Seismic evidence for reverse activation of a paleo-rifting system in the East Sea (Sea of Japan)

Abstract

The Japanese Islands were separated from the Eurasian plate due to continental rifting during the Oligocene to mid-Miocene, which caused the opening of the East Sea (Sea of Japan). Such tectonic evolution in the East Sea is important for understanding the evolution of back-arc regions with active convergent margins. To understand the evolution of the paleo-rifted back-arc region, we investigate seismicity, crustal seismic anisotropy, focal mechanism solutions and ambient stress field around the Korean Peninsula. The Korean Peninsula displays diffused seismicity with small and moderate earthquakes. Shallow earthquakes rarely occur in the central East Sea. The crustal fast shear-wave polarization directions in the Korean Peninsula are observed to vary in azimuth between 40° and 90°. The focal mechanism solutions are calculated by long period waveform inversions. The ambient stress field is calculated from the focal mechanism solutions. The compressional stress field in the Korean Peninsula is observed to be in ENE-WSW, which is consistent with the fast shear-wave polarization directions. The compressional stress directions in the East Sea progressively change from ENE to SE with increasing longitude. The rapid change of compressional directions in the central East Sea prohibits accumulation of stress, causing rare shallow seismicity. High seismicity of reverse faulting events is observed at the fringes of the East Sea, in particular, around the east coast of the Korean Peninsula and the west coast of Japanese Islands, which correspond to paleo-rifted margins where compressional stresses are accumulated. The compressional stress field and active thrustal events suggest reverse activation of paleo-normal faults that were developed during the opening of the East Sea.