Distant Quake Could Hit Los Angeles Hard

05/31/06
New simulations reveal a funnel effect guiding large waves into the L.A. basin. Intensity of the ground shaking would be greater than estimated by earlier models, says USC researcher.
By Carl Marziali
Location map for the TeraShake simulation project.

An earthquake at the south end of the San Andreas fault could inflict unexpected damage on Los Angeles, according to simulations commissioned by the Southern California Earthquake Center at USC.

A strong earthquake near the Salton Sea, more than 100 miles from downtown Los Angeles, could launch seismic waves that would be amplified and guided into the Los Angeles area by the sedimentary basins south of the San Gabriel Mountains.

The intensity of the ground shaking in the sedimentary basins would be greater than estimated by previous models, said Thomas Jordan, SCEC director and professor of earth sciences in USC College.

“L.A. is pretty far from this fault, and we’re surprised by how high the amplitudes are,” Jordan said.

The findings appeared in the April 16 issue of Geophysical Research Letters, recently mailed to subscribers. Maps and animations are available at http://www.scec.org/sanandreas.

Older simulations failed to account for two important factors, Jordan said: the direction of fault rupture, which tends to send energy ahead of itself, like a crack spreading through plate glass, and the role of sedimentary basins in amplifying ground motion.

“We know that sedimentary basins shake like bowls of jelly,” Jordan said. “In Southern California, most people live in these sedimentary basins, Los Angeles being the biggest and deepest.”

In the hardest hit areas, the speed of ground motion could exceed two meters (over six feet) per second. Any shaking over one meter per second is considered very strong, Jordan said.

The SCEC study does not include damage estimates. But the overall impact on the Los Angeles area could be far more severe than for the Northridge earthquake of 1994.

"It may not be much worse than Northridge [was] in the San Fernando Valley," Jordan said, "but this earthquake would generate shaking much stronger than Northridge in the L.A. basin and basins to the east."

The simulations, carried out at the San Diego Supercomputer Center at the University of California in San Diego, assume a magnitude 7.7 fault rupture toward the northwest.

Damage would be considerably lighter if the fault ruptured in the opposite direction, away from the Los Angeles basin.

The research team chose the southernmost portion of the San Andreas fault because it has gone the longest without rupturing (since 1690, compared to 1857 for the central portion and 1906 for the portion near San Francisco).

Jordan described it as the part “where the spring has been wound tightest.”

“This is one of the most likely segments of the San Andreas fault to rupture in a large earthquake,” he said.

SCEC conducted the study as part of its ongoing TeraShake simulation project, funded by the National Science Foundation. Kim Olsen of San Diego State University led the research team.

SCEC is a consortium of 54 institutions headquartered at USC, with funding from the National Science Foundation and the U.S. Geological Survey.