M. B. Magnani

Discriminating between natural versus induced seismicity from long-term deformation history of intraplate faults

Long-term fault slip history can diagnose natural versus induced earthquakes, independent of correlations with fluid injection. 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.

Stress Orientations in the Fort Worth Basin, Texas, Determined from Earthquake Focal MechanismsStress Orientations in the Fort Worth Basin

Since 2008, the Fort Worth basin (FWB) in northern Texas has experienced more than 30 M 3.0+ earthquakes, including one M 4.0. Earthquakes have primarily occurred on Precambrian basement faults and within the overlying Ellenburger limestone unit, which is the primary wastewater disposal formation used in the basin. Using data recorded by local seismic networks, we generate 240 focal mechanisms for the Azle–Reno, Irving–Dallas, and Venus sequences using P-wave first-motion and Sto P-wave (S/P) amplitude ratio data. The mechanism solutions describe primarily northeast (NE)–southwest (SW)-trending normal faults for each sequence and display a surprising lack of intersequence variability. Formal focal mechanism (FMF) stress inversions indicate maximum regional horizontal stress in the basement strikes 20°–25° east (E) of north (N), consistent with borehole breakout data collected from the overlying sedimentary succession, suggesting that the majority of seismogenic faults in the basin are optimally oriented for failure. We show via Mohr diagrams that increases in porefluid pressure at fault depths, with magnitudes similar to those observed at other induced-seismicity sites, are capable of inducing slips along the causative faults of the 2013–2015 Azle–Reno, 2014–present Irving–Dallas, and 2015 Venus earthquake sequences in the FWB. Electronic Supplement: Details about the Southern Methodist University (SMU) earthquake catalog and network coverage, figures showing the eight velocity models used for earthquake location and focal mechanism generation, parameter information for focal mechanism calculations, additional information on stress results, and the data files described in the main article.