Chance of Big Bay Area Quake in Next 30 Years Is 70%, Study Says

There is a 70% chance that a large earthquake will shake the San Francisco Bay Area in the next 30 years, according to a report released Thursday.

The study, by the U.S. Geological Survey, defines “large” as an earthquake at or above magnitude 6.7, the strength of the 1994 Northridge quake that killed 57 people and caused $20 billion in damage in Southern California.

The study’s conclusion: There are a number of faults slicing through heavily developed areas around San Francisco, more chances that they will rupture, and more people who will be affected when they do.

“We’re at ground zero. We’ve had large earthquakes. We will have large earthquakes in the future and the only question is where, when and how large,” USGS geologist David Schwartz said as he announced the findings at an earthquake conference.

The new estimate, which comes with an uncertainty factor of plus or minus 10%, is more dire than earlier predictions because it spreads the earthquake hazard over a much broader region, from the Pacific Ocean to the Sacramento Delta, about 40 miles inland.

“There’s really no escape,” Schwartz said.

Much of that area has seen rapid development since 1989, when the Loma Prieta earthquake, centered near Santa Cruz, shook the area with a 6.9 magnitude quake just as a World Series game in San Francisco was about to start.

In 1990, the USGS determined there was a 67% chance that a magnitude 7 or greater quake would strike the region before 2020.

However, Schwartz said that can’t really be compared with the new forecast because the latest figures include factors that weren’t looked at in 1990. Those include slippage on faults in the absence of earthquakes and the effect of the 1906 earthquake in reducing activity throughout the region.

The new study focused on quakes of 6.7 or greater because of the havoc caused by the Northridge quake. It also analyzed five additional faults.