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.