This story was updated at 8:20 a.m.
Many Bay Area residents got an early Monday morning wake-up jolt when they were shaken by an earthquake with preliminary magnitude of 4.0 centered on the Hayward Fault in El Cerrito.
Coordinates released by the U.S. Geological Survey place the epicenter just south of the next to the Hayward Fault, which runs through the East Bay hills. An earlier USGS report placed the epicenter at the golf course.
The agency reported the shaking as two separate quakes just seconds apart, with the first one registering 2.9-magnitude at 5:33 and 12 seconds at the golf course, and the 4.0-magnitude quake striking eight seconds later.
Two apparent aftershocks also were reported this morning by the USGS – one 2.0-magnitude at 6:03 a.m. at the golf course and another measuring 1.1-magnitude at 6:29 a.m. just northeast of the golf course on Vista Heights Road.
Within 45 minutes of the temblor, El Cerrito police said they have had no reports of damage.
As often happens in moderate-sized quakes, 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.
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