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Fault Geometry in the 2023-05-11 Mw 5.5 Lake Almanor, California, Earthquake Sequence, Revealed by Precise Aftershock Locations and Focal Mechanisms From a Nodal Deployment

The 2023-05-11 M_w 5.5 Lake Almanor earthquake, and a M_w 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 M_w 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 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 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^o. We find that the 2023 M_w 5.5 sequence locates northwest of the 2013 M_w 5.7 sequence without spatial overlap, and occurred on a large underlying fault structure with potential for a M6+ earthquake.

Session: Innovative Applications of Seismic Nodal Technology for Hazard Mitigation and Earth System Monitoring - I

Type: Oral

Room: Key Ballroom 9

Date: 4/15/2025

Presentation Time: 03:00 PM (local time)

Presenting Author: Clara Yoon

Student Presenter: No

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