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. But when several reach magnitude 3 or higher, residents notice — and worry.

What is an earthquake swarm?

An earthquake swarm is a burst of seismic activity in a confined area over a short period, typically without one dominant quake. Swarms are common near volcanoes and geothermal fields, where underground heat and fluids shift pressure inside the earth’s crust.

But they also occur in places without volcanoes, especially in geologically complex regions.

Why do they happen in non-volcanic areas?

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. “A fractured-up area with a lot of small faults. Sometimes they light up.”

Rather than one fault releasing stress in a single rupture, swarms reflect stress shifting across many small breaks in the rock.

Do earthquake swarms mean a big quake is coming?

This is the question people most often ask — and the answer is usually no.

Scientists say swarms can slightly increase the statistical likelihood of a larger earthquake, but only by a very small amount. In California, past swarms have not been followed by major quakes in the same areas.

“These things turn on and turn off,” Schwartz said. “They typically have not led to larger earthquakes. But they do scare the heck out of people.”

Importantly, swarms do not always “relieve” pressure on major nearby faults. In other words, they are not guaranteed to make a big earthquake elsewhere less likely.

What’s happening underground?

Scientists believe underground fluids can play a role in triggering swarms. In volcanic regions, magma can change pressure in cracks deep below the surface. In other areas, water moving through fractured rock may alter stress along small faults, allowing them to slip.

“There are a bunch of little cracks that are letting fluid flow and causing these small quakes,” said Sarah Minson, a research geophysicist with the U.S. Geological Survey. “It doesn’t look like the behavior you see with large earthquakes.”

Still, researchers emphasize that no single cause explains all swarms. Fault geometry, stress from nearby fault systems and the movement of fluids can all interact in complex ways.

Where else do earthquake swarms occur in California?

Several regions of the state experience swarms from time to time, including:

• The Geysers in Sonoma and Lake counties
• Mammoth Lakes in the eastern Sierra Nevada
• Brawley in Imperial County near the California-Mexico border

In some cases, swarms can last days; in others, weeks or even longer. Many involve hundreds of tiny earthquakes, with only a few strong enough for people to feel.

“Usually these clusters come and go and are over in a few weeks,” said Roland Bürgmann, a professor of earth and planetary science at UC Berkeley and a researcher at the Berkeley Seismology Lab.

Should people be concerned?

For most residents, the answer is reassurance rather than alarm.

“People probably shouldn’t be concerned,” Minson said. “Is this going to cause a ginormous earthquake? This place has had swarms before and nothing larger happened.”

Earthquake swarms are a reminder that California sits atop an active and complicated geological system. While they can be unnerving, scientists say they are typically part of the earth’s normal behavior — not a warning of an impending disaster.

What matters most, experts emphasize, is not reacting to any single swarm but staying prepared in earthquake country: securing heavy furniture, knowing safe places to take cover, and having an emergency plan in place for whenever the ground does move again.