Dangerous Sea Phenomena in the Southern Part of Matua Island, Kuril Ridge

— The results of an analysis of wave observations in Dvoinaya Bay at the southern part of Matua Island (Kuril Ridge) are reported. Experimental studies were carried out in order to determine the probability of amplifying the incoming waves in the coastal zone of the island. The observations provided the records of waves at two points of the bay located at depths of ~10 and 20 m, with a discreteness of 1 s. The spectral analysis of the time series allowed wave processes with a period of 40 s and in the range of periods of 1–12 min to be identified. It is shown that the former are related to infragravity waves; in the range from 1 to 7.3 min, they are seiches of Dvoinaya Bay. In this case, oscillations with a period of about 1.8 min can be manifested in the form of harbor oscillations with a significant increase in their energy during a storm. Waves with a period of ~12 min are of seiche nature as well and are associated with the zero Helmholtz mode, while the effect of “swinging” of these oscillations may occur in the case of a tsunami wave arrival.