This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

The Mechanics of Earthquakes and Faulting

Physical factors likely to affect the genesis of the various fault rocks—frictional properties, temperature, effective stress normal to the fault and differential stress—are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge—breccia—cataclasite series—pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10–15 km.

Fault rocks and fault mechanisms

The traditional view of tectonics is that the lithosphere comprises a strong brittle layer overlying a weak ductile layer, which gives rise to two forms of deformation: brittle fracture, accompanied by earth- quakes, in the upper layer, and aseismic ductile flow in the layer beneath Although this view is not incorrect, it is imprecise, and in ways that can lead to serious misunderstandings The term ductility, for example, can apply equally to two common rock deformation mechanisms: crystal plasticity, which occurs in rock above a critical temperature, and cataclastic flow, a type of granular deformation which can occur in poorly consolidated sediments Although both exhibit ductility, these two deformation mechanisms have very different rheologies Earthquakes, in turn, are associated with strength and brittleness—associations that are likewise sufficiently imprecise that, if taken much beyond the generality implied in the opening sentence, they can lead to serious misinterpretations about earthquake mechanics Lately, a newer, much more precise and predictive model for the earthquake mechanism has emerged, which has its roots in the observation that tectonic earthquakes seldom if ever occur by the sudden appearance and propagation of a new shear crack (or 'fault') Instead, they occur by sudden slippage along a pre-existing fault or plate interface They are therefore a frictional, rather than fracture, phenomenon, with brittle fracture playing a secondary role in the lengthening of faults 1 and frictional wear 2  This distinction was noted by several early workers 3 , but it was not until 1966 that Brace and Byerlee 4 pointed out that earthquakes must be the result of a stick-slip frictional instability Thus, the earthquake is the 'slip', and the 'stick' is the interseismic period of elastic strain accumula- tion Subsequently, a complete constitutive law for rock friction has been developed based on laboratory studies A surprising result is that a great many other aspects of earthquake phenomena also now seem to result from the nature of the friction on faults The properties traditionally thought to control these processes— strength, brittleness and ductility—are subsumed within the over- arching concept of frictional stability regimes Constitutive law of rock friction In the standard model of stick-slip friction it is assumed that sliding begins when the ratio of shear to normal stress on the surface reaches a value ms, the static friction coefficient Once sliding initiates, frictional resistance falls to a lower dynamic friction coefficient, md, and this weakening of sliding resistance may,

/pdf/earthquakes-and-friction-laws-22mbgrykdx.pdf

Earthquakes and friction laws

Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

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.

/pdf/laboratory-derived-friction-laws-and-their-application-to-44s35v6won.pdf

Laboratory-derived friction laws and their application to seismic faulting

Cataclastic deformation features in crystalline rocks from the Cajon Pass drill hole, located some 4 km NE of the San Andreas fault trace in southern California, appear to have developed at slow strain rates. There is no clear evidence of seismic deformation. Most of the observed structures and microstructures are inferred to have formed during pre-Pliocene distributed deformation, before the San Andreas fault became active in this area. Extension fractures are filled by laumontite which increases in grain size from a fine amorphous habit to coarser prismatic crystals unidirectionally across fractures and does not generally display crack-seal textures. Fragments within fractures were derived from adjacent grains without rotation or shear. Fracture opening and cementation were therefore synchronous and slow. Fluids were present at all times during fracture opening, but there was no repeated hydrofracturing after fracture formation. However pore fluid pressures probably stayed close to the least principal stress (σ3) during subsequent fracture growth. Plagioclase compositions in all fractured areas change from oligoclase to albite or anorthite as a consequence of albitization and laumontization. Particle size distributions in dilational areas, and in extension and in shear fracture fillings, show that alteration of the major phase, plagioclase, is the fundamental process of grain size reduction in a variety of rock types. Cataclastic stresses and strain rates were controlled by alteration reaction rates in a coupled process that is a form of transformation-modified deformation. Fracturing leading to weakening was assisted by stresses due to the 60% volume increase accompanying in situ laumontization of plagioclase and by stress corrosion and subcritical crack growth facilitated by alteration. Deformation was also enhanced by replacement of plagioclase by weaker laumontite. Such dilatant cataclasis is consistent with fluid pressure levels having remained high during deformation with effective least principal compressive stress close to zero. This transformation-modified deformation at low temperatures (90° < T < 250°C) may be a common process in feldspar-rich rocks. The structures and microstructures described here could be used to distinguish the products of slow and fast Cataclastic deformation.

