Moment tensors are calculated by using the P-wave first motion peak amplitudes of 59 microseismic events with high signal-to-noise ratio. These events are from a surface microseismic data set gathered during hydraulic-fracture stimulation of the Marcellus gas shale in Washington County, Pennsylvania, USA. The majority of these 59 events have a horizontal nodal plane ( a few degrees) characteristic of a dip-slip/horizontal-slip moment tensor. If the horizontal nodal plane is an auxiliary, the vertical nodal plane has a pure dip-slip motion, which is inconsistent with the opening motion for vertical hydraulic fractures that enables proppant loading. This points to slip on horizontal nodal planes with the auxiliary vertical nodal planes aligned with the local maximum horizontal stress orientation as indicated by drilling-induced fractures in nearby vertical wells. These 59 microseismic events are caused by slip on horizontal mechanical discontinuities such as bedding planes during the opening of vertical hydraulic fractures, a model first proposed by research teams headed by Rutledge and Eisner, respectively. During several stimulation stages in the Washington County Marcellus gas shale, a pattern of opposite slip direction develops within “double lineaments” of microseismic clouds. This suggests that fracking fluid is not only able to move in the direction of fracture propagation, but it can also spread sideways into previously unstimulated rock. A secondary microseismic cloud consistently initiates at approximately 133 m (400 ft) from the position opposite the central perforation toward the unstimulated heel of the horizontal wells. From these moment tensors, we have concluded that microseismic focal mechanisms with horizontal nodal planes are direct evidence of the presence of treatment fluid in open hydraulic fractures.