Free oscillation of the Japan Sea excited by earthquakes—I. Observation and wave‐theoretical approach

Abstract

Tsunamis caused by the 1964 Niigata and the 1983 Japan Sea earthquakes continued for a long time and appeared to have excited the free oscillation of the almost closed Japan Sea. The spectral analysis of the tide‐gauge records of those tsunamis shows that there are distinct peaks above the noise level in the frequency range between 0.08 and 0.4 mHz. Finite‐difference computations of tsunami for the whole Japan Sea using a supercomputer reproduce those peaks and confirm that they represent the free oscillation of the Japan Sea. The computation is also made for a region including only the SE half of the Japan Sea with an artificial radiation condition on the N and W boundaries. A comparison between the results of these two computations shows the effect of reflections from the NW coast and of the geometry and size of the whole Japan Sea. The simulated spectrum from the SE half region almost agrees with that from the whole Japan Sea for the Niigata tsunami, indicating that the Niigata earthquake excites mainly the regional oscillations. For the 1983 Japan Sea earthquake, only a few peaks are reproduced by the computation for the SE half region, indicating that the free oscillation of the whole Japan Sea is the essential part of the spectrum for the Japan Sea event. Thus the excitation of the free oscillation depends on the earthquake location. Copyright © 1988, Wiley Blackwell. All rights reserved