bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure near Explorer Ridge from a marine deep seismic sounding survey
Authors: Malecek S. J. , Clowes R. M.
Published: February 2008

In 1974, two reversed, 75‐km, deep seismic sounding profiles were recorded in the Explorer Ridge region of the northeast Pacific, one parallel and the other perpendicular to the ridge. Multichannel seismograms recorded at distances beyond 4 km were stacked, filtered, and statics‐ and amplitude‐corrected before compilation as record sections. The data have been interpreted using both travel time analysis and amplitude studies with the aid of synthetic seismograms. For the reversed profiles (74‐2 and 74‐2R) across the ridge, significantly different velocity‐depth curves were derived. The uppermost 2.5 km (subbottom) is similar with a rapid increase in velocity from about 2.5 to 5.6 km/s. However, the depth to and the form of the increase from about 5.6 to 6.8 km/s are different. Profile 74‐2 shows a small steplike increase followed by a substantial velocity gradient, while 74‐2R has a large steplike increase followed by a lesser gradient. The deeper crust as shown by 74‐2 consists of a monotonie velocity increase, but with changing velocity gradient, to the upper mantle. On 74‐2R, a deep crustal low‐velocity zone was required. It is suggested that this may be associated with a magma chamber beneath Explorer Ridge. For the two profiles (74‐1 and IR) parallel to the ridge, large velocity gradients in the uppermost crust also are derived. Our preferred interpretation to explain travel time delays and offsets involves crustal faulting with vertical offsets of approximately 5 km. An alternative interpretation in terms of a low‐velocity zone also is given. The total subbottom thickness of the oceanic crust varies between 8 and 10 km, except in the faulted regions, where it is thin. In the upper mantle, reversed velocities of 7.9 and 7.3 km/s are determined in directions perpendicular and parallel to the ridge; anisotropy is proposed to explain these different velocities. The interpreted velocity‐depth models are discussed in terms of petrologic models of the oceanic crust and recent determinations of the seismic velocity and lithologie structure of a New‐foundland ophiolite complex. In the uppermost crust, the large velocity gradients probably result from the closing of large cracks and pore spaces within the basalt. In the lower crust, our results are consistent with proposals involving gradual transitions between material of differing composition.
Present obliquity oscillations of Mars: Fourth‐order accuracy in orbital e and I
Authors: Ward William R.
Published: February 2008

A long period analysis of solar system orbital evolution, correct to fourth order in orbital eccentricities and inclinations (Bretagnon, 1974), and an improved value of the planet's moment of inertia (Reasenberg, 1977) have been incorporated in a recalculation of the obliquity oscillations of Mars. A linearized solution predicts a maximum oscillation amplitude of 13.6° centered on a long‐term average value of 24.4°. A numerical integration of the obliquity for the past 107 years is also presented. Epochs of minimal oscillation like the present occur at intervals of the order of 4 m.y.
A comparison between burg's maximum entropy method and a nonrecursive technique for the spectral analysis of deterministic signals
Authors: Swingler D. N.
Published: February 2008

It is demonstrated that the anomalous behavior of the Burg method on processing sinusoids is due to the Toeplitz structure inherent in the derivation of the recursion algorithm. A straightforward non‐recursive approach is described which (for sinusoids) not only is shown to be free of these anomalies but also seems to offer superior noise performance.
Planetary geodetic control using satellite imaging
Authors: Duxbury Thomas C.
Published: February 2008

A new data type for planetary geodetic control using natural satellite imaging is presented. Spacecraft images of natural satellites against the planet give a direct tie between inertial space and surface features surrounding the satellite image. This technique is expected to offer a factor of 3–10 improvement in accuracy over present geodetic reduction for Mars. A specific example using Viking imaging of Phobos against Mars is given.
The low temperature impact properties of the meteorite hoba
Authors: Johnson Alan A. , Remo John L. , Davis Robert B.
Published: February 2008

Charpy V‐notch impact specimens prepared from the high nickel ataxite Hoba have been tested at 77° and 195°K. The specimen tested at 77°K was fully brittle, and the one tested at 195°K was fully ductile. It is concluded that the transition to complete brittleness occurs below about 140°K or 150°K. Thus, high nickel material such as this is ductile at the ambient temperature of an asteroid, which is about 150°K, and does not fragment in a brittle manner when meteoritic material is formed by the collision of asteroids. This confirms a hypothesis put forward by Remo and Johnson [1974] according to which this failure of high nickel material to fragment explains the large average size of high nickel meteorites. The microstructure of Hoba was studied using optical microscopy, scanning electron microscopy, and electron beam microprobe analysis. It was found, in agreement with the results of earlier workers, that Hoba has a fine plessitic structure with prolific small mineral inclusions. Some troilite inclusions were shown to have transverse bands of daubreelite within them.
Inherent instability of quasi‐static creep slippage on a fault
Authors: Weertman J.
Published: February 2008

The inherent instability of quasistatic creep slippage is studied for two examples. One example is that of a symmetric transform fault at which a single spreading center terminates. The other example is the transform fault between two spreading centers. It is assumed that the friction stress of a fault decreases with increasing slip velocity.
Radioactive disequilibrium of uranium and actinium series nuclides in soil
Authors: Megumi Kazuko
Published: February 2008

