The Influence of Geomorphic Setting on the Frequency Distribution of Energy Delivered to Steep Coastal Cliffs from Environmental Forcings

Wind generated waves have long been acknowledged as major contributors to coastal cliff erosion and long-term morphologic evolution. Ambient seismic noise measurements on coastal cliffs provide useful information about the mechanical response from ocean waves, reflecting both wave-wave interactions from remote sources in the deep oceans and local cliff-wave interactions. Concurrent observations of seismic displacements, atmospheric, and oceanic forcings recorded nearby Banneg Island, an uninhabited island located in the Molène archipelago, Brittany, France, characterized by steep rocky cliffs exposed to highly energetic wave conditions were examined to explore the wave-cliff interaction and to better understand the effects of different environmental forcing proxies. Additional datasets were collected from another coastal cliff site at Camaret, located on the continent, to investigate differences in wave-cliff interaction between these two sites . Integrated power spectral density (IPSD) of coastal cliff-top displacement using 5 minute windows were computed to find the response to wave and meteorological impacts characterized for different frequency bands , (1) Infragravity band (IG < 0.05 Hz), (2) Single-frequency band (0.05 Hz < SF < 0.1 Hz), (3) Double-frequency band (0.1 Hz < DF < 0.3 Hz), (4) low-frequency shaking band (0.3 Hz < LFS < 25 Hz), and (5) high-frequency shaking band (25 Hz < HFS < 50 Hz) . The IPSD magnitude for frequencies higher than 0.05 Hz was always greater for Banneg Island than Camaret. Response to off-shore significant wave height had consistent peak correlations in the HFS band and frequencies between 0.5 - 1 Hz for Banneg Island (r² = 0.87), whereas correlations peaked in the LFS band for Camaret (r² = 0.84). The seismic response to the total water level that includes wave runup on cliffs showed similar correlation pattern to the off-shore wave height with peak correlations found in the HFS band for Banneg Island and LHS band for Camaret. Our results suggest that coastal configuration may have a critical role in wave-cliff interaction, where incident waves are transformed by wave refraction, shoaling, bottom friction, and wave breaking before energy is transferred to the cliff. This study provides an example of analytically interpreting seismic signals generated from environmental forcings, and broadens our understanding of the energy transfer mechanism from ocean waves to seismic waves for steep rocky cliffs exposed to energetic wave conditions.