An earthquake of magnitude M4.4 (M4.4) struck the outskirts of Los Angeles in California at 9.35 am on January 15, 2014, with another quake of M3.2 on January 16th.
The L.A. quake occurred at a depth of just over 3km and had its epicentre close to San Bernadino. Initial reports from the United States Geological Survey indicate that the tremor, which occurred at depth of just 2.6km, was felt across much of the Los Angeles area.
UPDATE: Just 24 hours later, on January 16th, a second, smaller tremor struck in the same area, with a magnitude of 3.2 and at a depth of 1.6km. Although no detailed information is available, the location of the epicentre (just a few hundred metres away) suggest that the two events are related, with the second being an aftershock for the first.
LA Quake: Context of California’s Earthquakes
California is known for its earthquakes: The San Francisco earthquake of 1906 is notorious for the death and destruction it wreaked on the city, and it’s by no means unique, as the earlier earthquake of M7.9, which struck Fort Tejon in 1857, demonstrates. Tectonically speaking, California is bisected by the plate margin which separates the Pacific plate from the North American continent. Here the two plates are sliding past one another at varying rates.
Although it’s largely known as the San Andreas Fault, the plate margin is in fact a whole series of broadly parallel largely strike-slip (laterally moving) faults. These faults are characterised by slow movement, or creep, although as strain builds up it is released at intervals in earthquakes whenever a particular threshold (varying according to a host of geological and environmental factors) is exceeded.
No detailed information is available on the tremor of the 15 January, but a close look at fault maps suggests that it took place on one of the subsidiary faults to the San Andreas Fault; the San Jacinto fault.
The San Jacinto fault runs along the western edge of the Salton Trough (where movement is extensional rather than lateral) before joining the San Andreas close to San Bernadino. This Southern California fault is one of the most active in the region, and small earthquakes there are by no means uncommon.
California Earthquakes, Past and Future: Big Quakes
Much drama has been made of the California’s earthquake zone, and there’s frequent online speculation about the likelihood of the next ‘Big One’ to match the major earthquakes of the past. Realistically, and despite decades of research, it remains virtually impossible to predict the timing and magnitude of an individual earthquake, even though the areas which are vulnerable to such earth movements are well known and, in the case of California, extensively studied. Even smaller earthquakes are by no means certain precursors of ‘The Big One’, given that major earthquakes can, and frequently do, strike with little or no preceding seismic activity.
Instead, scientists rely on detailed models which generate probability forecasts based upon a range of geologic and geophysical data, along with information on past earthquakes. In 2008 the USGS produced a detailed forecast which suggested that “The probability of a magnitude 6.7 or larger earthquake over the next 30 years striking the greater Los Angeles area is 67%… For the entire California region, the fault with the highest probability of generating at least one magnitude 6.7 quake or larger is the southern San Andreas (59% in the next 30 years)”.
The current quake, and the many smaller ones which occur in California each year, is far too small to fulfil this forecast – an M6.7 is two order of magnitude larger. But this L.A. Quake does continue to illustrate the ongoing seismic activity in California – and the need for awareness and preparation.