Overview
- More than 300 quakes have been logged since Nov. 9 near San Ramon, including a magnitude 4.0 on Friday and a 3.9 on Saturday during a renewed Dec. 21 burst.
- Over 2,000 people reported feeling the weekend shaking to the USGS, with reports stretching from Sacramento to San Jose.
- The events are clustered on the northern Calaveras fault near Mount Diablo, which is viewed as a fault with notable long‑term hazard despite no modern large rupture.
- USGS scientists point to fluids and complex fault geometry as likely drivers, noting San Ramon has experienced at least six recorded swarms since 1970 that did not lead to major quakes.
- Experts say short‑term odds of a larger event rise slightly after any quake but emphasize swarms are unreliable for prediction and urge standard preparedness measures.