The SanrikuOki, Japan, Earthquake of December 28, 1994 Mw 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. al., 1985; Nishizawa etal., 1992; Suyehiro and Nishizawa, 1994). The results suggest hat a 50-km-wide aseismic zone exists along the Japan trench. Byrne et al. [1988] suggest hat the primary cause for this type of aseismic zone is the stable slip properties of the unconsolidated or semiconsolidated sediments in that zone. The purpose of this paper is to answer two key questions: did the 1994 Sanriku-oki event rerupture the largest asperity of the 1968 Tokachi-oki earthquake? Did the 1994 event rupture the aseismic zone near the Japan trench? In order to do so, we determine the source process of the 1994 Sanriku-oki earthquake from seismic, tsunami, and geodetic data. Seismological Analysis