Landslides in the Upper Submarine Slopes of Volcanic Islands: The Central Azores

Small landslides in the upper submarine slopes of volcanic islands present potential hazards locally because of their high frequency. We examine evidence for landsliding in high-resolution bathymetric data from Faial, Pico, São Jorge and Terceira islands of the Azores. Because the rugged morphology of the upper slopes makes landslides difficult to interpret, we develop two classification schemes for the 1227 identified slope valleys. One scheme addresses how recognizable the valleys were as originating from landslides (whether scarps are prominent or indefinite), whereas the other scheme addresses valley types (whether apparently produced by single or multiple failures). Size distributions are used to assess the relative occurrence of large versus small landslides. Thirteen landslides are predicted to have generated tsunami heights at source of >1 m and one with height of >7 m. Some slopes have gradients far above 30˚, the angle of repose of incohesive clastic sediment, so the seabed in those areas is strengthened perhaps by carbonate cementation, by seismic shaking or by the presence of coherent lava or lava talus. Using all types of slope valleys, Faial and Pico have smaller affected volumes per unit slope area than those of São Jorge and Terceira. These differences could be associated with varied seismic activity, with more frequent earthquakes beneath Faial and Pico preventing the build-up of sediments on their slopes. Submarine landslide statistics are therefore potentially useful for assessing long-term earthquake hazards of volcanic islands in seismically active environments such as the Azores.