Hydraulic-fracture microseismicity is interpreted for cases in which persistent dip-slip or strike-slip mechanisms exhibit shear planes prevalently aligned close to the principal stress direction. Explaining the preferential alignment in terms of stress changes driven by elevated pore pressure on a Mohr diagram is difficult. Instead, it is proposed that the microseismic shearing is driven primarily by the strain of hydraulic-fracture opening. The interpretation is based on the analog of structures and deformation commonly associated with natural joint growth in layered rock. Specifically, by considering the alternate, horizontal nodal plane as the slip plane for the aligned dip-slip events, one can associate the shearing with a vertical hydraulic fracture stepping over or jogging along bedding planes and generating bedding-plane slip. For the aligned strike-slip events, it is proposed that the critical shearing is generated by breakup of the hydraulic fracture into en echelon fringe fractures near bedding surfaces. In both cases, the shearing represents a tearing mode (mode III) in which the microseismicity highlights the deformation at and near bedding boundaries as the hydraulic fracture rips through or along layer interfaces.

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