Bottom-simulating reflectors in the Korea Plateau, East Sea (Japan Sea)

Abstract

Multi-channel seismic reflection profiles from the trough areas of the southern South Korea Plateau, East Sea (Sea of Japan) (Fig. 1A), reveal bottom-simulating reflectors (BSRs). BSRs, recognized from various continental margin and deep-water settings, mark: (1) the base of gas hydrate-bearing sedi-ments or (2) the opal-A/opal-CT phase boundary. The gas-hydrate BSR results from a sharp acoustic impedance contrast between the high-velocity gas-hydrate stability zone above and low-velocity free gas below [Bryan, 1974]. The gas-hydrate BSR thus displays high seismic amplitude and a reverse po-larity with respect to the seafloor reflection. There are about 80 localities worldwide where the gas hy-drate has been inferred from various data; gas-hydrate samples have been recovered at over 20 of the-ses locations [Kvenvolden and Lorenson, 2002]. In contrast, the opal-A/opal-CT BSR has been recog-nized in a limited number of cases [e.g., Davies and Cartwright, 2002]. This BSR results from diagene-sis of biogenic silica from opal-A to opal-CT during burial [Murata and Nakata, 1974]. Therefore, a den-sity increase occurs across the interface between the two forms of opal, producing an acoustic imped-ance contrast adequate to cause a strong reflection with a positive polarity. In this study, we inferred the origin of the BSR observed in the multi-channel seismic reflection data from the southern South Korea Plateau in the East Sea (Japan Sea). We analyzed instantaneous fre-quency, reflectivity, and acoustic impedance to explore the validity of our interpretation. We also esti-mated the geothermal gradients from the sub-bottom depth of the BSR.