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

A global plate motion model, named NUVEL-1, which describes current plate motions between 12 rigid plates is described, with special attention given to the method, data, and assumptions used Tectonic implications of the patterns that emerged from the results are discussed It is shown that wide plate boundary zones can form not only within the continental lithosphere but also within the oceanic lithosphere; eg, between the Indian and Australian plates and between the North American and South American plates Results of the model also suggest small but significant diffuse deformation of the oceanic lithosphere, which may be confined to small awkwardly shaped salients of major plates

Current plate motions

SUMMARY 
We determine best-fitting Euler vectors, closure-fitting Euler vectors, and a new global model (NUVEL-1) describing the geologically current motion between 12 assumed-rigid plates by inverting plate motion data we have compiled, critically analysed, and tested for self-consistency. We treat Arabia, India and Australia, and North America and South America as distinct plates, but combine Nubia and Somalia into a single African plate because motion between them could not be reliably resolved. The 1122 data from 22 plate boundaries inverted to obtain NUVEL-1 consist of 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. We determined all rates over a uniform time interval of 3.0m.y., corresponding to the centre of the anomaly 2A sequence, by comparing synthetic magnetic anomalies with observed profiles. The model fits the data well. Unlike prior global plate motion models, which systematically misfit some spreading rates in the Indian Ocean by 8–12mm yr−1, the systematic misfits by NUVEL-1 nowhere exceed ∼3 mm yr−1. The model differs significantly from prior global plate motion models. For the 30 pairs of plates sharing a common boundary, 29 of 30 P071, and 25 of 30 RM2 Euler vectors lie outside the 99 per cent confidence limits of NUVEL-1. Differences are large in the Indian Ocean where NUVEL-1 plate motion data and plate geometry differ from those used in prior studies and in the Pacific Ocean where NUVEL-1 rates are systematically 5–20 mm yr−1 slower than those of prior models. The strikes of transform faults mapped with GLORIA and Seabeam along the Mid-Atlantic Ridge greatly improve the accuracy of estimates of the direction of plate motion. These data give Euler vectors differing significantly from those of prior studies, show that motion about the Azores triple junction is consistent with plate circuit closure, and better resolve motion between North America and South America. Motion of the Caribbean plate relative to North or South America is about 7 mm yr−1 slower than in prior global models. Trench slip vectors tend to be systematically misfit wherever convergence is oblique, and best-fitting poles determined only from trench slip vectors differ significantly from their corresponding closure-fitting Euler vectors. The direction of slip in trench earthquakes tends to be between the direction of plate motion and the normal to the trench strike. Part of this bias may be due to the neglect of lateral heterogeneities of seismic velocities caused by cold subducting slabs, but the larger part is likely caused by independent motion of fore-arc crust and lithosphere relative to the overriding plate.

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

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

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at approximately 2-kilometer depth on planes parallel to the San Andreas fault.

https://science.sciencemag.org/content/sci/238/4830/1105.full.pdf

New evidence on the state of stress of the San Andreas fault system

This paper presents an investigation of the factional properties and stability of frictional sliding for simulated fault gouge. In these experiments we sheared gouge layers (quartz sand) under saturated drained conditions and at constant normal stress (50–190 MPa) between either rough steel surfaces or Westerly granite surfaces in a triaxial apparatus. Surface roughness (60 to 320 grit) and gouge layer thickness (0–4.0 mm) were varied in the experiments with granite samples. Porosity ϕ was monitored continuously during shear. Our measurements indicate that granular gouge exhibits strain hardening and net compaction for shear strains γ less than 0.5–1.0. For γ > 0.5–1.0, sliding occurs at approximately constant shear stress and net compaction from one load/unload cycle to the next ceases. Dilatancy occurs at 1/3 to 1/2 the shear stress required for sliding and d2ϕ/dγ2 becomes negative at about the peak stress in a given loading cycle, indicating the onset of shear localization. Oblique shear bands appear in the layers at γ = 1.3–1.5. Experiments with an initial gouge layer exhibit velocity strengthening (the coefficient of friction increases with slip velocity), and initially bare granite surfaces exhibit velocity weakening. The magnitude of velocity strengthening varies inversely with normal stress and directly with gouge thickness and surface roughness. In the gouge experiments the dilatancy rate dϕ/dγ also varies with slip rate. Using a simple energy balance to relate volume change and frictional resistance, we find quantitative agreement between the measured change in dilatancy rate and friction following changes in slip rate. This indicates that velocity strengthening within granular gouge is the result of dilatancy. The slip rate dependence of dϕ/dγ increases with gouge thickness and surface roughness, in agreement with the friction data. Our data therefore suggest that slip within unconsolidated granular material, such as some natural fault gouges, is inherently stable. The results thus provide an explanation for (1) the tendency of gouge accumulation to stabilize slip in laboratory samples, and (2) the tendency for aseismic slip within shallow (< 3–5 km) unconsolidated fault gouge and within unconsolidated sediments such as shallow alluvium and accretionary prisms.

