Chilean Earthquake Shakes Up Prediction Methods

At 3:34 a.m. last February 27, the ground surged violently under the sleeping townsfolk of Maule, Chile. Though the shaking lasted for just seconds in some areas, it left hundreds dead and thousands homeless. But the event, though tragic, has yielded a glimmer of hope for scientists trying to forecast similar quakes. By analyzing satellite measurements of the Maule quake, geologists at the German Research Centre for Geosciences in Potsdam are now one step closer to being able to forecast where the next earthquake could strike and how big it might be.

Chile sits on the border of two slabs of the Earth’s crust: the Nazca tectonic plate, which is part of the Pacific Ocean floor, and the South American plate. The Nazca plate is being pulled under the South American plate at a rate of two and a half inches per year, and the friction between them causes earthquakes.

“Chile hosts about 25 percent of global [seismic] energy release, and because of that, is one of the most perfect natural laboratories to study these things,” said Onno Oncken, one of the geologists from the German Research Centre for Geosciences who analyzed the Maule quake data.

Oncken and his colleagues discovered that before the February earthquake, sections of the two plates were locked in place near Maule. Since the last major earthquake there in 1835, nearly two centuries worth of pressure had built up between the plates, as they were unable to move past each other, Oncken explained. Then, when the slabs of rock could no longer withstand the forces pushing them together, they gave way, and all the pent-up energy between the plates was released in a single, catastrophic event. No one knows exactly how the plates shifted deep in the earth’s crust, whether they broke, buckled, or in some way melted, but the result was the Maule earthquake.

Oncken’s team published its findings in the September 9 issue of Nature. The scientists examined data that research groups from many different institutions and countries had compiled over the last decade using Global Positioning System (GPS) satellites. The GPS readings took note of hundreds of markers placed on Chile’s tectonic boundary and recorded, down to the millimeter, how far each segment of land had moved. Scientists confirmed the area near Maule had been locked for 175 years.

By looking for similarly locked areas along active tectonic boundaries all over the world, geologists like Oncken believe that they may be able to discern which locations are at greatest risk for the next “big one.”

Christopher Scholz, a geophysicist at Columbia University in New York City, said that the GPS readings analyzed for the study provided sound data for what before had been simply a widely accepted theory. “They didn’t discover anything new, but they took everything to a higher level of understanding,” he said.

With more knowledge of the potential magnitude and location of the next big earthquake, both Scholz and Oncken believe that population centers in areas at risk can better prepare for disasters. Cities in areas along locked tectonic plates will be able to improve building codes, renovate old structures to withstand intense shaking, and hopefully decrease future casualties.

The town of Maule and its neighbors will probably have another 175 years to prepare, said Oncken. However, he cautioned that Santiago, Chile’s capital, is sitting on a locked area that last “failed in 1906 and might go at any time.”