Long-period long-duration (LPLD) seismic events that have been observed during hydraulic stimulation of shale-gas and tight-gas reservoirs appear to represent slow shear slip on relatively large faults. Within the limitations of the recording geometry, we determine the areas in the reservoirs where the events are located in two case studies in the Barnett shale. In one data set, LPLD events appear to occur in the region where the density of natural fractures as well as the fluid pressure during pumping were highest. In the other data set, the LPLD events are observed to occur between two wells and seem to establish a hydraulic connection between them. In both data sets, the LPLD events occur in areas with very few located microearthquakes. A combination of factors such as high fluid pressure and/or high clay content is potentially responsible for the slowly slipping faults. The LPLD events appear to be occurring only on faults large enough to produce a sequence of slow slip events. We suggest that these slowly slipping faults contribute appreciably to the stimulation of these extremely low-permeability reservoirs and hence mapping the distribution of faults and fractures and areas with rock properties that favor slow, sustained slip, can help in optimizing production.