The earth trembled once more in Los Angeles today, with the occurrence of a magnitude 4.2 (M4.2) tremor which struck in the hills north of Santa Monica in the early hours of 2 June 2014.
The NBC news channel reported that the tremor was felt across several areas of northern Los Angeles, a fact which was borne out by the United States Geological Survey’s shake map for the event.
Tectonic Setting of the 6 June Earthquake
Tectonically speaking, California is always associated in the popular perception with earthquakes along the San Andreas Fault – and, in particular, with concerns about the next ‘big one.’
Almost inevitably, the true situation is more complex. The San Andreas Fault Zone marks the boundary between the North American and Pacific plates, with the latter moving northwards relative to the former.
But in fact, California is geologically the product of a coming together of many different slivers of crust – and these have generated their own stresses and strains, as the map of faults in the area shows.
The main San Andreas Fault crosses the top right if the image, and other faults bound the Santa Gabriel and Santa Monica mountains, alongside yet more which criss-cross much of the state.
This most recent earthquake, which occurred at a depth of just 4.5km, is not directly associated with any individual fault. However, the quake’s epicentre lies just to the north of the junction between the Malibu Coastal Fault, which extends offshore, and the Hollywood Fault which marks the northern margin of the Los Angeles tectonic basin.
Such faults are capable of producing large earthquakes, although they rarely do so. Caltech’s Earthquake Data Center suggests that the Hollywood Fault is capable of producing a tremor of magnitude M5.8 or larger – though also noting that the time between major events is of the order of 1600 years.
Seismicity of the Los Angeles Region
With such a complicated network of stresses in the rock, it’s unsurprising that California is frequently shaken by minor earth tremors. In fact Caltech’s most recent weekly earthquake report, published on 29 May, includes ten tremors of at least M2.0 in southern California – a fairly typical figure, and one which excludes the very many smaller ones which also occurred.
March saw another large tremor in Los Angeles, with an M5.1 in La Habra, which was followed by numerous smaller aftershocks. Although this and the most recent event are relatively close in terms of the overall fault zone, a glance at the fault maps shows that they are tectonically distinct and it’s therefore unlikely that they are related.
California as a Whole: the Big One?
A major fault zone such as the San Andreas is capable of producing major, often damaging, tremors of M6.0 or greater – and indeed has done so, with the notorious San Francisco earthquake of 1906 standing out among them (though it wasn’t the largest recorded; that distinction belongs to the M7.9 which struck Fort Tejon in 1857).
Speculation about the next ‘Big One’ is regular and common in the media. But the San Andreas is only one of many active faults in California and seismicity maps show that Los Angeles itself may be more at risk from ruptures along other fault zones. Generally speaking, however, although LA will continue to shake on a regular basis, the majority of the earthquakes which occur will be small.