Surface roughness of ancient seismic faults exhumed from seismogenic depths (Gole Larghe Fault, Italian Alps): a combined LIDAR and high-resolution photogrammetric analysis

Fault surface roughness is a principal factor influencing earthquake mechanics, however little data currently exists 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 and hosted in granitoid rocks of the Adamello batholith (Italian Alps). We determined the geometry of fault traces over five orders of magnitude using terrestrial laser-scanning (LIDAR, >100 m to 1 m scale), and 3D mosaics of high-resolution rectified digital photographs (10 m to <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 used in this study has the advantage, over classical LIDAR-only surveys, that the spatial resolution of rectified photographs can be very high (0.2 mm/pixel in this study), allowing for detailed outcrop characterization. Fourier power spectrum analysis of the fault profiles revealed a self-affine behaviour over 3 to 5 orders of magnitude, with Hurst exponents ranging between 0.5 and 0.8. Parameters from the Fourier analyses have been used to reconstruct synthetic 3D fault surfaces with an equivalent roughness by means of 2D Fourier synthesis. Roughness anisotropy is always small to negligible for the Gole Larghe Fault Zone, while roughness of pre-existing joints is not significantly different from the seismogenic fault surfaces. These observations are consistent with the generally small offsets shown by individual seismogenic fault surfaces, and indicate that precursor joints have a strong influence on the roughness of the fault surfaces. 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.