A SUMMARY OF FORT WORTH BASIN INDUCED EARTHQUAKE SEQUENCES 2008-PRESENT

The Fort Worth basin (FWB) in north Texas has experienced over 30 magnitude (M) 3.0+ earthquakes and one M4.0 since 2008. Events have been linked to disposal of produced wastewater into the Ellenburger formation, which is in hydrogeologic fault contact with the crystalline basement. Here, seismicity rates have decreased as wastewater injection has decreased. The SMU North Texas Earthquake Study (NTXES) catalog now contains over 1600 earthquakes ranging between magnitude -1 to 4 that can be used to study fault processes and causative mechanisms for reactivation of this intraplate region. Monitoring currently takes place through the Texas Seismic Network and the SMU NTXES. The presentation provides an overview of earthquake catalogs, data availability, and current research conclusions regarding induced earthquakes in the Fort Worth basin.

Seismicity rates have decreased significantly since peak injection volumes in 2014 but earthquakes continue to occur into 2019. Earthquakes have occurred on northeast (NE)-southwest (SW) trending basement faults and to a more limited extent, within the overlying Ellenburger dolomitic limestone formation. The seismogenic faults do not appear unique relative to other similarly oriented faults in the system imaged using 2D and 3D seismic reflection data and do not appear to offset units younger than 300 Ma. Focal mechanisms indicate primarily normal faulting with maximum horizontal stress striking 20-30° NE. The faults appear optimally or near-optimally oriented for failure within the modern stress regime, suggesting that small perturbations in stress via poro-elastic or pore fluid pressure changes could trigger failure. Stress drop estimates are consistent with intraplate earthquakes in natural tectonic settings. The spatio-temporal history of earthquakes relative to wastewater injection data varies. It nows appears that the cumulative history of wastewater injection and fluid production within the Ellenburger, and the evolution of pressure in the injection formation and units in hydrogeological contact, are key drivers of the seismogenic evolution of the basin.