Analysis of the Cleburne, Texas, Earthquake Sequence from June 2009 to June 2010

On 9 June 2009, an M-bLg 2.8 earthquake shook Cleburne, Texas, a community not known to have previously experienced earthquakes. Over 50 small earthquakes followed by the end of December 2009. A temporary network of four and then five IRIS-Passcal broadband systems was deployed from June 2009 to June 2010, recording data that were used to locate 38 events with the most confident P- and S-arrival picks. Event locations were distributed along a 2 km long north-northeast trend. The location centroid was at 32.298 degrees N, 97.372 degrees W and at 3.6 km depth. This location is approximately 1.3 km from a saltwater disposal well that began injection in October 2007 and 3.2 km away from a second injection well that was active from September 2005 to late July 2009. Focal mechanisms estimated for the best-recorded events suggest a north-northeast-south-southwest-trending normal fault with a dip of similar to 50 degrees and a component of oblique motion (rake of similar to-80 degrees). This average solution is generally consistent with the north-northeast-trending extensional faults that are prevalent across parts of Texas, Oklahoma, Louisiana, and Arkansas. Stress drops calculated from P and S spectra for seven different events ranged from 3.9 to 90 bars, with most estimates between 40 and 50 bars, typical values for intraplate earthquakes. Because there were no known previous earthquakes, and the located events were close to the two injection wells and near the injection depth, the possibility exists that earthquakes may be related to fluid injection.