Free oscillation of the Japan Sea excited by earthquakes—II. Modal approach and synthetic tsunamis

Abstract

The free oscillations of the Japan Sea excited by the 1964 Niigata and the 1983 Japan Sea earthquakes are examined adopting a modal approach. Normal mode solutions with periods longer than 50 min are obtained numerically for the actual bathymetry provided by the grids of 20 km in size. The eigenvectors, or water height distributions, obtained show that the modes with longest eigenperiods are determined by the size and depth of the whole Japan Sea. Many modes with shorter eigenperiods are regionally trapped in a shallow part such as the continental shelf where the amplitude of eigenvectors is large. The Niigata earthquake excites mainly the regional modes whereas the Japan Sea event excites both kinds of modes equally. The difference in excitation is attributed to the different water depth at the source; the Niigata earthquake occurred on a continental shelf about 100 m deep whereas the depth at the source area of the Japan Sea event is about 2500 m. The synthetic tsunami is computed by a superposition of the normal modes. The spectra agree well with those observed in the period range of 50 to 210 min. Copyright © 1988, Wiley Blackwell. All rights reserved