Hydraulic-fracture microseismicity sometimes exhibit fairly uniform radiation patterns as revealed by systematic patterns of P- and S-wave first motions and amplitude ratios. These patterns suggest a predictable failure condition and allow composite source mechanism information to be obtained from sparse receiver coverage. Commonly, one of the nodal planes is aligned close to the hydraulic fracture. We present two case studies and interpret the composite radiation patterns in terms of expected stress conditions near the hydraulic fracture. In the first case, strike-slip failure is prevalently aligned with SHmax direction in a series of hydraulic fracture injections in the Cotton Valley tight sands. We interpret the alignment of nodal planes with principal stress direction as a preference for extension-shear fracturing. A preference for extension-shear fracturing requires conditions of low differential stress and high pore pressure, a condition produced immediately adjacent to a hydraulic fracture. In the second case dip-slip nodal planes are aligned with SHmax and hydraulic fracture direction in the Barnett shale. The alternate failure plane for a near-vertical dip-slip mechanism is a horizontal fracture plane. We interpret the uniform Barnett radiation patterns as an indication that shearing prevalently occurs on bedding planes, driven by and accommodating hydraulic fracture opening.

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