Geologic inheritance of the 2019 M7.1 Ridgecrest earthquake source faults and the resulting slip distribution

Johanna M. Nevitt, Benjamin A. Brooks, Jeanne L. Hardebeck, & Brad T. Aagaard

Published September 11, 2022, SCEC Contribution #12492, 2022 SCEC Annual Meeting Poster #166

Faults typically form through reactivation, growth, and linkage of pre-existing structures, often leading to nonplanar geometries and mechanical properties specific to the system’s geologic inheritance. The competition between fault geometry and other factors (e.g., lithology) to control slip at Earth’s surface is an open question that is central to our basic knowledge of fault processes and seismic hazards. In this work, we investigate the geologic inheritance of the 2019 M7.1 Ridgecrest, California, earthquake rupture geometry and test its impact on the slip distribution observed at Earth’s surface. The Ridgecrest ruptures occurred within the 148 Ma Independence Dike Swarm (IDS), with individual dikes easily identifiable in satellite and airborne lidar imagery. From this imagery, we made >5,000 measurements of dike orientations and show that spatial variations in rupture geometry generally follow those of the surrounding dike swarm. In outcrop, the dikes often are altered to epidote and chlorite and are accompanied by a set of subparallel fractures. We propose that the Ridgecrest source faults likely developed through reactivation of pervasive structural weaknesses, including dikes and fractures, within the IDS. At Earth’s surface, the M7.1 event resulted in a discrete ~12-km-long maximum slip zone (generally 3-4 m slip, compared to <1.5 m elsewhere), which coincides with where the surface rupture orientation is rotated ~20 degrees clockwise compared to elsewhere. We calculate fault tractions using previously published background stress field constraints and the observed variations in surface rupture geometry at the 500-m scale. The results indicate an elevated prestress ratio (i.e., shear traction/normal traction) along the maximum slip zone, indicating conditions more favorable for fault slip there. We use PyLith to run a series of mechanical finite element models that simulate slip distributions for a range of fault geometries under various loading conditions, including static stress changes due to the M6.4 foreshock. Through comparison of the model results with field data, we find that the inherited rupture geometry exerted a first-order control on the resulting slip distribution. These findings motivate revisiting the development of other large-magnitude earthquake ruptures (1992 M7.3 Landers, 1999 M7.1 Hector Mine) and tectonic provinces within the IDS in eastern California.

Citation
Nevitt, J. M., Brooks, B. A., Hardebeck, J. L., & Aagaard, B. T. (2022, 09). Geologic inheritance of the 2019 M7.1 Ridgecrest earthquake source faults and the resulting slip distribution. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)