Seafloor Topography Controls Primary Microseism Generation: New Insights from Cyclone-Forced Seismic Observations in Northeastern Australia

Primary microseisms are long-period seismic waves generated by the interaction of ocean surface gravity waves with the seafloor. Yet, the spatial distribution and excitation mechanisms of their sources remain poorly resolved. Here, we investigate the heterogeneous generation of primary microseisms associated with seasonal tropical cyclone activity and Cyclone Ita in North-east Australia (2014). Combining continuous seismic data from a regional network with high-resolution (30 m) bathymetric maps, we show that primary microseism excitation is highly localised in both time and space, and critically dependent on fine-scale seafloor roughness. Our analysis reveals that the most energetic Rayleigh wave bursts arise from regions with pronounced bathymetric variability, where coupling between ocean waves and the solid Earth is most efficient. The finding provides observational evidence to confirm the theoretical conjecture that topographic undulations at scales comparable to ocean wave wavelengths govern the strength of microseism sources in the 10 – 20 s period band. Our findings highlight the critical role of nearshore bathymetric roughness in shaping the spatial coherence of primary microseism excitation. The understandings are essential for guiding the future use of persistent seismic sources in seismic imaging of Earth’s near-surface structures and in monitoring the evolution of the seismic wavefield in near-future applications.