2021 Articles

The Shear Deformation Zone and the Smoothing of Faults With Displacement

Perrin, Clément; Waldhauser, Felix; Scholz, Christopher H.

We use high-resolution earthquake locations to characterize the three-dimensional structure of active faults in California and how it evolves with fault structural maturity. We investigate the distribution of aftershocks of several recent large earthquakes that occurred on continental strike slip faults of various structural maturity (i.e. various cumulative fault displacement, length, initiation age and slip rate). Aftershocks define a tabular zone of shear deformation surrounding the mainshock rupture plane. Comparing this to geological observations, we conclude that this results from the re-activation of secondary faults. We observe a rapid fall off of the number of aftershocks at a distance range of 0.06-0.22 km from the main fault surface of mature faults, and 0.6-1.0 km from the fault surface of immature faults. The total width of the active shear deformation zone surrounding the main fault plane reaches 1.0-2.5 km and 6-9 km for mature and immature faults, respectively. We find that the width of the shear deformation zone decreases as a power law with cumulative fault displacement. Comparing with a dynamic rough fault model, we infer that the narrowing of the shear deformation zone agrees quantitatively with earlier estimates of the smoothing of faults with displacement, both of which are aspects of fault wear. We find that earthquake stress drop decreases with fault displacement and hence with increased smoothness and/ or slip rate. This may result from fault healing or the effect of roughness on friction.