see in a second that it's not as simple as saying it got near the trench. It's actually just the release
of that built-up strain. It'll move back from now until the next earthquake, and then it will pop toward again.
We'll see it in just a second. If we look at kind of a cross section through the country drawn down here, we can actually
see what happened. Here is a cross section, slicing down through the earth. Here's the surface of the earth,
very schematically. Here's the top of that plate that is going down beneath Japan, mapped to that.
What happened was, the plates are stuck together, and it pulled the plate back while they were stuck in between the
earthquakes, from the forces of that subduction, and then popped forward during the earthquake. The coastline actually went
down while the offshore region actually went up. The offshore up is the translation of that
30 to 50 meters offshore. That's what made the coastline drop down vertically. Let me try to explain that--
Actually, one or two things and then explain that just a little bit more. Just one more interesting
observation from this GPS network. Here are the north, east, and vertical components
of the GPS measurements. That seismometer is the GPS. This is from a station by Tokyo. This is quite a long way away,
and in fact, a place where the ground shook strongly, but didn't do a huge amount of damage down there.
Look at these displacements. That's 50cm up, 30cm back; 40cm up, 20cm back. It's something like if you were
sitting in your chair right now, and suddenly you were moved back three feet, forward four feet, and then
it happens six times in a row. That's what's being shown by these GPS stations. We're calculating their global
position in every second. That's consistent with the accelerations we saw from the seismometers.
I got an email from a colleague who was in Yokohama, just actually about only half a day after the earthquake.
What he said in that email, was that from the earthquake itself, and then from the aftershocks and everything, it felt like
the ground was just continuously swaying without ever stopping, and that he knew many people who were feeling seasick,
feeling motion sickness because for hours and hours, the ground had not stopped moving. All right.
This is just kind of an interesting one. I said the earthquake waves spread all over the planet,
and in fact they do. The earthquake traveled all the way around the earth, and could be recorded
by instruments. We didn't feel it here in Wisconsin. This is actually a National
Science Foundation sponsored special experiment going on. It's called the US Array. It's a set of individual
seismometers spaced about 40 miles apart over this whole region. It actually started on the
west coast, and they're kind of and marching it across the country in two-year increments. It's to image the deep interior
of the earth. When the earthquake occurred, all these stations that are shown on these dots recorded
the earthquake happening. So what we're going to do, is we're going to watch the earthquake come across
the middle of the US, the Midwest, and we're going to see a line move across this record, which comes
from that station right there in, wherever that station is. I can't see very well, is it in Nebraska?
No, we're in Colorado? No, we're in Kansas. Thank you very much. I can do geography, yes I can.
All right, let's see. Let's watch this take place. So, the colors you see are up and down motions,
shown over here on the scale. I just did it again. I stopped my slide. I should have said that first.
You'll see the color change and get brighter and darker based on just those stations going up and down.
The earthquake occurred, here comes what we call the P wave, the primary wave that first came in from Japan.
We're going to see a wave that bounced off the core of the earth. And now we're going to see
the S wave. That's the main, what we call shear wave motion across the planet.
Then we get to the surface waves, which are the ones that have the largest amplitude and do the most damage there,
but move slower. They're waving across, because they're coming from Japan. If it doesn't look like
the right direction, remember the great circle pathway. If you fly to Tokyo, you go right over Anchorage, right?
They're coming from the right direction. Now, the things sort of move around for a while.
We watch those seismometers as the waves are just rippling around the earth, bouncing off the core, doing all kinds
of crazy things. Now look at what they're doing. The waves are coming back at us and look like they are going
back the other way. That's because they are going back the other way. The earthquake started, and
the waves started going around the earth every direction, so it went right by here, but then the ones coming back
from the other side are what you are looking at right now. >> We don't feel those? >> We don't really feel those.
The reason we don't feel them is, you look at the scale, that says 3 millimeters. The crust in Madison went up
and down by about a centimeter, but it's very slow. These are sped up a lot. Over the course of the minute,
it goes up a centimeter and back down. There is not much acceleration, so we don't feel it.
Actually, can you feel it right now? ( laughter ) The earthquake also excites
something in the earth called normal modes. Those are the vibrations, or the oscillation of the entire
earth's fundamental resonant frequencies. Have you ever seen those videos of the astronauts where they put
a little bubble or ball of water and they blow on it, and the water goes like this and vibrates back and forth?
