Do Big Quakes Increase Global Seismic Activity?
RENEE MONTAGNE, Host:
NPR's 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.
P: There was a real change in understanding about distant connections between earthquake 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 didn't cause widespread damage but like other big quakes, its seismic waves bounced around the world, and in fact...
P: 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 there's no question the big quake triggered the little ones. And seismologists have since found other examples.
P: There's 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.
P: 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: That said, Brodsky notes that there have been more big quakes than average since the devastating 9.1 quake hit Sumatra back in 2004.
P: In some sense, there is a possibility that this is an aftershock sequence of Sumatra - but I want to say that very cautiously. It's 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 you've got to wonder whether big quakes cluster together.
MONTAGNE: 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 there's an unusual pattern. He started off assuming what most scientists do, that earthquakes occur randomly.
MONTAGNE: It doesn't 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. You'll inevitably find some grains clumped together. And in fact, Michael finds that the clumping of earthquakes can be explained simply as randomness.
MONTAGNE: I think when people think there's an interconnectedness, then they think, well, there's been a lot of big earthquakes, they should be really worried right now that there's going to be more. I don't think that's the case.
HARRIS: It is still possible that a large quake might very occasionally trigger other large quakes, he says, but it doesn't happen often enough to be obvious in his data. Even so, the globally reverberating seismic waves do provide a possible mechanism. So he says it's a question well worth exploring. As Emily Brodsky puts it...
P: At risk of sounding like a scientist, more research is necessary.
HARRIS: Richard Harris, NPR News.
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.