M5.1 La Habra Earthquake: a Warning for Los Angeles?

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Recent earthquakes in California (last 30 days). Image credit: USGS

Recent earthquakes in California (last 30 days). Image credit: USGS

The magnitude 5.1 (M5.1) earthquake which struck close to La Habra, Los Angeles, on 29 March 2014 is yet another reminder of the high levels of seismic activity associated with the Golden State.

Although relatively modest in worldwide terms, the tremor was the largest to strike onshore in the state this year; the Los Angeles Times reported minor damage including rockslides, broken glass and fractured water mains, along with potential structural damage to a limited number of buildings.

Recent California Earthquakes

It’s well known that California is vulnerable to earthquakes, located as it is on the boundary between two of the earth’s major tectonic plates.

The Pacific and North American plates grind past one another along a boundary marked not just by the well-known San Andreas Fault, but by a series of other faults marking a broad zone of deformation which runs the length of the state. These faults have been the source of a number widely-reported tremors in recent weeks.

To the uninitiated, the apparent spate of earthquakes over the past month or so may make it seem as if the Golden State is shaking, rattling and rolling a little more than usual. In fact this isn’t the case.

California’s Department of Conservation notes that: “Each year, California generally gets two or three earthquakes large enough to cause moderate damage to structures (magnitude 5.5 and higher)” and a map of the earthquakes of at least M2.5 in the state over the past thirty days shows no unusual levels of activity beyond the large (but not unprecedented) M6.8 and its associated aftershocks off the north of the state.

A Closer Look at the M5.1 La Habra Earthquake

Surface and subsurface fault maps of LA. IImage created by Decoded Science, maps by USGS and Southern California Seismic Network

Surface and subsurface fault maps of LA – as you can see, the SCSN map shows much more detail, particularly when it comes to ‘blind faults.’ Image created by Decoded Science, maps by USGS and Southern California Seismic Network

The La Habra earthquake is remarkable not just for its size but for the fact that it occurred in a heavily-populated region on one of America’s largest cities.

Although California’s earthquakes are associated in the public consciousness with the San Andreas fault itself, as noted above the whole of the state is affected by deformation and faulting, even though the main San Andreas fault runs around the outskirts of LA rather than through its centre (unlike the situation in San Francisco, where the fault runs through the heart of the city).

Although maps show the major fault lines, they rarely show the true complexity of faulting below the city. USGS data suggest that it “could be associated with the Puente Hills thrust (PHT),” a northwards-dipping thrust fault (one in which compressional forces cause upwards movement).

The graphic shows the USGS map of the earthquake (taken from its real time earthquake map) in which the main fault shown is the Elsinore fault to the north, compared with a more detailed fault map produced by the Southern California Seismic Network.

The USGS map shows only those faults which appear at the surface while the SCSN map includes so-called ‘blind faults’ like the PHT – thrusts which don’t reach the surface but which are nevertheless capable of movement.

California’s Seismic Hazard

Seismic hazard in California. Image credit: USGS

Seismic hazard in California. Image credit: USGS

The key issue for the local population, then, is not just the magnitude of the earthquake, nor the main fault zone – it’s where there is risk from an earthquake, on any fault, which affects a significant number of people.

Obviously, magnitude is significant; but it’s only one component of seismic hazard.

One day the Big One will strike California and, as the USGS seismic hazard map shows, the risk is high for much of the state, both along and away from the San Andreas fault – but whether it hits on the Puente Hills Thrust or the San Andreas itself is, in a sense, irrelevant.

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