Inverse and Forward Modeling of the 1993 Hokkaido Tsunami

Abstract

We performed inverse and forward modeling of the 1993 tsunami around Okushiri Island from the Hokkaido Nansei-oki (Southwest off Hokkaido) earthquake, one of the best documented tsunamis by many field survey teams. For tsunami numerical computations, there are three important factors that control the results: initial conditions, governing equations, and output quantities to be compared with observations. In order to accurately estimate the initial water surface displacement, we first conducted inverse modeling from recorded tsunami waveforms on tide gauges. The linear long-wave theory was adopted to simulate the propagation, and the superposition principle was assumed. The tsunami source thus estimated is also supported by seismological and geodetic data. We next performed forward modeling using this water surface displacement as the initial condition, and computed tsunami run-up on Okushiri Island. The non-linear shallow water equations, including bottom friction and moving boundaries, were adopted. The computed results were compared with field observation data, including run-up heights, inundation area, flow velocity and direction. The computation roughly reproduced the inundation area and tsunami flow direction, but underestimated the run-up heights and velocity, showing the limitation of tsunami numerical modeling method.