Fault zone architecture and related permeability structures form primary controls on fluid flow in upper-crustal, brittle fault zones. We develop qualitative and quantitative schemes for evaluating fault-related permeability structures by using results of field investigations, laboratory permeability measurements, and numerical models offlow within andnearfaultzones.Thequalitativeschemecomparesthepercentageofthetotalfaultzone width composed of fault core materials (e.g., anastomosing slip surfaces, clay-rich gouge, cataclasite,andfaultbreccias)tothepercentageofsubsidiarydamagezonestructures(e.g., kinematically related fracture sets, small faults, and veins). A more quantitative scheme is developed to define a set of indices that characterize fault zone architecture and spatial variability.Thefaultcoreanddamagezonearedistinctstructuralandhydrogeologicunits that reflect the material properties and deformation conditions within a fault zone. Whether a fault zone will act as a conduit, barrier, or combined conduit-barrier system is controlled by the relative percentage of fault core and damage zone structures and the inherent variability in grain scale and fracture permeability. This paper outlines a frameworkforunderstanding,comparing,andcorrelatingthefluidflowpropertiesoffaultzones in various geologic settings.

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Fault zone architecture and permeability structure

This paper reviews rock friction and the frictional properties of earthquake faults. The basis for rate- and state-dependent friction laws is reviewed. The friction state variable is discussed, including its interpretation as a measure of average asperity contact time and porosity within granular fault gouge. Data are summarized showing that friction evolves even during truly stationary contact, and the connection between modern friction laws and the concept of “static” friction is discussed. Measurements of frictional healing, as evidenced by increasing static friction during quasistationary contact, are reviewed, as are their implications for fault healing. Shear localization in fault gouge is discussed, and the relationship between microstructures and friction is reviewed. These data indicate differences in the behavior of bare rock surfaces as compared to shear within granular fault gouge that can be attributed to dilation within fault gouge. Physical models for the characteristic friction distance are discussed and related to the problem of scaling this parameter to seismic faults. Earthquake afterslip, its relation to laboratory friction data, and the inverse correlation between afterslip and shallow coseismic slip are discussed in the context of a model for afterslip. Recent observations of the absence of afterslip are predicted by the model.

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Laboratory-derived friction laws and their application to seismic faulting

The concept of self-organized criticality was introduced to explain the behaviour of the sandpile model. In this model, particles are randomly dropped onto a square grid of boxes. When a box accumulates four particles they are redistributed to the four adjacent boxes or lost off the edge of the grid. Redistributions can lead to further instabilities with the possibility of more particles being lost from the grid, contributing to the size of each ‘avalanche’. These model ‘avalanches’ satisfied a power-law frequency‐area distribution with a slope near unity. Other cellular-automata models, including the slider-block and forest-fire models, are also said to exhibit self-organized critical behaviour. It has been argued that earthquakes, landslides, forest fires, and species extinctions are examples of self-organized criticality in nature. In addition, wars and stock market crashes have been associated with this behaviour. The forest-fire model is particularly interesting in terms of its relation to the critical-point behaviour of the sitepercolation model. In the basic forest-fire model, trees are randomly planted on a grid of points. Periodically in time, sparks are randomly dropped on the grid. If a spark drops on a tree, that tree and adjacent trees burn in a model fire. The fires are the ‘avalanches’ and they are found to satisfy power-law frequency‐area distributions with slopes near unity. This forest-fire model is closely related to the site-percolation model, that exhibits critical behaviour. In the forest-fire model there is an inverse cascade of trees from small clusters to large clusters, trees are lost primarily from model fires that destroy the largest clusters. This quasi steady-state cascade gives a power-law frequency‐area distribution for both clusters of trees and smaller fires. The site-percolation model is equivalent to the forest-fire model without fires. In this case there is a transient cascade of trees from small to large clusters and a power-law distribution is found only at a critical density of trees.

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Self-organized criticality

Despite concern about the effects of tropical forest disturbance and clearance on biodiversity1,2, data on impacts, particularly on invertebrates, remain scarce3,4,5,6,7,8. Here we report a taxonomically diverse inventory on the impacts of tropical forest modification at one locality. We examined a gradient from near-primary, through old-growth secondary and plantation forests to complete clearance, for eight animal groups (birds, butterflies, flying beetles, canopy beetles, canopy ants, leaf-litter ants, termites and soil nematodes) in the Mbalmayo Forest Reserve, south-central Cameroon. Although species richness generally declined with increasing disturbance, no one group serves as a good indicator taxon9,10,11,12 for changes in the species richness of other groups. Species replacement from site to site (turnover) along the gradient also differs between taxonomic groups. The proportion of ‘morphospecies’ that cannot be assigned to named species and the number of ‘scientist-hours’ required to process samples both increase dramatically for smaller-bodied taxa. Data from these eight groups indicate the huge scale of the biological effort required to provide inventories of tropical diversity, and to measure the impacts of tropical forest modification and clearance.

