Seismic to aseismic slip scaling during fluid injection experiments

Abstract

<p>A growing number of direct and indirect measurements and observation indicates that aseismic slip transients are often induced during fluid injection operation alongside with swarm-like seismicity. The detection of fluid-induced aseismic slip has made a paradigm shift on our understanding of the spatio-temporal evolution of earthquake activity during injection operation, classically interpreted as triggered by a diffusive front of high pore pressure. Instead, unclamping of the fault by pressurization induced by fluid injection creates the condition to nucleate synchronous aseismic and seismic slip transients.  In this scenario, the spatio-temporal evolution of the induced seismicity is driven by the stressing rate imparted at the leading edges of the aseismic rupture front. However, the relationship between the magnitude of aseismic slip and the hydraulic energy input in the system remains still elusive. A similar mechanism has been proposed for natural earthquake swarms triggered by shallow (5-10 km depth) slow slip events (SSEs), for which a robust power-law scaling has been demonstrated between seismic and aseismic slip. Notably, the power-law moment scaling of shallow SEEs and associated earthquake swarms has been interpreted with a mechanism of fault pressurization enhanced by intense fracturing in a seismogenic volume with abundance of crustal fluids. Similar fault conditions are at play for fluid-induced seismicity. Here, we collected several case studies of recorded induced aseismic deformation during injection experiments together with the accompanying seismic activity. We investigated the spatial distribution and temporal evolution of the seismicity with respect to the ongoing transient aseismic slip. We focused in particular on the seismic and aseismic slip budget of induced seismicity and compared it with previous scaling of SSEs. The aseismic and seismic moments of induced events are compatible with the power-law scaling of shallow natural earthquakes swarms triggered by SSEs, although a data gap exits for SSEs in 0-4 magnitude range, where no SSEs have never been recorded due to the low resolution of surface geodetic instrumentation. We performed also numerical simulation using a 3D hydro-mechanical model using realistic fault and hydraulic parameters in order to fill in the data gap. Our results serve as a basis to build up empirical models that incorporate aseismic slip together with injected volume and pressure to forecast seismicity during fluid-injection experiments.</p>