Fifteen Years of Observations Suggest Structural and Ocean-Wave Control on Seismicity at the Mendocino Triple Junction

Abstract The Mendocino triple junction (MTJ) is one of the most tectonically active regions in California. Seasonal variability in North Pacific storm activity generates strong ocean wave forcing along the California coast, raising questions about its role in modulating crustal stress and influencing MTJ seismicity. To investigate whether seasonal ocean forcing modulates seismicity, we analyze 15 yrs of seismic observations. Results indicate that seasonal ocean forcing modulates both seismicity rates and seismic noise amplitudes. Beamforming constrains the timing, direction, and wave-type characteristics of ocean-generated seismic energy, revealing azimuthally consistent seasonal oscillations in Rayleigh waves linked to the regional ocean wave climate, alongside more episodic and azimuth-dependent Love-wave behavior indicative of structural control. Seismicity rates lag seasonal stress peaks by several months, suggesting delayed fault response. Stress perturbations of only ± 3−4 kPa, comparable to typical ocean-loading stresses modulate seismicity rates by 10–40%, indicating that faults in the region are critically stressed and sensitive to weak external forcing. These findings highlight seasonal ocean forcing as an important contributor to MTJ seismicity modulation, with beamforming revealing a secondary influence of structural controls that shape the coupling of ocean-generated energy into the margin, and with potential implications under evolving coastal wave climates.