How do huge landslides occur in Hawaii? We can begin to address this question by examining an incipient landslide structure on the mobile south flank of Kilauea volcano. Between 1970 and 1989 this part of the south flank moved in a southeast direction (seaward by 4 m on its eastern end and by 10 m on its western end), parallel to the western boundary of a large offshore topographic bench. We show that abrupt decreases in cumulative flank movement correspond to abrupt decreases in hypocenter concentration. Most of the earthquakes occur in a 7- to 10-km-deep band beneath the south flank, and relative relocation of a subset of these events shows that they are derived from a single horizontal plane which appears to be a decollement. Focal mechanisms for this deep seismicity are dominated by thrust fault mechanisms, with the overlying block moving seaward.

A comparison of south flank bathymetry with decollement structures on other volcanoes suggests that the distal end of this decollement coincides with the distal end of the offshore topographic bench (30--50 km from shore and 5 km below sea level). The fault system forming the Hilina pali is connected with this decollement, so that slip on the Hilina pali's normal faults coincides with slip of the decollement and with reverse faulting of the distal end of the bench. (This interpretation is supported by studies of the events associated with the M7.2 Kalapana earthquake in 1975.) When the Hilina pali is connected to the outer bench with a decollement inferred from seismicity, a wedge 8-km thick adjacent to Kilauea's magma system and 1--2 km thick along the edge of the outer bench (an area over 3000 km2) is defined. The thickness adjacent to the magma system decreases eastward along Kilauea's East Rift Zone and southwestward from Kilauea caldera, whereas the thickness of the lip of the outer bench decreases only eastward. Geologic observations and geophysical studies of the subaerial flank reveal an extensive magma system beneath the Koae fault system as well as beneath Kilauea caldera and its East Rift Zone. We infer that the boundary of the proximal end of the mobile flank is the southern boundary of this magma system and that this boundary coincides with the southern boundaries of both the Koae fault system and Kilauea's East Rift Zone. ¿ American Geophysical Union 1995