Triggering of the Pohang, Korea, Earthquake (Mw 5.5) by enhanced geothermal system stimulation

Abstract

© 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.

An In‐Depth Seismological Analysis Revealing a Causal Link Between the 2017 MW 5.5 Pohang Earthquake and EGS Project

“…However, it is an earthquake that inflicted the most damage in the region (Choi et al, 2019;Kang et al, 2019). Concerned authorities suspected that the enhanced geothermal systems (EGSs) geothermal electricity project caused this earthquake (Ellsworth et al, 2019;Lee et al, 2019).…”

Induced and triggered seismicity below the city of Strasbourg, France from November 2019 to January 2021

“…A time delay of a few days between the end of the injection and a major induced earthquake was observed on several occasions. The regional case of Rittershoffen where seismicity was observed four days after the shut-in operations [Lengliné et al, 2017] and that of Pohang in South Korea (M5.5 triggered two months after the cessation of stimulation) [Ellsworth et al, 2019], are particularly illustrative. But many other cases are documented [Cuenot et al, 2008, Deichmann and Giardini, 2009, Diehl et al, 2017.…”

Variational Phase‐Field Modeling of Hydraulic Fracture Interaction With Natural Fractures and Application to Enhanced Geothermal Systems

“…On the contrary, stiff rocks generate higher overpressure for a lower injected volume. Because of the high fracture strength, high stiffness, and low permeability of basement crystalline rocks, during stimulation of a deep geothermal reservoir high overpressure can be achieved with relatively low values of injected volume (Ellsworth et al., 2019).…”