Abstract
The hydrographic basin of Ribeira Grande (S. Miguel Island, Azores) has a set of characteristics that enhance the occurrence of shallow slides that have been triggered by rainfall and earthquakes. Two landslide inventories were built according to the landslide triggers: Landslide Inventory 2 (LI 2), which includes 174 earthquake-triggered shallow slides occurred in 2005; and Landslide Inventory 1 (LI 1), which includes 442 shallow slides triggered by rainfall in several periods from 2005 to 2016. Both landslide inventories were characterized and compared from the morphometric point of view and were used individually to produce susceptibility models to failure using a simple bivariate state-of-the-art statistical method (the Information Value). The landslide susceptibility Models were validated using success rates, prediction rates, and Kappa statistics. The results show that shallow slides triggered by rainfall and earthquakes in the study area have different morphometric characteristics. It was verified that models produced with LI 1 are very effective in predicting the spatial location of LI 2, but the same does not happen in the inverse situation. Finally, landslide susceptibility models developed with LI 1 and LI 2 for the upper sector of the hydrographic basin (where most landslides occurred), and latter applied to the complete watershed, present more modest predictive results but are more reliable to characterize the landslide susceptibility in the study area.
Highlights
Landslides have received increasing attention from the scientific community in recent decades, and the efforts made to homogenize methodologies regarding the risk analysis associated with landslide hazards are very relevant [1,2,3,4,5].Shallow slides are the most common type of landslide worldwide [6,7,8], typically having a slip surface less than 1.5 m depth and affecting the soil mantle and the upper regolith [9]
The materials and methods used to produce landslide susceptibility models are described in the following subsections, which include the description of the study area and the landslide inventory process, the characterization and comparison of the morphometric characteristics of inventoried landslides, and the landslide susceptibility modelling strategy and explanation of the validation techniques
Results are presented in three sections following the adopted modelling strategy: (i) susceptibility models for the complete hydrographic basin (Model A); (ii) susceptibility models for the upstream sector of the basin (Model B); and (iii) susceptibility models for the complete hydrographic basin using the IV scores of Model B (Model C)
Summary
Shallow slides are the most common type of landslide worldwide [6,7,8], typically having a slip surface less than 1.5 m depth and affecting the soil mantle and the upper regolith [9]. When the relief energy is high, shallow slides tend to evolve into debris flows, which can affect infrastructure, houses, or agricultural land located downslope [8,9,10]. Shallow slides can be triggered by different natural and anthropogenic factors. The development of positive pore pressures in soil profiles resulting from the water infiltration in soils and colluvium during intense rainfall events is usually considered the major triggering mechanism [11,12,13,14]. Earthquakes have been responsible for extensive shallow slide events in different parts of the world, Geosciences 2019, 9, 268; doi:10.3390/geosciences9060268 www.mdpi.com/journal/geosciences
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