Abstract
Microearthquake activity at The Geysers geothermal area, California,
mirrors the steam production rate, suggesting that the earthquakes
are industrially induced. A 15-station network of digital, three-component
seismic stations was operated for one month in 1991, and 3,900 earthquakes
were recorded. Highly-accurate moment tensors were derived for 30
of the best recorded earthquakes by tracing rays through tomographically
derived 3-D V p and V p /V s structures, and inverting P- and S-wave
polarities and amplitude ratios. The orientations of the P- and T-axes
are very scattered, suggesting that there is no strong, systematic
deviatoric stress field in the reservoir, which could explain why
the earthquakes are not large. Most of the events had significant
non-double-couple (non-DC) components in their source mechanisms
with volumetric components up to approximately 30\% of the total
moment. Explosive and implosive sources were observed in approximately
equal numbers, and must be caused by cavity creation (or expansion)
and collapse. It is likely that there is a causal relationship between
these processes and fluid reinjection and steam withdrawal. Compensated
linear vector dipole (CLVD) components were up to 100\% of the deviatoric
component. Combinations of opening cracks and shear faults cannot
explain all the observations, and rapid fluid flow may also be involved.
The pattern of non-DC failure at The Geysers contrasts with that
of the Hengill-Grensdalur area in Iceland, a largely unexploited
water-dominated field in an extensional stress regime. These differences
are poorly understood but may be linked to the contrasting regional
stress regimes and the industrial exploitation at The Geysers.
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