The concentration of uranium series nuclides (238U, 234U, 230Th, 226Ra, and 210Pb) and actinium series nuclides (235U and 227Ac) were determined by γ ray spectrometry for size‐separated soil samples collected in Murho and originating from the weathering of granite rock. Each activity ratio of a descendant to 238U or 235U is found to be larger than unity. Extraction tests show that approximately 50% of the uranium series nuclides reside on the surface of soil particles and approximately 80% of the 230Th is surface correlated. Because of the anomalously high radioactivity levels, it was possible to obtain sufficient data to estimate the age of weathering of the soil. A weathering age of >103 years was estimated from the 230Th and 226Ra surface‐correlated activities.
Estimation of the ocean geoid near the Blake Escarpment using Geos 3 satellite altimetry data
Authors: Brammer Robert F.
Published: February 2008

The fundamental objective of this Geos 3 investigation was to determine the accuracy with which the local ocean geoid structure, expressed in terms of geoid undulations and vertical deflections, could be determined using satellite altimetry data. The approach taken to meet this objective was to estimate the undulation and the along‐track component of the vertical deflection for selected passes of Geos 3 near the Blake Escarpment and to compare these results with independent analogous estimates based on U.S. Navy surface gravimetric survey data. The results of these comparisons show agreement in the geoid undulation values generally to within 1 or 2 m. The nature of the discrepancy in the undulation values was primarily that of a bias error believed to be due essentially to radial orbit uncertainties. The agreement between the vertical deflection estimates was not significantly affected by orbit uncertainties over the track lengths considered in this study (1000–1500 km), and the comparisons show typical rms differences of between 1 and 2 arc sec. An additional objective of this study was to determine the capability of the altimeter to resolve short‐wavelength features of the geoid. This analysis involved spectrum and cross‐spectrum analysis of sets of closely spaced parallel subtracks to determine statistically significant short‐wavelength geoid resolution capability. The results of this analysis show that resolution can be achieved down to wavelengths as short as 30–80 km, depending on regional geoid variations.
Considerations of the indirect effect in marine gravity modeling
Authors: Chapman Michael E. , Bodine John H.
Published: February 2008

Correct procedures for the reduction of marine gravity data for the purpose of geophysical modeling, as established by previous authors (e.g., Bomford, 1962), include addition of a correction term necessitated by the undulation of the geoid surface on which gravity is measured. The variation of gravity that is due to the undulation of the geoid is known as the indirect effect. Although this correction is generally small and has not been commonly applied in marine geophysical modeling, there are cases in which it becomes significant. For regional studies in which the long‐wavelength components of gravity (λ > 500 km) are analyzed, adjustments must be made to remove the indirect effect and avoid the possibility of serious misinterpretation. For local studies in which shorter‐wavelength components are considered, gravity values may be satisfactorily approximated while neglecting this small gravity correction.
Current loop models of the Earth's magnetic field
Authors: Peddie Norman W.
Published: February 2008

Eleven models of the earth's magnetic field, of a new kind, were derived. They are based on sets of circular loops of filamentary electric current (current loops). Seven of the models consist of one to seven current loops that were constrained to lie on the surface of the earth's core. The other four models consist of one to four unconstrained current loops. The magnetic field of current loops was evaluated by numerical approximation of the Biot‐Savart formula. The parameters of the models were estimated by least squares using 412 uniformly distributed field vectors representing the observed field at 1975. The Marquardt nonlinear least squares method was used. Twenty preliminary computer runs, using randomly determined initial parameter values, were made for each model. Of the eleven models derived, only the one consisting of two unconstrained current loops is significantly more efficient than spherical harmonic analysis of comparable order at representing the static field. In the models consisting of four or fewer core‐surface current loops, the radii of the loops are greater than 2655 km. The currents flow generally westward and range in magnitude from 0.5 to 1.9 GA. The other core‐surface current loop models have, in addition, a small‐radius current loop in the western hemisphere and an eastward flowing current. In the models consisting of unconstrained current loops all the currents are located below the surface of the core. Their radii range from 812 to 2984 km.
A model for the earthquake cycle in underthrust zones
Authors: Thatcher Wayne , Rundle John B.
Published: February 2008

