A Magnitude 3.9 Earthquake at a depth of 6.9 km occurred roughly 150 km southeast of San Francisco slightly after 8.50 pm on Thursday 27 June 2013, local time (slightly after 3.50 am on Friday 28 June, GMT), according to the United States Geological Survey. The quake is not reported to have caused any damage or injuries, but was felt across much of Silicon Valley and as far north as San Francisco. The quake was followed by a Magnitude 2.8 aftershock which came about 40 minutes after the initial quake.
The location of the 27 June 2013 California Earthquake. Google Maps.
California is extremely prone to Earthquakes due to the presence of the San Andreas Fault, a tectonic plate margin that effectively bisects the state. The west of California, including Santa Barbara and Los Angeles, is located on the Pacific Plate, and is moving to the northwest. The east of California, including Fresno and Bakersfield is on the North American Plate, and is moving to the southeast. The plates do not move smoothly past one-another, but constantly stick together then break apart as the pressure builds up. This has led to a network of smaller faults that criss-cross the state, so that Earthquakes can effectively occur anywhere.
Witness accounts of Earthquakes can help geologists to understand these events and the underlying structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) then you can report it to the United States Geological Survey here.
See also Earthquake off the coast of Isla Vista, California, Northeast California shaken by Magnitude 5.7 Earthquake, Los Angeles shaken by Earthquake, Central California shaken by Earthquake and Earthquake swarm strikes southern California.
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