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,

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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.

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Laboratory-derived friction laws and their application to seismic faulting

‘A further result attending to the disturbances of the surface of the earth has been to produce rents or fissures in the rocks which have been subject to their violent movements, and to convert them into receptacles of metallic ores accessible by the labours of men .’ (Reverend William Buckland Geology and mineralogy with reference to natural theology . London: William Pickering (1836).) Analyses of the nucleation, propagation and stopping of moderate to large earthquake ruptures, coupled with high-precision studies of background and aftershock microearthquake activity, are yielding insights into the infrastructure of major transcurrent fault zones within the seismogenic regime. Curvature and echelon segmentation of a principal slip surface within a ca . 1 km wide main fault zone appear to exert major controls on the starting and stopping of ruptures. Geometrical irregularities of this kind, transverse to the direction of rupture propagation, may be usefully classified into dilational and antidilational jogs, referring to the area change in the plane defined by the slip vector and the pole to planar segments of the fault. Extensional fracture systems linking echelon fault segments across dilational jogs allow long-term transfer of fault slip, but appear to play an especially important role as kinetic barriers impeding or arresting rapid rupture propagation. In fluid-saturated crust the rapid transfer of slip is opposed by suctions arising from rupture-induced differential fluid pressures. Delayed slip transfer, delineated by intense aftershock activity, may then occur as fluid pressures re-equilibrate. Fossil examples of such linking extensional fracture systems are characterized by multi-episode hydraulic implosion breccias. Dilational zones of this kind may migrate along faults or remain fixed for relatively long time periods. They serve as particularly favourable sites for hydrothermal mineralization and may also localize magmatic activity along major lineaments.

Earthquakes and lineament infrastructure

Load-strengthening versus load-weakening faulting

High-angle reverse faulting in northern New Brunswick, Canada, and its implications for fluid pressure levels

Loading of faults to failure

Abstract Pseudotachylytes are ubiquitous within New Zealand's Alpine Fault Zone, occurring as: (i) thin fault veins parallel to existing hanging wall mylonitic foliation; (ii) thicker fault and injection veins around and within metabasite lenses in hanging wall fault rocks and on the footwall–hanging wall boundary; (iii) chaotic injected masses within footwall-derived, granitoid mylonites; and (iv) chaotic injected masses into cataclasites within the fault core. Overall, pseudotachylytes are not volumetrically dominant enough to have formed during all increments of earthquake slip on the Alpine Fault. We propose they were mostly generated during regular moderate magnitude events or during foreshock and aftershock sequences to larger earthquakes. The largest volume pseudotachylytes occur in footwall-derived mylonites (type (iii)). This may indicate that high-stress, anhydrous seismic slip is most common in the footwall. Most types (i), (ii) and (iii) pseudotachylytes formed at or near the base of the seismogenic zone, at temperatures up to 350 °C and at depths of 7–10 km or more. Ductilely overprinted pseudotachylytes represent the down-dip termination of large fault ruptures in a zone that would usually fail by aseismic creep. Type (iv) pseudotachylytes were formed at shallower depths (4–7 km) in a damage zone around the fault principal slip surface. Rare amygdules indicate that the fault zone locally contained free fluids. Supplementary material: Table of host, rock and pseudotachylyte major and minor oxide proportions is available at http://www.geolsoc.org.uk/SUP18490.

Richard H. Sibson

Papers

Earthquakes and lineament infrastructure

Load-strengthening versus load-weakening faulting

High-angle reverse faulting in northern New Brunswick, Canada, and its implications for fluid pressure levels

Loading of faults to failure

Diverse habitats of pseudotachylytes in the Alpine Fault Zone and relationships to current seismicity