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

Abstract

The 2019 Ridgecrest earthquake sequence initiated on July 4th with 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 dependency of the width of inelastic strain with the degree of fault extension and contraction, indicating wider fault zones under extension than under 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 the transition of strain beyond the tips of dextral faults leading to 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.

Structural Controls Over the 2019 Ridgecrest Earthquake Sequence Investigated by High‐Fidelity Elastic Models of 3D Velocity Structures

“…A slip of up to 2.5 m along the SWF is located slightly above the M w 6.4 hypocenter (Figures 5b, 5c and 7c). These slip patterns and magnitudes are consistent with the field/satellite observations (DuRoss et al., 2020; Gold et al., 2021; Milliner et al., 2021; Ponti et al., 2020; Jobe et al., 2020; Xu et al., 2020) and existing finite‐fault models (Chen et al., 2020; Goldberg et al., 2020; Jin & Fialko, 2020; Liu et al., 2019; Magen et al., 2020; Pollitz et al., 2020; K. Wang et al., 2020; Xu et al., 2020). Furthermore, we calculate the model resolution for the NWF and SWF (supporting information, Figure S4) based on the formulation provided by Loevenbruck et al.…”

“…Field observations and satellite imageries of the coseismic rupture suggest that the Ridgecrest earthquake sequence comprised incipient fault segments, splay faulting, flower structures, subparallel faults, step‐over structures, and pull‐apart zones (length < 10 km) (DuRoss et al., 2020; Gold et al., 2021; Jobe et al., 2020; Milliner et al., 2021; Ponti et al., 2020; Xu et al., 2020). The fault segments might be smoothed out or joined at depth, and current models assume a range of downdip geometrical complexities (Goldberg et al., 2020; Jin & Fialko, 2020; Pollitz et al., 2020; Ross et al., 2019; K. Wang et al., 2020).…”

Fault Friction Derived From Fault Bend Influence on Coseismic Slip During the 2019 Ridgecrest Mw 7.1 Mainshock

“…Such crustal rotations were observed in the current interseismic crustal velocity field using GNSS (Fialko & Jin, 2021). Although there is currently limited constraint of the paleoseismic history of the faults involved in the 2019 rupture sequence, it is thought that they are structurally immature due to the slow velocity of the mainshock rupture (Chen et al., 2020; Goldberg et al., 2020; Ross et al., 2019; K. Wang et al., 2020), the relatively unorganized fracture pattern (Ponti et al., 2020), wide zone of coseismic inelastic finite strain (Antoine et al., 2021; Milliner et al., 2021) and relatively low cumulative displacements (0.3–1.6 km) (Andrew & Walker, 2020; Milliner et al., 2021).…”

Monitoring creep along the Hayward Fault using structure-from-motion photogrammetry of offset curbs

“…An intermediate scale method yet to be tested on creeping faults would be to use a camera-equipped low-flying drone at a street scale for repeat SfM or optical pixel correlation (e.g. Milliner et al, 2021), which could help to quantify the localization of fault deformation occurring near the fault trace versus within the larger zone of deformation measured by other methods.…”