Abstract

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 S‐ to 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 pore‐fluid 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.

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