Stochastic poromechanical analysis forecasts a notable exceedance probability for the 2017 Pohang, South Korea, Mw 5.5 earthquake

5.5 earthquake is currently the largest seismic event induced by Enhanced Geothermal Systems. The high uncertainty on geological and mechanical conditions of the rupture fault of this earthquake has originated a debate on its triggering mechanisms. Here, we propose a stochastic poromechanical analysis approach that combines Monte Carlo sampling and poromechanical models to address the uncertainty problem. By conducting a large number of coupled poromechanical simulations varying the uncertain geomechanical parameters, we yield an exceedance probability of 7%-15% for the Pohang mainshock. Remarkably, this physics-based stochastic prior forecast is quite comparable to the posterior likelihood inferred from the magnitude-frequency relationship of recorded seismicity. Our results reveal a scaling relationship between the earthquake magnitude and the initial fault stability, which indicates a threshold of the initial Coulomb Failure Stress to differentiate if faults are initially, critically stressed, and thus, the earthquake magnitude. This Pohang threshold is -0.2 to -0.1 MPa, about one order of magnitude larger than that proposed for natural earthquakes. This study highlights that the reactivation of critically stressed faults may trigger damaging earthquakes even for small poromechanical perturbations and opens a promising avenue for assessing the likelihood of induced earthquakes based on physical understanding.