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\section{1. Introduction} \section{Introduction}  aaaaa There is abundant evidence for a close physical and chemical coupling between pore fluids and fracture in the upper crust ({Frank:1965wi, Hubbert:1959he, Nur:1973ju, Sibson:1981kt}). Increases in pore fluid pressure, for example due to fluid injection, have been observed to lead to rupture and an increase in seismic activity. This is readily explained by the Navier-Coulomb criteria for brittle failure and the decrease in effective stress as a result of the elevated pore fluid pressure ({Price:1966uh, Sibson:1981kt}). Conversely, the changes in groundwater levels and the surface effusions of warm water that sometimes occur along fault traces following earthquakes show that failure can also have a profound effect on fluid flow, heat and mass transport ({Nur:1974ht} Sibson et al., 1975, Sibson, 1981,). The flow is interpreted to result from either seismic pumping as a result of dilatancy diffusion type processes or a seismic valve mechanism in which fault rupture leads to leakage of an overpressured aquifer or reservoir of fluid (Nur, 1972, Nur et al., 1973, Sibson, 1981).