Tectonic deformation, subaqueous slides, and destructive waves associated with the Alaskan March 27, 1964, earthquake: an interim geologic evaluation

Abstract

The great earthquake which struck Alaska on Good Friday, March 27, 1964, caused severe damage to the coast of south-central Alaska mainly through vertical tectonic displacements, subaqueous slides, and destructive waves of diverse origins. Notable changes in land level occurred over an area in excess of 50,000 square miles in a broad northeast-trending belt more than 500 miles long and as much as 250 miles wide, which lies between the Aleutian Trench and the Aleutian Volcanic Arc. The northwest part of this belt, which includes most of the Kenai Peninsula and the Kodiak Island group, sank as much as 7 feet, bringing some roads, rail lines, docks, and settlements within reach of high tides and producing a fringe of salt-water-killed vegetation along the drowned coasts. The area to the southeast, including most of Prince William Sound and the adjacent continental shelf as far south as southern Kodiak Island, rose generally 4 to 8 feet, and locally at least 33 feet. Some beaches and surf-cut platforms were permanently raised above the reach of tides, resulting in mass extermination of intertidal faunas and floras and impaired usefulness of harbors, channels, and many shoreline installations. Surface faulting was confined to Montague Island, and was dominantly vertical and subsidiary to regional uplift. Of the two known faults one has been traced more than 16 miles on land and about 15 miles in the submarine topography to the southwest of the island. Maximum measured vertical fault displacement on land was 16 feet on one fault and about 18 feet on the other. Submarine uplift of the continental shelf generated a train of long-period large-amplitude seismic sea waves, the first of which struck the outer coasts of the Kenai Peninsula and Kodiak Island between 19 and 30 minutes after the initial shock. The highest waves inundated shorelines locally to elevations of 35 to 40 feet, causing 20 deaths and damage to property all along the coast of the Gulf of Alaska, especially in those areas that had been lowered relative to sea level by tectonic subsidence. The sea waves were recorded on tide gauges throughout the Pacific Ocean and resulted in casualties and local damage at points as distant as British Columbia, Oregon, and California. The earthquake caused widespread subaqueous sliding and sedimentation in Prince William Sound, along the south coast of the Kenai Peninsula, and in Kenai Lake. These slides carried away the port facilities of Seward and Valdez and the small boat harbor at Homer. Local violent surges of water, many of which were generated by known subaqueous slides that occurred during the earthquake, left swash marks as much as 170 feet above water level and caused heavy damage and took 85 lives at Seward, Valdez, Whittier, Chenega, and several smaller communities in Prince William Sound.