DETERMINING THE CAUSES OF RECENT SEISMICITY IN JOHNSON COUNTY, TX

North Texas has seen an increase in seismic activity around the Dallas/Fort Worth area since the early 2000’s, with activity in Johnson County in particular culminating in magnitude 3 and 4 events in 2011 and 2015 respectively. Previous analysis of the Johnson County sequence between 2009 and 2011 concluded that many of the events were induced by wastewater injection (Frohlich, 2012), however the earthquake database was small during this time period, and the differences between inducing and non-inducing injection wells were not clearly identified. This study addresses the causes of recent seismicity in Johnson County through an in depth characterization of the seismicity, industry operations, and regional and local geology in North Texas from 2009 to 2011. Seismic template matching using 3 USArray TA station recordings of all previously cataloged earthquakes in the study area provide a more complete temporal history of seismicity. Earthquakes cataloged by Frohlich (2012) were divided into similar clusters based on their template matching patterns, and were relocated using Elocate and HypoDD. Recorded seismicity seems to align along NE-SW trends consistent with regional stress orientations and preexisting structures related to the adjacent Ouachita thrust front. Monthly injection intervals, volumes, and pressures from nearby injection wells, along with completion dates from hydraulic fracturing wells, suggest a correlation between industry activities and the increased seismicity in the area.