Publications

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2012
Young, AP, Guza RT, Adams PN, O'Reilly WC, Flick RE.  2012.  Cross-shore decay of cliff top ground motions driven by local ocean swell and infragravity waves. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc007908   AbstractWebsite

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.

2011
Young, AP, Adams PN, O'Reilly WC, Flick RE, Guza RT.  2011.  Coastal cliff ground motions from local ocean swell and infragravity waves in southern California. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007175   AbstractWebsite

Ground motions atop a southern California, USA coastal cliff are compared with water level fluctuations observed at the cliff base, and with ground motions observed 10 km inland. At high tide, cliff top ground motions in three frequency bands were generated locally by ocean waves at the cliff base: (1) high-frequency (>0.3 Hz) "shaking" caused by waves impacting the cliff, and (2) gravitational loading-induced "swaying" at the frequency of the incident sea swell waves (0.05-0.1 Hz), and (3) slow "swaying" at infragravity frequencies (0.006-0.05 Hz). At high tide, at infragravity and incident sea swell wave frequencies, cliff top vertical ground displacement and cliff base water level fluctuations are coherent and oscillate in phase (with occasional deviation at sea swell frequencies), and spectral levels at the cliff top are much higher than at the inland seismometer. In contrast, at "double frequencies" (0.1-0.3 Hz) spectral levels of vertical motions are nearly identical inland and at the cliff top, consistent with a common (distant or spatially distributed) source. At low tide, when ocean waves did not reach the cliff base, power levels of vertical ground motions at the cliff top decreased to inland levels at incident wave frequencies and higher, and only infragravity-band motions were noticeably forced by local ocean waves.