Post-earthquake Fault Zone Overstrengthening Influences Slip during Future Earthquakes

Changes in fault-zone properties over successive earthquake cycles drive variations in fault slip behavior and seismic hazard. Typically, fault zones are believed to evolve towards tabular damage zones surrounding a low cohesion fault core and are characterized by increased fracture density and reduced elastic stiffness. However, interseismic mineral alteration and fracture healing can either weaken or strengthen fault zones, influencing future earthquake ruptures. Here, we document post-earthquake fault-zone-strength enhancement through field and laboratory observations of fault core and damage zone rocks from subsidiary faults partially activated during the 2019 Ridgecrest earthquake sequence. Analysis of coseismic slip observed with InSAR shows that only some portions of faults in the Spangler Hills experienced slip during the Ridgecrest earthquake sequence even though Coulomb failure stress change analysis predicts the entire length of the faults would have been stressed towards failure. Field investigations have revealed the presence of pseudotachylyte along the faults which is evidence of ancient earthquakes. Mineralogical analysis of healed pulverized rock within these fault zones suggests that these early earthquakes occurred near the brittle-ductile transition prior to exhumation. We measured the tensile and uniaxial compressive strength, Young's modulus, Poisson's Ratio, cohesion, and angle of internal friction of exhumed fault zone rocks and nearby plutonic rocks using a Split Hopkinson Pressure Bar. We find that post-earthquake healing via propylitic albitic alteration within the fault zone increased damage zone tensile and compressive strength and stiffness, and fault core cohesion by ~150% in the location where no slip was observed. These observations are further supported by multispectral Landsat and ASTER analyses, which indicate that surface slip along subsidiary faults is preferentially localized within zones of pre-existing phyllic hydrothermal alteration and terminates at the boundaries of propylitic alteration zones. Together, these results demonstrate that fault-zone cohesive healing can exert long-lasting control on fault slip behavior and seismic hazards.