Microseismic activity was monitored during stimulation of a horizontal well in a tight gas shale using a wide-aperture array of geophones deployed on the surface above the well location. The lateral was drilled perpendicular to the presumed maximum horizontal stress direction, but long, linear, well-constrained microseismic event trends developed at an angle to the wellbore. Source mechanisms of the events show the failure planes of the events were parallel to the microseismicity trends. However, in-situ stress analysis from a crossed-dipole shear log acquired in the well showed the event trends are not parallel to the maximum horizontal stress direction. This result has important implications for stress interpreted from source mechanism analysis and for the impact of natural fractures on the stimulation treatment. Prior knowledge of existing fractures in the reservoir may be critically important for deciding the deviation of horizontal wellbores in order to optimize the stimulation treatment and placement of subsequent wells for field development.