Narrow Results

Fundamental characteristics of tsunamis propagating over an oceanic ridge are investigated analytically based on the linear shallow water wave theory. The oceanic ridge model used has a triangular shape which is formed by the combination of a constant slope and a bottom of uniform depth. Wave pattern of on-ridge tsunamis varies depending on the obliquity of incident waves. Especially, standing waves are formed when the incident wave angle to the ridge exceeds the critical angle. Then no transmitted waves exist because of the evanescent waves. The present study is applied to the 1996 Irian Jaya tsunami which propagated over the South-Honshu ridge. The characteristics of on-ridge tsunamis observed at Chichijima and Hachijojima, which are close to the South-Honshu ridge, can be partly explained by using the solution derived in the present study.

Oceanic ridges could act as waveguides transferring tsunami energy to thousands of kilometers away, pumping large energy in to far-field regions as a secondary source. The shallow-water wave velocity $c={(gh)}^{1/2}$ can only predict the arrival time of the early signals accurately but can hardly estimate that of ridge-trapped waves. The present study provides a fast analytic prediction method to estimate the arrival time of the subsequent large trapped waves. The method is based on the energy velocity solution of trapped waves over the uniform parabolic-shaped submerged ridge. Records of two-tide gauge stations, located nearby Sand Island and Nawiliwili for the 2011 Tohoku tsunami are chosen to illustrate the application of this method. The Sand Island record shows typical open-ocean island characteristics that the maximum wave height is followed by rapid amplitude decay. While the Nawiliwili record is strongly affected by topographical trapping effect of the Hawaii Ridge, and several subsequent wave trains carrying large energy arrive within one day duration. Further investigations show that the present analytic method is able to estimate the arrival time for these distinguished subsequent trains.