bluuebunny/crossref_metadata_2025 huggingface dataset:
A compensation mechanism for the central Pacific
Authors: Sandwell D. T. , Poehls K. A.
Published: February 2008

Geos 3 derived geoid heights and sea floor topography were averaged into 256 square areas (203 km on a side) for a region in the central Pacific containing a large portion of the Hawaiian Island chain. The whole region is about 500 m shallower than normal sea floor of the same age. The major portion of the depth anomaly is the Hawaiian swell. Data were analyzed using a two‐dimensional fast Fourier transform. A transfer function was computed to determine the part of the observed geoid height that is coherent and in phase with the topography. A number of compensation models were tested against this function. Of these models no single physically reasonable model was found to have an acceptable fit. Accordingly, two models were introduced, one compensating short‐wavelength topography at a shallow depth (14 km) and the other compensating the longer wavelengths by a deep mechanism. Acceptable deep compensation models include Airy‐Heiskanen type compensation at depths between 40 and 80 km. Using the transition wavelength between the two models (1100 km), an estimate is made of the amplitude and shape of the heat anomaly needed to uplift the Hawaiian swell. The peak of the anomaly has an amplitude of 530 mW m−2 and is located 275 km east of Hawaii.
A steady state model for the distribution of stress and temperature on the San Andreas Fault
Authors: Turcotte D. L. , Tag P. H. , Cooper R. F.
Published: February 2008

The coupled problem of stress and temperature adjacent to a strike slip fault is solved for a steady state model. The fault is divided into two parts: an upper brittle zone and a lower plastic zone. A coefficient of friction is assumed to be applicable to the brittle zone, and motion on the plastic zone is assumed to take place by thermally activated creep. The transition from brittle to plastic behavior is assumed to occur at a depth where the shear stresses in the two zones are equal. The vertical distribution of stress and temperature on the fault and the surface heat flow are obtained for several values of the coefficient of friction. For a low value of the effective friction coefficient, near 0.15, the depth of the brittle zone is near 15 km, which is about the depth of the deepest earthquakes on the San Andreas fault; however, the predicted heat flow anomaly immediately adjacent to the fault exceeded the scatter in the measured values. Alternative explanations for this discrepancy are given.
Some constraints on levels of shear stress in the crust from observations and theory
Authors: McGarr A.
Published: February 2008

In situ stress determinations in North America, southern Africa, and Australia indicate that on the average the maximum shear stress increases linearly with depth to at least 5.1 km measured in soft rock, such as shale and sandstone, and to 3.7 km in hard rock, including granite and quartzite. Regression lines fitted to the data yield gradients of 3.8 MPa/km and 6.6 MPa/km for soft and hard rock, respectively. Generally, the maximum shear stress in compressional states of stress for which the least principal stress is oriented near vertically is substantially greater than in extensional stress regimes, with the greatest principal stress in a vertical direction. The equations of equilibrium and compatibility can be used to provide functional constraints on the state of stress. If the stress is assumed to vary only with depth z in a given region, then all nonzero components must have the form A+Bz, where A and B are constants which generally differ for the various components. This implies that the deviatoric stress changes linearly with depth, and the general solution also allows the directions of the horizontal principal stresses to change monotonically with depth. Solutions to the equations, a churning stress to vary with both z and x, were fit to the observations of Zoback and Roller, who measured stress along a horizontal profile near Palmdale, California. The results indicate that the average shear stress in the upper 8 km of the fault zone is about 3.5 MPa less than the shear stress in the far field, but this far field term, which is part of the solution and has the form A+Bz, cannot be evaluated using the existing constant depth data.
Lithospheric loading by the 1896 Riku‐u Earthquake, northern Japan: Implications for plate flexure and asthenospheric rheology
Authors: Thatcher Wayne , Matsuda Tokihiko , Kato Teruyuki , Rundle John B.
Published: February 2008

Under favorable circumstances the time‐dependent aseismic deformation resulting from the loading of the lithosphere by the stress drop of large dip slip earthquakes can be used to determine both the effective elastic plate thickness and the asthenospheric viscosity. The deformation has several similarities with the deflection of the lithosphere by surface loads and with movements due to postglacial rebound. Level changes obtained in the 80 years since the M = 7.5, 1896 Riku‐u earthquake, an intraplate thrust event in northern Honshu, provide convincing evidence that asthenospheric readjustments are responsible for the observed movements. Leveling surveys crossing the zone of surface faulting have been repeated five times since 1900 and delineate a localized depression that has subsided at a continually decreasing rate. The depression is centered close to the 1896 faulting, and its shape and width, about 75 km, are matched by our model using a plate thickness of 30 km. The decaying subsidence rate constrains the viscosity of the uppermost asthenosphere to be 1×1020 P. A linear viscous rheology matches the observed decay quite well, although measurements are sparse during the several decades following the earthquake.
Radar measurement of heterogeneous small‐scale surface texture on Mars: Chryse
Authors: Simpson R. A. , Tyler G. L.
Published: February 2008

