Dr. Robert C. Viesca, Assistant Professor of Geosystems Engineering at Tufts University, presents "Of faults and faucets" at the MIT Earth Resources Laboratory on April 1, 2016.

"Fluid injection in the deep subsurface, as part of energy production processes, is becoming an increasingly prominent concern for the potential earthquake hazard it may create. What volume of the subsurface is affected by this injection and how soon? One indication comes from observations of an enlarging cloud of microseismicity following injection. The diffusive growth of this cloud is thought to be a direct response to the diffusion of elevated pore fluid pressures reaching pre-stressed faults, triggering small instabilities. The observed high rates of this growth are interpreted to reflect a relatively high permeability of a fractured subsurface.

Here we investigate an alternative mechanism for growing a microseismic cloud: the elastic transfer of stress due to slow, aseismic slip on a subset of the pre-existing faults in this damaged subsurface. While this slip is initiated by the fluid injection, we show that, for critically stressed faults, such aseismic slip may spread as a diffusive process at rates far outpacing the diffusion of the elevated pore fluid pressure. Microseismicity in this case is triggered by the stress perturbation in the medium owed to the expanding region of slip on principal faults, which provides an alternative interpretation of the diffusive growth rates of the microseismic cloud.

As an addendum, we’ll also look at the genesis of the earthquake instability in response to such forcings: we’ll show how a fault is thought to transition from a creeping or nearly locked state to fast sliding by way of the acceleration of a small, but finite, patch of the fault. In the process, we’ll show that posing the problem with a current conception of sliding friction leads to a remarkable analogy to a seemingly disparate problem: the breaking up of a liquid jet into drops."