Distinct Aftershock Sequences of Moderate Magnitude Earthquakes Near the Mendocino Triple Junction in Northern California: 2021-12-20 Mw 6.1 and 6.0 Petrolia Sequence, and 2022-12-20 Mw 6.4 Ferndale Earthquake

Clara Yoon, & David R. Shelly

Published September 10, 2023, SCEC Contribution #13102, 2023 SCEC Annual Meeting Poster #011

The Mendocino Triple Junction (MTJ) in northern California, where subduction of the Gorda Plate beneath the North American Plate transitions into strike-slip motion along the San Andreas Fault, recently experienced three moderate magnitude strike-slip earthquakes. On 2021-12-20, an offshore Mw 6.1 earthquake triggered a deeper Mw 6.0 earthquake 11 seconds later on a different onshore fault ~30 km away, which in turn had a distributed aftershock zone extending ~10 km to the north (Yeck et al., 2023); these 2021 Petrolia earthquakes caused limited damage. Then a year later, on 2022-12-20, the Mw 6.4 Ferndale earthquake nucleated ~10 km northwest of the Petrolia aftershock zone, leading to structural damage in several Humboldt County communities.

Deep learning models for earthquake detection, phase-picking, and location were applied to continuous seismic data from 66 nearby Northern California Seismic Network (NCSN) stations, followed by relocation with waveform cross-correlation. This automatic workflow created a more complete, precisely located earthquake catalog for the MTJ region from December 2021 to June 2023, covering the Mw 6.1 and 6.0 Petrolia earthquakes, the Mw 6.4 Ferndale earthquake, and their aftershock sequences. This enhanced catalog contains over 10 times the number of events than in the routine NCSN-ComCat catalog.

Most Ferndale aftershocks occurred along a southwest-northeast oriented strike-slip fault, extending ~50 km inland to the east, at depths >15 km within the downgoing Gorda slab; this fault had little seismicity in the past 40 years, and is oriented differently from mapped surface faults. A Mw 5.4 triggered aftershock on 2023-01-01 happened on a separate fault located >20 km to the southeast. The enhanced catalog also found the 2021 Mw 6.1 and 6.0 Petrolia aftershock zone to be spatially distinct from, but adjacent to, the 2022 Mw 6.4 Ferndale aftershock zone. These results show that high-resolution earthquake catalogs enhanced by deep learning methods can identify new seismogenic fault structures and detailed aftershock information even in well-studied, seismically active regions such as the MTJ.

Key Words
aftershock, earthquake catalog, deep learning, Mendocino

Citation
Yoon, C., & Shelly, D. R. (2023, 09). Distinct Aftershock Sequences of Moderate Magnitude Earthquakes Near the Mendocino Triple Junction in Northern California: 2021-12-20 Mw 6.1 and 6.0 Petrolia Sequence, and 2022-12-20 Mw 6.4 Ferndale Earthquake. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology