MULTIPLET ANALYSIS AT COSO GEOTHERMAL

We have searched the Coso geothermal field (CGF) for microseismicity i n 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 eac h other. We subdivided hypocentral data into spatial clusters to reduc e the computational burden and evaluated multiple cross-correlation pa irs, 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 clus ter analysis routines were used. Among the 96 high-correlation pairs, 24 equivalence classes have been isolated. While most of these are dou blets, 8 classes include 3 or more cluster members and one class inclu des 16 members. Relative locations were calculated using phase shifts between corresponding events. Detailed analysis of hypocenter relocati ons shows elongate, vertical structure with apparent random temporal v ariations. The multiplets do not appear to be true repeating events; r ather, they are clusters of small, nearly identically oriented rupture s, perhaps representing swarms of fractures activated by fluid-pressur e fluctuations. Using the small volumes encompassing each multiplet, w e estimate fracture densities measure between 0.02 and 0.4 m(-1) and a re largest near injection wells.