Bookshelf Kinematics and the Effect of Dilatation on Fault Zone Inelastic Deformation: Examples from Optical Image Correlation Measurements of the 2019 Ridgecrest Earthquake Sequence

The 2019 Ridgecrest earthquake sequence initiated on July 4th by a series of foreshocks, including a Mw 6.4 event, that culminated a day later with the Mw 7.1 mainshock and resulted in rupture of a set of cross-faults. Here we use sub-pixel correlation of optical satellite imagery to measure the displacement, finite strain and rotation of the near-field coseismic deformation to understand the kinematics of strain release along the surface ruptures. We find the average off-fault deformation along the mainshock rupture is 34% and is significantly higher along the foreshock rupture (56%) suggesting it is a less structurally developed fault system. Measurements of the 2D dilatational strain along the mainshock rupture show a correlation between the degree of fault extension and contraction with the width of inelastic strain, and wider fault zones under extension than shear. Measurements of the vorticity along the main, dextral rupture show that conjugate sinistral faults are embedded within zones of large clockwise rotations caused by two mechanisms i) the transition of strain beyond the tips of dextral faults and ii) bookshelf kinematics. These rotations and bookshelf slip can explain why faults of different shear senses do not intersect one another and the occurrence of pervasive and mechanically unfavorable cross-faulting in this region. Understanding the causes for the variation of fault-zone widths along surface ruptures has importance for reducing the epistemic uncertainty of probabilistic models of distributed rupture that will in turn provide more precise estimates of the hazard distributed rupture poses to nearby infrastructure.