Massive swarm of tiny quakes rattles California geyser field. What’s going on?

Don Sweeney
Updated ·3 min read
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A huge swarm of earthquakes below 2.5 magnitude in 24 hours rattled the Geysers Geothermal Field in California, the U.S. Geological Survey reports.

The scores of quakes follow a 4.4-magnitude tremor at 10:39 p.m. on Tuesday, April 11, followed by several aftershocks in nearby Cobb, McClatchy News reported.

While most of the latest swarm fell below 2.5 magnitude, there was a 3.9-magnitude tremor at 12:42 a.m. Thursday, April 13, the USGS reported. Dozens of people reported feeling the quake.

But the swarm of quakes continued through a 0.7-magnitude tremor that hit at 7:51 a.m. Thursday.

The Press Democrat of nearby Santa Rosa reported at least 100 quakes in the previous 24 hours on Wednesday, April 12.

Small tremors are common at the Geysers Geothermal Field, which is about 90 miles north of San Francisco, even though the area has no major fault lines, the USGS said.

“However, activities associated with the withdrawal of steam for producing electric power cause or induce small quakes to occur in the field,” the agency said.

Withdrawing steam and heat from the rock causes it to contract, which can induce earthquakes, according to one theory. Pumping cold water into the heated rock also may cause tremors.

Magnitude measures the energy released at the source of the earthquake, the U.S. Geological Survey says. It replaces the old Richter scale.

Quakes between 2.5 and 5.4 magnitude are often felt but rarely cause much damage, according to Michigan Tech. Quakes below 2.5 magnitude are seldom felt by most people.

What to know about earthquakes

Earthquakes’ sudden, rapid shaking can cause fires, tsunamis, landslides or avalanches. They can happen anywhere, but they’re most common in Alaska, California, Hawaii, Oregon, Puerto Rico and Washington, according to the Department of Homeland Security.

If an earthquake strikes, it’s best to protect yourself right away. Here are tips from experts:

  • If you’re in a car: Pull over and stop. Set your parking brake.

  • If you’re in bed: Turn face-down and cover your head with a pillow.

  • If you’re outdoors: Stay away from buildings. Don’t go inside.

  • If you’re inside: Stay and don’t run outdoors. Stay away from doorways.

The best way to protect yourself during an earthquake is to drop, cover and hold on, officials say.

“Wherever you are, drop down to your hands and knees and hold onto something sturdy,” officials say. “If you’re using a wheelchair or walker with a seat, make sure your wheels are locked and remain seated until the shaking stops.”

Be sure to cover your head and neck with your arms, and crawl under a sturdy table if possible. If no shelter is available, crawl to an interior wall away from windows.

Once under a table, officials say you should hold on with one hand and be ready to move with it.

“There can be serious hazards after an earthquake, such as damage to the building, leaking gas and water lines, or downed power lines,” officials say. “Expect aftershocks to follow the main shock of an earthquake. Be ready to Drop, Cover, and Hold On if you feel an aftershock.”

How are earthquakes triggered and how are their magnitudes measured? What to know