4.0 Quake Rattles Bay Area Awake, Reminder of Hayward Fault Stress

It wasn't "The Big One" - not even close - but this morning's 4.0 earthquake rattled buildings and some nerves around the Bay Area this morning. The U.S. Geological Survey reported the temblor at 5:33 a.m. centered 5.5 miles below a country club in El Cerrito. Aftershocks of 2.0 and 1.1 magnitude followed.

Some in the East Bay say the quake jolted them awake. Others in San Francisco felt two sharp shakes. A computer glitch mistakenly registered the one quake as two, with a false report in southern Marin County near Belvedere. USGS quickly removed that report from its website.

History does give some indication that earthquakes can presage larger ones to come - "foreshocks", rather than aftershocks. But this one was probably not strong enough to be a foreshock.

"If we had a magnitude five, then a warning would go out that within the next 72 hours there's a 5-10% chance of something larger occurring," says Dr. David Schwartz, an earthquake geologist with USGS. "And this is based on California-wide earthquake staistics. So when we do see a magnitude-five, it is sometimes followed by something larger."

A five-pointer is about 31.6 times more intense than a four-pointer (see for yourself on the USGS's magnitude calculator). Schwartz says smaller quakes like today's just remind us of how high the pressure is in the Hayward Fault, considered overdue for a big one.

"This is a reminder to people living in the Bay Area," says Dr. Schwartz. "You know, we don't get earthquakes all that frequently. But when you have a (magnitude) four, it is felt over a fairly wide area, and it's a reminder that we will have larger earthquakes down the road. And people really should be serious about preparing (for) when that happens."

Berkeley residents may remember a series of earthquakes last year that struck near UC Berkeley's Memorial Stadium (which, ironically, is undergoing seismic retrofitting). Schwartz says those happened in a spot on the Hayward Fault known to be active, so it's hard to know exactly which smaller earthquakes will indicate larger ones to come.

"The Hayward Fault is a very, very complicated fault in terms of having parts of it that move all the time and parts of it that are stuck. And it's the stuck parts that really - that's where the stress builds up," Schwartz says.