Crustal structure beneath eastern Kyushu, Japan, derived from a wide-angle reflection and refraction experiment

Abstract

In 1994 and 1996, seismic refraction experiments were executed in the eastern part of Kyushu, Japan, he target of the experiments was to investigate the crustal structure composed of accretion materials associated with the paleo subduction process. The experiment in 1994 was composed of 3 shots, 195 observation stes, and 195 km measurement line. In the 1996 experiment, there were 3 shots, 236 observation sites, and 228 km measurement line, which extended north. Observation sensors were set at the same places except for the extended line of the second experiment. To establish a P wave crustal structure model, ray-trace forward modeling and inversion technique were used. The final P wave velocity structure model shows that the upper crust is composed of four layers: two sedimentary layers with velocities of 3-4 and 4-5km/s, basement layer, 5.2-5.8 km/s and granitic layer, 6 km/s. The sedimentary layers developed in the southern part of the profile, which corresponds to the North and South Shimant belts. The thickness of the sedimentary layers in the Southern Shimant belt, exceeds 4 km. The basement layer has a complicated form. We found two distinct irregularly folded parts of the basement layer beneath Shimant belts and an abrupt decrease of the amplitudes of seismic waves that pass through beneath those parts. We interpreted the sedimentary layers had developed under a subduction process in Cretaceous (Izanagi plate) and Tertiary (Pacific plate), and the 6 km/s layer rising beneath the Shimant belt might correspond to an under-plating fragment of a plateau