Recent Damaging Earthquakes on Conjugate Structures in the Walker Lane: Characteristics of the The Nine Mile Ranch Sequence (2016-2019) and Comparison to the Ridgecrest Sequence of 2019

Rachel L. Hatch, Ken D. Smith, Rachel E. Abercrombie, Christine J. Ruhl, William C. Hammond, & Ian Pierce

Published August 15, 2019, SCEC Contribution #9824, 2019 SCEC Annual Meeting Poster #263

Two damaging earthquake sequences have occurred on conjugate fault structures in the Walker Lane tectonic zone in the last 3 years. The Ridgecrest sequence (July 2019, M6.4 and M7.1) in the eastern California Shear Zone, the southern part of the Walker Lane, is the focus of much ongoing work. Further north in the Walker Lane, three moderate sized events (M5.6, M5.4, M5.5) occurred on Dec 28 2016, within an hour of each other near Nine Mile Ranch, southwest of Hawthorne, NV. We analyze the seismicity of the Nine Mile Ranch sequence and compare it with the Ridgecrest sequence to understand their similarities, and implications for seismotectonics and rupture dynamics. The Nevada Seismological Laboratory has located 7300+ events from the NMR sequence to date. We relocate 87% of events using an improved velocity model, with depths ranging from 3-12 km. We compute focal mechanisms and moment tensors to determine the orientation and sense of slip, and perform a stress tensor inversion and GPS analysis to investigate driving mechanisms. Relocations reveal previously unmapped conjugate structures. The largest structure detected strikes ~N57W, and dips ~60° to the NE. Two conjugate SW-striking, vertical fault planes lie at both ends of the larger fault. A similar orientation appears in the Ridgecrest sequence prior to the occurrence of the M7.1, but the NW plane is dipping. The three main events are located near one another at the intersection of the NW-striking fault plane and the southern-most SW-striking conjugate structure. Moment tensor solutions of the main events show high-angle strike-slip faulting, with focal mechanisms of smaller events showing strike-slip and normal ruptures. We interpret the first event as occurring on the SW-striking left-lateral plane, with the second occurring on the dipping NW-striking right-lateral fault plane, and the third event occurring more shallow on the SW-striking left-lateral plane. The Ridgecrest sequence also initiated on a SW-trending left-lateral fault plane. A preliminary Coulomb stress model of NMR indicates a stress increase on the right-lateral NW-trending fault if the left-lateral structure initiates the sequence. We observe differences in spatio-temporal evolution of both sequences: NMR main events ruptured within an hour of the initiation, while the Ridgecrest main events were ~36 hours apart. We see similarities in tectonic regime of both sequences, which occur just north of significant left-lateral step overs.

Key Words
conjugate structures, Walker Lane, Ridgecrest

Citation
Hatch, R. L., Smith, K. D., Abercrombie, R. E., Ruhl, C. J., Hammond, W. C., & Pierce, I. (2019, 08). Recent Damaging Earthquakes on Conjugate Structures in the Walker Lane: Characteristics of the The Nine Mile Ranch Sequence (2016-2019) and Comparison to the Ridgecrest Sequence of 2019. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes