It’s been a shaky week in California. Monday 10 March saw the north of the state rocked by the largest tremblor for years – an M6.8 off Cape Mendocino and its many aftershocks – and a couple of days later, Californians felt a tremor of M4.4 in the Sierra Nevada.
On Sunday 16 it was the turn of the populated coastal strip to feel the force of the planet with a relative tiddler of just M3.9 25km from Santa Clarita. And Monday 17 saw an M4.4 event strike just north of Santa Monica. Although there were no reports of damage or injuries at the time of writing, USGS maps indicate that the tremor was felt across much of Los Angeles.
California and the San Andreas Fault
The San Andreas fault and the many lesser faults which run broadly parallel to it comprise a major tectonic zone marking the geological boundary between the Pacific and North American plates.
The two plates are sliding past one another with the Pacific plate moving north relative to the North American. The sliding motion generates friction which is released as earthquakes.
Although in the common perception, earthquakes in California are generally associated with the San Andreas fault itself, the strains of the sliding continents cause extensive deformation along a broad area and seismic activity is widespread away from the main faults as well as along them.
This is the case in the tremor of 17 March; its epicentre was some distance from the physical expression of the fault itself and although there’s no detailed information available its shallow depth (around 8.5km) suggests that it was a locally triggered event rather than one directly associated with the San Andreas fault.
Very many earthquakes occur throughout California on a daily basis. A map of seismicity in the area over the last week clearly illustrates this.
It’s possible to pick out the fault lines in the state; but those lines are not the only locations for tremors. (And, incidentally, the line of earthquakes along the western margin of the Sierra Nevada indicates another smaller tectonic block).
The USGS estimates that there are around half a million detectable earthquakes worldwide each year, of which around 2% occur in the southern part of California. Humans feel very few of these, which means that tremors of the magnitude of the March 16 Santa Clarita tremor, though small, are noteworthy.
Plate Movement, Larger Quakes
Of course, the movement of the plates is capable of generating very much larger earthquakes, though thankfully these are relatively rare. Nevertheless, USGS records show that there were at least 60 tremors of at least M6.0 between 1812 and 2010 in California. The largest and most devastating earthquakes occur at or near the major faults.
Editorial Note: The author has updated this article to include information on the M4.4 quake of March 17, 2014.