Seismic Intensities, Ground Motions, and Basin Amplification from the 2022 Mw 6.4 Ferndale, California, Intraslab Earthquake

Abstract On 20 December 2022, a magnitude 6.4 earthquake occurred at a depth of 18 km within the subducting Gorda plate near the Mendocino Triple Junction, one of the most seismically active regions of the contiguous United States. This earthquake caused significant damage to local communities and is being investigated thoroughly as part of an ongoing scientific effort to understand subduction zone earthquake hazards. In this study, we document the seismic intensities, ground motions, and basin amplification effects observed during the event across northwestern California. Our field survey revealed modified Mercalli intensity values as high as VIII (severe) within the 3-km-deep Eel River sedimentary basin, with peak ground acceleration and peak ground velocity reaching 1053 cm/s2 and 52 cm/s, respectively. Comparisons with published ground-motion prediction equations indicate that these values align with the expected attenuation–distance patterns for intraslab earthquakes. Notably, frequencies above 1 Hz were amplified within the Eel River basin by factors greater than 2 when compared to reference stations outside of the basin and at distinct orientations to the fault. Our results suggest that sedimentary basin site effects were the primary cause of the observed peak ground motions, though rupture directivity may have further amplified shaking regionally. Although the Ferndale area is primarily affected by shallow intraslab earthquakes, these findings raise important questions about the potential site response and basin amplification effects from a future megathrust earthquake. The heightened seismicity of the southern Cascades highlights the value of comprehensive damage assessments and robust earthquake-resistant engineering designs along the northern California coast.