bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluctuation of the nondipole magnetic field and its implication for the process of geomagnetic polarity reversal in the Cox model
Authors: Honkura Yoshimori , Matsushima Masaki
Published: February 2008

In the Cox model on the mechanism of polarity reversal of the Earth's magnetic field, the nondipole magnetic field is assumed to behave differently from the dipole magnetic field; the former fluctuates at random with a time scale of the order of 1000 years, whereas the latter varies systematically with a longer time scale. Our analyses of Gauss coefficient data for various epochs show that the characteristic time of nondipole fluctuation is about 500 years, while paleomagnetic studies for lake sediments indicate 1000–2000 years. These estimates are comparable to the time scale for nondipole field fluctuation presumed in the Cox model. This result supports one aspect of the Cox model. It is also important to examine whether the Cox model actually presents a physically plausible mechanism of polarity reversal, in particular from the viewpoint of nonlinear dynamo action. In this respect, we first show that the Cox model is represented by a disk dynamo which is subject to magnetic perturbations exerted externally. Then we solve disk dynamo equations numerically and examine time evolution of the magnetic field of the disk dynamo with special attention to polarity reversals. We finally conclude that the mechanism of polarity reversal presented in the Cox model is basically operative. It is surprising, however, that excursionlike variations also appear as often as polarity reversals in this specific dynamo model. Detailed examination shows that the probability of magnetic field growth from the transitional state either in the previous direction (excursion) or in the opposite direction (reversal) is 1/2. We also conclude that the Cox model is capable of accounting for the exponential distribution of polarity intervals which has been found from paleomagnetic data.
Complex behavior of the geomagnetic field during three successive polarity reversals, 11–12 m.y. B.P.
Authors: Laj Carlo , Guitton Sylvie , Kissel Catherine , Mazaud Alain
Published: February 2008

Detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of Serravallian‐Tortonian boundary age in the island of Zackinthos (Greece). The three reversals are characterized by a complicated geometrical and temporal structure. Periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity. Different episodes of collapse and recovery of the field intensity and the presence of “rebounds” have been documented. Overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper Tortonian marine clays in Crete, in the same geographical area. The bearing of these results on some of the published reversal models is discussed, and it is also suggested that the greater complexity of the Zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses.
Magnetic stratigraphy of the Siwalik group, Bhittani, Marwat and Khasor ranges, northwestern Pakistan and the timing of neocene tectonics of the Trans Indus
Authors: Khan M. Javed , Opdyke Neil D. , Tahirkheli R. A. K.
Published: February 2008

Nine stratigraphic sections in the Siwalik Group, three from the Bhittani Range, five from the Marwat Range, and one from the Khasor Range, were measured and sampled for magnetostratigraphic studies. Progressive alternating field and partial thermal demagnetization of samples indicated the presence of two components of magnetization. These are a secondary component of low coercivity, which can be removed by alternating field demagnetization in fields up to 40 mT and/or temperatures to 400°6, and a stable primary component of magnetization which has blocking temperatures above 450°C. Therefore all samples were subjected to blanket demagnetization at 450°C and 500°C, and some to temperatures up to 600°C. Positive fold tests from the Bhittani and Marwat ranges indicate the successful isolation of a prefolding characteristic direction of magnetization. A magnetic polarity reversal sequence for each of the sampled sections is determined and correlated to the magnetic polarity time scale utilizing a fission track date and fauna from the sequences which have previously been shown correlated to the magnetic polarity time scale. This correlation shows that the Siwalik Group in the area of study ranges in age from the late Gilbert chron to the early Brunhes chron (about 0.5 m.y. B.P.). Deposition of the thick sandstone units of the Siwalik Group in the Marwat Range began at least by Gauss chron time. The correlation of observed magnetic polarity stratigraphy, of the Khassor Range, with the standard magnetic polarity time scale shows that the Siwalik Group of this area ranges in age from the early Gauss chron (about 3.2 Ma) to the Brunhes chron. The region of this study began to subside prior to 4 Ma and subsided at a rate of from 60 to 100 cm per 1000 years. The tectonism of the region began within the last 1 m.y. and may well be synchronous with folding in the Pabi Hill anticline on the other limb of the syntaxis near Jelum. This orogeny is therefore one of the world's youngest and is undoubtedly still in progress.
Reflection study of the Haughton impact crater
Authors: Hajnal Z. , Scott D. , Robertson P. B.
Published: February 2008

