Crustal Fluids and Earthquakes

The mechanical behavior of the upper crust over time periods of hours to thousands of years depends strongly on whether it is saturated with water or is primarily dry. There is much evidence to suggest that the crust, at least in some places, is saturated with water, possibly as deep as the brittle-ductile transition zone. If this is so, then the poroelastic behavior of the upper crust must be considered in studies of earthquake physics. The presence of fluid in an elastic, porous medium introduces a time-dependent element which cannot be dismissed as a secondary or insignificant factor. After briefly reviewing some of the arguments for the ubiquitous presence of crustal pore fluids, we will look at the physics of poroelastic materials, how it differs from elastic materials and the significance for earthquake physics. Computer simulations of poroelastic response following the large 1992 Landers, California earthquake will be presented and compared with postseismic data (aftershock occurence and deformation) to demonstrate the potential significance of pore fluids on the evolution of the physical state of the crust.