P-wave triggering of periodic fault sliding. Negative friction

Sliding of a fault with gouge leads to rotation of gouge particles. Since the particles are not spherical, their rotation in the presence of pressure normal to the fault can exhibit local negative shear stiffness. Another mechanism of local negative shear stiffness is rotations of couples of temporary connected particles. These rotations affect the relation between the shear sliding stress and normal stress creating the effect of apparent friction coefficient, which, in some locations, can become negative [1]. The value (and the sign) of the local stiffness and the apparent friction coefficient depend upon the initial pressure and the stiffness of the surrounding rock. When elastic p-wave approaches a fault in normal direction it causes both normal and shear oscillations of one fault face against the other. If the amplitude of the wave-generated normal oscillations exceeds a certain threshold which depends upon fault and particles’ geometry and rock stiffness, then the shear oscillations reach the negative stiffness stage and become unstable. This leads to unstable periodic fault sliding resulting in seismic events.

The proposed concept will form a basis for developing realistic models of sliding and periodic seismicity of fault with gouge. It will also facilitate developing models of monitoring of fractures affecting thermal spallation mechanics.

Acknowledgement. The authors acknowledge financial support from of the Australian Research Council through project DP250103594.

1. Pasternak, E. and A. Dyskin, 2025. Negative stiffness induced and controlled by constriction. Status Solidi B DOI: 10.1002/pssb.202500428 (in print).