Abstract: Geothermal resources are a promising approach to clean renewable energy; 90% of them are deep reservoirs of hot dry rock that require hydraulic fracturing to create a network of connections among wells to enable efficient heat exchange, known as an Enhanced Geothermal System (EGS). The Pohang EGS project in south Korea led to a devasting M_w5.5 earthquake, triggered by the reservoir’s EGS stimulation, the largest earthquake known to have been induced by EGS development. Detailed investigations have been conducted to understand the cause of the Pohang earthquake; the conclusion has been that overpressurized injected fluids migrated into an unknown fault triggering this large earthquake. Detailed velocity images for the source zone of the 2017 Pohang earthquake, which could be helpful for further understanding its inducing mechanism, are unavailable. However, we have assembled detailed aftershock data recorded by 41 local stations installed within about three months after the M_w5.5 Pohang earthquake, and have then applied the V_p/V_s model’s consistency-constrained double-difference seismic tomography method to determine the high-resolution three-dimensional V_p (compressional wave velocity), V_s (shear wave velocity), and V_p/V_s models of the source region that we report here, as well as earthquake locations within the source region. The velocity images reveal that the deep source area of the 2017 Pohang earthquake is dominated by low V_p, high V_s, and low V_p/V_s anomalies, a pattern that can be caused by overpressurized vapors due to high temperatures at these depths. Based on aftershock locations and velocity features, our studies support the conclusion that the 2017 Pohang earthquake was triggered by injected EGS fluids that migrated into a blind fault.