Abstract
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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