Shear wave splitting analysis in the Cerro Prieto Geothermal Field

Fluid extraction in geothermal fields could alter local stress in the crust and induce earthquakes. The Cerro Prieto Geothermal Field in Baja California, Mexico, is one of the four major geothermal fields in the Salton Trough tectonic province. It is located in the pull-apart basin between the Cerro Prieto fault and the Imperial fault. I have analyzed shear wave splitting data from local earthquakes to understand crustal stress variations in the vicinity of the geothermal field. Shear wave splitting parameters, the fast polarization direction, and the delay time, are measured from 103 earthquakes from 2014 to 2022 at three stations, CPX, GUVIX and UABX. Station CPX is about 6 km from and is nearest the geothermal field. After applying quality control criteria, 75 events are kept for further analysis. The primary fast direction at all three stations is generally NE-SW, inconsistent with the regional Imperial and Cerro Prieto faults trending NW-SE. Rather, the preferred fast direction coincides with the secondary faults formed by the extensional tectonics in the pull-apart basin. The average delay times at the three stations are similar from 0.041s at CPX to 0.048s at GUVIX. Further analyses show that the preferred fast directions vary with event depth at station CPX but not at UABX and GUVIX. Fast directions at CPX scatter in a large range for events at 0 to 15 km depths, indicating complex anisotropy in the upper and middle crust near the CPGF. It has been known that fluid extraction associated with the geothermal operation caused land subsidence, altering the crustal stress. The observed distinct anisotropy at CPX is probably the result of the geothermal activity near the station.