Anisotropy reveals contact sliding and aging as a cause of post-seismic velocity changes

Rocks exhibit astonishing time dependent mechanical properties, like memory of experienced stress or slow dynamics, which refers to a transient recovery of stiff-ness after a softening induced by almost any type of loading. This softening and transient recovery is observed in the subsurface and in buildings after earthquake shaking, or in laboratory samples. Our investigation of anisotropy of the slow dynamics effect under uniaxial loading shows that it is observed independent of propagation direction, while the loading effect shows the expected anisotropy originating from the opening and closing of cracks. These observations put a number of novel constraints on the enigmatic physics of slow dynamics. We conclude that transient changes in bulk stiffness are caused by sliding of oblique grain-to-grain contacts and resulting changes in frictional properties as empirically described by rate-and-state friction and observed in laboratory experiments across block contacts. Connecting the nonclassical nonlinearity of heterogeneous materials to the powerful framework of rate-and-state friction provides an elegant explanation for the long searched-for origin of slow dynamics and potentially adds a new perspective for the monitoring of very early stages of material failure when deformation is still distributed in the bulk and just starts to coalesce towards a fracture.