FIELD SURVEY REPORT ON TSUNAMI DISASTERS CAUSED BY THE 1993 SOUTHWESTHOKKAIDO EARTHQUAKE

Detailed field work at Okushiri Island and along the southwest coast o f Hokkaido has revealed quantitatively (1) the advancing direction of tsunami on land, (2) the true tsunami height (i.e., height of tsunami, excluding its splashes, as measured from the ground) and (3) the flow velocity of tsunami on land, in heavily damaged areas. When a Japanes e wooden house is swept away by tsunami, bolts that tie the house to i ts concrete foundation resist until the last moment and become bent to wards the direction of the house being carried away. The orientations of more than 850 of those bent bolts and iron pipes (all that can be m easured, mostly at Okushiri Island) and fell-down direction of about 4 00 trees clearly display how tsunami behaved on land and caused seriou s damage at various places. The true tsunami height was estimated by u sing several indicators, such as broken tree twigs and a window pane. The flow velocity of tsunami on land was determined by estimating the hydrodynamic force exerted on a bent handrail and a bent-down guardrai l by the tsunami through in situ strength tests. Contrary to the wide- spread recognition after the tsunami hazard, our results clearly indic ate that only a Few residential areas (i.e., Monai, eastern Hamatsumae , and a small portion at northern Aonae, all on Okushiri Island) were hit by a huge tsunami, with true heights reaching 10 m. Southern Aonae was completely swept away by tsunami that came directly from the foca l region immediately to the west. The true tsunami height over the wes tern sea wall of southern Aonae was estimated as 3 to 4 m. Northern Ao nae also suffered severe damage due to tsunami that invaded from the c orner zone of the sand dune (8 m high) and tide embankment at the nort hern end of the Aonae Harbor. This corner apparently acted as a tsunam i amplifier, and tide embankment or breakwater can be quite dangerous when tsunami advances towards the corner it makes with the coast. The nearly complete devastation of Inaho at the northern end of Okushiri I sland underscored the danger of tsunami whose propagation direction is parallel to the coast, since such tsunami waves tend to be amplified and tide embankment or breakwater is constructed low towards the coast at many harbors or fishing ports. Tsunami waves mostly of 2 to 4 m in true height swept away Hamatsumae on the southeast site of Okushiri I sland where there were no coastal structures. Coastal structures were effective in reducing tsunami hazard at many sites. The maximum Row ve locity at northern Aonae was estimated as 10 to 18 m/s (TSUTSUMI et al ., 1994), and such a high on-land velocity of tsunami near short is pr obably due to the rapid shallowing of the deep sea near the epicentral region towards Okushiri Island. If the advancing direction, true heig ht, and flow velocity of tsunami can be predicted by future analyses o f tsunami generation and progagation, the analyses will be a powerful tool for future assessment of tsunami disasters, including the identif ication of blind spots in the tsunami hazard reduction.