JOE PALCA, host:
This is SCIENCE FRIDAY. I'm Joe Palca. Ira Flatow is away.
A massive magnitude 8.9 earthquake struck off the northeast coast of Japan today. Hundreds people died in the quake and the tsunami that followed. There were tsunami warnings all around the Pacific Basin, prompting evacuations from low-lying areas.
Little damage was reported either in Hawaii or in the west coast of California, but people are still being urged to be cautious in those areas. Joining me now is Ross Stein. He's a geophysicist with the U.S. Geological Survey in Menlo Park. Welcome back to SCIENCE FRIDAY, Dr. Stein.
Dr. ROSS STEIN (Geophysicist, U.S. Geological Survey): Thank you, Joe.
PALCA: And if you'd like to join us, give us a call, our number is 1-800-989-8255. That's 1-800-989-TALK. Or tweet us, @scifri.
So 8.9. Massive is the word, I guess.
Dr. STEIN: Yes, and unprecedented in this part of Japan.
PALCA: Ah, and - but this is an earthquake zone, as I understand it.
Dr. STEIN: It is, but magnitude nine earthquakes are very rare. And unlike California and the Pacific west, northwest coast, they have a long history. So there's 400 years of earthquake records. Offshore the Tohoku area, where this earthquake struck, there's nothing like this in the record.
PALCA: So do we have any idea what's going on?
Dr. STEIN: Well, on the one hand, measurements by GPS receivers over the last decade have shown that the entire area that ruptured, the fault was locked, and stress was loading in a contiguous fashion. So from that vantage point, one can say, always in retrospect, this area was building up stress for a future earthquake, and that earthquake could be as large as the area that was locked.
PALCA: And what was the effect - the earthquake itself wasn't what was causing all the damage. It was the tsunami, right?
Dr. STEIN: That's right. Any earthquake where the - in this case the Pacific plate is shoving itself underneath the island of Honshu, Japan, and that causes the sea floor to move up and down, where - both rapidly and then permanently -and that excites the tsunami wave.
PALCA: Okay, so explain a little more about that. What is a tsunami wave as opposed another kind of wave?
Dr. STEIN: Well, the only kind of waves we have on the ocean are either driven by wind or by tsunamis. Those - that's the only show. And tsunami waves are special in the sense that they arise because huge portions of the sea floor are lifted and dropped, and these waves travel feeling the bottom.
They travel at about the same speeds that aircraft fly, and when they're over the deep part of the ocean, they're very, very broad and very shallow. They're maybe hundreds of kilometers or hundreds of miles wide and a few feet high.
But then, since they're always feeling the bottom, the sea bottom, when the sea bottom starts to shallow toward the coast, they get very, very steep and very short amplitude. And so that's when the destruction occurs.
PALCA: I've tried to - I've tried to understand, and I'm still not sure I do, why these things travel so fast. I mean, you just don't think of water traveling at 500 miles per hour.
Dr. STEIN: Well, its speed has to do with the depth of the ocean basin, and in fact, the very first measurement of the average depth of the ocean basin came from just knowing the arrival - the time of an earthquake versus the time of the arrival of the tsunami across the Pacific. And that gave a dead-on answer for the average, oh, one-mile-deep depth of the ocean basin.
PALCA: So there were - there's a system of buoys that are supposed to note the possibility of an impending tsunami. Did those work, in this occasion, as expected? And what did they tell us about - I mean, it seems that the destruction that was, at least warned about in Hawaii, failed to materialize.
Dr. STEIN: I don't - these are called the - the so-called DART buoys, which are basically buoys that are tethered by a long chain to the sea floor, and they are offshore.
I don't know if they worked, but I suspect they did. Japan is very, very well-wired into earthquake preparedness and earthquake warning systems, and they both - they have both warning for the arrival of tsunamis and for the arrival of the shaking.
And my guess is they had notification of the shaking before it hit, as well, I mean, seconds before, minutes before, but before.
PALCA: So why do some earthquakes, even the ones in the water, fail to cause tsunamis where others do cause tsunamis? Or do we know that?
Dr. STEIN: It's a really tough call. It appears that earthquakes that are deeper deform the sea floor less. Earthquakes that cause huge undersea landslides also trigger tsunamis, but they tend to more localized than these.
In general, the biggest the subduction earthquake, the more likely it is to produce a tsunami, but we do have lots of exceptions...
PALCA: Can you explain again how subduction works, just to remind us?
Dr. STEIN: Well, the Pacific Plate, which is stretching from Japan all the way to California, is being driven toward the northwest, driven toward Japan and Alaska, at about four inches a year. And it's jamming itself underneath the island of Honshu, the mainland of Japan.
And what happens is, in some places, that fault is very slippery, and it just goes down every day, goes down the hatch, and we never have any big earthquakes at all. And in others, the fault is quite stuck, very rough in character. And then one has these long waits before great earthquakes occur, when finally the stress overcomes the friction in the fault zone, and you - it suddenly leaps forward, 50 feet in this case.
PALCA: So it's a good reason not to get stuck.
Dr. STEIN: We wish that we could keep them all lubricated.
