12 years (2013-2025) of relocated earthquakes and focal mechanisms at Lake Almanor, Northern California, reveals a connection at depth between larger mapped surface faults

Clara E. Yoon, Robert J. Skoumal, Jeanne L. Hardebeck, Katherine A. Guns, Rufus D. Catchings, Mark R. Goldman, Joanne Chan, & Robert R. Sickler

Submitted August 30, 2026, SCEC Contribution #15285, 2026 SCEC Annual Meeting Poster #TBD

Lake Almanor, located in a region of transtensional deformation in northeastern California, experienced two moderate-magnitude earthquake sequences a decade apart. The 11 May 2023 Mw 5.5 Lake Almanor earthquake was followed ~11 hours later by an Mw 5.2 aftershock, while the 24 May 2013 Mw 5.7 Canyondam earthquake occurred ~4 km to the southeast. Both sequences occurred within 15 km of six dams and hydroelectric facilities, and neither sequence aligns with any mapped surface fault. The relatively sparse permanent seismic network was enhanced by a two-month-long, 34-station nodal seismometer deployment starting two days after the 2023 mainshock, which greatly improved the near-source and azimuthal coverage of the aftershocks. Seismic data from 2,864 ComCat earthquakes between 2013 and 2025 were uniformly processed through a multi-step workflow, including precise double-difference relocation with a new 1D velocity model, station corrections, and focal mechanisms from first-motion polarities and S/P amplitude ratios. The resulting 1,786 relocated earthquakes and 119 focal mechanisms reveal that the 2013 and 2023 sequences occupy adjacent, non-overlapping segments of a single curved fault, which transitions from steep dextral strike-slip faulting in the southeast aligned with regional fault orientation in the Sierra Nevada microplate, to shallower normal faulting in the northwest with orientation controlled by inherited basement faults. This curved geometry is consistent with a negative flower structure at a releasing stepover. We refer to this previously undocumented fault as the Southern Lake Almanor Fault (SLAF). InSAR data independently constrain the 2023 Mw 5.5 rupture depth to <9.0±0.7 km, consistent with its relocated hypocentral depth. The M5+ mainshocks ruptured updip from ~8-10 km to shallower depths, with no evidence of surface rupture. This SLAF alone is large enough to host a M6.1 earthquake, but it also fills a spatial gap between the larger mapped Almanor, Skinner Flat, and Butt Creek faults. A hypothetical joint rupture of these connected faults could exceed magnitude 7. This study demonstrates that nodal seismometer deployments in sparsely instrumented regions can reveal fault connectivity relevant for site-specific seismic hazard assessment near critical infrastructure.

Key Words
aftershock, earthquake relocation, focal mechanism, Northern California, normal faulting, nodal seismometer

Citation
Yoon, C. E., Skoumal, R. J., Hardebeck, J. L., Guns, K. A., Catchings, R. D., Goldman, M. R., Chan, J., & Sickler, R. R. (2026, 08). 12 years (2013-2025) of relocated earthquakes and focal mechanisms at Lake Almanor, Northern California, reveals a connection at depth between larger mapped surface faults. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology