The third modest earthquake in a week has shaken up northern Sonoma County, striking the area burned in the Kincade fire.
A magnitude 4.2 temblor was recorded in the sparsely populated Mayacamas Mountains about 13 miles northeast of Healdsburg just after 12:30 p.m. on Sunday. Strong shaking was forecasted to have occurred at the mountainous epicenter, but the most populated areas, Healdsburg and Geyserville, were expected to have encountered only weak or light shaking — not strong enough to cause major damage.
The Kincade fire, which at one point forced 200,000 people to evacuate from their homes, has burned through almost 78,000 acres and was 82% contained by Monday night. The fire destroyed 374 structures in Northern California’s wine country.
All three earthquakes occurred in the northern section of the Kincade burn zone and the vicinity of the Geysers geothermal plant run by Calpine Corp. The Geysers comprises 45 square miles along the borders of Sonoma and Lake counties and uses steam harvested from under the surface, heated by magma, to create electricity at 13 power plants for Northern California.
The Geysers facility is part of the Clear Lake Volcanic Field, one of eight volcanic areas classified by the U.S. Geological Survey to pose at least a moderate volcanic risk in the state. The most recent eruptions of the Clear Lake Volcanic Field occurred about 11,000 years ago around Mt. Konocti, above the southern shore of Clear Lake, the largest freshwater lake contained entirely in California.
Volcano scientists believe the Clear Lake Volcanic Field is in a long lull of activity. Between about 10,000 and 60,000 years ago, the area was believed to be far more busy, averaging one eruption every 1,800 years.
Earthquakes are fairly common at the Geysers, the largest complex of geothermal power plants in the world. According to the USGS, activities associated with the withdrawal of steam for producing electric power causes or induces small quakes to occur in the steam field.
The level of seismicity has been fairly stable since the mid-1980s, according to the geological survey, even as the amount of power produced has declined as steam reservoirs are depleted.
Energy production began at the Geysers in the 1960s.
The largest quake recorded at the Geysers was about magnitude 4.5, according to the USGS. It’s possible a magnitude 5 quake could occur in the area, the agency says, but a quake more powerful than that is probably unlikely.
Earthquake faults that are longer and therefore capable of more powerful temblors are miles away from the steam field.
There have been 11 earthquakes of magnitude 3 or greater in the Geysers area this year, including the three recorded in the past week.