The time‐dependent surface deformation due to thrust faulting in an elastic plate overlying a viscoelastic half space is examined and compared with geodetic measurements of earthquake‐related crustal movements from Japan. The model fault is two‐dimensional, and the half‐space rheology is that of a Maxwell solid whose instantaneous response to applied loads is purely elastic but which subsequently flows to relieve imposed shear stresses. A characteristic feature of shallow‐angle underthrusting shown by these computations is that buried slip produces predominantly land uplift, while asthenospheric relaxation due to surface faulting results principally in surface downwarping. These distinctive patterns, also reflected in the measured deformation, are particularly useful in formulating a viable model for the entire earthquake cycle. For the model constructed in this way, coseismic faulting in the upper part of the plate transfers a load to the lower lithosphere and asthenosphere, inducing postseismic slip downdip of the seismic rupture and relaxation in the asthenosphere, these transients eventually merging into the steady buried slip and smoother asthenospheric flow that characterize the interseismic phase of the deformation cycle. Aseismic slip and asthenospheric relaxation cause the load supported by the plate‐bounding fault to be gradually transferred back to the shallow locked segment of the fault, effecting the strain buildup for a subsequent earthquake, and the cycle is repeated. No explicit account of plate‐driving forces is made; their net effect is presumed to be the steady buried slip that persists throughout the cycle and provides the energy that drives it. Characteristic features of the observed deformation in southwest Japan, site of the 1946 Nankaido earthquake, and in the South Kanto district, where the great 1923 earthquake occurred, are matched by this model using conventional values of lithospheric thickness (60 km) and asthenospheric viscosity (1021 P, or 1020 N s/2), although the model is not strongly tied to these exact values. Postseismic movements are adequately explained by episodic slip that occurs below the coseismic rupture, is 10–30% of the seismic slip, and acts in the same sense. Superimposed on these movements are the lesser effects of asthenospheric relaxation, which in the South Kanto district contribute significantly to the postseismic vertical level changes because post‐1923 buried slip is comparatively small and involves largely strike slip movements. Interseismic subsidence in both regions is explained well by asthenospheric relaxation and less significant deformation effects due to steady aseismic slip on the lower part of the plate boundary.
No giant meteorite crater in Wilkes Land, Antarctica
Authors: Bentley Charles R.
Published: February 2008

Schmidt (1962) suggested that a large gravity anomaly in Wilkes Land, Antarctica, was caused by the impact of a huge meteorite and that the impact was also the australite source predicted by Barnes (1961). His suggestion was recently revived by Weihaupt (1976). However, Weihaupt's analysis does not stand up to critical examination, and Schmidt's (1962) arguments have been negated by the subsequent discovery of much higher subglacial relief than was previously realized. Recent definitive airborne radio‐echo sounding that shows no evidence for the supposed crater has effectively disproved the hypothesis.
On three‐dimensional inversion of P wave time residuals: Option for geological modeling
Authors: Christoffersson A. , Husebye E. S.
Published: February 2008

In this paper some simple algorithms are introduced which give added flexibility to the Aki et al. (1977) inversion technique of two‐dimensional travel time data. The new options include, besides computational efficiency, a block equalization scheme which may be used for smoothing poorly sampled blocks, testing whether geological surface contours have a counterpart in the deeper lithosphere and also to restrict the anomalous volume to parts of the lithosphere only. This novel computational scheme is demonstrated on a large Norsar data set, and the main results were as follows. Even a single heterogeneous layer at a depth of around 170 km, i.e., a two‐dimensional model, gives a not unreasonable fit to the observational data (explained variance of the order of 60–65%, dominant surface geological features have no counterparts in the deeper lithosphere, though a seismic counterpart of the Oslo graben (associated with a marked gravity high) is found in the crustal layer. Changes in the basic model parameters like number and thickness of layers affect only the secondary features of the final three‐dimensional seismic image of parts of the lithosphere. Physical smoothing of the final results in terms of equalizing poorly sampled blocks (e.g., number of hits less than 6) to nearby ones is demonstrated to be a viable alternative to introducing specific smoothing kernels.
The melting curve of gold, silver, and copper to 60‐Kbar pressure: A reinvestigation
Authors: Mirwald Peter W. , Kennedy George C.
Published: February 2008

The melting curves of gold, silver, and copper were redetermined by differential thermal analysis to 60‐kbar pressure. The experiments were carried out with an end‐loaded piston‐cylinder high‐pressure apparatus using a recently developed low‐friction cell. Significant improvements in the precision of the temperature determination above 40 kbar were obtained. We observed a slight lowering of the melting temperature of copper when graphite capsules were used, which is consistent with our past experience and indicates some alloying of the copper and carbon. The following parabolic functions (Tm = a3 + [a2(P − a1)]½ 10 8a2) were fitted to the raw data, and the data corrected for the influence of pressure on the emf of the thermocouples (in parenthesis): gold, Tm = −2439.28 + [0.262850×10−4(P + 322.668)]½ × 2628.50 (Tm = −1773.88 + [0.309303×10−4(P + 249.167)]½ × 3093.03); silver, Tm = −261.759 + [0.661750×10−4 (P + 99.085)]½ × 6617.50 (Tm = −194.145 + [0.668583 × 10−4(P + 89.347)]½ × 6685.83); and copper, Tm = 129.171 + [0.134011×10−3(P + 122.305)]½ × 134.011 (Tm = 199.265 + [0.135218×10−3(P + 105.960)]½ × 135.218). The precision of the pressure measurements at the 90% confidence level is 0.3±0.5 kbar. The precision in the temperature is ±5°C. A revision of a previously made suggestion for a high‐pressure calibration scale based on the melting temperatures of pure elements is proposed.
Volcano‐ice interactions on Mars
Authors: Allen Carlton C.
Published: February 2008