Radar measurements from Chryse Planitia in 1978 confirm earlier estimates of 4°–5° rms slope (on meter scales) over 45°–50°W longitude. The new measurements also confirm a skewing of the measured spectra. This has now been interpreted as a combined result of large‐scale surface tilt and a side effect of the decrease in small‐scale roughness as the subradar point moves from the basin floor toward the west.
Material properties for mantle convection consistent with observed surface fields
Authors: Kaula William M.
Published: February 2008

The spherical harmonic spectra of plate velocities, gravity, topography, and heat flow are closely approximated by forms Ml−n, where l is harmonic degree. The magnitudes M and slopes n of heat flow and gravity, compared to those of the poloidal velocity, imply a lithospheric viscosity of ∼1023 g/cm/s, similar to stress/strain rate ratios inferred from studies of flexure at subduction zones. The correlations among the field as functions of degree also appear explicable as the consequence of properties of the lithosphere (including its subducted extension). Applying boundary layer theory to plate velocities implies a Rayleigh number (based on observed heat flux) of ∼108 for whole mantle convection and ∼107 for convection confined to the upper mantle. To estimate the constraints on thermal and rheological parameters therefrom, a temperature curve consistent with phase transition data is constructed. The effective thermal expansivity is increased by phase transitions at depths of 400–700 km, but this effect is less important than a marked decrease from ∼3×10−5 to 10−5/°K at the bottom of the mantle. The Rayleigh number for upper mantle convection then is approximated rather well by viscosities corresponding to standard nonlinear strain rate stress laws below 60 km. The Rayleigh number for whole mantle convection requires a moderate increase in viscosity with depth, such as could plausibly arise from a decrease with depth in the activation volume for creep partially counteracting the effect of pressure. Hence constraints on the depth and other properties of mantle convection will require more detailed modeling utilizing relationships between the harmonic coefficients, as well as relative spectral magnitudes, of the surface fields.
A reservoir analysis of the Denver earthquakes: A case of induced seismicity
Authors: Hsieh Paul A. , Bredehoeft John D.
Published: February 2008

Injection of fluid wastes into the fractured Precambrian crystalline bedrock beneath the Rocky Mountain Arsenal near Denver triggered earthquakes in the 1960's. An analysis, based on the assumption that fluid flow in the fractured reservoir can be approximated by flow in a porous medium, is presented. The configuration and hydrologic properties of the reservoir are determined from two lines of evidence: (1) locations of earthquake hypocenters determined by seismic arrays installed at the Arsenal and (2) observed long‐term decline in fluid levels in the injection well. Together these two sets of data indicate that a long, narrow reservoir, aligned in the direction N 60°W, exists. The reservoir is 3.35 km in width, extends 30.5 km to the northwest and infinitely to the southeast, and spans a depth interval from 3.7 to 7.0 km below land surface. It has a transmissivity of 1.08 × 10−5 m2/s and a storage coefficient of 1.0 × 10−5. Computed pressure buildup along the length of the reservoir is compared with the spatial distribution of earthquake epicenters. The comparison shows that earthquakes are confined to that part of the reservoir where the pressure buildup exceeds 32 bars. This critical value is interpreted as the pressure buildup above which earthquakes occur. The migration of earthquake epicenters away from the injection well, a phenomenon noted by previous investigators, can be accounted for by the outward propagation of the critical pressure buildup. The analysis is extended to examining the effects of rapid flow in fractures opened by high injection pressure. The results show that the effect is confined to a small region within 1 km of the injection well. The existence of a critical pressure buildup above which earthquakes occur is completely consistent with the theory on the role of fluid pressure in fault movement as presented by Hubbert and Rubey.
Interpretation of adiabatic decompression measurements of the Grüneisen parameter
Authors: Stacey Frank D.
Published: February 2008

The most direct measurement of the thermodynamic Grüneisen parameter γ of a material is by observation of temperature changes accompanying sudden (adiabatic) compressions or decompressions. Measurements over a wide pressure range permit the dependence of γ upon density ρ to be inferred, but if measurements are made below the Debye temperature of the material, interpretation of the results may be confused by the temperature dependence of γ. If the temperature dependence is measured for the same samples, the density dependence may be adjusted to refer to a specimen held at a constant fraction of its Debye temperature, instead of constant temperature, by the equation [−(∂ 𝓁n γ/∂ 𝓁n ρ)T/θ] = [−(∂ 𝓁n γ/∂ 𝓁n ρ)T] × [1 + αT(d 𝓁n θ/d 𝓁n ρ)] − (∂ 𝓁n γ/∂ 𝓁n T)P(d 𝓁n θ/d 𝓁n ρ) where (∂ 𝓁n γ/∂ 𝓁n ρ)T/θ is the more fundamental quantity and for crystals with high Debye temperatures, will commonly be smaller in magnitude than (∂ 𝓁n γ/∂ 𝓁n ρ)T (both being negative) because (∂ 𝓁n γ/∂ 𝓁n T)P is positive at T < θ. Ideally, for geophysical purposes, we are interested in (∂ 𝓁n γ/∂ 𝓁n ρ)T>θ which is probably not very dependent on T and for which (∂ 𝓁n γ/∂ 𝓁n ρ)T/θ at T < θ appears to be a good approximation, but (∂ 𝓁n γ/∂ 𝓁n ρ)T for T < θ is not. However, the difference appears to be small for Cu, NaCl, CaF2 and α‐SiO2.
Deep electrical structure of the Colorado Plateau as determined from magnetotelluric measurements
Authors: Pedersen Jens , Hermance John F.
Published: February 2008

