津波に対する島のレンズ効果

Abstract

Focusing effect of islands on the 1983 Nihonkai-chubu (central part of the Japan Sea) earthquake tsunami was identified at eight coasts facing to islands from peak formations of the maximum inundation heights. Defining parameters of the peak height (H) at a coastal focus, the background average height (H0), the peak width (Wd), the coastal focus distance from the island (L) and the island size (L0), we discussed relations among them. As the result peak width Wd is approximated asWd/L0=0.43 (L/L0)1.0Amplification ratio H/H0 is about 1.5 for islands of epicentral distances smaller than 400km and shows an increase with the epicentral distance for islands of epicentral distances larger than 400km. These facts are explained as a focusing effect of islands on tsunamis, in which incident wave, refracted in a sloped region around the island after divided into two, superposes on each other in the back side. In islands distant from the source the incident wave is coherent and the amplification ratio increase. The amplification ratio and relative peak width are compared with those in the another focusing effect, previously found on the 1993 Hokkaido nansei-oki earthquake tsunami. The similar proportionality of peak width and amplification ratio between two tsunamis suggest that the amplification is caused by the same mechanism.