Influence of Hydraulic Fractures on Induced Seismicity: A Case Study of the Pohang Mw 5.5 Earthquake

Abstract

Identifying the mechanisms of induced seismicity in Enhanced Geothermal Systems (EGS) remains a challenge. In this paper, we propose a multimechanism approach to explain the induced seismicity sequence of the 2017 Mw 5.5 earthquake in the Pohang Enhanced Geothermal System, based on on-site seismicity data. We investigated two mechanisms, fluid pressure and tensile stress transfer from hydraulic opening, and examined their contributions to distinct stages of the seismicity sequence using a combined flow and geomechanical model. We suggest that tensile stress transfer was activated instantaneously when the hydraulic fractures opened during injection, contributing to the moderate Mw 1.67 event around the time injection stopped. At this point, fluid pressure had not yet reached the fault due to its finite propagation speed and the fault remained inactive. After injection stopped, fracture closure led to the deactivation of both stress and pressure mechanisms, potentially explaining the seismically dormant period observed post-injection. When the pressure front eventually reached the fault, continuous pressure increases led to resumed seismicity. The pressure buildup combined with the delayed arrival of the pressure front may have triggered the Mw 5.5 earthquake which occurred 58 days after injection stopped.