M4.6 Earthquake in California and its Aftershocks


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Location and setting of the July 5 M4.6 earthquake and its aftershocks. Image credit: USGS

Location and setting of the July 5 M4.6 earthquake and its aftershocks. Image credit: USGS

The earthquake of magnitude 3.1 (M3.1) which struck on 7 July 2014 is the latest in a series of aftershocks which followed the tremor which struck in the San Bernadino mountains two days previously.

At the time of writing, the sequence of tremors, centred on an area around 10km from Running Springs and around 200km from the Los Angeles conurbation, included over 100 individual tremors.

These tremors, ranging from those of M0.5, which register on seismometers but are not felt, to the largest shock of M4.6 which was felt across the Los Angeles area and beyond.

Tectonic Setting of the California Earthquake and its Aftershocks

California is known for its earthquakes, strongly associated in the public mind with the San Andreas fault zone where the Pacific plate slips past the North American plate, with associated stresses, deformation – and earthquakes. (Notoriously, it was an earthquake on the San Andreas fault which destroyed much of San Francisco in 1906.) But the forces associated with continental converging can generate deformation away from the narrow corridor of the fault itself.

The most recent series of earthquakes occurred some way from the main fault zone, within the San Bernadino mountains, which are one of several crustal blocks which have accreted to create what we know today as the state of California. The San Andreas fault zone bounds the mountains to the south west and there are several other faults around its margins.

There’s little available information on the earthquake of 5 July and its aftershocks but from their location and their depth (around 10km) it seems likely that they are the result of faulting within the block as a result of broader deformation, rather than a result of movement along the notorious San Andreas itself.

California: Earthquake Central

A week's worth of earthquakes in California. Image credit: USGS

A week’s worth of earthquakes in California. Image credit: USGS

The ‘Big One’, the major quake which threatens large parts of California’s urban sprawl, is a very real probability – but the most recent outbreak is unlikely to be a precursor of it. In fact, an examination of the numbers of earthquakes to strike the Sunshine State reveals an astonishing number.

In a single day the United States Geological Survey’s real time earthquake map reveals literally hundreds of tremors in the state. Over the course of a week (at the time of writing) the number exceeds a thousand. Many of these are barely detectable; but in the 30 days preceding the most recent tremors in the San Bernadino mountains, over a hundred earthquakes of M2.5 or more appeared on the map. And, according to the USGS, southern California can expect to experience over 10,000 tremors annually.

California Shakin’ – Should We Be Worried?

On the basis of the most recent activity, it appears that there’s little reason to fear an imminent major ‘quake (although of course this does not rule out the occurrence of a quake triggered quite separately). But tremors are common for the people of California – even if most of them are so small they aren’t even noticed.

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