NON-DOUBLE-COUPLE EARTHQUAKE MECHANISMS AT THE GEYSERS GEOTHERMAL AREA, CALIFORNIA

Inverting P- and S-wave polarities and P:SH amplitude ratios using lin ear programming methods suggests that about 20% of earthquakes at The Geysers geothermal area have significantly non-double-couple focal mec hanisms, with explosive volumetric components as large as 33% of the s eismic moment. This conclusion contrasts with those of earlier studies , which interpreted data in terms of double couples. The non-double-co uple mechanisms are consistent with combined shear and tensile faultin g, 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) compon ents in some mechanisms may indicate rapid fluid flow accompanying cra ck opening.