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.

Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

SUMMARY
We describe best-fitting angular velocities and MORVEL, a new closure-enforced set of angular velocities for the geologically current motions of 25 tectonic plates that collectively occupy 97 per cent of Earth's surface. Seafloor spreading rates and fault azimuths are used to determine the motions of 19 plates bordered by mid-ocean ridges, including all the major plates. Six smaller plates with little or no connection to the mid-ocean ridges are linked to MORVEL with GPS station velocities and azimuthal data. By design, almost no kinematic information is exchanged between the geologically determined and geodetically constrained subsets of the global circuit—MORVEL thus averages motion over geological intervals for all the major plates. Plate geometry changes relative to NUVEL-1A include the incorporation of Nubia, Lwandle and Somalia plates for the former Africa plate, Capricorn, Australia and Macquarie plates for the former Australia plate, and Sur and South America plates for the former South America plate. MORVEL also includes Amur, Philippine Sea, Sundaland and Yangtze plates, making it more useful than NUVEL-1A for studies of deformation in Asia and the western Pacific. Seafloor spreading rates are estimated over the past 0.78 Myr for intermediate and fast spreading centres and since 3.16 Ma for slow and ultraslow spreading centres. Rates are adjusted downward by 0.6–2.6 mm yr−1 to compensate for the several kilometre width of magnetic reversal zones. Nearly all the NUVEL-1A angular velocities differ significantly from the MORVEL angular velocities. The many new data, revised plate geometries, and correction for outward displacement thus significantly modify our knowledge of geologically current plate motions. MORVEL indicates significantly slower 0.78-Myr-average motion across the Nazca–Antarctic and Nazca–Pacific boundaries than does NUVEL-1A, consistent with a progressive slowdown in the eastward component of Nazca plate motion since 3.16 Ma. It also indicates that motions across the Caribbean–North America and Caribbean–South America plate boundaries are twice as fast as given by NUVEL-1A. Summed, least-squares differences between angular velocities estimated from GPS and those for MORVEL, NUVEL-1 and NUVEL-1A are, respectively, 260 per cent larger for NUVEL-1 and 50 per cent larger for NUVEL-1A than for MORVEL, suggesting that MORVEL more accurately describes historically current plate motions. Significant differences between geological and GPS estimates of Nazca plate motion and Arabia–Eurasia and India–Eurasia motion are reduced but not eliminated when using MORVEL instead of NUVEL-1A, possibly indicating that changes have occurred in those plate motions since 3.16 Ma. The MORVEL and GPS estimates of Pacific–North America plate motion in western North America differ by only 2.6 ± 1.7 mm yr−1, ≈25 per cent smaller than for NUVEL-1A. The remaining difference for this plate pair, assuming there are no unrecognized systematic errors and no measurable change in Pacific–North America motion over the past 1–3 Myr, indicates deformation of one or more plates in the global circuit. Tests for closure of six three-plate circuits indicate that two, Pacific–Cocos–Nazca and Sur–Nubia–Antarctic, fail closure, with respective linear velocities of non-closure of 14 ± 5 and 3 ± 1 mm yr−1 (95 per cent confidence limits) at their triple junctions. We conclude that the rigid plate approximation continues to be tremendously useful, but—absent any unrecognized systematic errors—the plates deform measurably, possibly by thermal contraction and wide plate boundaries with deformation rates near or beneath the level of noise in plate kinematic data.

Geologically current plate motions

We suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound. This mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes, and magmatic thickening of the crust. Our hypothesis is based on two critical concepts. First, the lithosphere retains finite mechanical strength or flexural rigidity during extension. Second, isostatic rebound (uplift) of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation. We develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks. We first investigate the isostatic consequences of finite simple slip on an initially planar, dipping normal fault cutting the entire lithosphere. When the lithosphere retains flexural rigidity during extension, the topography resulting from this model resembles a half graben, and the footwall rift flank is flexurally uplifted. This simple normal faulting model explains free-air gravity anomalies and topography observed at rift flanks in oceanic lithosphere (such as Broken Ridge in the eastern Indian Ocean, the Caroline ridges-Sorol Trough in the western equatorial Pacific, and the Coriolis Trough behind the New Hebrides island arc). We then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates. When the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical, the surface of slip can be regarded as a detachment. By simplification, our general model accounts for pure shear extension of the lithosphere that is uniform with depth. In this case, detachments have no meaning in the geologic sense. However, the kinematics of depth-independent pure shear may nevertheless be described in terms of a surface, which we term a kinematic reference surface, at some depth in the lithosphere. We speculate that the depth of this surface may be rheologically controlled. The magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface. In contrast, if local isostasy is assumed when the lithosphere undergoes a given amount of depth-independent pure shear, the resulting topography will be the same regardless of how the kinematics of that extension are formulated. The basin and rift flank topography and free-air gravity anomaly over young continental rifts, such as the Rhine graben, can be satisfied using our general extensional model with a small amount (<5 km) of extension along a listric-shaped detachment soling into the crust-mantle boundary. Because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains, we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension.

Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension

Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous

Intraplate seismicity, deformed oceanic crust and sediments, and local areas of abnormally high heat flow characterize significant internal deformation of the Indo–Australian plate in the northeastern Indian Ocean. The deformation appears analogous to the buckling of an elastic plate with brittle failure at its surface. Itis not clear whether the Indo–Australian plate can be resolvedinto two component parts or whether a new convergent plate boundary is likely to form in the deformed region. The onset of intraplate deformation in the late Miocene seems to be connected to the Himalayan orogenic stage of the collision between India and Asia.

Deformation of the Indo–Australian plate

Evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from Seasat altimeter data. Subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast-moving Pacific and Indo-Australian plates. In particular, for the east central Pacific Ocean, the Seasat-derived data sets reveal lineations with (1) wavelengths in the range 150–500 km (perhaps increasing with plate age), and (2) peak-to-trough amplitudes of 5–20 mGal for gravity anomalies. The lineated pattern over the Pacific plate, which first becomes discernable over seafloor 5- to 10-m.y.-old west of the East Pacific Rise, is clearly oblique to the trends of the prominent Pacific-Farallon fracture zones. Onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 Pa s (1019 P), which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies. Recent numerical studies of convection in fluids with a temperature- and pressuredependent viscosity support an early onset time for convective instability and low-viscosity zones in the upper mantle beneath young lithosphere. Such low-viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves.

Jeffrey K. Weissel

Papers

Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension

Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous

Deformation of the Indo–Australian plate

Evidence for Small-Scale Mantle Convection From Seasat Altimeter Data

Evolution of the Tasman Sea reappraised