bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges
Authors: Magde Laura S. , Sparks David W.
Published: February 2004

In contrast to the along‐axis uniformity observed at the East Pacific Rise (EPR), crustal accretion at the Mid‐Atlantic Ridge (MAR) appears to be a highly complex and heterogeneous process. Besides spreading rate, one of the first‐order differences between the EPR and the MAR is the much higher degree of ridge segmentation observed in the Atlantic. Circular lows in the mantle Bouguer anomaly (MBA bull's‐eyes) are common at centers of spreading segments of the MAR, suggesting crustal thickness variations of up to 4 km along individual segments. We use a three‐dimensional numerical model of mantle flow to examine the effect of ridge segmentation on mantle upwelling and the resulting overall crustal production and along‐axis variations in crustal thickness. Mantle flow in our model is driven by both buoyant forces and segmented plate spreading. Various asthenospheric viscosity structures, plate spreading geometries, and mantle potential temperatures are explored. We find that a combination of buoyant mantle flow and three‐dimensional melt migration can reproduce crustal thickness variations similar to those inferred from gravity. Buoyant flow gives rise to variations in upwelling velocity at along‐axis wavelengths greater than 150 km but does not contribute to short‐wavelength variations. However, three‐dimensional melt migration may greatly enhance crustal thickness variations along all segments, independent of the wavelength of buoyant upwelling. We present an idealized model, in which melt first rises vertically and then flows along the base of the lithosphere toward the ridge axis, that easily produces crustal thickness variations greater than 4 km. The models also predict that the average crustal thickness should decrease with increasing amount of segmentation and decreasing spreading rate. Therefore the thinner, more heterogeneous crust observed at the MAR may result from the combined effects of slower spreading rate and more pervasive ridge segmentation.
Distribution and mechanism of Neogene to present‐day vertical axis rotations, Pacific‐Australian Plate Boundary Zone, South Island, New Zealand
Authors: Little Timothy A. , Roberts Andrew P.
Published: February 2004

Remarkably little knowledge exists about mechanisms of vertical axis rotation in continental crust. Steeply dipping basement rocks in South Island, New Zealand, provide an opportunity to map the distribution of rotations across the Pacific‐Australian plate boundary zone, and to delineate boundaries of rotated blocks in unusual detail. We synthesize new structural data with new and existing paleomagnetic data, with geodetic data, and with patterns of Neogene‐Quaternary faulting in the strike‐slip Marlborough fault system. For the past 20 m.y., vertical axis rotations have been hinged about two crustal‐scale boundaries near the east coast. The NE hinge accommodated ∼50° of early‐middle Miocene clockwise rotation, which caused deformation of the eastern ends of the Alpine‐Wairau and Clarence strike‐slip faults. The SW hinge has accommodated a further 30°–50° of finite clockwise rotation since ∼4 Ma and deflects active fault traces. The locus of rotation has shifted southwestward astride a subduction margin that is lengthening in that direction. Rotating rocks are pinned to the south against a locked collision zone where the continental Chatham Rise impinges against the margin. Slip on inland strike‐slip faults is transformed seaward across a zone of fault termination into rigid body rotation of a large continental block that has been thrust eastward over the downgoing subducted slab of the Pacific plate. The rotation mechanism is a “migrating hinge,” which resembles a flexed telephone book. Strike‐slip faults are translated through a brecciated hinge region that does not coincide with a fixed material line in the rock.
Numerical modeling of surface deformation and mechanical stability of Vesuvius volcano, Italy
Authors: Russo G. , Giberti G. , Sartoris G.
Published: February 2004

