Injection-induced sequences give us insights about what is happening at depth during natural earthquake swarms

Natural earthquake swarms occur in various geological contexts, and are usually interpreted as driven by fluid pressure diffusion. However, little is known about their fluid-driving processes, as no direct observations of either fluid and deformation are possible at such depths. To improve our understanding of the processes involved in swarms, we develop a quantitative comparison between natural and injection-induced swarms. Fluid injections in the crust, for instance geothermal reservoir development or wastewater storage, are accompanied by a prolific seismicity, that can be related to the fluid-pressure perturbation and potentially in association with aseismic slip at depth. It is well-accepted that the released seismic moment scales with injected fluid volume, but proposed relations usually not consider the contribution of aseismic deformation. Constraining such a relation might provide information on what happens at depth during natural earthquake swarms. Indeed, based on the numerous similarities observed between natural and injection-induced swarms, we confirm that both types of sequences seem to obey the same physics. In our work, we establish a framework to relate seismic observables to the fluid volume circulating at depth. This allows us to quantify aseismic slip for all types of swarms, but also to estimate the volume of fluids circulating at depth during natural earthquake swarms. By focusing on several natural swarms, this sheds a new light on the processes driving swarms of seismicity.