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Shaken, not stirred…. 4.0 earthquake on the Hayward Fault

Oakland Hills, Bay Bridge

So, maybe THIS is what awakened me this morning…. Then I dozed for another half hour! 😉

Bay Area residents were jolted awake Monday morning when a 4.0 earthquake rattled the region.

The quake struck at 6:49 a.m. with an epicenter about one mile north of Piedmont along the Hayward Fault, according to the United States Geological Survey. Maps provided by the USGS indicate the quake was centered near Modoc Avenue east of Mountain View Cemetery in Oakland.

People reported feeling the temblor, which had a depth of about 3 miles, in San Francisco, further inland in the East Bay and in the North Bay. There were no immediate reports of damage.

Several aftershocks, the strongest of which was a 2.4, followed. A USGS Interactive map is here. The USGS report of the event is here. USGS says THIS about the Hayward Fault:

The Hayward Fault is a mostly right-lateral, strike-slip fault with approximately 5 mm/yr (1/5 inch/year) of creep. The 2003 Working Group for California Earthquake Probability, in agreement with previous working groups going back to 1990, assigned a slip rate on the Hayward Fault to be about 9 mm/yr (1/3 inch/year). This implies that approximately 4 mm/yr of motion is taken up in a stick-slip fashion, leading to the generation of earthquakes.

The Hayward Fault runs from San Pablo Bay in the north to Fremont in the south, passing through the cities of Berkeley, Oakland, Hayward, and Fremont. South of Fremont the fault branches into a complex set of surface faults that connect the Hayward Fault to the central part of the Calaveras Fault. The Hayward and Calaveras Faults may have a simpler connection at depths more than 5 km (3 miles), joining in the subsurface just south of the Calaveras Reservoir (site of the October 30, 2007 M5.4 Alum Rock earthquake). The Hayward Fault may be segmented into a northern and southern segment in the vicinity of Berkeley or Oakland.

SFGate.com reports that “Approximately 40,000 people felt strong shaking and 510,000 felt moderate shaking, according to estimates by the USGS. More than 4.5 million people likely felt weak or light shaking, officials said.”

The strongest earthquake that I experienced PERSONALLY in California was this one (later upgraded to a 5.7, I believe) on the Calaveras Fault that I blogged about in the fall of 2007! I was in my recliner, and it seemed to last for about 30 seconds! The earthquake magnitude scale is LOGARITHMIC, so you math wizards out there can figure out the difference between my 2007 quake and the one reported this morning.

-Bill at

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