May 12, 2006

Study Details Quake at San Andreas Fault

LOS ANGELES -- A powerful earthquake on the southern San Andreas fault, which hasn't ruptured in over three centuries, is capable of producing prolonged periods of strong shaking in the greater Los Angeles region, a new study finds.

The study offers one of the most detailed analyses yet of what would happen if a magnitude-7.7 temblor strikes along a 125-mile stretch of the fault between San Bernardino and Imperial counties.

The southern San Andreas last snapped in 1690, unleashing a strong quake that caused relatively little damage because few people lived in the area. But as Los Angeles and neighboring cities have become populated and built up over the decades, scientists now say a Big One could be devastating.

Computer simulations show the Los Angeles basin will experience some of the strongest ground shaking if the fault unzips from south to north.

That's because seismic waves fanning from the epicenter will have to travel through a chain of sedimentary basins between San Bernardino and downtown Los Angeles, trapping energy and channeling it toward the Los Angeles basin. The result will be strong and localized vibration.

The basin could potentially experience several minutes of "roller coaster motion," said lead researcher Kim Olsen of San Diego State University.

"A large part of the Los Angeles area would definitely get a good shake," he said.

But Olsen said the shaking in the region likely won't produce as much damage as areas near the epicenter because the traveling seismic waves will have weakened by the time they reach the greater Los Angeles region.

If the San Andreas ruptures from north to south, the areas most at risk of violent shaking include the Imperial Valley and northern Mexico, the study found.

The study was published in the journal Geophysical Research Letters this month.

Earlier this year, the U.S. Geological Survey received a $2 million federal grant to monitor the southern San Andreas fault, which has been building up stress that could lead to a big shaker.

Scientists have said the southern segment, which is overdue, has a high chance of rupturing in the next few decades, producing a quake of magnitude-7.5 or greater.


On the Net:

U.S. Geological Survey: http://www.usgs.gov

San Diego State University: http://www.sdsu.edu