南海トラフ沿いに発生する歴史的巨大地震の短周期地震波発生の特徴

Abstract

Geographical relationships between short-period seismic wave radiation zone (SPRZ) and large slip area (called asperity in this study) on the fault plane were examined for the 1944 Tonankai and the 1946 Nankai earthquakes in the Showa era, and reappearances of the SPRZs from the Ansei and the Hoei events were also investigated. The SPRZs were evaluated from the inversion analysis of seismic intensity data in the previous study. Most of the centroids of the SPRZs were found at almost the same areas in every event and they were located at the edge of corresponding asperities of the Showa Nankai and Tonankai earthquakes in the forward direction of fault rupture. This suggests that the short-period seismic waves were radiated from the terminal slip points of asperities of the events also in Ansei and Hoei eras on assumption of the same rupture propagation process as those of the Showa events. The sequence of these megathrust events along the Nankai trough included both single-segment rupture events and the multiple-segment rupture events. Magnitude of the short-period seismic wave radiation was deflned and calculated for each SPRZ of the analyzed earthquake in the previous study. We used the result to discuss an influence of multiple-segment rupture for the excitation of the short-period seismic waves from asperities. It was found that the sum of the magnitude values of the SPRZs for a segment of the multiple-segment rupture event was larger than that for the single-segment rupture event occurring from the same segment.