Resonant oscillations in the western part of the Peter the Great Gulf in the Sea of Japan

The research aims to study the resonant properties of the water area of the Peter the Great Gulf, in the Sea of Japan. The coast of the gulf is exposed to the threat of tsunamis due to relatively high seismic activity in the Sea of Japan. On May 26, 1983, a tsunami hit several bays of the gulf with wave heights reaching 5 m. As a response, the coasts were equipped with three sophisticated sea level measuring stations. Field measurements with installation of autonomous pressure gauges were carried out August 18–26, 2016, to study the peculiarities of free surface oscillations in the western part of the Peter the Great Gulf. Spectral analysis of the recorded data and the data obtained from the Posyet automated post of the Russian Tsunami Warning Service showed, in particular, that there were several clearly pronounced peaks at periods of 40, 48, 79 and 89 min. These oscillations are coincident for the Sea of Japan and the studied water area with a wide entrance. A finite-volume numerical model was used to calculate free oscillations in the computational domain covering the Sea of Japan region. We performed model calculations with an irregular triangular grid. This allowed us to describe both the outer water area of the Sea of Japan and separate small-scale relief elements in the water area under study, which are important for the formation of local seiches. Isophases and spatial patterns of several resonant oscillations spectra at 15–105 min periods were obtained for the Peter the Great Gulf area. Model calculations carried out in this work showed an evident spatial structure of resonant oscillations. Such research, including field measurements in adjacent coastal waters, is required to identify and monitor potentially hazardous coastal areas. The amplification of resonant oscillations depends on how effectively the specific atmospheric disturbance generates corresponding eigenmodes of the Sea of Japan. In some bays, the amplitudes of the Sea of Japan eigen oscillations can increase due to resonant properties of coastal water areas, even with a low quality factor. If the amplitudes are large, such a double resonance can lead to a meteorological tsunami.