Magnetotelluric measurements made on the Colorado Plateau near Farmington, New Mexico, characterize the province as consisting of conductive surficial sediments underlain by a more resistive crystalline crust down to about 28 km. Beneath this zone the average resistivity is anomalously low (13 ohm m) down to at least 130 km. On the other hand, seismic evidence indicates a crustal thickness of approximately 40 km for the interior of the plateau, with lower than normal velocities in the upper mantle. The two sets of measurements are compatible providing an electrical transition zone is present in the lower crust separating a highly resistive upper crust from more conductive mantle material below 40‐km depth. Constraints on the allowable upper mantle resistivity values, when combined with the theoretical analysis of Shankland and Waff on the effects of partial melt concentrations, suggest that the bulk resistivity of the mantle is being influenced by the presence of a small degree of partial melt (a few percent by volume). Temperature estimates associated with the allowable resistivity variations lie within the range 1100°–1200°C at 50‐km depth and yield a geothermal gradient over the same depth interval of 3°–4°C/km or less. A comparison with electrical data that are available in the surrounding tectonic provinces indicates that the upper mantle beneath the Colorado Plateau has resistivities at least as low as those for the Basin and Range, Rio Grande rift, and Great Plains and may in fact be substantially more conducting than any of these surrounding tectonic provinces.
The impact of refraction correction on leveling interpretations in Southern California
Authors: Strange William E.
Published: February 2008

Recent investigations show that the refraction corrections in leveling are large but can be adequately modeled. The corrections are proportional to the temperature differences and the square of the sight lengths. Changes in sight length with time have resulted in apparent crustal movements when non‐refraction‐corrected data were used. Applying refraction corrections to leveling data in southern California significantly alters the size and configuration of the southern California uplift. Uplift at Palmdale and Gorman‐Lebec relative to Los Angeles (BM V32) during the 1953–1978 time period is no greater than 10 cm. The bulk of the motion, about 6 cm, occurs as localized motion along the San Gabriel fault during the 1953–1964 time period. This motion along the San Gabriel fault and uplift of 8 cm directly north of the San Fernando fault between 1955 and 1968 may represent precursors to the 1971 San Fernando earthquake.
Rectangular harmonic analysis applied to the geomagnetic field
Authors: Alldredge L. R.
Published: February 2008

Spherical harmonic analysis of the earth's magnetic field is limited in the resolution that can be obtained. This limitation is caused by inadequacies of computers and of available data sets. The fundamental wavelength in spherical harmonic analysis is the circumference of the earth. To resolve wavelengths as short as 100 km would require a spherical harmonic analysis carried out to a degree and order 400 involving 160,800 coefficients. This is impractical even with modern computers. This limitation of spherical harmonic analysis can be overcome by using rectangular harmonic analysis in successively smaller areas so that the data are more fully utilized. Rectangular harmonic analysis is illustrated for data from Europe and then again for a subset of the data from a smaller area of Europe. The magnetic field at 15 observatories for the smaller area can be computed to an rms residual of only 7 nT for all three components using two sets of rectangular harmonic coefficients and the AWC/75 world chart model. Rectangular harmonic analysis and spherical harmonic analysis are complementary.
The nature of grain boundaries in dunite and lherzolite xenoliths: Implications for magma transport in refractory upper mantle material
Authors: Waff Harve S. , Holdren George R.
Published: February 2008

Using X ray photoelectron spectroscopy (XPS), we have determined that the composition of intergranular regions on olivine crystals of unaltered dunite and lherzolite xenoliths from flows in Hawaii, Australia, and Arizona are markedly different from the compositions of both the olivine and the host transporting lavas. The boundaries are without low‐melting components, such as alkalis, but are rich in aluminum, silicon, magnesium, and iron. While XPS spectra offer only semiquantitative analyses, the compositions of the intergranular regions appear to be such that they would remain unmelted at temperatures existing in the xenolith source regions. We infer that within their source regions, refractory dunite bodies, of which the Hawaiian dunites are samples, have solid mechanical contacts along the olivine grain boundaries. Furthermore, we feel that the compositions of surface films are established so as to minimize the surface free energy along intergranular faces as melt is gradually removed. This is consistent with the observed inability of the host lavas to penetrate these xenoliths during their ascent to the surface. As such, the parent refractory material within the mantle appears to be effectively nonporous to the magma, and hence magma transport within the upper mantle would have to proceed by some mechanism other than the migration of melt along channels developed along wetted intergranular surfaces.
Radiative thermal conductivity in obsidian and estimates of heat transfer in magma bodies
Authors: Stein Judy , Shankland T. J. , Nitsan Uzi
Published: February 2008

