Monday morning’s M4.2 earthquake in San Ramon is still producing registerable aftershocks as of Tuesday afternoon.
More than 4,000 people felt the ground shake beneath them around 7 a.m. Monday morning, after a 4.2 magnitude earthquake struck the East Bay city of San Ramon, according to information published by the United States Geological Survey (USGS). (The earthquake was even strong enough to be felt in eastern parts of San Francisco, most noticeably in the Marina and Embarcadero, two areas infamous for their liquefaction.)
Good morning Northern California! Did you feel the magnitude 4.3 earthquake about 2.5 miles southeast of San Ramon at 7:01 am PT. The #ShakeAlert system was activated. See: https://t.co/rj03jGPInq@Cal_OES @CalConservation pic.twitter.com/zpL1OrpKOV
— USGS ShakeAlert (@USGS_ShakeAlert) February 2, 2026
The initial 4.2 magnitude tremor, which was downgraded from a 4.3 upon further analysis, has set off a cascade — a “swarm,” if you will — of aftershocks. As of publishing, over 20 smaller earthquakes, each ranging between M2.5 and M3.6 in strength, have been recorded by USGS; the most recent aftershock occurred earlier this afternoon, over 36 hours after the aforementioned large quake.
No significant damage has been reported … but some anxiety is undoubtedly hanging around the area. Records from USGS show the East Bay city has experienced at least six earthquake swarms since the 1970s. However, despite its well-recorded seismic activity, the fault line these swarms populate around is unlikely to produce a “big one.”
The Calaveras Fault, where this most recent swarm struck, has just an 11% chance of producing a magnitude 6.7 or larger earthquake in the next 30 years. And because of its geology, seismologists suggest the fault line is actually incapable of producing a quake larger than 1984’s M6.4 Morgan Hill earthquake that epicrented on its central segment. The last sizeable earthquake (read: a tremor producing noticeable, significant damage) was in October 2007, when the M5.4 earthquake shook near Alum Rock.
Fun fact: A M2.1 earthquake produced by this recent swarm struck the exact same area where 2007’s Alum Rock tremor occurred. So that’s fun.
Seems like now’s a good time as any to make that emergency earthquake kit you’ve been putting off for … well, years.
