© 2019 Seismological Society of America. All rights reserved. On the afternoon of 15 November 2017, the coastal city of Pohang, Korea, was rocked by a magnitude 5.5 earthquake (Mw, U.S. Geological Survey). Questions soon arose about the possible involvement in the earthquake of the Republic of Korea's first enhanced geothermal system (EGS) project because the epicenter of the earthquake was located near the project's drill site. The Pohang EGS project was intended to create an artificial geothermal reservoir within low-permeability crystalline basement by hydraulically stimulating the rock to form a connected network of fractures between two wells, PX-1 and PX-2, at a depth of ∼4 km. Forensic examination of the tectonic stress conditions, local geology, well drilling data, the five high-pressure well stimulations undertaken to create the EGS reservoir, and the seismicity induced by injection produced definitive evidence that earthquakes induced by high-pressure injection into the PX-2 well activated a previously unmapped fault that triggered the Mw 5.5 earthquake. Important lessons of a general nature can be learned from the Pohang experience and can serve to increase the safety of future EGS projects in Korea and elsewhere.

© 2019 Seismological Society of America. All rights reserved. On the afternoon of 15 November 2017, the coastal city of Pohang, Korea, was rocked by a magnitude 5.5 earthquake (Mw, U.S. Geological Survey). Questions soon arose about the possible involvement in the earthquake of the Republic of Korea's first enhanced geothermal system (EGS) project because the epicenter of the earthquake was located near the project's drill site. The Pohang EGS project was intended to create an artificial geothermal reservoir within low-permeability crystalline basement by hydraulically stimulating the rock to form a connected network of fractures between two wells, PX-1 and PX-2, at a depth of ∼4 km. Forensic examination of the tectonic stress conditions, local geology, well drilling data, the five high-pressure well stimulations undertaken to create the EGS reservoir, and the seismicity induced by injection produced definitive evidence that earthquakes induced by high-pressure injection into the PX-2 well activated a previously unmapped fault that triggered the Mw 5.5 earthquake. Important lessons of a general nature can be learned from the Pohang experience and can serve to increase the safety of future EGS projects in Korea and elsewhere.