Locating an Active Fault Zone in COSO Geothermal Field by Analyzing SeismicGuided Waves from Microearthquake Data

Abstract

Active fault systems usually provide high-permeability channels for hydrothermal outflow in geothermal fields. Locating such fault systems is of a vital importance to plan geothermal production and injection drilling, since an active fault zone often acts as a fracture-extensive low-velocity wave guide to seismic waves. We have located an active fault zone in the Cos0 geothermal field, California, by identifying and analyzing a fault-zone trapped Rayleigh-type guided wave from microearthquake data. The wavelet transform is employed to characterize guided-wave's velocity-frequency dispersion, and numerical methods are used to simulate the guided-wave propagation. The modeling calculation suggests that the fault zone is -200m wide, and has a P wave velocity of 4.80km/s and a S wave velocity of 3.OOkrn/s, which is sandwiched between two half spaces with relatively higher velocities ( P wave velocity 5.60kds, and S wave velocity 3.20kd/s).