[SSS14-P09] Analysis of microtremors from aftershock observation in damaged area of the Kumamoto earthquake using seismic interferometry
First, a cross-correlation function between the observation points in Mashiki town and a rock site in the earthquake observation by Yamanaka et al. (2016) was calculated from the vertical records. The cross-correlation function is characterized by dispersive features of the surface wave (Rayleigh wave). It was confirmed that this major phases is surface wave components of the Green function between the two stations. Furthermore, we found that the amplitude of the surface wave was different by the observation point pairs, indicating that there was a change in the subsurface structure in the center of the Mashiki town in the damaged area.
Next, we estimated the Rayleigh wave group velocity between the two points by multiple filter analysis of the cross-correlation function. We made a one-dimensional model from the existing subsurface structural data of the Headquarters for Earthquake Research Promotion, and compared the theoretical group velocity value for the one-dimensional model with the observation value calculated from the cross-correlation function. The theoretical group velocity is similar to the observed ones in non-damaged area, while they are different at the observation point pairs across the damaged areas.
We finally conducted two-dimensional simulation of wave propagation due to a vertical surface source using two models. The first model was derived from the existing data of deep soil layers with almost horizontally flat interfaces. The second model has a fault structure with a rapid step-structure with the low S wave velocity on the near-surface layers. We simulated the propagation of two-dimensional P-SV waves using a difference method The virtual epicenter was placed on the surface of the observation point to extract vertical components of the Rayleigh wave. In the calculation of the previous model, the dispersion of the surface wave has uniform characters in the whole area of the model. In the synthetic motions of the fault structure model, the complex secondary-generated waves such as the wave which had been diffracted by the soft layers from the fault location. We also compared the synthetic motions with the observed correlation function, and found that the computed waveform of the fault structure model is much more similar to the features seen in the observation correlation function than the existing model.