Research: San Andreas quakes all sizable

Earthquakes along the San Andreas fault over the past 1,000 years have all been above magnitude 6.5, with no signs of small temblors that might have relieved pent-up pressure, scientists reported in the current issue of the journal Geology.

By matching up dry creek beds on either side of the fault 120 miles northwest of Los Angeles, the researchers estimated that 95 percent of the slippage in the last six earthquakes was caused by large, but rare quakes with magnitudes of about 7.5 to 8.

The smallest quake had a magnitude just below 7, according to the study conducted by researchers from the California Institute of Technology and Central Washington University.

“When we do have an earthquake along this section of the fault, it will probably have a substantial size,” said Charles Rubin, a geologist at Central Washington University and a co-author of the study. The last one was in 1857, and had a magnitude near 8.

The dry creek beds have provided scientists with one of the oldest and most detailed records of earthquake activity in the world.