Inland Quake Could Cause One Near L.A.

Three scientists said Monday that computer modeling found that a magnitude 7.0 quake in the Inland Empire could in rare circumstances trigger a 7.5 to 7.8 temblor on the Sierra Madre fault close to Los Angeles.

That would be an unusual event, occurring perhaps once every 10,000 years, the scientists stressed. But they said such a triggered series of temblors could be more damaging than a magnitude 8.0 quake on the San Andreas fault.

The comments came on the opening day of the annual San Francisco meeting of the American Geophysical Union and will be the subject of an article to be published Friday in the journal Science by Greg Anderson, Brad Aagaard and Kenneth Hudnut of the U.S. Geological Survey.

Triggered quakes have been the subject of considerable study since the magnitude 7.3 Landers temblor of June 28, 1992, triggered smaller quakes as far away as Yellowstone National Park.

The Sierra Madre-Cucamonga fault system follows a line along the San Gabriel Mountains from just northwest of San Bernardino through Pasadena. It was the locale of a magnitude 5.8 quake near Pasadena in 1991.

The scientists say their computer modeling indicates it is unlikely the fault system could trigger a quake on the San Jacinto fault system to the southeast. But a triggering in the reverse direction, from the San Bernardino-Riverside area toward the northern edge of the Los Angeles metropolitan area “is quite possible,” Hudnut said.

It is also possible that a big quake on the Sierra Madre fault could trigger a larger quake on the San Andreas fault, according to the scientists.

Some triggered earthquakes, such as the 1999 magnitude 7.1 Hector Mine quake, take years to develop after a nearby shock. Others can occur immediately.

The Northridge quake of Jan. 17, 1994, which was a magnitude 6.7, cost 57 lives and did $40 billion in damage.

A 7.5 to 7.8 on the Sierra Madre fault would be eight to 10 times more powerful, and could cause considerably greater casualties and damage.

Some scientists believe the Sierra Madre fault is potentially the most dangerous in the Los Angeles area.

No quake larger than Northridge has occurred in 220 years of recorded history in the area. But that is a comparatively short time in geologic terms.

Anderson, the lead author of the study, compared the kind of triggered quake the group of scientists is researching to the magnitude 7.9 quake on Alaska’s Denali fault last year. That began with a 7.2 temblor on a nearby fault, which immediately triggered larger quakes on the Denali fault.

The densely populated Los Angeles region is bounded by a large network of thrust and horizontal strike-slip faults similar to the Denali system, Anderson said.

He and his colleagues created sophisticated three-dimensional computer models of the Los Angeles region, including the geometry of the faults in the area and the physics of earth movement.