A swarm of at least six earthquakes reaching up to magnitude 2.9 rattled San Ramon near San Francisco, the U.S. Geological Survey reports.

The other quakes in the Saturday, Dec. 13, swarm ranged from magnitude 1.3 to 2.3, according to the USGS.

San Ramon earlier was hit by a swarm of more than 90 quakes starting Nov. 9, The Sacramento Bee previously reported.

The latest series of quakes, including the 2.9 temblor, began at 11:35 a.m. Saturday, Dec. 13, the USGS reported. More followed over the next several hours, with the most recent striking at 7:51 a.m. Sunday, Dec. 14.

The 5-mile deep 2.9 quake hit at 1:44 p.m., according to the USGS.

More than 40 people from as far away as San Francisco and Oakley reported feeling the tremors to the agency.

What to know about earthquakes

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 magnitude 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.

Earthquakes’ sudden, rapid shaking can cause fires, tsunamis, landslides or avalanches.

Quakes can happen anywhere, but they’re most common in Alaska, California, Hawaii, Oregon, Puerto Rico and Washington, according to the U.S. Department of Homeland Security.

What is an earthquake swarm?

An earthquake swarm is a series of small quakes that “may occur in a region for months without a major earthquake, according to Britannica.

Such swarms are commonly associated with volcanic activity but can occur elsewhere.

How are earthquake swarms different from aftershocks?

Aftershocks are a series of smaller quakes that typically follow a larger tremor, the U.S. Geological Survey said.

The magnitude of the smaller shocks, which normally start about an order of magnitude below the main earthquake, tends to fall off over a period of time from weeks to even decades.

“However, not all earthquake sequences follow this pattern,” the U.S. Geological Survey said.

In a swarm, there’s often no identifiable main shock or the largest quake may take place in the middle of the swarm, the agency said.

Also, many of the quakes may be around the same magnitude, geologists said.

“Sometimes sequences don’t decay over time, but rather remain steady or even increase in their activity rates over periods of days, weeks, or even months,” the U.S. Geological Survey said.

There’s no precise technical division between aftershocks and swarms, though.

“In reality, the distinction is not sharp,” the agency said.

What causes earthquake swarms?

Researchers are still trying to figure out what causes earthquake swarms, the U.S. Geological Survey said, but they have a few ideas.

“Based on past research, we understand that swarms probably indicate that an extra ‘ingredient’ is involved where the earthquakes are happening — an ingredient that isn’t as prevalent in mainshock-aftershock sequences,” geologists with the federal agency said.

That special factor could be a fault that is slipping slowly, popping off “small, sticky patches” — or magma pushing up through the Earth’s crust, the U.S. Geological Survey said.

Most often, quake swarms are caused by fluids — typically water — interacting with fault lines, the federal agency said.

A fault slipping in an earthquake becomes permeable and when it’s saturated by water, can result in further slipping and more tremors, geologists said.

“We still have much to learn about earthquake swarms and their underlying physical processes,” the USGS said.

What is happening in San Ramon?

The San Ramon earthquake swarm is one of at least six such recorded swarms in the community dating back to 1970. It’s probably a result of fluid interacting with earthquake faults, SF Gate reported.

“This has happened many times before here in the past, and there were no big earthquakes that followed,” Sarah Minson, a research geophysicist with the U.S. Geological Survey’s Earthquake Science Center at California’s Moffett Field, told SF Gate. “We think that this place keeps having earthquake swarms due to a lot of fluid-filled cracks, thanks to very complex fault geometry — unlike, say, the San Andreas Fault, which is this nice clean edge.”

The swarm might last as long as “a couple of months, waxing and waning,” UC Berkeley seismologist Roland Burgmann told SF Gate.

How to stay safe during an earthquake

If an earthquake strikes, it’s best to protect yourself right away, according to the Department of Homeland Security.

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.”

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