[S-SS09] Strong Ground Motion and Earthquake Disaster

[SSS09-05] Numarical simulation of seismic wave propagation in norteastern Japan: effects of source location and structure model

*Yasumaro Kakehi¹, Kazuki Shimomoto¹ (1.Department of Planetology, Graduate School of Science, Kobe University)

Keywords:seismic wavefield, numerical simulation, finite difference method, source location, velocity structure, northeastern Japan

In order to investigate the effects of source location and underground structure on seismic wavefield in detail, we systematically study the changes of seismic wavefield for three different velocity structure models with different complexity in northeastern Japan by numerical simulations using three-dimensional finite difference method. For the source, upper intraslab earthquake is assumed, and the station is put in the land area. Simulations are done for "near," "middle," and "far" cases of source locations, each of which is identified by the epicentral distance from the land station. For FDM calculation, GMS (Ground Motion Simulator) by NIED is used. In the case of "near" source, direct wave is dominant in the calculated wave and the waveform is simple, accompanied with later phases of very small amplitudes. In the case of "middle" source, S-wave becomes complex with later phases with amplitudes larger than that of the direct wave, and the Rayleigh wave follows. In the case of "far" source, the later phases of S-wave and the Rayleigh wave have larger amplitudes, waveforms becomes more complex, and the waveform duration becomes longer. As the the velocity structure model becomes more complex, the wavefield becomes more complex and the later phases have longer duration, independent of source locations. In case of "near" source, however, difference of the waveforms is small, since the amplitudes of the later phases are very small. Simulation results show that the seismic field is affected by the source location in the form of "focal depth and epicenter distance" and "incident angle of direct waves to the discontinuous boundary", and by the velocity structure in the form of "complexity of discontinuous boundaries." In case the source is close and deep for the observation station, the incident angle of the direct wave becomes high angle, and the amplitudes of the reflected waves become very small. And the surface wave does not develop, since the source is deep. Therefore, the waveform becomes simple, dominated by the direct wave. On the other hand, in case the source is far and shallow, the incident angle of the direct wave becomes low angle, and the total reflection (or reflection close to it) occurs, resulting in the large-amplitude reflected waves and the small-amplitude direct wave. Additionally, the surace wave develops well since the source is far and shallow, and the waveform becomes complex. As the velocity structure becomes more relalistic and complex, the structure of the discontinuous boundaries becomes more complex. Then the reflected waves and the surface wave sensitive to the structure are strongly effected by it. In case the source is far and shallow, the effect of the structure appears remarkably on the waveform, while in case the source is close and deep, the effect of the structure is small since the amplitudes of the reflected wave is very small. In case of observing seismic waves from the "far and shallow" earthquake at the land station, the effect of the velocity structure on the wave field becomes strong for the reasons described above. In the evaluation of the wavefield in such a situation, structure modeling with sufficient accuracy becomes significant. Additionally, it becomes difficult to extract source information from the waveforms on land stations. For accurate evaluation of the source term, the observation close to the source is required, in order to make the situation close to the "high-angle incidence" situation bringing simple wavefield. This means seismic observation in the ocean area is significant.