Scripps Institution of Oceanography

The tsunami which devastated the shores of the Hawaiian Islands on April 1, 1946, was caused by a movement of the sea bottom on the northern slope of the Aleutian Trough, south of Unimak Island. The waves traveled southward to Hawaii with an average speed of roughly 490 miles an hour, a wave length of nearly 100 miles, and a height in the open sea which is thought to have been 2 feet or less. The height and violence of the wave attack on Hawaiian shores varied greatly: at some points the waves dashed up on the shore with great violence and to heights as great as 55 feet above sea level; elsewhere they rose slowly and without turbulence. A detailed account is given of the heights the waves reached on the shores of the major islands, and damage done is described. Waves were generally highest on those sides of the islands which were toward the wave origin, and at the heads of sub- marine ridges projecting into deep water. They tended to be lower at the heads of submarine valleys, along shores protected by wide coral reefs, and at the ends of peninsulas projecting into deep water without corresponding submarine ridges. Waves were refracted around circular or nearly circular islands much more effectively than around elongate and rectangular islands. Locally, storm waves superimposed on the crest of the broader swells of the tsunami did considerable damage. The areas suffering heavy damage in the 1946 tsunami are in general those most subject to damage by future tsunamis originating in the North Pacific. Eastern and southern coasts are subject to damage by tsunamis from South America. Western coasts are comparatively safe, although they have suffered some damage from Japanese tsunamis. All Hawaiian shores are subject to possible damage from tsunamis of local origin. Damage from future tsunamis can be lessened by avoiding construction in known danger areas; by construction of suitable sea walls, with open strips behind them; by limiting construction in heavily populated areas of danger to reinforced concrete or other wave-resistant structures; and by raising on stilts the frame structures in rural areas. Loss of life can be lessened or eliminated by establishing a suitable warning system, the warnings being based on instrumental detection of the small preliminary water wave, or the observation of ocean waves by shore stations around the borders of the Pacific Ocean and on mid-Pacific islands nearer the origin of the waves.