Image: USGS Earthquake map

Central California is currently in the middle of an earthquake swarm, with up to 18 (and counting) tiny quakes shaking things up over the course of a single day. The situation is not, however, as ominous as it may seem.




KTLA reported that the quakes began in the Bay Area late last night and have continued through today. With nothing larger than a magnitude of 3.7, all of the quakes have been relatively small. But should the rising tally give alarm? Probably not. Central California is just in the middle of a run-of-the-mill earthquake swarm.

The precise mechanics of what sets an earthquake swarm off aren’t clear, but they’re pretty common in geologically active areas. The USGS defines them as simply a bunch of small earthquakes clustered around the same locale and time. There’s no set limits for either the time or area. In fact, the quakes in a swarm don’t even need to stem from the same fault lines.


Is a swarm a sign of something bigger on its way, though? Probably not. Although the question of whether swarms are a precursor to something larger is often asked, there’s no evidence linking them directly to larger quakes or eruptions so far. In fact, they usually just pop up in geologically-active areas and then just stop without incident.

Yellowstone National Park, for instance, is famous for being hit by regular earthquake swarms, including large ones in 2004, 2009, and again in 2010. The swarm in 2010 lasted for over a month and included more than 2,000 earthquakes over that period, several with a magnitude of over 3.0.

Despite how much bigger and longer that swarm was than California’s current on-going one, residents of the state should be comforted to know that Yellowstorm’s 2010 swarm simply petered out on its own, with no larger quake or event at the end.