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. Most are minor, often too weak to be felt.