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The USGS has so far recorded more than four dozen quakes in Imperial County, many near the city of Brawley. The first quake, a magnitude 3.9 event, struck at 10:02 a.m. (PST) northwest of Brawley and was followed by a series (swarm) of other quakes in the same general area, the USGS said in a statement.
As of 3 p.m. (PST), there have been no reports of serious damage or injuries, according to authorities in the region.
Some buildings in downtown Brawley did receive minor damage, however. Captain Jesse Zendejas of the Brawley Fire Department described the damage as "cosmetic" and said it occurred in at least three older buildings. Assessments were still ongoing as of press time.
Earthquake swarms are events where a local area experiences sequences of many earthquakes occurring over a relatively short period of time. The length of time used to define a swarm varies, but the USGS points out that an event may be on the order of days, weeks, or months, depending on circumstances.
Swarms are differentiated from aftershocks by the observation that no single earthquake can be attributed as the main shock. Earthquake swarms are generally grouped as events that precede volcanic eruptions in areas where volcanoes exist.
