2018 Volume 18 Issue 4 Pages 4_111-4_126
During the M 7.3 Kumamoto earthquake of April 16, 2016, the long-period ground motion was observed in Tokyo metropolitan area about 900 km away from the epicenter. It is important to understand the characteristics of the incident seismic motion to the Kanto basin and the response of the basin to the incident for evaluation of the earthquake ground motion in the metropolitan area during a large earthquake in the western part of Japan. First, in order to confirm the incident wave, we examined the velocity traces of F-NET stations from the vicinity of the epicenter to around the Kanto region. A remarkable wave packet with the dominant period of about 10 seconds and with the duration time of about 60 seconds was recognized in the transverse component, and it propagated from the epicentral region to the Kanto basin with the apparent velocity of about 3.3 km/s. This wave packet showed dispersion characteristics and was considered to be Love wave. In addition, the waveforms in the Kanto Mountains area were similar to the waveforms in the western edge of the Kanto basin, and the wave packet was considered to be an incident wave to the basin. Next, we examined the waveform features in the Kanto basin. The wave packet was amplified and the duration time of wave packet was extended in the basin area. However, the dominant period of the seismic motion was about 10 seconds at any observation points. It suggested that the influence of the incident wave to the motions in the basin was so large in this event case. The duration of the wave packet increased with the propagation in the basin and tended to be longer on the east side than the west side of the Tokyo bay. From the multiple filter analysis of the velocity waveform and the semblance analysis for several station sets, it was confirmed that the dispersion of the seismic waves influenced the extension of the duration, and that the wave packets were more dispersed at the eastern observation points. Although the seismic ground motion of the north-south component was dominant in the west side of the Tokyo bay, the large wave packets of the east-west component was recognized in the latter part of the wave traces in the east side. Examining the velocity locus showed that the dominant direction of vibration changed with the lapse of time. In addition, the semblance analysis showed that the direction of wave propagation changed with lapse of time. Those results suggested that bending propagation path of surface waves due to the irregularity of the basin also influenced the generation of later arrivals.
