Focusing Effect of Islands on a Tsunami

Part 1. A Case of the 1993 Hokkaido Nansei-oki Earthquake Tsunami

Abstract

Local amplifications of tsunami were found on the maximum inundation heights at Japanese 10 coasts facing islands in the 1993 Hokkaido nansei-oki earthquake tsunami. For each island-coast geography a peak height (H) at a coastal focus, a background average height (H₀), a peak width (Wd), a coastal focus distance from the island (L) and an island size (L₀) are defined on the space distribution of maximum heights obtained in the surveys and the relations are discussed.
Main results are as follows: Amplification ratio H/H₀, plotted as a function of coastal focus distance from an island, are distributed in the vicinity of 2. The peak width relative to the island size Wd/L₀ is proportional to the focus distance relative to the island size L/L₀. There is a negative correlation relation between the amplification ratio and the relative peak width. These facts suggest that the amplification was caused by a superposition of incident waves on the sea in the rear of the island slope after being separated into two and refracted on the slope. Thus, the amplification at the coast facing islands is explained from a focusing effect of islands on a tsunami.