Monday, 27 August 2012

Earthquake swarm strikes southern California.

On Saturday 25 August 2012, at 8.30 am local time (3.30 pm GMT) southeastern California suffered three magnitude 2.5 Earthquakes within the space of a few minutes, the beginning of a swarm of quakes that would persist for several days, with several quakes exceeding magnitude 5.0 on Sunday 26 August. No casualties have been reported, but considerable damage to homes and businesses has been reported around the town of Brawly, where a hospital was also evacuated after repeated power outages.

Map showing southern California and adjacent states of Mexico and the United States. USGS.

The quakes occurred in an area known as the Brawly Seismic Zone, which lies between the Imperial and San Andreas Faults. The Imperial Fault is an extension of the East Pacific Rise, an area of sea-floor spreading that separates the Pacific from the Cocos and Nazca Plates, which runs under the Gulf of California as far north as Imperial County California. The San Andreas Fault is a transform fault which runs under most of the length of the state of California, separating the northward moving Pacific Plate on the west from the southward moving North American Plate to the east.

The Brawly Fault Zone seems to go through periodic bursts of activity, producing swarms of quakes that go on for a number of days, before settling down again. The most recent episodes were in 2005 and 1981; there were a number of episodes in the 1960s and 1970s. The nature of the swarms is not well understood, but it is thought that the Zone is beginning to undergo a process of extension.

Map showing the plate margins underlying the east Pacific and southern North America. Bangor University.

See also Los Angeles Shaken by Earthquake, Earthquake in the Gulf of California, Oaxaca region of Mexico struck by second major Earthquake, Mexico Shaken by major Earthquake and Earthquakes on Sciency Thoughts YouTube.

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