The western flank of the Haughton impact crater was imaged with a 10.25‐km‐long multifold reflection profile. The seismic waves mapped 10 acoustic discontinuities in the stratified sedimentary target rocks. The impact disturbed the entire 1900 m of the highly competent Paleozoic strata. The average seismic velocity of over 6000 m/s exhibited by these rock units is a clear indication of the undisturbed impact area's unusually high elastic strength. The dynamic and destructive forces of the cratering process are manifested on the seismic profile by a dense and complex system of faults. These steeply dipping faults penetrated the sedimentary rocks but showed no recognizable extension into the crystalline basement. The recognition of normal faults 3 km west of the faulted annulus suggests a structural diameter of 24 km, some 4 km larger than postulated from earlier geological data. It has been found that the surface morphologically distinct ring zones do not have seismically recognizable subsurface signatures. In the central interior region of the crater, coherent reflection signals are lacking. However, some weak local acoustic discontinuities are recognized. The underlying crystalline basement rocks did not exhibit any seismically mappable impact related zones of alteration.
Prograde Phase Relations in the State 2–14 Well Metasandstones, Salton Sea Geothermal Field, California
Authors: Cho Moonsup , Liou J. G. , Bird Dennis K.
Published: February 2008

Seventy‐five core samples of medium‐ to fine‐grained metasandstones from the State 2–14 borehole were studied petrologically and mineralogically to evaluate compositional variations of geothermal minerals and prograde metamorphic reactions. Three distinct metamorphic zones are defined based on the appearance of index minerals. With increasing grade, they are (1) chlorite‐calcite (Chl‐Cc) zone at depths between 610 and 2480 m; (2) biotite (Bt) zone between 2480 and 3000 m; and (3) clinopyroxene (Cpx) zone between 3000 and 3180 m. All of the studied metasandstones at depths greater than 906 m contain epidote (Ep) and quartz (Qtz) with or without albite (Ab). Characteristic assemblages (+ Ep + Qtz ± Ab) are (1) Chl + Cc ± K‐feldspar (Ksp) ± phengitic muscovite for the Chl‐Cc zone; (2) Bt + Chl ± Ksp ± anhydrite (Anh) and Bt + Act ± Chl for the Bt zone; and (3) diopsidesalite + Act ± Bt ± oligoclase ± actinolitic hornblende for the Cpx zone. Epidote exhibits a compositional variation ranging from 0.11 to 0.42 XFe (= Fe/(Fe+Al)). Many amphibole grains of the clinopyroxene zone show patchy or lamellar intergrowths of both actinolite and actinolitic hornblende on a scale less than a few tens of micrometers. These amphiboles show distinct compositional discontinuities such as in Al2O3 contents (2.7 – 4.8 wt %) and Fe/(Fe+Mg) ratios (0.41 – 0.55). The Cpx zone samples thus mark the transition from the greenschist to amphibolite facies. Other important secondary phases include rutile or anatase (depths<1427 m; 2745 – 3020 m), aluminous titanite (>940 m), peristerite (1235 m), and Mn‐salite (2484 m). The observed assemblages and compositions of minerals are chemographically interpreted utilizing an epidote projection in the simple system, Al2O3 ‐ K2O ‐ MgO ‐ CaO ‐ SiO2 ‐ H2O ‐ CO2. Reactions for the appearance of index minerals are elucidated in a qualitative T ‐ XCO2 diagram. The results indicate that (l) the compositions of Chl and Bt are highly variable depending on the bulk composition; however, Chl in the Chl‐Cc zone becomes enriched in Mg with increasing depth; (2) Bt forms by a discontinuous reaction, Ksp + Chl + Cc = Bt + Ep + Qtz + H2O + CO2; hence the first appearance of Bt is greatly affected by the fluid composition; (3) the hydrothermal fluid in equilibrium with the analyzed assemblages is very low in XCO2 (<0.03); and (4) the absence of wairakite and prehnite is attributed to the high fO2 values of the Salton Sea geothermal system.
ABSENCE OF EVIDENCE FOR A SHALLOW MAGMA CHAMBER BENEATH LONG VALLEY CALDERA, CALIFORNIA, IN DOWNHOLE AND SURFACE SEISMOGRAMS
Authors: Hauksson Egill
Published: February 2008

A downhole seismometer at 900‐m depth and a temporary network of surface stations were deployed to use rays from local microearthquakes to study the upper and middle crust beneath the Long Valley caldera. The downhole seismograms show S waves with high apparent amplitudes from earthquakes located 2–20 km to the south of the downhole seismometer. In contrast, S waves from earthquakes located in the distance range 20–30 km to the south have low apparent amplitudes. If P and S amplitudes are normalized relative to the respective coda amplitudes, the S to coda amplitude ratios appear to remain constant but the P to coda amplitude ratios vary significantly with takeoff angle. A comparison of the calculated radiation patterns for a double couple in a uniform halfspace and focal mechanisms of the recorded earthquakes suggest that the observed variations in P and S amplitudes are caused by radiation pattern effects. Reanalysis of possible travel time delays found by Elbring and Rundle (1986), who used a subset of the borehole data analyzed in this study, shows that they underestimated the epicentral distances to three of the earthquakes and hence generated an artificial kink in the reduced travel time versus depth curve. Synthetic calculations of reduced travel time versus depth suggest that an apparent velocity of 5.7 km/s gives less scatter than 6.0 km/s used by Elbring and Rundle (1986). Plots of ts/tp versus depth show that contrary to the findings of Elbring and Rundle (1986), the Vp/Vs ratio stays fairly constant with depth and a small (<3‐km diameter) magma chamber cannot easily be resolved. Furthermore, combined analysis of downhole and surface data shows that neither data set requires a low‐velocity zone or a zone of anomalously high Vp/Vs at depth below the Casa Diablo area.
Introduction to Special Section on How Volcanoes Work: Part 3
Authors: Tilling Robert I.
Published: February 2008

