We use high-resolution earthquake locations to characterize the
three-dimensional structure of active faults and how it evolves with
fault structural maturity. We investigate the distribution of
aftershocks of several recent large earthquakes that occurred on crustal
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 0.06-0.25 km from the main fault surface of mature faults, and
0.7-1.5 km from the fault surface of immature faults. The total width of
the active shear deformation zone surrounding the main fault plane
reaches ~1.5 km and 2.5-6 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 an existing dynamic rough fault model, we show 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 compare this evolution of fault
structure with several attributes of earthquakes, and find that
earthquake stress drop decreases with fault displacement and hence with