The optical transmission spectra of four rhyolitic obsidian samples were measured in order to determine the importance of radiative heat transfer in granitic magmas. The spectra, obtained in the temperature range 25–800°C, show that the radiative spectral window in these samples is limited by a charge transfer band in the UV (400 nm) and Si‐O stretching overtone in the IR (4500 nm). Within this window the main obstacles to radiative transfer, in order of decreasing importance, are background scattering, a water band centered at 2800 nm, and an Fe2+ crystal field band at 1100 nm. Unlike crystalline silicates, the absorption bands in obsidian do not broaden significantly as temperature increases. As a result, the temperature dependence of the calculated radiative thermal conductivity KR is dominated by the T3 term. Actual values of KR increase from 9 × 10−5 to 1 × 10−3 cal cm−1 s−1 deg−1 between 300° and 800°C, the high‐temperature value being comparable to the lattice thermal conductivity in obsidian and a lower limit for KR in granitic melts. As the scattering coefficient in melts is probably significantly lower than in obsidian, the radiative conductivity in active plutons is likely to be much higher. As an example, if scattering and the water band are removed from the observed spectra of the obsidian samples, calculated values of KR could increase by a factor of 5, to about 5 × 10−3 cal cm−1 s−1 deg−1 at 1000°C.
Acoustic properties of northern Alaska shelves in relation to the regional geology
Authors: Houtz Robert E. , Eittreim Stephen , Grantz Arthur
Published: February 2008

More than 100 airgun‐sonobuoy records from the northern Alaska shelves have been reduced to yield an average of six layers from each record. Seafloor sound velocities, regional velocity functions, and high‐resolution velocity‐depth inversions were computed. Low seafloor velocities in the northwest part of the otherwise Cretaceous seafloor of the Colville foredeep represent thin Tertiary sediments that overstep the Barrow arch from the north. Anticlinal cores of ‘mid‐shelf arches’ (Grantz et al., 1981) appear as 200‐m/s velocity increases in the Neogene seafloor of the western Beaufort shelf. Very recent or currently, active sedimentation in the central Hope basin is indicated by low seafloor sound velocities. The value of K in the least squares determination of V = V0 + Kt (t is one‐way vertical travel time) decreases toward the east from 1.74 km/s2 just east of Point Barrow of 1.67 km/s2 around Prudhoe Bay, to 1.53 km/s2 near the Mackenzie River. This decrease in K corresponds with an eastward increase in the thickness of the clastic rocks within the youngest part of the Brookian sequence (Early Cretaceous to Quaternary). A 2.1‐km/s2 value of K is the Brookian sequence of the north Chukchi basin suggests a change in the relative proportions of Cretaceous and Tertiary sediments between the basin and the Beaufort shelf. High‐resolution velocity‐depth inversions, using closely digitized travel time data, yield velocities at about 40‐m vertical spacings. Calculated values of near‐surface velocity gradients determined by velocity‐depth plots of these data are generally not in good agreement with the values predicted from the velocity functions, which are statistical generalizations based on conventional refraction picks that resolve layers no thinner than about 250 m. The discrepancies are minimized by use of independently determined seafloor velocities.
Ice in the polar regions of the Moon
Authors: Lanzerotti L. J. , Brown W. L. , Johnson R. E.
Published: February 2008

Recent laboratory measurements of the erosion of H2O ice are used to estimate the loss rates of possible ‘trapped’ volatiles in the cold lunar polar regions. We conclude that a significant accumulation of water ice is unlikely to occur.
The gravity field in the central Pacific from satellite‐to‐satellite tracking
Authors: Marsh James G. , Marsh Bruce D. , Williamson Ronald G. , Wells William T.
Published: February 2008

Satellite‐to‐satellite Doppler tracking between the ATS 6 and the GEOS 3 spacecraft is used to measure the high‐degree and ‐order gravity field over an 80° area in the central Pacific Ocean. Doppler tracking provides velocity measurements of GEOS 3 as a function of time, which are easily converted to line‐of‐sight accelerations. Since the low‐degree and ‐order gravity field is known well, each GEOS 3 revolution is reduced relative to a twelfth‐degree and ‐order field model. The method is then capable of measuring wavelengths in the range from about 3500 to 1000 km. Accelerations computed along 40 revolutions, roughly half of them descending and the others ascending, can be contoured into a map of the gravity field at the GEOS 3 altitude of 840 km. The dominant wavelength of the map is about 2000 km. A comparison of this field with an altimeter‐derived geoid and the GEM 10B gravity field shows good agreement. Since each map is essentially independent of the others, this comparison is critical in establishing the validity of any of the maps. Each map shows virtually the same areas of positive and negative anomalies. These areas are essentially the same in the central Pacific as those found in our earlier study. Compared with surface amplitudes these anomalies are only one‐tenth as large at the GEOS 3 altitude. This observation and the correlation of gravity with residual depth anomalies imply that the cause of some of these anomalies may be due to bumps in the upper surface of the lithosphere that are compensated at depths of about 30–100 km. Some of the anomalies apparently do not correlate with residual depth anomalies.
Depth resolution of earthquakes in central Asia by moment tensor inversion of long‐period Rayleigh waves: Effects of phase velocity variations across Eurasia and their calibration
Authors: Romanowicz Barbara
Published: February 2008

