The Sanriku-oki, Japan, earthquake of December 28, 1994 (M(W) 7.7): Rupture of a different asperity from a previous earthquake

Fault geometry, depth, and slip distribution of the Sanriku-oki earthquake of December 28, 1994 (Ms 7.5) are estimated from seismic waveforms, geodetic measurements, and tsunami waveforms, and compared with those of the 1968 Tokachi-oki earthquake (Mw 8.2), the most recent large earthquake in the epicentral region. Seismic wave inversions indicate a shallowly dipping thrust type mechanism and the focal depth of 22-28 km, representing an underthrust event at the subduction interface. The source time function has an initial stage of 20 s followed by a main pulse of 30 s duration, a shape similar to that of the 1968 Tokachi-oki earthquake. Joint inversion of geodetic and tsunami data shows that the region of the largest slip, 1.7 m, corresponds to a region of relatively small slip for the 1968 Tokachi-oki earthquake. Furthermore, the joint inversion shows that the region of the largest asperity of the 1968 earthquake had essentially zero slip during the 1994 Sanriku-oki earthquake. These results indicate that the dominant asperities ruptured by the 1994 Sanriku and 1968 Tokachi-oki earthquakes are different. The result of the joint inversion also shows that the aseismic zone near the Japan trench was not ruptured by the 1994 earthquake.