1. Introduction
Faults and fault materials are a major controlling factor for superficial and shallow processes such as slope stability, groundwater flow and surface hydrology, underground excavations, hydrocarbons extraction and storage, and mining (De Joussineau & Aydin,2007; Bense et al., 2013; Laubach et al., 2014). Localized deformations at low confining stresses cause the formation of zones characterized by heterogeneous and anisotropic properties (Frankel et al., 2007; Gudmundsson, 2011). As a consequence, landslide susceptibility assessment (Dai et al., 2002: Wang et al., 2014), groundwater flow modeling (Faulkner et al., 2010; Bense et al., 2013) and design of superficial and underground structures (Aydin et al., 2004), require a detailed description of the zones affected by faulting (Faulkner et al., 2010). Fault core and damage zone are definitions which embrace the entire rock mass volume around a fault “plane” (Faulkner et al., 2010; Laubach et al., 2014). Such a volume can be affected by a more or less important deterioration due to the stress and displacement concentration. The fault core is the zone where most of the displacements are accommodated. The damage zone is the portion of rock mass characterized by secondary structures including mainly fractures, secondary faults and zones with more abundant micro-fracturing, porosity and groundwater flow. In landslide susceptibility mapping, the distance from fault core has been frequently used as an index to quantify the potential triggering of fault-related landslide (Wang et al.,2014). However, spatial differences in fault-controlled geometrical characteristics (e.g. fracture density) and the effects of faulting on the mechanical properties of rock (e.g. rock mass strength) are typically defined empirically or at a mesoscale with limited field evidence (Faulkner et al., 2010; Mizoguchi and Ueta, 2013; Laubach et al., 2014), limiting their value. Consequently, we suggest this distance should be the main focus in the geological characterization of fault damage and its engineering importance.
In the geomorphological literature, it has been recognized that the geometrical and mechanical characteristics of a rock mass are both important in controlling relief and stability of slope (Burbank et al., 1996; Crosta et al., 2014; DiBiase et al., 2018; Wang et al., 2020). However, the fault-controlled spatial variation of geometrical characteristics (i.e. fracture density) and a quantitative description of the effects of faulting on the mechanical properties of the rocks within a specific threshold area have rarely been quantified (Caine et al., 1996; Faulkner et al., 2010; Laubach et al., 2014). Such quantification is often hampered by certain conditions mainly including: (1) large faults could result in varying rock mass characteristics within a specific area; (2) changes in lithology along and around the fault could render it difficult to have comparable conditions; (3) the effects of topography and vegetation obscuring damaged rock mass outcrops, limiting their number, size and distribution and then the possibility to build a robust data set; (4) the local erosion of sections of weaker rock mass. At the same time, some of the above listed features can support the characterization and analysis of these damaged zones, as by back analysis of landslides in areas with different landslide types and abundance. The availability of high-resolution topographic data (i.e. laser scanner and photogrammetric point clouds) can be of help at studying both small and large features supporting the description of the degree of fracturing at different spatial scales (Oskin et al., 2007).
As a consequence, in order to assess the landslide susceptibility of the rock mass strength for construction, it is important to define some basic rules for the identification, mapping, sampling and testing of the extent of these zones and the properties of the involved materials (e.g. breccias, cataclasite, mylonite). The total thickness of the fault zone will depend on the size of the fault, the total amount of cumulated displacement, the type of fault, the overburden depth for the considered zone of the fault, the affected lithology. Many of the same factors will also controls the physical, chemical and mechanical characteristics of the fault materials (Laubach et al., 2014). Using recent technologies including Unmanned Aerial Vehicle (UAV), terrestrial laser scanning, and photogrammetry and point cloud analysis software tools (e.g. AgiSoft, Photoscan and Coltop; Jaboyedoff et al.,2007), we attempted to determine the best procedures, investigation approaches, evidence and criteria for defining the threshold distance for damage zones around faults. Combining geometrical, mechanical characteristics and published thermal evidence (Quidelleur et al., 1997), quantitative description of the effects of faulting on rock mass physical and mechanical properties were quantified to reveal the dynamic action of fault.