Central volcanic eruptions beneath terrestrial glaciers have built steep‐sided, flat‐topped mountains composed of pillow lava, glassy tuff, capping flows, and cones of basalt. Subglacial fissure eruptions produced ridges of similar composition. In some places the products from a number of subglacial vents have combined to form widespread deposits. The morphologies of these subglacial volcanoes are distinctive enough to allow their recognition at the resolutions characteristic of Viking orbiter imagery. Analogs to terrestrial subglacial volcanoes have been identified on the northern plains and near the south polar cap of Mars. The polar feature provides probable evidence of volcanic eruptions beneath polar ice. A mixed unit of rock and ice is postulated to have overlain portions of the northern plains, with eruptions into this ground ice having produced mountains and ridges analogous to those in Iceland. Subsequent breakdown of this unit due to ice melting revealed the volcanic features. Estimated heights of these landforms indicate that the ice‐rich unit once ranged from approximately 100 to 1200 m thick.
Morphology and distribution of common ‘sand’ dunes on Mars: Comparison with the Earth
Authors: Breed Carol S. , Grolier Maurice J. , McCauley John F.
Published: February 2008

Massed crescentic ridges are the most common dunes in the north circumpolar erg and crater floor dune fields on Mars; they are similar in plan to dunes that are typical of many desert basin ergs and dune fields on Earth. This correspondence implies that the dynamics of dune formation are similar on the two planets, despite martian constraints on dune formation that include much higher velocity winds required to move ‘sand’ in saltation, the possible inhibition of sand movement by absorbed water vapor and the seasonal ‘snow’ cover in the north circumpolar erg, and a probably sparse sand supply. Scale ratios derived from measurements of martian dunes at latitude 72°N, longitude 53° and latitude 47.5°S, longitude 331° are almost identical to scale ratios for much smaller gypsum dunes at White Sands, New Mexico. Isolated, scattered barchan dunes probably show thinning of dune sediments toward the southern margin of the north circumpolar erg on Mars, indicating a diminished sand supply and/or deflation of parts of the region by high speed winds. A complex dune formed by the apparent collision of barchans on Mars suggests that dunes of different sizes there, as on Earth, migrate at different rates. The present activity of the martian dunes, however, is not known, and they may be at least seasonally inactive.

Uniform dune patterns in areas of ‘sand’ accumulation on Mars, as on Earth, reflect the long‐term adjustment of eolian bedforms to wind regimes, sand supply, and topographic variations. Boundary ridges of sand that delineate the margins of many topographically controlled dune fields and ergs on Earth also occur on Mars. Complex aklé patterns on Mars, as on Earth, probably reflect locally complex wind regimes. Wind erosion grooves in some of the martian dunes, and the apparent superposition of some dune patterns in the north circumpolar erg may record changes in the eolian regime there. Preservation of these features implies condsiderable cohesion of the dune sands. Large through‐going longitudinal dune belts, common on Earth, are not seen on Mars. The absence of ‘sand‐passing’ dune of this type and the restriction of ‘sand‐trapping,’ massive crescentic dunes to a few sites on Mars, suggest that much of the available sand has been removed from the martian plains. Mars may have a long eolian history in which much of the sand suitable for saltation has already been transported to the north polar erg and crater floor fields. On Earth the formation of ‘mature’ ergs is estimated to require 2×106 years or more. By analogy, the massive dunes on Mars probably required as much or more time to accumulate. The age of their formation is not known; it may extend to periods when climatic conditions were possibly more earthlike than at present.
Optical properties of solid CO2: Application to Mars
Authors: Ditteon Richard , Kieffer Hugh H.
Published: February 2008

Using a special low‐temperature cell, thick layers of solid CO2 are grown on an infrared transparent window. The transmission spectra of solid CO2 is measured from 4000 to 360 cm−1 (2.5–27.8 μm). The complex index of refraction is derived for samples of different thicknesses. Scattering in the samples is the major source of error. Away from the major absorption bands the imaginary index is very low. The absorption coefficient is of the order of 1 cm−1. The emissivity in the Viking IRTM 20‐μm band of a solid surface of CO2 is estimated at
. Snowlike deposits have similar or lower emissivity. The results of this paper relieve the need for exceptional meteorology to explain Martian brightness temperatures below the CO2 frost point.
Viking radio occultation measurements of the atmosphere and topography of Mars: Data acquired during 1 Martian year of tracking
Authors: Lindal Gunnar F. , Hotz Henry B. , Sweetnam Donald N. , Shippony Zvi , Brenkle Joseph P. , Hartsell Gene V. , Spear Richard T. , Michael William H.
Published: February 2008

