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

Acknowledgements 1. Introduction 2. Electromagnetic wave propagation 3. The absorption of light 4. Specular reflection 5. Single particle scattering: perfect spheres 6. Single particle scattering: irregular particles 7. Propagation in a nonuniform medium: the equation of radiative transfer 8. The bidirectional reflectance of a semi-infinite medium 9. The opposition effect 10. A miscellany of bidirectional reflectances and related quantities 11. Integrated reflectances and planetary photometry 12. Photometric effects of large scale roughness 13. Polarization 14. Reflectance spectroscopy 15. Thermal emission and emittance spectroscopy 16. Simultaneous transport of energy by radiation and conduction Appendix A. A brief review of vector calculus Appendix B. Functions of a complex variable Appendix C. The wave equation in spherical coordinates Appendix D. Fraunhoffer diffraction by a circular hole Appendix E. Table of symbols Bibliography Index.

Theory of Reflectance and Emittance Spectroscopy

Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.

Gradients in silicic magma chambers: Implications for lithospheric magmatism

Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp-derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south-central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.

Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones

Phase relations in a portion of the granite system, KAlSi3O8-NaAlSi3O8-SiO2-H2O have been studied in the pressure range 4 to 10 kb. At 10 kb, melting begins at 625°C at an isobaric invariant point involving albite + orthoclase + quartz + liquid + vapor. The anhydrous composition of the liquid at this isobaric quaternary eutectic corresponds to 21 orthoclase: 56 albite: 23 quartz in contrast to that at the isobaric quaternary minimum at 0.5 kb, where the liquid corresponds to 31 orthoclase: 30 albite: 39 quartz. This represents a shift of the liquid, in anhydrous projection, almost directly toward NaAlSi with increasing P. The quaternary minimum in this system becomes a eutectic at approximately 3.6 kb. Compositional comparisons among granites, aplites, zoned pegmatites, and the quaternary minimum and eutectic suggest that igneous pegmatites and aplites represent magmas that were saturated or nearly saturated with water when they began to crystallize. Trends in bulk composition among pegmatites further suggest selective losses of alkalies during crystallization, probably by diffusion through aqueous fluids coexisting with the silicate melts and their solid products.

The Granite System at Pressures of 4 to 10 Kilobars

Wallace Creek is an ephemeral stream in central California, the present channel of which displays an offset of 128 m along the San Andreas fault. Geological investigations have elucidated the relatively simple evolution of this channel and related landforms and deposits. This history requires that the average rate of slip along the San Andreas fault has been 33.9 ± 2.9 mm/yr for the past 3,700 yr and 35.8 + 5.4/−4.1 mm/yr for the past 13,250 yr. Small gullies near Wallace Creek record evidence for the amount of dextral slip during the past three great earthquakes. Slip during these great earthquakes ranged from ∼9.5 to 12.3 m. Using these values and the average rate of slip during the late Holocene, we estimate that the period of dormancy preceding each of the past 3 great earthquakes was between 240 and 450 yr. This is in marked contrast to the shorter intervals (∼150 yr) documented at sites 100 to 300 km to the southeast. These lengthy intervals suggest that a major portion of the San Andreas fault represented by the Wallace Creek site will not generate a great earthquake for at least another 100 yr. The slip rate determined at Wallace Creek enables us to argue, however, that rupture of a 90-km-long segment northwest of Wallace Creek, which sustained as much as 3.5 m of slip in 1857, is likely to generate a major earthquake by the turn of the century. 

In addition, we note that the long-term rates of slip at Wallace Creek are indistinguishable from maximum fault-slip rates estimated from geodetic data along the creeping segment of the fault farther north. These historical rates of slip along the creeping reach thus do represent the long-term—that is, millennial—average, and no appreciable elastic strain is accumulating there. 

Finally, we note that the Wallace Creek slip rate is appreciably lower than the average rate of slip (56 mm/yr) between the Pacific and North American plates determined for the interval of the past 3 m.y. The discrepancy is due principally to slippage along faults other than the San Andreas, but a slightly lower rate of plate motion during the Holocene epoch cannot be ruled out.

Holocene activity of the San Andreas fault at Wallace Creek, California

Results of experimental studies in the system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O, at 4 kb and from 650° to 1000°C, have been used to generate composition paths for liquid and crystal fractions as functions of temperature and bulk composition. In both the experimental work and the analyses of crystallization, attention has been devoted mainly to silicate liquid that is saturated with quartz and an alkali feldspar but unsaturated with respect to an aqueous vapor phase. Such liquid can be represented on a compound T-X surface that slopes, with decreasing H 2 O content in the system, toward SiO 2 for NaAlSi 3 O 8 -rich compositions and away from SiO 2 for KAlSi 3 O 8 -rich compositions. Compositions of the crystal fraction that separates from the liquid under equilibrium conditions in the temperature range 700° to 655°C have been calculated for an array of bulk compositions. Equilibrium crystallization history is characterized by numerous shifts and some reversals along paths that describe the composition of the crystal fraction. The various paths demonstrate that intensive parameters such as temperature, pressure, and H 2 O fugacity cannot be uniquely determined from the composition of a specific fraction. Paths also have been outlined, for a variety of bulk compositions, from the maximum temperature for separation of quartz ± alkali feldspar to the solidus temperature of 655° C under conditions of perfect fractional crystallization in which successive precipitates are immediately isolated from the silicate liquid. These crystal fraction paths are marked by discontinuities that indicate abrupt, discrete changes in composition, and with falling temperature, they tend to focus toward a compositional area centering about 20 NaAlSi 3 O 8 , 45 KAlSi 3 O 8 , 35 SiO 2 . The melting of crystalline assemblages that represent the haplogranite system can be considered as the inverse of equilibrium crystallization, fractional crystallization, or some combination of these idealized processes. Thus the composition path for an equilibrium crystal fraction can also serve for the tracing of compositional changes in a residue formed during equilibrium melting. A residue of given composition can be produced at constant elevated pressure under conditions representing many different combinations of temperature and bulk composition. The experimentally determined complexities of crystallization are reasonable indications of what can occur under natural plutonic conditions. They are potentially most useful, however, for the testing of genetic models based upon detailed studies of rocks in a geologic and petrographic context. They also lead to the conclusions that (1) the bulk composition of a granitic plutonite is not by itself sufficient for estimating conditions under which the rock was formed and (2) experimental data obtained for the haplogranite system under conditions that include the presence of an aqueous fluid phase ( P H 2 O ≈ P fluid = P tot ) are highly restricted in their pertinence to the crystallization histories of most granitic plutonites.

Richard H. Jahns

Papers

The Granite System at Pressures of 4 to 10 Kilobars

Holocene activity of the San Andreas fault at Wallace Creek, California

Crystallization of Alkali Feldspar and Quartz in the Haplogranite System NaAlSi3O8-KAlSi3O8-SiO2-H2O at 4 Kb

Mineralogy and geochemical evolution of the Little Three pegmatite-aplite layered intrusive, Ramona, California

Presentation of the 1965 Mineralogical Society of America award to Peter J. Wyllie