Characterizing geologic formations near a borehole is critically important for the energy and environmental industry. Better understanding anisotropic properties of subsurface formations in a geothermal field is essential for effective fracture stimulation to extract geothermal energy from enhanced geothermal systems. We apply a shear-wave splitting analysis method, based on eigenvector rotation, to six induced micro-earthquakes recorded using a 3-component geophone array deployed in a borehole at the Utah FORGE (Frontier Observatory for Research in Geothermal Energy) site. We determine that the faster S1-wave is along the radial direction at about an azimuth of E12.7oS, while the slower S2-wave is in the tangential direction perpendicular to that of S1-wave. This radial direction is consistent with major-semi axis orientation of induced micro-earthquake distribution. The averaged S-wave splitting rate (SSR) is 0.83%, indicating the average S1-wave velocity is faster than that of S2-wave by 0.83%. Average SSR values for receivers in the sedimentary and granite rocks are 0.91% and 0.72%, respectively, implying that the natural fracture density in the sedimentary rock is higher than that in the granite rocks. We can apply the shear-wave splitting analysis method to induced micro-earthquake data during fracture stimulation for time-lapse monitoring of EGS reservoirs.

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