Determining Principal Stress Orientations and Magnitudes in the Fort Worth Basin, Texas

Since 2008, the Fort Worth Basin in northern Texas has experienced over 30 M3.0+ earthquakes, including one M4.0. 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. The most productive earthquake sequences (Azle, Irving- Dallas, and Venus) have been recorded using local seismic networks containing a mix of SMU broadband, IRIS short period, and USGS NetQuakes stations and by regional seismic networks operated under the Advanced National Seismic System and the Texas Seismic Network (TexNet). Using the data recorded by the local seismic networks, I generate 240 focal mechanisms describing the fault plane orientations of the 2013-2015 Azle, 2014-present Irving-Dallas, and 2015-present Venus earthquake sequences using P-wave first motion and S to P-wave amplitude ratio data. The focal mechanism solutions describe primarily NE-SW trending normal faults for each sequence and display a surprising lack of inter-sequence variability. Single focal mechanism and formal focal mechanism stress inversions indicate that the maximum regional horizontal stress in the crystalline basement rocks strikes 20-25° E of N. This maximum horizontal stress orientation is consistent with prior collected borehole breakout data collected from the overlying sedimentary succession, suggesting that the majority of seismogenic faults in the basin are optimally oriented for failure. I show via Mohr diagrams that increases in pore fluid pressure at fault depths (3-4 km), with magnitudes similar to those observed at other induced seismicity sites, are capable of inducing slip along the causative faults of the Azle, Irving-Dallas, and Venus earthquake sequences in the Fort Worth Basin. Preliminary calculations studying the potential sources of the increase in stress at the Irving-Dallas site indicates that Coulomb stress changes associated with the other FWB sequences, and poroelastic stress effects related to injection activities in Johnson county alone, would not account for the necessary stress change to trigger slip on the Irving- Dallas causative fault