We extend a full-waveform modeling method to invert source focal-plane mechanisms for microseismic data recorded with dual-borehole seismic arrays. Combining inverted focal-plane mechanisms with geomechanics knowledge, we map the pore pressure distribution in the reservoir. Determining focal mechanisms for microseismic events is challenging due to poor geometry coverage. We use the P-wave polarities, the P- and S-wave similarities, the SV/P amplitude ratio, and the SH/P amplitude ratio to invert the focal-plane mechanisms. A synthetic study proves that this method can effectively resolve focal mechanisms with dual-array geometry. We apply this method to 47 relatively large events recorded during a hydraulic fracturing operation in the Barnett Shale. The focal mechanisms are used to invert for the orientation and relative magnitudes of the principal stress axes, the orientation of the planes slipping in shear, and the approximate pore pressure perturbation that caused the slip. The analysis of the focal mechanisms consistently shows a normal faulting stress state with the maximum principal stress near vertical, the maximum horizontal stress near horizontal at an azimuth of N60°E, and the minimum horizontal stress near horizontal at an azimuth of S30°E. We propose a general method that can be used to obtain microseismic focal-plane mechanisms and use them to improve the geomechanical understanding of the stimulation process during multistage hydraulic fracturing.