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

An assessment of high voltage electron microscopy (HVEM). An invited review

Rheological properties of the upper mantle of the Earth play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues as the dominant mechanisms of flow have not been well resolved. A synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the Earth's upper mantle. The hot and shallow upper mantle flows by dislocation creep, whereas cold and shallow or deep upper mantle may flow by diffusion creep. When the stress increases, grain size is reduced and the upper mantle near the transition between these two regimes is weakened. Consequently, deformation is localized and the upper mantle is decoupled mechanically near these depths.

https://science.sciencemag.org/content/sci/260/5109/771.full.pdf

Rheology of the Upper Mantle: A Synthesis

The concept of strength envelopes, developed in the 1970s, allowed quantitative predictions of the strength of the lithosphere based on experimentally determined constitutive equations. Initial strength envelopes used an empirical relation for frictional sliding to describe deformation along brittle faults in the upper portion of the lithosphere and power law creep equations to estimate the plastic flow strength of rocks in the deeper part of the lithosphere. In the intervening decades, substantial progress has been made both in understanding the physical mechanisms involved in lithospheric deformation and in refining constitutive equations that describe these processes. The importance of a regime of semibrittle behavior is now recognized. Based on data from rocks without added pore fluids, the transition from brittle deformation to semibrittle flow can be estimated as the point at which the brittle fracture strength equals the peak stress to cause sliding. The transition from semibrittle deformation to plastic flow can be approximated as the stress at which the pressure exceeds the plastic flow strength. Current estimates of these stresses are on the order of a few hundred megapascals for relatively dry rocks. Knowledge of the stability of sliding along faults and of the onset of localization during brittle fracture has improved considerably. If the depth to the bottom of the seismogenic zone is determined by the transition to the stable frictional sliding regime, then that depth will be considerably more shallow than the depth of the transition to the plastic flow regime. Major questions concerning the strength of rocks remain. In particular, the effect of water on strength is critical to accurate predictions. Constitutive equations which include the effects of water fugacity and pore fluid pressure as well as temperature and strain rate are needed for both the brittle sliding and semibrittle flow regimes. Although the constitutive equations for dislocation creep and diffusional creep in single-phase aggregates are more robust, few data exist for plastic deformation in two-phase aggregates. Despite the fact that localization is ubiquitous in rocks deforming both in brittle and plastic regimes, only a limited amount of accurate experimental data are available to constrain predictions of this behavior. Accordingly, flow strengths now predicted from laboratory data probably overestimate the actual rock strength, perhaps by a significant amount. Still, the predictions are robust enough that uncertainties in geometry, mineralogy, loading conditions and thermodynamic state are probably the limiting factors in our understanding. Thus, experimentally determined rheologies can be applied to understand a broad range of topical problems in regional and global tectonics both on the Earth and on other planetary bodies.

Strength of the lithosphere: Constraints imposed by laboratory experiments

https://www.whoi.edu/fileserver.do?id=180587&pt=2&p=59967

Water in the oceanic upper mantle: implications for rheology, melt extraction and the evolution of the lithosphere

THE mechanism of deep-focus earthquakes has been a puzzle since their discovery almost 70 years ago1, 2, because brittle fracture and frictional sliding at depths in excess of 100–200 km would require unrealistic rock strengths3, 4. Rock strength does increase with pressure, but a few hundred MPa (equivalent to 10–20 km depth) suffices to inhibit most fracture, and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength. A range of mechanisms has been proposed for deep earthquakes, including plastic instabilities5–7, shear-induced melting8–11 and instabilities accompanying recrystallization12, 13 or polymorphic phase transformation14–23. Each of these proposed mechanisms has exhibited certain inherent weaknesses (see Kirby22 for review and discussion). Here we report experimental observations of high-pressure faulting of metastable Mg2GeO4 olivine as it undergoes incipient transformation to a spinel 24, 25 structure. We present a model in which this faulting arises from the generation, propagation and linking-up of spinel-filled anticracks26. When applied to the olivine → spinel transformation in the Earth's mantle, the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding.

A new self-organizing mechanism for deep-focus earthquakes

The Mechanics of Deep Earthquakes

Earthquakes are observed to occur in subduction zones to depths of approximately 680 km, even though unassisted brittle failure is inhibited at depths greater than about 50 km, owing to the high pressures and temperatures. It is thought that such earthquakes (particularly those at intermediate depths of 50-300 km) may instead be triggered by embrittlement accompanying dehydration of hydrous minerals, principally serpentine. A problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2-4 GPa (60-120 km depth), above which brittle fracture mechanics predicts that the instability should be quenched. Here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine-grained solid reaction products formed during dehydration. This phenomenon was observed under all conditions tested (pressures of 1-6 GPa; temperatures of 650-820 degrees C), independent of the sign of the volume change of reaction. Although this result contradicts expectations from fracture mechanics, it can be explained by separation of fluid from solid residue before and during faulting, a hypothesis supported by our observations. These observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth, as long as there are hydrous minerals breaking down under a differential stress.

Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change.

Diamonds and other ultrahigh pressure (UHP) minerals have been reported previously from the Luobusa ophiolite of Tibet, but these minerals have thus far been found only as individual grains. Here we report the occurrence of diamond as an inclusion in OsIr alloy and coesite as part of a silicate assemblage rimming a grain of FeTi alloy, both of which were recovered from chromitite. These occurrences confirm the presence of UHP minerals in the Luobusa chromitite requiring minimum pressures of ∼2.8–4 GPa. Individual coesite “crystals” have external form similar to that of stishovite and are polycrystalline, suggesting pseudo morphic replacement and implying a pressure >9 GPa. We propose that the UHP minerals were incorporated into the chromitites in the deep upper mantle or that they have an impact origin; the preponderance of evidence favors the former.

Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet

The abundance of FeTiO 3 and chromite precipitates in olivine of the Alpe Arami peridotite massif, Switzerland, requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers. Three previously unknown crystal structures of FeTiO 3 were identified that indicate that the originally exsolved phase was the high-pressure perovskite polymorph of ilmenite, implying a minimum depth of origin of 300 kilometers. These observations, coupled with the unique lattice preferred orientation of olivine, further suggest that the original composition of olivine may reflect that of a precursor phase, perhaps wadsleyite, that is stable only at depths greater than 400 kilometers.

https://science.sciencemag.org/content/sci/271/5257/1841.full.pdf

Harry W. Green

Papers

A new self-organizing mechanism for deep-focus earthquakes

The Mechanics of Deep Earthquakes

Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change.

Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet

Alpe Arami: A Peridotite Massif from Depths of More Than 300 Kilometers