Fractures in rock cause seismic anisotropy because they are oriented mechanical discontinuities that may occur in sets with preferred orientations. However, anisotropy from orientation can be altered or masked by gradients in stress and fluids. Such gradients in stress occur naturally in the Earth or are induced through anthropogenic activities. For instance, stress increases with depth in the Earth because of lithostatic forces, and in a given region, local gradients in horizontal stresses can arise through the tectonic history of the region (e.g., folding, faulting, etc.). In addition, gradients in fluid distributions as well as pore pressures occur when fluids are either injected or withdrawn from a rock formation.

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