Using information once jealously guarded by oil and gas companies, scientists have for the first time identified a major fault under downtown Los Angeles of a kind whose existence there they previously could only infer. They say it is capable of producing a devastating earthquake.
Scientists have for years believed that such faults, called blind thrusts, exist beneath the city. But this is the first solid proof that one is actually present, said Dr. James Dolan, a professor of geology at the University of Southern California.
The research adds support to the theory, emerging in the last decade, that Los Angeles is threatened more by faults like this one, which lie concealed underground right inside the city, than by the famous and much larger San Andreas fault, a visible scar running across the landscape 60 miles east of downtown.
Asked how many blind thrust faults might lie beneath Los Angeles, experts said yesterday that they did not know. But Dr. Dolan said there was unlikely to be another the size of the newly identified one, which would measure 250 square miles on the surface and is believed to be at least nine miles deep. It is ''the poster child of blind thrust faults underneath Los Angeles,'' he said.
Dr. Dolan, who was not among the researchers, said they had ''nailed its location'' and determined that it was under increasing strain.
Dr. Tom Henyey, co-director of the Southern California Earthquake Center, a consortium of academic and government institutions with headquarters at U.S.C., said the finding would add to the data that policy makers consider in evaluating building codes and the design of crucial complexes like hospitals.
The fault was identified by Dr. John Shaw, a structural geologist at Harvard University, and Dr. Peter Shearer, a seismologist at the Scripps Institute of Oceanography in San Diego. Their work is described in today's issue of the journal Science.
Thrust faults, which exist elsewhere and have long been thought to lie beneath Los Angeles, in part because of surface features found within the city, are so called because of the activity they induce in earthquakes. In a quake caused by what is known as a strike-slip fault, like the San Andreas, the opposing blocks of earth slide past each other horizontally. But in a quake caused by a thrust fault, whose fracture of the earth dips along at angles that can be either shallow or steep, the blocks move diagonally, almost vertically.
Such movement may cause rocks to fold just below the surface. Or it may create small hills on the surface. But that evidence of a quake can easily erode away, or be bulldozed to make room for apartment buildings, leaving the fault invisible, or ''blind.''
The blind thrust faults of Los Angeles, Dr. Shaw explained, are caused by the relative motion of the North American and Pacific plates, two huge masses of crust that are sliding past each other along the San Andreas fault. Because of a dogleg in the fault, the Los Angeles basin is being squeezed like an accordion between these masses of earth. The ground under the city is compressing by a fifth of an inch a year, Dr. Shaw said, and some of that stress is building up on one fault or another somewhere out of sight.
The first sign that the Los Angeles basin had potentially dangerous blind thrust faults came in 1987 with the Whittier Narrows earthquake, a magnitude-6 temblor that killed eight people and caused property damage of $358 million. No fault had been mapped in the area, and among the findings of the latest research is that this quake was caused by the newly identified fault.
The magnitude-6.7 Northridge earthquake of 1994, also on a blind thrust fault that had not been mapped, killed 61 people and caused damage worth $35 billion.