Control of Seismicity Migration in Earthquake Swarms by Injected Fluid Volume and Aseismic Crack Propagation

The evolution of fluid injection-induced seismicity, generally characterized through the number of events or their seismic moment, depends on, among other factors, the injected fluid volume. Migration of seismicity is observed during those sequences and might be caused by a range of mechanisms: fluid pressure diffusion, fluid-induced aseismic slip propagating along a stimulated fault, interactions between earthquakes. Recent theoretical and observational developments underline the important effect on seismicity migration of structural parameters, like fault criticality, or injection parameters, like flow rate or pressurization rate. Here, we analyze two well-studied injection-induced seismic sequences at the Soultz-sous-Foret and Basel geothermal sites, and find that the evolution of the seismicity front distance primarily depends on the injected fluid volume. Based on a fracture mechanics model, we develop new equations relating seismicity migration to injected fluid volume and frictional and structural properties of the fault. We find that the propagation of a fluid-induced aseismic slip front along the stimulated fault, triggering seismicity, explains well the observations made on the two sequences. This model allows us to constrain parameters describing the seismicity front evolution and explains the diversity of migration patterns observed in injection-induced and natural earthquake swarms. Injection of fluids at depth may induce earthquakes, which can represent a hazard both for reservoir activities and populations. The magnitude and number of earthquakes depend, at first order, on the injected fluid volume. Induced seismicity has also been observed to migrate away from the injection area, with a diversity of velocities and patterns. Migration has been attributed to several mechanisms like the diffusion of a pore pressure perturbation along the fault, earthquakes triggering each other, or the propagation of a silent aseismic slip. Here, we combine recent theoretical and observational developments to propose a new model to explain the migration of seismicity in two well-studied induced sequences in France and Switzerland. Our model directly relates migration distance to injected fluid volume and other physical parameters describing the frictional or stress properties of the stimulated faults. This generic model explains seismicity propagation and provides constraints on the relevant physical parameters controlling it. We apply our approach to earthquake swarms occurring in natural context, and reconstruct fluid circulation dynamics during those sequences. Induced seismicity front position correlates with cumulative injected fluid volumeAn aseismic crack model driven by fluid injection explains the observed slip distribution and seismicity migrationsOur results enable the reconstruction of a wide range of migration patterns within earthquake swarms and their injection history