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Multiplet analysis at Coso geothermal

Department of Geology and Geophysics Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109

Abstract

We have searched the Coso geothermal field (CGF) for microseismicity in seismic doublets, co-located hypocenters that appear to have nearly identical waveforms. Using 1085 high-quality events from 1993 to 1994, we identified numerous doublets, some occurring within minutes of each other. We subdivided hypocentral data into spatial clusters to reduce the computational burden and evaluated multiple cross-correlation pairs, assigning scores to each pair. As an example, one spatial cluster includes 183 events yielding 96 high-correlation (>0.6) paired events. To isolate potential multiplets, equivalence class analysis and cluster analysis routines were used. Among the 96 high-correlation pairs, 24 equivalence classes have been isolated. While most of these are doublets, 8 classes include 3 or more cluster members and one class includes 16 members. Relative locations were calculated using phase shifts between corresponding events. Detailed analysis of hypocenter relocations shows elongate, vertical structure with apparent random temporal variations. The multiplets do not appear to be true repeating events; rather, they are clusters of small, nearly identically oriented ruptures, perhaps representing swarms of fractures activated by fluid-pressure fluctuations. Using the small volumes encompassing each multiplet, we estimate fracture densities measure between 0.02 and 0.4 m^–1 and are largest near injection wells.

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