Natural and anthropogenic processes involving the injection or migration of fluids within rock formations are well known driving mechanisms causing earthquakes, either induced or triggered. A variety of field observations has led to the formulation of three different modeling paradigms for the estimation of the seismic hazards as function of injected volume ($V$) [1], useful to develop risk mitigation strategies [2]. All of them are based on proportional hazard models but imposing different constraints caused by finite size effects: 

A) events are small and their number is proportional to injected volume: ($N(m)\propto V$) [3,4];

B) events are bounded in size and magnitude ($m$) by the extension of the stimulated area and other physical constrains [5];

C) event magnitudes are statistically indistinguishable from tectonic earthquakes [6].

Based on the simulation results of a simplified conceptual model accounting for the non-homogeneous pore-pressure stimulation [8] caused by fluid injection in a pre-stressed region with stick-slip mechanics [7], we provide a fundamental explanation on how all three paradigms can naturally coexist. The loading history and heterogeneity of the host medium determine which of the three paradigms dominates. In non-tectonic settings two populations of events triggered at different pore-pressure levels with different Gutenberg-Richter $b$-values are superposed. Stress-levels an d $V$ determine their proportions. In active tectonic settings, fluid injection triggers tectonic earthquakes.

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