Deformation processes adjacent to active faults - examples from eastern California

Major seismogenic faults occur within bands of inelastic off-fault deformation (OFD), where both distributed displacement and modification of rock properties occur. Active distributed displacement may affect slip-rate estimates, understanding of seismic energy radiation and geodynamic models. This study addresses the role of OFD in the displacement history and mechanical behavior of major seismogenic faults. Here I present a multi-site study of planar and linear geologic features next to active strike-slip faults in the central Mojave Desert. Conservative estimates suggest that distributed displacement accommodates 19 +/- 3% of the total displacement over zones of 1 to 4 km width. Most of this displacement occurs within 100-200 m of faults and decreases exponentially with distance from the main fault. Distributed displacement is accommodated by a combination of block rotation and simple shear across secondary faults. Analysis of block dimensions show that blocks tend to decrease in size toward faults. Conservative estimates indicate that the cumulative length of secondary faults is at least a factor of ten greater then the length of the main fault. Finally, I argue that distributed displacement is an active feature and suggests that zones of diminished rigidity near faults may be at least in part driven by formation and lengthening of secondary faults and their associated OFD zones.