Biodiversity inventories, indicator taxa and effects of habitat modification in tropical forest

A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones

New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluate possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large-displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged host rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that (1) the maximum principal stress was oriented at large angles to principal fault planes, (2) strain was partitioned between simple shear in the fault cores and nearly fault-normal contraction in the damaged zones and surrounding host rock, and (3) the principal faults were weak. Microstructures and particle size distributions in the damaged zone of the SGF imply deformation was almost entirely cataclastic and can be modeled as constrained comminution. In contrast, cataclastic and fluid-assisted processes were significant in the cores of the faults as shown by pervasive syntectonic alteration of the host rock minerals to zeolites and clays and by folded, sheared, and attenuated cross-cutting veins of laumontite, albite, quartz, and calcite. Total volume of veins and neocrystallized material reaches 50% in the fault core, and vein structure implies episodic fracture and sealing with time-varying and anisotropic permeability in the fault zone. The structure of the ultracataclasite layer reflects extreme slip localization and probably repeated reworking by particulate flow at low effective stresses. The extreme slip localization reflects a mature internal fault structure resulting from a positive feedback between comminution and transformation weakening. The structural, mechanical, and hydrologic characteristics of the Punchbowl and San Gabriel faults support the model for a weak San Andreas based on inhomogeneous stress and elevated pore fluid pressures contained within the core of a seismogenic fault. Elevated fluid pressures could be repeatedly generated in the core of the fault by a combination of processes including coseismic dilatancy and creation of fracture permeability, fault-valve behavior to recharge the fault with fluid, post-seismic self-sealing of fracture networks to reduce permeability and trap fluids, and time-dependent compaction of the core to generate high pore pressure. The localized slip and fluid-saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events, which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. In addition, such a dynamic weakening mechanism is expected only in mature fault zones and thus could help explain the apparent difference in strength of large-displacement faults from smaller-displacement, subsidiary seismogenic faults.

Internal structure and weakening mechanisms of the San Andreas Fault

The methods and first results of a new approach to examining fault gouge are described. Samples of undisturbed fault gouge from the exhumed Lopez fault zone in the San Gabriel Mountains, California were impregnated with low viscosity epoxy resin and sectioned to produce microscope slides. The slides were photographed using optical and electron microscopy with magnifications ranging in factors of 2 from 12.5 to 1600. At all scales, the particles appeared angular with planar faces, suggesting tensile failure. No shear zones were discernable. The particle size distribution was studied. At each magnification the particles were sorted by diameter into four classes, differing in mean diameter by factors of 2. The numbers in each class were then scaled by the characteristic class dimension. The process revealed a remarkable degree of self-similarity. Over the range examined, the fractal dimension was within 5% of 2.60.

The kinematics of gouge deformation

The frictional properties of a simulated gouge having a fractal particle distribution

The particle-size distribution of a natural fault-gauge has been determined over the range from 5 μm to 40 cm. The gouge is self-similar over the range from 5 μm to 1 cm having a fractal dimension of 2.60 ± 0.11. The lower and upper fractal limits were also determined. The lower fractal limit occurs at a dimension of about 1–10 μm where mineral cleavage and intergranular porosity dominate the cataclasis. The upper fractal limit occurs at particle sizes on the order of 1 cm where the scaled particle density decreases abruptly by a factor of about three. By analogy to soil-mechanics shear box tests and laboratory rock friction experiments, it is argued that the upper fractal limit of the gouge determines the characteristic displacement parameter in stick-slip friction models. A characteristic displacement on the order of 1 cm is consistent with estimates based on numerical simulations of faulting.

Fractals, fault-gouge, and friction

The fault zone of a mature large-displacement fault may be idealized as a nested hierarchical structure consisting of a core of extremely fine grained material surrounded by coarser granulated gouge and breccia which is in turn bordered by fracture-damaged wall rock in which the fracture density decreases with distance to a regional background level While there are significant variations in the symmetry of this structure, virtually all fault zones have a core of deformed granular rock within which most of the displacement appears to have occurred, often on an extremely narrow prominent slip surface In many faults, the gouge and breccia layer is missing from one or both sides of the fault zone For strike-slip faults, this appears to be associated with variations in the lithology of the wall rock For dip-slip faults it is most likely a consequence of the exhumation of only one wall by the fault motion In normal faults, the layered structure appears on the footwall while the hanging wall shows almost no damage For reverse faults it is the hanging wall which contains the layered structure Mechanisms proposed to explain the formation of these fault zone structures are reviewed with an eye toward whether they shed any light on the earthquake process Conversely, mechanical models for earthquake nucleation, propagation, and arrest are reviewed to see how fault zone structure might affect these processes A key parameter in these models is the characteristic displacement D c required to reduce friction from its static to its dynamic value In nucleation, D c determines the size of the smallest earthquake The observation of magnitude zero earthquakes, implies D c is on the order of microns, comparable to values measured in the laboratory For propagation, D c is closely related to the fracture energy, which is the integrated value of stress times displacement from zero to D c  Dynamic models of large earthquakes find fracture energies on the order of 1 MJ/m 2 and D c on the order of centimeters The implication is either that only large earthquakes nucleate on large faults, or that the D c which controls nucleation is different from that which controls propagation because they reflect different processes Two such processes that might produce a larger D c for propagation are the frictional heating of pore fluid and off-fault damage Finally, models for earthquake arrest based on barriers requiring large values of fracture energy also yield fracture energies on the order of MJ/m 2 

Ronald L. Biegel

Papers

Internal structure and weakening mechanisms of the San Andreas Fault

The kinematics of gouge deformation

The frictional properties of a simulated gouge having a fractal particle distribution

Fractals, fault-gouge, and friction

Relating Fault Mechanics to Fault Zone Structure