New Quake Faults Found Beneath L.A.

LOS ANGELES - Scientists from two California universities announced yesterday the discovery of two faults under downtown Los Angeles and Hollywood that are believed capable of generating magnitude 6 earthquakes.

But without further research, such as "trenching" projects in which scientists dig across the fault structure to detect evidence of historic ground movements, seismologists cannot estimate how often earthquakes might occur on the newly discovered fault lines.

In this century, no destructive quakes have occurred directly beneath downtown Los Angeles. The closest, about 15 miles to the east, was the 5.9 magnitude Whittier Narrows earthquake on Oct. 1, 1987. It occurred on a deeply buried fault that may lie beneath at least one of the newly detected surface faults.

The faults have been named the MacArthur Park and the Echo Park by the scientists who discovered them - geologist Kerry Sieh and research fellow James Dolan of California Institute of Technology and geologist Thomas Rockwell of San Diego State University.

"We believe that the faults and folds that we have discovered are connected at depth to a large buried fault that runs for almost 20 miles beneath Los Angeles," Dolan said.

Traces of the Echo Park Fault, after crossing East Los Angeles, disappear in the Los Angeles River bed and reappear farther west in the Hollywood area.

Traces of the MacArthur Park Fault were found beneath downtown Los Angeles. They run beneath a major freeway and stretch to the northwest before tailing off in Hollywood. Major downtown and Hollywood landmarks could be severely shaken by a magnitude 6 quake on either fault.

By studying 70-year-old topographic maps, Dolan and his colleagues identified land forms created by the faults, despite their frequent partial concealment by urbanization since the 1920s. They also examined horizontal shifts in former channels of the Los Angeles River, indicating movement on the Echo Park Fault.

"Until recently, most people working on earthquake hazards in Los Angeles focused on the effects of a `Big One' on the San Andreas Fault," Dolan said.

"But in the past few years, some of us have also become concerned about faults that may be active in the urban area," he said. "Even moderate earthquakes on these faults could be very damaging because they are so close to major population centers."

The Whittier Narrows temblor and last year's 5.8 magnitude Sierra Madre earthquake led the scientists to take a closer look at faults beneath Los Angeles.

Dolan said that a study of the ground above the MacArthur Park fault line indicates that it must have experienced multiple quakes over thousands of years.

A key question is how long ago any movement occurred on the newly discovered faults.

Under California law, after the state designates a fault as active, meaning that it had a surface earthquake within the past 11,000 years, local authorities can ban construction within 50 feet of the fault line. But that ban does not affect existing buildings. Altogether, about a dozen major faults might traverse the Los Angeles area, Dolan said.