This study was undertaken with the aim of contributing to the risk evaluation of Vesuvius. We calculate the surface displacements due to an increase in pressure in a shallow reservoir and assess the mechanical instability of the volcanic edifice and of the feeding system caused by pressure on the reservoir's walls and by regional stresses. We consider axisymmetric models that take into account topography, gravity, homogeneous and heterogeneous country rocks, spheroidal magma chambers with different aspect ratios and variable depths, and subvertical intrusions from the top of the chamber to various depths. We impose both symmetric and asymmetric regional stresses increasing with depth as boundary conditions. The models are static. The ground deformation and the stress distribution are calculated, in the framework of linear elasticity, by a numerical finite element method. The surface displacements refer to an overpressure of 10 MPa. This value is considered an upper limit for the fracture of surrounding rocks. We find the maximum vertical displacements to be of a few centimeters in the most favorable case and the displacement gradients to be at the lower limit of measurability. To evaluate the mechanical stability, we calculate the stress distributions of a prolate ellipsoidal reservoir within a heterogeneous medium. We consider a total hydrostatic magmatic pressure starting from the value of 50 MPa at the reservoir's top and three regional stress regimes from symmetric tensile to asymmetric tensile to strike‐slip. In the last two cases we use three‐dimensional models. The criteria adopted for instability are (1) the development of tensile tangential stress and (2) the Navier‐Coulomb criterion, in compression. In no case have we found instability near or on the wall of the reservoir, whereas the slope of the volcanic edifice exhibits a shear failure instability, which increases with greater regional stress anisotropy.
Magnetic domains and domain walls in pseudo‐single‐domain magnetite studied with magnetic force microscopy
Authors: Pokhil Taras G. , Moskowitz Bruce M.
Published: February 2004

Magnetic domain and domain wall structures in pseudo‐single‐domain grains (5–20 μm) of magnetite (Fe3O4) were studied using magnetic force microscopy. Many of the observed micromagnetic features can be explained by the magnetostatic effects of surfaces and grain edges and interactions within and between walls. Domain walls were frequently subdivided into 1–3 opposite polarity segments separated by Bloch lines, although some walls contained no Bloch lines. Subdivided walls display a characteristic zigzag structure along the easy axis direction, where zigzag angles can be as high as 20°–40°. The zigzagging structure, in addition to wall segmentation, further minimizes the magnetostatic energy of the walls. Bloch lines can be (de)nucleated during wall displacement or after repeated alternating field (AF) demagnetization. Within individual walls, the number of Bloch lines and their pinning locations were found to vary after repeated AF demagnetization demonstrating that walls, like individual grains, can exist in several different local energy minima. The number of Bloch lines appears to be independent of domain state, but frequently the polarity of the wall was coupled with the direction of magnetization in the adjoining domains, such that wall polarity alternates in sign between adjacent walls across an entire grain. Even after the domain magnetization is reversed, the same sense of wall chirality is maintained across the grain producing unique grain chiralities. For one particular grain it was possible to reconfigure a likely three‐dimensional (3‐D) domain structure. The body and surface structures result primarily from a combined volume magnetostatic interaction between all grain surfaces and magnetocrystalline anisotropy. Finally, commonly observed open‐flux features within the interior of grains or along grain edges terminating planar domains are inconsistent with the prediction of edge closure domain formation based on recent 2‐D micromagnetic models. Our observations suggest that 3‐D micromagnetic models are required to model results even for grains larger than 1 μm.
Topography, boundary forces, and the Indo‐Australian intraplate stress field
Authors: Coblentz David D. , Zhou Shaohua , Hillis Richard R. , Richardson Randall M. , Sandiford Michael
Published: February 2004

The relative contribution of topographic (e.g., ridge push, continental margins, and elevated continental crust) and plate boundary (e.g., subduction and collisional) forces to the intraplate stress field in the Indo‐Australian plate (IAP) is evaluated through a finite element analysis. Two important aspects of the IAP intraplate stress field are highlighted in the present study: (1) if substantial focusing of the ridge push torque occurs along the collisional boundaries (i.e., Himalaya, New Guinea, and New Zealand), many of the first‐order features of the observed stress field can be explained without appealing to either subduction or basal drag forces; and (2) it is possible to fit the observed SHmax, (maximum horizontal stress orientation) and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes (e.g., tens of megapascals, averaged over the thickness of the lithosphere) throughout the plate. This study therefore presents a plausible alternative to previous studies of the IAP intraplate stress field, which predicted very large tectonic stress magnitudes (hundreds of megapascals) in some parts of the plate. In addition, topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental India and Australia, and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions. A central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin. We note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field, and this nonuniqueness continues to make modeling the IAP stress field problematic. While our study is an important step forward in understanding the sources of the IAP intraplate stress field, a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin.
Forearc structures and tectonic regimes at the oblique subduction zone between the Hikurangi Plateau and the southern Kermadec margin
Authors: Collot Jean‐Yves , Davy Bryan
Published: February 2004

