EVOLUTION OF SLIP PARTITIONING DURING FAULT REORGANIZATION OF EXPERIMENTAL RELEASING BENDS WITH DIFFERING STRENGTH

Active releasing bends along strike-slip faults display a range of fault patterns. For example, the hot and weak crust hosting the Brawley seismic zone accommodates slip along many distributed and interconnected faults, which is in sharp contrast to the more localized fault network of the Death Valley fault system in cooler and stronger crust. Analog experiments allow us to directly document the complete evolution of releasing bend systems under differing strength conditions. Here, we use a split-box apparatus filled with wet clay of differing strengths to run and analyze two releasing bend experiments. Precut vertical discontinuities within the clay activate with displacement of the basal plate followed by the propagation of new secondary faults. Of note, the weaker clay experiment produces many secondary faults and cross faults that completely replace the precut releasing fault segment while the stronger clay experiment produces fewer secondary faults with no cross faults, and a lasting precut fault. The kinematic efficiency (i.e., strike-slip rate:total velocity) of the respective overall systems differ with the initial fault reorganization stages, but both systems converge to ~85% efficiency. Interestingly, the weaker clay experiment maintained steady kinematic efficiency even during later stages that had significant fault reorganization.

In addition to assessing the overall evolution and kinematic efficiency of faults, we track slip rates at particular sites along faults. The partitioning of strike- and dip-slip among faults depends on the clay strength. In the stronger clay, the precut releasing bend maintains the highest slip rates for the entire experiment. In contrast, some secondary faults in the weaker clay accommodate more slip than portions of the precut releasing bend, which leads to greater variability of slip rates with time. Fault systems under different strength conditions produce different patterns of either delocalized or localized fault evolution. Furthermore, the greater slip rate variability among faults within a delocalized system, such as the Brawley seismic zone, means that seismic hazard estimates based on past slip rate record may be unreliable estimates for future hazard for faults within weak crust.