(KRON) — A preliminary magnitude 3.8 earthquake shook Sonoma County around 2:30 p.m. Sunday, according to the USGS. The quake struck a few miles northwest of The Geysers, a geothermal field that spans around 30 square miles.
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Ground tilting, caused by the buildup of stress in the rocks, may precede a large earthquake, but not always. Water levels in wells fluctuate as water moves into or out of fractures before an earthquake. This is also an uncertain predictor of large earthquakes.