Coseismic Variations in Slip Orientation from Curved Striations and Projectile Playa Soils during the 2019 M7.1 Ridgecrest Earthquake

Elizabeth K. Haddon, Scott E. Bennett, Jason R. Patton, Katherine J. Kendrick, David D. Oglesby, Brian Olson, Christopher B. DuRoss, & Alexandra Pickering

Published August 15, 2019, SCEC Contribution #9843, 2019 SCEC Annual Meeting Poster #220

Porpoising fault striations and overturned soil horizons in China Lake playa record coseismic oscillations in fault slip vector orientation during the July 5, 2019 M7.1 Ridgecrest earthquake. We investigated aspects of the dynamic rupture process by measuring fault slip vectors, building 3D SfM models, and mapping ~2 km of the rupture, where the largest displacements occurred ~2-4 km south of the M7.1 epicenter. Here, the 80°NE–85°SW-dipping primary rupture strikes 325° ± 5°, locally consisting of two or more anastomosing traces up to 50 m apart. Surface rupture is characterized by mole tracks, pressure ridges, pull-apart grabens, and open fissures in lacustrine clay and clay breccia. Fault offset of streams, pebble lags, and sand dunes indicate right-lateral separation averaging ~2 m and up-to-the-NE vertical separation up to 1 m, with a maximum displacement of ~5.5 m. The evolution of slip is recorded by ridge-in-groove style slickenlines preserved on ~10–100 cm-high fault scarp free faces. Striations tend to terminate with a gentle NW plunge, indicating dominantly lateral slip. Some sites preserve remarkably complex striations, characterized by early porpoising reversals in plunge angle, varying ~60° over a <1 m horizontal distance. Striations vary in the sense of vertical slip at the onset of rupture and record two brief periods of up-to-the-SW motion. Flakes of the uppermost few cm of vesicular soil were overturned and flung up to 5 m NE of the final SW-facing scarp, indicating that initial up-to-the-SW motion may have produced vertical peak ground accelerations exceeding 1 g. The final up-to-the-NE scarps along the primary rupture trace resulted from a subsequent coseismic change in slip orientation that produced a second set of convex-down striations cross-cutting the first. During this latter period, at least ~80–100 cm of right-lateral slip and ~20–30 cm of up-to-the-NE vertical slip occurred on curving striations that shallow from ~25°NW to 10°NW. The magnitude (~1.7–2 m) and plunge (~10°NW) of the convoluted, full slip vector is consistent with a near-surface slip estimate based on the finite fault model. Our simulation of the dynamic rupture process will further describe coseismic changes in slip orientation relative to the rupture propagation direction. Cross-cutting relations between striations likely result from coseismic changes in shear traction or localized interactions among the geometrically complex faults due to fault-block rotations.

Key Words
Ridgecrest, Earthquake, Slickenlines, China Lake, Coseismic Slip, Striations

Citation
Haddon, E. K., Bennett, S. E., Patton, J. R., Kendrick, K. J., Oglesby, D. D., Olson, B., DuRoss, C. B., & Pickering, A. (2019, 08). Coseismic Variations in Slip Orientation from Curved Striations and Projectile Playa Soils during the 2019 M7.1 Ridgecrest Earthquake. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes