California is earthquake country, haunted by the twin spectres of the 1906 San Francisco earthquake and a forthcoming ‘Big One’; but in reality, although it’s shaken by hundreds of earthquakes each week, most of them are relatively small, with only two events of magnitude 5 (M5.0) or greater occurring on land in the state since the beginning of 2014.
Bearing this in mind, the M4.2 which struck in the San Gabriel Mountains on 3 January 2014 is both noteworthy (in the day-to-day context) and unremarkable (in terms of the state’s potential for large tremors). But it does offer the opportunity to examine the tectonic context of the Los Angeles area and its seismic potential.
The January 4 M4.2 California Earthquake
The initial available information on the earthquake from the United States Geological Survey (USGS) indicates that the earthquake occurred at the relatively shallow depth of around 9km some 26km from Santa Clarita and the northern suburbs of Los Angeles.
The tremor was the largest in series of dozens of (mostly very small) earthquakes to strike the area between 3-4 January.
Early shake maps indicate that shaking was light to moderate within immediate are of the epicentre and that weak shaking occurred as far away as central LA, suggesting that the tremor may have being felt, albeit weakly, by millions of local residents. News reports suggest that there were no reported injuries and no damage a result of the earthquake.
The Tectonic Setting of California
California is dominated by the San Andreas Fault Zone, the margin between the Pacific and North American plates. The margin is a strike-slip zone, with lateral movement as the plates slide past one another obliquely, causing a zone of broadly parallel faults.
The July 3 tremor occurred at the western end of the Clearwater Fault, which lies between the San Andreas Fault itself and the broadly parallel San Bernardino Fault. Both of these are strike slip faults, while the Clearwater Fault itself is described as a reverse fault, implying that although it’s tectonically part of the wider San Andreas Zone, it’s different orientation and type of movement result from stresses between those two parts of the San Andreas zone.
Not Just San Andreas: The Los Angeles Area
But as seismologist Robert Yeats notes: ‘The San Andreas Fault is the principal tectonic displacement zone, but not the only one.”
The San Andreas Fault Zone runs the length of California, but in the region near Los Angeles, where the most recent earthquake occurred, the fault zone is intersected at an angle by the series of mountains known as the transverse ranges — mountains which are associated with a series of broadly northwest-south east trending faults.
As the associated maps show, the outcome of this is a highly complex network of active faults throughout the region — and a high number of minor earthquakes associated with them. Clearly, the many and complex stresses within the Earth’s crust in this region mean that we can reasonably expect that the Golden State will be shaking again, on many different faults, on plenty of other occasions in 2015.