Swarm

It has been almost a year since I predicted a major earthquake would hit California in the near future -http://boggythicket.blogspot.com/2011/09/earthquake-prediction.html – and no, it hasn't happened yet.

This week, though, Southern California has experienced an "earthquake swarm” – something I didn’t know existed.

More than 400 earthquakes greater than magnitude 1.0 have been recorded in Imperial County since Saturday evening, said U.S. Geological Survey geophysicist Elizabeth Cochran. The largest were a 5.3 and a 5.5 about midday Sunday.

Scientists say the reason is not fully understood, but there is a clue: Earthquake faults work much differently south of the Salton Sea than they do closer to Los Angeles.

Take, for instance, the San Andreas fault as it runs through Los Angeles County. It’s a fault where, generally speaking, two plates of the Earth’s crust are grinding past each other. The Pacific plate is moving to the northwest, while the North American plate is pushing to the southeast.

South of the Salton Sea, the fault dynamic changes. The Pacific and North American plates start to pull away from each other, Cochran told The Times from her Pasadena office. (That movement is what created the Gulf of California, which separates Baja California from the rest of Mexico.)

So Imperial County is caught between these two types of faults in what is called the Brawley Seismic Zone, which can lead to an earthquake swarm, Cochran said.

The  last major swarm was in 2005, Cochran said, when the largest magnitude was a 5.1. The largest swarm before last weekend's occurred in 1981, when the biggest quake topped out at 5.8. Before that, there were swarms in the 1960s and 1970s.

The Imperial County Board of Supervisors declared the emergency after getting more information about damage. According to the Imperial Valley Press, 19 mobile homes have been red-tagged, several buildings -- including a school auditorium -- have been closed because of damage and water pipes broke. There has been no total damage estimate.

Most of the damage has been cosmetic, and of course a lot of stuff fell off of shelves.