The temblor occurred at 8:31 a.m., according to the US Geological Survey.
The epicenter was located one mile north of The Geysers geothermal steamfield, five miles west southwest of Cobb and seven miles west northwest of Anderson Springs, at a depth of 1.7 miles, the survey reported.
Several smaller quakes followed that were centered one to two miles north of The Geysers – the largest a 2.0, according to US Geological Survey records.
By noon the survey received 15 shake reports from nine zip codes for the 4.0-magnitude quake, including reports from Lakeport, Middletown and Kelseyville, Calistoga and St. Helena in Napa County, the Sonoma County communities of Santa Rosa, Cloverdale and Windsor, and a report from Arcata, 248 miles away in Humboldt County.
A 3.2-magnitude quake occurred three miles east of The Geysers on Monday, as Lake County News has reported.
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