Directivity analysis of the December 28, 1994 Sanriku-oki earthquake (M(w)=7.7), Japan

On December 28, 1994 a segment along the northern Japan trench failed in a M(w) = 7.7 underthrusting earthquake, the Sanriku-oki earthquake. Less than 30 years earlier, the great Tokachi-oki underthrusting earthquake of May 16, 1968 (M(w) = 8.2) occurred in the same region. We examine the rupture process of the recent event, using teleseismic body and surface wave data, and compare our results to the rupture characteristics of the Tokachi-oki earthquake, as estimated by various authors in previous studies. We use an empirical Green's function technique to obtain relative source time functions spanning a large range of the directivity parameter. The strong azimuthal dependence of the durations of these source time functions indicates a significant unilateral component of rupture in a down-dip direction (west-northwest). The main rupture initiated about 35 km southwest of the epicenter approximately 20 seconds after the onset of a low energy precursor. The main Tokachi-oki rupture was also preceded by low moment release. However, it is unclear whether the low moment release preceding both earthquakes represents part of the rupture process or a distinct foreshock. The main rupture areas (asperities) of the Sanriku-oki and the Tokachi-oki earthquakes do not significantly overlap.