Focal mechanisms of recent earthquakes in the Southern Korean Peninsula

We evaluate the stress field in and around the southern Korean Peninsula with focal mechanism solutions, using the data collected from 71 earthquakes (M-L D 1.9-5.2) between 1999 and 2004. For this, the hypocentres were relocated and well-constrained fault plane solutions were obtained from the data set of 1270 clear P-wave polarities and 46 SH/P amplitude ratios. The focal mechanism solutions indicate that the prevailing faulting types in South Korea are strike-slip-dominant-oblique-slip faultings with minor reverse-slip component. The maximum principal stresses ( (1)) estimated from fault-slip inversion analysis of the focal mechanism solutions show a similar orientation with E-W trend (269(-)-275(-)) and low-angle plunge (10(-)-25(-)) for all tectonic provinces in South Korea, consistent with the E-W trending maximum horizontal stress ( (Hmax)) of the Amurian microplate reported from in situ stress measurements and earthquake focal mechanisms. The directions of the intermediate ( (2)) and minimum ( (3)) principal stresses of the Gyeongsang Basin are, however, about 90 deg off from those of the other tectonic provinces on a common (2)-(3) plane, suggesting a permutation of (2) and (3). Our results incorporated with those from the kinematic studies of the Quaternary faults imply that NNW- to NE-striking faults (dextral strike-slip or oblique-slip with a reverse-slip component) are highly likely to generate earthquakes in South Korea.