The south flank of Kilauea volcano in Hawai'i is the site of an active submarine landslide, the Hilina slump. The on-land expression is the Hilina fault system, which has exhibited seismicity and significant coseismic ground motion. The offshore extent a nd structure of the slump (e.g., the nature and location of the detachment fault) are very poorly known. Four slope parallel (NE/SW) and seven slope perpendicular (NW/SE) 2-D seismic reflection lines, collected on the R/V Maurice Ewing in January and February of 1998 over the submarine expression of the Hilina slump, have provided insights into this problem.

The upper slopes of the submarine flank of Kilauea are covered by a landward thickening package of sediments, presumably fragmental debris derived from the ocean entry of lava. Bedding is subparallel to the seafloor and laterally continuous; locally, sediments are disturbed and/or truncated at the seafloor. A midslope bench is well defined on several lines; a topographically rough ridge delimits its seaward edge, ponding well bedded sediments behind. In places, the sediments are back-rotated, uplifted, a nd truncated at the seaward edges suggesting post-depositional deformation and erosion. A fairly continuous, deep reflection can be identified on most lines, at a depth of 5-7 km below the sea surface. We interpret this to be the top of the Cretaceous oceanic crust upon which the volcano is built, defining a potential sliding surface. Seaward shallowing reflections rise from the deep ocean crust reflector beneath the upper flank and midslope bench suggesting the presence of compressional faults at depth t hat may be responsible for the back-rotation of the bench and basin. Intermediate depth reflections near the base of the sediments beneath the upper slope parallel the seafloor, and are potential sites for shallow detachments. The outer bench slope is oversteepened, perhaps due to local slumping. Several irregular blocks seaward of the toe of the slope may represent old landslide debris.