The Effects of Locked-in Pressure on the Mechanics of Faulting

Abstract

Static friction and dynamic friction between two surfaces are generally proportional to the normal force between the surfaces. Pressure in the Earth is approximately equal to the lithostatic overburden. It follows that the failure strength of a fault and its resistance to sliding should increase with depth below the surface. Or should it? Stress describes internal forces that act to deform a material. It can be separated into shear stress, which acts to change the shape; and pressure, which acts to change the volume. It can also be separated into environmental stress and locked-in stress. The environmental stress results from fixed displacements, surface tractions, point loads, body forces, and thermal expansion or contraction. The locked-in stress remains after all environmental factors have been removed; it is locked into the shape of the material. A simple example would be the stress that results from cementing an ellipsoid volume of material into a spherical cavity. Another example is the pressure that is locked into a solid material that comes into existence in a high-pressure environment. We need to consider the nature and effects of this locked-in pressure. Sitting on my desk is a Wham-O Super Ball. It's a two-inch-diameter, solid-latex, high-bounce ball developed in 1965 by Wham-O Manufacturing (the same company that brought us the Hula Hoop and the Frisbee). The Wham-O Super Ball was made by first formulating a recipe for an extremely strong latex. Melt the latex and inject it under 3,500 pounds per square inch (psi) of gage pressure into a mold.1 Then maintain the pressure while the latex cools and hardens. Without the pressure in the manufacturing process, you only get a normal soft rubber ball with little resiliency or “bounce.” More information is available at http://www.superballs.com. The 3,500 psi of pressure was locked into the …