Earthquakes have unsettled San Ramon residents for months, with dozens of small quakes peppering the Tri-Valley area since November.
The area around San Ramon has recorded 91 quakes stronger than magnitude 2 over November and December, through Tuesday morning. That tally is the highest on record for the area for a two-month period, according to a Chronicle analysis of U.S. Geological Survey data.
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The Tri-Valley region has a history of earthquake swarms, when earthquakes repeatedly strike a small area and don’t follow a typical mainshock–aftershock sequence. Swarms struck the area around Alamo, Danville and San Ramon in 1970, 1976, 1990, 2002, 2003, 2015 and 2018, said Annemarie Baltay, a research geophysicist with the U.S. Geological Survey.
“We have these earthquakes in San Ramon somewhat frequently,” Baltay said. Before this year, San Ramon’s highest two-month tally for small quakes was 89, in the fall of 2015…