Characteristics of source types revealed from FMT solutions of nine earthquakes in Pohang using the hierarchical Bayesian inversion

Full moment tensor solutions (FMT) delineate complex source processes for various source types of earthquakes that cannot be fully understood by a shear dislocation. However, induced seismicity has illustrated diverse FMT solutions without a specific pattern, even for seismicity linked to fluid injections. The enhanced geothermal system project in Pohang, South Korea, caused small-to-moderate earthquakes during and after several injection periods. An induced nature of the Mw 3.2 earthquake has been recognized due to its spatiotemporal correlation with hydraulic stimulations and the largest Mw 5.4 mainshock was triggered with subsequent aftershocks after the termination of injections. We analyzed FMT solutions of nine events (Mw ≥ 3.2) and interpreted characteristics based on the posterior probability density. The hierarchical Bayesian method enables one to obtain an ensemble of solutions having the posterior probability density and allows us to consider the noise that governs the model complexity. The posterior distribution of the Mw 5.4 event showed a large compensated linear vector dipole (CLVD) of 29 - 65% and isotropic (ISO) parts of 0 - 29%, varying on time shift, frequency bands, and noise model. We note that FMT solutions with near zero ISO were obtained at an intermediate frequency band (0.05-0.1 Hz). These suggest that their source processes were likely to reflect complex faulting at shallow depths rather than a direct relation with fluids. In contrast, the Mw 3.2 event displayed pure-DC mechanisms (80 - 100%) with a lower CLVD (0 - 17%) and limited ISO (< 11%), and we confirmed the reliability of their posterior distribution through further synthetic tests. This indicates that detection for the ISO component of an induced event is not always ensured. Other aftershocks also showed non-DC components but with low contributions. In particular, the two Mw 3.5 events reported 36 - 48%, 1 - 42% of CLVD, and 18 - 25%, 16 - 29% of ISO, respectively. It is implied that the complexity of faulting or some volumetric change may be acted in a specific region on the fault segments. As a result, our results highlight that the different significance of individual non-DC components should be considered before discussing further details of FMT solutions, and this would be possible by dealing with its probabilistic uncertainties.