Shallow sedimentary rock as a fragile geological feature; effects of clay content and hydrology on frictional strength

Strong long-period ( approximately 3 s) seismic waves impose dynamic strains on the shallow subsurface. The dynamic strain is the dynamic velocity divided by the phase velocity of the waves. The dynamic stress is the strain times the shear modulus. A testable hypothesis is that the shear modulus of the rock self-organizes so that the rock barely fails in friction with typical imposed dynamic strains. The predicted value of stiffness divided by depth is then independent of depth for constant rock density and constant coefficient of friction with the water table at the surface. Predicted stiffness divided depth deviates from constancy for finite-water-table depth. Prior laboratory studies indicate that the coefficient of friction is lower in clay-rich rocks than in clay-free rocks. These effects provide appraisal of the concept in which hydrology and lithology are constrained. Four boreholes near Parkfield, California, qualitatively exhibit the predicted effects. There is some indication of the predicted effect of water-table depth within accumulating sediments penetrated by borehole McGlincy (MGCY) in the Santa Clara Valley of California, but the effect of clay is not well resolved, due to a dearth of clay-rich beds.