The moment tensor inversion method for long‐period Rayleigh waves is discussed from the point of view of event‐depth resolution. In this method, depth is usually determined by finding the minima in appropriate least squares residuals versus depth curves. We show that depth cannot be resolved if the data are sampled only at one frequency and a choice of frequencies spanning a wide range of values yields better resolution. We also show that the variance reduction at the minimum depends strongly on biases in the data, provided they are not simple π‐periodic functions in azimuth from the source. Since such biases are introduced, in particular, when the propagation corrections are inaccurate, this provides a means by which to study regional trends of phase velocities. This method is then applied to obtain an estimate of lateral heterogeneity in Central Asia. Starting with a reference point in the Pamir mountains and a set of accurately known phase velocities on paths to Eurasian WWSSN stations, we study the Rayleigh waves at these stations, in the period range 30 to 90 s, for eight events located in or near the Tibet Plateau, at stances from the reference point, heterogeneity in phase velocity is thus found to be about 1000 to 1500 km in this period range. Constraining the source parameters of the most distant events by body wave modeling, we determine new phase velocities on paths to WWSSN stations from the center of the Tibet Plateau. The variation of phase velocity with source location reaches 1 to 2%, and the low velocity within Tibet clearly has a dominant effect. We then discuss the regionalization of phase velocities as a possible way to determine a calibration curve for a given source‐to‐receiver path. We find that although the regionalized phase velocities in Eurasia are stable to first order, they do not seem accurate enough to be used for moment tensor inversions with arbitrary source locations. This may be due to non ray theoretical effects causing the breakdown of phase velocity additivity when higher order accuracy is needed. On the other hand, reference phase velocities on source‐receiver paths can be obtained reasonably fast for events large enough so that body wave modeling is possible to constrain depth and source mechanism. We demonstrate that these phase‐velocity curves are accurate enough for the calibration of propagation effects for another event located as much as 400 to 500 km away. This provides an encouraging approach to study both structure and source mechanisms of smaller events within a highly heterogeneous continental region. The only constraint is to find several events with the right magnitude of both long‐period body waves and surface waves.
Isostasy of the southern Mid‐Atlantic Ridge: Long‐wavelength and short‐wavelength effects
Authors: Kostoglodov V. V. , Kogan M. G. , Magnitskaya E. I.
Published: February 2008

The isostatic transfer function for the southern Mid‐Atlantic Ridge was computed from 25 profiles of gravity and topography normal to the ridge trend. As follows from the comparison with previous studies, the transfer function for the Mid‐Atlantic Ridge is insensitive to the tectonic setting of profiles used. The experimental transfer function was employed to remove all coherent short‐wavelength features in the gravity and topography from 17 long profiles across the southern Mid‐Atlantic Ridge. The profiles were then recomputed as a function of t, age of the lithosphere, and averaged over equal ages. The observed regional topography, regional gravity anomaly, and altimeter geoid anomaly agree well with conductive cooling models of the spreading lithosphere. An observed slope of the topography versus t1/2 and of the geoid versus t can be explained with a single set of model parameters. Comparison with the estimates by Haxby for the northern Mid‐Atlantic and Reykjanes Ridges suggests a systematic decrease in the temperature of the mantle from 1500°–1600°C near Iceland to 1000°–1100°C at the southern Mid‐Atlantic Ridge. The theoretical gravity anomaly deduced by the analytical method of Haxby and Turcotte is sensitive to the thermal parameters only in the age range 0–20 m.y. The fit of the theoretical to the observed gravity anomaly is good outside the axial strip 0–3 m.y. The poor fit at the axis is attributed to inadequacy in the boundary condition of the model.
Gravity model and structural implications of the Goddard Pendant, Sierra Nevada, California
Authors: du Bray Edward A. , Oliver H. W.
Published: February 2008

A subsurface model for the Goddard pendant is constructed from a residual gravity high of about 7 mGal over the pendant. The model, which is the simplest and most geologically reasonable possibility, shows a metamorphic block that tapers with depth and extends about 3.5 km below the surface. The structures in the Goddard pendant are similar in style and orientation to those in other Sierra Nevada pendants, indicating that the country rock was neither deformed nor rotated during pluton emplacement. Consequently, emplacement must have been a passive rather than a forceful process. The pendant itself represents a piece of country rock trapped between plutons which are dome shaped in cross section.
Geochemistry and petrology of Mid‐Tertiary ash flow tuffs from the Sierra el Virulento Area, eastern Chihuahua, Mexico
Authors: Moll Elizabeth J.
Published: February 2008

