The U.S. Geological Survey reports more than 390 earthquakes with a magnitude greater than 1.0 have been recorded in Imperial County since Saturday evening. 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 want to grind past each other. The Pacific plate wants to move to the northwest, while the North American plate wants go southeast.
But 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.
So, could the quake swarm trigger something worse?
Past swarms have not triggered larger quakes.
But one theoretical concern is that last weekend’s swarm was located near the southern part of the San Andreas, southern San Jacinto and Imperial faults, and “all of those are capable of producing larger events.”
After the Easter Sunday earthquake in 2010, experts said the 7.2 quake in Mexicali placed pressure on two Southern California fault lines: the Elsinore and the San Jacinto, making quakes more likely. Sure enough, about three months later, a moderate 5.4 quake hit Southern California.
As for last weekend’s swarm, there is no evidence of any “triggered” quakes on nearby faults, said Lucy Jones, seismologist with the USGS.
By midday Monday, the quakes appeared to be decreasing in frequency.