Science News - July 30, 2011

Time since last big
quake on southern
San Andreas fault
180
years

Average interval of big quakes on southern

Floods may have triggered quakes
Geologists find evidence for natural disaster one-two punch

The team also probed 1,200 years of the
lake bed sediment, which records ancient
floods from the nearby Colorado River.
Sharp tilts in the sediment revealed past
motion on the faults, including four slips
that happened at about the same time as
earthquakes on the San Andreas.

By Devin Powell

SouthernSanAndreasfault

Before Hoover Dam was built, Colorado River floodwaters may have triggered small earthquakes in southern California. These tremors coincided with — and perhaps helped kick off — at least one big temblor on the nearby San Andreas fault, researchers report online June 26 in Nature Geoscience.

10 km Salton Sea

Source fault

Daniel Brothers, a marine geologist at the U.S. Geological Survey Woods Hole Coastal and Marine Science Center in Massachusetts, and his colleagues mapped out several faults beneath California’s Salton Sea, which sits at the southern tip of the San Andreas fault.

At least four out of 17 small fault slips coincided with floods, though whether quake or deluge came first is anyone’s guess. In at least one case, a big quake on the San Andreas occurred at about the same time.

“Overall, it’s a story that makes sense,” says Ken Hudnut, a geophysicist at the

USGS Pasadena, Calif., field office.

Double disaster A computer simulation suggests that the rupture of a fault beneath the Salton Sea could stress the nearby San Andreas (red indicates more stress).

Lake Cahuilla, the Salton Sea’s ancient predecessor, was up to six times as deep as the present-day lake. In the team’s computer simulations, stress caused by the water’s weight and pressure was