Quantifying the effects of injection parameters on fault response under spatially homogenous and heterogenous pore-fluid conditions

Heightened seismic activity due to human activities, such as wastewater injection, carbon storage and geothermal energy production, has been a rising problem in recent years. Various injection parameters and geological conditions have been shown to affect fault behaviour differently when fluid is injected on the faults, although existing observational studies about their effects often show contradictory results. Aseismic slip is also known to affect seismicity, but its exact contribution remains elusive.

To address these, we perform numerical modelling to study the effects of various injection parameters on fault slip behaviour. Our fully dynamic fault model is governed by the rate-and-state friction laws and spontaneously resolves all stages of an earthquake cycle and long-term fault slip. Our results show several interesting observations on the role of injection volume and rate: First, the injected volume can advance or delay the next earthquake if no earthquakes are directly triggered during perturbation. Second, if earthquakes are triggered, the number of triggered earthquakes is controlled by the rate at which fluid is injected, while the timings of the triggered earthquakes are controlled by the injected volume. Large triggered earthquakes are usually preceded by smaller precursors. Third, the pore-pressure threshold at which earthquakes are triggered changes depending on the injection parameters. In most cases, it increases with the volume of injected fluid, but in some cases when the injection is slow, it can also depend on the rate of injection. The change with respect to injection rate is not a smooth positive trend, however, as increasing the rate causes aseismic transients to grow stronger and transition into seismic events, thus advancing the triggering time and causing decrease in the threshold pore pressure in the process. Overall, the effects of perturbation do not end as soon as injection stops. Instead, heightened aseismic activities, as well as oscillating earthquake timings and magnitudes occur for multiple seismic cycles after the end of pore-pressure perturbation. We also see large variations in aseismic moment release under different perturbation scenarios and its intricate relationship with the resulted seismicity pattern, which confirms the vital role of aseismic slip in earthquake triggering. Similar to previous studies, we find that energy on the fault is primarily released aseismically.

Our results thus far are based on spatially uniform pore-pressure evolution, and we are currently developing models that resemble environments with temporally and spatially heterogeneous pore pressure by coupling the temporal evolution of pore pressure with spatial diffusion. We are also incorporating geologic information of the crustal medium, which will be more fitting for modelling realistic scenarios such as the injection-induced earthquakes in Oklahoma.