A swarm of shallow earthquakes is being registered under the city of Brawley, California since December 31, 2016. Mayor Sam Couchman said the quakes have placed the city of 26 000 on edge.
By 15:25 UTC on January 2, 2017, more than 260 small to moderate earthquakes have been registered by the USGS.
The strongest so far was M3.9 which struck directly under Brawley at a depth 1.6 km (1 mile) at 23:06 UTC on December 31, 2016. Brawley is about 273 km (170 miles) SE of Los Angeles.
Caltech seismologist Egill Hauksson said the earthquakes are being registered 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."
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 48.3 km (30 miles) from the city of Brawley, across the Salton Sea’s southern half, and ends near Bombay Beach.
Hauksson was closely monitoring the swarm that began Saturday, as there was a chance that an earthquake of magnitude 5 or larger could be triggered, The Los Angeles Times reports.
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.
“There’s always reason to be concerned for a bigger earthquake,” Hauksson said. But by Sunday night, the possibility of the swarm triggering a larger event had largely receded.
The southern Brawley Seismic Zone is close in proximity to the Imperial fault which caused two major earthquakes in recent decades - M6.5 in 1979 and M7.1 in 1940.
The last major earthquake to hit Brawley was M5.4 in 2012.
According to the USGS, catastrophic earthquakes seem to strike along the southern San Andreas Fault about once every 150 years. The estimate was based on studies examining the past 1 400 years.
The last time a powerful earthquake on the San Andreas fault struck Southern California was in 1857.
Below is a list of all earthquakes near Brawley since December 27, 2016:
Image credit: USGS
Data source: USGS (updated 15:27 UTC on January 2, 2017)
Featured image credit: USGS