Bulk of the recent quakes are not the fault of the tectonic plates

Red Sismológica Nacional graphic
Of eight earthquakes last week on two (in blue) were caused
by tectonic plates, said the Red Sismológica.

Central Valley residents felt a sharp shock at 10:40 p.m. Sunday when the seventh significant earthquake of the day made itself known.

This one was estimated to have been a kilometer southeast of Dulce Nombre de Cartago, and the magnitude was estimated by all three monitoring agencies at 3.8.

Another significant quake Sunday was at 2:23 a.m. about 9 kilometers west of Hacienda Murcielago de la Cruz in northwestern Costa Rica. That was estimated at a 3.6 magnitude. There also was a 3.4-magnitude quake Sunday at 8:16 a.m. a kilometer northeast of Sandalo de Golfito in southern Costa Rica, said the Observatorio Vulcanológico y Sismológico de Costa Rica.

The Laboratorio de Ingenieria Sismica reported a 4.3-magnitude quake Friday about 6 kilometers (less than 4 miles) south southwest of Yolillal de Upala in northern Costa Rica. That was at 10:36 p.m.

The Laboratorio also reported a 3.9-magnitude quake about 6.6 kilometers southeast of Pittier de Coto Brus, also in the southern part of the country at 7:32 a.m. Saturday.

Despite the proliferation of quakes, the Red Sismológica Nacional said that many are the result of local faults and not a result of the Cocos tectonic plate grinding against the lighter Caribbean plate.

The 10:36 p.m. Friday quake was so deep at 177 kilometers, about 110 miles, that it was hardly felt, said the Red Sismológica. By contrast a flurry of three quakes less than four kilometers deep were felt last week in Escazú even though the magnitudes were less.

The Laboratorio said that 2005 was the year with the fewest quakes of 4.5 or more in 39 years. That year saw 59 earthquakes.

Before the weekend flurry of quakes, the Laboratorio calculated that there had been 140 quakes of 4.5 or better this year in Central America, including three greater than 7.0 magnitude within 72 days. That includes the Sept. 5 quake off the coast of the Nicoya peninsula.

That is unusual because only 1876, 1983 and 1992 were years in which there were two quakes of that magnitude.

The year with the most quakes in Central America was 1992, said the Laboratorio. That was the year of the devastating earthquake in Nicaragua, and there were 235 quakes.

The Laboratorio said that 2005 was the year with the fewest quakes of 4.5 or more in 39 years. That year saw 59 earthquakes, the Laboratorio said.

The region averages 114 quakes of 4.5 or better per year.

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