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.
“What immediately struck us was the unusual location of such a big quake,” said Thorne Lay, professor of earth sciences at the University of California at Santa Cruz, a co-author of one of three articles in Nature about the quake. Normally, a quake of this type is caused by a rupture along a single line. But this was a messy event, rupturing at several places and in different directions at once, Lay said.
“We could not have predicted such a complexity, and this is quite annoying,” said Matthias Delescluse, the lead author of another article.
The scientists are also puzzled by the exceptional depth of the quake, a factor that contributed to its vigor. It would have been expected to occur at about 12 miles into the earth, but it apparently reached down to about 20 miles. “Down there, we expect the earth to be like honey,” said Jean Paul Ampuero, “but how do you generate an earthquake in honey?” The assistant professor of seismology at the California Institute of Technology co-authored a study on the quake published in Science last month.
Ampuero and other scientists argue that potential earthquakes in California should be reevaluated because of the recent findings. The geological structure of the region is in some ways similar to the one of the April 11 quake, which was more than five times stronger than the 1906 earthquake that devastated San Francisco.