  Natural landslides exhibit scaling properties revealed by power law
relationships. These relationships include the frequency of the size (e.g.,
area, volume) of the landslides, and the rainfall conditions responsible for
slope failures in a region. Reasons for the scaling behavior of landslides are
poorly known. We investigate the possibility of using the Transient Rainfall
Infiltration and Grid-Based Regional Slope-Stability analysis code (TRIGRS), a
consolidated, physically-based, numerical model that describes the
stability/instability conditions of natural slopes forced by rainfall, to
determine the frequency statistics of the area of the unstable slopes and the
rainfall intensity (I) - duration (D) conditions that result in landslides in a
region. We apply TRIGRS in a portion of the Upper Tiber River Basin, Central
Italy. The spatially distributed model predicts the stability/instability
conditions of individual grid cells, given the local terrain and rainfall
conditions. We run TRIGRS using multiple, synthetic rainfall histories, and we
compare the modeling results with empirical evidences of the area of landslides
and of the rainfall conditions that have caused landslides in the study area.
Our findings revealed that TRIGRS is capable of reproducing the frequency of
the size of the patches of terrain predicted as unstable by the model, which
match the frequency size statistics of landslides in the study area, and the
mean rainfall D, I conditions that result in unstable slopes in the study area,
which match rainfall I-D thresholds for possible landslide occurrence. Our
results are a step towards understanding the mechanisms that give rise to
landslide scaling properties.
