New approach could help forecast earthquakes

Predicting earthquakes and tsunamis has long been seen as a fool's errand, which makes preparing for the next "big one" in quake-prone places like California, Japan and Indonesia particularly difficult.

Now, an international team has identified a geological phenomenon known as slow slip events -- motion in the Earth that occurs over weeks or months -- which they believe could improve long-range forecasting of certain earthquakes and, crucially, how big they might be.

"Nobody to my knowledge thinks we can predict earthquakes. We can't say anything about when they are going to happen," University of South Florida professor Tim Dixon, who was part of an international team whose quake study appears in the Proceedings of the National Academies of Sciences.

"What we can say is that we think we can forecast where they are going to happen and how big they are going to be," he said. "That was the point of the paper. If we could at least do that, communities would have a better chance prepare."

The problem with earthquake predictions comes down to the fact that earthquakes originate miles underground where conditions cannot be observed directly. There is no way to place sensors in locations where faults are locked, storing up energy that is eventually released as an earthquake. Nor is there a way to image them.

Nobody predicted the 2004 tsunami that left 230,000 dead nor the Japanese quake and tsunami in 2011. And in Italy, six Italian scientists and a government official were sentenced to six years in prison over statements made prior to a 2009 earthquake that killed 309 in the town of L'Aquila. Their conviction - for allegedly downplaying the threat of a large quake - was overturned earlier this month.

Using high precision GPS to measure the slight shifts on a fault line in Costa Rica, the team -- which included scientists from Observatorio Vulcanológico y Sismológico de Costa Rica, the University of California-Santa Cruz, Portland State, Pacific Geoscience Center in Canada and Georgia Tech -- studied the Sept. 5, 2012 earthquake on the Costa Rica subduction plate boundary, as well as motions of the Earth in the previous decade.

They recorded numerous slow slip events in the decade leading up to the 2012 earthquake. The scientists made their measurements from a peninsula overlying the shallow portion of a megathrust fault in northwest Costa Rica.

William Leith, a senior science advisor for earthquake and geological hazards at the U.S. Geological Survey, said the slow slip events alone are not a great forecasting tool. But he said the paper provides greater understanding into the connection between slip events and major quakes which makes it "pretty interesting."

"That is what the seismological community has been looking for, some evidence of what happens deep in the plate boundary and it's connection to large earthquakes," Leith said. "That's the problem they are attacking with this paper."

Dixon acknowledged that the larger benefit might be helping predict the size of quakes, since past forecast efforts have often failed to consider the role slow slip played before they occurred.

"We used to think all strain would get released in earthquakes but it didn't," Dixon said. "The reason is that slow slip events become an important part of the story. They release some of the strain. It's like energy from a spring being released in little bits."

Factoring in the slow slip, he said, could help disaster response teams direct resources to areas most vulnerable and away from subduction zones -- a place where one plate is pushed beneath another -- that in the end might just see a small quake.

"If you couldn't measure slow slip for some reason, you have to assume all energy of that spring would be released in one event. You would overestimate the size of next earthquake," Dixon said. "You would have everyone preparing for magnitude 9. This gives us a tool to refine our estimate."

The technique could also go a long ways to helping understand why Central America has not been hit by a giant earthquake, which is defined as magnitude 9, he said.

"While it has had major earthquakes, it hasn't had giant earthquakes. It has always puzzled scientists," Dixon said. "Now, we think we understand. Up to now, a lot of the strain in Costa Rica has been released by these slow slip events."