/pdf/aseismic-fracturing-and-cataclasis-involving-reaction-r9zx4vkq1v.pdf

Aseismic fracturing and cataclasis involving reaction softening within core material from the cajon pass drill hole

Abstract Earthquakes give rise to three principal effects – fault rupture (with subsidiary fracturing), ground shaking from wave propagation, and stress (±fluid-pressure) changes – that influence a wide range of geological processes at varying distances from the source. Many intermittent events are recognizable in the geological record (e.g. an individual turbidite bed in a flysch sequence; renewal of fracture permeability followed by hydrothermal precipitation) and are potentially attributable to seismic events, but positive identification of an earthquake as the cause is often difficult. Earthquake Geology has grown enormously as a discipline since the advent of palaeoseismic techniques for dating prehistoric fault ruptures as an aid to hazard assessment, but has largely focused on surface processes. There is a need to expand our concept of Earthquake Geology to include subsurface as well as surface effects in the full gamut of tectonic settings. The likelihood is that many Earth processes that have hitherto been considered smooth and progressive are, in fact, intermittent and tied to the seismic stress cycle that is integral to deformation in the Earth's crust and lithosphere.

The scope of earthquake geology

Deformed Neogene basins, active faulting and topography in Westland: Distributed crustal mobility west of the Alpine Fault transpressive plate boundary (South Island, New Zealand)

Abstract The base of the continental seismogenic zone is defined within individual fault zones by the transition with depth from pressure-sensitive factional (FR) faulting to temperature-sensitive quasi-plastic (QP) ductile shearing. The depth of this FR-QP transition fluctuates principally as a consequence of variations in geothermal gradient and crustal lithology but other factors (e.g. fluid pressure level, strain rate) also play a role. For quartz-dominant and feldspar-dominant lithologies, respectively, it corresponds approximately to isotherms at 300–350°C and c. 450 °C., defining an undulating transition zone in the mid-crust with a relief of the order of 5–10 km. This transition zone correlates with the greenschist-facies metamorphic environment where the bulk of mesozonal Au-quartz lodes form in mixed continuous-discontinuous shear zones. In areas of crustal convergence and thickening, where fluid release results from prograde metamorphic dehydration, especially at the greenschistamphibolite-facies transition in the middle to deep crust, the seismogenic carapace acts as an upper crustal stress guide and low-permeability lid to overpressured metamorphic fluids migrating through shear zone conduits. Under appropriate combinations of stress and fault architecture in the brittle carapace, substantial fluid volumes may be trapped beneath this elastic lid at sufficient overpressure to generate dilatant fault-fracture meshes discharging episodically by fault-valve action following fault rupture, with permeability locally enhanced by aftershock activity distributed about the rupture zones. Topographic irregularities in the seismic-aseismic transition determine rupture nucleation sites and probably play a critical role in focusing the discharge of overpressured metamorphic fluids into the seismogenic layer. This helps to account for the observed spacing of mesozonal lode systems along transcrustal shear zones.

Au-quartz mineralization near the base of the continental seismogenic zone

Evidence exists in both active and ancient fold-thrust belts for extensive hydraulic connectivity over distances of kilometres to tens of kilometres along-strike during fold growth. Components of 3D structural permeability contributing to along-strike flow include fold closures with interlayered high and low permeability beds, continually resheared bedding planes, saddle reef structures with incrementally created hinge-line gapes, and various thrust flat-ramp and fault-fracture intersections. Structurally controlled directional permeability may arise both in ‘normally’ pressured crust (i.e. where fluid pressures are approximately hydrostatic) and at depth in strongly overpressured submetamorphic environments where fluid pressures approach lithostatic. Along-strike flow parallel to fold hinges is particularly likely where non-uniform shortening occurs along a fold-thrust belt during compressional inversion or continental collision. Interpretation of fluid redistribution from cross-strike sections coupled with 2D numerical modelling may then be highly misleading.

Richard H. Sibson

Papers

Aseismic fracturing and cataclasis involving reaction softening within core material from the cajon pass drill hole

The scope of earthquake geology

Deformed Neogene basins, active faulting and topography in Westland: Distributed crustal mobility west of the Alpine Fault transpressive plate boundary (South Island, New Zealand)

Au-quartz mineralization near the base of the continental seismogenic zone

Hinge-parallel fluid flow in fold-thrust belts: how widespread?*