Dynamic fault interaction during a fluid-injection induced earthquake: The 2017 Mw 5.5 Pohang event

Abstract

The November 15th, 2017 Mw 5.5 Pohang earthquake (South Korea) has been linked to hydraulic stimulation and fluid injections, making this the largest induced seismic event associated with an Enhanced Geothermal System (EGS). To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction and 1D subsurface structure. A guided fault reconstruction approach that clusters spatio-temporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersects under a shallow angle of 15°. Based on simple Mohr-Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify preferred two fault plane scenarios that well reproduce observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, while local stress conditions constrained by borehole logging generate the observed thrust faulting component. Our preferred model is characterized by overpressurized pore fluids, non-optimally oriented but dynamically weak faults and a close to critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection induced earthquakes and that local stress perturbations dominate over the regional stress conditions. These findings, therefore, have important implications for seismic hazard in active geo-reservoir.