The nine papers in this issue represent the third, and final, part of the special section on “How Volcanoes Work.” Part 1 of this special section was published in the December 1987 [Tilling, 1987] and part 2 in May 1988 [Tilling, 1988] all three parts will be published together as a separate volume titled “How Volcanoes Work” by the American Geophysical Union. In its entirety, the special section gives a good sampling of the nearly 300 papers presented at an international symposium of the same name held in Hilo, Hawaii, in January 1987 in commemoration of the Diamond Jubilee (75th Anniversary) of the founding of the Hawaiian Volcano Observatory [Wright and Decker, 1987].

The breadth of topics covered in all three parts of the special section (Table 1) amply attests to the multidisciplinary nature of modern studies of volcanic phenomena. Collectively, these studies also comprise a most fitting tribute to Thomas A. Jaggar, Jr., who founded the Hawaiian Volcano Observatory in 1912 and was a dominant force in quantifying the science of volcanology. Not only was Jaggar a scientific visionary, but he also stressed that the scientific knowledge on volcanoes must be applied to reduce death and destruction from volcanic hazards. It is clear from the papers contained in the special section of the Journal of Geophysical Research that great strides have been made in our scientific understanding of how volcanoes work since Jaggar's time. But the destructive eruptions at Mount St. Helens (United States, May 1980), E1 Chichón (Mexico, March–April 1982), and Nevado del Ruiz (Colombia, November 1985), each causing the worst volcanic disaster in the recorded history of each of these countries [Tilling and Newhall, 1987] are tragic reminders that commensurate advances in reducing volcanic risk on a global basis have not yet been achieved.
Existence of an S wave reflector near the upper plane of the double seismic zone beneath the Southern Kanto District, Japan
Authors: Obara Kazushige , Sato Haruo
Published: February 2008

The amplitude of coda from local shallow earthquakes in the southern Kanto district of Japan does not decay smoothly with time. A clear phase appears in the S coda at lapse time from 15 to 45 s measured from the S wave arrival, and it is predominant for frequencies around 8 Hz. This phase seems to be an S wave reflected from a discontinuity inclined westward at upper mantle depths since it is predominantly on the horizontal components and propagated westward with a high apparent velocity. Applying an inversion method to the travel times of the phases read from 8‐Hz band seismograms, we locate the S wave reflector near the upper plane of the double seismic zone associated with the subducting Pacific plate. The S wave reflector is resolved east of the volcanic front at a depth of 80–100 km. If we put Qs−l = 10−3, the reflection coefficient is estimated to be around 0.5; however, the scatter is large. The S wave impedance contrast at the interface seems to be very large. It may suggest the existence of a zone containing many liquid bodies.
Paleomagnetism of late Quaternary marine sediments from Santa Catalina Basin, California continental borderland
Authors: Brandsma Dan , Lund Steve P. , Henyey Thomas L.
Published: February 2008

Paleomagnetic secular variation (PSV) records have been recovered from three marine sediment cores from Santa Catalina basin, California continental borderland, in order to more accurately date these late Quaternary sediments. The PSV records are not high in resolution due to low sedimentation rates coupled with a 3‐cm sampling interval and some inherent smoothing of the PSV signal during remanence lock‐in. However, the PSV waveforms are sufficiently clear to permit their correlation among the three separate cores. These PSV records can also be correlated with four independent PSV calibration curves from western North America that have detailed radiocarbon age control. The four calibration curves are developed in this paper to improve the dating and regional comparison of PSV records from western North America. The PSV correlations establish time‐depth curves for the three cores which indicate that the sediments are all younger than about 11,000 years B.P. The relative accuracy of the time‐depth curves is approximately 200 years, which represents an order‐of‐magnitude improvement in the chronology of these sediments. Sedimentation rates derived from the three time‐depth curves indicate a constant rate of 20–25 cm/kyr for the last 6700 years throughout Santa Catalina basin, and more variable rates (but constant within each core) of 13–86 cm/kyr prior to 6700 years B.P. In all three cores, the change in sedimentation rate corresponds to a subtle but distinct change in lithology. These changes probably indicate a major shift in paleoceanographic processes within Santa Catalina basin 6700 years B.P.
A Morphological method of geometric analysis of images: Application to the gravity anomalies in the Indian Ocean
Authors: Martel C. , Flouzat G. , Souriau A. , Safa F.
Published: February 2008

