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,

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

Earthquakes and friction laws

To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad-scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first-order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate-driving forces. These buoyancy forces are just one of several categories of “second-order” stresses, or local perturbations, that can be identified once the first-order stress patterns are recognized. These second-order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second-order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two-dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin-normal extension derived from sediment-loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first-order (plate boundary force-derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first-order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

/pdf/first-and-second-order-patterns-of-stress-in-the-lithosphere-595hs86ljh.pdf

First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

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

Borehole elongations or breakouts in central California show that the direction of regional maximum horizontal stress is nearly perpendicular to the San Andreas fault and to the axes of young thrust-related anticlines. This observation resolves much of the controversy over shear-stress magnitude in the crust and around the San Andreas fault specifically. A low shear stress of 10–20 MPa (100–200 bar) or less on the San Andreas fault, suggested by heat-flow and seismic observations, is compatible with a high regional deviatoric stress (100 MPa, 1 kbar) when the observed principal stress directions are considered. Therefore, the San Andreas fault is a nearly frictionless interface, which causes the transpressive plate motion to be decoupled into a low-stress strike-slip component and a high-stress compressive component. These observations suggest that standard concepts of transpressive wrench tectonics—which envisage drag on a high-friction fault—are wrong. The thrust structures are largely decoupled from the strike-slip fault.

State of stress near the San Andreas fault: Implications for wrench tectonics

Present-day stress directions interpreted from well bore breakouts adjacent to two crastal-scale, active, strike-slip faults (the San Andreas fault in California and the Great Sumatran fault in Sumatra) indicate that the maximum horizontal stress direction (SH) is oriented at a high angle (70°–90°) to both faults. The regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to SH and are therefore in the thrust-fault orientation. These observations indicate a decoupling of the strike-slip and compressional components of the deformation within these broadly transpressive zones. Borehole breakouts in 118 wells in western California indicate a regionally consistent stress pattern with SH generally oriented NE-SW, nearly perpendicular (80°–90°) to the strike of the San Andreas fault. The orientation of SH nearly perpendicular to the San Andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the Coast and Transverse Ranges indicating active compressional deformation, fault plane solutions for recent dip-slip-style earthquakes, principal stress directions determined from inversion of earthquake focal mechanisms, and induced hydraulic fracture orientations. A stress trajectory map for western California is computed using an iterative statistical algorithm in which observed directional data, such as breakout directions, are used to obtain a model regional stress field. Analysis of well bore breakouts in 25 wells within the central and southern oil districts of Sumatra indicates that the regionally defined SH adjacent to the active Great Sumatran strike-slip fault is oriented at a high angle (70°–80°) to the fault This orientation of SH is consistent regionally with geologic stress indicators and focal mechanisms of dip-slip earthquakes. Preliminary analysis of the stress field in the vicinity of the Philippine and Alpine faults suggests SH is also oriented at a high angle to these active strike-slip faults. Similarly, the minimum horizontal stress Sh is oriented at a high angle to the the Kane and Dead Sea transforms. The observation of SH and Sh in the vicinity of active strike-slip faults being oriented nearly perpendicular and parallel to the faults suggests that large, crustal-scale strike-slip faults may, in general, be inherently weak surfaces.

Present-day stress orientations adjacent to active strike-slip faults: California and Sumatra

\n Three quantitative regional transects across the Saih Hatat and Jebel Akhdar anticlines in the Central and Southern Oman Mountains and the Northern Ghaba Basin have been constructed based on surface, well and seismic data. Interpreted large-scale structural geometries suggest that the Saih Hatat and Jebel Akhdar anticlines are basement-involved compressional structures, underlain by north-dipping, high-angle, blind, reverse faults located beneath their southern limbs. A compressional deformation event initiated in the Oligocene (constrained by apatite fission track data) involving the high-angle reverse faults is interpreted in which pre-Permian strata and Permian-through-Lower Cretaceous strata, exposed in the Saih Hatat and Jebel Akhdar anticlines, were parautochthonous - uplifted over the underlying reverse faults, and not displaced a great distance laterally. The allochthonous Hawasina and Sumeini sedimentary rocks and the Semail Ophiolite complex are interpreted to have been emplaced onto the carbonate platform during the Late Cretaceous, and have subsequently been parautochthonous during the Tertiary deformation.\n The upper portion of the pre-Permian section in the Ghaba Basin consists predominantly of a thick (>4 kilometers) sequence of Cambrian-through-Silurian, predominantly non-marine to shallow-marine, clastics of the Haima Supergroup. In contrast, out of the Ghaba Basin proper in the Central Platform or Musallim High region, the Haima Supergroup is generally less than 2 kilometers thick, and interpreted to thin to the north. The fundamental difference in pre-Permian strata exposed in the Saih Hatat and Jebel Akhdar Anticline windows is the thick (>3.4 kilometers) section of Ordovician age, shallow-marine strata (Amdeh Formation) present in the Saih Hatat Anticline, but absent in the Jebel Akhdar Anticline. In our interpretation, the shallow-marine clastics exposed in the Saih Hatat Anticline represent the northern extension of the Early Paleozoic Ghaba Basin, which have been uplifted over a high-angle reverse fault in the Early Tertiary deformation event. The cross-section through Jebel Akhdar is located to the northwest of the Ghaba Salt Basin, along the Musallim High. In this area the thickness of the Ordovician strata deposited is interpreted to be less than in the Ghaba Basin. The Ordovician section is not present in the Jebel Akhdar structure - the thinned section likely eroded in a Late Paleozoic deformation event.

Regional Structural Style of the Central and Southern Oman Mountains: Jebel Akhdar, Saih Hatat, and the Northern Ghaba Basin

A strategy to balance cross sections of complex structures is documented and illustrated by the interpretation of a compressional structure in the deep-water Gulf of Mexico. The strategy is applicable to structures formed in sedimentary rocks under low temperatures in both compressional and extensional environments, and involves the comparison of the observed structure with simple, balanced, forward models. Forward models generated using fault-related fold theory help in understanding the processes and kinematics involved in the deformation. Further, forward models are completely constrained and easy to balance, whereas it is difficult to balance data. Therefore, forward models are useful in evaluating ideas without completely solving the structure. Models are constructed assuming parallel behavior (preservation of layer thickness, no net distortion where layers are horizontal, and conservation of bed length). Unmetamorphosed, sedimentary rocks are generally observed to deform obeying the assumptions of parallel behavior in field, map, well, and seismic data.

Van S. Mount

Papers

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

State of stress near the San Andreas fault: Implications for wrench tectonics

Present-day stress orientations adjacent to active strike-slip faults: California and Sumatra

Regional Structural Style of the Central and Southern Oman Mountains: Jebel Akhdar, Saih Hatat, and the Northern Ghaba Basin

A Forward Modeling Strategy for Balancing Cross Sections (1)