Scientists say quake data are really just estimates

Laboratorio de Ingeniería Sísmica graphic Yellow dot shows the estimated epicenter of Monday's earthquake.

Expats sometimes wonder how an earthquake can be reported differently.

For example, a quake in the upper gulf of Nicoya Monday at nine seconds to 2 p.m. was of a magnitude 5.0, according to the Red Sismológica Nacional at the Universidad de Costa Rica. But the Laboratorio de Ingeniería Sísmica at the same university listed it as 4.8 magnitude.

The Red Nacional has published an article on this topic. It said that such discrepancies are inherent in the system.  Costa Rica has had regular earthquake monitoring for 40 years, but now there are three different systems that measure different aspects of an event, the Red Nacional noted.

In addition to the Red, earthquake reports in Costa Rica come from the Laboratorio and from a network run by the Observatorio Vulcanológico y Sismológico de Costa Rica at Universidad Nacional in Heredia.

When an earthquake hits, scientists quickly try to estimate the magnitude and the depth as well as the location. They also try to figure out why the earthquake took place. Is it the result of movement in a volcano? On the coast the usual culprit is the subduction of the Cocos plate beneath the Caribbean tectonic plate. There also are local faults. Finding the location is not just academic. The location and magnitude can have a bearing on the emergency response.

The Red Sismológica Nacional says that the U.S. Geological Survey has 150 sensing stations around the world. But these are not sufficient to collect data on small quakes. Usually the Survey only reports small quakes in the 2.5 magnitude range when they take place within the borders of the United States. Elsewhere the cutoff point is around 4.5 magnitude.

U.S. Geological Survey lists 26 events in the U.S. and the world Monday. Expats frequently express their frustration on online discussion lists when they cannot find an earthquake report in English on the  Geological Survey list. Usually that is because the magnitude was less than the threshold.

Costa Rica has networks of sensors, too. They are not only on land but also on the floor of the oceans.

That is why the Laboratorio de Ingeniería Sísmica, which maintains 24 sensing stations, can say that  the quake Monday was felt strongest in Paquera where the sensing device is in the Cruz Roja building, in Cóbano, in the Fraijanes branch of the Universidad de Costa Rica and the library at Cañas. The sensations were considered light to moderate. The Laboratorio estimated the depth at 43.8 kilometers, about 27 miles. The Red Nacional said 40 kilometers, about 25 miles.

As A.M. Costa Rica has reported, the Observatorio  is replacing analog equipment with digital sensors, geographical coverage is expanding and satellite technology brings an extra element to tracking earth movements.

The new network has 32 stations around the country, all with seismographs and real-time data transmission. Each station is on a solid base reaching bedrock and powered by solar cells. Communication with computers in Heredia is by internet where feasible, with radio connections for the more remote sites. The older system used radio communication with limited range.

The satellite system can measure ground movement as small as a millimeter.

Earthquakes happen all the time, but there are scientific predictions of major ones to come. Sceintists from the Observatorio Vulcanológico y Sismológico have been holding community meetings on the Nicoya peninsula because they expect a major quake to take place through the middle of the Gulf of Nicoya, perhaps running through the point where the quake happened Monday. They expect the peninsula to shift so that one coast subsides and the other is raised higher above sea level.

They base their predictions on previous quakes in records going back to 1827. There have been four 7.0 magnitude or better quakes in and around the peninsula since 1950. The last was March 25, 1990.

Costa Rica is one of the most earthquake-prone and volcanically active countries in the world, according to the University of California at Santa Cruz, which has studied the area extensively. Just off the west coast is the Middle America Trench, where a section of the sea floor, the Cocos Plate, dives beneath Central America, generating powerful earthquakes and feeding a string of active volcanoes, said researchers.

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.

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