Microseismicity , stress , and fracture in the Coso geothermal field , California

Microseismicity. stress, and fracture in the Coso geothermal field are investigated using seismicity, focal mechanisms and stress analysis. Comparison of hypocenters of microearthquakes with locations of development wells indicates that microseismic activity has increased since the commencement of fluid injection and circulation. Microearthquakes in the geothermal field are proposed as indicators of shear fracturing associated with fluid injection and circulation along major pre-existing fractures. High-seismicity zones are associated with the main fluid-flow paths within the geothermal system. Calculated stress patterns from focal mechanisms provide direct evidence for the boundary between significantly different stress regimes within the Coso geothermal field. Microseismicity in the Coso geothermal field is spatially but not temporally related to regional seismicity extending southeast of the field. The spatial distribution of these events defines a northwest-trending seismic-fracture zone, consistent with a previously defined northwest-striking zone. The abrupt decrease of seismicity below this fracture zone may provide seismic evidence for the existence of a brittle and ductile transition zone at shallow depth beneath the Coso geothermal field. @ 1998 Elsevier Science B.V. All rights reserved. ie ts If)