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SCV data will help quake predictions

Research done in 1999 improved understanding of how quakes work

Posted: July 30, 2008 12:54 a.m.
Updated: September 30, 2008 5:02 a.m.

Seismologists pouring over data surrounding Tuesday's earthquake in Chino Hills credit Santa Clarita for having provided valuable information needed to interpret what happened and to predict seismic activity.

"You can thank the Santa Clarita city fathers," said Gary Fuis, associate team chief of scientists for the U.S. Geological Survey. "Santa Clarita was very helpful to our survey people in understanding earthquake-producing machinery."

In 1999, a team of USGS scientists used sophisticated "seismic imaging survey" tools to paint a portrait of faults under Santa Clarita Valley.

They set off powerful explosions 12 miles underground and monitored blast response on those fractures in the earth's crust.

What they found was a cluster of faults that produce and relieve sideways pressure on the San Andreas Fault.

It was this "sideways pressure" on the San Andreas Fault that produced Tuesday's earthquake near Chino Hills, with a magnitude of 5.4, Fuis said.

It was the state's 14th largest quake in recent history, USGS scientists reported Tuesday.

Did the Chino Hills quake release enough pressure off the San Andreas Fault to reduce the risk of a large earthquake?

The short answer, Fuis said, is no.

The chance of a large earthquake following Tuesday's quake is still a 5 to 10 percent probability in the next seven days, he said Tuesday.

"There's no way a whole lot of itty-bitty quakes would empty the bucket," Fuis said. "What we need is a really big earthquake to significantly reduce the stress. These little quakes are a drop in the bucket."

Scientists have learned that the Earth's crust is fractured into a series of "plates" that have been moving very slowly over the Earth's surface for millions of years, according to the USGS.

Two of these moving plates meet in western California and the boundary between them is the San Andreas fault, cutting through the center of the state from north of San Francisco to an area south of Temecula.

In the 1999 fault study undertaken by the USGS team, scientists learned that the same dynamics that triggered the Whittier Narrows quake of 1987 and the San Fernando quake of 1971 involved the same "sideways pressure" fault system underneath Santa Clarita Valley.

Scientists at the time looked at a narrow area from Santa Monica, through Northridge and Santa Clarita, over to Mojave and the Sierra Nevada mountain range.

"We discovered the fault involved in the San Fernando quake of 1971 is very similar to the fault in Whittier Narrows and also to this earthquake today," Fuis said about the Chino Hills quake.

The activity along this series of faults producing "sideways pressure" is because of mountains trying to move south, he said. "In Santa Clarita, San Fernando and Chino Hills, what you're seeing is basically mountains that are trying to move south and southwest over the populated areas in the Los Angeles Basin and in the San Fernando Valley. North-dipping faults pushing the mountains south are relieving the sideways pressure on the San Andreas Fault," he said.

Fuis used the example of two sections of carpet being pushed together with one carpet moving over the top of the other, not evenly but rather at an angle.

It's the faults identified in the 1999 study (Santa Clarita/Whittier Narrows/San Fernando) that exert "sideways pressure." on the much larger San Andreas Fault. In October 1987, the Whittier Narrows earthquake measured 5.9 in magnitude, 10 times stronger that the one in Chino Hills, Fuis said.


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