Roughness, Off-fault Damage and Frictional Melt Distribution in an Exhumed Seismogenic Fault

The topography of fault surfaces causes significant variations in the near fault stress field, and it controls the nucleation, propagation and arrest of seismic ruptures, off-fault damage and, in some earthquakes, the production and redistribution of frictional melts. Here, we quantify the relations between fault surface topography, coseismic off-fault damage and melt distribution on a well-exposed seismogenic fault in the Italian Southern Alps.

The fault surface belongs to a kilometer-wide dextral transpressive fault zone accommodating ca. 1 km of slip within the granodiorite of the Adamello pluton. The selected fault patch is exposed for 20 m along strike and 4 m along dip, and hosts a single and continuous pseudotachylyte fault vein, suggesting that it experienced only one seismic rupture with a coseismic offset of c. 1 m. The fault preserves patches of hanging wall, allowing the original thickness of the pseudotachylyte fault vein to be measured. The footwall and hanging wall are crosscut by swarms of pseudotachylyte injection veins.

The fault surface was reproduced as a photogrammetric Digital Outcrop Model (DOM) with sub-millimeter resolution and millimeter accuracy. We characterized its topography and roughness by Fourier power spectral analysis. Injection veins and the thickness of the fault veins were mapped on the DOM.

The roughness of the fault surface has a self-affine distribution with Hurst exponent between 0.7 and 0.9, comparable with other fault surface profiles measured in the area. Injection veins are clustered along the releasing sides of asperities with wavelengths of 1 - 2 m. The thickness of the pseudotachylyte fault vein ranges between less than 1 mm in restraining bands, to more than 1 cm in releasing bands. A significant volume of frictional melt, between 1.3 and 1.7 l/m², is drained within injection veins swarms.

Our data suggest that the distribution of macroscopic off-fault damage and the migration of frictional melt on the fault surface are controlled by fault waviness of the same order of magnitude as the coseismic slip. Both rupture dynamics and dynamic weakening are thus influenced by meter-scale fault waviness in this 1-meter-slip event, pointing to complex energy partitioning and seismic radiation patterns at the metric scale.