Analysis of seismograms from a magnitude 3.9 seismic event on 6 August 2007 in central Utah reveals an anomalous radiation pattern that is contrary to that expected for a tectonic earthquake and which is dominated by an implosive component. The results show that the seismic event is best modeled as a shallow underground collapse. Interestingly, large transverse surface waves require a smaller additional noncollapse source component that might represent either faulting in the rocks above the mine workings or deformation of the medium surrounding the mine. Seismic moment tensor results for nuclear explosions, explosion and other mining cavity collapses, and tectonic earthquakes are compared, and the separation of the different populations indicates that the seismic moment tensor may be used for source-type discrimination.