[S-SS26_30PO1] Crustal Structure

[SSS26-P12] Depth variation of the P- and S-wave velocities in the Kanto sedimentary basin inferred from seismic interferometry

*Kazuo YOSHIMOTO¹, Yuma SUGAWARA¹, Kengo KAJIKAWA¹, Manabu KOBAYASHI¹, Kei MASUDA¹, Shunsuke TAKEMURA¹, Naoshi HIRATA², Shin'ichi SAKAI², Hiroshi SATO², Hisashi NAKAHARA³ (1.Nanobioscience, Yokohama City Univ, Yokohama, Japan, 2.ERI, Univ. of Tokyo, Tokyo, Japan, 3.Geophysics, Science, Tohoku Univ, Sendai, Japan)

Keywords:seismic interferometry, Kanto sedimentary basin, sedimentary structure, seismic velocity structure

Information on the seismic velocity structure of the Kanto sedimentary basin is necessary for evaluating the strong and long-period ground motions in the Tokyo Metropolitan area for future large-to-great earthquakes. However, there are few studies on the depth variation of both P and S wave velocities of the sediment, except for the vertical seismic profiling (VSP) measurements at a limited number of deep boreholes. In this presentation, we will report the characteristics of the depth variation of both P and S wave velocities of the sediment on the basis of the seismic interferometry for P and S waves of local earthquakes. Seismic waveforms of 160 local earthquakes recorded by the MeSO-net were analyzed in this study. The autocorrelation of vertical displacement waveform of P wave and SH displacement waveform from a single event were stacked over all events available at each station, respectively, to obtain the P- and S-wave reflection responses of the Kanto sedimentary basin. We successfully found clear P- and S-wave reflections from the bedrock (seismic basement) at 266 observation points. This result indicates that the P-wave seismic interferometry is effective for the exploration of deep sedimentary basin as well as S-wave interferometry. In our dada, two-way travel time between the free surface and the bedrock of P-wave and S-wave (hereafter Tp and Ts, respectively) ranges from 0.5 s to 4.0 s and 2.0 s to 8.0 s, respectively. A graph showing Tp-Ts relation reveals that the trend of its variation is very similar to that reported at Iwatsuki deep boreholes from VSP measurement, even though there is a large scatter of data. Our results indicate that the ratio of P-wave velocity and S-wave velocity of the sediment is approximately 4 at a shallow depth (< 0.5 km) and decreases down to 2 or less at a deep depth (> 2.0 km) in the Kanto sedimentary basin. ACKNOWLEDGMENTS Data provided by the MeSO-net is gratefully acknowledged. This research was partially supported by the Earthquake Research Institute, University of Tokyo.