Inferring earthquake mechanics from exhumed faults
Destructive earthquakes nucleate at 7-15 km depth; therefore monitoring active faults at the Earth's surface or interpretation of seismic waves yields limited information regarding earthquake mechanics. A complementary approach involves the integration of field studies of fossil seismic sources now exhumed at the Earth's surface with laboratory friction experiments that reproduce deformation conditions typical of seismic slip. Microstructural and geochemical comparison of the natural and experimental fault rock materials can be used to constrain boundary conditions for theoretical earthquake models. Here we will discuss the preliminary results of a project that takes such an integrated approach. In particular, rock friction experiments, including experiments in a cutting-edge high-velocity-rock-friction apparatus recently installed in Italy, suggest coseismic fault lubrication at seismogenic depths for a variety of host lithologies and tectonic settings. This result is consistent with estimates from field observations and theoretical analysis of rock friction at seismic slip rates. Moreover, experimental and natural fault products have strikingly similar microstructural and geochemical features, suggesting that experiments reproduce natural deformation processes.
High velocity friction experiments were performed on smooth surfaces and under low normal stress, so direct extrapolation to seismogenic depths should be performed with caution. For instance, the presence of bumps along natural faults might impede the smooth sliding observed in the experiments. To resolve this scaling issue we measured the fault surface roughness of natural seismogenic faults exposed in large glacially polished outcrops over a range of scales (from 100 m to 10 microns) using (1) terrestrial laser-scanning (LIDAR), (2) orthorectified mosaics of high-resolution digital photographs and, (3) scans of thin sections from cores of the slipping zones. LIDAR scans and photomosaics were georeferenced in 3D using a Differential Global Positioning System and gOcad to reproduce large fault surfaces. A numerical model using approximate friction constitutive equations deduced from experiments was developed to investigate the effects of fault roughness on friction. Preliminary results suggest that fault roughness may only slightly reduce the overall lubricating effects operating at high velocities, primarily because slip resistance encountered at geometric barriers is counteracted by opening in extensional zones.