SUBMARINE SLIDES

Abstract

Areas of the seafloor mapped with multibeam sounding instruments show morphological evidence of considerable mass movement on seafloor slopes from the shore to the deep trenches. Slides and slumps are especially prevalent where large earthquakes occur. Submarine landslides have damaged coastal infrastructure and are costly, they can strike without warning and have been deadly. Key factors in determining whether a landslide will be tsunamigenic are slide volume and the acceleration of a detached mass. Modern oceanographic instrumentation can resolve remnant features in areas of potential future slides and outline areas where past slides indicate dangerously unstable slopes. Monitoring critical areas with bottom instrumentation is a means to show where future slope failure can be anticipated. But the distribution and magnitude of tsunamigenic landslides has yet to be fully appreciated. To raise the proper degree of awareness, the hazards need to be assessed quantitatively which requires more data on the physical properties related to slope failure. Long term assessments of all natural hazards should be part of an area’s infrastructure. Landslides are common on inclined areas of the seafloor, particularly in environments where weak geologic materials such as rapidly deposited fine-grained sediment or fractured rock are subjected to strong environmental stresses. Such stresses are inflicted by earthquakes, large storm waves, and high internal pore pressures. Submarine landslides can involve huge amounts of material and can move great distances: slide volumes as large as 20,000 km3 and runout distances in excess of 140 km have been reported. They occur at locations where the down-slope component of stress exceeds the resisting stress, causing movement along one or several concave to planar rupture surfaces. Some recent slides that originated near shore were conspicuous by their direct impact on human life and activities. Most known slides, however, occurred far from land in prehistoric time and were discovered by noting distinct to subtle characteristics, such as headwall scarps and displaced sediment or rock masses, on acoustic-reflection profiles and side-scan sonar images. Submarine landslides can be analyzed using the same mechanics principles as are used for occurrences on land. However, some loading mechanisms are unique, for example, storm waves. There is a potential for landslides to transform into sediment flows that can travel exceedingly long distances and this is related to the density of the slope material and the amount of shear strength that is lost during catastrophic slope failure.