Probability of earthquake occurrence and magnitude estimation in the post shut-in phase of geothermal projects

Induced seismicity in geothermal projects is observed to continue after shut-in of the fluid injection. Recent experiments show that the largest events tend to occur after the termination of injection. We use a probabilistic approach based on Omori's law and the Gutenberg-Richter magnitude-frequency distribution to demonstrate that the probability of exceeding a certain maximum magnitude still increases after shut-in. This increase is governed by the exponent of Omori's law q and the Gutenberg-Richter b value. For a reduced b value in the post-injection phase, the probability of occurrence directly after shut-in can be even higher than the corresponding probability for an ongoing injection. For the reference case of q = 2 and a 10% probability at shut-in time t (S) to exceed a given maximum magnitude, we obtain an increase to 14.6% for t = 2t (S) at a constant Gutenberg-Richter b value after shut-in. A reduction of the b value by one quarter leads to a probability of 20.5%. If we consider a constant probability level of occurrence for an event larger than a given magnitude at shut-in time, this maximum magnitude increases by 0.12 units for t = 2t (S) (0.26 units for a reduced b value). For the Soultz-sous-Forts (France) injection experiment in 2000, recent studies reveal q = 9.5 and a b value reduction by 14%. A magnitude 2.3 event 9 h after shut-in falls in the phase with a probability higher than for the continued injection. The probability of exceeding the magnitude of this post-injection event is determined to 97.1%.