Costa Rican Geography Enables Unique Earthquake Studies

The unique geography of the Nicoya Peninsula in Costa Rica, which sits on the boundary of two tectonic plates, has allowed geologists to learn how major earthquakes rupture, and perhaps even predict their occurrence.

The peninsula is one of the few known places where land sits atop a subduction zone - the meeting point of two tectonic plants where one subducts (or sinks) beneath the other - which is where the largest earthquakes are generated.

When enough pressure is built up at a subduction zone, the plates essentially slip out of place, creating a large earthquake, which often generate tsunami. The most major earthquakes in recent history have all occurred along subduction zones, including a magnitude 9.2 earthquake in December 2004 that occurred along a subduction zone off the coast of Sumatra, Indonesia. That earthquake and subsequent tsunami killed more than 200,000 people in 14 countries. Other recent major earthquakes, including the magnitude 8.8 quake in Chile in 2010 and the magnitude 9.0 earthquake in Japan in 2011, also occurred along subduction zones. Each of these major earthquakes also triggered tsunami that devastated huge swaths of coastline.

The Nicoya Peninsula is the site of a magnitude 7.5 earthquake that struck Sept. 5, 2012. Two people died in the quake, one from a heart attack, another from falling debris; a tsunami warning was issued after the quake, but later canceled.

"Fortunately there was very little damage considering the earthquake's size," said Marino Protti of the Costa Rica Volcanological and Seismological Observatory (OVSICORI), lead author of a new study in the journal Nature Geoscience. "The historical pattern of earthquakes not only allowed us to get our instruments ready, it also allowed Costa Ricans to upgrade their buildings to be earthquake safe."

Earthquakes of this magnitude strike the Nicoya Peninsula region roughly twice a century, with the last large quake occurring there in 1950. Since then, scientists were preparing for the 2012 quake through a number of geophysical studies, the most recent of which used GPS to map out the area along the fault that stores the energy that is released in a large earthquake.

"This is the first place where we've been able to map out the likely extent of an earthquake rupture along the subduction megathrust beforehand," said Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

Susan Schwartz, professor of earth sciences at the University of California, Santa Cruz, called the Nicoya Peninsula the ideal natural lab for studying large earthquakes "because the coastline geometry uniquely allows us to get our equipment close to the zone of active strain accumulation."

A few months before the 2012 earthquake, the researchers published a study describing a particular fault path that had the most potential for the region's next large earthquake.

"It occurred right in the area we determined to be locked and it had almost the size we expected," Newman said, but he noted that similar studies in other regions around the world will be more difficult.

"Nicoya is the only place on Earth where we've actually been able to get a very accurate image of the locked patch because it occurs directly under land," Newman said. "If we really want to understand the seismic potential for most of the world, we have to go offshore."

"If we want to understand the potential for large earthquakes, then we really need to start doing more seafloor observations," Newman said. "It's a growing push in our community and this study highlights the type of results that one might be able to obtain for most other dangerous environments, including offshore the Pacific Northwest."