The anisotropy of permeability under simulated crustal stress conditions: Experimental evidence of flow enhancement parallel to the intermediate principal stress

Fluid circulation in Earth’s crust influences fault slip, mineralization, and volcanic activity. The geometry and efficiency of fluid transport depend on the orientation and magnitude of crustal stresses, but laboratory insights into this relationship remain limited. Most experiments use simplified stress states and one-directional flow measurements, missing the full three-dimensional behavior of permeability. We present results from a novel apparatus that quantifies the directional permeability of cubic rock samples when subjected to three independent principal stresses. We identify two regimes for stress-controlled anisotropic fluid flow in initially isotropic basalt. First, permeability decreases due to anisotropic crack closure, with higher flow reduction in the direction of maximum principal stress. Then, once new crack damage develops, permeability increases sharply along the intermediate principal stress direction. These results provide laboratory evidence that stress anisotropy alone can reorganize fluid pathways, with implications for crustal fluid flow during seismic rupture, volcanic unrest, and ore formation.