The United States Geological Survey recorded a Magnitude 3.5 Earthquake at a depth of 12.6 km, south of the community of Anza in Riverside County, California, at about 6.10 am local time (about 1.10 pm GMT) on Sunday 22 September 2013. This was followed by a second quake at roughly the same spot eight minutes later. Neither of these quakes was large enough to present any danger, though they were felt over a large area of southern California.
The approximate location of the 22 September 2013 Anza Earthquakes. 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 (for the first quake) or here (for the second quake).
See also Magnitude 2.6 Earthquake in Kern County, California, Magnitude 2.5 Earthquake in San Bernardino County, California, Magnitude 3.1 Earthquake to the north of the Gulf of California, Magnitude 3.2 Earthquake in southern California and Magnitude 4.3 Earthquake hits Kern County, California.
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