Three-stage rupture process of the 28 December 1994 Sanriku-Oki earthquake

We investigated the rupture process of the 28 December 1994 Sanriku-Oki earthquake (M(w) = 7.7) using broadband seismograms recorded at local distances. The earthquake rupture nucleated at the eastern end of the aftershock area. As it propagated towards the west, a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture, releasing most of the seismic energy of this earthquake. About 24 s later, another subevent followed at the western end of the aftershock area, emitting large high-frequency waves. The whole rutpure time is about 55 sec. The three-stage rupture process is very similar to the earlier-stage rupture process of the 1968 Tokachi-Oki earthquake (M(w) = 8.2). The difference is that the rupture of the Tokachi-Oki earthquake further propagated towards the north in the following stage instead of terminating at the western end. The occurrence of the Sanriku-Oki earthquake is critical to the validity of the characteristic earthquake so far believed to exist in this part of the plate boundary.