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The SE continental shelf of the Korean Peninsula was deformed by back-arc rifting associated with the separation of the SW Japan Arc in the Cenozoic. However, the deformation character of the inner continental shelf of the peninsula is not well known. High-resolution seismic profiles reveal a structure representative of a strike-slip duplex on the inner shelf that consists of bounding master faults joined by an array of splay faults. Structural interpretation of this duplex enables us to explain the deformation of the inner shelf with three tectonic stages: (1) generation of normal rift faults in the early stage of back-arc rifting in the late oligocene, (2) sinistral slip on the rift faults under northward compression induced by back-arc closing since the Middle Miocene, and (3) dextral slip on the rift faults induced by ENE–WSW compression since the early pliocene. In the first stage, back-arc rifting generated NNE–SSW oriented rift faults. In the second stage, the interior region between a pair of outer rift faults was subjected to NE–SW extension, creating a duplex with the generation of second-order splay faults oriented NW–SE. In the third stage, the interior region was deformed by ENE–WSW compression, causing convex upward stratigraphic reflectors in individual fault blocks. Our study suggests that the geologic deformation of the continental shelf of the SE Korean Peninsula, up to the inner part, is tied closely to the tectonic evolution in the back-arc region in the Pacific subduction zone. [ABSTRACT FROM AUTHOR]

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 Mw 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 Mw 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 Mw 5.0 earthquake is suggested as the source structure, with its attitude consistent with the focal mechanism solution. We propose that the Mw 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 Mw 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. [ABSTRACT FROM AUTHOR]