Table 2-1 Three pillars for earthquake-andtsunami-prevention measures
3. The fundamental plan for disaster prevention in Okushiri Port
The fundamental ideas for measures against earthquakes and tsunamis is to determine disaster- prevention measures which effectively combine the 3 pillars shown in Table 2- 1 in relation to the measures to solve problems selected according to the comprehensive disaster- prevention plan, indicated in the "Okushiri Town Regional Disaster- Prevention Plan and the "Plan to Build a Port Resistant to Earthquakes," in order to ensure safety and property of the residents around Okushin Port.
The fundamental plan for Okushiri Port, taking into consideration the following points, is suggested:
(1) The earthquake and tsunami which followed the Hokkaido Nansei- Oki Earthquake caused the most serious damage in history to the areas along the Sea of Japan.
(2) It was found that a blank area (an area where no seismic centers have been recorded) existed in the Sea of Japan coastal areas along Hokkaido and Tohoku. It is feared that tsunami damage due to earthquakes may occur in the future.
(3) Port facilities are constructed mainly against forces such as ordinary wind waves. Thus, they are not designed to withstand forces, such as tsunamis, which exceed the design force.
(4) There is no policy for regional disaster- prevention plans for the area around Okushiri Port.
(5) Regarding disaster- prevention systems, the degree of awareness of evacuation determined whether people lived or died. However, as the speed of the tsunami was unexpectedly fast, even islanders who expected the tsunami were killed.
Based on the above points, the basic plans for disaster prevention are summarized in the following table.
4. Examination of Disaster-prevention Measures
4-1 Examination of disaster-prevention facilities
(1)Outlying facilities which consider the reduction of tsunami force
The following items are considered measures for the outer facilities to reduce the tsunami force.
To avoid losing the function of the breakwater, the tsunami force should be considered as a design external force.
To determine the location of the outlying facilities, the extension of the water areas within the port should be considered (the establishment of offshore breakwaters or similar facilities).
To prevent tsunamis from overflowing the breakwaters, the crown heights of outlying facilities should be increased.
Possible measures for improving the present layout of the port (Fig. 4- 1) are to increase the crown heights of outlying facilities and to improve the north breakwater, which flipped from the tsunami caused by the Hokkaido Nansei- Oki Earthquake, such that it will not do so again.
When these 2 possibilities were compared by computer simulation, the improvement of the north breakwater was determined to be more effective in decreasing the height oftsunami waves.
Furthermore, because expansion of the water area within the port by building an offshore breakwater is thought to be effective in reducing tsunami runup heights, examination of detailed arrangement plans of future breakwaters should be required (Fig. 4-2).
