The general lack of a correlation between earthquakes and surface faulting in the
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...
We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.
Oblique convergence in the St. Elias orogen of southern Alaska and northwestern Canada has constructed the world's highest coastal mountain range and is the principal driver constructing all of the high topography in northern North America. The orogen originated when the Yakutat terrane was excised from the Cordilleran margin and was transported along margin‐parallel strike‐slip faults into the subduction‐transform transition at the eastern end of the Aleutian trench. We examine the last 3 m.y. of this collision through an analysis of Euler poles for motion of the Yakutat microplate with respect to North America and the Pacific. This analysis indicates a Yakutat‐Pacific pole near the present southern triple junction of the microplate and predicts convergence to dextral‐oblique convergence across the offshore Transition fault, onland structures adjacent to the Yakutat foreland, or both, with plate speeds increasing from 10 to 30 mm/yr from southeast to northwest. Reconstructions based on these poles show that NNW transport of the collided block into the NE trending subduction zone forced contraction of EW line elements as the collided block was driven into the subduction‐transform transition. This suggests the collided block was constricted as it was driven into the transition. Constriction provides an explanation for observed vertical axis refolding of both earlier formed fold‐thrust systems and the collisional suture at the top of the fold‐thrust stack. We also suggest that this motion was partially accommodated by lateral extrusion of the western portion of the orogen toward the Aleutian trench. Important questions remain regarding which structures accommodated parts of this motion. The Transition fault may have accommodated much of the Yakutat‐Pacific convergence on the basis of our analysis and previous interpretations of GPS‐based geodetic data. Nonetheless, it is locally overlapped by up to 800 m of undeformed sediment, yet elsewhere shows evidence of young deformation. This contradiction could be produced if the overlapping sediments are too young to have accumulated significant deformation, or GPS motions may be deflected by transient strains or strains from poorly understood fault interactions. In either case, more data are needed to resolve the paradox.
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