Frictional behavior and constitutive modeling of simulated fault gouge

The magnitude of deviatoric stresses in the earth's crust and uppermost mantle, and especially the magnitude of shear stresses that resist plate motions across the major plate boundaries, is a matter of central importance to solid earth geophysics. Since currently available estimates vary by at least a factor of 10, even when the same features or set of observations are involved, such fundamental issues as the driving mechanism(s) of plate tectonics and the energetics of crustal faulting are entirely unresolved. More generally, there is very little in the study of active geologic processes of geophysical interest that does not involve material deformation of some kind or another; in understated terms the analysis of material deformation to reveal causative processes is poorly constrained when the magnitude of the deviatoric stresses involved is so uncertain.

Apart from the obvious problem of our inability to measure directly deviatoric stresses in the earth's crust and uppermost mantle through all but a small fraction of its depth—or sample directly this region in any other way at the present time—a confounding difficulty exists in the great range of time scales involved in geophysical processes of interest, from fractions of seconds for the spontaneous failure of small crustal earthquakes to billions of years for the relaxation of certain topographic and gravimetric features of the continental crust. It is probably naive then to expect that the same magnitude of deviatoric stresses should exist for all these geologic processes, given the great range of time scales over which they operate. Moreover, the earth's crust and uppermost mantle are heterogeneous enough, especially in continental regions, that significant spatial variations probably exist as well. Certainly, an important problem is whether the actual variations in space and time of deviatoric stresses in the earth's crust and uppermost mantle can be large enough to accommodate the order of magnitude differences as presently perceived or whether our present perceptions are only imagined, in one sense or another. Even so, we can be assured that at least some of our perceptions must be imagined, when deviatoric stress estimates differing by an order of magnitude or more are offered to explain the same basic processes or the same observations of them.

The conference on magnitude of deviatoric stresses in the Earth's crust and uppermost mantle

A visit to China of an American seismological delegation, which took place October 5 to November 5, 1974, is covered in this report. The Americans were sponsored by the Committee on Scholarly Communication with the People's Republic of China (CSCPRC), and the hosts in China were the Scientific and Technical Association and the State Seismological Bureau. The CSCPRC is sponsored jointly by the National Academy of Sciences, the Social Science Research Council, and the American Council of Learned Societies.

The visit had its origins in an invitation extended in January 1973 by Carl Kisslinger, as President of the Seismological Society of America, to the Chinese Academy of Sciences to send representatives to a Symposium on Earthquake Prediction Research. Although the Chinese declined to participate, this invitation was one step towards a reciprocal exchange of seismologists between the United States and the People's Republic of China. Several months after Kisslinger's letter the CSCPRC visited Peking. Their purpose was to arrange an exchange program with the Chinese Scientific and Technical Association. Prompted by Kisslinger, the committee's proposals for American delegations included seismology. Not only was this particular exchange accepted, but the Chinese in turn suggested that a Chinese seismology group visit the United States.

Earthquake research in China

Laboratory of the California Institute of Technology, was awarded the 1991 William Bowie Medal for outstanding contributions to fundamental geophysics and unselfish cooperation in research. The Bowie Medal, AGU's most distinguished honor, was presented to AGU Past-president Anderson on May 29 at the annual Spring Meeting Honors Night in Baltimore, Md.

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C. Barry Raleigh

Papers

New evidence on the state of stress of the San Andreas fault system

Frictional behavior and constitutive modeling of simulated fault gouge

The conference on magnitude of deviatoric stresses in the Earth's crust and uppermost mantle

Earthquake research in China

Anderson receives 1991 Bowie Medal