Three‐Dimensional Pore Fluid Pressures in Source Region of 2017 Pohang Earthquake Inferred From Earthquake Focal Mechanisms

We analyzed 91 focal mechanism data with relocated hypocenters to estimate the 3‐D pore fluid pressure field in the source region of the 2017 Pohang earthquake in South Korea using the method of focal mechanism tomography. The upper limit of pore fluid pressure was 8 ± 3 MPa in a location northwest of the enhanced geothermal system. The overpressurized fluids were localized in the footwall of the causative fault. It was directly connected to the open hole section of PX2, and fluid injection into this hole could have triggered the mainshock. Plain Language Summary The 2017 Pohang earthquake (Mw 5.5) raised a serious social debate as to whether it was triggered by the fluid injections at the Pohang enhanced geothermal system (EGS). In March 2019, the Overseas Research Advisory Committee organized by the Korean government authoritatively concluded that the Pohang event was triggered by high‐pressure fluid injection at the EGS. However, the level of the pore fluid pressure driving the mainshock and the vigorous aftershocks remains unclear. In this study, we estimated the 3‐D pore fluid pressure field in the Pohang EGS from 91 earthquake focal mechanism data, using an inversion method based on the Bayesian statistical inference and Akaike's Bayesian Information Criterion. We found that the pore fluid pressure was enhanced in regions with horizontal and vertical length scales of 2–3 and 1–2 km, respectively, around the hypocenter of the mainshock in the Pohang EGS. The maximum excess pore fluid pressure above hydrostatic was at most 8 ± 3 MPa. This overpressurized fluid reservoir was directly connected to the open hole section of one of the injection wells. This study provided with new evidence for involvement of fluids in the Pohang earthquake. Key Points Three‐dimensional pore fluid pressures were estimated from earthquake focal mechanisms in the source region of the 2017 Pohang earthquake Overpressurized fluids concentrated in the footwall of the mainshock fault were directly connected to the Pohang EGS The upper limit of pore fluid pressure was 8 ± 3 MPa in a location northwest of the EGS