Neotectonics at the SE Continental Margin of the Korean Peninsula: Implications for the Back-Arc Region Behind the SW Japan Arc

Abstract

The geological evolution of the SE continental margin of the Korean Peninsula resulted from crustal extension with back-arc rifting to spreading (from the Late Oligocene to the Middle Miocene) and crustal shortening with back-arc closing (from the Middle Miocene to the present), associated with the separation of the SW Japan Arc. Earthquakes occur more frequently in this region compared to other offshore regions of the Korean Peninsula; among them, the M-w 5.0 earthquake that occurred in 2016 is the largest event ever recorded instrumentally. We investigate the geological structure of the epicentral area of the M-w 5.0 earthquake and address neotectonic activity at the margin. Seismic reflection profiles reveal abundant faults in the epicentral area that make up strike-slip fault systems. A fault system encompassing the epicenter of the M-w 5.0 earthquake is suggested as the source structure, with its attitude consistent with the focal mechanism solution. We propose that the M-w 5.0 earthquake occurred due to the reactivation of an extensional fault created during back-arc rifting which currently induces dextral slip under the ENE-WSW-oriented compressional stress field in and around the Korean Peninsula. The maximum magnitude of earthquakes expected at the margin is estimated as no higher than M-w 6.0. Restoration of seismic profiles indicates that the current stress field was established after 5.5 Ma. The S-wave velocity structure of the uppermost mantle shows asthenospheric upwelling elongated along the continental margin, which may be considered an important regional source of the current stress field by inducing convection in the uppermost mantle toward the Korean Peninsula lithosphere.