The Sierra el Virulento, located in the Mexican Basin and Range province of eastern Chihuahua, is composed of over 700 m of siliceous high‐K calc‐alkalic ash flow tuff ranging in age from 33.7 ± 0.3 to 29.5 ± 0.5 m.y. At least 70% of phenocrysts in the tuffs are zoned feldspar crystals that have andesine cores and anorthoclase rims. Subordinate phenocrysts include clinopyroxene, biotite, ilmenite, magnetite, sanidine, hornblende, quartz, and orthopyroxene. Pyroxenes are Mg‐rich and have a restricted range of Mg‐Fe ratios. Fe‐Ti oxide minerals equilibrated at high temperatures (880°–1055°C) and oxygen fugacities (‐log fO2 = 8.3–11.0). The tuffs are strongly enriched in light rare earth elements (LREE) and have moderate negative Eu anomalies. Initial 87Sr/86Sr ratios range from 0.7043 to 0.7065 and show a rough correlation with SiO2, Sr, and Rb/Sr. The Sr‐isotopic compositions suggest that the rocks formed from an isotopically heterogeneous source material and were modified by later crustal contamination. Dacite tuffs may have originated by either fractional crystallization of more mafic magmas or by partial melting of crustal material. The series dacite to rhyolite seems to have evolved by a combination of low‐pressure fractionation in shallow magma chambers and crustal interaction. If variations from dacite to rhyolite are the result of crystal fractionation, the mineralogy and major‐element data suggest that anorthoclase was the dominant fractionating phase.
Statistics of narrow structures of the gravity field of the Earth: 1. General theory
Authors: Bertotti Bruno
Published: February 2008

With the increase in resolution and accuracy of gravity measurements it is becoming possible and increasingly important to concentrate on local analysis and on the role of elongated geophysical features. This paper deals with the statistical analysis of such features and develops for this purpose a new tool, the three‐ and four‐point correlation functions. Using an integration over the manifold of the rotation group, expressions are derived for these functions in terms of the spherical harmonic coefficients; I also discuss the corresponding quantities for a flat earth and show how this approximation is recovered for small features. To gain an understanding of the structure of these functions, a simple, heuristic model is constructed in which the surface gravity field is expressed as a random superposition of elliptical elementary disturbances with random orientations. The three‐point correlation function shows a characteristic behavior when the angle between its two vector arguments is of the order of the ratio of the axes of the ellipse. Another important piece of information contained in this function is the skewness of the gravity field.
Ellipsoidal corrections for geoid undulation computations using gravity anomalies in a cap
Authors: Rapp Richard H.
Published: February 2008

Ellipsoidal correction terms have been derived for geoid undulation computations when the Stokes equation using gravity anomalies in a cap is combined with potential coefficient information. The correction terms are long wavelength and depend on the cap size in which its gravity anomalies are given. Using the regular Stokes equation, the maximum correction for a cap size of 20° is −33 cm, which reduces to −27 cm when the Stokes function is modified by subtracting the value of the Stokes function at the cap radius. Ellipsoidal correction terms were also derived for the well‐known Marsh/Chang geoids. When no gravity was used, the correction could reach 101 cm, while for a cap size of 20° the maximum correction was −45 cm. Global correction maps are given for a number of different cases. For work requiring accurate geoid computations these correction terms should be applied.
Dissipation due to obliquity‐induced tides in a Kelvin‐Voigt viscoelastic body
Authors: VanArsdale William E.
Published: February 2008

The local and total dissipation rate of mechanical energy is calculated for an incompressible, Kelvin‐Voigt body in a circular orbit with finite obliquity. This material is characterized by a shear modulus and a viscosity. While in principle both of these material constants could be determined experimentally, only the shear modulus is known. The variation of the total dissipation rate with the viscosity is studied. For ‘small’ values of the viscosity the Kelvin‐Voigt material approximates the behavior of an elastic solid. In this range, results for the earth‐moon system are similar to those obtained by Peale and Cassen (1978) using a modified form of the elastic solution. For ‘large’ values of the viscosity the results are qualitatively different from those in the ‘elastic’ range, as viscous effects dominate the material response.
Velocity structure of the Pamir‐Hindu Kush Region: Possible evidence of subducted crust
Authors: Roecker S. W.
Published: February 2008

The arrival times of compressional (P) and shear (S) waves from approximately 580 microearthquakes recorded by a temporary array in the Pamir‐Hindu Kush region in central Asia are used to deduce one‐ and three‐dimensional velocity structures of this region. The results for one‐dimensional structures imply that the Moho is at 70±5 depth. Also, there is a velocity reversal near 160 km depth, which is inferred to be the beginning of the low velocity zone. This reversal continues to depths of approximately 230 km. Below 230 km, velocities are somewhat higher than those of normal mantle at similar depths (9.3 km/s versus 8.4 km/s for P waves). The outstanding feature of the results for three‐dimensional velocity structures is a broad (>40 km), centrally located region with 8 to 10% lower velocities than those in the surrounding regions. This low velocity region envelopes the seismic zone at depths between 70 and 150 km. The region may actually extend beyond these depths, but the results for shallower and deeper structure lack sufficient resolution to decide. Several tests, using both hypothetical and real data, were performed to estimate the reliability of the three‐dimensional solutions. The results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks, although one must be cautious of the effects of averaging in interpreting the solution. The low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust. Therefore, while subduction has occurred in the Pamir‐Hindu Kush, the results of the three‐dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km.

