Ⓔ Injection-Induced Seismicity and Fault-Slip Potential in the Fort Worth Basin, Texas by Peter H. Hennings, Jens-Erik Lund Snee, Johnathon L. Osmond, * Heather R. DeShon, Robin Dommisse, Elizabeth Horne, Casee Lemons, and Mark D. Zoback Abstract The rate of seismicity in the hydrocarbon-producing Fort Worth Basin of north-central Texas, which underlies the Dallas–Fort Worth metropolitan area, increased markedly from 2008 through 2015, coinciding spatiotemporally with injec- tion of 2 billion barrels of wastewater into deep aquifers. Although the rate of seis- micity has declined with injection rates, some earthquake sequences remained active in 2018 and new clusters have developed. Most of this seismicity occurred away from regionally mapped faults, challenging efforts to constrain the continuing hazards of injection-induced seismicity in the basin. Here, we present detailed new models of potentially seismogenic faults and the stress field, which we use to build a probabi- listic assessment of fault-slip potential. Our new fault map, based on reflection seismic data, tens of thousands of well logs, and outcrop characterization, includes 251 base- ment-rooted normal faults that strike dominantly north-northeast, several of which extend under populated areas. The updated stress map indicates a relatively consistent north-northeast–south-southwest azimuth of the maximum horizontal principal stress over seismically active parts of the basin, with a transition from strike-slip faulting in the north to normal faulting in the southeast. Based on these new data, our probabi- listic analysis shows that a majority of the total trace length of the mapped faults have slip potential that is equal to or higher than that of the faults that have already hosted injection-induced earthquake sequences. We conclude that most faults in the system are highly sensitive to reactivation, and we postulate that many faults are still uniden- tified. Ongoing injection operations in the region should be conducted with these understandings in mind. Supplemental Content: Tabulation of data used for the interpretation of S H max in the Fort Worth basin. Introduction The Fort Worth basin (FWB) underlies a broad area of north-central Texas (Figs. 1 and 2), including most of the Dallas–Fort Worth (DFW) metropolitan area. The FWB hosts conventional and unconventional hydrocarbon-producing intervals including the Mississippian Barnett Shale (Pollastro et al., 2007). Like many areas of the south-central United States, the FWB experienced a sharp increase in earthquakes over the last 10 yr that has been linked to the disposal of oil- field wastewater into deep wells, with most disposal targeting the carbonate-rich Ordovician Ellenburger Group (Ellsworth, 2013; Weingarten et al., 2015). This process is called saltwater disposal (SWD). Only two earthquakes were recorded in the basin between 1970 and 2007, but 18 M w ≥ 2:5 events were cataloged in 2008 through 2010, 101 from 2011 through 2015, and 6 since 2015 (U.S. Geological Survey [USGS] Advanced National Seismic System Comprehensive Catalog [ComCat], see Data and Resources). The largest earthquake was an M w 4.0 event that occurred in Johnson County near Venus in 2015. The rate of SWD in the basin increased considerably beginning in 2005 and peaked at 28 MMbbl/month in late 2011, but the largest number of earthquakes per month occurred in late 2013 (Fig. 3). Months with large numbers of earthquakes continued to occur throughout 2014 and 2015 although the average monthly rate of SWD in the basin decreased by 43% during that time. This trend of a marked increase and decrease in seismicity with a time delay following an increase and decrease in the rate of SWD roughly mirrors the behavior *Also at Centre for Earth Evolution and Dynamics (CEED), P.O. Box 1028 Blindern, NO-0315 Oslo, Norway. BSSA Early Edition / 1 Bulletin of the Seismological Society of America, Vol. XX, No. XX, pp. –, – 2019, doi: 10.1785/0120190017 Downloaded from https://pubs.geoscienceworld.org/ssa/bssa/article-pdf/doi/10.1785/0120190017/4801529/bssa-2019017.1.pdf by Univ of Texas-Austin user on 25 July 2019