  While the hydraulic fracturing technology, aka fracking (or fraccing, frac),
has become highly developed and astonishingly successful, a consistent
formulation of the associated fracture mechanics that would not conflict with
some observations is still unavailable. It is attempted here. Classical
fracture mechanics, as well as the current commercial softwares, predict
vertical cracks to propagate without branching from the perforations of the
horizontal well casing, which are typically spaced at 10 m or more. However, to
explain the gas production rate at the wellhead, the crack spacing would have
to be only about 0.1 m, which would increase the overall gas permeability of
shale mass about 10,000$\times$. This permeability increase has generally been
attributed to a preexisting system of orthogonal natural cracks, whose spacing
is about 0.1 m. But their average age is about 100 million years, and a recent
analysis indicated that these cracks must have been completely closed by
secondary creep of shale in less than a million years. Here it is considered
that the tectonic events that produced the natural cracks in shale must have
also created weak layers with nano- or micro-cracking damage. It is numerically
demonstrated that a greatly enhanced permeability along the weak layers, with a
greatly increased transverse Biot coefficient, must cause the fracking to
engender lateral branching and the opening of hydraulic cracks along the weak
layers, even if these cracks are initially almost closed. A finite element
crack band model, based on recently developed anisotropic spherocylindrical
microplane constitutive law, demonstrates these findings.
