Long-term repeating earthquakes and physical drivers of the 2022 Ferndale and 2024 Offshore Cape Mendocino earthquake sequences

Saeed Y. Mohanna, Grant Kawamoto, Lingsen Meng, Max Liu, John Wellik, & Roland Bürgmann

Submitted September 7, 2025, SCEC Contribution #14247, 2025 SCEC Annual Meeting Poster #TBD

The Mendocino Triple Junction (MTJ), where the Pacific, Juan De Fuca, and North American tectonic plates meet, is a geometrically complex region that has experienced several episodes of intense seismicity. The 2022 Mw 6.4 Ferndale earthquake ruptured close to where the Juan De Fuca subducts underneath the North American plate, while the 2024 Mw 7.0 Offshore Cape Mendocino earthquake ruptured closer to the MTJ itself. In this study, we enhanced the Northern California Seismic Network (NCSN) earthquake catalog using machine learning and template matching approaches to detect lower magnitude earthquakes. We then obtained absolute and relative earthquake relocations using the HypoInverse, HypoDD, and GrowClust algorithms so that we could analyze the physical drivers of both of these sequences. To assist our analysis, we searched for repeating earthquakes over the past 8 years. Also, we examined changes in groundwater levels during the 2022 sequence using data from the Humboldt County Department of Public Works. Our enhanced catalogs revealed ~5-8 times more events than the NCSN catalog. We found that most repeating earthquake families that had more than 2 members were periodic. Most families were also close to the MTJ, near the 2024 sequence’s aftershock zone. This suggests that while aseismic slip plays a role in the deformation zones of both sequences, its effects are particularly strong closer to the MTJ. Additionally, the 2022 sequence coincided with sharp changes in groundwater levels, and strain modeling suggests that, despite the earthquake being relatively deep, an elastic strain mechanism could explain the changes in groundwater levels.

Key Words
Repeating Earthquakes, Mendocino, Groundwater, Physical Drivers

Citation
Mohanna, S. Y., Kawamoto, G., Meng, L., Liu, M., Wellik, J., & Bürgmann, R. (2025, 09). Long-term repeating earthquakes and physical drivers of the 2022 Ferndale and 2024 Offshore Cape Mendocino earthquake sequences. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology