Dependence of Injection-Induced Seismicity on the Injection Scenario

It is well established that subsurface fluid injection may increase dramatically the micro-seismicity rate and induce some large earthquakes, like the 2011 Mw 5.7 in Oklahoma. As well, it may essentially provoke aseismic slip, with later micro-earthquakes as an indirect effect. The injection scenario (injection pressure and pressure rate) was observed to be a key parameter controlling the features of induced seismic activity. However, its effect was never fully investigated. In this study, we aim to outline the principal parameters governing the magnitudes and the rate of induced earthquakes. We present a Dietrich-Ruina heterogeneous 2-D fault producing aseismic slip and micro-seismicity, coupled with a 1-D fluid diffusion model along the fault segment. We test the impact of the well head pressure as well as the injection pressure rate. Following fluid injection, the seismicity rate increases significantly. It then experiences a 1/t decay. It is in direct correlation with the pore pressure rate along the fault segment, and not the pore pressure itself. The Mw content changes as well, with an increased proportion of larger magnitudes. The duration of this induced seismicity is entirely controlled by the fluid diffusion. On the other hand, maintaining a small injection pressure or pressure rate can lead to no amplification of the seismicity. In fact, the seismicity rate increase is proportional to the injection pressure rate. The latter governs also the magnitudes of the induced events, whereas a higher value can trigger more numerous events but with lower proportions of large magnitudes than a smaller value.