Geological Origins of the Southern California Earthquake Swarm of August 2012


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Earth tremors are common: seismic activity in Southern California in 2012. Image courtesy of USGS

Two earth tremors measuring magnitude 5.5 and 5.3 (M5.5 and M5.3) rocked Southern California on August 26, 2012.

A slightly smaller tremor (M5.2) also took place on 27 August some distance to the south in the Gulf of California.

The epicentre of the largest shock was just 3 miles from the town of Brawley in California, and although at the time of writing no deaths or serious injuries has been reported, the Los Angeles Times reported that some buildings have been damaged.

Latest data from the United States Geological Survey show that these tremors were accompanied by dozens of minor tremors (foreshocks and aftershocks) of between M2.0 and M5.0, and it is likely that aftershocks will continue to occur.

Southern California Quakes: Seismic Setting

Located on the boundary between two of the Earth’s tectonic plates, Southern California is seismically active and earthquakes of such magnitude are by no means unusual or unexpected.

The earthquake swarm of the weekend of 26-27 August occurred in the Salton Trough, at the southern end of the notorious San Andreas Fault Zone. Its focus was close to the point where the plate boundary changes in nature.

Plates moving apart have created the Gulf of California. Image courtesy of Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC

To the north, the boundary is a transform fault along which the Pacific and North American continental plates move laterally relative to one another, causing friction and generating earthquakes. To the south, the boundary is a constructive boundary, where an ocean ridge (the East Pacific Rise) effectively reaches the shore. Here, rather than moving past one another, the two plates are moving apart. Extensional forces, which occur when the plates move apart, cause faulting and earth movements result. It is these extensional movements that have caused the Saltoun Trough.

Glancing at a map of Mexico and California makes it easy to see where this rift occurs – the ridge runs up the middle of the Gulf of California. Over a very long time, this process is likely to split Baja California away from the main continent, with the Gulf eventually evolving into a new ocean.

Earthquake History of Southern California

As David Alles of Western Washington University notes in the paper, Geology of the Salton Trough, “this is an area with a high level of historical seismicity, and also it has recently been seismically active.” Northern California has also suffered notable major earthquakes along the San Andreas Fault zone, including those at San Francisco in 1906, and Fort Tejon in 1857.

The largest recent earthquake to strike southern California occurred in 2010 at Sierra El Mayor: at M7.2 it was many times larger than the largest shock of August 2012. Other significant earthquakes in the area have occurred in 1940 (M6.9), 1934 and 1915.

West Coast Earthquakes

The whole of the western coast of North America is prone to repeated earthquakes – a product of the major and dynamic processes which move the earth and create continents, mountain ranges and oceans. The events of 26-27 August 2012, though noteworthy, are by no means unusual in the geological history of the region, and are playing a part in shaping a continually-evolving landscape.


Alles, David L. (ed). Geology of the Salton Trough. (2011). Western Washington University. Accessed August 27, 2012.

Los Angeles Times. California earthquake swarm ‘pretty bad,’ buildings damaged. (2012). Accessed August 27, 2012.

USGS. Latest Earthquakes in the World – Past 7 days – Latest Earthquakes Magnitude 2.5 or Greater in the United States and Adjacent Areas and Magnitude 4.5 or Greater in the Rest of the World – Last 7 days. (2012). Accessed August 27, 2012.

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