Source processes of industrially-induced earthquakes at The Geysers geothermal area

• Main Authors: Alwyn Ross, G. R. Foulgerz, Bruce R. Julian
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1999
• Subjects: Grensdalur; Hengill; Iceland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.523.9369; http://community.dur.ac.uk/g.r.foulger/Offprints/ross_geysers.pdf

Microearthquake activity at The Geysers geothermal area, California, mirrors the steam production rate, sug-gesting that the earthquakes are industrially induced. A 15-station network of digital, three-component seis-mic stations was operated for one month in 1991, and 3,900 earthquakes were recorded. Highly-accurate mo-ment tensors were derived for 30 of the best recorded earthquakes by tracing rays through tomographically de-rived 3-D VP and VP=VS 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 mecha-nisms with volumetric components up to »30 % of the total moment. Explosive and implosive sources were ob-served 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 with-drawal. Compensated linear vector dipole (CLVD) com-ponents 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 contrast-ing regional stress regimes and the industrial exploitation at The Geysers.