Rupture process of the 1946 Nankai earthquake estimated using seismic waveforms and geodetic data

Abstract

The rupture process of the 1946 Nankai earthquake (MJMA 8.0) was estimated using seismic waveforms from teleseismic and strong motion stations together with geodetic data from leveling surveys and tide gauges. The results of joint inversion analysis showed that two areas with large slip are more confined than in previous studies. In our inversion, we assumed spatially varying strike and dip angles and depth of each subfault by fitting those to the actual complex shape of the upper surface of the Philippine Sea plate in the Nankai Trough region. As a result, we calculated the total seismic moment, M0 = 5.5 × 1021 Nm; the moment magnitude, Mw = 8.4; and a maximum slip of 5.1 m, occurring at a point south of Cape Muroto. The estimated slip distribution on the west side of the fault plane appears somewhat complicated, but it explains well the vertical deformations at Tosashimizu and in the vicinity of Inomisaki. Arguments have been made that the westernmost part slipped slowly after the earthquake over a period of days or months as an afterslip because the seismic waveforms can be largely explained without the slip in this part. However, in order to explain the displacement recorded by the tide gauge at Tosashimizu, we conclude that the westernmost part slipped simultaneously with the earthquake. Splay faulting, which was suggested in previous studies, is not required in our model to explain the seismic waveforms and geodetic data.