A 4.4 magnitude earthquake hit three miles from The Geysers in Sonoma County Sunday night, according to the USGS.
The Geysers is one of the most active areas of Northern California with small earthquakes hitting the area nearly every week. A 4.4 quake is on the large size for The Geysers. The UGSG Website says the largest quake ever to be recorded there was 4.5.
The U.S. Geological Survey said Sunday's 4.4 quake hit at 8:47 p.m. at a shallow depth of approximately 0.2 miles.
There were no reports of damage or injuries. People reported feeling the earthquake from San Francisco, Half Moon Bay and as far away as Santa Clara and Palo Alto.
Here is an explanation from the USGS as to why there are so many earthquakes in this particular area:
The Geysers geothermal field is located in a tectonically active region of Northern California. The major seismic hazards in the region are from large earthquakes occurring along regional faults that are located miles away from the geothermal field, such as the San Andreas and Healdsburg-Rodgers Creek faults. However, activities associated with the withdrawal of steam for producing electric power cause or induce small quakes to occur in the field. These smaller quakes are frequently felt by those who work at the field and by nearby residents.
Seismicity at The Geysers was poorly documented when power generation commenced in the 1960's, but since 1975 high-quality seismic monitoring data has been available, and it has been demonstrated that increased steam production and fluid injection correlates positively with changes in earthquake activity. The level of seismicity has been fairly stable since the mid-1980s, even though power production has declined in the field with the depletion of the steam reservoirs. Sheet 3 of Open-File Report 2002-209 illustrates how seismicity has spatially and temporally expanded with geothermal production.
Seismologists have proposed several mechanisms to explain why earthquakes are being induced. The operators of the geothermal field are withdrawing mass (steam boiled from water) and heat, both of which cause the surrounding rock to contract, which in turn can induce earthquakes as a result of the contractional stresses. In addition, the operators condense the extracted steam and flow the water back into the steam reservoir at depths of one to three km in order to extend the life of the field. Furthermore, reclaimed water from nearby Lake County and Santa Rosa is pumped to The Geysers and flowed into the steam reservoir. The condensed steam and reclaimed water is cold, whereas the rock is hot, and it appears that this thermal contrast is a significant factor in inducing the earthquakes. It is also possible that the hydraulic pressure of the injected fluid finds its way into faults and facilitates fracturing due to increased fluid pressures.
To date, the largest quake recorded at The Geysers is approximately M4.5. It is possible that a magnitude 5 could occur, but larger earthquakes are thought to be unlikely. In order for a larger earthquake to occur, it is necessary that a large fault exist. For example, the 1906 magnitude (M) 7.8 San Francisco earthquake ruptured nearly 300 miles of the San Andreas Fault. At The Geysers no such continuous fault is known to exist. Rather, there are numerous small fractures in the rock located near the many steam and injection wells. The activities described above result in local stress changes which, when added to the prevailing regional tectonic stresses, induce the quakes on these small faults.