The Onset of Convection in a Fault Zone: Effect of Anisotropic Permeability

Abstract

The condition for the onset of convective instability in a water saturated, fault or fracture zone in which the permeability is anisotropic is derived. For the case in which the walls of the fault are assumed to be insulated, impermeable boundaries the results show that if the permeability ratio, y=Kv/Kh is less than unity, the critical Rayleigh number is substantially less than for an isotropic medium; whereas if y is greater than unity the critical Rayleigh number is greater that the isotropic medium. The flow takes the form of rolls with axes parallel to the short side of the box, the number of rolls contained in the fault zone depend upon the fault zone geometry (length/depth ratio) as well as upon the permeability ratio y. The number of rolls increases as y increases. The convection model is a applied to the linear, fault controlled array of hot springs in the Double Hot Springs area of northern Nevada. If the spring system is a manifestation of roll like convection within the fault, the results suggest that y>1.