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Was quake the ‘Big One’ experts predicted?

From the print edition

By Vanessa I. Garnica and Hannah J. Ryan | Tico Times Staff

“This is the quake we were waiting for,” said Leopold Linkimer, seismologist at Costa Rica’s National Seismological Network, following a powerful magnitude-7.6 earthquake that rattled the country Wednesday morning. “Regardless, we have to be ready for any contingency because nature can surprise us.”

The earthquake was registered 8 kilometers (5 miles) west of Sámara in the Nicoya Peninsula, in the northwestern province of Guanacaste. After the quake, which damaged several buildings in the province, shaken residents wondered, was this the big one that experts had predicted would strike the peninsula?

Scientists in Costa Rica have been contemplating the possibility of a big quake in the Nicoya region for more than two decades. Marino Protti of the Volcanological and Seismological Observatory (Ovsicori), who has been warning of a Nicoya quake for years, disagreed with Linkimer, saying that this week’s quake may or may not have been the “big one”.

The last time an earthquake of a similar magnitude was registered in the area was 1950. On average about 18 small tremors are registered daily in Costa Rica, Linkimer said. At press time, more than 530 aftershocks were registered, the biggest occurring Thursday morning, with a magnitude of 5.1.

Following the quake, emergency officials gathered at National Emergency Commission (CNE) headquarters in the western San José district of Pavas to discuss initial damage reports.

“We are not declaring a state of emergency,” said Vanessa Rosales, CNE president. “We have begun to receive damage reports taking place around the country. Overall, we encouraged at this point, even though it was a significant quake.”

Wednesday’s temblor is considered the second biggest in the country’s history, behind a 1991 earthquake measuring magnitude-7.7. That quake, centered in Limón province along the Caribbean coast, killed 75 people and injured more than 600.

Linkimer said the official magnitude of the quake could be adjusted in coming days, pending further studies, and several additional aftershocks are expected.

The cause of Wednesday’s quake was a process called subduction, where tectonic plates located in the Earth’s outer layer constantly move on top and under each other, causing the ground to move in the form of a temblor. The Nicoya fault zone is caused by subduction of the Cocos plate under the Caribbean plate along the peninsula’s western side. 

Geologists, geophysicists, oceanographers and other experts have analyzed the degree of pressure buildup over the past 50 years – the seismic recurrence interval in Nicoya Peninsula is about 50 years – allowing them to predict that a “big one” would occur with a magnitude in the range of 7.7-7.9. With Wednesday’s quake registering magnitude-7.6, experts had accurately predicted the event within magnitude-0.1 – if it was the one they were waiting for. Seismologists said that this week’s quake released about 80 percent of the pressure buildup, meaning that another event is possible. 

“Part of the problem in that part of the subduction zone is that the Caribbean plate (on which the Nicoya Peninsula lies) seems to be stuck to the subducting oceanic crust of the Cocos plate,” geologist Jonathan Harris said in an email.

That hampers subduction, and the release of pressure in the form of energy occurs only sporadically, he added.

Another result of the phenomenon is that the Caribbean plate is dragged down with the oceanic crust of the Cocos plate, causing land to sink along the Pacific coastline.

“Indeed, sea levels have appeared to rise – not the result of global warming, but rather of the sinking crust,” Harris said.

Seismologists consider Costa Rica a very seismically active country and apt for an earthquake at any given time. Linkimer said a big earthquake had been expected in the Nicoya Peninsula since the early 1990s.

“There are certain [scientific] models that describe that some earthquakes will occur with regular occurrence,” he said.

Because there are signs of cracking in the crust as far away as the Arenal Volcano in north-central Costa Rica, some have speculated that “the Big One” could also lead to more volcanic activity in that area. Linkimer said earthquakes can cause landslides in areas near the epicenter, but not necessarily volcanic activity. However, volcanic activity was reported at the Irazú and Rincón de la Vieja volcanoes in 1991.

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