The convergent southern Kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic Hikurangi Plateau. Swath bathymetric data have been combined with other geophysical data to reveal that the South Kermadec inner trench wall is segmented into distinct upper, middle, and lower margins. The lower margin is a nongrowing, compressionally unstable accretionary wedge that is dissected by transcurrent faults. The wedge has been thinned and narrowed as a result of tectonic erosion. The middle margin is stable by comparison and separated from the lower margin by the 220‐km‐long Awanui Fault. This fault is a landward dipping, strike‐slip, reverse fault. The middle upper margin, including East Cape Ridge (ECR), forms a deforming backstop with a sharp along‐strike change in tectonic regime. South of 36°40′S, transpression characterizes the ECR shear zone. North of 36°40′S, the ECR is marked by uplift, by extension, and farther north by subsidence. It is suggested that subsidence and subsequent tectonic erosion of the margin north of 35°50′S, in the wake of Hikurangi Plateau subduction, have broken the link between once continuous segments of the ECR‐Tonga Platform. The unstable nature of the southern Kermadec wedge under compression, together with a comparison of its taper characteristics with those of modern convergent margins, implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime. Strike‐slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the Hikurangi Plateau.
New and refined constraints on three‐dimensional Earth structure from normal modes below 3 mHz
Authors: Resovsky Joseph S. , Ritzwoller Michael H.
Published: February 2004

We present the results of generalized spectral fitting (GSF) regressions which estimate normal mode structure coefficients for the observable spheroidal and toroidal free oscillation multiplets below 3 mHz. The size, accuracy, and precision of our new catalogue of modal constraints make it a powerful new tool for assessing and refining three‐dimensional Earth models. The estimates include more than 3100 coefficients for 90 multiplets and 25 pairs of coupled multiplets, including several deep mantle overtones previously obscured by fundamental modes. The coefficients constrain mantle structures of both even and odd spherical harmonic degrees, through degree 12 in some cases. Improvements in accuracy and precision have been achieved with three innovations: the development of GSF, an enhancement of the established spectral fitting technique which incorporates both Coriolis and structural coupling between multiplets; the application of GSF to an edited, high signal‐to‐noise and geographically diverse data set of more than 4500 seismograms from 33 high moment earthquakes; and the assignment of coefficient uncertainties using a Monte Carlo method to simulate the effects of seismic noise, theoretical errors, and coefficient covariances. The results of GSF are assessed by examining the internal consistency of estimated coefficients and through comparisons with recent mantle models. The new catalogue of structure coefficients and uncertainties is available as an electronic supplement to this paper and through the University of Colorado internet site.
Compaction‐driven evolution of porosity and permeability in natural mudstones: An experimental study
Authors: Dewhurst David N. , Aplin Andrew C. , Sarda Jean‐Paul , Yang Yunlai
Published: February 2004

This paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones. Two intact samples of London Clay with clay fractions of 40% and 67% were compacted to 33 MPa effective stress. Clay fraction, permeability, porosity, pore size distribution, and specific surface area were measured and their evolution was monitored throughout the compaction process. Electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress. In both cases, porosity loss occurred primarily by the collapse of large pores. This process is more obvious in the coarser‐grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius. Consistent with the pore size distributions, the permeability of the coarser sample ranges from ∼ 10−10 m s−1 to 10−12 m s−1 while that of the finer‐grained sample ranges from ∼4 × 10−12 m s−1 to 5 × 10−14 m s−1 during progressive compaction from 2 to 33 MPa. The compressibility of the finer‐grained sample is greater than that of the coarser‐grained sample (0.15 as opposed to 0.07). However, in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales. The demonstration that both porosity and lithology (clay fraction) influence the permeability of mudstones should allow the development of more realistic porosity‐permeability relationships which take into account lithological variations exhibited by mudstones.
Whole Earth structure estimated from seismic arrival times
Authors: Vasco D. W. , Johnson Lane R.
Published: February 2004

