Are This Week's Earthquakes Related?

In Samoa, more than 100 people died after a magnitude 8 quake triggered a tsunami on Tuesday. Less than a day later, a magnitude 7.6 earthquake caused more than 400 deaths and widespread damage in Indonesia. A third earthquake struck in Peru, magnitude 5.9, on Wednesday. Scientists are studying those and other quakes to see if patterns emerge.

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I'm Steve Inskeep.

Hundreds of people are dead following an Indonesian earthquake yesterday. And even as rescuers searched for survivors, the country was hit by another earthquake today. And these are just two of four big earthquakes over the last couple of days. The others were in Samoa and Peru. All are considered part of the Pacific Rim, which has the world's most active fault lines.

NPR's Richard Harris has been looking at whether the tremors were related.

RICHARD HARRIS: You've got to wonder, did the first big one trigger the others. Ross Stein at the U.S. Geological Survey in Menlo Park, California, says it's not a crazy idea.

Dr. ROSS STEIN (United States Geological Survey): The interesting thing is, any earthquake of let's say magnitude six or larger, sends out its ripple of seismic waves, just like throwing a stone into a pond. And for a magnitude six or larger, those waves encircle the entire globe. And every single sand grain on the planet is dancing to that music.

HARRIS: So when it comes to seismic waves, the world really is all interconnected.

Dr. STEIN: But the tough question is, okay, you shook the region but did you trigger an earthquake, which is something else.

HARRIS: That's much harder to do. In the case of this week's events, it's true that the Samoa quake and the Sumatra quake took place on opposite edges of a giant slab of Earth called the Australian Plate.

Dr. STEIN: However, the plate itself as a whole doesn't move. The plate is basically a thin slab of rubber sliding over honey, and it bangs and bends on the borders but the entire plate isn't moving at once.

HARRIS: So there's no expectation that a single plate-wide event could be behind this one-two punch. Stein says the quakes, along with one in Peru, all occurred in places where big quakes happen a lot.

Dr. STEIN: Would you expect them all to occur in a 24-hour period? I don't know and I haven't looked at that.

HARRIS: But a colleague of his down the hall has looked at whether big quakes trigger other quakes. Tom Parsons says the answer is a resounding, yes, but with a very important caveat.

Dr. TOM PARSONS (Seismologist, United States Geological Survey): They tend to happened within about a thousand kilometers of their source...

HARRIS: Which is 600 miles or so.

Dr. PARSONS: Right. And in this case, the Sumatran-Samoan events are more than 10,000 kilometers apart.

HARRIS: Parsons has poured through the records of all potentially damaging earthquakes over the past 30 years, magnitude seven or higher. He looked at more than 200 quakes in all to see if big quakes trigger other big quakes far away, and he simply doesn't se that pattern.

So what can explain the pair of big quakes this week? Well, Parsons says there are, on average, six or seven quakes a year with magnitudes greater than seven. So it's not too likely that any of these will happen on the same day. But it certainly can happen.

Parsons has a simpler explanation for the large quake near the Indonesian island of Sumatra.

Dr. PARSONS: Of course, in the Sumatra area is very active at the moment because of the giant earthquake that happened there back in 2004. So if you were going to blame this most recent event on anything, I'd probably look at that big magnitude nine event that happened then as the root cause, more than the Samoa event.

HARRIS: That 2004 event was the quake that caused the devastating tsunami that swept across the Indian Ocean. And ever since then, we've paid a lot more attention to big quakes, even those that strike way out at sea.

Richard Harris, NPR News.

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