Mutual relations between stress drop of induced earthquakes and fault cohesion

In some regions a significant stress drop characterizes earthquakes induced by underground fluid injections or productions. In addition, long-term fluid operations in the underground can influence a seismogenic reaction of the rock per unit volume of the fluid involved. 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 earthquakes. Our model suggests that a high stress drop can result from a decrease in cohesion of initially inactive faults that are seismically activated by long-term fluid operations. On the one hand, the increasing stress drop can lead to an increase in the seismogenic index with the time of fluid operations. On the other hand, a production/injection caused change of the pore pressure can also cause a systematic increase in the stress drop. This can provide an additional contribution to the growth of seismogenic index (and thus to the seismic risk) with operation time of reservoirs.

The case study of Groningen gas field provides interesting information in this respect. A significant stress drop of some induced earthquakes at Groningen can be explained by activating preexisting cohesive normally-stressed fault systems. Seismic events on such faults lead to the drop of their cohesion due to the rupture process. This cohesion drop contributes directly to the earthquake stress drop. The production-related increase of the differential stress in the reservoir leads to an increasing number of seismically activated more cohesive faults. This leads in turn to an increasing seismogenic index. The seismogenic index seems to be quite low at Groningen. However, it increases systematically with the production time. One of reasons of this behavior can be related to the average cohesion of involved faults as it is mentioned above. An additional effect contributing to this increase is a systematically increasing stress drop due to the production-related pressure depletion increasing the effective stress in the reservoir. A growing seismogenic index can result in an increasing with time maximum possible magnitude, Mmax.