Abstract

A significant stress drop characterizes sometimes earthquakes induced by injection or extraction of fluids in rocks. Moreover, long-term fluid operations in underground reservoirs can impact a seismogenic reaction of the rocks per a unit volume of the involved fluid. The seismogenic index is a quantitative characteristic of such a reaction. We derive a relationship between the seismogenic index and the stress drop. We propose a simple and rather general phenomenological model of the stress drop of induced events in various faulting regimes. Our results suggest that high stress drops of some earthquakes induced by long-term underground fluid operations may be controlled by drops of cohesion of more cohesive faults getting seismically activated due to gradually increasing with time differential stresses. On the one hand, this effect can result in an increase of seismogenic index with production time. On the other hand, a production-caused depleting of the pore pressure can also cause a systematic increase of the stress drop. This provides an additional contribution to the growth of seismogenic index with production time at such reservoirs.