THE 1958 LITUYA BAY LANDSLIDE AND TSUNAMI — A TSUNAMI BALL APPROACH

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Abstract
Many analyses of tsunami generation and inundation solve equations of continuity and momentum on fixed finite difference/finite element meshes. We develop a new approach that uses a momentum equation to accelerate bits or balls of water over variable depth topography. The thickness of the water column at any point equals the volume density of balls there. The new approach has several advantages over traditional methods: (1) by tracking water balls of fixed volume, the continuity equation is satisfied automatically and the advection term in the momentum equation becomes unnecessary. (2) The procedure is meshless in the finite difference/finite element sense. (3) Tsunami balls care little if they find themselves in the ocean or inundating land. We demonstrate and validate the tsunami ball method by simulating the 1958 Lituya Bay landslide and tsunami. We find that a rockslide of dimension and volume (3 - 6 × 107m3) generally consistent with observations can indeed tumble from 200–900 m height on the east slope of Gilbert Inlet, splash water up to ~ 500 m on the western slope, and make an impressive tsunami running down the length of the fiord. A closer examination of eyewitness accounts and trimline maps, however, finds a "rockslide only" tsunami somewhat lacking in size outside of Gilbert Inlet. This discrepancy, coupled with fact that ~ 3 × 108m3of sediment infilled the deepest parts of Lituya Bay between 1926 and 1959, suggests that the source of the 1958 tsunami was not one landslide, but two. The initial rockslide generated the famous big splash and cratered the floor in front of Lituya Glacier. We propose that the impact of the rockslide destabilized the foundation of the Glacier and triggered a second larger, but slower moving subglacier slide. The subglacier slide induced the fresh normal faults on the collapsed glacier above, helped to bulk up the rockslide tsunami outside of Gilbert Inlet, and supplied most of the infill evident in post-1958 bathymetric charts.

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