For Two Weeks, This California Town Has Averaged One Earthquake Per Hour

At 408 quakes so far, San Ramon sets a shaky new record
San Ramon Earthquake Swarm
A map of some of the earthquakes that hit the town of San Ramon. USGS/ComCat catalog

Large earthquakes can be incredibly serious, like the one that struck Afghanistan on Monday. But not all earthquakes are devastating, earth-shattering monsters. Sometimes, they’re just…there.

Over the past two weeks, over 408 earthquakes have rattled the town of San Ramon, California. That’s a little over an earthquake every hour, and sets a record for the area, beating out a 2003 swarm which lasted for a month and had 120 earthquakes. It’s an impressive accomplishment, but San Ramon has a long way to go if it wants to beat seismic heavyweights like Yellowstone National Park, which recorded 3,000 earthquakes over 3 months in 1985.

The largest earthquake in the swarm was a magnitude 3.6 on October 19. Magnitude is a measure of how large an earthquake is. A 3.6 magnitude earthquake is a fairly moderate-size earthquake that is unlikely to cause damage. Many of the other earthquakes in the swarm were so small that they weren’t felt by residents.

The United States Geological Survey (USGS) says that the swarm could last for “several more weeks”. But that isn’t a reason to panic.

As unsettling as the shaking is, the USGS says that the chance of these smaller earthquakes presaging a larger earthquake is very small, and the chance of a damaging earthquake (larger than a magnitude 6.7) happening along the fault causing the shaking is only about 8 percent.