According to catastrophe modelling firm AIR Worldwide, a swarm of low to moderate magnitude earthquakes has hit Southern California, approximately 30 miles north of the Mexican border. Concentrated in Imperial County a few miles north of the city of Brawley, CA (population estimated at around 25,000 in 2010), over 120 events of magnitude 2.5 or greater have struck since Sunday morning, local time. Due to the low magnitude of these earthquakes and California’ stringent building codes, insured losses are not currently expected to be significant.
The strongest of these—a M5.5 (depth of 9 km) and a M5.3 (depth of 12.3 km)—hit in the early afternoon on Sunday, according to the United States Geologic Survey (USGS). No deaths or serious injuries have been reported, and damage caused thus far has been minor. Data from the USGS indicates that the swarm activity has subsided since yesterday.
“A seismic swarm is defined by the USGS as a localized surge of earthquakes, with no one magnitude being conspicuously larger than all other magnitudes of the swarm,” said Dr. Bingming Shen-Tu, senior principal scientist at AIR Worldwide. “In contrast, earthquakes in a main shock/aftershocks sequence typically involve a large main shock, followed by much smaller aftershocks. The current earthquake sequence in Southern California is, therefore, considered by seismologists to be a swarm.”
“This swarm is located in the Brawley Seismic Zone, which connects the southern end of the San Andreas Fault to the Imperial Valley Fault Zone. The two major strike-slip faults in the north (San Andreas Fault) and south (Imperial Valley Fault) form a right-step in the Salton Sea area. Most of the events in this area have extensional or trans-extensional faulting mechanisms.”
Dr. Shen-Tu continued, “The Salton Sea and the Brawley Seismic Zone are known for volcanic and geothermal activity along the great San Andreas Fault Zone, a geological condition not conducive to the accumulation high seismic energy. Instead, low to moderate magnitude earthquake swarms are relatively common. The previous swarm was in 2005, and before that in 1981 (during which the largest recorded event in the Brawley Zone, a magnitude 5.8, occurred). Historically, swarm activity in this region has not been followed by large earthquakes on nearby faults.”