The arrival times of 10 compressional and shear phases (P, S, PP, PcP, SS, ScS, PK Pab, PK Pbc, PK Pdf, SK Sac) and two sets of differential times (SS‐S410S and SS‐S660S) are used to infer the Earth's three‐dimensional structure. With the exception of SS‐SdS, the raw data are composed of arrival times from the International Seismological Centre (ISC) catalogue. The ISC events have been relocated using P and S phases and the ak135 radial Earth model. A total of 818,655 summary arrival times constrain lateral velocity variations in the crust, mantle, and core. We also determine relocation parameters for all events, stations corrections for each phase, boundary topography (410 km, 660 km, core‐mantle boundary, inner core boundary), and inner core anisotropy variations. Our model of P and S velocity variations in the crust and mantle contains the general features observed in other body wave inversions: a low‐velocity ring around the Pacific Ocean basin in the depth range 35–400 km, high‐velocity anomalies associated with subducting slabs most prominent in the depth range 660–1070 km, and high‐velocity anomalies surrounding the Pacific basin just above the core‐mantle boundary. In the outer core there is evidence of a latitudinal variation in velocity, approaching 0.6% in magnitude, with lower P velocities at the poles and higher velocities near the equator. This pattern is observed throughout the outer core but is most prominent in the depth range 3671–4071 km and near the inner core boundary. Synthetic tests indicate that outer core features are moderately resolvable and inversions of random arrival times indicates that the anomalies are a factor of 2 above the estimated parameter noise level. Lateral heterogeneity in the inner core trades off with anisotropy resulting in poor resolution of isotropic velocity variations. Anisotropy in the inner core is predominantly subparallel to the Earth's axis and of the order of 3% but this value can decrease if more inner core isotropic lateral heterogeneity is allowed. A comparison of structure above and below the core‐mantle boundary (CMB) indicates that velocity heterogeneity is uncorrelated across the CMB, but there is a moderate correlation between CMB topograpy and outer core heterogeneity. An inversion in which the PK Pdf residuals were absent indicates that the latitudinal pattern of outer core velocity variations is not due to the influence of inner core velocity structure.
Properties of the mantle transition zone in northern Eurasia
Authors: Ryberg T. , Wenzel F. , Egorkin A. V. , Solodilov L.
Published: February 2004

Short‐period, three‐component recordings of peaceful nuclear explosions (PNEs) in northern Eurasia are used to constrain the P wave velocity structure of the mantle transition zone. The properties of the upper mantle discontinuities play an important role in understanding the nature of mantle processes. Data from several PNE seismic sounding profiles reveal reflections and refractions from upper mantle discontinuities at 410 and 660 km depth. The amplitude and the sharpness of these velocity discontinuities contain important information to assess models of upper mantle phase changes and chemical layering. The absence of strong critical and precritical reflections from the 660 km discontinuity is characteristic for all the data in northern Eurasia. By studying the P wave reflections and refractions from the 660 km discontinuity, several velocity models were derived. To construct a generalized model, 18 shots observed on seven profiles in Russia were stacked to eliminate local effects. Synthetic seismograms were calculated and compared with the observations to test different velocity models. A comparison of the observed data with synthetic data predicted by a standard reference model (e.g. International Association of Seismology and Physics of the Earth's Interior 1991, (IASP91)) shows a systematic difference between the mantle phases. While the observed phases related to the 410 km discontinuity are consistent with the phases predicted by the IASP91 model, models with a smaller velocity step (∼50%) across the 660 km discontinuity or with a transitional 660 km discontinuity of several tens of kilometers thickness provide the best fit to the observed data for the 660 km phases. As long‐period data predict a sharp 660 km discontinuity, we have to postulate that under northern Eurasia the velocity increase across the bottom of the mantle transition zone is only about 50% of the value predicted by the IASP91 model. We present a generalized P wave velocity model, called Generalized Northern Eurasia Model (GNEM), for the upper mantle beneath northern Eurasia based on all available long‐range PNE data.
Thermodynamics of lateral dike propagation: Implications for crustal accretion at slow spreading mid‐ocean ridges
Authors: Fialko Yuri A. , Rubin Allan M.
Published: February 2004

