Crustal structure of the southern Korean Peninsula from seismic waves generated by large explosions in 2002 and 2004

In order to investigate the velocity structure of the southern part of the Korean peninsula, seismic refraction profiles were obtained along a 294-km WNW-ESE line and a 335-km NNW-SSE line in 2002 and 2004, respectively. Seismic waves were generated by detonating 500-1000 kg explosives in drill holes at depths of 80-150 m. The seismic signals were recorded by portable seismometers at nominal intervals of 1.5-1.7 km. Separate velocity tomograms were derived from first arrival times using a series expansion method of travel-time inversion. The raypaths indicate several mid-crust interfaces including those at approximate depths of 2-3, 15-17, and 22 km. The Moho discontinuity with refraction velocity of 7.8 to 8.4 km/s has a maximum depth of 37-39 km under the southern central portion of the peninsula. The Moho becomes shallower as the Yellow Sea and the East Sea are approached on the west and east coasts of the peninsula, respectively. The depth of the 7.6 km/s velocity contour varies from 29.4 km to 36.5 km. The discrepancy in depth between the seismological Moho and the interpreted critically refracting interface may result from the presence of a gradual transition between the crust and mantle. The velocity tomograms show particular crustal structures including (1) the existence of an over 70-km wide low-velocity zone centered at 6-7 km depth under the Okchon fold belt and Ryeongnam massif, (2) existence of high-velocity materials under the Gyeongsang basin, and (3) the downward extension of the Yeongdong fault to depths greater than 10 km.