Seismic Hazard Due To Fluid Injections

Earthquakes can be induced by natural and anthropogenic processes involving the injection or migration of fluids within rock formations. A variety of field observations has led to the formulation of three different and apparently contradicting paradigms in the estimation of the seismic hazard associated with fluid injections. We introduce a unified conceptual model accounting for the nonhomogeneous pore-pressure stimulation caused by fluid injection in a prestressed region, to prove how all three paradigms can naturally coexist. Within our model framework the loading history, accounting for both the fluid injections and natural tectonic loading, together with the heterogeneity of the host medium determine which of the three paradigms prevails at a given time. We identify a superposition of two populations of events triggered at different local stress levels with different Gutenberg-Richter b values. Our findings suggest that both the observed diversity of b values across different fluid-induced settings and the dominating hazard paradigm are a consequence of such a superposition leading to an effective b value.