Slow-moving quakes may contain key to predictions

Scientists at Penn State University think they have found a way to predict earthquakes particularly in areas like Guanacaste where so-called slow-moving or silient quakes take place.

Monitoring slow earthquakes may provide a basis for reliable prediction in areas where slow quakes trigger  normal  earthquakes, according to the Penn State geoscientists, the university said in a summary.

“This has the potential to change the game for earthquake monitoring and prediction because if it is right and you can make the right predictions, it could be big,” the university quoted  Chris Marone, a professor of geophysics, as saying.

Costa Rica is one of the most earthquake-prone and volcanically active countries in the world. Just off the west coast is the Middle America Trench, where a section of the sea floor called the Cocos Plate dives beneath Central America, generating powerful earthquakes and feeding a string of active volcanoes, researchers have reported for years. This type of boundary between two converging plates of the earth’s crust is called a subduction zone ― and such zones are notorious for generating the most powerful and destructive earthquakes.

The  University of California at Santa Cruz, local universities, the national emergency commission and other agencies maintain a host of monitoring stations on the Nicoya peninsula and on the sea floor.

One discovery is that the peninsula experiences what researchers call silent earthquakes or slow slips. A slow slip event involves the same fault motion as an earthquake, but it happens so slowly that the ground does not shake. It can be detected only with networks of modern instruments that use the Global Positioning System to measure precisely the movements of the earth’s crust over time. The monitoring showed that in 2007 the peninsula experienced the equivalent of a 6.9 magnitude earthquake over a period of 30 days instead of the usual 10 seconds for a quake to release the tension on the earth’s crust.

At Penn State, Marone and Bryan Kaproth-Gerecht, a recent doctoral graduate, looked at the mechanisms behind slow earthquakes and found that 60 seconds before a slow slip began in their laboratory samples, a precursor signal appeared, the university reported.

Normal earthquakes typically move at a rate of three to 33 feet per second, but slow earthquakes, while they still stick and slip for movement, move at rates of about 0.004 inches per second taking months or more to rupture, said the university in a summary released Thursday. However, slow earthquakes often occur near traditional earthquake zones and may precipitate potentially devastating earthquakes, the summary added.

The researchers at Penn State conducted more than 50 experiments using serpentine under stress to simulate a slow earthquake. The signals that may provide a way to predict quakes comes from the seismic waves generated by the tension and movement.

The Observatorio Vulcanológico y Sismológico of the Universidad Nacional is heavily involved in earthquake monitoring and efforts at predictions. Marino Protti there, who received his doctorate at Santa Cruz, is the leading Costa Rican expert on this topic.

Protti and colleagues have identified at least five slow-slip quakes in the area of the Cocos and Caribbean tectonic plates in the last decade. One seems to take place every 15 to 27 months, these researchers have estimated. This is still a new research area that many scientists hope will lead to earthquake predictions. The term slow earthquake was not even coined until 1992.

Slow slip quakes have been detected all over the world, including in Nicaragua, California, Italy and New Zealand. The Great East Japanese Quake is believed to have been caused by tension created by a previous slow-slip event.

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