What are earthquake swarms? Why do they occur and what they mean for large earthquakes

Earthquakes usually strike without warning. But sometimes they come in clusters — dozens or even hundreds of small earthquakes concentrated in one area over days or weeks. Geologists call these groups “earthquake swarms,” ​​and although they can be alarming, scientists say they rarely indicate that a major earthquake is imminent.

Unlike the familiar pattern of one large earthquake followed by aftershocks, swarms consist of many small earthquakes without an obvious main shock. Most are minor, often too weak to be felt. But when a number of them reach 3 degrees or higher, residents notice this and become concerned.

What is an earthquake swarm?

An earthquake swarm is a wave of seismic activity in a confined area over a short period, usually without a single dominant earthquake. The swarms are common near volcanoes and geothermal fields, where underground heat and fluids change the pressure within the Earth's crust.

But they also occur in places where there are no volcanoes, especially in geologically complex areas.

Why do they occur in non-volcanic areas?

In many parts of California, the subsurface land is fractured by networks of small, sometimes unnamed, faults sandwiched between larger fault systems. The pressure from these larger faults can activate these smaller faults in rapid succession, triggering a wave of minor earthquakes.

Related: A 4.2-magnitude earthquake shakes the northern Gulf

“It's like dropping a piece of pottery and having pieces of different sizes all over the ground,” said David Schwartz, a geologist and scientist emeritus with the US Geological Survey. “A broken area with lots of little imperfections. Sometimes it lights up.”

Instead of a single fault releasing stress in a single rupture, swarms reflect the transfer of stress across many small breaks in the rock.

Do earthquake swarms mean a big earthquake is coming?

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

Scientists say swarms can slightly increase the statistical probability of a larger earthquake, but by a very small amount. In California, previous locust swarms have not been followed by major earthquakes in the same areas.

“These things turn on and off,” Schwartz said. “It usually doesn't lead to bigger earthquakes. But it scares people really bad.”

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

What's happening underground?

Scientists believe that underground fluids could play a role in triggering the swarms. In volcanic areas, magma can change pressure in cracks deep beneath the surface. In other areas, water moving through broken rocks may change the pressure along small faults, allowing them to slide.

“There are a bunch of tiny cracks that allow fluids to flow and cause these small earthquakes,” said Sarah Minson, a research geophysicist at the USGS. “It doesn't look like the behavior you see with large earthquakes.”

However, researchers stress that there is no single cause that explains all swarms. Fault geometry, stress from nearby fault systems, and fluid movement can interact in complex ways.

Where else do earthquake swarms occur in California?

Several areas of the state see crowds from time to time, including:

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

In some cases, swarms can last for days; In other cases, weeks or even longer. Many involve hundreds of small earthquakes, and only a few are strong enough for people to feel them.

“Usually these clusters come and go and are over within a few weeks,” said Roland Borgmann, a professor of Earth and planetary sciences at UC Berkeley and a researcher at the Berkeley Seismic Laboratory.

Should people be worried?

For most residents, the answer is reassurance, not alarm.

“Maybe people shouldn't be worried,” Minson said. “Will this cause a massive earthquake? This place has had swarms before and nothing bigger has happened.”

Earthquake swarms are a reminder that California sits atop an active and complex geological system. Although they may be unnerving, scientists say they are part of the Earth's natural behavior, and not a warning of impending disaster.

Experts stress that the most important thing is not to respond to any single swarm, but rather to remain prepared in earthquake country: securing heavy furniture, knowing safe places to hide, and developing an emergency plan whenever the ground moves again.

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