How Fault-Normal and Shear-Parallel Stiffness Influence Frictional Sliding Behavior

The potential of faults to show earthquake-generating slip instabilities depends not only on the intrinsic frictional properties of the fault zone, but also on the elasticity of the surrounding material. A velocity-weakening fault is expected to show increasingly unstable frictional behavior with decreasing local elastic stiffness around the fault zone. Fault zone roughness can cause slip in the shear direction to be accompanied by fault-normal movement, modulated by fault-normal elastic properties, however these effects are poorly understood. Here, we systematically vary the stiffness surrounding the fault in both the shear-parallel and fault-normal directions, to investigate the origin of slip instabilities and changes in friction constitutive properties. We confirm the transition from stable sliding through slow slip to stick-slip due to reduced fault-parallel stiffness, and that the occurrence of different types of slip events can be explained by the ratio between shear and critical stiffness. In contrast, reducing the fault-normal stiffness produces stick-slip instabilities under conditions where the conventional critical stiffness criterion predicts stable sliding, and does not produce transitional slow slip events. Our data suggest that: (a) the stability criterion for frictional slip should be modified to incorporate fault-normal stiffness, and (b) the unexpected slip instabilities may represent wrinkle-like slip pulses, possibly due to a stiffness asymmetry introduced by lowering the fault normal stiffness on one side of the fault. This implies that earthquakes may occur when the fault-normal stiffness, or bulk modulus for natural faults, is decreased and/or asymmetric across the fault zone, both of which may be common in nature. Earthquakes occur on fault planes, but whether a fault will show stable sliding or host earthquakes, or their lab equivalent called "stick-slip", depends only partly on the fault properties but also on how the fault and the surrounding rock respond to stress changes (the stiffness). Previous studies have shown that the transition between stick-slip instabilities and stable sliding along a fault, can be triggered by changing the stiffness in the direction along the fault plane. However, faults commonly also have movement perpendicular to the fault plane, for example, if the fault surface is very rough, and therefore we study the effect of stiffness changes both perpendicular and parallel to the fault plane. We confirm previous observations when changing the stiffness parallel to the fault plane, but also observe unexpected stick-slip instabilities when we decrease the stiffness perpendicular to the fault plane. This means that: (a) we have to adjust the stiffness criterion where stick-slip instabilities are expected to include the fault-normal stiffness, and (b) the unexpected stick-slip instabilities at low fault perpendicular stiffness may be a different type of slip instability, called wrinkle-like slip pulses, that may occur when one side of the fault is stiffer than the other. Reduced fault-normal stiffness alone can cause stick-slip instabilities, but not slow slip in quartz gouge Systematically reducing both fault-normal and shear-parallel stiffness shows highly irregular slip events Adapting the critical stiffness criterion for fault-normal stiffness shows slip instabilities are more likely