Seismic spectral signatures of individual wave impacts on coastal cliffs

Coastal cliffs are vulnerable to erosion from both marine forcing and subaerial processes, which presents a hazard to coastal property and infrastructure. Understanding of erosion processes and mechanisms of failure remains limited, making predictions of future erosion uncertain, especially for evaluating effects of sea level rise. A key uncertainty concerns the way in which marine processes affect coastal cliffs through direct wave impacts. In previous work, seismometers have been installed at cliff-tops to measure ground motion resulting from wave impacts on cliffs, and to infer energy transfer. However, attention has mainly focussed on averaged hourly statistics, rather than ground motion measured at a wave-by-wave scale. It is necessary to better constrain the nature of wave-cliff interaction at the scale of individual impacts in order to investigate the interplay between the frequency and magnitude of wave impact events. Are frequent, low magnitude impacts more important than rare, violent impacts in driving erosion?

We carried out fieldwork in Taranaki, NZ on a cliff that is exposed to broken, breaking, and unbroken wave impacts at different tidal stages. Wave pressure sensor, seismometer, and video data were collected to observe a range of wave impact types under different marine conditions. Impacts were classified according to the stage of wave transformation at the moment of impact. Eight different stages of transformation were defined and impacts were manually categorised using video data. The corresponding seismic signal was then analysed to compare ground motion and impact types. Analyses confirm that high-magnitude peaks in ground motion were associated with wave impacts which enclosed a gas pocket between the wave front and the cliff face. Spectral characteristics of impact types suggested that different impacts produce peaks in energy at different frequencies, though the total energy transfer may by similar for different impact types.

This research provides a first step in analysing the spectral signature of wave impacts at an individual wave scale, and provides a new method of quantifying wave-cliff interaction with seismic signals. This study helps link marine forcing to the timing and magnitude of cliff erosion events, and therefore increase confidence in future erosion predictions.