Giant Earthquake Predictions Aided by Historical Data

Stefan Lovgren
for National Geographic News

September 15, 2005

Predicting when the next giant earthquake will occur is a shaky proposition. But new analysis of an unusual seismic fault in Chile could help scientists better understand when and where giant earthquakes are likely to recur.

Geologists believe that the longer a seismic system goes without experiencing an earthquake, the more strain it accumulates and the larger the next quake will be.

This theory applies only to so-called subduction zones, where one tectonic plate moves over or under another. Strike-slip faults, where plates grind against each other, store stress differently.

But the behavior of a seismic fault in a subduction zone in Chile has confounded seismologists' predictions.

A 9.5 magnitude earthquake—the biggest ever recorded—rocked that region in 1960. The energy released should have taken several centuries to build up. But the area also experienced earthquakes in 1837 and 1737.

A new study shows that the earlier quakes probably did not release much of the stress building on the fault. Instead, strain kept accumulating on the fault ever since a large quake occurred there in 1575.

The findings are reported today in the science journal Nature.

"It [would be] very difficult to expect a very large earthquake along a portion of a fault that has recently been ruptured," said Sergio Barrentos, a scientist at the Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna, Austria.

Barrentos, who was not involved with the research, wrote a commentary published with the study.

Giant Tsunami

The 1960 earthquake in Chile resulted from a rupture along a fault where one tectonic plate slides under another at the west coast of South America. The break was about 620 miles (1,000 kilometers) long and 93 miles (150 kilometers) wide.

The monster quake produced a giant tsunami on both sides of the Pacific Ocean, with crests as high as 30 to 50 feet (10 to 15 meters) in Chile and 20 feet (6 meters) in Japan.

In addition, the average fault displacement—the distance two plates move in relation to each other during a seismic event—after the quake was 66 feet (20 meters).

Such a large fault displacement should have occurred only if stresses along the fault had been building up for several hundred years. But the region had experienced an earthquake in 1837.

"A recurrence time of 130 years is not consistent with the fault displacements associated with the 1960 event," Barrentos said.

It should have taken closer to 300 years to accumulate the average displacement that the 1960 quake showed, he said.

Studies of the region's buried soil and sand layers might solve the mystery. A large quake would trigger a tsunami that would deposit considerable amounts of sand farther inland.

Historical accounts suggest the 1837 earthquake may have been a magnitude 8 and that a 20-foot-high (6-meter-high) tsunami associated with the event crashed into Hawaii.

But the thickness of soil and sand layers along the South American coast suggests that the quakes of 1837 and 1737 did not produce sizeable tsunamis in the region. The tsunami that hit Hawaii in 1837 may have originated outside the study area, researchers say.

The scientists believe the 1837 and 1737 quakes were therefore not strong enough to significantly release the stress building on the fault since the last great earthquake struck in 1575.

Further study of the soil and sand layers allowed the scientists to identify a sequence of eight large earthquakes that have occurred in the region over the past 2,000 years.

"[This is] confirmation that these very large earthquakes recur regularly in southern Chile once every 300 years," Barrentos said.

Predicting Quakes

The Chilean earthquake study could help scientists anticipate when and where giant quakes are likely to recur elsewhere in the world.

"Subduction zones known to have produced merely great earthquakes … might be capable of larger earthquakes," said Brian Atwater, a U.S. Geological Survey geologist based at the University of Washington in Seattle.

Atwater, who co-authored the Nature study, says the findings in Chile could help explain the enormity of the December 26, 2004, earthquake centered west of Sumatra in Southeast Asia.

That quake was a magnitude 9 and produced a giant tsunami that swept across the Indian Ocean, killing more than 170,000 people.

"[That] earthquake took a lot of scientists by surprise, because the largest earthquakes known along the rupture were no larger than magnitude 8," Atwater said.

"In light of the history leading up to the 1960 earthquake [in Chile], it isn't so surprising that successive earthquakes can differ in size through storage of plate motion," he said.

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