Poroelastic and Earthquake Nucleation Effects in Injection Induced Seismicity

Abstract:

The standard model of injection-induced seismicity considers changes in Coulomb strength due solely to changes in pore-pressure. We consider two additional effects: full poro-elastic coupling of stress and pore-pressure, and time dependent earthquake nucleation. Stress and pore-pressure due to specified injection rate are modeled in a homogeneous, poroelastic medium. Stress and pore-pressure are used to compute seismicity rate through the Dieterich [1994] model. For constant injection rate, the time to reach a critical seismicity rate scales with t ~ r^2/(c f_c), where r is distance from the injector, c is hydraulic diffusivity, and f_c is a factor that depends on mechanical properties, and weakly on r. The seismicity rate decays following a peak, consistent with some observations. During injection poro-elastic coupling may increase or decrease the seismicity rate, depending on the orientation of the faults relative to the injector. If injection induced stresses inhibit slip, abrupt shut-in can lead to locally sharp increases in seismicity rate; tapering the flux mitigates this effect. The maximum magnitude event has been observed to occur post-injection. We suggest the seismicity rate at a given magnitude depends on the nucleation rate, the size distribution of fault segments, and if the background shear stress is low, the time varying volume of perturbed crust. This leads to a roll-over in frequency magnitude distribution for larger events, with a corner that increases with time. Larger events are absent at short times, but approach the background frequency with time; larger events occurring post shut-in are thus not unexpected.