Comparing the strength of Landslide Path Dependency in the downslope and lateral directions using simulated annealing and space-time clustering

Landslide susceptibility is traditionally determined by analyzing various topographic, geological, and hydrological factors, which influence the probability of landslide occurrence. Recent research in Italy and Nepal has shown that landslide susceptibility is also controlled by landslide path dependency (LPD), where previous landslides locally and temporally influence the future landslide susceptibility. Our study focusses on Collazzone (Italy), a region predominantly affected by shallow landslides, supported by multi-temporal landslide inventory from 1939 to 2014. Here, we are comparing the impact of earlier landslides on landslide susceptibility in the downslope and lateral directions. We hypothesize that the LPD has more impact in downslope direction than in lateral direction due to the crucial role played by formation of positive feedback loop of the soil-landslide system. In the downslope direction, landslides can create weakly permeable soil layers that increase the water saturation, thus increasing the probability of subsequent landslides. In contrast, the lateral direction lacks this feedback mechanism, making subsequent landslides less likely.

For testing our hypothesis, we used simulated annealing to make artificial landslide inventories which approximate the real landslide inventory in terms of topographic positioning, but that lack any LPD. After that we calculate Ripley’s K by using a space-time cuboid for these control inventories and for the real inventory. Generalized additive models (GAM) were used to analyze the ratio between real and control Ripley’s K values. GAM results indicate that there is a nonlinear relationship between ratio of real to control Ripley’s K and time difference (dT), lateral (dL) and downslope distance (dD) between consecutive landslides. This ratio has a negative relationship with dL, and dD, while the relationship with dT is weak. Moreover, we found that earlier landslides have a stronger impact on future occurrence of landslides in downslope direction than in lateral direction. Our results provide clear evidence that downslope direction plays a significant role in landslide path dependency.