Image of earthquake damage

Earthquake swarm in California is reason for caution but not panic

An unusual cluster of hundreds of smaller earthquakes has shaken Southern California in recent days. Fortunately no lives have been lost, and the damage caused by this tremor event has been limited. But is it a sign of worse things to come?

After several hundred earthquakes rocked an area south of the Salton Sea in southern California during the last week of August 2012, the message from U.S. Geological Survey (USGS) scientists was that we shouldn't worry too much. This event is an "earthquake swarm", a name given to a sequence of quakes that doesn’t fit the usual pattern of a sudden “mainshock” followed by a rapid decay of “aftershocks”. Instead, this occasional phenomenon is more like the build-up and fading of traffic on the freeway around rush hour.

USGS seismologists aren’t so worried about the recent swarm for a couple of reasons: firstly, because the earthquakes are not really getting bigger or smaller; if things look relatively stable, there’s a good chance they will stay that way. Secondly, this particular area of southern California is known to be tectonically a little different. The Salton Sea, which is itself below sea level, sits on top of a relatively thin crust and lithosphere. This leads to a slightly higher rate of heat escaping from the earth than in surrounding regions. Higher heat flow has been linked to increased flow of fluids within the earth’s crust as well as more stable styles of deformation, which have both been related by researchers to earthquake swarms.

So, should we be worried about the swarm in Southern California?  There’s no real reason to be any more worried than normal, since this type of thing happens quite frequently. However, there are also plenty of reasons to be cautious. Maybe you remember the 2009 L’Aquila earthquake in Italy that killed over 300 people. By some standards, this could have been termed part of an earthquake swarm as it was preceded by a period of increased seismic activity. A statement made shortly before the earthquake by Italian scientists seeking to reassure the public was later used against them, and they are currently standing trial for manslaughter. No doubt the USGS scientists have been very careful with their words when explaining the Salton Sea swarm.

In addition to the lesson from Italy, we must also take into account that the Southern San Andreas Fault, just to the north of the current earthquake swarm, has been referred to as “ten months pregnant” by professor Tom Rockwell of San Diego State University. It’s therefore easy to find reasons to err on the side of caution. Every earthquake shifts around the elastic forces that are stored in the Earth’s crust. Sometimes this leads to nothing and sometimes this leads to larger earthquakes. One thing to bear in mind, though: the San Andreas fault cannot remain pregnant forever.

There is a noticeable difference in the distribution of earthquake magnitudes over time when comparing a typical earthquake sequence to an earthquake swarm

Map from the U.S. Geological Survey of earthquakes in the Salton Sea area recorded during the seven days prior to the 30 August 2012. Red lines show the major active faults in the area.

Published 2 September 2012

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