TSUNAMI FROM THE MARIANA EARTHQUAKE OF APRIL 5, 1990 - ITS ABNORMAL PROPAGATION AND IMPLICATIONS FOR TSUNAMI POTENTIAL FROM OUTER-RISE EARTHQUAKES

The tsunami generated by the Mariana earthquake of April 5, 1990 was observed on the Japanese and Pacific islands as far as Hawaii. The observed tsunami amplitudes are not a simple function of distance from the source but vary with large-scale bathymetry. Numerical computations of tsunami propagation are made for actual bathymetry and the computed amplitudes are compared with the observations. From the comparisons, the seismic moment is estimated to be 1.4 x 10(20) Nm, very similar to that from seismic waves, and indicates that the seismic waves and tsunami are equally excited. The numerical computations also show that the tsunami propagated toward the Japanese coast through two different paths: one is through the trench system with high velocity and small amplitude, the other along the ridge system with low velocity but large amplitude. The two kinds of tsunami are identified on recently-installed ocean bottom pressure gauges. Since the Mariana earthquake was an outer-rise earthquake in a weakly coupled subduction zone, where the size of outer-rise event is larger than underthrusting events, it is possible that even larger earthquakes occur in this region. The tsunami potential from such events must be considered, including the unusual tsunami propagation.