Evolution of the off-fault deformation of strike-slip faults in a sand-box experiment

Surface deformation associated with strike-slip faults can be distributed in space, with deformation located either along the primary fault strand or around it and referred to as off-fault deformation (OFD). Fault displacement hazard evaluation require to identify and estimate surface slip rates along active fault strands. We calculate the horizontal and vertical displacement of the analogue models surfaces with optical image correlation and photogrammetry, to investigate the OFD's development with increasing cumulative deformation. The criterion uses the gradient of the horizontal displacement norm perpendicular to the basal fault. Below 0.005 (noise level), there is no deformation, up to 0.03, it is off-fault-deformation, above 0.03, it is on-fault. We confirm previous observations made on analogue models that the surface deformation starts with a broad diffuse deformation, then produces fault strands alternating with relay zones that may be abandoned and reactivated. OFD is located first between Riedels, then between synthetic shears, and finally takes place in the relay zones. We also show that the OFD initially accommodates 100% of the applied slip (no faults), then decreases abruptly during the Riedels stage down to 20 to 30% to finally remain stable for the rest of the experiment. The abandonment and reac-tivation of the relay zones has the consequence of maintaining the OFD ratio on a stable value. Our experiments show that, like the OFD ratio, the width of the fault zone decreases with cumulative displacement to reach a stable value. Consequently, the OFD is correlated with this fault zone width and its geometric complexities. The ratios of OFD observed in this study are also consistent with measurements of OFD made on seven natural faults that exhibit different cumulative displacements. Hence our models suggest that strike-slip faults will never reach a continuous, linear geometry, and will always maintain a minimum amount of OFD.