The role of diffraction effects in extreme run-up inundation at Okushiri Island due to 1993 tsunami

Abstract

The tsunami generated on 12 July 1993 bythe Hokkaido–Nansei–Oki earthquake (Mw D7.8) broughtabout a maximum wave run-up of 31.7 m, the highestrecorded in Japan during the 20th century, near the MonaiValley on the west coast of Okushiri Island (HokkaidoTsunami Survey Group, 1993). To reproduce the extremerun-up height, the three-dimensional non-hydrostatic model(Flow Science, 2012), referred to here as the NH-model, hasbeen locally applied with open boundary conditions suppliedin an offline manner by the three-dimensional hydrostaticmodel (Ribeiro et al., 2011), referred to here as the H-model.The area of the H-model is sufficiently large to cover the entirefault region with one-way nested multiple domains. Forthe initial water deformation, Okada’s fault model (1985) usingthe sub-fault parameters is applied.Three NH-model experiments have been performed,namely without islands, with one island and with two islands.The experiments with one island and with two islands giverise to values close to the observation with maximum runupheights of about 32.3 and 30.8 m, respectively, while theexperiment without islands gives rise to about 25.2 m. Thediffraction of the tsunami wave primarily by Muen Island,located in the south, and the southward topographic guidingof the tsunami run-up at the coast are, as in the laboratorysimulation (Yoneyama et al., 2002), found to result in the extremerun-up height near Monai Valley. The presence of HiraIsland enhances the diffraction of tsunami waves but its contributionto the extreme run-up height is marginal.