An earthquake swarm has been in progress along the northern segment of the Calaveras fault, about 3 to 4 km (1.8 to 2.5 miles) southeast of the center of San Ramon, California, since November 9. Data from the United States Geological Survey (USGS) show at least 40 recorded events ranging in magnitude from M0.6 to M3.8 at depths between 5 and 10 km (3 to 6 miles).
The strongest earthquake, M3.8 at 09:38 local time (17:38 UTC) on November 9, was followed within minutes by several M3.0 and M2.5 events.
The clustering pattern, absence of a clear mainshock, and tight spatial concentration indicate a typical fault-segment swarm rather than a standard aftershock sequence…