Integrating field observations and fracture mechanics models to constrain seismic source parameters for ancient earthquakes

Most of our understanding of earthquake rupture comes from interpretation of strong-ground-motion seismograms; however, near-rupture-tip fields of stress and particle motions are difficult to resolve. In particular, the decay of frictional resistance from a peak value at the leading tip of the rupture to a residual kinetic value and subsequent healing characterizes the earthquake process, yet the nature of this evolution in situ is still unclear. Knowledge of this coseismic frictional constitutive behavior has been supplemented by laboratory experiments, yet scaling laboratory experiments to natural faults is non-trivial because these experiments do not exactly reproduce the boundary conditions governing natural earthquakes. Field investigations of exhumed faults provide the spatial resolution needed to integrate remote seismological observations, laboratory experiments, and numerical models of the rupture process for natural ruptures. Here we build on previous work that showed that the orientation of pseudotachylyte injection veins found along the Gole Larghe fault zone in granitoid rocks of the Italian Alps can be used to infer rupture directivity and velocity. We demonstrate that the length of these veins can be used to further constrain the rupture size, slip weakening distance, stress drop, and fracture energy. The results are consistent with seismological observations and recent friction experiments incorporating rapid accelerations, placing constraints on coseismic frictional evolution in granitoid rocks.