TSUNAMI GENERATION OF THE 1993 HOKKAIDO-NANSEI-OKI EARTHQUAKE

Heterogeneous fault motion of the 1993 Hokkaido Nansei-Oki earthquake is studied by using seismic, geodetic and tsunami data, and the tsunam i generation from the fault model is examined. Seismological analyses indicate that the focal mechanism of the first 10 s, when about a thir d of the total moment was released, is different from the overall foca l mechanism. A joint inversion of geodetic data on Okushiri Island and the tide gauge records in Japan and Korea indicates that the largest slip, about 6 m, occurred in a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shal lowly to the west. The slip on the northernmost subfault, which is dip ping to the east, is about 2 m, while the slips on the southern subfau lts, which are steeply dipping to the west, are more than 3 m. Tsunami heights around Okushiri Island are calculated from the heterogeneous fault model using different grid sizes. Computation on the smaller gri ds produces larger tsunami heights that are closer to the observed tsu nami runup heights. Tsunami propagation in the nearly closed Japan Sea is examined as the free oscillation of the Japan Sea. The excitation of the free oscillation by this earthquake is smaller than that by the 1964 Niigata or 1983 Japan Sea earthquake.