Thursday, June 22, 2006
L.A. called ripe for a big quake
Study: South San Andreas fault overstressed


THE ASSOCIATED PRESS


LOS ANGELES
New earthquake research confirms that the south end of the San Andreas fault near Los Angeles is overdue for a big one.

The lower section of the fault has not produced a major earthquake in more than 300 years. The new study, which analyzed 20 years of data and is considered one of the most detailed analyses yet, found that stress has been building up since then, and that the fault could rupture at any moment.

"The southern section of the fault is fully loaded for the next big event," said geophysicist Yuri Fialko of the Scripps Institution of Oceanography in La Jolla.
 

Predicting when that might happen, however, is beyond scientists' ability. The analysis was published in today's issue of the journal Nature.

Experts have estimated that an earthquake of magnitude-7.6 or greater on the southern San Andreas could kill thousands of people in the densely populated greater Los Angeles area and cause tens of billions of dollars in damage.
 

It was the 800-mile San Andreas fault, which runs down California like a scar, that caused the 1906 San Francisco quake that led to about 3,000 deaths.

But scientists know very little about the dormant, 100-mile southern segment, which slices through Southern California from San Bernardino, east of Los Angeles, to near the Mexican border.
 

The section last released its pent-up energy in 1690, producing an estimated 7.7-magnitude quake, but it caused little injury or damage because hardly anyone lived there at the time.
 

Using satellite radar and global-positioning data, Fialko measured the movement of the southern San Andreas between 1985 and 2005. Small movements along a fault can relieve strain. Calculating those subtle movements allows scientists to figure out how much strain is building up.

Fialko found that the southern end of the fault has shown little movement and that strain is building up.
 

Surprisingly, he found that the two sides of the southern San Andreas behaved differently, with one side showing more flexibility than the other. This could help scientists understand potential earthquake risks, he said.