We consider solidification of hot fluid flowing through a rigid‐wall channel of infinite extent. The calculated “thermal arrest” lengths are used to investigate the role of magma freezing in limiting the propagation distance of lateral dike intrusions. Our results demonstrate that for reasonable parameters the propagation distances of meter‐wide dikes do not exceed the wavelength of crustal thickness variations or transform fault spacing along slow spreading ridges. This suggests that thermal controls on the crustal melt delivery system could be an important factor in modulating these variations. Unlike published results for a finite channel, which predict unlimited meltback of the channel walls if the prefreezing fluid velocity exceeds some critical value, any flow into an infinite channel will eventually freeze, provided that shear heating in the magma is negligible. The thermal arrest distances depend strongly on the average dike thickness h (∝h4 for dikes driven by an along‐strike topographic slope and ∝h2 for dikes driven by an excess source pressure). Thermal erosion of the country rocks associated with lateral dike intrusions is likely to be confined to a very small region near the magma source. Substantial correlations between the along‐strike bathymetry and geochemistry of the erupted lavas along individual ridge segments may be consistent with high‐level basalt fractionation in the laterally propagating dikes.
Formation of chromitites by assimilation of crustal pyroxenites and gabbros into peridotitic intrusions: North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada
Authors: Bédard Jean H. , Hébert Réjean
Published: February 2004

The lowermost crust of the North Arm Mountain massif, Bay of Islands ophiolite, is dominated by peridotitic intrusions that assimilated and reacted with host gabbros, pyroxenites and peridotites. Cr‐spinel is a common accessory and locally forms chromitite layers, schlieren, or rims. Thermobarometric calculations and fractionation models suggest that the Cr‐spinels formed at oxidation states between QFM and QFM+1.5. Postcumulus and subsolidus diffusional exchanges with silicate minerals increased the Fe/Mg ratio of Cr‐spinels at North Arm but cannot explain the diversity of Cr‐spinel Cr/Al. Nor can closed or open‐system fractional crystallization of a single parental magma in cotectic proportions generate the required range of Cr/Al in Cr‐spinel. Either a variety of primary magmas was present or the Cr/Al ratio of the melt was perturbed by assimilation of host rocks, with assimilation inducing crystallization of spinel in greater than cotectic proportions. Outcrop‐scale structures and phase relationships suggest that high‐Cr/Al chromitites that are associated with contacts between intrusive peridotites and host pyroxenites are by‐products of the incongruent dissolution of pyroxenes into a Cr‐spinel‐saturated magma. Aluminous chromitites occur higher in the section, in association with hybrid feldspathic peridotites and gabbros that formed by disaggregation of feldspathic cumulates into peridotitic intrusions. These rocks commonly have disseminated anhedral Cr‐spinels and may also contain thin chromitite schlieren, both with low Cr/Al spinels. The disseminated Cr‐spinels fill dissolution pits in feldspar xenocrysts, implying that they formed by incongruent dissolution of feldspar. The H2O needed to stabilize the amphibole included in the low‐Cr/Al Cr‐spinels was probably derived from the reactant gabbros, many of which were previously affected by subseafloor alteration. Locally, chromitites and disseminated Cr‐spinels have higher concentrations of Ti, V, Fe2+, and Fe3+, which we attribute to assimilation of gabbros rich in Fe‐Ti oxides. The composition of Cr‐spinels in chromitites at North Arm depends more on the nature of the minerals being assimilated than on the composition of the magma.
Geothermics and climate change: 2. Joint analysis of borehole temperature and meteorological data
Authors: Harris Robert N. , Chapman David S.
Published: February 2004

Long‐period ground surface temperature variations contained in borehole temperature‐depth profiles form a complementary climate change record to high‐frequency, but noisy surface air temperature (SAT) records at weather stations. We illustrate the benefits of jointly analyzing geothermal and meteorological data for two regions in Utah where both high‐quality temperature‐depth measurements and century long SAT records exist. Transient temperature‐depth profiles constructed from SAT time series reproduce in considerable detail borehole transient ternperature‐depth profiles. Typical rms differences between these transient temperature profiles are less than 13 mK. The analysis yields a preobservational mean (POM) temperature, a parameter describing the long‐term mean surface temperature prior to the onset of SAT measurements (i.e., prior to the 20th century). The average POM for these two regions is 0.6°±0.2°C cooler than the 1951–1970 average SAT, suggesting that 20th century warming represents a real and significant departure from 19th century surface temperature values. In certain cases, borehole temperature profiles might be used as an independent check on long‐wavelength adjustments made to SAT data.
A reinterpretation of seismicity associated with the January 1983 dike intrusion at Kilauea Volcano, Hawaii
Authors: Rubin Allan M. , Gillard Dominique , Got Jean‐Luc
Published: February 2004

