STORM WAVES BREAK AND MOVE LARGE STRATIFIED ROCKS

Where layered rock crops out under shoaling waves, large slabs can be moved, broken, or overturned during severe, but not extraordinary, storms. The resulting rock damage may be misinterpreted as tsunami damage. Waves in shallow water passing over stratified rock exert significant uplift pressure on the base of the uppermost stratum, if the bedding plane is significantly porous, as is often the case. Analyses of this condition shows, for example, that 6m waves are sufficient to vertically displace jointed rock slabs with thicknesses up to 2m, or to rupture in bending relatively thick slabs of unjointed rock. Loosened slabs up to one third of a meter thick can be overturned by wave-induced bottom velocities associated with such waves. All three mechanisms - uplift, rupture, and overturning - are favored when the density difference between rock and water is reduced. This density difference may be reduced by boring biota in rocks or by suspended sediment in the water. All three mechanisms probably are more effective on old outcrops than on recently exposed outcrops. These failure mechanisms caused by storm waves do not require extraordinary storms, and their effects on rock would be difficult to distinguish from the effects attributed to tsunamis.