Assessing landslide-tsunami hazard in submarine canyons, using the Cook strait canyon system as an example
Résumé
Tsunami generated by submarine landslides are now recognised as an important hazard, following
several historical events. Submarine landslides can occur in a variety of settings such as on
continental slopes, volcanic slopes, and submerged canyons and fjords. While significant progress has
been made in understanding tsunami generation processes on open slopes, the problem of tsunami
generation by landslides within submarine canyons has received less attention. In this paper we
examine the tsunami hazard posed by submarine landslides in the Cook Strait canyon system, near
Wellington, New Zealand. Understanding of the hazard posed by this tsunami source has practical
value because of its proximity to a populated coast. Our studies also provide general results
highlighting the differences between tsunami generation on open coasts and tsunami generation
within canyons. Geotechnical and geological studies of the Cook Strait region reveal evidence for
many large landslide scars in the canyon walls, these are interpreted to be failures of consolidated
material which descend the slopes on the sides of the canyon. Scouring of the base of the canyon
slopes by strong tidal currents is believed to be an important process in bringing slopes to the point of
failure, with most large failures expected to occur during earthquake shaking. We present the results
of computer simulations of landslide failures using simplified canyon geometries represented in either
2D (vertical slice) or 3D. These simulations were made using Gerris, an adaptive-grid fluid dynamics
solver. A key finding is that the sudden deceleration of the landslide material after reaching the
canyon floor, leads to larger amplitude waves in the back-propagation direction (i.e. in the opposite
direction to the initial landslide motion).