Can hydrocarbon extraction from the crust enhance or inhibit seismicity in tectonically active regions? A statistical study in Italy

A number of oil- and gas-producing leases have been operating in Italy in the last decades,many of which are located in the surroundings of tectonically active regions. Identifying human-induced seismicity in areas with high levels of natural seismicity is a difficult task for whichvirtually any result can be a source of controversy. We implemented a large-scale analysisaiming at tracking significant departures of background seismicity from a stationary behavioraround active oil and gas development leases inItaly. We analyzed seismicity rates before andafter hydrocarbon peak production in six oil-producing and 43 gas-producing leases, andevaluate the significance of possible seismicity rate changes. In a considerable number ofcases seismicity rate results stationary. None of the observed cases of seismicity rate increaseafter the peak production is statistically significant (at as.l.0.05). Conversely, consideringcases of seismicity rate decrease after peak production, our results suggest that the seismicityrate reduction is statistically significant (s.l.0.05) around one oil-producing lease (Val d’Agri,Basilicata) and around a cluster of gas-producing leases in Sicily. Our results put in evidencecorrelated changes between the rates of shallow seismicity and hydrocarbon production inthese areas, which are then identified as hotspots requiring more detailed research; assessingactual causal relationships between these processes will require further physically-basedmodelling. If a physical causative link between these processes exists, then the observedseismicity rate reduction could either be due to increased seismicity during the progressiveincrease in production before reaching its maximum, or to an actual seismicity rate reductionafter that peak. Considering that there is evidence of seismicity occurring before the start ofhydrocarbon production, which contrasts withthe evident reduction of events observed afterthe peak production, we think it likely that the seismicity inhibition is a plausible hypothesis.Using a simple model we also calculate Coulomb stress changes in planes optimally orientedfor failure, and we show that under some conditions the inhibition of seismicity is feasible in atleast one of these cases. We conclude that more efforts to study the mechanisms and thepossible consequences of anthropogenically-driven seismicity inhibition are required.