Tsunami Resonance Curve from Dominant Periods Observed in Bays of Northeastern Japan

Abstract

Seiche oscillations were observed under quiet sea conditions in 36 bays in the Sanriku district of northeast Japan from September 24, 2001 to August 11, 2002. A semiconductor pressure gauge was installed at quays in heads of bays to detect a water level for 6 hours with a sampling interval of 1 minute. Dominant periods were determined from the maximum levels in the amplitude spectra. The resulting dominant periods varied from 7 to 60 minutes, with an average of 25 minutes. Most of them corresponded to those of the fundamental mode. The dominant periods were correlated with amplification factors of tsunamis in the same bays using inundation heights observed in four large tsunamis: the 1896 Meiji Sanriku, the 1933 Showa Sanriku, the 1960 Chilean, and the 1968 Tokachi-oki tsunamis. The amplification factor was defined as the ratio of wave heights between head and mouth. The latters were obtained using a continuous function applied to half of inundation heights observed at the open coast. The amplification factors show a clear dependence on the dominant period, having the maximum levels at 12, 7, 47, and 28 minutes for the 1896, 1933, 1960, and 1968 tsunamis, respectively. These periods nearly coincide with dominant periods of 10, 9.8, 49, and 22 minutes observed at Ayukawa tide station for the 1896, 1933, 1960, and 1968 tsunamis, respectively. This verifies that the response curve from the space distribution is similar to the tsunami spectra from the time history. The amplification factors were plotted versus the tsunami periods normalized by the dominant periods of bays in a tsunami resonance curve. The tsunami resonance curve is a unified expression of tsunamis having the maximums at resonance periods. It was shown that the observed dominant periods are adequate for the resonance curve.