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Scientists Claim Los Angeles More Prone to Earthquakes

By Robert Lee Hotz
and Kenneth Reich
Los Angeles Times

The Southern California temblor in January, the costliest U.S. earthquake since 1906, measureably rearranged more than 1,900 square miles of metropolitan Los Angeles, leaving portions 2 feet higher than before, according to a wide-ranging report released today.

The severe shock - now re-assessed at 6.7 magnitude - also forced scientists to revise their estimates of the seismic hazard facing the densely inhabited suburbs of metropolitan Los Angeles.

In the most comprehensive assessment so far, the scientists of the U.S. Geological Survey and the Southern California Earthquake Center say that people in the Los Angeles area should expect at least one earthquake every year of magnitude 5.0 or more for the foreseeable future. There is a 45 percent likelihood of another aftershock of the Jan. 17 Northridge quake above magnitude 5.0 by the end of next July.

The scientists said that, with almost 100 faults in the Los Angeles area known to be capable of a magnitude 6.0 or greater earthquake, emergency planners must recognize that urban earthquakes cannot be considered rare. Over the long run, contractors should expect that homes and buildings might be subjected to severe ground shaking - at the extremes of the stresses considered in present building codes - several times in the structure's lifetime.

"The full extent of the urban corridor from San Bernardino through Los Angeles and northwest to Santa Barbara is at risk from both the thrust faults (like Northridge) and the San Andreas fault, and the two risks are comparable," the scientists said.

Accordingly, the scientists suggested "that the northern Los Angeles region faces one of the greatest seismic hazards in Southern California."

The report's conclusions, reported in Friday's issue of the journal Science, represent an unusual scientific consensus on the events of Jan. 17, when 61 people were killed, 9,253 injured and more than 20,000 left homeless.

The Northridge quake produced the most severe ground motions ever recorded in a North American city, giving researchers their best close-up look at a thrust quake. After evaluating their findings, the scientists concluded that such devastating shaking should be considered the norm near any large thrust earthquake.

"There was a debate over what happens when you are right on top of an earthquake," said Lucile M. Jones, a USGS seismologist who led the team that prepared the report. "That argument is over."

The Northridge earthquake, Jones said, "really rammed home the nature of the complicated fault zone that we are sitting on. This is an incredibly complex web of faults underlying all of Los Angeles - more complex than we thought."

The entire Los Angeles Basin is becoming more seismically active, and the data from the Northridge quake reinforced the expectation that the area should continue to experience at least one 5.0 temblor or greater every year.

Although many minor controversies are still unsettled, the scientists reached agreement on the temblor's major lessons relatively quickly - barely nine months after the earthquake.

In part, the researchers reaped the benefit of the large number of sensors that lace the region to record subtle ground motion and changes in position, experts said. They also were able to evaluate their conclusions quickly, in part due to computer communications on the Internet, which enabled researchers on distant campuses to share the latest findings.

The earthquake left more than 3,000 buildings unsafe for habitation, but the scientists blamed much of that damage on faulty construction and inadequate building inspections.

Even simple cost-effective safety measures, such as securing computers, lights, water pipes and bookcases, could save billions of dollars in future earthquakes.