Do Big Quakes Increase Global Seismic Activity?

There have been three deadly earthquakes already this year — in Haiti, Chile and Turkey — and a fourth that caused damage in Taiwan. Is this a coincidence? Seismologists can't answer that question directly, but they say there's a growing realization that big earthquakes can trigger other earthquakes many thousands of miles away.

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The Earth has been moving a lot lately. In the space of just a few months, weve experienced deadly earthquakes in Haiti, Chile and Turkey. Yet another quake caused damage in Taiwan. Earthquake scientists dont see a direct link between these quakes, but earthquake activity has increased since the huge Indian Ocean earthquake in 2004 and its resulting tsunami. That has led some scientists to wonder whether big quakes can, in fact, increase seismic activity around the world.

NPRs Richard Harris has the story.

RICHARD HARRIS: After a big quake, there are always local aftershocks, but Earth scientists used to pooh-pooh the idea that a big quake in one part of the world could trigger another quake thousands of miles away. Emily Brodsky at the University of California, Santa Cruz says that belief started to give way in 1992.

Professor EMILY BRODSKY (UCSC): There was a real change in understanding about distant connections between earthquakes at the time of an earthquake in Southern California called the Landers earthquake.

HARRIS: That 7.3 magnitude quake hit a remote part of the California desert. It didnt cause widespread damage but like other big quakes, its seismic waves bounced around the world, and in fact...

Prof. BRODSKY: It triggered very small earthquakes as far away as Yellowstone National Park.

HARRIS: Brodsky says those quakes occurred just as the seismic waves got to Yellowstone. So theres no question the big quake triggered the little ones. And seismologists have since found other examples.

Prof. BRODSKY: Theres a beautiful one from Sumatra that shows up in Alaska, that as the individual pulses of the waves come through, there are individual earthquakes popping off.

HARRIS: The Chilean quake last month triggered tiny quakes in Nebraska, Brodsky says.

Prof. BRODSKY: The key question at this point is whether or not those distant interactions are just triggering little earthquakes, or can actually trigger something significant.

HARRIS: There's no reason to think that the magnitude 7 quake in Haiti triggered the other damaging quakes this year. After all, earthquakes are actually pretty common, so coincidences should be as well. There are, on average, 17 magnitude 7 quakes a year globally, and 130 quakes each year in the magnitude 6 range. We just notice the ones that do damage, and weve had a very unlucky streak.

That said, Brodsky notes that there have been more big quakes than average since the devastating 9.1 quake hit Sumatra back in 2004.

Prof. BRODSKY: In some sense, there is a possibility that this is an aftershock sequence of Sumatra - but I want to say that very cautiously. Its a possibility; it is not something we know.

HARRIS: Huge earthquakes do sometimes come in clusters - notably, the three truly enormous quakes of the 20th century all occurred within a span of 12 years: Russia in 1952, Chile in 1960, and Alaska in 1964. So youve got to wonder whether big quakes cluster together.

Mr. ANDREW MICHAEL (U.S. Geological Survey): Yeah, I can see why people think that, which is why we actually did the calculation.

HARRIS: Andrew Michael at the U.S. Geological Survey in Menlo Park, California, looked into the recent uptick of quake activity to see if theres an unusual pattern. He started off assuming what most scientists do, that earthquakes occur randomly.

Mr. MICHEAL: It doesnt mean that they are evenly spaced out in time. But randomly, it means that actually, there will be sort of what seem like clusters that are really just coincidence.

HARRIS: Imagine tossing a handful of rice on the floor at random. Youll inevitably find some grains clumped together. And in fact, Michael finds that the clumping of earthquakes can be explained simply as randomness.

Mr. MICHAEL: I think when people think theres an interconnectedness, then they think, well, theres been a lot of big earthquakes, they should be really worried right now that theres going to be more. I dont think thats the case.

HARRIS: It is still possible that a large quake might very occasionally trigger other large quakes, he says, but it doesnt happen often enough to be obvious in his data. Even so, the globally reverberating seismic waves do provide a possible mechanism. So he says its a question well worth exploring. As Emily Brodsky puts it...

Prof. BRODSKY: At risk of sounding like a scientist, more research is necessary.

HARRIS: They also agree that more big quakes are inevitable in the long run. So the world needs to be prepared for them, regardless of what sets them off.

Richard Harris, NPR News.

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