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.