Application of nonstationary ray decomposition to identifying deep seismic bedrock of the Kanto sedimentary basin, Japan

摘要

This study identifies the deep seismic bedrock in the Kanto sedimentary basin of Japan based on the nonstationary ray decomposition method. The reflections of SH waves can be observed from the local maxima of instantaneous power at the intersections of the upcoming and downgoing rays, namely the major boundaries of a multilayer sedimentary bedrock structure. In particular, the depth of the seismic bedrock can be clearly identified because the greatest instantaneous power appears at the top of the seismic bedrock, where the impedance contrast is the highest. Strongmotion data recorded at 12 KiK-net observatory stations in the Kanto area are used to identify the depth of the seismic bedrock. The velocity structures of these 12 sites have previously been investigated by the vertical seismic profiling (VSP) method up to the depth where the observation well could penetrate. The VSP method revealed the velocity structures up to the seismic bedrock at three sites, while for the remaining sites the seismic bedrock was not reachable. The method presented in this paper performs well on identifying deep subsurface structure and gives travel times of the SH wave to the seismic bedrock at the 12 sites, even at sites with extremely deep seismic bedrock, where the VSP method could not reach. Furthermore, assuming that the shear-wave velocity is constant from the bottom of the observation well to the top of the seismic bedrock, the depths of the seismic bedrock at the nine stations are obtained.