Moletrack mechanics: Deciphering surface rupture processes from mobile laser scanning and mechanical models

Abstract:

The only portions of modern earthquake ruptures that we can directly and readily observe are located at Earth’s surface. Processes of surface rupture, however, and their impacts on the observable deformation remain poorly understood, leaving open fundamental questions in earthquake science (e.g., how much slip reaches Earth’s surface?) and seismic hazard analysis (e.g., how is strain distributed near shallow infrastructure?).

Here we investigate the mechanics of surface rupture using high-resolution imaging and mechanical modeling of the 2019 M7.1 Ridgecrest, CA earthquake rupture. We target a 3 km long section of the maximum slip zone located within the China Lake basin, a Quaternary lacustrine deposit where the rupture is spectacularly exposed. We imaged the study area using mobile laser scanning 3 days after the event, from which we derived digital elevation models (DEMs) with 1 cm resolution. Within the DEMs, we identify fractures as depressions or shadows where no laser returns are recorded. We confirm the features are fractures through comparison with georeferenced ground- and air-based photographs of the study site.

Preliminary analysis indicates that the rupture consists of left-stepping echelon fractures with lengths ranging from ~1-10 m and trending <5° clockwise from the overall rupture trend (~320-345°), consistent with mode III fracture propagation from a parent fault at depth. Contractional steps along the array typically are defined by underlapping fracture tips (along-strike separation ~2-10 m) and across-strike widths of <1 m. The steps are sites of localized uplift (~0.5 m) and often contain two internal mutually-abutting fracture sets that trend ~40-55° clockwise and ~20-40° counterclockwise from the first set. In some cases, a zigzagging fracture through the step suggests formation by linkage of the two internal fracture sets.

We analyze how the fracture characteristics vary spatially with local fault zone orientation, along with the observed and inferred slip. Additionally, in order to give process-based insight into how the local stress state and rupture connectivity affect the magnitude of slip reaching Earth’s surface, we employ 3D mechanical models that calculate the evolving stress state as the propagating fractures interact with one another and Earth’s surface.