San Francisco skyline and Bay Bridge at sunset, California
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A magnitude 4.3 earthquake with its epicenter centered near Berkeley shook the entire Bay Area, according to U.S Geological Survey. It was the largest seismic event in the area since 2007. The earthquake happened in the early morning hours, waking up an estimated 22,000 people, but no injuries or damage were reported.
The San Andreas Fault is a roughly 800-mile (over 1.200 kilometers) fracture that runs much of the length of California and crosses the Bay Area. It is capable of producing magnitude 7+ earthquakes, powerful enough to cause widespread destruction. Yet it is not the only fracture separating the North American Plate from the Pacific Plate, and Monday’s earthquake happened along the Hayward Fault.
Like its more famous neighbor, the Hayward Fault is a strike-slip fault with approximately 5 millimeters/year (1/5 inch/year) of sideways creep.
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, another fault running parallel to the San Andreas Fault.
San Francisco Bay Area with major fault systems between Pacific and North America Plate.
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Geologic studies on the southern segment of the Hayward Fault have shown that the average interval between past earthquakes has been 140 to 170 years, yet with a large uncertainty of over 50 years.
The most recent major earthquake on the Hayward Fault occurred in the early morning hours on 21 October 1868. With an epicenter in the heart of the Bay Area, then having the largest population on the west coast with a total of 260,000 residents, this earthquake was one of the most destructive in California history, and remains the nation’s 12th most lethal earthquake. Property loss was extensive and 30 people were killed. Five deaths were reported in San Francisco, out of a population of 150,000, where the total property loss was estimated to be $350,000 (in 1868 dollars). This earthquake was known as the “great San Francisco earthquake” until the magnitude 7.9 shock on the San Andreas Fault on 18 April 1906.
