Slip distribution controlled by inherited fault geometry during the 2019 M7.1 Ridgecrest earthquake

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

Published August 14, 2020, SCEC Contribution #10550, 2020 SCEC Annual Meeting Poster #130

Though fundamental to mitigating seismic hazards, forecasting fault slip distributions remains a significant challenge due largely to mechanical and geometric crustal heterogeneities. During the 2019 M7.1 Ridgecrest earthquake, anomalously high right-lateral offset (3-4 m versus <1.5 m elsewhere) occurred within a sharply-defined 12-km-long section of the rupture in the epicentral region, which we refer to as the maximum slip zone (MSZ). We show that this slip concentration does not reflect changes in lithology or basin thickness. Rather, it closely follows changes in the surface rupture orientation; within the MSZ, the rupture trend is rotated ~20° clockwise compared to elsewhere. We also show that the varying rupture trend mimics that of the surrounding Independence Dike Swarm, which we characterize with >4500 trend measurements in satellite imagery. The common orientations suggest the fault system may have evolved from the pre-existing dikes, with implications for fault zone mechanical properties.

We construct quasi-static, plane strain finite element models in PyLith to test how fault geometry affects the slip distribution, assuming homogeneous linear elasticity and Coulomb fault friction. In the base case, slip is driven by prescribed initial tractions, which we calculate assuming a uniform background stress field with the average near-field orientation from Hardebeck (2020). We test three model fault geometries that approximate the natural rupture at different scales: planar, nonplanar at ~5 km scale, and nonplanar at ~500 m scale. The planar fault produces the expected elliptical slip distribution and poorly fits the field data. Nonplanarity at the 5 km scale greatly improves the model result, with elevated slip in the MSZ and reduced slip elsewhere. We attribute this result to the more favorable ~30° angle between the MSZ and the maximum compressive stress, compared to ~50° elsewhere. Nonplanarity at the 500 m scale leads to greater slip heterogeneity, matching local slip minima along releasing bends in the MSZ. We also test different initial tractions, accounting for nonuniform background stress orientations and stress changes due to the M6.4 foreshock, finding that the effects are relatively small (<5% change in slip in MSZ compared to base model).

Thus, fault geometry relative to the background stress can control the distribution of fault slip at Earth’s surface and, where well-constrained, should be included in seismic hazard analysis.

Citation
Nevitt, J. M., Brooks, B. A., Hardebeck, J. L., & Aagaard, B. T. (2020, 08). Slip distribution controlled by inherited fault geometry during the 2019 M7.1 Ridgecrest earthquake. Poster Presentation at 2020 SCEC Annual Meeting.

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