A swarm of more than 250 small earthquakes has struck since New Year’s Eve near the California-Mexico border, causing unease among residents and attention from scientists.
The earthquakes struck in the southern end of the Brawley Seismic Zone, a region that connects the San Andreas and Imperial faults, which can produce major earthquakes.
The earthquakes struck in the southern end of the Brawley Seismic Zone, a seismically active region where tectonic plates are moving away from each other and the Earth’s crust is getting stretched out “and basically adding land,” said Caltech seismologist Egill Hauksson.
The Brawley Seismic Zone is particularly important to watch because it is the region that connects the San Andreas and Imperial faults, both of which can produce damaging earthquakes. The seismic zone extends for about 30 miles from the city of Brawley, across the Salton Sea’s southern half, and ends near Bombay Beach.
The southern Brawley Seismic Zone is close in proximity to the Imperial fault. The Imperial fault has caused two major earthquakes in recent decades.
In 1979, a magnitude-6.5 earthquake sent violent shaking into El Centro, injuring 91 and causing so much damage to the concrete Imperial County Services Building that it had to be demolished.
The magnitude-7.1 earthquake that hit El Centro in 1940 claimed nine lives and swayed buildings as far away as Los Angeles. Irrigation systems were damaged, and railroad tracks were left warped where they crossed the fault.
Earthquake swarms that occur in the other end of the Brawley Seismic Zone — to the north — could trigger a major event on the San Andreas fault, one of California’s most dangerous, that could send catastrophic shaking into Riverside, San Bernardino and Los Angeles counties.
In late September, one such swarm began in the northern Brawley Seismic Zone, with three measuring above magnitude 4. That event led the U.S. Geological Survey to warn that chances of a magnitude 7 or greater earthquake on the San Andreas fault had risen as a result of the swarm.
Another swarm of small earthquakes, topping out at magnitude 3.5, struck the town of Niland near the eastern shore of the Salton Sea on Halloween.
Brawley Mayor Sam Couchman said the earthquakes have placed the city of 26,000 on edge since Saturday afternoon.
“We’re just kind of listening to it, and when you can hear it coming, it’ll rattle things. Last night, we had the rain, the earthquakes, and the fireworks. All we needed were frogs and locusts.”