An important characteristic of seismicity is the distribution of magnitudes of earthquakes. Fluid injection in rocks, aimed to create enhanced geothermal systems (EGS), can sometimes produce significant seismic events (e.g., Majer et al., 2007). This is rarely the case in hydraulic fracturing of hydrocarbon reservoirs. However, in any case the behavior of the seismicity triggering in space and in time is controlled by the process of stress relaxation and pore-pressure perturbation that was initially created at the injection source. This relaxation process can be approximated by pressure diffusion (possibly a nonlinear one) in the pore fluid of rocks (e.g., Shapiro and Dinske, 2009). At some locations the tectonic stress in the Earth's crust is close to a critical stress, causing brittle failure of rocks. Increasing fluid pressure in such a reservoir causes pressure in the connected pore and fracture space of rocks to increase. Such an increase in the pore pressure consequently causes a decrease of the effective normal stress. This leads to sliding along pre-existing, favorably oriented, subcritical cracks.

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