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[SSS10-P19] Long-duration ground motion simulation for the great earthquakes along the Nankai Trough
Keywords:Nankai Trough, SDWave, finite difference simulation
We have been performing long-period ground motion simulation by the finite difference (FD) method for various source models assumed along the Nankai Trough. However, it is sometimes difficult to calculate long-duration ground motion due to the instability in which the amplitude increases exponentially as the calculation time step increases. Although we have realized a long-duration ground motion calculation by smoothing the velocity structure model, we could not completely suppress the instability. Therefore, a Smoothing scheme with Diffusionized Wave equation (SDWave) proposed by Imai et al. (2018) was introduced to the FD calculation of GMS developed by NIED for the purpose of suppressing the instability (Maeda et al., 2021, SSJ Fall meeting). The SDWave applies the analogy of the convection-diffusion equation to solve the modified wave equation with diffusion effects, and can selectively attenuate short-period components. Therefore, by appropriately setting the coefficient to be multiplied by the diffusion term, it is expected to eliminate only the influence of instability that occurs in the short period band rather than the period band that is effective for ground motion simulation.
In this study, we performed ground motion simulations with different velocity structure models, source models, and coefficients of the diffusion term, and investigated the effect of the coefficients on the wave field. First, a study using a simple calculation model (half-space medium & point source) was conducted. When the Fourier spectral ratios of multiple calculation results with different smoothing scheme coefficients and calculation result without the smoothing schemes are taken, the spectral ratio can be approximated by a Gaussian function with a coefficient proportional to the smoothing scheme coefficients. Next, a study using a realistic calculation model (3D velocity structure & finite source) was conducted. By properly setting the coefficients, instability was suppressed and long-duration ground motion could be calculated. On the other hand, it was found that the degree of attenuation of the short-period component is not spatially uniform. It is an issue to study the setting policy of the coefficient of the wavefield smoothing scheme in a realistic calculation model and to improve the calculation method such as changing the coefficient temporally and spatially.
Acknowledgments: This study is part of the ' Research Project for Disaster Prevention on the great Earthquakes along the Nankai Trough' funded by the Ministry of Education, Culture, Sports, Science and Technology, Japan.
In this study, we performed ground motion simulations with different velocity structure models, source models, and coefficients of the diffusion term, and investigated the effect of the coefficients on the wave field. First, a study using a simple calculation model (half-space medium & point source) was conducted. When the Fourier spectral ratios of multiple calculation results with different smoothing scheme coefficients and calculation result without the smoothing schemes are taken, the spectral ratio can be approximated by a Gaussian function with a coefficient proportional to the smoothing scheme coefficients. Next, a study using a realistic calculation model (3D velocity structure & finite source) was conducted. By properly setting the coefficients, instability was suppressed and long-duration ground motion could be calculated. On the other hand, it was found that the degree of attenuation of the short-period component is not spatially uniform. It is an issue to study the setting policy of the coefficient of the wavefield smoothing scheme in a realistic calculation model and to improve the calculation method such as changing the coefficient temporally and spatially.
Acknowledgments: This study is part of the ' Research Project for Disaster Prevention on the great Earthquakes along the Nankai Trough' funded by the Ministry of Education, Culture, Sports, Science and Technology, Japan.