Shallow landslide susceptibility assessment in granitic rocks using GIS-based statistical methods: the contribution of the weathering grade map

Shallowlandslides(i.e.,slide,flow,andcomplex)arewidespreadaroundtheworld,affectingthesoilmantleandupperregolith as a result of the weathering of granitic bedrock, and periodically cause enormous social and economic damages. Shallow landslide hazards are predominantly due to the scarcity of warning signs during the pre-failure stage, high velocities reached in the postfailure phase, and an increase in mobilized volumes caused by the entrapment of material in the downhill path of the phenomena. Owing to the abovementioned aspects, susceptibility assessment of shallow landslides in weathered granitic rocks is a relevant issue for landuseplanninganddesignpurposes.Thisstudyproposesathreestepmethodologyforthesusceptibilityassessmentofthesephenomena. The methodology has been tested and validated at the 1:10,000 scaleovera30.4-km2areainsouthernItaly,whereweatheredgranitic rocks are periodically affected by shallow landslides. This methodology is divided into three successive steps: step 1 consists of database creation, with an emphasis on the weathering grade map (including five weathering classes, from class II to class VI, each one characterized by comparable mechanical behavior), and steps 2 and 3 focus respectively on susceptibility map calibration and validation through statistical analyses. The area under the ROC curve (AUC)showsvalues rangingfrom0.95instep 2 (calibration)to0.88 instep3(validation)andisatestamenttothegoodoverallpredictive accuracyofthemethodology.Theobtainedresultsdemonstrateboth the effectiveness and the consistency of the proposed methodology inperformingsusceptibilitymappingofshallowlandslidesinweathered granitic rocks, as well as the important role played by the weathering grade map.