Tremors No Prelude To The Big One: Experts Say

By Catherine Keogan

Earth tremors this week in Guanacaste may have left some feeling a little shaky, but they aren’t the forerunners of the big earthquake experts say is due to strike the Nicoya Peninsula.

Residents across the province were startled when three separate earthquakes rattled their homes Saturday, Sunday and Tuesday evenings. The seisms, considered light by geological standards, measured between 4.2 and 4.8 on the Richter scale.

However, seismologists say they were unrelated to a much stronger earthquake that is expected to rock the Peninsula sometime in the near future, predicted to be of magnitude 7.5 or greater on the Richter scale.

Just how soon that will be, however, is anyone’s guess.

“The Nicoya Peninsula is still waiting for a big earthquake,” confirms Dr Marino Protti, a seismologist with the Costa Rican Volcanic and Seismological Observatory (OVSICORI).

Records show at three major earthquakes of magnitudes 7 or greater have struck the peninsula in the last couple of centuries: in 1853, 1900, and 1950.

While it might seem the next one is well overdue, 58 years later, Dr Protti points out the trend is too short for exact predictions.

“Nature doesn’t follow strict statistics; it has been happening on an approximate 50-year cycle, but it can vary as much as ten years,” he notes.

Earthquakes are common in all Central America, but the frequency and intensity can vary as the fault line, running south of Mexico to south of Costa Rica, behaves differently in different segments.

These latest rumbles originated with the segment that extends north from the Peninsula Papagayo, to just below the Nicaraguan border.

“The fault line is weak toward Nicaragua, and so it releases easily,” he explains. “This week’s tremors are part of the weak fault segment north of Papagayo. This won’t trigger any earthquakes in the other segments.

“But below the Papagayo Peninsula the segment is very strong - we don’t feel a lot of earthquakes, and when it releases it does so all at once in a big earthquake. Then the fault closes up.”

© Photo Courtesy

THE BIG ONE: Dr Marino Protti, a Costa Rican seismologist, examines equipment in the field. Dr Protti’s research shows the Nicoya Peninsula is due for a major earthquake in the near future. (Photo Courtesy of Dr Marino Protti)

The root of all the ruckus lies deep underground, where solid matter floats in slabs on top of the earth’s molten core. Two of these slabs, the Cocos plate, which forms the ocean floor on the Pacific coast, and the Caribbean plate, which forms the continental land mass (including the Nicoya Peninsula), are moving together at a rate of about 90 mm a year.

This drives the Cocos plate underneath the land mass, catching at its underbelly and causing it to buckle.

Eventually, the pressure of this deformation grows too strong, and the land mass pops back into place — releasing a lot of energy in the form of an earthquake.

GPS stations record the downward movement of the land, giving seismologists a clear understanding of just how much stress is building.

“We can measure the deformation of the landmass,” says Dr Protti. “The west coast is lowering about 10 mm per year. We can see the effects in the way the tide is now taking old trees, and getting closer to the houses on the beaches.

“The Papagayo segment is very strong, it is resisting the Cocos plate, but it cannot resist forever.

There are deformations we can observe and they are not sustainable, it will have to snap back.

Once the earthquake happpens the continent will lift up quite a bit — as much as one to two meters.”

When the earthquake does strike, there will be little warning for residents, he warns.

“We have no way of alerting people when it does happen. We may see changes in our instrumentation, but we have no way of knowing what they predict. But every event will give us important information; that is why we are gathering data, so we can better predict in the future.”

Dr Protti encourages everyone to prepare themselves to be earthquake-ready, by making sure their homes are built according to Costa Rica’s seismic code (itself based on the US building code), and understanding what to do in the event of an emergency.

“What is happening in these segments should serve to remind people to prepare themselves,” he points out. “People should always have an awareness of what they would do in case of an earthquake. It should be like deciding whether to take an umbrella in case of rain, it should be part of the culture.”