TRIGGERED EARTHQUAKES AND DEEP-WELL ACTIVITIES

Earthquakes can be triggered by any significant perturbation of the hy drologic regime. In areas where potentially active faults are already close to failure, the increased pore pressure resulting from fluid inj ection, or, alternatively, the massive extraction of fluid or gas, can induce sufficient stress and/or strain changes that, with time, can l ead to sudden catastrophic failure in a major earthquake. Injection-in duced earthquakes typically result from the reduction in frictional st rength along preexisting, nearby faults caused by the increased format ion fluid pressure. Earthquakes associated with production appear to r espond to more complex mechanisms of subsidence, crustal unloading, an d poroelastic changes in response to applied strains induced by the ma ssive withdrawal of subsurface material. As each of these different ty pes of triggered events can occur up to several years after well activ ities have begun (or even several years after all well activities have stopped), this suggests that the actual triggering process may be a v ery complex combination of effects, particularly if both fluid extract ion and injection have taken place locally. To date, more than thirty cases of earthquakes triggered by well activities can be documented th roughout the United States and Canada. Based on these case histories, it is evident that, owing to preexisting stress conditions in the uppe r crust, certain areas tend to have higher probabilities of exhibiting such induced seismicity.