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It is well known that metals and some ceramics deform superplastically during polymorphic phase transitions, but this phenomenon has not been considered as a possible deformation mechanism in the earth's mantle. Although phase transitions in the mantle can be expected to move significant distances in response to changing surface loads, the time scale of this motion (of the order of 105 yr) is too long to allow simple applications of laboratory results in the absence of a well understood mechanism. To examine possible mechanisms, an apparatus capable of applying a shear traction parallel to a polymorphic phase boundary has been developed. Preliminary results with the β-α transformation in CsCl show that macroscopic deformation was localized at the phase boundary and that microscopic deformation appears to be restricted to the high-temperature phase, in agreement with the mechanism proposed by Greenwood and Johnson. Seismic evidence suggesting that flow is localized at the 400- and 650-km discontinuities in the earth's mantle is reviewed.

The largest earthquake to affect northern Delaware in 100 years occurred at 08:21 GMT on February 28, 1973. The event was perceptible over 15,000 km2 in New Jersey, Delaware, Pennsylvania, and Maryland with a maximum Modified Mercalli intensity of V-VI near the Fall Zone between Wilmington and Claymont, Delaware. The isoseismals are elongate in a northeast-southwest direction. The main shock had a magnitude of 3.8 and was located at 39°43.1′N, 75°26.4′W with a depth of 14.1 km as calculated by NOAA. The five aftershocks located in this study were centered within a few kilometers of the region of greatest intensity at about 39°47′N and 75°25′W with depths ranging from 5 to 8.4 km. The fault plane solution determined from the mainshock and aftershocks indicates dip-slip motion on a nearly vertical plane striking N28°E. The southeast side is down in agreement with geological observations of subsidence of the Coastal Plain and uplift of the Piedmont. The strike of this fault is similar to that of the border faults of a graben in the basement rocks about 30 km southwest along the strike from the epicenter suggesting that the seismic activity may be associated with such faults.

The elastic properties of NaCl have been measured ultrasonically as a function of pressure and temperature, from 0 to 8 kbar and from 300 to 800°K. Together with zero pressure data on thermal expansivity and heat capacity, these data permit the specification of the equation of state ( PVT ) throughout the P–T range measured. This permits extrapolation of high temperature ( T > θ D ) data to zero temperature for comparison with properties predicted for a static lattice. A Born lattice model calculation shows the importance of second nearest neighbor interactions, ionicity and the functional form of the potential. The measured equation of state permits detailed comparison with theory, and the separation of volume and temperature effects. Gruneisen's parameters is shown to be primarily dependent on volume, with small temperature dependence; it is not a linear function however.

S-Y The method of homogeneous static deformation is used to compute the pressure dependence of the elastic constants of spinel under the assumption of a Born-Mayer two-body potential. The method, originally used by Fuchs (1935), is first applied to a number of alkali halides and periclase which have the sodium chloride structure, and reasonable agreement with the measured pressure derivatives is obtained. When applied to the spinel structure, the theory predicts that dc,,/dP is positive at low pressure but becomes negative at pressures of a few hundred kilobars. The geophysical implications of a negative d~~~/dP for the spinel region of the mantle are discussed.

La zona de baja velocidad en regiones tectónicas y oceánicas es demasiado pronunciada para ser únicamente producto de altos gradientes de temperatura. La fusión parcial es consistente con la baja velocidad, la baja Q y con los límites abruptos de esta región del manto superior y también es consistente con los valores medidos del flujo de calor. Las bajas temperaturas de fusión que se infieren parecen indicar que la presión del agua es suficientemente alta para bajar el punto de solidus de 200°C a 400°C por abajo de las determinaciones de laboratorio del punto de fusión de silicatos anhidros. La inestabilidad mecánica de una capa fundida parcialmente el manto superior es probablemente una fuente importante de energía tectónica. La cima de la zona de baja velocidad se puede considerar como una superficie autolubricada sobre la cual pueden deslizarse la corteza y la cima del manto con muy poca fricción. El movimiento de alejamiento lateral de la corteza y del manto superior con respecto a los altos oceánicos se contrarresta con el flujo de material fundido en la capa de baja velocidad hacia el alto donde eventual-mente emerge como corteza nueva. Si este flujo lateral del material fundido no es tan activo como la remoción del magma de la cima, entonces migrarán las regiones de extrusión tales como los altos oceánicos.

