Influence of hydraulic fractures on induced seismicity: A case study of the Mw 5.5 Pohang Enhanced Geothermal Systems (EGS) earthquake

Yuan Tian, & Roland Horne

Submitted September 7, 2025, SCEC Contribution #14748, 2025 SCEC Annual Meeting Poster #TBD

Induced seismicity remains a critical issue in Enhanced Geothermal Systems (EGS) development. While stopping injection can effectively reduce seismicity during stimulation, large earthquakes may still occur unexpectedly after a prolonged and nearly quiescent period (approximately 2-months at Pohang). The mechanisms driving these discontinuous seismicity sequences with large and delayed events remain unclear.
We identified two key contributors to this pattern: (1) stress transfer from hydraulic fractures opening and (2) fluid pressure diffusion. Using a combined flow and geomechanics model, we computed stress perturbation history on a fault under both mechanisms. Based on the perturbed stress history, we evaluated fault slip velocity evolution as a proxy for seismic event magnitude using a spring-slider model with rate-and-state friction law.
During injection, stress transfer from hydraulic opening is activated first. As an elastic effect, it instantaneously perturbs stress field over a long range, likely contributing to the moderate earthquake towards the end of injection. Meanwhile, the fluid pressure front propagated at a finite speed governed by hydraulic diffusivity and had not yet reached the fault. After injection stopped, the pressure drop led to fracture closure and deactivated stress transfer, resulting in a seismically dormant period. Eventually, the delayed arrival of the pressure front on fault triggered the final large earthquake.
Our results show that the time lag between different mechanisms can explain the discontinuous seismicity pattern and the delayed occurrence of large events. These findings highlight the importance of physics-based models calibrated with in-situ data to quantify contribution from multiple mechanisms in induced seismicity hazard assessment. This understanding also allows for the formulation of strategies to reduce the risk of such events.

Key Words
Induced seismicity, Hydraulic fractures, Enhanced Geothermal Systems

Citation
Tian, Y., & Horne, R. (2025, 09). Influence of hydraulic fractures on induced seismicity: A case study of the Mw 5.5 Pohang Enhanced Geothermal Systems (EGS) earthquake. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)