Large California Quake Is Third in Five Days

Southern California is shaken by an afternoon earthquake Thursday, the third significant quake to strike California since Sunday. The strength of the tremors was measured at 4.9 on the Richter scale. Melissa Block talks with Dr. Kate Hutton, seismologist with the California Institute of Technology.

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MELISSA BLOCK, host:

A broad region of Southern California was rocked by a moderate earthquake this afternoon. It registered a magnitude of 4.9 and was centered about 70 miles east of Los Angeles, near the town of Yucaipa. There are no reports of serious damage or injuries, but there were lots of shaken nerves, since this was the third significant earthquake to strike California since Sunday. Joining us from Pasadena, California, is seismologist Dr. Kate Hutton of Cal Tech.

Dr. Hutton, three quakes since Sunday; that includes a large one on Tuesday that led to a tsunami warning for the West Coast. Are these events connected?

Dr. KATE HUTTON (California Institute of Technology): Well, I don't know if we can answer that question at this moment. There is a theory which covers triggered earthquakes where either the seismic waves from one quake or the strain change in the surrounding area can trigger off another earthquake if it's very close to happening anyway. But without a significant amount of analysis, we really aren't able to say for sure. And there are some seismologists who are saying that it can't be true; it has to be a statistical coincidence.

BLOCK: So it wouldn't be that common a thing--Or would it?--for earthquakes to cluster like this, to happen over a short period?

Dr. HUTTON: Well, most earthquakes don't seem to cluster, but we have seen cases in the past where we've had several earthquakes that were of moderate size in the same week or whatever. So it has happened in the past.

BLOCK: Now today's earthquake, we mentioned, registered 4.9. What did it feel like? I'm assuming you felt it yourself.

Dr. HUTTON: Most everyone in this building felt it, and at first I think we weren't sure. But you know, when the computer screens, monitors, start waving back and forth, then it's pretty clear that there's an earthquake going on. I suppose you could say that an earthquake feels a little bit like riding on a train. You know, it's sort of bumping back and forth a little bit, without the clicking sounds, of course.

BLOCK: Earlier this week, on Tuesday, there was an earthquake off the coast of Northern California, and that one registered 7.0. It triggered tsunami warnings from Vancouver down to the Mexico border. How did it seem people responded to those warnings, and do you think there are any lessons that were learned from that incident?

Dr. HUTTON: Well, I think there's a lot of lessons that are learned from it. I think the emergency services people haven't had much opportunity to deal with tsunami warnings on the West Coast, anyway. And I guess you could consider this to be an exercise in a way, because there was no actual tsunami. But it definitely got everybody going. And by the way, that earthquake is now called a 7.2.

BLOCK: Seven point two, not 7.0?

Dr. HUTTON: Right.

BLOCK: For people in the Los Angeles area now who've been rocked by a few earthquakes in a row now, any way of telling at all whether there might be a few more in store in this recent cluster?

Dr. HUTTON: Well, we'll certainly be having aftershocks, and there's always--any time we have any earthquake, a small chance that it's a foreshock or a precursor to something else that's larger. And usually the probability is about 1 in 20, or 5 percent. And unfortunately there's no way to tell a foreshock from any other earthquake; they look exactly alike. So we just have to rely on the statistics; we can't say for sure if it's a foreshock or not.

BLOCK: Well, Dr. Hutton, thanks very much.

Dr. HUTTON: OK.

BLOCK: Dr. Kate Hutton is a seismologist at Cal Tech. She spoke with us from Pasadena.

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