f Effective stress drop of fluid induced seismicity

In this paper we test how the effective stress drop is comparable to the static stress drop of a single earthquake rupturing the same fault portion. To this purpose, we compare the spatiotemporal evolution of the seismic moment release and analyze the uncertainties of the resulting stress drop estimates. We show that the effective stress drop is only comparable to earthquake stress drops in specific cases. In particular, the effective stress drop values significantly underestimate the earthquake stress drops in the presence of aseismic deformation. Furthermore, the values are only scale-independent if pre-stress and post-stress conditions are uniform in space. Our analysis of data from injection-induced seismicity, natural earthquake swarms and aftershock sequences shows that in most cases the effective stress drop estimate is rather stable during the cluster evolution. Slightly increasing estimates for injection-induced seismicity are indicative for the local forcing of the system, while overall low effective stress drop values hint to the important role of aseismic deformations. While normal values up to 1MPa are found for seismicity associating geothermal reservoirs stimulation, anomalous small effective stress drop occur in case of fracking tight sands and shales, which may indicate aseismic deformation during these treatments.