南海トラフの中規模地震で観測された東京湾岸のやや長周期地震動

——2009年駿河湾の地震と2016年三重県南東沖の地震——

抄録

The characteristics of long-period strong ground motions observed in the Tokyo Bay area from the 2009 Suruga Bay earthquake (M_j 6.5) and the 2016 southeast off Mie Prefecture earthquake (M_j 6.5) were studied. The epicenters of two events are in the same direction but the epicentral distance of the 2016 event from Tokyo bay area is about three times as large as that of the 2009 event. The high-frequency ground motion of the 2009 event is significantly larger than that of the 2016 event. However, the spectral amplitudes at frequencies below 0.1 Hz were almost equal level. The multiple-filter analysis and frequency-wavenumber spectral analysis were performed on the low-frequency ground motions around the Tokyo Bay area. The records of the 2016 event were more dispersive than that of the 2009 event and the duration of the ground motion was extended due to the dispersion of surface waves. The surface waves propagated from almost the same direction at frequencies below 0.15 Hz for both event and the coming direction changed from the direction of epicenter to West at higher frequencies. In addition, Love waves propagating from the direction perpendicular to the epicenter direction were detected only in the records of the 2009 event. From these results, it is considered that the surface waves are predominant in both records, but the incident wave fields are different. In order to clarify the characteristics of the seismic wave incident to the Kanto Plain, the records of both earthquakes in the Kanto Mountains and Izu Peninsula were investigated. The record of the 2009 event was short in duration and body waves were predominant. On the other hand, the record of the 2016 event had a long duration and it was thought that surface waves were predominant. The spectral amplitudes at frequencies below 0.1 Hz were almost equal and the spectral ratios of two events were almost the same in the western mountain area and the Tokyo Bay area. The characteristics of the incident waves to the Kanto Plain are reflected in the seismic motions of the Tokyo Bay area. It is presumed that the Nankai Trough accretionary prism amplified the seismic motions of the southeast off Mie Prefecture event, and the amplitude became same level to those of the Suruga Bay event.