Analysis of the Cleburne earthquake sequence from June 2009 to June 2010.

摘要

A MLgGS 2.8 earthquake shook Cleburne, Texas on June 9, 2009. The area had no seismic history. Over fifty earthquakes followed by the end of December 2009, all smaller than MLgGS 2.8. A temporary network of four and then five IRIS-Passcal broadband seismometers was deployed from June 2009 to June 2010. The TexFlex program was used to locate the 38 events with the most confident P and S arrival picks. Event locations were distributed along a north-northwest trend that was approximately 2 km in length. The average location was 32.298° N, 97.372° W, and depths ranged from 1.62 to 4.97 km, with an average of 3.55 km. This location is about 1.3 km from a salt water disposal well that had commenced injection in October 2007. It was 3.2 km away from another injection well that had commenced injection in September 2005 and subsequently shut in late July 2009. Focal mechanisms estimated for the events with the best local network distribution suggest a NNE or SSW trending normal fault with a dip of ∼50° and a component of oblique motion in the direction of strike (rake of ∼ -80). This is consistent with the quaternary NNE trending extensional faults that are prevalent across parts of Texas, Oklahoma, Louisiana, and Arkansas. Stress drops were calculated automatically from the source spectra and ranged from 3.9 to 90 bars for 7 different events, with most stress drop estimates between 40 and 50 bars, a typical range for intraplate earthquakes. Because there were no historical earthquakes, and because of the proximity to two injection wells, injecting around the depth of the events, the possibility that the earthquakes were induced cannot be ruled out even though both the fault plane solutions and the kinematic source parameters calculated are consistent with the orientation of the regional stress field and with previous small intraplate earthquakes.