Field observations of meteotsunami locally called “abiki” in Urauchi Bay, Kami-Koshiki Island, Japan

Abstract

A seasonal scale field observation extending over a period of 82 days was conducted in Urauchi Bay on Kami-Koshiki Island, to record meteotsunami events, disastrous secondary oscillations locally known as “abiki.” The bay has an elongated T-shape topography with a narrow mouth opening westward to the East China Sea. The area has suffered the effects of meteotsunami causing flooding in residential area and damage to fishing fleets and facilities. A comprehensive observation system for sea level, ocean currents and barometric pressure was deployed to cover the regions within and offshore from Urauchi Bay and the open sea near the island of Mejima in the East China Sea. Vigorous meteotsunami events, where the total height exceeded 150 cm, were observed over five-day periods during the observation period. One or two hours prior to the arrival of meteotsunami events at Kami-Koshiki Island, abrupt 1–2 hPa pressure changes were observed at the Mejima observation site. Pressure disturbances were found to travel eastward or northeastward. The propagation speed was found to nearly coincide with that of ocean long waves over the East China Sea, and as a result, resonant coupling should be anticipated. The incoming long waves were also amplified by geometric resonance with eigen oscillations inherent in the T-shape topography of Urauchi Bay.