Well, the earth did that in response to this earthquake. It's like the ringing of a bell, and it's still doing it today,
those normal modes are still vibrating from this earthquake on March 11. We can't feel them, but they're
detectable earlier on, now I'd call it just a calculation. I'm having fun with questions, but I'm going slower
than I planned to. I do want to show you more things about the tsunami, for sure, and some other stuff.
The earthquake happened, the waves start propagating. You saw that there are different waves that leave the earthquake.
One called the P wave, that's the first one that goes. It goes the fastest, but it's not the biggest.
After that comes the secondary wave, or shear wave, and then the surface waves. Those are the ones that do
most of the damage. They go slower. Japan has implemented an early warning system for earthquakes.
This is not a prediction of earthquakes, it's a system that once those P waves are detected, if they're big enough, a
computer algorithm automatically sends out a warning to the country. It turns out that this worked
remarkably well, but this was really the first test. This is live television, was a live television feed
from Japan. This is actually Parliament. ( tones chiming ) That's the earthquake warning
coming up on the screen on everyone's television who was watching this. That sound.
Basically, what they're saying is an earthquake happened off this coastline. It just did it again.
Let's see if I can just make it keep going. If I back it up, it'll keep going.
So, no human intervention, an automatically generated earthquake warning appears on every television screen
that's on, and also was sent out as a text message to people's cell phones. They call it the reverse 9-1-1.
It sends everyone a message, earthquake just happened, it's big enough you should take cover.
Then as we'll see, not long after this, the tsunami warning comes in. The translation
is down at the bottom. A short time before the vibrations arrive. You can see in Parliament,
they still haven't felt it yet. >> Are they aware of it at this point? >> No, because they're not
watching TV. They may be just getting them at this point. You'll see them start
to respond. People are looking up.
So, everybody is sort of sitting there. Earthquakes are so frequent in Japan, that just feeling one
won't even stop people in the middle of a sentence. The meeting will just go on, until they realize how big
the event was.
That's the announcer on the television. ( man speaking Japanese ) And eventually here, they'll
break in. So they break in. ( tones chiming ) ( man speaking Japanese )
The tsunami warning has now been issued. The earthquake was first. Now the tsunami warning,
as an initial calculation, says there is danger of a real tsunami happening. ( man speaking Japanese )
In the interest of time, I'm going to speed this ahead a little bit-- attempt to speed it ahead a little bit.
Here we go. So, by another minute or so later, they had a tsunami warning out for this particular
portion of the coastline. And a more minor tsunami warning for other regions along the coastline.
The bottom line is, the earthquake warning was issued several seconds before the S wave even got
to the coastline. It was a tremendous success in earthquake early warning. It was basically a split second
thing that took place. People along the coast, even in the areas damaged badly, got somewhere between
five or 10 minutes' warning of the impending tsunami. That probably saved very, very many lives.
It looks like 27,000 people lost their lives in the tsunami, but probably several times that would have without
the early warning system. The earthquake itself acts as an early warning. If you feel the earthquake,
especially if you are Japanese and live on the coast, you know to move away or move to higher ground if you can.
The scale of this earthquake was such that people were not able to escape it, or underestimated what they had to do to escape it
and that's a different story. So, the earthquake happens, the warning is issued. The earthquake did some damage,
but not a lot of much damage, even in the close-by regions, not that many buildings failed. You probably heard
that story. Earthquake engineering is good in Japan. And it turns out that these
offshore earthquakes, although they shake the ground a lot, they don't shake it with those very high frequency vibrations
that tend to really damage the earthquakes the worst. But the tsunami of course is another story.
And so, the tsunami arrival at the coast was much greater inundation than was expected over that large an area.
But it was not much greater than had been seen in some parts of this region many times in the past.
Actually relatively recently in the past as well. I have a little bit of video of the tsunami coming in.
Well, this image is showing us, there is actually a seawall here that was built to guard against up to a five-meter tsunami.
