Abstract
Inverting P- and S-wave polarities and P:SH amplitude ratios using
linear programming methods suggests that about 20\% of earthquakes
at The Geysers geothermal area have significantly non-double-couple
focal mechanisms, with explosive volumetric components as large as
33\% of the seismic moment. This conclusion contrasts with those
of earlier studies, which interpreted data in terms of double couples.
The non-double-couple mechanisms are consistent with combined shear
and tensile faulting, possibly caused by industrial water injection.
Implosive mechanisms, which might be expected because of rapid steam
withdrawal, have not been found. Significant compensated-linear-vector-dipole
(CLVD) components in some mechanisms may indicate rapid fluid flow
accompanying crack opening.
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