Earthquake Centered in El Cerrito Rattles East Bay

Many Bay Area residents received an early morning wake-up jolt today, Monday, at 5:33 when they were shaken by an earthquake with preliminary magnitude of 4.0 centered in El Cerrito.

Coordinates released by the U.S. Geological Survey place the epicenter at the along the Hayward Fault, which runs through the East Bay hills.

Within 45 minutes of the tremblor, El Cerrito police said they have had no reports of damage.

As often happens in moderate-sized quake, BART trains were halted temporarily and service soon resumed.

The USGS gave the quake a preliminary 4.0 magnitude at a depth of 5.5 miles.

"Boy did we feel that one!" El Cerrito resident Larry Craighill said in a message posted on Patch. "The first bump woke me up, and the second had our hearts racing."

Craighill offered an explanation of how the USGS quickly traces the epicenter: "The bump was the P (pressure) wave, and the second was the S (shear) wave. The time delay between the two is how the USGS calculates the distance from the epicenter. Once they have three readings, they can pinpoint the location in three dimensions."

The Hayward Fault is considered by many seismologists to be a prime candidate for a major earthquake. The 2010 update of the report, "On Shaky Ground," by the Association of Bay Area Governments, reported that there's an estimated 62 percent probability of a destructive quake of magnitude 6.7 or greater in the Bay Area in the years 2003-2032.

Responses to this morning's quake sent to the USGS "Did you feel it?" Web page came from as far away as San Luis Obispo and Eureka.

Please tell what you experienced in the comments below.