Fault roughness at seismogenic depths from LIDAR and photogrammetric analysis

Abstract—Fault surface roughness is a principal factor influencing
earthquake mechanics, and particularly rupture initiation,
propagation, and arrest. However, little data currently exist on fault
surfaces at seismogenic depths. Here, we investigate the roughness
of slip surfaces from the seismogenic strike-slip Gole Larghe Fault
Zone, exhumed from ca. 10 km depth. The fault zone exploited
pre-existing joints and is hosted in granitoid rocks of the Adamello
batholith (Italian Alps). Individual seismogenic slip surfaces generally
show a first phase of cataclasite production, and a second
phase with beautifully preserved pseudotachylytes of variable
thickness. We determined the geometry of fault traces over almost
five orders of magnitude using terrestrial laser-scanning (LIDAR,
ca. 500 to\1 m scale), and 3D mosaics of high-resolution rectified
digital photographs (10 m to ca. 1 mm scale). LIDAR scans and
photomosaics were georeferenced in 3D using a Differential Global
Positioning System, allowing detailed multiscale reconstruction of
fault traces in Gocad . The combination of LIDAR and high-resolution
photos has the advantage, compared with classical LIDARonly
surveys, that the spatial resolution of rectified photographs can
be very high (up to 0.2 mm/pixel in this study), allowing for
detailed outcrop characterization. Fourier power spectrum analysis
of the fault traces revealed a self-affine behaviour over 3–5 orders
of magnitude, with Hurst exponents ranging between 0.6 and 0.8.
Parameters from Fourier analysis have been used to reconstruct
synthetic 3D fault surfaces with an equivalent roughness by means
of 2D Fourier synthesis. Roughness of pre-existing joints is in a
typical range for this kind of structure. Roughness of faults at small
scale (1 m to 1 mm) shows a clear genetic relationship with the
roughness of precursor joints, and some anisotropy in the selfaffine
Hurst exponent. Roughness of faults at scales larger than net
slip ([1–10 m) is not anisotropic and less evolved than at smaller
scales. These observations are consistent with an evolution of
roughness, due to fault surface processes, that takes place only at
scales smaller or comparable to the observed net slip. Differences
in roughness evolution between shallow and deeper faults, the latter
showing evidences of seismic activity, are interpreted as the result
of different weakening versus induration processes, which also
result in localization versus delocalization of deformation in the
fault zone. From a methodological point of view, the technique
used here is advantageous over direct measurements of exposed
fault surfaces in that it preserves, in cross-section, all of the
structures which contribute to fault roughness, and removes any
subjectivity introduced by the need to distinguish roughness of
original slip surfaces from roughness induced by secondary
weathering processes. Moreover, offsets can be measured by means
of suitable markers and fault rocks are preserved, hence their
thickness, composition and structural features can be characterised,
providing an integrated dataset which sheds new light on mechanisms
of roughness evolution with slip and concomitant fault rock
production.