HOKKAIDO NANSEI-OKI EARTHQUAKE TSUNAMI AND DISASTER PREVENTION.

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1) Based upon a re-analysis of long-period (20-50 seconds) surface waves, time-and space-distributions of dislocation of the 1993 earthquake are determined. Total earthquake moment amounts to 6.5 x 10^<20> Nm. Moment is large at three points ; shallow point near the epicenter, deep point at the southern edge of the north fault and the whole south fault. A large upheaval near Okushiri island is concluded from the analysis.2) A theory of entraptment of long waves around a conical island is derived. Theoretical results are compared with the numerical and hydraulic experiments. The theory is applied to the 1993 tsunami, and the runup distribution around Okushiri island is explained except for Monai where the maximum runup was found and except for Hamatsumae where runup of 20 m was found although the village is in the sheltered area.3) In order to simulate the high runup at Hamatsumae, the most important is the location of the south fault and its strike relative to the axis of the Okushiri Spur. The spatial grid as fine as 50 m is necessary. In order to simulate the maximum runup at Monai which was obtained at the bottom of a narrow valley, the spatial grid as fine as 5 m is used. The runups at the two places are well simulated.4) Driftwood and floated fishing boats showed a destructive effect on houses. Theories derived on several assumption are used to determined the shape of formulas, and the coefficients are fixed by experiments of large-scale. The formulas for practical use are thus obtained for the first time.5) Problems and difficulties in an emergency are learned from recent experiences and reviewed carefully for establishing an evacuation manual. An example of tsunami hazard map is obtained, based upon the yearly-change of earthquake generation probability, magnitude of earthquakes, assumed position of faults and probability of the highest tsunami height.