Fault geometry from 12 years of relocated earthquakes (2013-2025) near Lake Almanor, Northern California, aided by a nodal deployment

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

Submitted September 7, 2025, SCEC Contribution #14597, 2025 SCEC Annual Meeting Poster #TBD

The 2023-05-11 Mw 5.5 Lake Almanor earthquake, and a Mw 5.2 aftershock ~11 hours later, ruptured normal faults in a sparsely instrumented region in Northern California, with only one strong-motion seismic station within 10 km and two broadband stations within 25 km epicentral distance. Previously, the 2013-05-24 Mw 5.7 Canyondam earthquake and its aftershocks had occurred ~5 km southeast of the May 2023 earthquakes (Chapman et al., 2016). The presence of several nearby dam facilities within 15 km motivates a detailed study of the causative fault structure(s) and geometry at depth, to inform site-specific seismic hazard.

Within two days of the 2023 mainshock, the U.S. Geological Survey (USGS) deployed 34 nodal seismometers for ~2 months, from 2023-05-13 to 2023-07-27, to record the Lake Almanor aftershocks at close (<5-10 km) epicentral distances with dense azimuthal coverage. These nodal seismometers, spaced ~3 km apart, recorded three-component continuous waveforms sampled at 200 Hz. For over 100 ComCat M 1.0 to 4.1 earthquakes between 2023-05-13 and 2023-07-24 (USGS, 2017), contributed by Northern California Seismic Network, we use nodal waveforms to improve event locations and focal mechanism estimates. Automatic P and S picks on 15-second nodal event waveforms, made by the PhaseNet deep-learning model (Zhu and Beroza, 2019), result in hypocenters that are 2-5 km deeper than those from ComCat. Additional first-motion P-wave polarities and S/P amplitude ratios from the nodal data reduce the uncertainty in focal mechanism orientation by ~25o.

Using catalog picks and waveform cross-correlation, ~1,700 ComCat earthquakes between 2013-04-01 and 2025-07-01 were double-difference relocated. These results reveal that the 2023 Mw 5.5 sequence was located northwest of the 2013 Mw 5.7 sequence without spatial overlap, and occurred on the same steeply-dipping fault structure; the underlying structure may be large enough to potentially host a M6+ earthquake.

Key Words
aftershock, earthquake catalog, earthquake relocation, deep learning, focal mechanisms, Northern California, nodal seismometer

Citation
Yoon, C. E., Skoumal, R. J., Hardebeck, J. L., Catchings, R. D., Goldman, M. R., Chan, J., & Sickler, R. R. (2025, 09). Fault geometry from 12 years of relocated earthquakes (2013-2025) near Lake Almanor, Northern California, aided by a nodal deployment. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology