Preliminary Shear Wave Splitting Inversion Results for Puna Geothermal Field

Abstract

A new method of linear inversion for sub-surface crack density is presented. The method is based on shear wave splitting (SWS) observations (time delay and polarization angle) which are a direct measure of the shear wave velocity anisotropy analogical to birefringence in optics. The relationship between crack density and SWS observations is non-linear. By fixing the crack dip to vertical and the strike to the SWS polarization angle the relationship becomes approximately linear and can be solved using LSQR. By including observations where the earthquake does not display SWS, the inversion is further constrained thus improving the resolution. A crack density inversion is calculated for a large (~9500) data set of earthquakes detected at the Puna geothermal field. This data is detected by a local seismic array consisting of eight borehole seismometers. Results indicate that there is a large area of high crack density in the south of the field. The results also indicate two linear features (one of which correlates well with drilling records) in the shallow (1.8km) regions of the field.