Simulating Swarm-to-Mainshock Evolution at the St. Gallen Geothermal Project

SeongJu Jeong, & Junle Jiang

Submitted September 7, 2025, SCEC Contribution #14555, 2025 SCEC Annual Meeting Poster #TBD

Induced seismicity can evolve from earthquake swarms to damaging mainshock sequences, but the physics of this transition remains poorly understood. We analyze the 2013 earthquake sequences at the St. Gallen geothermal site in Switzerland, where localized fluid injection triggered swarm-like seismicity and a subsequent unintended gas kick—interpreted by prior studies as a spatially distributed pore-pressure increase—preceded an ML 3.5 mainshock sequence that terminated operations. To investigate this transition, we develop 2-D physics-based fault models with rate-and-state friction, pore pressure diffusion, aseismic slip, and dynamic rupture. The model incorporates heterogeneous fault properties, such as depth-dependent normal stress and initial shear stress, as well as alternating velocity-weakening (VW) and velocity-strengthening (VS) patches. Ruptures propagate across multiple VW patches by overcoming intervening VS barriers, enabling larger earthquakes. Our simulations qualitatively reproduce observed features of the St. Gallen sequence, including event timing, magnitude evolution, and stress drop variability. Localized injection produces swarm-like, self-arresting ruptures, whereas the gas-kick-induced, more distributed pressure perturbation initiates a mainshock sequence modeled as a runaway rupture. The transition between these behaviors depends on the balance between pore pressure perturbation and pre-rupture shear stress. Notably, pore pressure increases alone are insufficient to produce a mainshock; runaway rupture occurs only when pore pressure effects are coupled with foreshock activity and associated aseismic slip. Simulated events range in magnitude from Mw 0.02 to 4.39, with stress drops from 0.37 to 14.03 MPa, consistent with observed values except for the largest event. In our model, enhancing permeability following the gas kick suppresses the mainshock and minimally alters stress drop variability. Larger stress drops are associated with ruptures that propagate into VW patches with more negative (a–b) values. These results underscore the critical roles of aseismic slip and fault property heterogeneity in controlling rupture dynamics. Monitoring aseismic deformation and avoiding fluid injection near critically stressed fault zones may be key strategies for mitigating induced seismic hazards.

Key Words
Earthquake Simulation, Induced Seismicity, Stress drop

Citation
Jeong, S., & Jiang, J. (2025, 09). Simulating Swarm-to-Mainshock Evolution at the St. Gallen Geothermal Project. Poster Presentation at 2025 SCEC Annual Meeting.

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