February 16, 2009

Scientists Study “˜Silent Earthquakes’ In Costa Rica

Researchers are using modern technology to study "silent earthquakes" along a major fault zone beneath the Nicoya Peninsula in Costa Rica.

Also known as slow slip events, they involve the same motion as an earthquake, but they occur so slowly that they can only be detected by networks of modern instruments.

Scientists are using a network of 13 Global Positioning System (GPS) monitoring stations and 13 seismic stations to measure precisely the movements of the Earth's crust on the peninsula over time.

"At least two slow slip events have occurred beneath the Nicoya Peninsula since 2003," said Susan Schwartz, a professor of Earth and planetary sciences at the University of California, Santa Cruz.

"When we recorded the first one in 2003, we had only 3 GPS stations. By 2007, we had 12 GPS stations and over 10 seismic stations, so the event that year was very nicely recorded."

A 2007 slow slip event in Costa Rica involved movement along the fault equivalent to a magnitude 6.9 earthquake. But it took place over a period of 30 days rather than the 10 seconds typical for an earthquake of that size.

The slow slip phenomenon has also been observed in the Cascadia fault zone off the coast of Washington and British Columbia and Japan's Nankai Trough.

"The newest discovery is the occurrence of these slow slip events. But there has been a decade of focused effort in this area that has significantly advanced our knowledge of the Central America seismogenic system," Schwartz said.

"Initially, we focused on areas of the fault that are locked up, which slip in an earthquake. The slow slip is occurring in regions that are not strongly locked, and a big question is whether that is loading the locked area, making it more likely to break, or relieving stress on the fault."

Costa Rica is one of the most earthquake-prone countries in the world thanks to being located near a boundary between two converging plates of the Earth's crust, known as a subduction zone.

Schwartz says slow slip gives researchers the "perfect opportunity to study the seismogenic zone using a network of land-based instruments."

Scientists say it is hard to determine the true tell-tale signs of an earthquake.

"Will we ever be able to predict them? I don't know," said Leigh Royden of the Massachusetts Institute of Technology on Sunday at a meeting of the American Association for the Advancement of Science.

"There have been a few signals associated with quakes, but only in hindsight," she said. "And those quakes are rare."


Image Caption: Nicoya Peninsula in Costa Rica. NASA


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