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Varying Costa Rica quake readings confusing, but normal

March 13, 2009

Two moderately strong earthquakes hit Costa Rica´s Southern Zone on Wednesday and were felt throughout much of the national territory, that much everybody could agree on. What caused a bit more consternation was just how big those earthquakes were, with measurements of everything from magnitude 5.2 to 6.3.

The U.S. Geological Survey (USGS) preliminary reported a magnitude 5.7 for the first quake, which struck at 11:24 a.m. near the mouth of the Golfo Dulce, and magnitude 5.9 for the second, which hit 3:03 p.m. a few kilometers farther north in the gulf. USGS have since revised the first quake´s measurement to a 5.9 as well.

Local instruments, on the other hand, registered different numbers. The National Seismological Network (RSN) reported magnitudes of both 5.2 and 5.5 for the first quake, while the widely cited Volcanological and Seismological Observatory of Costa Rica (OVSICORI) measured it at magnitude 6.3.

For the afternoon quake, RSN and the observatory came closer with their measurements, at 5.7 and 5.8, respectively.

Bruce Presgrave, a geophysicist at the USGS´ National & International Earthquake Center in Golden, Colorado, said all the readings were accurate and the discrepancies normal. “(Earthquakes) don´t send out the same amount of energy in the same directions,” he said, adding that it was typical in his experience that readings for Wednesday´s quakes could range from the high magnitude 4 range to the low 6 measurements.

Julie Dutton, Presgrave´s colleague at the institute in Golden, however, called the range between magnitude 5.2 and 6.3 for readings on the first quake a “fairly large discrepancy.”

The RSN, a division of the University of Costa Rica, and OVSICORI, based at the National University in Heredia, use readings from local instruments. USGS readings, on the other hand, come from stations all over the world that pick up waves from tectonic movements, send the information to a USGS satellite, which in turn relays the information to USGS Colorado offices, where it is compiled.

Dutton said stations as far-flung as Uzbekistan, Antarctica and Japan will pick up readings from waves generated by Wednesday´s quakes in Costa Rica. Undulations from the recent temblors would take about seven to 10 minutes to reach Japan, she said.

“Definitely, you´re going to have a better location and magnitude (readings) … if you´ve got a greater coverage around the earth. It´s a matter of triangulation: If you have more spread out information, your results are going to be more precise,” said Dutton.

But Presgrave insisted on the accuracy of all the readings. “I wouldn´t say (local institutes´) solutions are wrong: They´re different.”

USGS receives readings in Costa Rica from OVSICORI´s monitoring station in Juntas de Abangares in the northwest province of Guanacaste, just above the Gulf of Nicoya. They also have stations near Barra Colorado Island in Panama and El Rosal in Colombia to the south, and just outside Tegucigalpa, Honduras, and Tepiche on Mexico´s Yucatan Peninsula.

Thursday at 5:23 p.m., a quake centered in the waters off Panama was felt in Costa Rica´s Central Valley. OVSICORI reported a magnitude of 6.5, but by that time, local media directly cited USGS, which recorded magnitude 6.2

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