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    Solid earth sciences


    Progress and application of the synthesis of trans-oceanic tsunamis

    Shingo Watada

    Trans-oceanic tsunami, Historical tsunamis recorded in tide gauge, Tsunami in the gravitationally coupled Earth system, Tsunami phase delay, Tsunami traveltime delay, Tsunami in the eformable Earth, Earthquake fault parameters studied by tsunami waveforms, Tsunami simulation by the phase correction method, Eulerian and Lagrangian variables, Free oscillation and forced oscillation of the Earth with an ocean layer, Compressible density-stratifed ocean

    The tsunami generated by the 2011 Tohoku-Oki earthquake took almost a day to cross the Pacific Ocean, and its details were recorded by the tsunami stations (red triangle) installed on the deep seafloor. The observed tsunami (white line) and the predicted tsunami based on the classic linear long-wave theory (yellow line) are compared at the two stations circled in white.

    Abundant high-quality distant tsunami records from the 2010 Maule (Chile) and 2011 Tohoku-Oki earthquakes have revealed two distinctive features compared to long-wave tsunami simulations. The records show that the traveltime delay of the tsunami increases with distance from the earthquakes, and the initial phase reversal of tsunamis appears and grows systematically. The conventional tsunami theory cannot explain the observed waveforms and traveltimes of distant tsunamis, leading to the need for a new theory to explain and synthesize distant tsunamis. The propagating elevated sea surface of a tsunami compresses seawater and deforms the seafloor and the solid Earth. A propagating tsunami changes the mass distribution of the Earth and results in a spatiotemporal change in gravity, thereby altering the propagating tsunami itself. Incorporating these physics, we developed a new tsunami propagation theory in which a tsunami is naturally treated as a wave in a gravitationally and elastically coupled Earth system composed of solid Earth layers and an ocean layer. Two distinct tsunami simulation techniques based on the new tsunami propagation theory were introduced and confirmed to produce nearly identical tsunami waveforms. One technique treats tsunamis as free waves within a deformable Earth system, while the other treats tsunamis as external pressure and gravitational forces acting on the surface of a deformable Earth system. With the new techniques, the waveform and traveltime differences between the observed and simulated distant tsunamis disappear. Past distant tsunamis recorded by coastal tide gauges, which were not previously studied due to the traveltime and waveform mismatch problems, have become the focus of quantitative tsunami studies analyzing waveforms. New tsunami propagation techniques have been applied to the analysis of distant tsunami waveforms from the past 19 events and have helped to unveil the slip distributions of the past large earthquakes and to determine the earthquake origin time of the trans-Pacific tsunami events recorded by tide gauges since 1854.