A Magnitude 4.5 Earthquake at a depth of 19.3 km occurred roughly 20 km off the coast of Eureka in north California at 7.25 pm local time on Friday 2 August 2013 (2.25 am on Saturday 3 August, GMT), according to the United States Geological Survey. This is a moderately large quake, but unlikely to cause any problems this deep and this far offshore, though it was felt throughout the Eureka area.
The approximate location of the 2 August 2013 Eureka 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 3.4 Earthquake in northern California, Magnitude 3.9 Earthquake in north California, Earthquake off the coast of Isla Vista, California, Northeast California shaken by Magnitude 5.7 Earthquake and Los Angeles shaken by Earthquake.
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