2 New Faults Found Under Center of L.A.

Scientists from Caltech and San Diego State University reported Sunday the discovery of traces of two faults under Los Angeles’ Eastside, downtown Los Angeles and Hollywood that they believe are capable of generating magnitude 6 earthquakes.

But without further research--such as trenching projects in which scientists dig along the fault structure to detect evidence of ground movements--experts cannot estimate how often quakes might occur.

All they know is that in this century there have been no destructive quakes directly under the center of Los Angeles. The closest, about 15 miles east, was the 5.9 magnitude Whittier Narrows earthquake on Oct. 1, 1987, which occurred on a deeply buried thrust fault that may underlie at least one of the surface faults whose traces have now been detected.

The newly discovered faults have been named the MacArthur Park Fault and the Echo Park Fault by the scientists who found them--Caltech geologist Kerry Sieh, Caltech research fellow James Dolan and San Diego State geologist Thomas Rockwell.

“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, adding that trenching could provide indirect indications of recurrence intervals for quakes on the buried fault.

Echo Park Fault traces cross the Eastside, disappear in the sediments of the Los Angeles River bed and reappear farther west along the Hollywood Freeway. Traces are surface indications of underground faults.

MacArthur Park Fault traces were found along Wilshire Boulevard downtown. They traverse the Harbor Freeway and stretch northwest toward a point west of Western Avenue, tailing off before reaching Santa Monica Boulevard in Hollywood.

Dodger Stadium is about a mile away from the nearest of the two faults, as is the main Hollywood Boulevard business section. But the Los Angeles Civic Center appears to miss the Echo Park Fault by an eighth of a mile or less. All major downtown and Hollywood landmarks are close enough to be severely shaken by a magnitude 6 quake on either fault.

The scientists also have more precisely mapped the Hollywood and Santa Monica faults along the base of the Hollywood Hills and the Santa Monica Mountains.

Sieh and Rockwell were out of the country, but Dolan discussed the discoveries in interviews and will present the results of the research today at the opening of the annual American Geophysical Union meeting in San Francisco.

By studying a set of 70-year-old topographic maps, Dolan and his colleagues identified landforms created by the faults, despite the land’s partial concealment by urbanization since the 1920s. They also examined horizontal shifts in former channels of the Los Angeles River that indicate strike-slip 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. 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 inspired the scientists to take a closer look at faults in Los Angeles. The San Andreas Fault is at least 40 miles away from the city, and the portions of it considered most likely to rupture are at least 60 miles away.

The MacArthur Park Fault is visible, Dolan said, if one stands at the northwest corner of Wilshire Boulevard and Alvarado Street in Los Angeles and looks north. The lip of a 25- to 75-foot high scarp can be seen beginning south of 6th Street and continuing into the densely developed area to the north.

A scarp is a slope, initially quite abrupt but in this case weathered by time, formed by a quake ground rupture.

Distinct scarps, often as high as five or six feet, formed over the 45-mile desert rupture zone in June’s 7.5 magnitude Landers earthquake.

If that rupture had occurred in an urban area such as Los Angeles, it would almost certainly have caused major property damage because, in addition to the vertical scarps, horizontal offset ranged up to 22 feet. This means that two points directly across the fault from each other before the quake became separated by as much as 22 feet.

Dolan said the height of the MacArthur Park Fault scarp indicates that the fault caused multiple quakes over thousands of years.

A key question is how long ago any movement occurred on the newly discovered faults. The trenching of the surface faults can lead state authorities to determine that a fault is active, and that finding can have major legal consequences for development. A fault is considered active if it is believed to have had a surface earthquake along it in the past 11,000 years.

Under the state’s Alquist-Priolo law, local authorities can ban construction within 50 feet of an active fault line. But this ban does not affect existing structures, and it appears that many homes and businesses may be directly over the newly discovered faults.

Sieh, who is known for his trenching work on the San Andreas Fault, said in the announcement Sunday that a small trenching project on the Santa Monica Fault indicated that although it was thought to be inactive, it has ruptured within the past several thousand years.

“That may seem like a long time,” Sieh said, “but don’t forget that until this summer, the faults involved in the Landers earthquake hadn’t ruptured for several thousand years either.”

Altogether, about a dozen major faults may traverse the Los Angeles Basin, Dolan said. He said the scientists have found preliminary indications of one under the southeast part of Los Angeles. Others, such as the Newport-Inglewood Fault, which slipped in the 6.3 magnitude Long Beach quake of 1933, are well-identified.

Newly Discovered Faults

Caltech and San Diego scientists have discovered geological traces of two previously unknown faults close to downtown Los Angeles and Hollywood. The MacArthur Park Fault is believed to have formed a scarp (a slope formed by a quake ground rupture) 25 to 75 feet high north of Alvarado Street and Wilshire Boulevard. The Echo Park Fault closely follows the Hollywood Freeway in places. Both are believed to be capable of generating magnitude 6 earthquakes. The scientists also have more precisely mapped the already known Hollywood and Santa Monica faults. Another fault-the Newport-Inglewood-is well known for the 1993 Long Beach earthquake.