A powerful earthquake under the Indian Ocean on April 11 was followed by a series of smaller earthquakes around the globe, including off the West Coast, pointing to a possible chain reaction that many geologists found surprising.
I didn’t think this could happen, said Ross Stein, a geophysicist with the U.S. Geological Survey in Menlo Park, Calif., who co-authored a study published Wednesday in the science journal Nature describing the trigger effect of the quake.
The team of scientists led by Fred Pollitz compared global records of earthquakes before and after April 11. They found that only one quake stronger than a magnitude 5.5 occurred in the six days before the quake, but 16 were recorded in the six days after. The long-term average is five. The increase was strongest right after the quake.
Stein concedes that the study’s conclusions will be controversial. I think most of my colleagues will be skeptical; I would be, too, he said.
Susan Hough of the USGS in Pasadena, Calif., finds the discovery interesting, but points to the fact that the increased occurrence of earthquakes could be a coincidence not related to the April 11 event. If the less quakes before are a fluke, are we sure the increase afterwards isn’t as well? she said.
Previous studies found that a quake can trigger others, although with a much smaller reach and power.
We don’t have a smoking gun, Stein said. We have to build a statistical case, and we have a strong one.
While the April 11 quake was powerful magnitude 8.6 it was more notable for being the strongest observed of its kind, Stein said.
The most forceful earthquakes usually happen at the boundaries between tectonic plates, when one of them violently slides under another. But the April 11 quake was produced by rupturing inside a plate, like a stone cracking when compressed. That set seismic waves shooting around the planet multiple times before finally dying away. The whole earth was vibrating like a bell, Stein said.
These waves were of a type more likely to destabilize potential earthquake spots than the ones set off by other types of earthquakes, he said. Most of the earthquakes after April 11 were found in the tracks of those waves.
Minutes after the quake, it was already clear to geologists that it was extraordinary in many ways. Earthquake researchers around the world raced to best characterize it.
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