The harbor is out here. The water has risen, and it's simply risen enough that now it's overtopping the seawall
and coming over. I might as well show a little of this additional video. I've got to jump out of my
Power Point to do that. There's a great special running on National Geographic right now, you should try to catch it,
"Witness to Disaster." It has a lot of these clips. I'm going to use a little of this clip here.
The tsunami is arriving in a couple of the cities that got the largest inundation from the tsunami.
Those walls are to guard against a five-meter high tsunami, about 18- to 20-foot high tsunami. As it came to the coastline,
people have mostly moved into the higher regions, assuming they were safe. In some cases they were, but in
other cases, they were not. I think probably people have mostly seen a lot of these kinds of clips.
The story was not that they were unprepared for tsunami, but that the tsunami overwhelmed the preparation and defenses.
It gets a little shaky there. But you can see what it's doing. Sometimes it's like a breaking wave, but other times it's just
like a rapidly rising tide that comes up into the harbors and overwhelms the defenses. Of course, the hydraulic power
of the water did the damage, knocked the buildings down, and so forth.
That's a truck. I'm assuming that people saw these videos on the news at the time.
But that coastline of Japan, a lot of it's very rugged and mountainous, and there are a lot of narrow harbor towns.
There are little valleys that meet the sea. It was difficult to get out of the way, to get far enough
inland or up-slope. That's what we just saw, a still from a minute ago. The water is black, because it
has picked up shallow water mud along the way. This is the city of Sendai. I'm going to skip ahead,
because it makes you seasick to watch this part. Actually, I think we've probably watched enough of this stuff.
They are on YouTube, exactly. Watch the National Geographic special if you get a chance. It's really good.
Let's wake up Power Point. Let's talk about the tsunami generation. I showed you my stick and how
it pops forward, and I said it's the displacement of the sea floor. Let's make sure we have a grasp
of what's going on offshore to make that happen. Here are the two plates merging, the stickiness pulling that
plate back, and then it's the pushing up of the sea floor that pushes all the water above it now, so the water
suddenly has a many meters bulge in it, set up over just a few seconds or tens of seconds. Water can't make a bulge,
water will spread out as a wave. Water wants to be at sea level. That's the source of the tsunami.
The source of the tsunami is the displacement of the sea floor upward by the unsticking of that upper plate
as it moves. Here's the same thing, if that was confusing. In a diagram, the subducting
plate is stuck, it bends back, and that distortion, that elastic distortion just like with my stick
pulls the offshore part down, but moves the on shore part up. For the hundreds of years between earthquakes, the land
has been slowly rising out of the sea. And then during the earthquake, it suddenly pops forward,
it launches the tsunami and also caused the coastline to drop down by that 70 centimeters. That's what happened
in Indonesia in 2004, and the coast of Chile last year. So the tsunami is this sort of obviously exaggerated cartoon
version bulge here. There is the nearby tsunami, and the far traveled one. Let's look at those.
The nearby tsunami, again, was one for the record books, but maybe not quite as much as you might think.
There are observations of bigger tsunamis that have happened on the planet. This region in Japan though,
the same coastline, just recently, this was just posted online by the Earthquake Research Institute about two
days ago, has gone out and done initial surveys of how far the water came in offshore. In the open ocean, the tsunami
height was just a few meters, two to four meters, or so, in the open ocean. That's where the water is deep.
Just like any wave, as it comes in to shallow water, the wave slows down and piles up. When it comes into the harbors
that are like funnels, it piles up more and more. The maximum-- This is hard to read, I realize,
but these are meters over here, observations from 2011. The maximum measured value is somewhere close--
More than 35, close to 40 meters of water height during the tsunami in one of those towns. That is 125 or 130 feet
that the water rose during that tsunami. That is, you know, it would cover my building, Weeks Hall,
and then actually cover it twice over if the tsunami came in of that elevation. We'll come back to the rest of
what's on that in a few minutes. Actually, no, I put it in here. Let's talk about this right now. It turns out that you could say
was the tsunami unprecedented? And the answer is, along this northern coast, this is north of Sendai.
Tokyo is to the south somewhere. This was not unprecedented. In fact, it's happened twice in the past 120 years, or so,
in very similar scales. This is a view from kind of looking towards the south. I know it's a little confusing,
but there's that same coastline shown over here. Hokkaido is here and now Tokyo is over there.
