5:15 PM - 5:30 PM
[S07-6-04] Long-period later phases observed in the Echigo Plain, Japan during the deep earthquake in the west off Ogasawara Islands of May 30, 2015
On May 30, 2015, a deep earthquake of magnitude 8.1 occurred at a depth of 682 km off the west of the Ogasawara Islands. In case of a deep earthquake, the incident wave to a basin structure can be regarded as a body wave, and therefore the surface wave generated at the basin edge can be evaluated separate from incident surface waves. In addition, since almost the same incident wave can be expected for inside and outside of the basin, it is considered to be effective for evaluation of site factor. The Echigo Plain, Japan is known as an area where the seismic bedrock is deep and long-period ground motion is dominant. To see the influence of the basin structure, we examined the waveform change along the line from Mt. Yahiko to the Echigo Mountains across the Echigo Plain. The waveform of S wave part was common to all observation points. On the other hand, no significant later arrivals were recognized in the waveform of the mountain region, whereas there were complicated later arrivals and the duration was long in the waveform of the plain part. Also, it was able to be confirmed that the travel time delay of the S wave by the low-velocity sedimentary layers and the later phases seemed to propagate from the edge to the center of the basin from the comparison of wave traces. Numerical simulations were carried out to confirm the generation of the later phases by the underground structure. For the numerical calculation, a two-dimensional finite difference method was used. The numerical simulation was able to reproduce the amplification of ground motion and the generation of later arrivals in the basin. However, the reproduction of the duration of the ground motion was insufficient.