An underground thunderstorm of seismic activity, officially called a “seismic swarm,” hit a farm town called Brawley in the Imperial Valley yesterday — gearing up around sunrise with some magnitude 2s and 3s, peaking with a magnitude 5.3 and 5.5 just after noon, and rolling into Monday morning with some magnitude 4s and 3s.

This, according to U.S. Geological Survey geophysicist Nancy King, who says the quakes are “still going” at this very moment. “There have been over 200 events so far,” she says.

This marks the first “significant” swarm in the area since…

… 2005, according to King. (USGS seismologist Lucy Jones, however, tells City News Service that “what we're seeing is a classic Brawley seismic swarm. We haven't seen one of these since the 1970s, and there was another one back in the 1930s.” So believe who you will.)

Peak of the swarm. (Click to enlarge.); Credit:

Peak of the swarm. (Click to enlarge.); Credit:

When the quakes peaked around 2 p.m., they could be felt as far south as Mexico, as far north as the L.A. County coast and as far east as Arizona, according to a nifty “Did You Feel It?” crowd-sourcing map on the USGS website (see left), which garnered thousands of responses.

Residents and businesses within Brawley itself, at the epicenter of the swarm, reported power outages and minor structural damages, including “20 mobile homes that suffered damage to their foundations,” “merchandise shaken from store shelves” and “part of a home's terra-cotta roof [that] collapsed,” according to the Los Angeles Times.

More quake science from City News:

“Our system is choking on so many earthquakes,” Jones said. “This area of California is deep soils, and we do not get as precise data as we do over the rest of the state, and that makes our data a little less precise.”

… Jones said the quake swarm was about midway between fault complex on the west side of the Imperial Valley, and the main branch of the San Andreas Fault, which runs from near Palm Springs to enter Mexico just west of Yuma. “These don't seem to be related to earthquakes on the San Andreas itself, other than in a general way,” she said. “It's pretty far away.”

The “swarm” concept is still something of a seismic mystery, King tells L.A. Weekly.

It more or less defies the theory that there's no such thing as earthquake weather, throwing the USGS computer system for a loop as each quake bleeds into the next. “Sometimes there are thousands of events in these swarms,” says King. “We don't know when it will stop… [and] we don't really understand why some places have swarms and others don't.”

One thing she can say: “There is a slightly higher probability of a larger event in the area when you have swarms like this.” (Let's just hope it won't be the Big One.)

On an unrelated but creepy note, a massive 7.3 magnitude earthquake hit El Salvador early this morning, as the SoCal swarm still rumbled away in the Imperial Valley. And in other weather-disaster news, Hurricane Isaac is threatening to dash New Orleans just days before the anniversary of the Katrina catastrophe.

“People need to remember: We live in earthquake country,” says King with the USGS. “Just like people in hurricane country have to be ready, we have to be ready for earthquakes.”

[@simone_electra / / @LAWeeklyNews]

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