Percolation in the Crust

Considering the Earth's Crust as a heterogeneous medium, we show here that the general features of the seismic, electrical, and mechanical properties of the Crust can be conveniently described with a unified model derived from percolation theory. The basic assumption is that fracture density is a variable parameter which decreases with depth and as a result there exists a critical threshold below which fractures are only connected within finite clusters. This threshold corresponds to a critical fracture density and to a critical depth. Fluids circulate through a connected network of fractures down to the critical depth Z(c). Below Z(c), anisotropic fracture clusters result in seismic reflectors and high electrical conductivity. Earthquakes are restricted to depths shallower than Z(c), where friction controls the mechanical resistance to fluid infiltrated pre-existing fractures.