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We investigate possible links between seismicity and fluid injection in the Williston basin in the north‐central United States, focusing on the region around the Bakken formation unconventional hydrocarbon play. Here, we show earthquakes are rarer near injection wells in the Williston basin than in the Fort Worth basin of Texas or in central Oklahoma. To identify earthquakes, we analyze seismograms collected by EarthScope USArray temporary stations, deployed on a grid with 70 km spacing. During the September 2008–May 2011 study period, we identified only nine regional earthquakes; of these only three were situated near injection wells. The reason why Williston basin earthquakes are so scarce is unclear. In both the Bakken and Barnett Shale play regions, injection volumes increased significantly in late 2007, and both areas have very low levels of natural seismicity. Oklahoma has experienced much higher rates of apparently induced seismicity than either region, possibly because injection volumes are higher in some wells in Oklahoma.

On 9 June 2009, an ![Graphic][1] 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° N, 97.372° 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 ∼50° and a component of oblique motion (rake of ∼−80°). 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. Online Material: Tables of absolute and relative earthquake locations and a figure summarizing focal mechanism solutions. [1]: /embed/inline-graphic-1.gif

INTRODUCTION As earthquake seismologists, we routinely plot “beachball” focal mechanisms describing the pattern of P -wave first-motions for ordinary, garden-variety double-couple earthquakes. But, how does one plot the pattern for Sh and Sv waves or for earthquake sources with isotropic or compensated linear vector dipole (CLVD) components? This is one of those questions that has always threatened me with a feeling of vague discomfort because I was sure I should know the answer, but I wasn't sure I did, even though I thought it was probably easy. Recently I needed to know the answer for two reasons: to check some software I had written for constructing synthetic seismograms and to determine whether observations of S waves at certain specific seismic stations had the same sign as the radiation predicted by the published Harvard centroid moment tensors (CMT). So, I resolved to spend a “few hours” writing a program to plot nodal...