Abstract
AbstractSmall 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 1,227 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.
Highlights
Three areas of high acceleration occur around the slopes of Faial and São Jorge associated with the largest earthquakes in the International Seismological Centre (ISC) catalog
If our suspicion is correct that the landslides have formed over the Holocene, all of these slopes may have experienced as much as 0.5 g peak horizontal acceleration (PHA) over that timescale for at least two reasons
To overcome difficulties of interpretation arising from the rugged morphology of the upper submarine slopes, valleys were first categorized based on the levels of recognition as landslides and whether they appear to have formed by single, multiple or retrogressive failures events
Summary
Giant landslides have been found around many volcanic ocean islands (those located away from convergent margins), including the Canary archipelago (Masson et al, 2002, 2006; Mitchell et al, 2002; Wynn & Masson, 2003), the Hawaiian Islands (Lipman et al, 1988; McMurtry et al, 2004; Moore et al, 1989, 1994), Cape Verdes (Barrett et al, 2019; Masson et al, 2008; Ramalho et al, 2015), Madeira archipelago (Quartau et al, 2018; Santos et al, 2019), Reunion Island (Labazuy, 1996; Ollier et al, 1998), and islands near midocean ridges (Mitchell, 2003). Smaller submarine landslides have been found in the steep upper flanks of volcanic islands and they have generated tsunamis (Alberico et al, 2018; Casalbore et al, 2011, 2018; Casas et al, 2016; Chiocci et al, 2008; Fornaciai et al, 2019; Kelfoun et al, 2010; Rahiman & Pettinga, 2006; Tinti et al, 2005; Tommasi et al, 2008), but their threats are less well known. One motivation for this research was to investigate the sizes and geometries of upper-slope submarine landslides among the islands, in order to assess whether the landslides were large enough to generate tsunamis
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