This paper presents a geometric approach to the processing of two‐dimensional geophysical data. The study has been carried out as an alternative to harmonic analysis, and it uses the properties of an image analysis method: mathematical morphology. Some examples illustrate the most interesting characteristics of the morphological approach which is then applied to the analysis of the gravity anomalies in the Indian Ocean. Two kinds of results have been obtained. From a methodological point of view, morphological analysis shows the interest of a method that does not need either a priori information on the spatial characteristics of the objects under study or global processings needing long computer times. From the geophysical point of view, the extraction and characterization of the undulations allow one to retrieve the geophysical phenomena recently discovered in this region and to reveal some features and structures that the harmonic analysis methods have not yet detected.
Flexure and isostatic residual gravity of the Sierra Nevada
Authors: Kennelly Patrick J. , Chase Clement G.
Published: February 2008

The Sierra Nevada of California displays an isostatic gravity anomaly of +35 mGal over the western foothills and −45 mGal at the crest of the range. A flexural isostatic model designed to explain the rapid Neogene uplift of the Sierra Nevada is consistent with these observations and can explain a portion of both gravity anomalies by contributions from Moho depths. The present‐day crustal root causing regional compensation in this model differs from the simple local Airy compensation assumed for the Sierra in calculating the isostatic residual gravity. Two‐dimensional gravity modeling indicates that this difference results in a residual gravity high of +9 mGals over the western foothills, which are buoyed up by the excess root to the east, and a low of −16 mGals over the crest of the Sierra. The remainder of the isostatic gravity anomaly is attributed to upper crustal density contrasts because high‐density rocks underlie the foothills, and a low‐density batholith forms the crest. Any model for the isostatic residual gravity must assume, implicitly or explicitly, a flexural response of the lithosphere. With the stiff Sierran lithosphere, the isostatic deflection of the Moho due to the upper crustal bodies will be small. If a similar gravity model were to assign no stiffness to the lithosphere (local Airy compensation), the mass anomalies of the upper crustal bodies would necessitate a change in depth of the Moho to achieve compensation. This deflection of the Moho would significantly reduce the amplitude of the anomaly produced by the upper crustal block. For the unbroken, pre‐uplift case of the Sierra Nevada of 10 Ma, our model predicts an isostatic gravity low over the range crest of approximately −80 mGals and a flanking high of +55 mGals. These values are larger than would be predicted for a more typical eroded magmatic arc that does not have abnormally low heat flow and consequent high flexural rigidity.
Splitting of normal modes and travel time anomalies due to the magnetic field of the Earth
Authors: Tanimoto Toshiro
Published: February 2008

We examine the effects of a toroidal magnetic field on two types of seismic data: first, on splitting of normal modes and, second, on body wave travel times. The main contribution of this paper is the derivation of formulas for these problems. Comparison between data and theory indicates that the magnitude of the toroidal magnetic field (T20) has to be as large as 250 T (tesla) to explain the data. There are, however, some inconsistencies among the fit to the data set. For example, some modes require a strong magnetic field near the core mantle boundary, while other modes require a weak field. Also the travel time difference between polar and equatorial paths is not explained by the T10 or T20 field. A field with odd angular degree 𝓁 (T𝓁0 or S𝓁0) is needed to produce the travel time difference between the polar and equatorial paths. We also point out that ohmic dissipation due to a strong magnetic field with 100 T is about 4–5 orders of magnitude larger than the present heat flow output of the Earth. The analysis suggests that the magnetic field in the outer core is not the cause of seismic anomalies.
The recurrence time of dislocation events on the antitethic margins of an elastic plate welded to a Maxwell viscoelastic half‐space
Authors: Bonafede Maurizio
Published: February 2008

The dynamics of an elastic plate subject to a “tectonic” stress field and controlled by finite strength boundaries is studied, following the stress relaxation in a Maxwell asthenosphere due to a long sequence of dislocation events, localized on two antitethic transform boundaries. The model is plane bidimensional, and each dislocation event, cutting the whole strike of the boundary, is interpreted as a sequence of “earthquakes.” The time of each dislocation event is determined as the time when the overall stress field reaches given threshold values on each boundary. If two boundaries only are considered, a stationary dislocation cycle is attained only if events on the two boundaries release opposite stress drops; if this is not the case, very slow diffusion of stress takes place in the medium, well beyond the two boundaries, and a stationary regime is not achieved even after a time lag greater than the age of the Earth. A periodic boundary model is then devised, which avoids introducing plates with infinite width. In this second model, stationary dislocation cycles are obtained even if the strength values of the antitethic boundaries are different from each other. From these simulations of the earthquake cycles, inference could be drawn, in principle, on tectonic parameters, and a formal equivalence can be stated between an elastic lithosphere affected by boundary earthquakes and a viscoelastic continuum lithosphere, subjected to the same tectonic stress field. Even if this plate model cannot be applied to any real plate of the Earth, nevertheless, the role of the finite widths of plates and of stress diffusion from very distant boundaries is proven to be important in modeling the seismic cycle and the deformation of plates. However, the extreme sensitivity of the model from very distant features and the trade‐off existing between the long‐time and the long‐distance behavior imply that horizontally stratified, two‐dimensional dislocation models cannot be profitably employed to simulate the seismic cycle in realistic situations.
Spatial patterns of aftershocks of shallow focus earthquakes in California and implications for deep focus earthquakes
Authors: Michael Andrew J.
Published: February 2008

