WAVE-INDUCED CYCLICAL STRAIN OF SEACLIFFS

Rates of seacliff retreat along active margin coasts are among the most rapid geomorphic rates witnessed in landscape evolution. As retreat progresses, the nearshore shelf bathymetry evolves and feeds back into wave transformation and wave energy dissipation. Previous workers have characterized the dominant processes operating at the seacliff interface as mechanical (abrasion) or hydraulic (water hammer/block removal). We propose that wave-induced cyclical strain is a significant, though heretofore underappreciated, process contributing to seacliff rock fatigue and prepares the cliff face for large block removal during intense, infrequent storms. During the Spring of 2003, we collected 2Hz water level data with a wave gage deployed in 4m of water, 50Hz ground motion data with a portable broadband seismometer on top of the adjacent bedrock seacliff, and digital video footage of wave run-up on the platform directly in front of the seismometer. Analysis of the video footage of waves and their characteristic seismic signature indicates that seacliff flexure corresponds to the timing of wave run-up loading of the platform. Spectral analyses of data from the wave gage and the seismometer were compared and reveal that the seacliff “flexes” with the same frequency as that of the incoming wave field. To investigate this seacliff flexure, we set up a second seismometer at two separate distances away from the reference sensor at the seacliff edge. The ground motion displacement amplitude ratios from the two inland sensor locations define a spatial strain curve that decays exponentially with distance from the seacliff edge. This spatial distribution of strain fosters our hypothesis that bedrock near the seacliff face is subjected to frequent, high-amplitude micro-flexing that causes bulk rock fatigue and propagates cracks into the seacliff material. Eventually the cracks loosen the bedrock, allowing large-scale block removal to occur during intense storm wave action.