Roughness, off-fault damage and frictional melt distribution in anexhumed seismogenic fault: quantitative high resolution data from aDigital Outcrop Model study

The topography of fault surfaces causes significant variations in the near fault stress field, thus controllingfault plane processes such as nucleation, propagation and arrest of earthquake ruptures, off-fault damage andproduction and redistribution of frictional melts during coseismic slip. All these processes affect the earthquakeenergy budget and the pattern of the radiated energy. Here, we quantify the 2D relationships between fault surfacetopography, coseismic off-fault damage and melt distribution on a well-exposed seismogenic fault reproduced in ahigh-resolution photogrammetric digital outcrop model.The fault surface is about 20 m long along strike and 3 m high, strikes EW and dips of 50◦to the South, andis part of a wide fault zone composed of hundreds of fault segments crosscutting the granodiorite of the Aviopluton in the northern Adamello batholith (Italy). The fault experienced dextral transpressive activity, and, sinceone pseudotachylyte fault vein with average thickness of 3 mm could be identified from field and microstructuralobservations, it likely hosted only one main seismic rupture. The coseismic offset could not be measured in thefield, but, based on a wide dataset of coseismic offsets measured on similar fault strands in the area, it was likelyon the order of 1 m. An unknown amount of slip was accumulated at subseismic slip rate before the earthquakepropagation, as suggested by the occurrence of a cataclasite layer crosscut by the pseudotachylyte vein. Thefault footwall crops out on a recently deglaciated rock cliff and preserves thin patches of hanging wall, whereit is possible to measure the thickness of the pseudotachylyte fault vein. The footwall is crosscut by swarms ofpseudotachylyte injection veins aligned in direction roughly perpendicular to the slip vector.The digital outcrop model was reconstructed from 2972 photographs by using the VSFM software. The resultingpoint cloud has sub-millimeter resolution and millimeter accuracy. We extracted several profiles from the pointcloud, both parallel and perpendicular to the slip vector, and characterized their topography and roughness byFourier power spectral analysis. The point cloud was also interpolated to obtain a triangulated mesh, successivelytextured with selected high-resolution field pictures and used to digitize the relevant structures, i.e. injection veintraces, hanging wall and pseudotachylyte patches.Our preliminary analysis suggests that fault wavy topography controls the position of injection veins clusters andthe thickness of the pseudotachylyte fault vein. Injection veins swarms are located along the restraining side ofasperities with wavelength of around 1m. The thickness of the pseudotachylyte fault vein ranges between lessthan 1 mm in restraining segments, to more than 1 cm in releasing bands over similar wavelengths. A significantvolume of frictional melt, between 1.3 and 1.7 l/m2, is drained within injection veins swarms. We propose that thelength scale of fault waviness controlling the distribution of macroscopic off-fault damage and the thickness of thecoseismic frictional melt layer has a scale comparable with that of the coseismic slip.