By David Jacobson, Temblor
Yesterday afternoon, at approximately 1:30 p.m., a M=3.7 earthquake shook Geysers, California, 110 km (70 mi) north of the city of San Francisco. According to the USGS, the earthquake occurred at a shallow depth of 1.4 km and caused light shaking near the epicenter. In total, 47 “Did You Feel It” reports were filed on the USGS website came in from as far away as Santa Rosa 35 km (20 mi) south of the epicenter.
Geysers, CA is one of the most seismically active regions in California. Therefore, this quake does not come as a surprise. Even though there are major active faults in the area, the majority of earthquakes near Geysers are induced. In the 1960’s, steam withdrawal for power production commenced at Geysers. While high quality seismic data only dates back to 1975, scientists have seen a strong correlation between increased steam production, its associated fluid injection and the number of earthquakes in the area.
The operations being undertaken at this geothermal field are believed to be causing earthquakes in three ways. First, when steam is withdrawn, both heat and mass is removed. The result of this is that the surrounding rock contracts, which can cause earthquakes. Additionally, once the steam is extracted from the ground and used to produce electricity, it is condensed and pumped back into the steam reservoir to prolong the life of the geothermal field. In addition to replenishing the field with condensed steam, operators are pumping reclaimed water from Lake County and Santa Rosa into the reservoir. However, the condensed steam and reclaimed water is cold, while the surrounding rock is hot. These contrasting temperatures are believed to be a significant factor in causing induced earthquakes. Lastly, scientists believe it that the injected fluids could be finding their way into fractures in the rock, and generating earthquakes in a way similar to what is seen in Oklahoma. So, while geothermal energy may be renewable, the associated earthquakes at Geysers is a downside.
While small earthquakes are extremely common at Geysers, and are likely to continue, larger magnitude earthquakes are not common because no large faults are in the immediate vicinity of the geothermal field. Instead, only small fractures are known to exist, which are not capable of generating large earthquakes. Having said that, in December last year, a M=5.0 earthquake shook the area. Additionally, faults nave the capability to link up in larger earthquakes. Therefore, the area is continuously monitored.