Previous workers have pioneered statistical techniques to study the spatial distribution of aftershocks with respect to the focal mechanism of the main shock. Application of these techniques to deep focus earthquakes failed to show clustering of aftershocks near the nodal planes of the main shocks. To better understand the behavior of these statistics, this study applies them to the aftershocks of six large shallow focus earthquakes in California (August 6, 1979, Coyote Lake; May 2, 1983, Coalinga; April 24, 1984, Morgan Hill; August 4, 1985, Kettleman Hills; July 8, 1986, North Palm Springs; and October 1, 1987, Whittier Narrows). The large number of aftershocks accurately located by dense local networks allows us to treat these aftershock sequences individually instead of combining them, as was done for the deep earthquakes. The results for individual sequences show significant clustering about the closest nodal plane and the strike direction for five of the sequences and about the presumed fault plane for all six sequences. This implies that the previously developed method does work properly. Nonrandom behavior was also found about the slip directions, the P axis, the T axis, and the B axis, but this is probably caused by the lack of independence between these axes and the previously mentioned features of the focal mechanisms. Given that the method does work and that deep aftershocks were not shown to cluster about the main shock nodal planes, the shallow focus data were used to simulate the deep focus study. The goal is to determine if there are artificial factors that make clustering in the deep focus data unobservable. To more closely mimic the work on deep earthquakes, the largest aftershocks from each of the six sequences were combined and studied with respect to their respective main shock focal mechanisms. This reduced the significance of the clustering about the focal mechanism parameters, but not below 95% confidence. Gaussian noise was then added to the aftershock hypocenters in order to determine if the larger hypocentral and focal mechanism errors in the deep focus data could account for the previous negative result. The conclusion is that the following reasons are sufficient to explain the lack of clustering about the main shock nodal planes for the deep focus aftershocks: the need to combine aftershocks from several sequences, the size of the hypocentral location and focal mechanism errors, and the alignment of distant aftershocks with the Wadati‐Benioff zone.
Electrical conduction in olivine
Authors: Schock Robert N. , Duba Alfred G. , Shankland Thomas J.
Published: February 2008

This paper reports detailed measurements of electrical conductivity σ and thermoelectric effect S in the mineral olivine and in synthetic forsterite as functions of temperature in the range from 1000° to 1500°C and oxygen partial pressure in the range from 10−10 to 104 Pa. The two most striking observations are strong conductivity anisotropy in forsterite and a sign change in S in olivine at 1390°C. These results are interpreted to show that both materials have mixed ionic and extrinsic electronic conduction under these conditions. On the basis of these interpretations, we infer that forsterite conductivity is dominated by electronic conduction in the a and b directions and probably by movement involving magnesium vacancies in the c direction, where far higher, PO2‐independent conductivity is observed. Olivine appears to show mixed conduction under all the circumstances observed; at low temperatures, electron holes dominate but are superseded by magnesium vacancies at high temperatures.
Introduction to the Special Section on the Southwestern Nevada Volcanic Field
Authors: Vogel Thomas A. , Byers F. M.
Published: February 2008

The southwestern Nevada volcanic field (SWNVF) is a classic area for studying the relationships among ash flow sheets, lava flows, calderas, and high‐silica magma evolution. Ash flow sheets are fundamental to our understanding of the evolution of high‐level silicic bodies because they represent a sample of a large portion of a magma body at an instant in time. This special section is a report of research conducted on four caldera complexes that form a large part of the southwestern Nevada volcanic field. These complexes erupted over a period of 16–6.5 m.y. ago, and their eruptive products cover an area about 160 km long by 110 km wide.
A model for oxygen isotope transport in hydrothermal systems
Authors: McKibbin Robert , Absar Ahsan
Published: February 2008

As groundwater passes through porous rocks, exchange of oxygen between the fluid and the solid matrix causes a change in the oxygen isotope concentrations in both water and rock. If the rate at which the exchange takes place can be estimated (as a function of the isotope concentrations and temperature), then the time taken for a rock/water system to come to equilibrium with respect to isotope concentration might be calculated. In this paper, conservation laws are used to derive equations for the isotope transport in a porous medium; these are combined with fluid flow equations and a simple equation describing the rate of oxygen isotope exchange between the water and the rock. The resulting set of equations is solved analytically for the zero‐dimensional (lumped‐parameter) model, and numerically, using typical parameters, for the one‐dimensional case.
Three‐dimensional structure of the upper mantle beneath the Atlantic Ocean inferred from long‐period Rayleigh waves: 1. Group and phase velocity distributions
Authors: Mocquet A. , Romanowicz B. , Montagner J. P.
Published: February 2008

