09:30 〜 09:45
[MAG39-03] A highly attennuative zone beneath the Tokyo Metropolitan area.
キーワード：attenuation, tomography, MeSO-net
The material properties of the complex subduction zone beneath the Tokyo Metropolitan area can be estimated by the seismic attenuation Q of seismic waves observed at local seismic stations. Previous studies have provided us only with the large scale attenuation structure for all Japan (Jin & Aki, 2005; Nakamura et al., 2006; Edwards & Rietbrock, 2009) or only for the shallow part inside the Kanto basin (Kinoshita, 1994; Yoshimoto & Okada, 2009). In this study we aim to derive a detailed picture of the attenuation structure in the crust and upper mantle beneath the Kanto basin. The waveform data used in this study are recorded at the dense seismic array of the Metropolitan Seismic Observation network (MeSO-net). The station network is distributed on five lines with an average spacing of 3 km and in an area with a spacing of 5 km in the central part of Kanto plane. The 296 MeSO-net stations are equipped with a three-component accelerometer at a bottom of a 20-m-deep borehole, signals from which are digitized at a sampling rate of 200 Hz with a dynamic range of 135 dB. The attenuation of seismic waves along their path is represented by the t* attenuation operator that can be obtained by fitting the observed seismic wave amplitude spectrum to a theoretical spectrum using an omega square source model. In order to accurately fit the spectral decay of the signal, only earthquakes that are recorded with intensity greater than 1 in the Japan Meteorological Agency (JMA) intensity scale are selected. The waveforms of 154 earthquakes were selected from the JMA unified earthquake list from January 1st 2010 to May 31st 2011. A grid search method is applied to determine the t* values by matching the observed and theoretical spectra. The t* data where then inverted to estimate a 3D Qp structure under the Tokyo Metropolitan area, using a layered initial Q model. Grid points were set at 15 km spacing in the horizontal direction and with 10 km spacing at depth. We implemented the 3D velocity model estimated by Nakagawa et al., 2012 and in addition we set the initial Q values at 116 for the 0 km grids and to 400 for all the grids below them. The obtained model suggests average Q values of 50〜100 inside the Kanto basin. Furthermore, a low Q zone is observed in the area where the Philippine Sea plate meets the upper part of the Pacific sea plate. This area is located at approximately 40 km depth, beneath the north-east Tokyo and west Chiba prefecture areas and is represented by Q values of 100〜200. Earthquakes occurring on the Pacific plate pass through this low Q area inside the Philippine sea plate and are attenuated significantly. Combined with the detailed velocity structure beneath the Kanto basin, our results help us to understand the material properties of the subducting plates. The implementation of our findings in strong motion simulation studies could help towards a better understanding of the damage area of future earthquakes and mitigate the disaster of the affected areas.