  We examine the hypothesis proposed in recent years by several authors that
the crust is in a self-organized critical (SOC) state by exploring how the SOC
concept can help in understanding the observed earthquake clustering on
relatively narrow fault domains and the phenomenon of induced seismicity. We
review the major reported cases of induced seismicity in various parts of the
world and find that both pore pressure changes (+/-p) and mass transfers (+/-m)
leading to incremental deviatoric stresses of <<1 MPa are sufficient to trigger
seismic instabilities in the uppermost crust with magnitude ranging up to 7.0
in otherwise historically aseismic areas. Once triggered, stress variations of
at least 1 order of magnitude less but still larger than the ~0.01 MPa tidal
stress are enough to sustain seismic activity. We argue that these observations
are in accord with the SOC hypothesis as they show that a significant fraction
of the crust is not far from instability and can thus be made unstable by
minute perturbations. Not all perturbations, however, trigger seismic activity;
this is also compatible with the SOC hypothesis which embodies naturally the
existence of large heterogeneities in the stress field. In conclusion, the
properties of induced seismicity and their rationalization in terms of the SOC
concept provide further evidence that potential seismic hazards extend over a
much larger area than that where earthquakes are frequent.
