Spatial Variability of Earthquake Ground Motions in the Eel River Basin, Northern California, from Dense Aftershock Arrays

Abstract Sedimentary basins in seismically active regions can exhibit strong spatial variability in earthquake ground motions that are often not adequately captured by sparse permanent seismic networks. In northern California, the Eel River basin is of particular interest due to its thick, poorly consolidated sediments, complex basin morphology, and proximity to the Mendocino Triple Junction (MTJ), which produces diverse and potentially large-magnitude earthquakes. The occurrence of an Mw 7.0 earthquake on 5 December 2024, along the MTJ, provided the opportunity to record ground motions of aftershocks with two densely spaced seismic arrays focused on an ∼300 km2 populated area of the Eel River basin. The full deployments included 114 three-component 5 Hz geophones and 17 temporary accelerometers that recorded seismic activity for over 30 days, with captured events reaching Mw 5.3. Empirical, distance-normalized peak ground velocity analysis reveals persistent localized amplification along basin edges and within narrow river corridors, and consistent deamplification across an extensive fluvial terrace. Single-station horizontal-to-vertical spectral ratios indicate dominant resonance peaks between approximately 3 and 6.5 Hz at basin sites, potentially generated from thin (<100 m) surficial sediments. Secondary phases observed between P- and S-wave arrivals are interpreted as conversions at deeper crustal or slab-related interfaces. These results demonstrate that ground motions in the Eel River basin are strongly influenced by local sedimentary structure and topography, highlighting the value of dense arrays for seismic hazard assessment in populated basins.