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Crustal structure near the Arctic Mid‐Ocean Ridge
Authors: Jackson H. R. , Reid I. , Falconer R. K. H.
Published: February 2008

Seismic reflection and crustal refraction studies were carried out at the FRAM I ice station near the Arctic Mid‐Ocean Ridge crest. This ridge is spreading at the very slow rate of 5 mm yr−1. Upper mantle Pn arrivals with apparent velocities averaging 7.9 km/s are observed at distance ranges less than 15 km, with corresponding intercept times of typically 1.1 s of which 0.5 s can be attributed to the travel path through a sedimentary layer. Anisotropy of about 6% appears to be present. These early Pn arrivals are observed on seven of the eight refraction lines studied, but crustal velocities are not well constrained. Modeling was done by computing travel time curves for a number of possible velocity structures for comparison with the data and suggests a crustal thickness between 2 and 3 km. This thin crust is associated with low‐amplitude magnetic anomalies. The remaining refraction line was shot within a region of enhanced magnetic anomaly amplitudes the Yermak H zone, where the data interpretation indicates a crustal thickness of about 8 km. This thicker crust and associated strong magnetic anomalies may be due to the ‘Yermak hot spot,’ a region of high magmatic activity. The generally thin crust is probably due to low basaltic productivity at the ridge crest, as a result of increased cooling at the very slow spreading rate.
Magnetotelluric soundings on the Idaho National Engineering Laboratory Facility, Idaho
Authors: Stanley William D.
Published: February 2008

The magnetotelluric (MT) method was used as one of several geophysical tools to study part of the Idaho Engineering Laboratory (INEL) facility. The purpose of the geophysical study on INEL was to investigate the facility for a possible site to drill a geothermal exploration well. A successful geothermal well would be used to provide hot water for a chemical processing plant. The MT method was employed to map any large‐scale structures or conductivity anomalies that might prove interesting as geothermal exploration targets. In addition to the MT data, direct current resistivity soundings, gravity data, aeromagnetic data, and seismic refraction data were obtained in the course of the geophysical study. In the MT survey described in this paper, an additional goal was to provide a better understanding of the electrical units mapped in the regional study of the Snake River Plain (SNRP) by Stanley et al. (1977). It was thought that a widespread conductive layer found beneath surface basalts in the 1977 study could be categorized petrologically by a deep well and additional MT soundings done nearby. Also, INEL is located on the margin of the SNRP, and it was desired to have MT data in the area to study the electrical nature of the margin of the plain. The MT sounding interpretations did not indicate any conductivity anomalies or significant structures near the chemical processing plant which could be used to guide the location of the proposed geothermal well to be drilled to a depth of 3 km. The initial interpretation of the MT sounding data was done with one‐dimensional models consisting of four or five layers, the minimum number required to fit the data. After the test well (INEL‐1) was completed, the electric log was used to guide an improved one‐dimensional ID interpretation of the MT sounding data. Profile models derived from the well log provided good agreement with velocity models derived from refraction seismic data. A resolution study using generalized inverse techniques shows that the resolution of resistive layers in the lower part of the MT models is poor, as is the definition of a shallow, altered basalt unit. The only major structure observed on the MT data was the faulted contact between the SNRP and basin and range structures on the west. Modeling of the data near this structure with a two‐dimensional computer program showed that the MT data near the fault require a model similar to the seismic refraction models and that structure on a deep crustal conductor is also required.
Polar wandering and the forced responses of a rotating, multilayered, viscoelastic planet
Authors: Sabadini Roberto , Yuen David A. , Boschi Enzo
Published: February 2008

Changes in the earth's rotation produced by the waxing and waning of large mobile ice sheets can be used to infer the viscosity of the mantle from a comparison with the recent secular polar drift as revealed in the International Latitude Service data and also in the estimates of the nontidal acceleration of the length of the day. We have developed by using an analytical approach a physical model of a rotating earth consisting of an elastic lithosphere, a viscoelastic mantle, and an inviscid core. Forcings from the two major ice sheets, Laurentide and Fennoscandia, have been considered. Sensitivity analysis of the parameters governing the external forcings, such as the length and the time of termination of the deglaciation phase, show in general that there exist two families of mean mantle viscosities, 0(1022 P) and 0(1023 P), which can match the data satisfactorily. However, we find it an improvement to incorporate some amount of shrinkage from the Antarctic ice sheet, obtaining simultaneously a better fit with the two rotational data sets to the lower viscosity solutions. Hence forcings from the southern hemisphere may be able to remove the ambiguity associated with the extraction of mantle viscosity from rotational data. We have also solved the initial value problem of the components of instantaneous angular velocity by means of the Laplace transform for applied loads characterizing the continental ice sheets, which since the mid‐Pliocene have periodically expanded and contracted over the northern hemisphere. In consequence of this continual forcing, polar wander of around 5–10 degrees can take place for a mean mantle viscosity 0 (1022 P). Recent reanalysis of paleomagnetic data in conjunction with hot spot tracks have revealed that several degrees of true polar wander could have occurred in the last 5 m.y. These calculations thus provide a physical explanation for the observed movement of the rotation axis. Membrane stresses of 0 (10) bars can develop from this secular polar motion.
Horizontal deformation in the Imperial Valley, California, between 1934 and 1980
Authors: Snay Richard A. , Cline Michael W. , Timmerman Edward L.
Published: February 2008

