A guide to recognising slow-moving subaqueous landslides in seismic and bathymetry datasets

Slow-moving subaqueous landslides (SMSL) have been recognised in many studies from seafloor bathymetric and seismic data, based on shared morphological characteristics with slow-moving subaerial landslides. SMSL are considered a significant geohazard based on results from previous research studies, and the number of studies on these mass movements is actively growing. Since their initial discovery forty years ago, many morphological characteristics have been considered appropriate SMSL identification criteria. However, SMSL vary in their shape, geophysical properties and emplacement processes. Despite this variety, many SMSL studies use the same nomenclature for different processes and products, and a unified classification scheme, defining the different types of SMSL processes and morphological characteristics, has not been established. In the present study, a process-based classification scheme is proposed based on a review of the published literature comprising more than fifty studies from the past forty years. Four distinct SMSL process classes (creep, spread, slow flank slump and slow mudflow) can be inferred from the morphological characteristics and geometrical relationships of thirty-two SMSL examples observed in bathymetry and seismic data (2D and 3D). Internal block character (shape, distribution and relative position, side alignment), lateral variations in seismic facies, fault geometry and transport direction are the main aspects characterising deformation style of SMSLs. Different SMSL classes can coexist in a same depositional environment. However, observations suggest that SMSL classes are not end members of a spectrum and do not appear to evolve into one another. Instead, it may be shown that at least two of these classes (creep and lateral spread) have their own evolutionary pattern of deformation development, affecting the variability of the morphological characteristics of their deposits.