Magnani Maria Beatrice, Blanpied Michael L, DeShon Heather R, Hornbach Matthew J
Roy Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75205, USA.
U.S. Geological Survey Earthquake Hazard Program, Reston, VA 20192, USA.
Sci Adv. 2017 Nov 24;3(11):e1701593. doi: 10.1126/sciadv.1701593. eCollection 2017 Nov.
To assess whether recent seismicity is induced by human activity or is of natural origin, we analyze fault displacements on high-resolution seismic reflection profiles for two regions in the central United States (CUS): the Fort Worth Basin (FWB) of Texas and the northern Mississippi embayment (NME). Since 2009, earthquake activity in the CUS has increased markedly, and numerous publications suggest that this increase is primarily due to induced earthquakes caused by deep-well injection of wastewater, both flowback water from hydrofracturing operations and produced water accompanying hydrocarbon production. Alternatively, some argue that these earthquakes are natural and that the seismicity increase is a normal variation that occurs over millions of years. Our analysis shows that within the NME, faults deform both Quaternary alluvium and underlying sediments dating from Paleozoic through Tertiary, with displacement increasing with geologic unit age, documenting a long history of natural activity. In the FWB, a region of ongoing wastewater injection, basement faults show deformation of the Proterozoic and Paleozoic units, but little or no deformation of younger strata. Specifically, vertical displacements in the post-Pennsylvanian formations, if any, are below the resolution (~15 m) of the seismic data, far less than expected had these faults accumulated deformation over millions of years. Our results support the assertion that recent FWB earthquakes are of induced origin; this conclusion is entirely independent of analyses correlating seismicity and wastewater injection practices. To our knowledge, this is the first study to discriminate natural and induced seismicity using classical structural geology analysis techniques.
为了评估近期的地震活动是由人类活动引起还是源于自然，我们分析了美国中部（CUS）两个地区的高分辨率地震反射剖面的断层位移：德克萨斯州的沃思堡盆地（FWB）和密西西比河下游河谷（NME）。自2009年以来，CUS的地震活动显著增加，众多出版物表明，这种增加主要是由于废水的深井注入引发的诱发地震，这些废水包括水力压裂作业产生的返排液以及伴随油气生产产生的采出水。另一些人则认为，这些地震是自然发生的，地震活动的增加是数百万年来发生的正常变化。我们的分析表明，在NME内，断层使第四纪冲积层以及从古生代到第三纪的下伏沉积物发生变形，位移随地质单元年龄增加，记录了长期的自然活动历史。在FWB这个正在进行废水注入的地区，基底断层显示元古代和古生代地层发生了变形，但较年轻地层几乎没有或没有变形。具体而言，宾夕法尼亚纪之后地层中的垂直位移（如果有的话）低于地震数据的分辨率（约15米），远低于这些断层在数百万年里积累变形时的预期值。我们的结果支持了近期FWB地震是由诱发因素导致的这一论断；这一结论完全独立于将地震活动与废水注入实践相关联的分析。据我们所知，这是第一项使用经典构造地质学分析技术区分自然地震活动和诱发地震活动的研究。
