Full Moment Tensor Inversion and 3-D Seismic Velocity Structure Analysis of Micro-earthquake Data in “WY” Geothermal Field

Abstract

A two-month micro-earthquake (MEQ) monitoring had been conducted in the "WY" geothermal field during fluid injection in 2005. MEQ data provide information with regards to processes developed in geothermal reservoirs. We have analyzed 198 MEQ data, starting from phase identification to location determination using Geiger's method and joint hypocenter determination. A travel time tomography inversion was performed in order to obtain a better 3-d seismic velocity model and more accurate hypocenter locations, and provide high resolution images of the geothermal reservoir. From the updated velocity model and hypocenter locations, we analyzed focal mechanisms of 32 MEQ events that were selected based on hypocenter clustering and signal quality. We used a full moment tensor inversion algorithm which utilizes MEQ waveforms as data input. Our inversion result shows that most of those MEQs have more than 50% non-double-couple component (compensated linear vector dipoles and volumetric). This result may indicate that those MEQ mechanisms could possibly be related to steam flow from the southern part of reservoir which caused explosive mechanism at the steam zone. This possibility is supported by information from Vp/Vs anomaly zones from travel time tomography results, which indicate the existence of steam. Moment tensor inversion results show positive volume change at the steam zone in northern part of reservoir, and negative volume change in southern part of reservoir. We also identified two oblique faults with NE and NNW strike directions. These faults are combinations of shear fracturing and extensional fracturing.