Interaction between regional stress state and faults: Complementary analysis of borehole in situ stress and earthquake focal mechanism in southeastern Korea

Abstract

We characterize the present-day stress tensor in southeastern Korean Peninsula using two different sets of data (geotechnical in situ stress data and earthquake focal mechanism solutions), to understand the regional contemporary stress state and its relationship to the population of faults. Both sets of data show a comparable result of ENE–WSW maximum compression direction, which is in accord with the first order pattern of tectonic stress direction in the eastern Eurasian plate. More rigorous analyses of in situ stress as well as the inversion of focal mechanism show that the current stress field exhibits a systematic heterogeneity in its orientations and magnitudes, possibly caused by the influence of faults. The minimum and maximum horizontal principal stresses normalized by vertical stress at the shallow depths where stress measurements were conducted vary spatially. It turns out that the magnitude of stress field appears to be inversely correlated with the density of regional scale faults. This suggests that a stress release due to faulting may be one of the major factors that contribute to the low stress regime in the region. As a way to confirm the inference, we examine the attitudes of recently activated Quaternary faults with respect to the current stress field. A majority of the faults are oriented in the optimal directions for slip, as indicated by the overall high ratios of shear to normal stress acting on fault planes for the given stress condition, which implies that they might sustain the current stress field. The contemporary earthquake distribution indicates that the lower stressed region has a denser population of seismic activities, suggesting that fault strength in the corresponding region may be at frictional limit with the contemporary stress state. This may imply that the heterogeneity of the regional stress state is a result of the heterogeneity of the strength of faults.