Group and phase velocity distributions of Rayleigh wave fundamental modes are determined in the Atlantic Ocean using a method of regionalization without a priori constraints, for a period range from 50 to 200 s. Eighty‐six direct source‐station paths are studied, and 38 out of the best data from Weidner's (1974) phase velocities are added to our data set for periods lower than 100 s. The importance of shallow layer corrections is described in detail. The general features of the distributions agree with global models, and smaller‐scale heterogeneities are revealed. A spatial resolution of 1500 km is needed to detect fine lateral heterogeneities in the Atlantic. The small lateral extent of young oceanic regions and strong lateral velocity gradients do not allow to quantify velocity versus age relationships using our data set. On the other hand, the regionalization without a priori constraints of phase velocities confirms the presence of a high‐velocity anomaly beneath the central part of the Atlantic for periods greater than 100 s.
Application of internal gravitational field equations to geophysical measurement of G
Authors: Rubincam David Parry , Chao B. Fong , Schatten Kenneth H.
Published: February 2008

We present the spherical harmonic equation for the gravitational potential inside the Earth. The equation satisfies Poisson's equation and converges uniformly. It obviates the need for downward continuation of the exterior potential with its attendant convergence difficulties but of course requires some knowledge of the Earth's density distribution. Equipped with the equation, we derive the general expression for the geophysical measurement of the gravitational constant G made inside the Earth, such as in boreholes and mine shafts. We also present numerical evidence that the long‐ to intermediate‐wavelength gravity anomalies can masquerade as the “fifth force” if not properly corrected for.
Uncertainty estimates in geomagnetic field modeling
Authors: Langel R. A. , Estes R. H. , Sabaka T. J.
Published: February 2008

Geomagnetic field models usually consist of coefficients of a truncated spherical harmonic analysis derived by a weighted least squares analysis. The data are typically assumed to be uncorrelated, and an estimated, diagonal, data covariance matrix is incorporated as an inverse weighting function. Accuracy estimates on the derived coefficients are taken to be the usual output covariance matrix. This procedure does not take into account the field from the truncated terms, the presence of crustal fields, or the presence of external fields, such as Sq. The resulting accuracy estimates are believed to be too low by factors of 2–10. A formalism is presented within which approximate account can be taken of these neglected fields. This formalism includes a weight matrix in which the observations are suitably correlated. A priori statistics describing the truncated main field, the crustal field, and the Sq field are required. An estimate of the properties of the crustal field is made which is consistent with both surface and satellite data. The properties of the Sq field are inferred from the model of Malin [1973]. Given these estimates, computation of the elements of the weight matrix is straightforward. For surface data the weight matrix is adequately approximated as diagonal for moderate sized data sets and, in those cases, is easily incorporated into the least squares formalism. However, for satellite data and for large quantities of surface data, the matrix is “full,” and an approximation is adopted to make the analysis tractible. Models are derived using a selected subset of Magsat data, both with and without the correlated weight matrix. Inclusion of the correlated weight matrix increases the error estimates of the coefficients by a factor of up to 7.5. Calculation of such models using no approximations in the correlated weight matrix are limited by computer size to observation sets of about 1500 points or less. However, using the approximation, much larger data sets can be accommodated, and it becomes practical to perform the calculations on satellite data sets. The GSFC(12/83) model, based on Magsat data, was rederived using this method. The coefficient uncertainty estimates increased by factors of 4–70, while the coefficient values remained essentially the same.
An active thermoelastic rift mechanism
Authors: McMullen Richard J. , Mohraz Bijan
Published: February 2008

Finite element solutions for stresses and displacements during rift development are obtained for a variable thickness, elastic lithosphere plate subjected to thermoelastic loading by conductive heating from an axial line source in the asthenosphere. The asthenosphere is modelled as a fluid foundation. The prerift lithosphere is modelled as an infinite continuous plate. A semi‐infinite plate is used to characterize the rifted lithosphere. Horizontal restraints are removed at the rift axis to model the transition from a continuous to separated lithosphere plate during rifting. Lithosphere strength envelope curves derived from olivine flow laws are used to determine the elastic lithosphere depth in the solutions. Conductive heating of a variable thickness, continuous lithosphere plate by a 2000 W/m line source at 60 km depth produces high stresses, oriented orthogonal to the forming rift axis, and small vertical displacements. The maximum uplift obtained in the continuous lithosphere plate models subjected to thermoelastic loading was 0.12 km at a distance of 120 km from the forming rift axis. This small uplift is accompanied by shallow tensile stresses of 80 MPa at a distance of 5 km from forming rift axis which decrease to 6 MPa at a distance of 210 km. Shallow horizontal thermoelastic tensile stresses are over 5 times greater than the horizontal stresses produced by thermal buoyancy loads near the forming rift axis. The transition from a continuous to rifted lithosphere is accompanied by an increase in lithosphere uplift at the rift axis from 0.11 to 0.42 km and a reduction in shallow horizontal stress magnitude by a factor of 30. Thermoelastic loading may be a source of shallow tensile stress in the lithosphere and a contributing factor for the initiation of rifting. Thermal loading may also produce postrifting topography changes. Thermoelastic loading of the lithosphere is proposed as a possible driving mechanism for rift formation which produces separation and the transition from a continuous to rifted lithosphere plate.
Seismic properties of tuffs from the epithermal system of Creede, Colorado
Authors: Mayrand Louis J. , Fountain David M. , Smithson Scott B.
Published: February 2008

