Seismic Hazard Is Greater Than Believed, Scientists Say

The Northridge temblor in January, the most costly American earthquake since 1906, measurably rearranged more than 1,900 square miles of metropolitan Los Angeles, leaving portions two feet higher than before, according to a wide-ranging report released today.

The severe shock--now reassessed at magnitude 6.7--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 of the Northridge earthquake 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% likelihood of another Northridge aftershock above magnitude 5.0 by the end of 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 can no longer be considered rare events. 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 study’s conclusions, reported in today’s issue of the journal Science, represent an unusual scientific consensus on the events of Jan. 17, when as a result of a few seconds of brutal whiplash--in which the Earth rippled at speeds up to five feet per second--at least 57 people were killed, 9,253 injured and more than 20,000 left homeless. As scientists evaluated their findings in the months since, they have adjusted the magnitude of the quake several times. Most recently they had assigned it a 6.8 figure, but now consider it a 6.7 temblor.

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.

The fault responsible for the Northridge earthquake belongs to a system of thrust faults related to a bend in the San Andreas Fault between Ft. Tejon and the Cajon Pass. There are so many hidden faults underlying the basin, however, that any safety efforts must be regional in scope, the scientists said.

“We can’t focus our attention on individual faults . . . because there are so many blind faults and it’s so complicated, we have to look at this in a regional sense,” said Thomas L. Henyey, executive director of the Southern California Earthquake Center at USC.

The detailed scientific portrait of the Northridge quake and of the implications for future earthquakes in Los Angeles was the work of scientists from Caltech, USC, NASA’s Jet Propulsion Laboratory, Columbia University and UC San Diego.

“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 because of computer communications on the Internet, which enabled researchers on distant campuses to share the latest findings with great speed.

In assessing the engineering implications of the Northridge earthquake, the scientists paid special attention to building codes in California and how well the construction standards anticipate the severe shocks of even moderate earthquakes.

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.

Federal and state emergency officials are still collecting damage reports. On Thursday, officials at the County-USC Medical Center announced that the complex sustained $1 billion in quake damage, more than any U.S. hospital ever has sustained because of a natural disaster.

“The emphasis on protecting against loss of life says nothing about the cost of repair to restore the buildings to usability,” said Douglas Dreger, a research seismologist at UC Berkeley who praised the group’s findings. “Under the present codes, the buildings may not collapse but may not ever be usable again. It may be advisable to strengthen the codes to prevent much more of the damage that can occur in strong earthquakes.”

* HOSPITAL DAMAGE: County-USC quake damage tab estimated at $1 billion. B1

(Southland Edition, A28) Analyzing the Quake

Here are some findings of a wide-ranging report on the Northridge earthquake:

* The slip on the fault was greater than normal for thrust faults with a rupture surface of less than 97 square miles. The maximum slip exceeded 10 feet.

* Almost 100 faults in the Los Angeles area have been identified as capable of magnitude 6 or greater earthquakes. But only a few of these will produce such events in our lifetimes.

* Earthquakes as large as the Northridge one probably occur an average of every 40 years in the Los Angeles area, according to geologic and geodetic estimates of slip on faults in Los Angeles.

* Earthquakes similar to Northridge will occur several times within the lifetime of a structure. Therefore, the focus of design practice on avoiding collapse should be reconsidered.

Sources: U.S. Geological Survey and the Southern California Earthquake Center