Why earthquake swarms happen and what they mean for California

"Earthquakes usually strike without warning. But sometimes they come in clusters dozens or even hundreds of small quakes concentrated in one area over days or weeks. Geologists call these clusters earthquake swarms, and while they can be unsettling, scientists say they rarely signal that a major quake is imminent. Unlike the familiar pattern of a single large earthquake followed by aftershocks, swarms consist of many small quakes without a clear mainshock."

"In many parts of California, the earth beneath the surface is fractured by networks of small, sometimes unnamed faults layered between larger fault systems. Pressure from those major faults can cause these smaller faults to activate in rapid succession, producing a flurry of minor earthquakes. It's like dropping a piece of pottery and having pieces of different sizes spread across the floor, said David Schwartz, a geologist and scientist emeritus with the U.S. Geological Survey."

Swarms are bursts of seismic activity concentrated in a small area over days to weeks, typically lacking a single dominant mainshock. They are common near volcanoes and geothermal fields where shifting heat and fluids change subsurface pressure, but also occur in geologically complex, non-volcanic regions. Networks of small, often unnamed faults between larger fault systems can activate in rapid succession as stress transfers, producing many minor quakes. Most swarm events are minor and imperceptible; when several reach magnitude 3 or higher, people notice and become concerned. Swarms only rarely precede a major earthquake.

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