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Thrust Faults Pose Brutal Danger to Basin

January 18, 1994|Robert Lee Hotz and Kenneth Reich | Times Staff Writers

The earthquake that convulsed the San Fernando Valley early Monday demonstrated as brutally as possible the danger posed by a complex web of deeply buried thrust faults underlying the Los Angeles Basin.

Seismologists said Monday that dozens of such faults underlay the basin, many unmapped and unknown until they abruptly announce their presence with a powerful shudder. Although they may lack the capacity to generate the devastating force of the Big One, these faults can cause severe injury and widespread property damage.

"There are so many (faults) that can produce stress of this magnitude," said Hiroo Kanamori, director of the Caltech Seismological Laboratory. "We have to be prepared for this sort of thing."

Monday's 6.6 quake, the strongest in the basin's modern history, was powerful enough to raise parts of Northridge, the Northridge hills and the Santa Susana Mountains "a foot or two," while areas of the northeast Valley south of the city of San Fernando slumped an equal amount, said Lucille M. Jones, a seismologist at the Pasadena field office of the U.S. Geological Survey.

Movement on the fault may have been as much as two yards on each side, several seismologists said.

Any of the other short, thrust faults hidden under the basin could produce the same kind of strong, jarring upward motion. A thrust fault, unlike the more common horizontal strike-slip fault, moves vertically.

Within hours of the frightening, early morning jolt, scientists from around the state headed for Southern California to study the Northridge earthquake, and portable seismographs were installed through the quake area.

As they carefully evaluated their seismic charts and the results of preliminary aerial surveys, scientists started to construct a rational frame of facts and measurements around Monday morning's long seconds of tumult.

By late afternoon, scientists had yet to determine which fault was responsible for the damage. They hoped that the pattern of aftershocks would help them locate it more precisely and give a clearer insight into the stresses that underpin the region.

Monday's quake lasted no more than 10 seconds at its source, with up to an additional 20 seconds of reverberations elsewhere. That was followed by more than 86 noticeable aftershocks, including three of at least 5.0 in magnitude by Monday night. The strongest was a 5.3 jolt.

Aftershocks are expected to continue with some decreasing frequency through the coming months, scientists said. The chance of an aftershock of more than 5.5 in magnitude in the weeks ahead is 1-in-4; and for an aftershock of 6.0 or more in the coming year is 1-in-10, they said.

"The fault is still uncertain," said Pat Jorgenson, spokeswoman for the Geological Survey office in Menlo Park. "Anybody who's going on a limb and identifying a fault is not being cautious."

The quake occurred on what Jones, Kate Hutton of Caltech and other scientists initially said was an unnamed, virtually unknown fault on a plane shallowly dipping toward the Santa Monica Mountains.

They believe that the fault probably intersects with the Elysian Park Fault belt, which extends about 50 miles from Whittier all the way to the ocean near Point Dume. That fault generated the powerful Whittier Narrows thrust quake of Oct. 1, 1987.

But one of Caltech's most prominent seismologists, Kerry Sieh, said late Monday afternoon that there is at least a chance that the quake occurred on the Elysian Park Fault itself. The fault intersects with two others near Reseda and Roscoe boulevards, about a mile south of Cal State Northridge, which was severely damaged Monday. Those two faults, called the Devonshire and the Frew, had been mapped years ago by oil company geologists.

The quake could have begun on any of the three, Sieh said. Not until the aftershock pattern is further studied will scientists know which.

Jones and Hutton noted that almost all of the aftershocks occurred well north of the Elysian Park Fault (sometimes also known as the Santa Monica Mountain Thrust Fault). The two scientists continued to favor one of the lateral faults, probably the Frew.

Jim Mori, director of the Pasadena field office of the USGS, said the quake was on "an east-west trending thrust fault with the aftershocks to the north." Based on this, he said the Elysian Park Fault was not directly involved. A helicopter flight by Sieh and another Caltech seismologist, James Dolan, Monday morning showed no indication of a ground rupture from the quake. This was also the case with the magnitude 5.9 Whittier Narrows temblor and is the trademark of a deeply buried fault.

After Whittier Narrows, some scientists said the quake danger in the Los Angeles Basin had been underestimated. There may have been some past quakes that were unknown, they explained, because they did not break the surface.

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