The Dallas-Fort Worth Earthquake Sequence: October 2008 through May 2009

This paper describes a sequence of small earthquakes (m(b) <= 3.3) that occurred at the Dallas-Fort Worth Airport, Texas, between 30 October 2008 and 31 May 2009. Analysis of records at regional station WMOK identified more than 180 earthquakes in the sequence; about 90 percent occurred in four clusters on 30 October-1 November, 20 November, 26 December and 15-17 May. After the sequence began, a six-station temporary local network obtained high-quality three-component records for 11 earthquakes occurring between 20 November and 1 December. Analysis of these data demonstrated that all 11 earthquakes originated from a focus near 32.855 degrees N, 97.051 degrees W, with an estimated depth similar to 4.4 km. This location is less than 0.5 km from a well completed in August 2008 that extends to a depth of 4.2 km, drilled to dispose of brines collected during flowback of hydraulic fracturing fluids associated with the production of natural gas. Brine disposal commenced at the well on 12 September 2008. Seismograms and (S-P) intervals for the earthquakes are similar though not identical, and relative locations indicate they occurred along a north-northeast-south-southwest trend with horizontal and vertical dimensions of similar to 1.1 km and 0.2 km, respectively. This trend is approximately coincident with that of a mapped normal fault in the subsurface, and consistent with the maximum horizontal in situ stress direction. Because of the absence of previous historical earthquakes, the proximity of the brine disposal well, and the similarity with other documented cases of induced seismicity, it seems likely that fluid injection induced the 2008-2009 sequence.