In January 1983, a dike intrusion/fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16‐km segment of Kilauea's Middle East Rift Zone. We searched the Hawaiian Volcano Observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation. Over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation. Some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm, consistent with generation near the propagating dike tip, while others were active for several days. The two multiplets nearest the origin of the seismic swarm include events from the preceding days and months. Most multiplets span only 50 to 100 m following relocation, are located at about 3 to 4 km depth, and appear to deepen downrift. The catalog depths of those earthquakes in multiplets and those not in multiplets are similar, suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface. By analogy with a mechanical model used to explain a similar clustering of background seismicity in the Upper East Rift in 1991, we infer that the earthquakes are generated in regions of high stress concentration immediately above Kilauea's deforming deep rift body. This conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1.
Role of sulfur and carbon in the electrical conductivity of the middle crust
Authors: Raab S. , Hoth P. , Huenges E. , Müller H. J.
Published: February 2004

Electrical conductivity measurements were made on black shale samples from deep boreholes under laboratory conditions corresponding to more than 10 km burial depth. At these conditions, large differences in resistivity were observed for shales of different diagenetic/metamorphic grade, although the total organic carbon (TOC) and sulfide content are comparable. A bituminous black shale (low diagenesis) showed reversible, temperature dependent changes of resistivity from 105 Ωm at room temperature to 104 Ωm at 300°C and 250 MPa. These are typical values for dry crustal rocks. For a very low grade metamorphic black shale (transition from high diagenesis to beginning of metamorphism) a decrease in resistivity of at least 1 order of magnitude to a value of about 1 Ωm was observed at the same p, T conditions. This type of black shale shows permanent changes in resistivity. The high conductivity achieved in this sample is mainly caused by the transition of primary pyrite to pyrrhotite and the increase in connectivity between the conducting phases produced by the more elongated geometry of the newly formed sulfides. Both factors result in an increase in conductivity at in situ conditions. These results confirm that the known parameters of TOC content, amount of primary sulfides, and coalification stage are important for determining the conductivity of black shales. But the analytical investigations of the samples after the experiments indicate that the degree of foliation and sulfide transformation at low metamorphic conditions also plays an important role. The foliation and the coalification as well as the geometrical form of the newly formed sulfides are connected with the diagenetic/metamorphic overprint of the shales. Midcrustal conditions (temperature, pressure, low oxygen fugacity) seem to promote the mobilization of sulfur and/or iron and result in increased connectivity of conducting components. Therefore some highly conducting zones in Earth's crust might be caused by a combination of organic carbon at least in a weak metamorphic stage and sulfides.
Mainshock and aftershock analysis of the June 17, 1996, deep Flores Sea earthquake sequence: Implications for the mechanism of deep earthquakes and the tectonics of the Banda Sea
Authors: Tinker Mark Andrew , Beck Susan L. , Jiao Wenjie , Wallace Terry C.
Published: February 2004

One of the largest (Mw = 7.8) deep earthquakes (595 km) in recorded history occurred beneath the Flores Sea on June 17, 1996. We analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake. The source time function has a small initial subevent (5 s duration) followed by three major subevents with a total source duration of 29 s. Directivity analysis indicates an unilateral rupture to the east‐southeast with a subhorizontal rupture length of 75 km. A time‐independent moment tensor (TIMT) inversion gives a total seismic moment of 5.4×1020 N m ( Mw = 7.8). A time‐dependent moment tensor (TDMT) inversion indicates rupture on a near‐vertical fault (strike of 130°, dip of 80°SW, rake of −166°) during the first 5 s, releasing 8% of the seismic moment. The major part of the moment was released along a shallower dipping plane (strike of ∼100°, dip of ∼55°, rake of ∼−45°). This change of faulting geometry may imply that the rupture initiated at a structural, thermal, or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone. The mainshock produced at least 23 (mb>2.9) aftershocks within the first 3 days. Seventeen of the largest aftershocks were located using the Joint Hypocenter Determination algorithm. The aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km, respectively. The aftershock locations correlate with the southeast striking nodal plane of the moment tensor mechanism.
On the dayside region between the shocked solar wind and the ionosphere of Mars
Authors: Szegő K. , Klimov S. , Kotova G. A. , Livi S. , Rosenbauer H. , Skalsky A. , Verigin M. I.
Published: February 2004

