Deformation processes adjacent to active faults: Examples from eastern California

Major seismogenic faults are embedded within narrow zones of inelastic off-fault deformation (OFD), where both distributed displacement and modification of rock properties occur. Active distributed displacement may affect slip rate estimates, seismic energy radiation and geodynamic models. This study addresses the role of OFD in the displacement history and mechanical behavior of seismogenic faults, by multisite study of deformed geologic features adjacent to 30-60 km long active strike-slip faults of <10 km of dextral displacement in the Mojave Desert, eastern California. We find that distributed displacement accommodates 0 to ̃25% of the total displacement over zones of one to two kilometers width. Displacement occurs mostly within 100-200 m of faults and decreases nonlinearly away from the main fault. We show evidence for distributed displacement through simple shear in the form of parallel secondary faults and progressive deformation of linear markers adjacent to the Calico fault. We also find evidence for shear via rotation and progressive fragmentation adjacent to the Harper Lake fault. Analysis of block dimensions show that blocks tend to decrease in size toward faults and that cumulative length of secondary faults is longer than the main fault by at least a factor of 10. Based on crosscutting relationships and the relationship of OFD to geophysically imaged compliant zones around active faults, we argue that distributed displacement is an active process and suggest that zones of diminished rigidity near faults may be at least in part driven by secondary faulting during the rupture propagation along the main fault.