Uncertainty analysis on the potential triggering mechanisms of the Pohang Mw5.5 earthquake

The Pohang earthquake is the largest seismic event induced by any enhanced geothermal system (EGS) project and occurred approximately two months after hydraulic stimulation was completed. Although it is now commonly accepted that the earthquake was induced by high-pressure hydraulic stimulation, which activated a mature fault, the detailed triggering processes and mechanisms remain under debate. To shed light into them, we first make a comparative analysis on the relative likelihood of the several fault planes and in-situ stress states that have been proposed, which present a wide range of values, based on the linear Mohr-Coulomb criterion. From this analysis, we propose two likely oblique-slip faulting patterns to analyze the problem of induced seismicity: a reverse faulting pattern with a strike-slip component (S-RF), which involves a reverse fault being in a three dimensional (3D) reverse faulting stress field; and a strike-slip faulting pattern with a reverse component (R-SS), which considers a strike-slip fault being in a 3D strike-slip faulting stress field. We then apply two analytical solutions to evaluate the coupled hydro-mechanical responses in the quasi-expanding reservoir at the instance of the mainshock for both fault slip patterns. We also perform a parametric space analysis to analyze the effect of the uncertainty in the geological properties on induced seismicity. Following this result, we carry out the Monte Carlo simulations to assess the probability of inducing the Pohang earthquake. From our analyses, we find that the pressure changes and stress variations caused by fluid injection can induce fault reactivation for both patterns, being the R-SS pattern more critical than the S-RF pattern. The probabilities of inducing a felt earthquake (Mw>3.0) and a Mw5.5 earthquakes for the former reach to ~15% and ~10%, respectively. Parametric space analysis shows that the magnitude of in-situ stress and the rock properties are playing a more significant rol