Ultrasonic P wave velocity, effective porosity, grain and bulk density, residual water saturation and petrography of 46 tuff samples from the epithermal system of Creede, Colorado, were analyzed with stepwise multiple curvilinear regressions to predict dry bulk density, P wave velocity (Vp), and acoustic impedance (I). Changes in porosity best predict variations of all three parameters; residual water saturation is significantly correlated with Vp and I for coarse‐grained matrix tuffs; and high aspect ratio cracks have a small but significant ability to predict Vp and I. The importance of porosity suggests that seismic and gravity work could be used to outline porosity‐related structures of crucial importance to our understanding of Creede's epithermal system. In areas where lithologies are comparable to our samples, seismic velocity or acoustic impedance measured remotely with seismic surveys may yield estimates of porosity and provide important quantitative information on rocks that cannot be studied directly through a comprehensive drilling program.
Introduction to special section on the Northern Chugach Mountains‐Southern Copper River Basin Segment of the Alaskan Transect
Authors: Page Robert A.
Published: February 2008

This special section of the Journal of Geophysical Research is the second part of a collection of papers on the nature and evolution of the lithosphere in southern Alaska, specifically beneath the Chugach Mountains and Copper River Basin. The studies in this collection were conducted under the Trans‐Alaska Lithosphere Investigation [Stone et al., 1986]: a coordinated geological and geophysical transect of the Alaskan lithosphere along the north‐south, trans‐Alaska oil pipeline corridor between Valdez and Prudhoe Bay and across the Pacific and Arctic continental margins. The Trans‐Alaska Crustal Transect (TACT) project of the U.S. Geological Survey was the primary source of support for these studies, including all the seismic refraction and reflection profiling and most of the geologic field investigations.

The first part of the collection appeared in the April 1989 issue and focused on the geology and tectonics of the northern Chugach Mountains and southern Copper River Basin and on the interpretation of a 107‐km‐long seismic reflection profile along the transect. Appended to those papers, but not included in the introduction [Page, 1989], was an interpretation of the upper crustal structure of the Chugach terrane derived from a transverse (east‐west) seismic refraction profile along the axis of the Chugach Mountains [Wolf and Levander, 1989].
Crustal structure of the Chugach Mountains, southern Alaska: A study of peg‐leg multiples from a low‐velocity zone
Authors: Flueh Ernst R. , Mooney Walter D. , Fuis Gary S. , Ambos Elisabeth L.
Published: February 2008

A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high‐velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg‐leg multiples generated from within a low‐velocity zone between the second and third upper crustal high‐velocity layers. The third high‐velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low‐velocity zone that produces a seismic shadow zone wherein peg‐leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high‐ and low‐velocity layers. Synthetic seismogram modeling indicates that a low‐velocity zone with transitional upper and lower boundaries generates peg‐leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long‐offset (90–225 km) data. This may be due to a lower crustal waveguide whose top is the high‐velocity (7.6 km/s) layer and whose base is the Moho. A deep (∼54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg‐leg multiples have been interpreted on some long‐range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg‐leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide‐angle reflection profiles than previously recognized.
Reflectivity and seismic properties of the deep continental crust
Authors: Christensen Nikolas I.
Published: February 2008

The recovery of drill core from a section of upper amphibolite facies terrane located in the Inner Piedmont of South Carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust Seismic reflection profiles over the Inner Piedmont in the vicinity of the drill hole reveal subhorizontal high‐amplitude reflections. The origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core. Synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamorphic layering, probably originating from lower crustal ductile flow. Fine tuning of the metamorphic layering, which is temperature and pressure related, can produce bright spots in the deep crust. Anisotropy, resulting from preferred mineral orientation, is another important property of this section. Velocities are low for vertical wave propagation, whereas horizontal velocities are relatively high. Horizontal velocities show only a slight variation with azimuth, which is unlikely to be detected using standard refraction techniques. Reflectivity decreases with increasing anisotropy. Critical to this study has been the continuous coring of a major section of high‐grade metamorphic terrane, which has provided the necessary stratigraphic and lithologic control for the generation of accurate synthetic seismograms.
Thermal stresses, differential subsidence, and flexure at oceanic fracture zones
Authors: Wessel Pål , Haxby William F.
Published: February 2008