These are the heights of tsunamis recorded in 1933, from a smaller earthquake that just affected this coastline,
but up to 23 meters then. In 1896, the same coastline, same little villages and towns, up to 38 meters at that time.
So similar magnitude, well-known to occur. They built the tsunami walls, but they really never built them
to match these kinds of scales. The business about the nuclear reactor, that's down here along this part of the coast.
It turns out this region south of the city of Sendai has not experienced a tsunami of this kind of scale in,
it looks like, about 1100 years, over 1,000 years. But they had experienced tsunami before, and there was certainly
not any lack of understanding that tsunami could hit that coastline, including where the power plant is.
Let's talk about the far traveled tsunami now. I don't want to overrun my time by too much.
You probably saw the news that day, and saw that they were saying a tsunami is coming towards Hawaii, and
going to arrive at such and such a time, and hours from now. Let's look at what happens when a tsunami travels
across the ocean. This is actually another early warning detection success story. All around the Pacific Ocean,
there are a number of these buoyed stations called the DART system. What they are is, there's
a sensor on the sea bed, and it's basically just got a pressure gauge in it. It measures the pressure
in the water very precisely. So, when the water height, or a column of water produces certain pressure--
So you can see the tidal signal over several days in this DART station, that's what the blue line going up and down
is over here, and that's just the tide going up and down out in the open ocean. This station was several hundred
miles offshore near Japan. But when the sudden spike in the water height comes in, they know that's got to be something
different. In fact, that's the tsunami arriving at that station several hundred miles offshore.
That triggered a warning. I should say the earthquake triggered a warning to the Pacific Tsunami Warning Center
in Hawaii. These DART stations were then used to detect that a tsunami had in fact actually been made
by that earthquake, and that the warning had to go out. What that does actually, is feed into a computer model
immediately, within about an hour after the earthquake, this he computer simulation was done by NOAA, and they show
the earthquake occurring, this down here is the record at some of the nearby stations that are these little squares
up here, and now the tsunami is spreading across the Pacific Ocean. It turns out the wave travels
at a speed that's determined solely by water depth. It's just like a shallow wave tank experiment in a physics
class, so you can predict what time it will get to Hawaii and what time to the west coast of the US
with a lot of accuracy. I you know a little about the size of the tsunami initially, you can predict the size.
They did a great job. They actually predicted the size in Hawaii spot-on, and the time spot-on, as well as warning
Crescent City, California, Santa Cruz, and all those other places that the tsunami was coming.
If we look at a version of that that just shows the predicted wave height from the simulation around the ocean, it shows
up to just 240 centimeters, like I said, just a couple of meters in the ocean, spreading across the Pacific.
You can kind of see that it doesn't spread evenly. That's because water depth varies around the Pacific.
It's not one constant depth, it varies a lot. You can see that it sort of aimed the far distance waves
out to the open ocean, hit Hawaii over here. There's this interesting finger that looks like a kind of
wave height, you know, concentration headed straight for the coast of California. It turns out that's actually
a sea bed feature, called the Mendocino Fracture Zone. It's a shallow water feature that extends across
the ocean floor, and acted as a wave guide for the tsunami, carrying it in that direction, and causing a larger magnitude
right there of the tsunami, where it did more damage than anywhere else, north and south along the coast.
Damage happened in Hawaii. The news sort of overwhelmed this story, but actually, there was a lot of economic damage,
I don't think anybody lost their lives in Hawaii, along the Kona coast, if you've ever been there on vacation, a bunch of those
hotels had water wash right through the first floor. Okay, a little bit of historical perspective on earthquakes.
We have heard the story that, you know, the tsunami or the earthquake took us by surprise at some level,
and that's partly true and partly not true. There is the map of some of the earthquake and aftershocks.
Here are the ruptured areas of known previous earthquakes in the 20th century, give or take a little bit.
These are all areas that are magnitude 7-ish earthquakes. Remember, the magnitude scale is a logarithmic scale, so a 9
is 100 times larger than a 7, actually in energy released it's 300 times larger. But so magnitude 7 earthquakes
have happened frequently. The year is shown for each of those. Two of those are the ones that
generated the tsunami I just showed you the records of. What had never happened, though, in historical records
was an earthquake that spanned this whole region. Northern area had had tsunamis, southern area hadn't really had
tsunamis. But it turns out that that record is just not long enough to capture all the events.
