Precise aftershock locations and focal mechanisms for the 2023-05-11 Mw 5.5 Lake Almanor, Northern California, earthquake from a two-month nodal seismometer deployment

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

Published September 8, 2024, SCEC Contribution #13990, 2024 SCEC Annual Meeting Poster #053

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. The presence of several nearby dam facilities 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 mainshock, 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 known ComCat magnitude 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 ComCat. Subsequent double-difference relocation illuminates northeast-dipping fault geometry with changing dip along the fault strike. Additional first-motion P-wave polarities and S/P amplitude ratios from the nodal data reduce the uncertainty in focal mechanism orientation by ~25 degrees.

Key Words
aftershock, earthquake catalog, 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. (2024, 09). Precise aftershock locations and focal mechanisms for the 2023-05-11 Mw 5.5 Lake Almanor, Northern California, earthquake from a two-month nodal seismometer deployment. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology