Distinct Yet Adjacent Earthquake Sequences near the Mendocino Triple Junction: 20 December 2021 Mw 6.1 and 6.0 Petrolia, and 20 December 2022 Mw 6.4 Ferndale
Creators
Abstract
Two earthquake sequences occurred a year apart at the Mendocino Triple Junction in northern California: first the 20 December 2021 M_w 6.1 and 6.0 Petrolia sequence, then the 20 December 2022 M_w 6.4 Ferndale sequence. To delineate active faults and understand the relationship between these sequences, we applied an automated deep‐learning workflow to create enhanced and relocated earthquake catalogs for both the sequences. The enhanced catalog newly identified more than 14,000 M 0–2 earthquakes and also found 852 of 860 already cataloged events. We found that deep‐learning and template‐matching approaches complement each other to improve catalog completeness because deep learning finds more M 0–2 background seismicity, whereas template‐matching finds the smallest M < 0 events near already known events. The enhanced catalog revealed that the 2021 Petrolia and 2022 Ferndale sequences were distinct in space and time, but adjacent in space. Though both the sequences happened in the downgoing Gorda slab, the shallower Ferndale sequence ruptured within the uppermost slab near the subduction interface, while the onshore Petrolia sequence occurred deeper in the mantle. Deep‐learning‐enhanced earthquake catalogs could help monitor evolving earthquake sequences, identify detailed seismogenic fault structures, and understand space–time variations in earthquake rupture and sequence behavior in a complex tectonic setting.
Copyright and License
© 2024 The Author(s). Published by Seismological Society of America. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Acknowledgement
Waveform data, metadata, or data products for this study were accessed through the Northern California Earthquake Data Center (NCEDC; doi: 10.7932/NCEDC). The authors acknowledge the contributions of the technical and monitoring staff at the Northern California Seismic System, jointly operated by the U.S. Geological Survey (USGS) and Berkeley Seismo Lab for seismic data collection and the ComCat earthquake catalog used in this study. The authors thank all developers and maintainers of open‐source seismology software listed in Data and Resources. Jeff McGuire, Daniel Trugman, and an anonymous reviewer provided helpful article reviews, and Editor‐in‐Chief Keith Koper facilitated the submission process. Luke Blair, Jeanne Hardebeck, and Shane Detweiler helped us with the data release process for the supplemental data sets. Jeff McGuire sent us the velocity model, Gabrielle Tepp shared the most recent dML station correction file (Uhrhammer et al., 2011) to calculate local magnitudes, Sarah Minson helped with Generic Mapping Tools (GMT) plots, and Rob Graves reviewed conference abstracts about this work. Jack Wilding and Jonny Smith helped with setting up and running EikoNet and HypoSVI. The authors had helpful discussions about Mendocino Triple Junction (MTJ) earthquakes with Andy Barbour, Max Liu, Kathryn Materna, Saeed Mohanna, Jay Patton, and the USGS Operational Aftershock Forecasting team. The authors thank all presenters and participants in the special Ferndale earthquake session at the virtual 2023 Northern California Earthquake Hazards Workshop for sharing information. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Data Availability
ComCat catalog (doi: 10.5066/F7MS3QZH), contributed by Northern California Seismic System (NCSS; https://earthquake.usgs.gov/fdsnws/event/1/, last accessed July 2023). Continuous seismic data were accessed from Northern California Earthquake Data Center (NCEDC) (https://ncedc.org/, last accessed July 2023), with data contributed by BK (doi: 10.7932/BDSN), CE (doi: 10.7914/b34q-bb70), NC (doi: 10.7914/SN/NC), NP (doi: 10.7914/SN/NP), and PB (https://www.fdsn.org/networks/detail/PB/, last accessed July 2023) networks. Seismology software: EQTransformer (https://github.com/smousavi05/EQTransformer, last accessed May 2023), REAL (https://github.com/Dal-mzhang/REAL, last accessed May 2023), EikoNet (https://github.com/Ulvetanna/EikoNet, last accessed May 2023), HypoSVI (https://github.com/Ulvetanna/HypoSVI, last accessed May 2023), GrowClust (https://github.com/dttrugman/GrowClust, last accessed May 2023). Scripts for creating enhanced catalog from seismic data, plotting (https://gitlab.com/cyoon1/ferndale2022, last accessed March 2024). ObsPy 1.4.0 (doi: 10.1088/1749-4699/8/1/014003; https://www.obspy.org/, last accessed May 2023) was used for seismological processing and visualization, and Generic Mapping Tools (GMT) 6.4.0 (doi: 10.1029/2019GC008515; Wessel et al., 2019; https://www.generic-mapping-tools.org, last accessed May 2023) for maps. U.S. Geological Survey (USGS) Quaternary Faults database (Figs. 1 and 3–5; https://www.usgs.gov/natural-hazards/earthquake-hazards/faults; last accessed May 2023). Major plate boundaries (Fig. 1; doi: 10.1029/2001GC000252). The supplemental material includes Tables S1–S6 with input parameters; Figures S1–S9 with analysis details; Data Sets S0–S4 with station data and earthquake catalogs in text format (Yoon and Shelly, 2024; doi: 10.5281/zenodo.10621116); and References.
Supplementary data
Conflict of Interest
The authors acknowledge that there are no conflicts of interest recorded.