Cessation of nearby low-frequency earthquake activity following the December 2024 Mw 7.0 Mendocino earthquake: the likely role of pore pressure reductions

David R. Shelly, & Kathryn Materna

Submitted August 30, 2026, SCEC Contribution #15145, 2026 SCEC Annual Meeting Poster #TBD

Earthquakes affect surrounding fault systems through multiple mechanisms. Here, we examine a rare cessation of activity in 27 families of low-frequency earthquakes (LFEs) that likely reflect fault creep at the southern edge of Cascadia subduction, near the Mendocino triple junction. These sources usually exhibit nearly daily LFE activity but were shut off for ~7-25 days following the December 2024 moment magnitude (Mw) 7.0 Mendocino fault earthquake, which occurred at a distance of ~110 km from the LFE sources. Based on modeling, we infer that this cessation was not caused by induced shear stress, which was slightly encouraging of slip, but rather by combined clamping normal stress and reduced pore pressure. This mechanism, which isn’t captured in the common effective friction simplification, may have strengthened the fault and inhibited creep during this time. Observed variations in LFE cessation times are not predicted by modeled stress changes alone and may instead reflect differing source dimensions or subtle variations in LFE fault geometry. These observations highlight the role of the full stress change tensor – including normal stress and pore pressure changes in addition to shear stress – in generating earthquake interactions on LFE sources that are highly sensitive to transient stresses. These findings also emphasize the importance of LFE observations to detect otherwise-hidden fault slip variations.

Key Words
Mendocino triple junction, fluids, earthquake triggering, slow slip

Citation
Shelly, D. R., & Materna, K. (2026, 08). Cessation of nearby low-frequency earthquake activity following the December 2024 Mw 7.0 Mendocino earthquake: the likely role of pore pressure reductions. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)