On Tuesday 29 May 2012 slightly before 10.15 pm, local time, (slightly before 5.15 am on Wednesday 30 May, GMT), an Earthquake centered in the California Channel Islands, 85 km west of central Los Angeles City, shook the southern part of Los Angeles County, according to the United States Geological Survey, who measured the quake as 4.0 on the Richter Scale, at a depth of 16.4 km. There are no reported casualties or injuries, and residents of the areas where the quake was felt were reportedly unconcerned.
Map showing the areas where the quake was felt (shading), the point where it was centered (red star) and known geological faults in the area (red lines). USGS.
California lies on the boundary between two tectonic plates; the Pacific to the west and the North American to the East. These plates are moving past one-another, the Pacific moving to the north, the North American to the south along the San Andreas Fault (a transform fault). This passes to the north of Los Angeles, but the plates do not move past one-another cleanly, but drag on one-another causing friction, and leading to many smaller faults on either side of the plate boundary, such as the one upon which Tuesday's Earthquake occurred.
The San Andreas Fault. Geology.com.
See also Earthquake in the Gulf of California, Understanding the subduction zone beneath Mexico, Mexico shaken by major Earthquake, Hydrothermaly heated active seeps from the continental shelf margin, south of Costa Rica and Earthquake off the coast of Vancouver Island.
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