Effects of deviatoric stress on undrained pore pressure response to fault slip

Abstract

The influence of deviatoric stress on coseismic water level changes is examined using an empirical relation between induced pore pressure and stress. Skempton [1954] characterized undrained pore pressure response in clayey soils to triaxial loading using two coefficients A and B. The undrained pore pressure response under axial loading can be either larger (A>1/3) or smaller (A<1/3) than that predicted from the mean normal stress alone. For Indiana limestone the undrained pore pressure response under axial loading is characterized by A<1/3. Skempton's equation is generalized to a polyaxial state of stress by making the undrained pore pressure a linear combination of the mean normal stress and the octahedral shear stress, which is proportional to the square root of the second invariant of the deviatoric stress tensor. Dependence of the undrained pore pressure on the deviatoric stress components is consistent with coseismic water level changes observed on the trace of the fault plane near Parkfield California, for an A value less than 1/3. Regions of pore pressure decrease are enlarged relative to regions of pore pressure increase.