The region down here is called the Sendai Plain. Sendai City is there, and the nuclear plant, I should
be able to point right to it. It's somewhere in this region right here. The Sendai Plain had not
experienced a very large tsunami in recent historical time, but this tsunami washed up to six or seven kilometers inland
along that low elevation region. Of course, that's where a lot of the damage and loss of life happened.
It turns out that 10 years ago, a paper was published by clever tsunami experts that showed that this had happened
before, and it happened before about in the year 869. It was known there was a large earthquake in Japan at that
time, from historical accounts, and it was known that a tsunami had come inland. But the size, you know,
we didn't know much about. Historical records in Japan are phenomenal, they have the best in the world, but they're
not good enough to completely estimate the magnitude. What the folks did though, was they go on land,
and maybe this map doesn't show it very well, but here's the coastline. They go on land along
a transect and dig trenches or holes down into the ground. In the upper few meters of the soil, they found these sandy
marine sand deposits. Those deposits are paleo-tsunami deposits. They're the archaeological
records, sort of, of a tsunami. It turns out that they could correlate the sheets of sand from different trenches at
different distances on shore, and then they used things like radio carbon to date the ages, roughly speaking.
There are some error bars on those, but at any rate, they came up with an estimate that 1100 years ago, or in 869,
the tsunami had traveled inland by about the same distance, of four to six kilometers, and furthermore, that it had
happened twice before that over the span of 3,000 years or so. So, approximately once every 1,000 years, instead of
a magnitude 7, this region seems to have a magnitude 9 scale. I left part of the story out, but they basically model
the source of the tsunami back from what they measure in the trenches on land. So this is real sleuthing out
pre-historical records of tsunamis. The picture we're left with is that we can have smaller
events that we thought were the biggest that region had, but every once in a while, the whole thing sort of unzips
in one big event that covers the zone of several of those smaller ones. This is actually from the paper.
More than 1100 years have passed since that Jorgan tsunami and the possibility of a large tsunami striking
the Sendai Plain is high. They estimated, actually, they were conservative and underestimated the scale.
But what they wrote down and actually told the people who run TEPCO several years ago, was that their nuclear plant
was at risk of a large tsunami inundating the plant, to much larger than the protective walls could cover.
The story of why that didn't happen is a political one, not a scientific one. Let's see, I think what I want
to do is just bring up a couple more things. The scale of the earthquake, I want to put in perspective
for you a little more. I talked about it at the beginning. Here's a little chart that we
kind use to illustrate the magnitude scale for earthquakes running from below 1, again, it's an algorithmic scale,
up to the largest earthquake that's ever been recorded, which was 1960, off Chile, another subduction zone,
at magnitude 9.5. You know, they're very rare. Once or a few times a century for that scale.
Then there are 20 or more magnitude 7s around the world every single, year, year in and year out.
You don't always hear about them if they don't do a lot of damage. There are thousands
of magnitude 5s, and there are millions of magnitude 2s that happen every year around the planet.
The earthquake that just happened, and the Sumatra earthquake, were almost the same size.
They fall right up here along with the biggest ones that we have a record of. Notice the energy equivalent
scale. Here's a lightning bolt, or the Oklahoma City bomb This is a bit of an old map.
The Hiroshima atomic bomb released the energy equivalent to the slip on a magnitude 5.5 earthquake.
The largest nuclear test was close to an 8. These are way above there, and above any other man-made event
that happens on the planet. Here's another way of actually looking at earthquakes, and why subduction zone earthquakes
like this one are so important relative to other earthquakes. This is a pie chart of the energy released by all
the earthquakes on the planet from 1906 to 2011. So a century of earthquakes, everything we've ever recorded.
Well, all the ones less than magnitude 6 are just that piece of the pie. All the ones from magnitude
from 6 to 7 are that piece of the pie. All the ones for magnitude 8 to 7 are that piece of the pie.
