EGS: The Geysers: What is the risk of a large, damaging earthquake at The Geysers?
The rate, amount, location, and maximum size of earthquakes are controlled by a number of factors. As stated above, the size of an earthquake (or how much energy is released) depends upon how much slip occurs on the fault, how much stress there is on the fault before slipping, how fast it fails, and over how large an area it occurs. Damaging earthquakes (usually greater than magnitude 5) require the fault surfaces to slip over relatively large areas (kilometers). The overall volume of seismicity (seismogenic zone) is controlled by the volume of rock that is capable of building up stress and the forces that are acting to deform that volume. These forces are the forces that are fundamentally generated by the dynamic nature of the whole earth.
In most regions where there are economic geothermal resources, there is usually tectonic activity, such as in the western United States. At The Geysers, it is clear that one of the main causes of seismicity is the injection of the water to prevent pressures in the steam reservoir from falling below uneconomic levels. As has been pointed out, the number of earthquakes is related to the volume of water being injected into the steam reservoir. The size and number of the events are also related to the volume and the rate at which the injected water alters the stresses in the subsurface. In The Geysers, it is rare to observe an event much deeper than five to six kilometers below a sea level datum. This is a strong indication that in The Geysers, stress release in the form of an earthquake (sudden slip) would not occur deeper than five or six kilometers. What this means is that the seismogenic zone is shallow (five or six kilometers) compared 20 to 25 kilometers outside of The Geysers in California. This shallow earthquake zone is thought to be caused by the high temperatures below The Geysers, making the rock less brittle at relatively shallow depths. What this also means is that there is a limited depth range at The Geysers over which earthquakes can occur, thus placing an upper limit on the maximum size of an event. If we assume that this limit is five kilometers, then it is highly unlikely that an event of greater than magnitude five would occur. Last but not least, although there are many small faults in The Geysers field (small faults crisscross the entire field), there is no one large fault that has been mapped that would support a large event. This “broken up” nature of The Geysers is probably why it is such a good geothermal resource. Although there are over 30,000 events per year at The Geysers exceeding magnitude 0.0, the largest event recorded since 1960 registered a magnitude 4.6. Assuming a b-value of 1.25, we would have expected to have at least one magnitude 5 during this time.