Middle to lower crustal earthquakes in the western East Sea (Sea of Japan) and their implications for neotectonic evolution

Abstract

The western East Sea (Sea of Japan) is a seismically active region. A major earthquake in the western East Sea may cause damage in the eastern Korean Peninsula. The nature of seismicity in the western East Sea is important information for mitigation of seismic hazards. The seismicity in the western East Sea occur around the paleo-rifting structures where seismic and geophysical properties apparently change. Both reverse faulting and strike slip earthquakes occur in the region. Earthquake locations and focal depths are refined. The maximum focal depth generally increases from ∼10 km in the coast to ∼32 km in the continental slope of the western East Sea that is located in distance of ∼60 km from the coast. Mid to lower crust earthquakes continue around the paleo-rifting structures that may extend to the lower crust and Moho. Offshore earthquakes rarely occur in oceanic basins of bathymetry with >2 km. Clustered episodic earthquakes occur in four local offshore areas including the stress-induced areas by the 12 September 2016 ML5.8MW5.4 Gyeongju and 15 November 2017 ML5.4MW5.5 Pohang earthquakes. The spatiotemporal clustering of earthquakes suggests stress accumulation at localized paleo-rifting structures and episodic stress release. The mid to lower crustal earthquakes are confirmed from phase arrival time analyses with collocated vertically-separated sensors. The laterally progressive focal depth changes suggest neotectonic evolution of thrust across the crust in the paleo-rifting structure.