Geosat geoid undulations over four Pacific fracture zones have been analyzed. After correcting for the isostatic thermal edge effect, the amplitudes of the residuals are shown to be proportional to the age offset. The shape of the residuals seems to broaden with increasing age. Both geoid anomalies and available ship bathymetry data suggest that slip must sometimes occur on the main fracture zone or secondary faults. Existing models for flexure at fracture zones cannot explain the observed anomalies. A combination model accounting for slip and including flexure from thermal stresses and differential subsidence is presented. Our model accounts for lateral variations in flexural rigidity from brittle and ductile yielding due to both thermal and flexural stresses and explains both the amplitudes and the shape of the anomalies along each fracture zone. The best fitting models have mechanical plate thicknesses that are described by the depth to the 600°–700°C isotherms.
The 40Ar/39Ar geochronology of the Pelona schist and related rocks, southern California
Authors: Jacobson Carl E.
Published: February 2008

Seventeen 40Ar/39Ar ages for hornblende, celadonitic muscovite, and biotite from the Pelona, Orocopia, Rand, and Portal Ridge (POR) schists range from 39 to 85 Ma. The 85 Ma is from Portal Ridge, consistent with the proposed correlation of that body with the schist of Sierra de Salinas. Two muscovites and one hornblende from the Rand Schist have ages of 72 to 74 Ma, indistinguishable from the K‐Ar age of 74 Ma for hornblende from a posttectonic granodiorite that intrudes the schist, but younger than the 79 Ma U‐Pb age of the intrusion. Four muscovite and two hornblende ages for schist and mylonite from the East Fork area of the San Gabriel Mountains range from 55 to 61 Ma, similar to K‐Ar ages from nearby upper plate rocks. A third hornblende has an age of 73 Ma but is not well constrained. Concordance of schist and upper plate ages confirms structural and metamorphic evidence that the Vincent thrust in the San Gabriel Mountains has not undergone significant postmetamorphic disruption. Ages from the Orocopia Mountains are 75 Ma for hornblende from nonmylonitic upper plate, 52 Ma for muscovite from structurally high Orocopia Schist that is mylonitic, and 41 Ma for muscovite from nonmylonitic Orocopia Schist. These are consistent with field evidence that the Orocopia “thrust” is a postmetamorphic normal fault. Muscovite and hornblende from the Gavilan Hills have ages of 48 to 50 Ma, younger than ages from the San Gabriel Mountains but similar to schist ages from the Orocopia Mountains. The geochronologic and structural complexities of the Vincent, Chocolate Mountains, Orocopia, and Rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism (subduction) of the POR schists. The 40Ar/39Ar data indicate substantial uplift of the POR schists prior to middle Tertiary detachment faulting, which confirms other geochronologic evidence of uplift in southern California and southern Arizona during the Late Cretaceous‐early Tertiary.
Boundary element‐image method approach to seismic modeling
Authors: DuBroff R. E.
Published: February 2008

The boundary element‐image method is proposed as a method for generating synthetic seismograms in a system of piecewise homogeneous layers. In contrast with many other methods (such as finite difference) in which the boundary conditions are represented simply by a change of material properties, the present approach uses the boundary conditions explicitly in order to develop a system of integral equations. As in the indirect boundary element method, the scattered seismic waves are presumed to result from the action of fictitious sources. However, as in the image method, these fictitious sources are located outside of the domain through which the scattered waves propagate. This combination of the image and boundary element methods thereby avoids introducing additional singularities (in the form of fictitious sources) into the wave equation for the scattered waves. Although this method can be formulated in a general way, specific computational advantages occur when the basis and testing functions are chosen as harmonically related complex sinusoids. Physically, this corresponds to a plane wave expansion of the scattered waves. Computationally, when considering piecewise linear interfaces, this results in a (possibly) large system of simultaneous algebraic equations having matrix elements which can be calculated analytically. This method is applied to the reflection of seismic waves from a V‐shaped reflector subjected to two different incident waves: a plane wave and a cylindrical wave created by a line source. The synthetic seismograms for both cases are presented (in the form of time sections) and then processed in an attempt to reconstruct the geometry of the reflector (in the form of a depth section).
The Martian polar cap: Radiative effects of ozone, clouds, and airborne dust
Authors: Lee Lindner Bernhard
Published: February 2008

The solar and thermal flux striking the polar cap of Mars is computed for various ozone, dust, and cloud abundances and for three solar zenith angles. Ozone does not significantly affect the total energy budget of the polar cap. Hence the observed hemispherical asymmetry in ozone abundance causes only an insignificant hemispherical asymmetry in the polar caps. Vertical optical depths of dust and cloud ranging from zero to 1 cause little change in the total flux absorbed by the polar cap near its edge but increase the absorbed flux significantly as one travels poleward. Hemispherical asymmetries in dust abundance, cloud cover, and surface pressure combine to cause a significant hemispherical asymmetry in the total flux absorbed by the residual polar caps, which helps to explain the dichotomy in the residual polar caps on Mars. Other processes which affect the energy budget of the polar cap are proposed and reviewed, particularly with respect to their interaction with the radiative effects of clouds and dust.