Why Are So Many Quakes Jarring Indonesia?

Indonesia has been rattled by dozens of earthquakes over the last two days, including three severe shocks, causing at least 10 deaths and at least 100 injuries. The Indian Ocean basin is still on alert for tsunamis. What's causing the spate of seismic activity?

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MICHELE NORRIS, Host:

Indonesia has been rattled by dozens of earthquakes over the last two days, including three severe shocks. At least 10 people have died and more than 100 have been injured. Those living in the Indian Ocean Basin have been on alert for tsunamis.

Joining us to talk about the seismic activity there is NPR science correspondent Richard Harris. Richard, first, what is causing these earthquakes?

RICHARD HARRIS: Well, when you get right down to it, what's really causing these earthquakes is continental drift. You have one huge chunk of the Earth's crust called the Australia Plate and it's moving at the rate of about two inches a year going to the northeast.

And off the island of Sumatra, it's colliding with another one of these giant continental plates, and that's called the Sunda Plate. All that stress has to go somewhere, so periodically the rocks just can't stand it anymore and one plate ends up slipping under the other and when that happens, you get an earthquake.

NORRIS: So why so many and so many days?

HARRIS: Well, really - let's go back and look at the history about this a little bit. The last uptake of activity really started about yesterday morning, our time. It was an 8.4 magnitude earthquake. That's a really big earthquake. And then, about 12 hours later, there was 7.9, another really big earthquake. And a few hours after that, one that's about a seven. So there have been many, many more also in the magnitude of five or six throughout this time. So it's just been - I can't imagine living in Sumatra right now. It must be really nerve-wracking to think more earthquakes are on the way.

And no one can predict exactly when something like this is going to happen. But seismologists have been watching this area for quite a long time, a decade or more, and been very worried because of the build up of seismic stress. So they knew there was going to be seismic activity here and big seismic activity. We just didn't know when.

NORRIS: Well...

HARRIS: Yeah, when it starts, though, you know, that you're going to have not only big quakes but you're going to have all these aftershocks. So that's we're seeing.

NORRIS: So you say that they've been watching this area for the past decade. Is this related to the quake that struck the same region at the end of 2004?

HARRIS: It's the same general stress building up. That one was a 9.1 magnitude earthquake, which is one of the biggest that was ever measured on Earth. And that happened at a slightly different junction of continental plates. It was farther to the north, but it was still off the coast of Indonesia, it was still in this general area. But even though the plates - they were slightly different, yet it is, essentially, the same seismic activity, the same portion of these plates, one against the other. And as it happens, four plates come together in this one part of the world, which is one reason why there are always so many earthquakes around Indonesia.

NORRIS: And as we remember that 2004 quake triggered a devastating tsunami. Why not this one?

HARRIS: Well, that...

NORRIS: Or the series of quakes?

HARRIS: Yeah. Well, they've been lucky, really, because there've been tsunami alerts and warnings and concerns about it. But in 2004, the quake there ruptured a very long length of the fault under the seafloor. And what happened was it caused that area to bounce, a very big bounce, like a 10-foot bounce on the seafloor, and that, of course, created this enormous surge of water upward and that really is what caused the tsunami; that huge bounce of the seafloor. These have been quite lucky, the ground motion has not been the same. And I don't know exactly what's going on right now, but that was really the difference between these.

NORRIS: Now, those here in the U.S. those here in the U.S. may be wondering how those quakes differ from the ones along the faults close at home at San Andreas.

HARRIS: Right. Well, San Andreas sort of slips north and south that rubs against - one plate rubs against to another, more than one diving under another, so you don't get the same kind of bouncing motions as much. So, they are, obviously, the same process of continental drift, but you get very different grand motion from that.

NORRIS: That was NPR's Richard Harris. Richard, thanks so much.

HARRIS: My pleasure.

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