The United States Geological Survey recorded a Magnitude 2.5 Earthquake at a depth of 8.5 km on the boundary between San Diego and Riverside Counties slightly before 9.40 am local time (slightly before 4.40 pm GMT) on Thursday 26 September 2013. This is not a large quake, and is unlikely to have caused any damage or injuries, but is likely to have been felt locally.
The approximate location of the 26 September 2013 San Diego/Riverside County boundary 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 Magnitude 2.8 Earthquake in San Benito County, California, Two Earthquakes in Riverside County, southern California, Magnitude 2.6 Earthquake in Kern County, California, Magnitude 2.5 Earthquake in San Bernardino County, California and Magnitude 3.6 Earthquake in Inyo County, northern California.
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