DataSheet1_Can Hydrocarbon Extraction From the Crust Enhance or Inhibit Seismicity in Tectonically Active Regions? A Statistical Study in Italy.ZIP

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 which virtually any result can be a source of controversy. We implemented a large-scale analysis aiming at tracking significant departures of background seismicity from a stationary behavior around active oil and gas development leases in Italy. We analyzed seismicity rates before and after hydrocarbon peak production in six oil-producing and 43 gas-producing leases, and evaluate the significance of possible seismicity rate changes. In a considerable number of cases seismicity rate results stationary. None of the observed cases of seismicity rate increase after the peak production is statistically significant (at a s.l. = 0.05). Conversely, considering cases of seismicity rate decrease after peak production, our results suggest that the seismicity rate 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 evidence correlated changes between the rates of shallow seismicity and hydrocarbon production in these areas, which are then identified as hotspots requiring more detailed research; assessing actual causal relationships between these processes will require further physically-based modelling. If a physical causative link between these processes exists, then the observed seismicity rate reduction could either be due to increased seismicity during the progressive increase in production before reaching its maximum, or to an actual seismicity rate reduction after that peak. Considering that there is evidence of seismicity occurring before the start of hydrocarbon production, which contrasts with the evident reduction of events observed after the 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 oriented for failure, and we show that under some conditions the inhibition of seismicity is feasible in at least one of these cases. We conclude that more efforts to study the mechanisms and the possible consequences of anthropogenically-driven seismicity inhibition are required.

近数十年来，意大利境内运营着大量油气开采特许权区块，其中多数位于构造活跃区域周边。在天然地震活动水平较高的区域识别人工诱发地震（human-induced seismicity）是一项极具挑战性的工作，几乎任何相关研究结果都可能引发争议。本研究开展了大规模分析，旨在追踪意大利境内活跃油气开发区块周边背景地震活动性（background seismicity）偏离平稳状态的显著异常情况。我们针对6个产油区块与43个产气区块，分析了油气产量达到峰值前后的地震活动率（seismicity rates）变化，并评估了地震活动率改变的统计显著性。在相当一部分案例中，地震活动率保持平稳。产量峰值后观测到的地震活动率上升案例，均未达到0.05的显著性水平（s.l. = 0.05）。与之相反，针对产量峰值后地震活动率下降的案例，研究结果表明，其中1个产油区块（巴西利卡塔大区的瓦勒达格里（Val d’Agri））以及西西里岛的一群产气区块周边的地震活动率下降具有统计显著性（s.l. = 0.05）。本研究结果揭示了上述区域浅源地震活动率与油气开采活动间的关联变化，这些区域因此被认定为需要开展更深入研究的热点区域；而要明确二者间实际的因果关系，则需进一步开展基于物理机制的建模工作。若二者间确实存在物理成因关联，那么观测到的地震活动率下降，既可能是因为产量达到峰值前的持续增产阶段地震活动有所增强，也可能是产量峰值后实际的地震活动率降低。考虑到已有证据表明油气开采启动前便存在地震活动，这与产量峰值后观测到的地震事件明显减少形成鲜明对比，我们认为地震活动抑制是一个合理的假说。通过构建简单模型，我们还计算了最优失稳取向平面的库仑应力变化（Coulomb stress changes），并证明在部分条件下，至少在其中一个案例中，地震活动抑制是可行的。综上，我们认为需要投入更多精力研究人类活动诱发的地震活动抑制的机制与潜在后果。