PALCA: Right, okay. Why don't we take a call now and go to Nila(ph) in Newport, Oregon. Nila, you're on the air. Welcome to SCIENCE FRIDAY.
NILA (Caller): Hi there. I listened carefully, and it sounds like the northernmost thing I've heard about was in Crescent City. And I'm actually in I-5 going home to Newport, Oregon. And so I'd like to know if you have reports of damage north of there.
Dr. STEIN: Well, I don't. I'm not involved in the coastal alerts. The expectation was that we could have up to about six feet along the West Coast of the United States. And my understanding is some of that has materialized. But I don't know any of the details.
PALCA: And Nila, I talked to my colleague, Christopher Joyce, just before we went on the air. And he seemed to indicate that the waves were not that high, coming in to the California coast. So perhaps you'll be in good shape.
NILA: Okay, thanks so much.
PALCA: I hope so, anyway. So if you're going to go and study these tsunamis - I mean, let's say - what's your first thing you want to do if you were in Japan and you wanted to study this? What are you going to look at?
Dr. STEIN: Well, among - there are many interesting things about this earthquake. Why is it that in this historical record, we only see, let's say, sevens and seven and a halves throughout this area? They are so small compared to this earthquake, they're about one-thousandth the size, that they don't just don't release appreciable stress.
It looks like this site has been building up stress for 1,000 years. Why so? Why is it that we don't see this more commonly? Another question that arises is that this earthquake did have a foreshock sequence, magnitude 7.2 earthquake that occurred probably in or near the epicentral area, to produce an unusually high rate of aftershocks.
We definitely know that these foreshock sequences are not seen before most great earthquakes, including the nines that we have in Sumatra in 2004 and last year in Chile. Nevertheless, is there anything that we are going to learn from that foreshock sequence that would give it away as a harbinger of something great to come?
PALCA: Right. And I just wanted to clarify something we were talking about earlier. When we talk about this wave moving at 500 miles per hour, it's not the water itself that's moving along at 500 miles an hour but in fact the height of the wave that's moving along.
Dr. STEIN: That's exactly right. There's kind of a circular and cross-section, or vortical motion of the particles. But you're right. You're not going to be jetted along on the water surface at 500 miles an hour.
And as you can see from the imagery, one of the problems of a tsunami is not just its height. It's its density. It entrains all this debris. So it's really not water when it hits shore. It's like a moving mass of very, very dense material that's entrained in it. And that's one of the reasons why it's so destructive.
PALCA: Let's take another call now and go to Michael(ph) in Cleveland, Ohio. Michael, you're on the air.
MICHAEL (Caller): Hi, thank you. I was just wondering if the recent volcanic activity in Hawaii is linked to the earthquake in any way.
Dr. STEIN: Well, I certainly hope not. I would be flabbergasted if that were the case. Generally speaking, we don't see how earthquakes and volcanoes could communicate over such vast distances. And by the same token, I would be very surprised if this earthquake in Japan triggered some activity on the west coast of the United States or in Alaska or South America. As far as most of us feel, those distances are too great.
On the other hand, this earthquake could trigger some very, very large aftershocks in and around Japan, and Tokyo lies just to the south of this rupture zone. So there are some very real follow-on processes that could happen, but we don't think something, particularly of a volcanic nature in the mid-Pacific, could be related to this.
PALCA: But this is kind of, I guess, alarming in a way. If you look at something where you say, well, this is something - I mean, this quake is significantly larger than the ones we've seen up until now and more than we were expecting, does that mean you have to sort of rethink what you knew about the size of a quake someplace?
Dr. STEIN: Joe, this is a very humbling field to be in. You know, if you want to be smug, don't go into it, because we keep being surprised. And we keep falling victim to the kind of human frailty of thinking that what will happen next will look an awful lot like what happened before.
And so one of the best examples is the Pacific Northwest, where in our short history, there really aren't any large earthquakes at all, but two geologists discovered that there was a magnitude nine in 1700. You know, here again, we were fooled into not realizing the potential high magnitude of this earthquake.
I think the message of the Mali earthquake, the Sumatra earthquake and this one, is in any subduction zone, we have to consider the unlikely possibility of a super-large earthquake.
PALCA: Yikes. And just finally, is there a time when, you know, the all-clear can be blown? Or is this something that could go on for a few weeks, even, in the Japan area?
Dr. STEIN: Strike that and say a decade or two. It's a great question.
PALCA: That's a long to remain vigilant and keep the sandbags around.
Dr. STEIN: That's right. Well, here - I'm not talking about a tsunami so much as a subsequent earthquake. And consider that aftershocks, the frequency of aftershocks, how often they occur, diminishes very rapidly with time - but not their magnitudes.
So we could get a very large aftershock, let's say a magnitude eight, in days, in years, in decades from now, that is still related to this main shock. The vigilance we have to maintain is really continuous and remarkable.
PALCA: Okay, we've run out of time. I want to thank my guest. Ross Stein is a geophysicist with the U.S. Geological Survey in Menlo Park. Thanks for joining us.
Dr. STEIN: Happy to be here.
PALCA: When we - we'll come back right after this short break.
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