Fracture mode influence on surface rupture: The 2019 M 7.1 Ridgecrest earthquake, California, USA

Earthquake surface rupture results from fracturing of the shallow crust, yet it is rarely studied within a fracture mechanics framework. Here, we combine mobile laser scanning and optical image correlation displacement data to analyze surface rupture fractures in the maximum slip zone of the 2019 M 7.1 Ridgecrest, California (USA), earthquake. The study area comprises a right-bending principal rupture trace with right-lateral and vertical slip of 1−5 m and 0−1 m, respectively, with decreased horizontal and increased vertical slip in the releasing bend. Fracture length tends to vary inversely with right-lateral slip. Segmented, echelon fractures up to ∼5 m long dominate outside the bend, whereas continuous fractures up to ∼20 m long occur within the bend. We estimate the fault-local 3-D displacement gradient tensor as a proxy for near-fault-tip loading and fracture propagation mode. We find that mode III fracturing dominates outside the bend, whereas mode I, mode II, and mixed-mode (I-II-III) fracturing characterize the bend interior. This transition may explain the fracture length distribution. During mode III propagation, fractures twist from the parent tipline into discontinuous segments, whereas for pure modes I and II, the propagating tipline remains intact, favoring longer fractures. Spatial variations in fracture mode may reflect parent fault geometry, neighboring fault strand interactions, rheological conditions, and dynamic effects. Our results show that local fracture mode, rather than slip magnitude alone, influences surface rupture patterns, with implications for fault displacement hazard analysis.