Landslide Susceptibility Assessment at the Basin Scale for Rainfall- and Earthquake-Triggered Shallow Slides

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.Geosciences 2019, 9, 268 2 of 22 namely in active seismic zones [15][16][17][18]. The effect of propagation of seismic waves can be explained by the subjection of geological materials to alternating cycles of loading and decompression, which produce changes in the interstitial pressure, shear strength, and material cohesion [19][20][21]. The occurrence of landslides following a seismic event is, thus, a consequence of the transient voltages associated with the propagation of the seismic waves [19]. According to Lin et al. [22,23], earthquakes can also increase the weakness of the superficial rocks and soils near the epicentre, thus increasing the susceptibility to landslide occurrence. Indeed, in some cases, earthquakes do not trigger, but they act as a preparatory factor, providing a significant increase in the density of shallow slides triggered by rainfall in the next months or years.In the landslide literature, the idea has been reinforced that landslides with different triggering mechanisms (e.g., rainfall and earthquakes) do not present the same spatial incidence, nor are they similarly conditioned by the same predisposing factors. Thus, several authors have emphasized the importance of an individualized treatment of landslides according to their trigger, namely in what regards the evaluation of landslide susceptibility [24][25][26][27][28].At another level, statistically based landslide susceptibility models explore the relationships between landslides and landslide predisposing factors based on the contrast between conditions associated with un...

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