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The 2022 M 6.4 Ferndale, California Earthquake Sequence: A Piece of the Puzzle at the Mendocino Triple Junction

David R. Shelly, Dara E. Goldberg, Robert J. Skoumal, Jeanne L. Hardebeck, Kathryn Z. Materna, Clara E. Yoon, William L. Yeck, & Paul Earle

Submitted September 10, 2023, SCEC Contribution #12821, 2023 SCEC Annual Meeting Poster #028

Seismicity near the Mendocino Triple Junction is highly active, yet enigmatic, accommodating complex relative motion among the Pacific, North America, and Gorda Plates. The December 20, 2022, Mw 6.4 Ferndale earthquake occurred within this zone, macroscopically as left-lateral strike slip faulting nucleating within the subducting Gorda slab. Here, we examine the aftershock sequence via waveform-correlation-based detection, relocation, and focal mechanisms. We find that aftershocks are confined to 16+ km depth, likely within the Gorda slab, with focal mechanisms reflecting a mix of strike-slip and normal faulting. We also model the mainshock slip distribution using geodetic and seismic data. Although the unconstrained slip distribution suggests that faulting may have extended updip into the overriding North America plate, the bulk of the data can also be fit by deeper slip alone. Based on this analysis, we conclude that the rupture was most likely confined within the slab itself, with little-to-no rupture within the upper plate, and high aftershock rates within the likely-hydrated uppermost Gorda slab. We further examine possible evidence that the mainshock was followed by ~2 months of aseismic slip on the subduction interface, reversing generally eastward coseismic motion observed at Global Navigation Satellite System (GNSS) stations south of the rupture. Although the small displacements and relatively sparse distribution of GNSS receivers allow for alternate interpretations, the tectonic complexity of this zone provides ample opportunity for interactions among the different fault systems.

Key Words
aftershocks, finite fault modeling, postseismic slip, earthquake relocation, subduction, fault interactions

Shelly, D. R., Goldberg, D. E., Skoumal, R. J., Hardebeck, J. L., Materna, K. Z., Yoon, C. E., Yeck, W. L., & Earle, P. (2023, 09). The 2022 M 6.4 Ferndale, California Earthquake Sequence: A Piece of the Puzzle at the Mendocino Triple Junction. Poster Presentation at 2023 SCEC Annual Meeting.

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