A triangulation‐trilateration survey was performed by the National Ocean Survey/National Geodetic Survey four months prior to the October 15, 1979, Imperial Valley M 6.6 earthquake. The survey was repeated in February–March 1980 to gauge the coseismic displacement. These data combined with triangulation surveys performed in 1934–1935, 1939, 1941, 1954–1955, and 1967 provide a historical record of crustal deformation over a time period that also includes the 1940 El Centro M 7.1 earthquake. The total data set was analyzed in terms of a dislocation model that relates the observed surface deformation to slip on prescribed geologic faults. According to the model, the 1941–1980 fault slip at depths 10 km and greater is 1.9±0.3 m. This value is approximately twice the 1.0±0.1 m estimated for the 1941–1980 slip on the Imperial fault in the 0‐ to 10‐km depth range. This discrepancy can be explained by a range of possibilities including the following: (1) some of the deep fault slip is due to aftereffects of the 1940 earthquake, in which case the seismic cycle over which deep slip and shallow slip are equal is not the interval between large earthquakes but is delayed a few years; (2) the deep slip should be equated to the shallow slip over several parallel faults; (3) the crust experienced permanent inelastic deformation; and (4) deep deformation is accommodated by a mechanism other than slip.
The Oroville Earthquakes: A study of source characteristics and site effects
Authors: Cohn Stephen N. , Hong Tai‐Lin , Helmberger Donald V.
Published: February 2008

Half‐space modeling, in the time domain, of broadband seismic records from the Oroville aftershock sequence demonstrates that these events have simple sources. Systematic scaling between the far‐field source duration and seismic moment for the aftershocks studied here and other California earthquakes confirms Brune's (1970) source model and shows that stress drops for California earthquakes range between 10 and 100 bars. These events also show no trend in stress drop as a function of size for 3.0 ≤ ML ≤ 6.4. Analysis of static deformation associated with the main shock shows that slip occurred on a fault 81 km2 in area corresponding to the zone defined by the first 3 days of aftershocks. Teleseismic studies indicate that the seismic source area was 50 km2, corresponding to a zone completely free of aftershocks in the middle of the aftershock zone. Both results imply a stress drop of 29–39 bars and seismic moment between 5.7 and 8.7×1024 dyn cm. The evidence suggests that faulting occurred seismically on the central portion of the fault and propagated out slowly, enlarging the fault plane. Complicated waveforms recorded for the same events at nearby sites on alluvium indicate the critical role that propagation through complex structure plays in determining the character of the observed seismic signal at close‐in stations. Modeling of these records shows that dipping structure may account for the observed waveform complexities.
Thermal conductivity and temperature structure of the Reydarfjordur borehole
Authors: Oxburgh E. R. , Agrell S. O.
Published: February 2008

One hundred and ten measurements of thermal conductivity have been made on samples over a 1900‐m depth interval in the Reydarfjordur borehole comprising largely basaltic flows and intrusives. The mean conductivity of both flows and intrusives increases with depth by about 25%, reaching a maximum value of about 1.9 W m−1 °C−1. This increase results from the growth of secondary minerals which progressively fill voids with depth and in some cases replace phases having lower values of thermal conductivity. The conductive heat flow between 650 m and the surface is about 100 mW m−2. Below 650 m the conductive heat flow appears to be less than half the near‐surface value, and there is strong evidence that in this depth interval the thermal structure is modified by hydrothermal circulation.
Rivera Ocean Seismic Experiment (ROSE) overview
Authors: Ewing J. I. , Meyer Robert P.
Published: February 2008

The Rivera Ocean Seismic Experiment (ROSE) was designed as a combined sea and land seismic program to utilize both explosive sources and earthquakes to study a number of features of the structure and evolution of a mid‐ocean ridge, a major oceanic fracture zone, and the transition region between ocean and continent. The primary region selected for the experiment included the Rivera Fracture Zone, the crest and eastern flank of the East Pacific Rise north of the Rivera, and adjacent areas of Baja California and mainland Mexico. These areas were to be instrumented with land and ocean bottom seismographs in order to determine good source parameter and location data for natural events and to record these events along a large number of paths crossing various parts of the region. Explosive charges were to be detonated at sea to supplement the natural events. However, the necessary permission to conduct the experiment was not received from Mexican authorities; therefore an alternate plan was implemented whereby the marine program had to be moved southward outside of territorial waters. This had the effect of transforming this experiment into three, almost independent components: (1) an experiment to study the East Pacific Rise south of the Orozco Fracture Zone primarily using ocean bottom recording and explosive sources, (2) a seismicity program at the Orozco, and (3) a land‐based program of recording natural events along the coastal region of Mexico. A considerable amount of useful data was obtained in each of the three subprograms. In the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle, structure and dynamics of the East Pacific Rise, seismicity of the Orozco Fracture Zone, and partitioning of energy transmission between the ocean volume and the crust/lithosphere. On land, the fortuitous occurrence of the Petatlan M7.6 earthquake of March 14, 1979, permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event, which provide new insight into the filling of a major seismic gap in the region. This overview describes the scientific rationale and the design of the experiments, along with some general results. Other articles in this volume give preliminary scientific results from certain components of the overall experiment or, in some cases, report on other data pertinent to the scientific goals of ROSE.