Massive Chile Earthquake May Not Be the 'Big One'

A big earthquake of magnitude 8.2 shook the coast of northern Chile on Tuesday night, setting off small landslides and a small tsunami and killing at least five people. But scientists say the quake, while large, was not the "big one" that is predicted for the region.

"The big question is, is this a foreshock to an even bigger earthquake to come?" Rick Allmendinger asks. Allmendinger, a geologist who specializes in earthquake analysis at Cornell University in Ithaca, New York, is a visiting professor at the Universidad Católica del Norte in Antofagasta, Chile.

"It probably has not released all of the stored-up energy on the subduction earthquake fault in northern Chile," he says. "For the sake of all of our friends in the region, we're hoping that there isn't a bigger one still to come."

Scientists don't know when a possible larger quake might strike along the subduction zone, an area where one tectonic plate slides beneath another. It could happen at any time. (See "What Caused the Chile Earthquake?")

During the past 140 years, the faults off the Chilean coast have shifted repeatedly because of its proximity to the "Ring of Fire," an area around the edges of the Pacific Ocean with high levels of earthquake and volcano activity.

But there is a section of the South American tectonic plate boundary off Chile that has not ruptured since 1877, when an earthquake of magnitude 8 to 8.9 struck, Allmendinger says. That indicates a high probability that part of that plate has stored up considerable seismic energy.
 

"Were that segment to rupture all at once, it would generate a much larger earthquake than did occur [last night]," says Allmendinger.

Foreshock?

The important question is whether the recent quake put additional stress on that section of the plate, and whether that can help it overcome the frictional coupling with its adjacent plate, says Allmendinger. That would make this week's quake a foreshock to something bigger.

"It's sort of like a zipper; if you pull more on it, it could unzip," he says.

There is reason for concern, he adds, because the Chile quake has some similarities to the 2011 earthquake in Tohoku, Japan, which set off a massive tsunami and caused extensive damage. There were foreshocks for about two weeks before both events, and the shock waves from both quakes generally spread in similar patterns. (See "Japan Tsunami: 20 Unforgettable Pictures.")

In the past few weeks, there were 50 to 100 small quakes off Chile, the largest being a 6.7 temblor.

Since 1973, Chile has had more than a dozen quakes of magnitude 7.0 and above. In 2010, about 500 people died after an 8.8-magnitude earthquake struck. That temblor displaced a city about 10 feet (3 meters) to the west.

Limited Reports of Damage

Early analysis of the Chile event from the United States Geological Survey suggests that the quake started relatively deep and close to the coastline, says Allmendinger. The depth may help explain why damage reports have been relatively minimal.

The tsunami generated was a "relatively modest" 20 feet (6 meters) high, and has apparently not caused extensive damage. "There was probably some flooding in low-lying areas, but I haven't heard of any damage," he says.

The 1877 earthquake, in contrast, produced a 82-foot-high (25-meter-high) tsunami.

The tsunami generated by this week's quake will spread across the Pacific, "but is unlikely to cause damage," says Allmendinger.

"We can't say for sure that there will be a bigger one," he adds. "But the Earth is constantly surprising us." NationalGeographic