Comparison Study between North Texas Earthquake Sequences from 2008-2015

Abstract:

High-resolution earthquake locations and fault plane solutions are combined with information on subsurface geology, fault structure, well data, and 3D pore pressure modeling to provide further insight into the relationship between fluid migration at depth and modern seismicity in North Texas. Since 2008, the USGS has reported over 165 felt earthquakes in the Fort Worth (Barnett Shale) Basin located in northern Texas. Five earthquake sequences warranted deployment of local seismic networks, and there are currently 30+ temporary seismic stations operating in the basin. Event size has increased over time. The 2008/2012 DFW Airport and 2009 Cleburne sequences had maximum event magnitudes of 3.3 and 2.8, respectively. The 2013/2014 Azle-Reno and 2014/2015 Irving-Dallas swarms contain multiple M3.5 earthquakes, and the M4.0 May 2015 Venus earthquake is the largest event recorded to date. Causative faults strike NNE-SSW to NE-SW and are associated with normal faulting, consistent with fault reactivation in the current stress regime. The active faults range from 2-5 km in length, are steeply dipping (45-70º) and can dip to the SE (DFW Airport; Irving-Dallas) or NW (Azle-Reno; Venus). Events occur between 2-6 km depth in the DFW, Cleburne, and Venus sequences but extend to 8 km in the Irving-Dallas and Azle-Reno earthquake swarms. These depths are consistent with reactivation of ancient faults located in the basement granites and/or overlying sedimentary units. The top of the Ordovician Ellenburger group, which serves as a wastewater injection unit in the basin, ranges from 1.4-2.7 km depth across the seismogenic area, is ~1 km thick on average, and generally overlies the Precambrian basement. The DFW Airport, Cleburne, and Azle-Reno sequences have been linked to nearby wastewater injection and/or production activity associated with shale gas extraction. Causal studies of the 2014/2015 Irving-Dallas and 2015 Venus sequences are ongoing and will be updated here.