Tectonic stress orientations and magnitudes, and friction of faults, deduced from earthquake focal mechanism inversions over the Korean Peninsula

We characterize the present-day stress state in and around the Korean Peninsula using formal inversions of earthquake focal mechanisms. Two different methods are used to select preferred fault planes in the double-couple focal mechanism solutions: one that minimizes average misfit angle and the other choosing faults with higher instability. We invert selected sets of fault planes for estimating the principal stresses at regularly spaced grid points, using a circular-area data-binning method, where the bin radius is optimized to yield the best possible stress inversion results based on theWorld Stress Map quality ranking scheme. The inversions using the twomethods yield well constrained and fairly comparable results, which indicate that the prevailing stress regime is strike-slip, and the maximum horizontal principal stress (S_Hmax) is oriented ENE-WSW throughout the study region. Although the orientation of the stresses is consistent across the peninsula, the relative stress magnitude parameter (R-value) varies significantly, from 0.22 in the northwest to 0.89 in the southeast. Based on our knowledge of the R-values and stress regime, and using a value for vertical stress (S_v) estimated from the overburden weight of rock, together with a value for the maximum differential stress (based on the Coulomb friction of faults optimally oriented for slip), we estimate the magnitudes of the two horizontal principal stresses. The horizontal stress magnitudes increase from west to east such that S_Hmax/S_v ratio rises from 1.5 to 2.4, and the S_hmin/S_v ratio from 0.6 to 0.8. The variation in the magnitudes of the tectonic stresses appears to be related to differences in the rigidity of crustal rocks. Using the complete stress tensors, including both orientations and magnitudes, we assess the possible ranges of frictional coefficients for different types of faults. We show that normal and reverse faults have lower frictional coefficients than strike-slip faults, suggesting that the former types of faults can be activated under a strike-slip stress regime. Our observations of the seismicity, with normal faulting concentrated offshore to the northwest and reverse faulting focused offshore to the east, are compatible with the results of our estimates of stress magnitudes.