Experimental demonstration of a semi-brittle origin for crustal strain transients

Tectonic motions give rise to destructive earthquakes and transient slip events. These movements are often described by friction laws for stick-slip motion on brittle fault surfaces and gouge-filled zones(1,2). Yet, many transient slip events, such as slow earthquakes and aseismic creep, occur in rocks that exhibit mixed brittle-ductile rheology, where these friction laws are not clearly applicable(3,4). Here we describe the flow and evolution of fractures as observed in a semi-brittle rock analogue exposed to shear stress in laboratory experiments. We find that, depending on the strength of the rock-analogue material, and thus the magnitude of yield stress, the material exhibits either creep-like or stick-slip behaviour. At low yield stress, deformation occurs as constant creep along a main fracture, whereas at high yield stress, the material exhibits stick-slip behaviour. However, the deformation does not involve frictional behaviour; it is instead accommodated by the initiation and growth of a system of tensional and shear fractures. The opening and interplay of such fracture systems could generate tectonic tremor and slow slip. Our laboratory experiments thus support a frictionless alternative mechanism for the development of tectonic strain transients.