Centuries of stress caused biggest quake

Centuries of stress caused biggest quake
9.5 magnitude, 1960 Chile temblor killed 2,000, produced tsunami

Updated: 4:47 p.m. ET Sept. 14, 2005
LONDON - Nearly four centuries of pent-up stress and energy were brewing under the surface before the biggest ever recorded earthquake rocked Chile in 1960, scientists said on Wednesday.

The May 22 quake, whose epicenter was below the ocean floor off the coast of Chile, had a magnitude of 9.5.

It killed more than 2,000 people, left 2 million homeless, caused heavy damage in the cities of Concepcion, Valdivia and Puerto Montt towns and churned up a tsunami that hit as far away as Hawaii, Japan and the Philippines.

To produce an earthquake the size of 1960 event, scientists thought it would take several hundred years of energy and slippage to build up. It is believed that the longer the time since a previous quake the bigger the next one will be.

But two earthquakes, in 1737 and 1837, occurred in the same area which should have lessened the impact of the 1960 tremor.

Using soil and sand samples dating back 2,000 years and records of land movement along the fault line, researchers in Chile reconstructed a picture of the events that preceded the record-breaking quake to explain what made it so powerful.

"In these records, the 1960 earthquake ended a recurrence interval that had begun almost four centuries before, with an earthquake documented by Spanish conquistadors in 1575," Marco Cisternas, of the Pontificia Universidad Catolica de Valparaiso, said in a report in the science journal Nature.

Cisternas and his team said the two earlier quakes did not produce any subsidence or tsunami and probably left the fault with more accumulated energy that was eventually released in the record-breaker.

Ignacio Salgado, who worked on the research, said knowing the history of the quake and the behavior of such giant tremors is important because it could influence the magnitude of future earthquakes. "If the events are more frequent maybe they will be of lower intensity," he said in an interview.

The world's biggest earthquakes, including the one that triggered the deadly Indian Ocean tsunami in December 2004, occur in subduction zones where one of the earth's tectonic plates is driven below another. During the 1960 quake a rupture about 621 miles long occurred where the Nazca plate slipped under the South American plate.

"Thanks to the work of Cisternas et al, we now have more insight into the seismic cycle in south-central Chile," Sergio Barrientos, of the Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna, Austria said in a commentary on the research.

(c) Reuters 2005. All rights reserved. Republication or redistribution of Reuters content, including by caching, framing or similar means, is expressly prohibited without the prior written consent of Reuters. Reuters and the Reuters sphere logo are registered trademarks and trademarks of the Reuters group of companies around the world.