Chile in 1960, plus Alaska in 1964, plus Sumatra in 2004, plus Japan just now, is half of all the energy
all the earthquakes in the whole planet have released in a century, just for earthquakes. Here's another way of seeing the
same thing. We call it the cumulative moment release, maybe it's a kind of technical slide,
but it's the same thing through time. Earthquakes happen, they pop off, they release energy,
and then boom, in come -- There was one in 1952 that was pretty big, 8.7 or 8. And Chile is the biggest one,
and then Alaska, then Sumatra and Japan, with these periods where not much happened in between.
Another way of looking at that, here is just the earthquakes bigger than magnitude 8.8, since 1900.
Well, we say, and we have said for a long time in response to questions that earthquakes happen at different sizes
according to that statistical chart I just showed you, and they happen at random times. They happen randomly in time.
But all the magnitude 8.8 and larger earthquakes that we know about, that we have records for, basically happened either
in a cluster between 1952 and 1964, or in a cluster that maybe we are right in the middle of, at the end of,
we don't know, since 2004. That's the Indian Ocean. That's Chile last year. That's the one
that just happened. And so, there is at least a hypothesis, just a hypothesis, among seismologists that
actually these are like global trigger or global aftershock sequences, that when one really big one happens,
it shakes things up enough in other parts of the world that perhaps ones that are ready to go statistically pop off
more frequently. This isn't a very long historical record. Basically, when we have 1,000
years like this, we might be able to say whether that's really true or not. But it certainly looks like
that's possible. You can conceive of the physics. That's a controversial statement.
There are lots of people from the US Geological Survey on television in the past month who said these earthquakes are
totally unrelated to each other, it's just a coincidence that this has happened. I don't necessarily believe
it's a coincidence. I also don't necessarily believe they are related to each other. We don't really know.
But it's an interesting observation that that's it for magnitude greater than 8.8. So that's kind of the answer
to a fundamental question, are all the recent earthquakes related to each other? For a long time, the answer was
just no, except for aftershocks. Just the aftershocks on a given fault, yes, we knew they were related.
Two things that have happened in the last decade or two, suggest that within a given region of the earth, even
nearby, the change in stress when one earthquake occurs can kind of set off things in the adjacent region.
You know, if you pop one nail out of a board, it might make the next nail likely to pop, and the next likely to pop,
and so on. Now we don't really know the answer to long distances. Probably mostly no, but for
these very largest events people are starting to ask those questions. I'm going to end with a question
that people always want to know about earthquake prediction. And actually, the answer is really easy.
In terms of a useful prediction, something we could give you a warning that in the next three days,
or in the next two hours, or in the next month there's going to be an earthquake of magnitude 7.5 on this part
of the San Andreas fault, or this part of the Japan coast, no way, can't do it. We have not found anything
that holds up. There's lots of tantalizing things that happen for one earthquake, when we look back,
like those foreshocks, but when you look at them globally, we just haven't got anything. We really don't have anything.
no matter what you hear on the news. So, we don't have it. And in fact, some people argue
that it's impossible from basic physics, and other people think we just haven't measured the right thing.
I'm actually in the second camp, that we have not measured the right thing. That's a different story.
Long-term forecasting though, is sort of like long-term weather forecasting. Yeah, we can make probability
estimates based on past events. We know where the plate boundaries are, so we know which places might be likely
to have big earthquakes, and we issue risk factors. Those take a form like this, 30-year probability of exceeding
a certain intensity of earthquake. This is the official Japanese government's warning document.
We have one like it in the US. They really predicted a very high chance of earthquakes down here in the region that
I'm working in, because of that in part, not the only reason we're working there, actually, not at all.
This is one of the lowest probabilities that was mapped for Japan, where that earthquake just occurred.
So, is it humbling for seismologists? You bet it is. There's actually more I haven't
told you about why we really thought this region wasn't even capable of a magnitude 9. It just proved to us
that we were wrong about that. Okay, so that's the stuff I talked about earlier on. I don't have a slide like this
for Japan. The Japan earthquake was smaller, probably closer to this area.
But the region of the Sumatra earthquake compared to the Pacific Northwest is about the same, and the same size
earthquake can certainly happen up there, so they are putting tsunami warning signs all along the coastline.
If you go to the coast of Oregon, you will see the signs all over the place, and you should pay attention to them.
Thanks a lot. I appreciate your patience and I'm happy to answer more questions.
( applause )