1 Introduction

Post‐2008 earthquake rate increases in the central United States have been associated with large‐scale subsurface disposal of waste‐fluids from oil and gas operations (e.g., Ellsworth, 2013; Rubinstein & Mahani, 2015; Walsh & Zoback, 2015; Weingarten et al., 2015). The 31 October 2008 earthquakes at the Dallas‐Fort Worth International Airport (hereafter, DFW Airport) were the first documented earthquakes in the Fort Worth Basin (FWB) in the historic record (Frohlich et al., 2010, 2011, 2016; Frohlich & Davis, 2002) and the first of multiple earthquake sequences in the basin associated with waste‐fluid injection (Frohlich, 2012; Hornbach et al., 2015, 2016; Justinic et al., 2013; Lund Snee & Zoback, 2016; Reiter et al., 2012; Scales et al., 2017; Weingarten et al., 2015). As efforts to design mitigation strategies for induced earthquakes continues, questions regarding the temporal continuity in rates and magnitudes during and after reduction or cessation of subsurface injection remain unclear (Bommer et al., 2015). Understanding the spatiotemporal evolution of induced earthquake sequences like those in the FWB provides data to inform mitigation strategies.

Seismicity at the DFW Airport began on 31 October 2008 with a series of 8 m blg 2.6 to 3.0 earthquakes reported by the U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC). Following the 31 October events, seismologists from Southern Methodist University (SMU) deployed six 3‐component broadband seismographs and recorded 11 events of magnitude 1.7 to 2.3, including a cluster on 20 November not reported by the NEIC (Frohlich et al., 2010, 2011) (Figure 1). A m blg 3.3 in May 2009 prompted redeployment of four stations around the airport. Analysis indicated that the earthquakes were located within a 1 km radius of a Class II saltwater disposal (SWD) well that had begun injecting 7 weeks prior to the October earthquakes (Figure 2). Reflection data showed a regional NE‐SW trending normal fault (Railroad Commission of Texas, 2015), now called the Airport fault, present at earthquake and injection depths. The south airport well (hereafter, S‐well) ceased injecting in August 2009. Frohlich et al. (2011) concluded that the DFW Airport earthquakes were probably triggered on a preexisting fault by subsurface stress changes due to waste‐fluid injection. Reiter et al. (2012) reanalyzed the data collected during the SMU deployment, were able to detect more events, and reached the same conclusion as Frohlich et al. (2011) regarding cause.

Figure 1 Open in figure viewer PowerPoint 2010 2011 2012 2015 Study area for the DFW Airport earthquake sequence. The 2008–2009 SMU operated stations (yellow triangles), reanalyzed in this study, and recorded events (red asterisks) reported in Frohlich et al. (). Janská and Eisner ()reported additional events (hypocenters approximated by green asterisks) using stations operated by Chesapeake Energy (green triangles); CHKDFWS station is used in this study. Earthquakes reported by the 2013–2016 SMU earthquake catalog (blue asterisks) indicate continued seismicity at the DFW Airport (property boundary is dashed cyan line). The seismicity around station CPSTX is the Irving sequence. Regional normal faults mapped at the top of the Ellenburger formation are indicated by black lines (Railroad Commission of Texas,). The solid black line is the Airport fault. Inset: Map of Texas showing the three regional seismic stations used in this study. The circles denote the equidistant circumference used for relocation methods described in the text.

Figure 2 Open in figure viewer PowerPoint 2015 Spatiotemporal clustering of earthquakes sized by the number of events in a cluster. Clusters are binned per month and are colored based on time. The solid black line is the Airport fault, and the dashed line is unnamed (Railroad Commission of Texas,). The north injection well is 7.5 km north of the point C. Inset: Location of the north (N) and south (S) injection wells relative to the DFW Airport. The dashed red line coincides with the pore pressure diffusion model cross section shown in Figure S5

Beginning in 2009, temporary seismic stations were placed at the airport by Chesapeake Energy, which operated multiple production and two Class II SWD wells on the property (see Figure 2 for injection well locations). Janská and Eisner (2012)used these data to document a cluster of earthquakes about 4 km north of the S‐well beginning in May 2010 through January 2012 (hypocenters approximated by green asterisks, Figure 1). Deployment of local seismic stations in the area by SMU, including redeployment of AFDAD in 2014, revealed small‐magnitude earthquakes continued at the airport; earthquakes from 2013 to present are reported in the SMU local earthquake catalog (blue asterisks, Figure 1) (Hornbach et al., 2015; Magnani et al., 2017; Scales et al., 2017).

We hypothesize that migration of the earthquakes along the Airport fault follows diffusion of pore fluid pressure changes at depth due to injection of waste fluids at the airport injectors. In order to provide a more consistent understanding of the spatial and temporal evolution of the sequence, we reanalyze local seismic data and long‐running regional stations to develop a spatially and temporally consistent earthquake catalog (Figure 1, inset). We use the NEIC and local earthquake catalogs as detection templates for cross correlation of continuous waveform data at regional stations that recorded from 2008 to 2015. We employ a novel relative location technique to constrain epicentral location using regional data. We model the pore pressure changes in the Ellenburger formation that is targeted for waste‐fluid injection and in the Precambrian crystalline basement over the period of injection to December 2015 and consider pressure influences from both the south and north injection wells located at the airport (Figure 2).