Duration of 3-second quake depended on soil composition

Each green dot represents an epicenter. Most are clustered in the top 50 kilometers. Photo: Red Sismológica Nacional/A.M. Costa Rica

The quake that struck near Puriscal a week ago lasted just three seconds at the epicenter, but the time it was felt elsewhere depended on the composition and consistency of the soil, said the Observatorio Vulcanológico y Sismológico de Costa Rica.

Residents of Jacó only felt the quake for about 10 seconds, based on recordings from monitoring stations there. But in the Central Valley with the soil there, the earthquake lasted much longer, said the observatory at Universidad Nacional in Heredia.

The situation was similar to a shock applied to a dish of Jell-O, which will shimmy for some time.

The Red Sismológica Nacional at Universidad de Costa Rica reported that there were six aftershocks in the 2.0 to 4.0 range, but none was felt by humans. The quake itself was magnitude 6.0.

Mauricio Mora Fernández of the Red said that the quake originated in the Coco tectonic plate, and such events usually are greater than 50 kilometers (31 miles) deep. He
prepared a map showing quakes from 1985 to 2008 ranging from the Mesoamerican trench beneath the Pacific to the central volcanic mountains of Costa Rica.

The Coco plate goes to 100 kilometers (62 miles) below the central mountains. The quake Friday was 74 kilometers (46 miles) deep, he said. Other estimates were slightly higher.

Such quakes within the tectonic plates are infrequent, said Mora, and because they are deep the impact on people living on the surface is less.

He noted that a 7.0 quake took place Nov. 19, 1940, at a depth of 70 kilometers (43.5 miles) below the central mountains and that a 6.2 quake took place below Naranjo at 85 kilometers (53 miles) in 1991.

The quake Friday caused little damage although there were reports of items being knocked off shelves. A few cracks opened up in the asphalt of highways.

The epicenter was placed at 6 kilometers (3.7 miles) north of Santiago de Puriscal by the observatory and 5 kilometers (3.1 miles) southeast of Turrúcares de Alajuela by the Red. That’s about the same location.

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