A swarm of earthquakes rattled the Bay Area on the evening of Friday, December 19. The earthquakes occurred over a 30-minute span, according to the U.S. Geological Survey.

There have been no reports of damage or casualties, and no tsunami warnings have been issued as of reporting.

Why It Matters

In recent weeks, the East Bay Area has been rattled by multiple earthquakes.

Over 150 earthquakes affected the area near San Ramon in November, according to a report from public radio station KQED. Then on December 8, 10 earthquakes hit the area. These earthquakes had a preliminary magnitude of 2.5 or above, as per KQED.

Earthquakes can be a regular occurrence in California, and these range from minor tremors which go unnoticed to major natural disasters. Aftershocks are often a possibility when an earthquake occurs.

There are more than seven million people in the wider Bay Area. Earthquake risks have serious implications for disaster preparedness, public safety and infrastructure resilience.

What To Know

The first earthquake struck at 7.49 p.m. and had a preliminary magnitude of 3.8. The epicenter was nearly 3 miles southeast of San Ramon.

Then, minutes later, an earthquake with a preliminary magnitude of 4.0 struck near San Ramon.

The earthquakes that followed had smaller preliminary magnitudes, and the latest struck at 8:10 p.m. That earthquake had a preliminary magnitude of 2.9.

People generally report feeling earthquakes that have a magnitude larger than about 2.5.

Earthquakes result from the abrupt release of energy along fault lines. These in turn cause the ground to shake, with seismic waves radiating away from the earthquake’s epicenter.

Modern earthquakes can knock objects off of shelves and cause concern, but they may not cause any visible damage. Regardless, each event is a reminder to residents to review their emergency plans and supplies.

The USGS estimates a 5 percent chance of a larger earthquake following a moderate one within a three-day period.

What People Are Saying

The United States Geological Survey (USGS) said in a post on its website addressing why there are so many earthquakes in Northern California: “The Geysers Geothermal Field is located in a tectonically active region of Northern California. The major seismic hazards in the region are from large earthquakes occurring along regional faults that are located miles away from the geothermal field, such as the San Andreas and Healdsburg-Rodgers Creek faults.

“However, activities associated with the withdrawal of steam for producing electric power cause or induce small quakes to occur in the field. These smaller quakes are frequently felt by those who work at the field and by nearby residents.”

What Happens Next?

The USGS and other seismological agencies continued to monitor seismic activity following the series of earthquakes.