The S (2.3 GHz) and X band (8.4 GHz) tracking links with the Viking orbiters have been used to study the atmosphere and topography of Mars at latitudes ranging from 74°S to 73°N. Data acquired in the troposphere show large meteorological changes with near‐surface temperatures ranging from 150° to 250°K. Inversion layers were observed above the polar caps and in areas engulfed by dust storms. At other locations the temperature was found to decrease with increasing height at a rate equal to the dry adiabatic lapse rate. Seasonal pressure variations, presumably caused by changes in the polar frost deposits, were observed. At the 5‐km altitude level the atmospheric pressure ranged from about 3.5 to 4.8 mbar during the Martian year. The measurements in the upper atmosphere yielded double‐ and single‐layered electron density profiles on the sunlit and dark sides of the planet, respectively. A comparison of the Viking occultation data with earlier Mariner measurements has revealed that the temperature and plasma scale height of the ionosphere appear to be functions of solar activity. The topographic occultation data agree well with the elevation contours that are shown on U.S. Geological Survey map M 25M 3 RMC except in a few areas such as the south polar region and the Alba Patera region of Mars. In the south polar region the occultation measurements yielded surface elevations ranging from 3 to 6 km relative to the reference gravity equipotential surface or reference areoid. The terrain around Alba Patera was found to be 6 to 7 km high.
The motion of the ground during an earthquake: 2. The case of a dip slip fault
Authors: Bouchon Michel
Published: February 2008

We present a theoretical study of the ground motion produced by shallow dip slip faults near the source. The earthquake is modeled as a finite three‐dimensional dislocation embedded in a layered medium, and the resulting surface displacement is computed at more than a thousand locations distributed all over the source region. By combining together all the seismograms obtained, the time history and spatial dependence of the strong ground motion can be inferred. Calculations are carried out by using for the source geometry the fault plane obtained for the San Fernando earthquake. Various crustal structures are considered. When the medium departs from a homogeneous half space, the ground displacement exhibits a high degree of complexity. This complexity increases when low‐velocity layers are present. Two of the major findings of the study are the striking directivity effect of the propagating rupture on the surface motion and the large amplification of horizontal motions by sedimentary layers.
Reference point equalization method for determining the source and path effects of surface waves
Authors: Patton Howard
Published: February 2008

A reference point equalization method is presented for the determination of source and propagation effects of surface waves. The method works on seismic events located in a small source region, allowing the assumption that all events share the same path effects to a given receiver. Two steps in the method are initialization and iteration. Initialization obtains the first reference events in order to compute initial estimates of phase velocity and attenuation coefficient. Iteration simultaneously refines the propagation parameters and determines source parameters of new earthquakes. This method was applied to nine earthquakes in the Pamir Mountains, Central Asia (reference point: 39.58°N, 73.55°E). Source parameters were determined using the linear moment tensor inversion on 26 to 60‐s Rayleigh wave complex spectra. It was necessary to modify the straight least squares inversion method because of its sensitivity to even just a few bad data points in the data set. Residuals obtained from repeated application of the moment tensor inversion over trial focal depths showed two minima: one at depths less than 20 km and the other at depths greater than 70 km, with values of the residuals at these minima close enough to cast doubt on the determination of focal depth. The inversion generally gave three‐couple force systems having significant nonzero intermediate component. These features of source parameter inversions are discussed in light of errors in the data set. Propagation parameters, in particular, the phase velocities, show good qualitative correlation with geologic and geographic features on the Eurasian continent.
Geochemistry of Chilean ophiolites: Evidence for the compositional evolution of the mantle source of back‐arc basin basalts
Authors: Stern Charles R.
Published: February 2008

Basalts within mafic complexes in southern Chile, interpreted as remnants of the mafic portion of the floor of an Early Cretaceous extensional back‐arc basin, exhibit a tholeiitic differentiation trend. The least differentiated basalts are olivine normative. Both Chilean basalts and the dominant basalt type sampled from present‐day back‐arc basins resemble, with respect to FeO*/MgO, Ni, and Cr, ocean ridge rather than island arc tholeiites. A wide range of K/Rb, Rb/Sr, and (Ce/Yb)Nis exhibited by Chilean basalts as well as basalts from present‐day back‐arc basins. The observed variations in K/Rb, Rb/Sr, and (Ce/Yb)Nin Chilean basalts are interpreted as secular compositional changes. Relatively low K/Rb ≈ 250, high Rb/Sr ≈ 0.075, and high (Ce/Yb)N≈ 1.7 basalts were emplaced during the early stages of development, and relatively high K/Rb ≈ 850, low Rb/Sr ≈ 0.020, and low (Ce/Yb)N≈ 0.6 basalts, compositionally indistinguishable from ‘normal’ ridge segment oceanic tholeiites, were emplaced during later stages of development of the back‐arc basin. The observed differences in K/Rb, Rb/Sr and (Ce/Yb)Nfor basalts of similar FeO*/MgO could not have resulted from different depths, degrees or single‐stage processes of melting of a chemically homogeneous mantle. Instead, these differences must reflect compositional differences in the mantle source region, as has also been inferred for ‘normal’ and ‘plume’ segment ocean ridge tholeiites. Secular variations in the composition of the mantle source may result from sequential tapping of different depths of a chemically layered mantle but may also result from progressive depletion in large‐ion‐lithophile elements of an initially homogeneous mantle due to removal of successive partial melt fractions. The data suggest that a limited volume of mantle source material which evolves compositionally by progressive partial melting and magma extraction is an appropriate model for the source regions of back‐arc basin basalts.
Whole planet cooling and the radiogenic heat source contents of the Earth and Moon
Authors: Schubert Gerald , Stevenson David , Cassen Patrick
Published: February 2008

