Isostatic consequences of giant landslides on the Hawaiian Ridge

Giant landslides, like melting glaciers, lead to a redistribution of mass w hich will have isostatic consequences. Three-dimensional numerical modeling experiments were devised to examine how this mass redistribution affects t he isostatic flexural curve. A debris avalanche of 10-40% of pre-slide Oahu is required to account for the 1200-5000 km(3) Nuuanu deposit, while only similar to 1% of pre-slide Hawaii Island is necessary to generate the 200-8 00 km(3) Alika I and II avalanche deposits. Trials were run using 25, 30, a nd 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 wa s calculated for the same volumes, respectively, using T-e = 40 km. A great er amount of uplift is expressed directly over the failed flank, causing th e edifice to tilt away from the calved-off portion. The landslide deposit d epresses the plate several meters beneath the debris held itself. Smaller s lides (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 s low moving, intact slumps where the failed blocks remain relatively close t o the island pedestal were examined for the case of the Hilina slump, makin g up approximately 10% of the Hawaii Island edifice. Perhaps more significa nt than the uplift for the Hilina slump, comparable to that for the 10% Nuu anu 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 pro cess, in the case of a relatively large landslide, should be considered as an added component to the evolutionary course of oceanic islands.