Cross‐shore decay of cliff top ground motions driven by local ocean swell and infragravity waves

Abstract

Ground motions at the frequencies (between 0.01 and 0.1 Hz) of ocean infragravity and swell waves were observed on a cross‐shore transect extending landward from the edge of a southern California coastal cliff. Cliff top ground motions are coherent and in phase with water level fluctuations at the cliff base. Vertical ground motions at infragravity and single frequencies decay rapidly with inland distance from the cliff edge (e‐folding scale is about 12 m), and at the edge decrease by several orders of magnitude between high tide when waves reach the cliff base, and low tide when the waterline is about 50 m from the cliff base. The observed cross‐shore decay scales are qualitatively consistent with gravitational loading and attraction of water waves at tidally modulated distances from the cliff base. At approximately constant distance from the waterline, ground motions vary roughly linearly with nearshore swell wave energy. In contrast to these locally forced ground motions, double frequency band (0.1–0.2 Hz) cliff top vertical ground motions are remotely generated with spatially uniform magnitudes approximately equal to those observed 14 km inland. Near the cliff edge, ground tilt dominates the observed large (relative to vertical) cross‐shore acceleration at infragravity frequencies, contributes significantly to cross‐shore acceleration at swell frequencies, and is a small fraction of cross‐shore acceleration at higher frequencies.