Assessing whether the 2017 M _w 5.4 Pohang earthquake in South Korea was an induced event

Abstract

The moment magnitude () 5.4 Pohang earthquake, the most damaging event in South Korea since instrumental seismic observation began in 1905, occurred beneath the Pohang geothermal power plant in 2017. Geological and geophysical data suggest that the Pohang earthquake was induced by fluid from an enhanced geothermal system (EGS) site, which was injected directly into a near-critically stressed subsurface fault zone. The magnitude of the mainshock makes it the largest known induced earthquake at an EGS site.

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

“…Based on the derived equation reported in McGarr () for the relationship between the maximum seismic moment ( M max ) and volume ( V ) of the injected fluid, inducing the M W 5.5 Pohang earthquake required at least a 500‐fold‐greater fluid injection volume. Despite the inconsistency between the observed and expected maximum seismic moments, the M W 5.5 event is suspected to have had an anthropogenic origin due to its spatiotemporal correlation with the injection workflow (Grigoli et al, ; Kim et al, ). To closely analyze the relationship between M max and V at the Pohang EGS site, we compared the seismic moment of the Pohang earthquake and largest prior events with the net injected volumes at their origin times for PX‐1, PX‐2, and both wells (Figure ).…”

“…Phase arrival times and cross‐correlation measurements used for relocating hypocenters, cut seismograms recorded from the KIGAM, the KMA, and the KHNP, and three versions of earthquake catalogs, used in this study are available at https://zenodo.org/record/3561903. Cut seismic data of Kim et al () are available at https://zenodo.org/record/1218738. This work was conducted during the Korean Government Commission (KGC) on the relations between the 2017 Pohang earthquake and EGS project, funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant from the South Korean government (MOTIE) (no.…”

Injection‐Induced Moment Release Can Also Be Aseismic

“…In particular, the Pohang, South Korea, earthquakes, possibly associated with a nearby EGS project (Grigoli et al, ; Kim et al, ) showed a moment release that exceeded the bound given by equation by several orders of magnitude. Because the 15 November 2017 M5.4 main shock caused considerable damage and injury in the City of Pohang (Kim et al, ), the reported possibility that this event was caused by an EGS project generated considerable consternation. In response, the Government of South Korea has convened an international panel of experts who will have access to a more comprehensive data set than is currently available for the purpose of evaluating the likelihood that the Pohang earthquakes were caused by an EGS project.…”

Fluid‐Induced Fracturing of Initially Damaged Granite Triggered by Pore Pressure Buildup

“…A similar mechanism has been proposed to explain induced seismicity at active fault zones, where microseismicity is triggered by fluid injection that results in a decrease in effective pressure on preexisting fault planes (e.g., Ohtake, ; Raleigh et al, ; Tadokoro et al, ). Recent increases in seismicity in relatively stable regions (e.g., Oklahoma in the United States and South Korea) likely resulted from the injection of wastewater and fluids from geothermal power plants (e.g., Keranen et al, ; Kim et al, ). Furthermore, Scholz et al () proposed a dilatancy diffusion model to explain the physical processes during earthquakes, which predicts changes in physical and transport parameters (e.g., Vp / Vs ratio and electrical resistivity) prior to earthquakes due to insufficient fluid flow into the damage zone, although such changes have not been systematically observed before earthquakes.…”