Tsunami Waves and Tsunami‐Induced Natural Oscillations Determined by HF Radar in Ise Bay, Japan

Abstract

AbstractTsunami waves and the subsequent natural oscillations generated by the 2011 Tohoku earthquake were observed by two high‐frequency (HF) radars and four tidal gauge records in Ise Bay. The radial velocity components of both records increased abruptly at approximately 17:00 (JST) and continued for more than 24 h. This indicated that natural oscillations followed the tsunami in Ise Bay. The spectral analyses showed that the tsunami wave arrivals had periods of 16–19, 30–40, 60–90, and 120–140 min. The three longest periods were remarkably amplified. Time‐frequency analysis also showed the energy increase and duration of these periods. We used an Empirical Orthogonal Function (EOF) to analyze the total velocity of the currents to find the underlying oscillation patterns in the three longest periods. To verify the physical properties of the EOF analysis results, we calculated the oscillation modes in Ise Bay using a numerical model proposed by Loomis. The results of EOF analysis showed that the oscillation modes of 120–140 and 60–90 min period bands were distributed widely, whereas the oscillation mode of the 30–40 min period band was distributed locally. The EOF spatial patterns of each period showed good agreement with the eigenmodes calculated by the method of Loomis (1975). Thus, the HF radars were capable of observing the tsunami arrival and the subsequent oscillations.