It is widely believed that the surface heat flows of the earth and moon provide good measures of the total amounts of radioactives in these bodies. Simple thermal evolution models, based on subsolidus whole mantle convection, indicate that this may not be the case. These models have been constructed assuming an initially hot state, but with a wide variety of choices for the parameters characterizing the rheology and convective vigor. All models are constrained to be consistent with present‐day surface heat fluxes, and many of the terrestrial models are consistent with the mantle viscosities indicated by post‐glacial rebound. For the earth the acceptable models give a radiogenic heat production that is only 65–85% of the surface heat output, the difference being due to secular cooling of the earth (about 50°–100°C per 109 years in the upper mantle). It is argued that the actual heat generation may be substantially less, since the models omit core heat, upward migration of heat sources, possible layering of the mantle, and deviations from steady convection. Geochemical models which are near to chondritic (apart from potassium depletion) are marginally consistent with surface heat flow. In the lunar models, heat generation is typically only 70–80% of the surface heat flow, even with allowance for the strong near‐surface enhancement of radioactives. Despite the simplicity of the models the persistence of a significant difference between heat generation and heat output for a wide range of parameter choices indicates that this difference is real and should be incorporated in geochemical modeling of the planets.
A surface wave dispersion study of the crustal and upper mantle structure of China
Authors: Pines Ira , Teng Ta‐Liang , Rosenthal Robert , Alexander Shelton
Published: February 2008

Repeated surface wave group velocity measurements over the same paths were made by using large aftershocks of several great (M>7) earthquakes recently occurred in China. Multiple‐filter technique was applied to the properly rotated three‐component digital data from Seismological Research Observatory stations, so that both Rayleigh and Love wave dispersion data were obtained over a number of paths crossing various tectonic provinces of China. In several cases, higher‐mode data were also derived. An estimate of uncertainty of these dispersion data was obtained from the repeated measurements with identical source‐receiver geometry. The generalized surface wave inversion technique was applied to these multimode dispersion data, and crustal and upper mantle structures were derived for various tectonic provinces of China. The results clearly demonstrate that the Chinese mainland is far from being laterally homogeneous, the lateral heterogeneities closely reflect the tectonic developments in the recent past. A particularly unusual crustal and upper mantle structure is found underlying the Tibet plateau.
Geophysical parameters of the Earth‐Moon System
Authors: Ferrari A. J. , Sinclair W. S. , Sjogren W. L. , Williams J. G. , Yoder C. F.
Published: February 2008

Doppler tracking data from Lunar Orbiter 4 have been combined with laser ranging data from lunar retroreflectors to yield a number of geophysical and geodetic parameters for the earth and moon. This joint solution gives values of (1) the lunar principal polar moment C/MR2 = 0.3905 ± 0.0023, (2) GME = 398600.461 ± 0.026 km3/s2, and (3) an earth/moon mass ratio at 81.300587±0.000049. Also determined are the harmonics of a complete lunar gravity field through degree and order 5, the obliquity of the lunar pole, selenocentric coordinates of the lunar retroreflectors, geocentric coordinates of the McDonald Observatory, and the lunar secular acceleration. The lunar potential Love number is weakly determined at 0.022±0.013, and a surprisingly large dissipation of rotational energy is inferred, though either solid body tidal dissipation or liquid core mantle interactions could be causes.
A study of amplitude anomaly and mb bias at Lasa Subarrays
Authors: Chang Andre C. , von Seggern David H.
Published: February 2008

Large‐aperture seismic array (LASA) subarray amplitude anomalies are investigated using 395 medium sized events, distributed in ten azimuthally divided sectors. Although LASA magnitudes, when averaged over all azimuths, are only slightly biased in relation to National Earthquake Information Services (NEIS) magnitudes, the amount of bias varies with azimuth and subarray, suggesting that a simple station correction for mb bias is not adequate. In addition, fluctuations among LASA subarrays are about 0.15 in standard deviation even when these magnitudes are calibrated in sectors. Details of such fluctuations are explained in part by local crustal and upper mantle heterogeneities under LASA. The amplitude anomalies are linearly related to the amount of travel time anomalies in each sector, implying that both effects are due to crustal focusing. Using a fixed effects model, the authors attempt to separate the cause of mb bias into sector (azimuth) effect, subarray effect, and subarray‐sector interaction. However, even this detailed modeling could not, with confidence, explain mb bias. Using the reciprocity principle, even with a well‐calibrated station, a factor of 2 in uncertainty results for predictions of a stations's amplitude for an event only 50 km away from a calibration event in a region as complex as LASA. The event magnitude uncertainty would probably be reduced by network averaging, or by a nonstatistical detailed crust and mantle structure which could be analyzed by ray tracing to remove receiver effects.
Geophysical evidence of shallow nearshore permafrost, Prudhoe Bay, Alaska
Authors: Rogers James C. , Morack John L.
Published: February 2008

