All San Andreas Quakes Found to Be Big

Earthquakes along the San Andreas fault over the last 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% 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 Caltech 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 coauthor of the study.

The last one was in 1857, and had a magnitude near 8.

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

Earthquakes cause land on either side of the fault to slip horizontally, putting a kink, or offset, in creeks that had been flowing straight across the fault.

By mapping buried creek beds or gullies on either side of the fault, the researchers were able to determine offsets for quakes that occurred hundreds of years ago.

Five of the last six earthquakes caused the fault to slip 15 to 25 feet, and one caused a slip of about 5 feet.