Science of a Costa Rica quake: Expect the aftershocks

January 9, 2009

Five small earthquakes felt throughout the Central Valley on Monday night and early Tuesday did little to steady the nerves of residents still shaken from last Thursday´s magnitude 6.2 earthquake.

But seismologists say such aftershocks are normal and to be expected in the wake of such a temblor.

The five mini-quakes, the majority of which qualified as aftershocks from Thursday´s, occurred between 6:45 p.m. Monday and 1 a.m. Tuesday. Three of them registered magnitudes over 4.0, with the last at 4.3, and were centered around the Poás Volcano, the epicenter of Thursday´s quake, and were felt throughout the Central Valley.

Costa Rica is situated over a “diffuse seismic zone,” said Javier Pacheco, a research scientist with the Volcanological and Seismological Observatory of Costa Rica (OVSICORI), based at Universidad Nacional (UNA) in Heredia, north of San José.

Unlike the San Andreas fault along the West Coast of the United States, which scientists have been able to clearly demarcate, Costa Rica is at the convergence of four plates – including one microplate – explained Tico seismologists. Mapping it – and therein identifying what specific tectonic movements are taking place, is an uncertain task – and debate persists among scientists as to what plates are in motion.

Pacheco described Thursday´s earthquake as local lateral movement between the Panama microplate, on which the majority of Costa Rican territory sits, and the Caribbean plate, which runs under only the country´s northern border area.

U.S. scientists, on the other hand, are sticking with the theory that the country is situated on just the three large plates. George Choy, a seismologist with the U.S. Geological Survey´s regional office in Denver, said the recent activity is the result of a strike-slip mechanism that occurs as the Cocos plate, under the Pacific Ocean, slides over the Caribbean plate. This type of conversion, called subduction, occurs at a rate of 77 millimeters per year, forming a trench over the western edge of Costa Rican territory. But, he added, most seismologists believe in the microplates hypothesis, too.

Both scientists said it is normal for the plates to shift and readjust in the wake of an earthquake such as Thursday´s. “With high seismic activity, there will be aftershocks,” said Pacheco. “After a fault breaks, all the other faults that are strengthened by it get set off.”

The aftershocks will likely last “a few months,” said Pacheco. “Of course, the intensity will go down over time until only seismological instruments register it.”

For now, scientists are taking as much information from the recent series of quakes as they can. “There´s still a lot to learn,” said Pacheco. “It´s such a complex system.”

See Friday´s print or digital edition of The Tico Times for more earthquake coverage and analysis.

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