An earthquake swarm that began near downtown San Ramon, California, in October remained active on December 15, with three earthquakes recorded within a few hours by the U.S. Geological Survey. The swarm is occurring in a structurally complex fault zone near the Calaveras Fault, where stress is released through repeated small ruptures rather than a single larger earthquake.

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Over 40 earthquakes were recorded along the Calaveras fault southeast of San Ramon, California, between November 9 and 10, 2025. The largest event, M3.8, occurred at 09:38 LT (17:38 UTC) on November 9 at a depth of 9.2 km (5.7 miles). Geological and seismic studies show that the Calaveras fault is capable of generating an event in the M6.5 to M7.0 range over long-term timescales.

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