Quakes Wednesday do not mean the big one is due

Map shows the direction of tectonic plates with the Mid-Atlantic trench just offshore. Green dots are epicenters of past earthquakes.

Experts say they do not believe that the earthquake at the mouth of the Gulf of Nicoya Wednesday was an indication of a much larger event that has been long expected.

The earthquake actually was a series of four, said the Observatorio Vulcanológico y Sismológico de Costa Rica. The first took place at 1:01 and was measured between 5 and 5.4 magnitude by various monitoring agencies.

Eight minutes later there was a quake of about 2.4 magnitude in approximately the same place. Then at 1:11 another quake took place, and that was measured at 2.7 magnitude.

The epicenter moved north from about 38 kilometers south of Paquera on the Nicoya peninsula to 33 kilometers south and then 28 kilometers south. That is about 23.5 miles to a bit more than 17 miles.

Then at 1:23 p.m. a fourth quake took place. This was estimated at a 3.2 magnitude, said the observatory, which is affiliated with Universidad Nacional in Heredia.

The initial quake was felt lightly in the Central Valley and more strongly at sites bordering the gulf. The observatory said that the only significant damage was to a home in Cóbano on the peninsula where cracks appeared in a wall.

Juan Segura, observatory director, said in a news release that the location was in the same area as a March 25, 1990, quake. That one caused considerable damage in central Costa Rica and was estimated at a magnitude of 7.0, according to the observatory. There are thousands of tiny earthquakes in that area, according to measuring devices, but few are felt, said the observatory.

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 called 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.

Scientists have been trying to prepare Nicoya residents for what they believe is an inevitable major earthquake, In fact, the Red Sismológica Nacional at the Universidad de Costa Rica held a general meeting for Nicoya residents a year ago to alert them.

According to Cal Poly Pomona University earth scientist Jeff Marshall, the peninsula is unique because it is one of the few land masses along the Pacific Rim located directly above the seismogenic zone of what he calls a subduction megathrust. He is one of the researchers who has been studying the peninsula in detail for years.

The area even is the location of university summer programs in earthquakes.

The reasoning behind the earthquake prediction is that major quakes in the 7.0 magnitude range have taken place in 1853, 1900 and 1950. In addition to the 7.0 quake in 1990, another took place in 1992 north of the peninsula in Nicaragua. Experts expect the new epicenter to be somewhere between these two points and the magnitude to be in the 7.6 to 7.8 range.

Both Marshall and the observatory’s Marino Protti expect the western beaches of the peninsula to jerk up perhaps as much as two meters or a bit more than 6.5 feet. Land on the east shore of the peninsula is expected to subside. That is what happened in the 1950 quake, Marshall noted in a 2008 book chapter.

Scientists say there are plenty of substandard buildings on the Nicoya peninsula and that the quake would cause considerable damage in the Central Valley. The April 12, 1991, earthquake in Limón province also caused major damage in the Central Valley. That was a 7.5 magnitude quake that killed 53 persons.

Local television stations are edgy and already have their emergency plans set. When a moderate quake takes place in the Nicoya area, news teams from San José rush by helicopters to the area to videotape the damage. Several times viewers have been treated to footage of normal street scenes in Cóbano because there was no significant damage.

This entry was posted in Costa Rica News. Bookmark the permalink.