Extensional tectonics at The Geysers geothermal area, California
Journal of Geophysical Research, 91 (B11):
11463--11476 (October 1986)DOI: 10.1029/JB091iB11p11463
Abstract
Comparisons between the yearly volume of steam withdrawn from The
Geysers, California, geothermal reservoir and the number of earthquakes
indicate that most of the seismicity is induced. To establish the
inducing mechanism, an estimate of the stress field orientation was
calculated from inversion of 210 fault plane solutions. Below 1 km
depth The Geysers is undergoing uniaxial extension with the least
principal stress sigma3 oriented near horizontal at approximately
105 deg azimuth. The consistency between the local and regional stress
fields indicates that the regional tectonic stress is much larger
than the stresses induced locally through geothermal activities.
The stress data are equivocal as to whether earthquakes are induced
due to conversion of aseismic to seismic slip or by increased shear
stresses arising from reservoir contraction. The observed tendency
for downward growth of seismicity may arise from a preferential fracture
orientation that is near vertical in conjunction with a stress state
where the gradient of sigma1 exceeds that of sigma3. Evidence of
extension at The Geysers and in the surrounding Clear Lake region
indicates a direct relationship to the presence of the Quaternary
Clear Lake Volcanics.