Marine seismic refraction data taken in and near Prudhoe Bay, Alaska, are presented. The higher compressional sound wave velocity in ice‐bonded materials as compared to unbonded materials was used to locate areas of subsea permafrost and depths to the top of the permafrost. These data are compared favorably with similar information from available drilling data. A map is presented which shows the extent of permafrost within 40 m of the ocean surface along approximately 40 km of the Beaufort Sea coast. The permafrost surface is shown to vary irregularly in depth without regard to the distance from shore. Large areas without shallow permafrost are attributed to possible paleo‐river‐valleys and to the lack of clays which may have been removed in these valleys.
On the internal structure of a moon of fission origin
Authors: Binder Alan B.
Published: February 2008

Internal structure models of the moon, based in part on petrological and thermal models of a moon of fission origin, indicate that the moon has an iron or iron‐rich core with a radius between 200 and 400 km. The derived STP density of the lower mantle3(>200‐km depths) is between 3.35 and 3.51 g/cm3 and indicates that the Mg' of the lower mantle is between 73 and 88. This result, together with the Mg' of the upper mantle (≃70) and of the crust (≃70), leads to Mg' between 72 and 84 for the bulk moon (minus the core). The Vp in the mantle of the moon is found to be between 7.6 and 7.9 km/s; this result is in general agreement with that derived from seismic data.
Frequency dependence of shear wave internal friction in the continental crust of eastern North America
Authors: Mitchell Brian J.
Published: February 2008

Fundamental‐mode Rayleigh wave attenuation data for eastern North America can be explained by a frequency‐independent model of shear wave internal friction (Qβ−1) as well as by numerous frequency‐dependent models of the crust. Assuming that Qβ−1 (ω, z) = C(z)ω−ζ, fundamental‐mode Rayleigh wave data were inverted to obtain several frequency‐dependent Qβ−1 models corresponding to different distributions of ζ. Limiting values of ζ were then sought by computing attenuation coefficients for the higher Rayleigh modes for the various models. The calculated attenuation coefficients were compared to reported values for Lg and the first higher Rayleigh mode as observed at periods between 1 and 10 s in eastern North America. If it is assumed that ζ is constant over the entire period range between 1 and 40 s, then its most likely value lies between 0.3 and 0.5. The frequency‐independent case (ζ = 0.0) can definitely be rejected. Better fits to the data can be achieved by allowing ζ to vary with period, increasing relatively rapidly with decreasing period at periods of 4 and less. It is possible that a peak in Qβ−1 occurs at periods between 1 and 4 s. Additional higher‐mode data will be needed in that range, however, before the existence of such a peak can be verified or rejected. All Qβ−1 models resulting from the inversion process include a low‐Q zone with a base at midcrustal depths. The low‐Q zone is, however, marginally resolvable for only some of the frequency‐dependent models of this study.
Paleomagnetic data and pelagic sediment facies and the motion of the Pacific Plate relative to the spin axis since the Late Cretaceous
Authors: Suárez Gerardo , Molnar Peter
Published: February 2008

In order to study the motion of the Pacific plate with respect to the spin axis during the Cenozoic and Upper Cretaceous we combined an analysis of the distribution of pelagic sediments on the sea floor with paleomagnetic data collected on the Pacific plate. The facies distribution of sediments is used to determine the time when an equatorial crossing took place at as many sites as possible. By combining these two independent data sets, bounds can be placed on the relative motion of the ‘hot spots’ with respect to the spin axis. The data strongly suggest that the Hawaiian hot spot has remained close to its present latitude for the last 70 m.y. with an uncertainty in the rate of latitudinal motion of about 1 cm/yr. A set of continental paleomagnetic data was selected on the basis of the number of samples used, the degree of secondary demagnetization, the uncertainty of the mean direction of magnetization, and the tectonic stability of the region. These data are used to obtain independently the apparent polar wander curve of the Pacific plate. If the plates were rigid and the finite rotations were not grossly in error, reliable continental paleomagnetic poles rotated relative to the Pacific plate would yield a pole path consistent with the sediment facies data and with Pacific paleomagnetic data. Whereas the data from the Pacific plate indicate a latitudinal motion of almost 30° during the last 70 m.y., the rotated paleomagnetic poles from the continents cluster around the present‐day north pole and require essentially no latitudinal displacement of the plate during this same period of time. Several hypotheses are considered to explain this large discrepancy: a nondipolar behavior of the paleomagnetic field, insufficient quality of the existing Cenozoic paleomagnetic catalogue, or a large amount of internal deformation in the Antarctic or Pacific plate from the Upper Cretaceous to the Lower Oligocene.
Cambrian paleomagnetism of the Llano Uplift, Texas
Authors: Watts Doyle R. , Van der Voo Rob , Reeve Scott C.
Published: February 2008

