In the 2003 Northern Miyagi Prefecture Earthquake with an Mj of 6.4, damage of wooden houses was observed in the focal area including Ishinomaki plain. Strong ground motion observed at Ishinomaki station of the K-NET in the area contains later phases with a dominant peak at a period of 1 second. This study aims at understanding the nature of the later phases observed in the focal area during the main shock from numerical simulation of ground motion using a large parallel computer. First, we conducted an inversion of S-wave motion from aftershocks to 1D S-wave velocity profiles for the deep sediments. Then we constructed a 3D basin model of the deep sedimentary layers from existing data of a gravity anomaly distribution map with the inverted profiles. The shallow soils of Quaternary layers were three-dimensionally modeled using existing data, too. After qualitative understanding of wave propagation in the basin model through 2D simulations, we calculated ground motions during the main shock and one of the aftershocks in the basin with a MPI-based 3D finite difference code. A large parallel computer, TSUBAME, at Tokyo Institute of technology with 200 CPUs was used to simulate ground motions at a period range of more than 0.75 seconds. We found the significant later phases in the synthetic motions at the Ishinomaki station in the basin and concluded that the later phases are the surface waves converted from the S-wave by irregularities of the sediments and amplified with the shallow low velocity layers. The peak ground velocities and seismic intensities calculated from the synthetic motions are also compared with those observed or estimated in previous studies. The calculated peak values and seismic intensities in the basin are similar to the observed ones.

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 A method for estimating near-surface attenuation of S-waves based on PS logging (downhole PS logging) method was examined to clarify its efficiency using the actual measuring data obtained at the test field composed of the horizontal soil layers of Quaternary age. The applicability of this estimating attenuation method was verified by comparing the estimated result with that obtained by the vertical array records (earthquake records). The S-wave attenuations estimated from both methods conform to each other at frequencies higher than around 10 Hz, which is the available frequency range for PS logging. It becomes clear that constant S-wave attenuation model derived from the PS logging method in the high frequency ranges can be applied to high-precision evaluation of earthquake ground motions for earthquake-resistant design.

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 Gravity survey is useful tool to estimate subsurface structure for large area. However, we may face sometime difficult problem to fix the trade-off between depth and density. One solution to solve this problem is to introduce different physical quantities, such as magnetic anomaly etc. Chandler et al. (1981) proposed a technique to estimate location of boundary of medium using Poissons' relation, which provides a relation between gravity and magnetic potentials. We propose a method to improve the Chendler's method for obtaining better solutions. Furthermore, we applied the proposed method to some numerical examples and a real data-set, and showed that appropriate results can be obtained.

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 Seismic interferometry has been extensively used to estimate group velocities of long period surface-wave with the long time series of mirotremors obtained at distant stations. For subsurface exploration in a relatively-narrow area, however, the seismic interferometry should be used in a wide frequency range with short time series of microtremors as possible. We conducted observations of microtremors with six stations distributed along an about 10 km survey line for 2 days, and verified the feasibility of using the seismic interferometry with the short time series of microtremors. We first discussed theoretically about estimation of Rayleigh-wave group velocities by using seismic interferometry as a function of wavelength and station distance. Next, we analyzed the observed microtremors by the seismic interferometry and estimated the Rayleigh-wave group velocities. Examination on the amplitudes of cross correlation functions indicated that the surface-wave signals could be retrieved in the cross correlation functions even if the data length is short. However, the group velocities were estimated appropriately only in the range suggested by the theoretical consideration of wavelength between about 1/2 and 1/50 of station distances. We finally concluded that the data at stations pairs with the short separation provides the surface-wave group velocities in short-period range.

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[in Japanese]

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