A series of 19 earthquakes shook the Bay Area early Monday, according to the U.S. Geological Survey.
The strongest quake, a magnitude 4.2, hit shortly after 7 a.m. just southeast of San Ramon. The shaking was felt across parts of the East Bay, including Oakland, Richmond, San Leandro and Hayward.
In San Francisco, residents reported rattling windows in neighborhoods such as Glen Park and NoPa, along with brief rumbling sensations.
The additional earthquakes ranged in magnitude from 2.5 to 3.9. All of them were centered in the same general area near Alcosta Boulevard in San Ramon, according to USGS data.
Following the quakes, BART issued a systemwide alert warning riders of delays. Trains were operating at reduced speeds while crews conducted track safety inspections, with delays of up to 20 minutes expected.
San Ramon has experienced recurring earthquake activity over the past month. From November through December, at least 87 earthquakes measuring magnitude 2.0 or higher were recorded near the East Bay city, according to a Chronicle analysis of USGS data. Seismologists note that the Tri-Valley area has a history of earthquake swarms, in which multiple quakes repeatedly strike a small, localized area.
The specific fault responsible for Monday’s earthquakes has not yet been identified. Scientists say earthquake swarms are not fully understood. The Tri-Valley sits atop a section of the Calaveras Fault that splits into smaller subfaults, and researchers believe the activity may be linked to water or other fluids moving upward from deep underground.
Experts emphasize that these smaller quakes do not necessarily indicate that a larger earthquake is imminent.