Fault Found Underneath Wilshire Boulevard : Geology: Rift extends under Hollywood and Beverly Hills. Experts say it could produce a quake of 5.7 magnitude or greater.

Geologists have discovered a buried thrust fault, like the one that triggered the Northridge earthquake, running for several miles beneath Wilshire Boulevard and extending under Hollywood and Beverly Hills in one of the most densely populated sections of Los Angeles.

The scientists cautioned that there is no evidence that the fault has produced a major quake since the city was settled, and there is no way to know if it ever will.

Nevertheless, the team of six geologists from Oregon State University and Caltech said the newly identified Wilshire Fault “poses a seismic hazard to some of the most expensive real estate in the world.”

The fault could trigger a quake of magnitude 5.7 or more, they said, and cause severe property damage in an area with many older buildings that have not been structurally reinforced to meet newer, more stringent building codes.

The Wilshire Fault, they said, represents a hazard common throughout the Los Angeles basin. They believe it poses no greater hazard than any of dozens of other so-called blind thrust faults believed to run under the metropolitan area.

Other major faults have been found nearby, one under MacArthur Park, another directly under Sunset Boulevard in Hollywood, and one under Santa Monica Boulevard running west to the coastline along the Pacific Palisades.

Buried thrust faults are difficult to detect even under the best of circumstances because they do not break the surface. The Wilshire fault is more than two miles deep.

When buried thrust faults do rupture, as happened on Jan. 17 in the Northridge quake, the energy is directed almost straight up in a violent vertical heave. Only recently have scientists begun to appreciate the danger such faults pose in Southern California.

“It is one of a couple of dozen faults we have identified under Los Angeles that are capable of producing damaging earthquakes,” said Caltech earthquake expert James F. Dolan, who helped prepare the research.

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The Geological Society of America plans to publish the research in the April issue of Geology.

The new fault is believed to run from east to west, uplifting the terrain from the Elysian Hills on the east to Beverly Hills on the northwest.

“There are a lot of variables and a lot of assumptions we had to make,” said Cheryl Hummon, one of two OSU graduate students who prepared the study. “This is our best guess about where the fault is located. We just don’t have enough data to pinpoint it.

The geologists located the fault by analyzing core samples from oil wells and test drilling conducted as part of subway construction. They also studied a pattern of six very small earthquakes in the immediate area of the fault between 1970 and 1992.

“As far as we can tell, this fold started growing a million years ago,” Hummon said. “Where there is a fold, there is a thrust fault or a reverse fault underneath it uplifting it. Folds grow during an earthquake on the fault underlying the fold,” she said. “That was true in this case.”

The fault has pushed up the sediments above it by about 1,300 feet over the millennia. Normally, the two sides of the fault move by each other at a speed barely exceeding one-tenth of an inch a year. But preliminary evidence suggests that the fault may have been responsible for about 2,000 serious “oblique slip” earthquakes since it started to lift the sediments above it about 800,000 years ago.

“Until 1987, we did not even realize that blind thrust faults could cause quakes,” Dolan said.

“Forewarned is forearmed. If people understand what they are up against, they will do a much better job of preparing for an earthquake,” he said.

New Fault Geologists at Oregon State University and Caltech have discovered a potentially dangerous buried thrust fault running along Wilshire Boulevard in Hollywood and Beverly Hills. There has not been a major earthquake on the fault in recorded history but if it did rupture, it could cause an earthquake of magnitude 5.7 or greater, geologists said.