Summary The method of long waves is used to compute the volume dependence of the elastic properties of MgO, γ-Al2MgO4 (spinel), and γ-Mg2SiO4 (spinel structure) from a rigidion, central force model. A comparison of theoretical and experimental elastic constants for the first two compounds shows that the neglect of non-central forces and ionic polarization severely limits the application of the model for oxides and silicates. The Coulombic contributions to the elastic constants for both the 3–2 spinel (γ-Al2MgO4) and the 2–4 spinel (γ-Mg2SiO4) are expressed as dimensionless Madelung-like constants and are computed as a function of the oxygen parameter. The distortion of the two spinels from c.c.p. is shown to be primarily an electrostatic effect. The elastic properties of γ-Mg2SiO4 are predicted using the bond parameters for Mg-O found from data on periclase and for Si-O found from stishovite. Although the rigid-ion model is too restrictive to give geophysically useful predictions, it shows how lattice dynamics may be used as a framework through which laboratory data may be used to interpret seismic velocity and density profiles.

The low velocity zone in tectonic and oceanic regions is too pronounced to be the effect of high temperature gradients alone. Partial melting is consistent with the low velocity, low Q and abrupt boundaries of this region of the upper mantle and is also consistent with measured heat flow values. The inferred low melting temperatures seem to indicate that the water pressure is sufficiently high to lower the solidus about 200 °C to 400 °C below laboratory determinations of the melting point of anhydrous silicates. The mechanical instability of a partially molten layer in the upper mantle is probably an important source of tectonic energy. The top of the low-velocity zone can be considered a self-lubricated surface upon which the top of the mantle and the crust can slide with very little friction. Lateral motion of the crust and upper mantle away from oceanic rises is counterbalanced by the flow of molten material in the low-velocity layer toward the rise where it eventually emerges as new crust. If this lateral flow of molten material is not as efficient as the upward removal of magma, then regions of extrusion, such as oceanic rises, will migrate.

Seismology and solid state physics place strong constraints on the constitution of the earth's interior.

The primary objective of theoretical equation-of-state work in geophysics has been to provide a framework with which the ultrasonic, X-ray compression, and shock data can be used in interpreting the seismic velocity and density profiles in the earth. Each of these experimental techniques falls short of the ultimate experiment of reproducing the temperature-pressure conditions at any depth in the earth and measuring V_p, V_s, and p of mantle-candidate mineral assemblages for direct comparison with the seismic profiles. The ultrasonic data give V_p, V_s, and p as a function of T and P, but the pressure range has been limited to ∼10 kb. This limit necessitates a large extrapolation for discussion of even the upper mantle. The X-ray static compression measurements have a pressure range to 300 kb, but are limited to room temperature and to only the pure compression properties K_T and p. Finally, although the shock-wave techniques can generate pressures comparable to those found throughout the earth's mantle and core, the shock-produced states are characterized by pressure and internal energy. Thus an E(T, P) equation of state is required before the data can effectively be used. Further, like static compression experiments, the shock technique now yields only K and p.

The finite strain expressions, originally derived by F. Birch (1938), for the compressional and shear velocities in an isotropic solid under hydrostatic pressure, when rederived to be complete to first order in the strain, are functions of both the second- and third-order elastic constants. The extra terms appearing in the complete first-order theory for the velocities and their pressure derivatives are of the order of the ordinary elastic constants. There is no longer any discrepancy between first-order finite strain theory and ultrasonic and seismic data.

The radiation field from explosions in a prestressed medium can be theoretically predicted on the basis of a dynamical theory of stress relaxation in the vicinity of the shock-induced fracture zone created by an explosion. The field consists of the normal compressional wave field resulting from the conversion of the shock wave to an elastic wave plus an anomalous part due to the release of strain energy. In this study we consider the nature of the radiation field to be expected from such a source in an inhomogenous earth and determine the stress field required to explain the observations from a large underground explosion. The field is described in terms of radiation patterns as functions of frequency or alternately as the amplitude and phase spectrums at particular distances. These theoretical predictions are compared to the Love- and Rayleigh-wave radiation patterns and spectra from the nuclear explosion Bilby. Using the known source parameters, we obtain agreement between the observed radiation patterns and the predicted patterns for a pure shear prestress field equivalent to a shear couple oriented approximately N10°W. Using the amplitude spectrum of the observed field adjusted for propagational effects, we find that the prestress was 70 ± 20 bars in the source area. We conclude that this approach can be utilized as a means of systematically measuring the stress field of the earth.