Seismologist Says Aftershocks Impossible to Predict
MELISSA BLOCK, host:
Here in China, we've been spending a lot of time talking about what's going on above ground - rescue, recovery, relief work. Well, now we're going talk about what's happening underground, about six miles below the surface. We're joined by Walter Mooney. He's a seismologist with U.S. Geological Survey in Menlo Park, California. He's also worked with Chinese authorities on earthquake studies for 25 years. Walter Mooney, welcome to the program.
Dr. WALTER MOONEY (Seismologist, U.S. Geological Survey): Thank you very much. Nice to be here.
BLOCK: We've been reporting for a couple of days now on a pretty widespread panic here, because there have been predictions made and broadcasts on local TV radio that a big aftershock is about to hit. Can scientists predict aftershocks with any accuracy at all?
Dr. MOONEY: Well, fortunately, we can say quite a bit about the nature of aftershocks. And the maximum aftershock that we expect would be about a 6.5. In this case, the largest aftershock so far has been about a 6.1. So that kind of prediction has held.
BLOCK: What would the warning signs be that something might be about to happen? What would you be looking for?
Dr. MOONEY: I think that we would be perhaps alarmed if there was a sudden burst of activity, in that there were a rapid increase in the numbers and magnitudes of the aftershocks. So given that we can't predict earthquakes, we can only look at patterns which are unusual. And in this case, I'm glad to say that we haven't seen any unusual pattern in the aftershocks.
BLOCK: I saw a quote from one of your USGS colleagues, Tom Parsons, who was quoted as saying there's a high risk of new aftershocks much bigger than those so far, especially in the area around Dujiangyan. That all sounds very specific.
Dr. MOONEY: Well, fortunately, we are able to see the kind of changes in the earth's crust that are accompanied by a large earthquake. You might imagine if you slip a rug, on one half of the rug, that the second half is kind of loaded and prepared to move. It's just the loading of adjacent faults by the main shock. As Tom Parsons has seen, this earthquake ruptured and moved in a northerly direction, actually northeasterly direction, and as a consequence the faults that are located in that area now have to carry an additional load from the Earth's crust.
So that's the area that we expect there to be greater numbers of earthquakes, and at some future time, a larger earthquake.
BLOCK: Larger than what we saw last week?
Mr. MOONEY: We wouldn't expect it to be larger than what we saw last week, but it would be larger than the average aftershock that's been accompanying this earthquake.
BLOCK: And when you say, at some future time, what sort of a time frame are we talking about?
Mr. MOONEY: The delay time between the main shock and the larger event that follows, can easily be as much as three months. And in other cases it can be twice that or longer. Unfortunately, we really can't say when the crust will unzip and the larger event will occur.
BLOCK: What are you hearing from your Chinese counterparts here, about what they've learned since the earthquake and what they still hope to learn?
Mr. MOONEY: The primary reaction that the - my Chinese colleagues have had - is one of disappointment and shock at the amount of destruction. They have tried for two decades to find a reliable technique that can give a warning that they can predict in the short term, the impending event of an earthquake of this magnitude.
And unfortunately the Earth is very complicated, and that process of predicting earthquakes is simply not possible. Now the challenge that's before them, is to think about their building codes, and the land use policy in China.
You know, despite the growing population, not every area is suitable for the construction of habitations and schools and hospitals, and people have to be relocated to more stable regions.
BLOCK: Would you say, knowing what you know what happened here and what might happen in the future, would you say that these communities that were the worst hit should not be rebuilt where they were, they should be relocated.
Mr. MOONEY: Many of the communities are located far too close to steep mountain sides, and you know, you can build a very strong building with modern engineering techniques and you can you have the best possible warning procedures for people to move quickly, as quickly as possible out of their residences once an earthquake occurs, but with a rapid moving landslide, you really don't have much of a change. So, in many cases, the communities simply cannot be rebuilt on the slopes of the very steep ravines and towering mountains, because the landslides are just inevitable.
BLOCK: Walter Mooney, thanks very much for talking with us.
Mr. MOONEY: You're very welcome.
BLOCK: Walter Mooney, a seismologist with the U.S. Geological Survey, he's also worked for 25 years studying earthquakes in China.
NORRIS: That's my co-host Melissa Block, from Chengdu.
Tomorrow she'll report on the giant pandas at the Chengdu Research Base of a giant panda breeding. She already shot some video of a panda bathing - I'm looking at it now on our Web site - actually pretty cute.
It's posted on our blog; that's npr.org/chinadiary.