Isostatic Consequences of Giant Landslides on the Hawaiian Ridge

Abstract

—Giant landslides, like melting glaciers, lead to a redistribution of mass which will have isostatic consequences. Three-dimensional numerical modeling experiments were devised to examine how this mass redistribution affects the isostatic flexural curve. A debris avalanche of 10–40% of pre-slide Oahu is required to account for the 1200–5000 km3 Nuuanu deposit, while only ∼ 1% of pre-slide Hawaii Island is necessary to generate the 200–800 km3 Alika I and II avalanche deposits. Trials were run using 25, 30, and 40 km elastic plate thicknesses (T e ). The island uplift resulting from the Nuuanu slide was calculated to be 23 m and 109 m for 10% and 40% volume slides, respectively, both using T e = 25 km. A rebound of 10 m and 49 m was calculated for the same volumes, respectively, using T e = 40 km. A greater amount of uplift is expressed direct lyover the failed flank, causing the edifice to tilt away from the calved-off portion. The landslide deposit depresses the plate several meters beneath the debris field itself. Smaller slides (e.g., Alika I and II) do not produce as much flexural response, with 17 m and 7 m uplift for T e = 25 and 40 km, respectively. The effects of slow moving, intact slumps where the failed blocks remain relatively close to the island pedestal were examined for the case of the Hilina slump, making up approximately 10% of the Hawaii Island edifice. Perhaps more significant than the uplift for the Hilina slump, comparable to that for the 10% Nuuanu debris avalanche, is the 114 m and 56 m of downwarp beneath its massive slumped foot (T e = 25 and 40 km, respectively). The landslide rebound process, in the case of a relatively large landslide, should be considered as an added component to the evolutionary course of oceanic islands.