Late Cambrian sandstones and limestones sampled from various members of the Riley and Wilberns formations of the Llano uplift show a progression of paleomagnetic pole positions as a function of age. The members, ages, and poles are the following for the Riley Formation: the Hickory Sandstone, Lower Dresbachian, 34°N, 145°E; the Cap Mountain Limestone, Dresbachian, 33°N, 140°E; the Lion Mountain Limestone, Upper Dresbachian, 24°N, 146°E. For, the Wilberns Formation they are the following: the Welge Sandstone/Morgan Creek Limestone, Lower and Middle Franconian, 24°N, 151°E; and the Point peak Shale, Upper Franconian, 6°N, 159°E. These poles are based on thermal, chemical, and alternating field demagnetizations and on vector analysis. Most directions are interpreted to be of reversed polarity, but the Cap Mountain Limestone also yielded normal polarity directions; all directions resided in hematite with blocking temperatures up to 680°C. Almost all Cambrian poles appear to fall in a broad streak between the equator at about 155°E (e.g., the poles from the Tapeats Sandstone and from the Late Cambrian Point Peak Shale of the Wilberns Formation) and about 60°N, 90°E, where Cambrian poles have been obtained by Elston and Bressler (1977) and French et al. (1977). Although, at the present time, partial to complete remagnetization or nondipole behavior of the geomagnetic field are adequate ad hoc hypotheses to explain some of the data, it is suggested that the simplest and most unifying hypothesis to explain all data involves a Cambrian loop of apparent polar wander with respect to North America. This loop occurs before the middle Late Cambrian, with the poles from the Llano uplift falling on the return track.
Paleomagnetic investigation of the Cambrian Waynesboro and Rome Formations of the Valley and Ridge Province of the Appalachian Mountains
Authors: Watts Doyle R. , Van der Voo Rob , French Rowland B.
Published: February 2008

The Lower to lower Middle Cambrian Waynesboro and Rome formations yield paleomagnetic directions with east‐southeast to southeast declinations and shallow positive inclinations; the corresponding pole positions fall at 28°N, 165°E (dp = 11°, dm = 22°) and 39°N, 142°E (dp = 6°, dm = 12°), respectively. The sampling concentrated on red sandstones and siltstones and crossed several styles of deformation and changes in structural trend in the central Appalachians. Thermal and alternating field demagnetizations were analyzed with the aid of vector diagrams and the combined results of the two formations yield a positive fold test at the 99% confidence level. Since the mean directions from these formations deviate substantially in a clockwise sense from directions predicted by previously published paleomagnetic poles obtained from Cambrian rocks, the possibility of local rotations of the sampling sites has been considered. Two arguments, however, can be raised against such rotations. First, there is no geological evidence for rotations of the Tennessee thrust sheets relative to one another or relative to the craton. Second, there are directions from Ordovician and late Precambrian‐Early Cambrian formations in the Tennessee thrust sheets that do not deviate in a clockwise sense from those predicted by results from the craton. Remagnetization and nondipole behavior of the Cambrian geomagnetic field have been suggested as alternative explanations for the North American Lower and Middle Cambrian paleomagnetic poles but are considered unlikely causes for some of the new results from this and a companion paper. Instead, Cambrian apparent polar wander is proposed to explain the distribution of the pole positions.
The use of mechanical twinning in minerals as a measure of shear stress magnitudes
Authors: Tullis Terry E.
Published: February 2008

Mechanical twinning of minerals is a deformation process that occurs to a first order under a critical resolved shear stress that is independent of temperature and strain rate. Thus unlike critical shear stresses for intracrystalline slip, extrapolations from laboratory measurements to the field situation are not necessary. The critical shear stress for twinning does not appear to be affected by hydrostatic pressure or by fluid pore pressure. Thus unlike the situation when coefficients of friction are used to infer stresses on faults, there need be no uncertainty in the shear stresses that caused twinning in nature, even if the depth of burial or the fluid pressure is unknown. Calcite, dolomite, and clinopyroxene are the rock‐forming minerals with the greatest promise for the use of this technique to determine the magnitudes of paleostresses. The critical shear stresses for calcite, dolomite, and clinopyroxene are about 100, 1000, and 1400 bars, respectively. These stresses are in an interesting range for helping to resolve controversies concerning the magnitudes of stresses accompanying faulting. The method has not yet been used extensively, but in separate previous studies, differential stresses as high as 1250 and 2800 bars appear to have accompanied thrust faulting.
Estimates of stress differences in the crust from isostatic considerations
Authors: Lambeck Kurt
Published: February 2008

Estimates of the stresses in the crust can be obtained from considerations of isostasy, and insofar as the earth's topography is generally near a state of isostatic compensation, these values reflect the strength of the crust subjected to loads on a geological time scale. While gravity is a poor indicator of the stress state in the crust, some limits can be established. Lower limits can be found using Jeffreys' criterion that the compensation is such as to minimize the maximum stress differences that occur in the loaded crust. Analysis of several large gravity anomalies using this approach suggests an average value of about 500 bars, with maximum values up to 1.5 kbar for the Tibetan plateau. This approach is only valid if the region is free from contemporary and recent tectonics, which the latter is not. An upper limit to the maximum stress differences is given by modeling the features using elastic plate theory. Such models often indicate stress differences in excess of 5 kbar, but they can be significantly reduced; beneath the large seamount, for example, stress differences need not exceed 1 kbar. Thus the limits of maximum stress differences are about 500 to perhaps as high as 1–1.5 kbar, with the maximum stress differences being confined to the upper crust above 15–20 km in the continents and shallower in the oceans.