HR: 16:45h
AN: H34C-04    [Abstracts]
TI: Swaying and Shaking of Sea Cliffs: Fatigue and Failure Mechanisms on Rocky Coasts
AU: * Adams, P N
EM: adamsp@wlu.edu
AF: Washington and Lee University, Department of Geology Washington and Lee Univ., Lexington, VA 24450 United States
AU: Anderson, R S
EM: Robert.S.Anderson@colorado.edu
AF: University of Colorado, Dept. of Geological Sciences and INSTAAR University of Colorado, Boulder, CO 80303 United States
AU: Storlazzi, C D
EM: cstorlazzi@usgs.gov
AF: U.S. Geological Survey, Pacific Science Center U.S. Geological Survey, Santa Cruz, CA 95060 United States
AB: Wave energy is imparted to sea cliffs in discrete, concentrated bundles every 4-25 seconds. While the resulting episodic sea cliff failure occurs when internal strength of sea cliff material is exceeded by forces associated with waves, the detailed mechanics of sea cliff failure processes are poorly understood. Failure events are difficult to predict and challenging to instrument. Ground motions associated with nearshore wave energy, microseisms, directly measure the response of sea cliffs to assailing waves. Through several microseismic field deployments from 2000 - 2003, we documented both the high-frequency shaking of sea cliffs, an instantaneous ringing response to the strike of waves against the sea cliff face, and a low-frequency (10 to 20 sec period) swaying of sea cliffs. This latter signal we interpret to reflect ground motion associated with water from the broken wave bore loading the wave cut platform that fronts the sea cliff face. We integrate microseismic velocities to obtain continuous swaying displacements of 50 micron and 10 micron amplitudes in horizontal and vertical directions, respectively. Shaking displacements, although discontinuous and less frequent, can be significantly greater ($>$100 microns) than swaying displacements. Displacement ellipsoids reveal simultaneous downward and seaward sea cliff motion with each incoming broken wave bore; time-registered video footage corroborates the downward sea cliff flex in response to the imposed water load on the wavecut platform. Gradients in displacement amplitudes in a direction orthogonal to the shoreline, documented using three seismometer locations, suggest longitudinal strain of the flexing sea cliff ranging up to 2.5 microstrain and shear strain ranging up to 3.5 microstrain during each wave loading cycle. Assuming an average wave period of 10 seconds, sea cliff flexure occurs approximately 3 million times annually. This continuous cyclical loading, and the associated incomplete recovery, may fatigue the sea cliff rock through development and lengthening of micro-cracks. Local sea cliff retreat rates of 10 cm/yr imply a given parcel of rock is flexed through roughly one billion cycles of increasing strain before exposure to direct wave attack at the cliff face. We suggest that this history of strain will make this material more susceptible to erosion upon reaching the cliff face, where high frequency shaking from direct wave impacts does the work.
DE: 1824 Geomorphology (1625)
DE: 1878 Water/energy interactions
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting