Introduction to Special Section: Mechanical Involvement of Fluids in Faulting

Abstract

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. These include the long term structural and compositional evolution of fault zones; fault creep; and the nucleation, propagation, arrest, and recurrence of earthquake ruptures. Besides the widely recognized physical role of fluid pressures in controlling the strength of crustal fault zones, it is also apparent that fluids can exert mechanical influence through a variety of chemical effects. The United States Geological Survey sponsored a Conference on the Mechanical Effects of Fluids in Faulting under the auspices of the National Earthquake Hazards Reduction Program at Fish Camp, California, from June 6 to 10, 1993. The purpose of the conference was to draw together and to evaluate the disparate evidence for the involvement of fluids in faulting; to establish communication on the importance of fluids in the mechanics of faulting between the different disciplines concerned with fault zone processes; and to help define future critical investigations, experiments, and observational procedures for evaluating the role of fluids in faulting. This conference drew together a diverse group of 45 scientists, with expertise in electrical and magnetic methods, geochemistry, hydrology, ore deposits, rock mechanics, seismology, and structural geology. Some of the outstanding questions addressed at this workshop included the following: 1. What are fluid pressures at different levels within seismically active fault zones? Do they remain hydrostatic throughout the full depth extent of the seismogenic regime, or are they generally superhydrostatic at depths in excess of a few kilometers? 2. Are fluid pressures at depth within fault zones constant through an earthquake cycle, or are they time‐dependent? What is the spatial variability in fluid pressures? 3. What is the role of crustal fluids in the overall process of stress accumulation, release, and transfer during the earthquake cycle? Through what mechanisms might fluid pressure act to control the processes of rupture nucleation, propagation, and arrest? 4. What is the chemical role of fluids in facilitating fault creep, including their role in aiding solid‐state creep and brittle fracture processes and in facilitating solution‐transport deformation mechanisms? 5. What are the chemical effects of aqueous fluids on constitutive response, fractional stability, and long‐term fault strength? 6. What are the compositions and physical properties of faultfluids at different crustal levels? 7. What are the mechanisms by which porosity and permeability are either created or destroyed in the middle to lower crust? What factors control the rates of these processes? How should these effects be incorporated into models of time‐dependent fluid transport in fault zones? 8. What roles do faults play in distributing fluids in the crust and in altering pressure domains? In other words, when and by what mechanisms do faults aid in or inhibit fluid migration? What are the typical fluid/rock rati...

Isotopic evidence for the infiltration of mantle and metamorphic CO2–H2O fluids from below in faulted rocks from the San Andreas Fault system

“…A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes (see Hickman et al, 1995). This is particularly true for the plate bounding strike-slip San Andreas Fault (SAF) system, California, where it has been hypothesized that geo-pressured fluids impact the energetics of the system and weaken the fault with respect to shear failure.…”

A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

“…Although it has long been recognised that seismically active faults must be pervaded by high pore-fluid pressures in order to permit slip without frictional heating [62][63][64][65], in the past there has been no way to demonstrate their presence. One of the major objectives of the current SAFOD (San Andreas Fault Observatory at Depth) plans to drill into the San Andreas Fault is to directly measure pore-fluid pressures on an active fault [66].…”

Fracture permeability and in situ stress to 7 km depth in the KTB scientific drillhole

“…It is generally accepted that fractures provide important pathways for fluid flow in the earth's crust [see review by Hickman et al, 1995]. Fluid-filled fractures can exist even at mid-crustal depth as has been shown by the detection of large flows of hot brine at depths to greater than 9 km in the deep wellbore of the Kola Peninsula [Kozlovsky, 1984].…”