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http://www.cnn.com/2001/US/01/01/quake.study.ap/index.html

Study: Southern California could be at greater risk of quake damage

January 1, 2001 Web posted at: 7:40 AM EST (1240 GMT)

LOS ANGELES, California (AP) -- A new picture of the ground beneath Southern California shows one of its most populous areas could be at greater risk of earthquake damage than previously realized.

A basin of soft sediment beneath the San Gabriel Valley, just east of downtown Los Angeles, is about 3 miles deep -- 11/2 times deeper than previously estimated. That could lead to more surface shaking during an earthquake, according to a study appearing in January's issue of Geology, the journal of the Geological Society of America.

Such basins shake "like big bowls of Jello," said Gary Fuis, a seismologist with the U.S. Geological Survey in Menlo Park who was the study's lead author.

"The potential damage threat is greater because we've found that the basin is deeper," but specifics will have to await computer modeling of the new data, Fuis said.

The study used data from a series of test explosions in 1994 which created shock waves that were picked up by seismographs as they bounced through underground rock and soil. The area mapped, to a depth of about 12 miles, stretches about 100 miles from Seal Beach through the San Gabriel Valley and across the San Andreas Fault to the Mojave Desert.

Egill Hauksson, a seismologist at the California Institute of Technology in Pasadena, said having a detailed picture of the subsurface is important for understanding earthquake hazards.

"I don't think anybody is going to run out and build a stronger building because of this paper, but it will help an earthquake engineer do better modeling," he said.

The study also suggests the presence of an immense, ramplike crack stretching under the San Gabriel Mountains that could be the deep generator of earthquakes along several Southern California faults.

The fault, called a decollement, rises from 12 miles deep near the San Andreas Fault to about 8 miles as it moves south. It connects the faults that caused the Whittier and Sierra Madre earthquakes of the late 1980s and early 1990s.

If the discovery is confirmed, it could help scientists make better guesses about where future quakes might occur in the region, based on how pressure from the San Andreas Fault is channeled.

"This was completely unexpected. We had no good idea of what the machinery down there driving earthquakes looks like," Fuis said. "Now we think we have a picture of how it all works.

He said scientists weren't sure if the large crack could generate earthquakes or if it transferred its deformation to shallower faults. "We'll start looking for deformations -- wrinkles that might reflect movement," Fuis said.

Southern California has long been considered among the places most at risk for earthquakes. Its densely packed cities were built above an area where two plates of the Earth's crust grind together, and it is lined with earthquake faults, many of them buried.

The San Gabriel Valley and the San Fernando Valley, just north of downtown, both sit on basins of sediment laid down by ancient seas and rivers.

The San Gabriel Valley was the center of the 1987 Whittier Narrows and 1991 Sierra Madre quakes, which together caused eight deaths and $700 million in damage. The San Fernando Valley was the site of the 1994 Northridge earthquake that killed dozens and caused $15.3 billion in insured losses.

-- K (infosurf@yahoo.com), January 01, 2001

Answers

DUH

I used to live in the San Gabriel Valley. I remember, right after the Northridge earthquake, I was looking at a seismograph as an aftershock hit. It was thrilling and chilling.

We moved to the east coast a couple of years later partially to get away from earthquake country. You can protect yourself from snow, but you can't do anything about an earthquake.

Sally

-- Sally Strackbein (Sally@SallysKitchen.com), January 01, 2001.