Dynamics of fluid-injection induced seismicity caused by the release of remnant stress

In natural earthquakes, faults slip when tectonic loading builds shear stress beyond the fault strength. Conversely, fluid-injection induced seismicity occurs when fault frictional strength is reduced below far-field shear stresses by comparatively rapid local changes in pore pressure – where remnant stresses control the potential energy release. We resolve controls on injection-induced stress drop size and dynamics using laboratory experiments in which fluid injection triggers the release of remnant shear stress. The experiments include fractures (bare surfaces) and mature faults (gouge) and show that rapid fluid pressure changes trigger stress drops by momentarily increasing the shear stress above its steady state. Fault friction modulates stress drop magnitude, but stress drop dynamics are largely controlled by the stiffness and permeability of the fracture. Stress release in low-stiffness gouge-filled fractures is comparatively slow, but rapid and dynamic for stiffer bare rock surfaces. Our results show that permeability is the most important parameter controlling the dynamics of stress drops, as permeability controls how fast the fluid front propagates. Stress drops become more dynamic for higher pore fluid pressures, therefore with ongoing fluid injection. Most importantly, when water is present on a surface, stress drops are larger than theory predicts.