Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

Abstract

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 rupture 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.