Seismic signatures of individual wave impacts on a coastal cliff

Abstract

AbstractThe mechanisms through which wave action induces cliff erosion are not well quantified, which limits our ability to model future cliff erosion. Previous studies have investigated using seismic measurements of cliff‐top ground motion from wave impacts as a proxy for potential wave driven cliff erosion. However, most attention has focussed on averaged hourly statistics, rather than at the individual wave impact scale. Higher temporal resolution studies are needed to determine the extent to which frequent, low magnitude impacts transfer wave energy into cliff rock, compared to violent impacts that occur much less frequently. We carried out fieldwork in Taranaki, New Zealand on a cliff that is exposed to broken, breaking, and unbroken wave impacts at different tidal stages. Synchronized seismometer, wave gauge, and video data were collected to observe a range of wave impact types under different marine conditions. Approximately 7500 wave impacts were manually classified into eight groups according to the stage of wave transformation at impact using video data. Corresponding seismic signals were then analysed to compare ground motion and impact types. The greatest peaks in ground motion were associated with impact classes identifiable by the wave breaking just in front of, or onto the cliff face. Waves classed as broken and unbroken had a median displacement of 4.2 μm and 5.1 μm, respectively, whereas breaking wave impact classes had a median displacement value of 25.8 μm. Spectral analyses of impact types suggest that different impacts produce peaks in energy at different frequency bands, and the total energy transfer is higher for breaking wave impact classes. This research provides a first step in assessing the spectral signature of wave impacts at an individual wave scale, providing a new method to investigate wave–cliff interaction.