HR: 1340h
AN: T13E-1635    [Abstracts]
TI: S-wave Velocity Structure in the Kanto Basin from Inverting the HZ Ratios of Rayleigh Waves
AU: * Tanaka, Y
EM: ystanaka@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Koketsu, K
EM: koketsu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Miyake, H
EM: hiroe@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: University of California, Santa Barbara, Santa Barbara, CA 93106, United States
AB: We are carrying out detailed modeling of velocity structure in the Tokyo metropolitan area in order to upgrade strong ground motion prediction. We have proposed an integrated velocity structure model by compiling refraction/reflection, borehole, microtremor, and gravity data by joint inversion method (Tanaka et al., 2005). We here perform further modeling of velocity structure of sedimentary layer up to the depth of seismic basement, by the tuning of S-wave velocity model using HZ ratios of Rayleigh waves extracted from continuously observed data of broadband seismographs. The velocity structure model of Tanaka et al. (2005) was estimated by the refraction/gravity joint inversion method with refraction data obtained by artificial exploding events and gravity data in the whole Kanto basin. The model consists of three sediment layers (Shimosa, Kazusa, and Miura layers).The depths of Kazusa/Miura and sediment/basement interfaces, and the basement velocity distribution are obtained to minimize the residuals of travel times and gravity data. Based on the above velocity model, we applied the H/V spectral peak frequency matching method (Suzuki et. al., 2005) to the tuning of velocity structure model. This method modifies the model to adjust theoretical H/V spectral ratio calculated from the model to observed one averaged from the data of moderate earthquakes. We however found the method had some problems; 1) Extracted spectra contain waves not only the assumed Rayleigh waves, 2) Some stations have not-obvious peak and did not show good match with observed spectra, 3) Locations of large earthquakes around the Kanto basin are unevenly distributed. We then applied the Rayleigh wave HZ ratio inversion method (Tanimoto and Alvizuri, 2006), as the method of extracting Rayleigh waves from long-term microtremor data, and inverting HZ ratio calculated from the whole spectral waveform, for the tuning of structure model including S-wave velocity structure. We show some improvements in this approach; 1) Picking correctly extracted Rayleigh wave with phase-shift characteristics, 2) Trying to fit the shape of Rayleigh wave HZ ratio with much information of spectra from previous method, 3) Using microtremors observed at each stations, and large earthquakes around Kanto basin covers azimuthally from each site. We applied the inversion of the HZ ratio of Rayleigh wave for the modeling of 1-D S-wave velocity structure under the F-net sites in the Kanto basin. We used 1-year data in the term of 1998-2006 from continuously observed data by broadband seismographs of F-net. As a starting model of HZ ratio inversion, we extracted 1-D velocity structure at each site from the result of joint inversion. We show the obtained S-wave structure model contains slightly higher velocity than the initial model. We will apply this method for the earthquake data as opposed to microseisms at highly distributed stations in the Kanto basin.
DE: 7200 SEISMOLOGY
DE: 7290 Computational seismology
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8015 Local crustal structure
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting