Three-dimensional finite-difference waveform modeling of strong motions observed in the Sendai basin, Japan

We perform three-dimensional (3D) finite-difference (FD) waveform modeling of strong motions in the frequency range 0.2 to 1.67 Hz observed in the Sen dai basin. Japan, during the Japan Meteorological Agency magnitude (M-J) 5. 0 1998 Miyagiken-Nanbu earthquake. In a previous, we estimated S-wave veloc ity structures above the pre-Tertiary bedrock at six sites in the Sendai ba sin based on array records of microtremors. To interpolate these velocity s tructures in space we conduct single-station microtremor measurements at a total of 61 locations and estimate the S-wave velocity structure at each si te by modeling the horizontal-to-vertical spectral ratio of fundamental mod e Rayleigh waves. An initial 3D model of the basin is constructed using the velocity structures estimated from both array and single-station microtrem or measurements, along with other information such as surface geology. This model encompasses a region 33 km long, 30 km wide, and 19 km deep. The fin al model is obtained through a trial-and-error process by fitting 3D FD syn thetic waveforms to the bandpass-filtered (0.2 Hz to 1.67 Hz) displacement records 12 stations for the 1998 Miyagiken-Nanbu earthquake. We compute the synthetics using a fourth-order staggered-grid 3D FD method with variable grid spacing. As the 3D model is modified, the source parameters (strike, d ip, rake, and seismic moment) are estimated by a grid search method using 1 D site-specific models derived from the modified 3D model. The observed wa veforms are reproduced well at most stations by the final 3D basin model. T his agreement suggests the validity of the final 3D basin model for theoret ical strong-motion prediction of large earthquakes in the frequency range f rom 0.2 to 1.67 Hz.