The objective of this paper is to explore the structure and properties of the region between the shocked solar wind and the planetary plasma on the dayside of Mars. This study is based on the observations obtained by the plasma instruments carried on board the Phobos 2 spacecraft. These instruments have independently identified several plasma boundaries in the dayside magnetosphere of Mars, the boundaries are located close to each other but exhibit different plasma features. This investigation leads us to conclude that a dayside region exists on Mars with the following characteristics: (1) the bulk of the shocked solar wind protons are deflected, (2) the total magnetic field increases, (3) a plasma depletion region is formed, (4) both shocked solar wind and planetary plasma are present, (5) accelerated electrons and heavy ions are also present, (6) intensive wave activity is seen in the 5 to 150 Hz frequency interval, and (7) a current layer and associated magnetic shears can be identified. We name this region, located between the ionosphere and the shocked solar wind, the dayside boundary layer or dayside mantle. Its key features are similar to those of the Venus mantle; the variety of names given to the Martian plasma boundaries merely reflects its complicated structure. We conclude that the two boundary layers at Venus and Mars are very similar and that the physical processes (i.e., the interaction of the shocked solar wind with the planetary plasma) at work within these boundary layers are probably similar.
Appleton anomaly increase following sunset: Its observed relation to equatorial F layer E × B drift velocity
Authors: Whalen J. A.
Published: February 2004

The upward E × B drift velocity of the postsunset F layer at the dip equator, in response to the prereversal electric field, and the resulting enhancement of the fountain effect plasma at the Appleton anomaly are observed nearly simultaneously near the same geomagnetic meridian. Measurements, the first under such controlled conditions, are by a chain of ionospheric sounders recording ionograms at intervals of 5–15 min on two consecutive days during equinox at solar maximum, both having quiet magnetic conditions and high solar fluxes. Measured is the following sequence of cause and effect: the equatorial F layer electron density that persists through sunset; the solar E layer electron density conjugate to this F layer that decays rapidly through sunset; the bottomside equatorial F layer altitude, h′F, as it drifts upward in response to the F region dynamo electric field; and the anomaly F layer electron density that increases with the arrival of the resulting enhanced fountain effect plasma at 16.0° and at 20.3° dip latitude. At the equator the maximum drift velocity measured by dh′F/dt is 70 m s−1 on the first day and 50 m s−1 on the second. The differences in drift velocity are reflected in the anomaly in latitudinal distributions of F layer maximum electron density between 0° and 30° dip latitude that show the anomaly crest to increase in magnitude and in latitude: rapidly on the first day, reaching a maximum of 55 × 105 el cm−3 18° dip latitude at 2100 LT, and more slowly on the second day, reaching ≥38 × 105 el cm−3 at ≤16° dip latitude, also at 2100 LT. In addition to the qualitative relationship, maximum drift velocity and anomaly crest maximum electron density and its maximum rate of increase are all reduced by about the same factor on the second day relative to the first.
Calculated and observed ionospheric parameters for a Magion 2 passage and EISCAT data on July 31, 1990
Authors: Korenkov Yu. N. , Klimenko V. V. , Förster M. , Bessarab F. S. , Surotkin V. A.
Published: February 2004

Ionospheric parameters, calculated by using a global numerically self‐consistent model of the thermosphere, ionosphere, and protonosphere (GSM TIP), are compared with experimental data. For July 31, 1990, there are ground‐based Common Programme 1 observations from the European Incoherent Scatter (EISCAT) radar station below about 600‐km height and simultaneous satellite data obtained onboard Magion 2 of the Active mission which passed near the EISCAT station around 1540 UT at an altitude of about 2500 km. The main goal of this paper is to estimate the ability of the first‐principles mathematical GSM TIP model to reproduce the real ionospheric situation in a wide altitude/latitude range. Numerical calculations were performed with the complete theoretical thermosphere‐ionosphere‐protonosphere model. The results presented show good agreement between the incoherent scatter radar measurements (Ne, Te, Ti, and ) and model calculations under certain model assumptions. Model runs were performed to study the influence of additional local and nonlocal heating sources, the role of vibrationally excited molecular nitrogen, and different convection patterns for the conditions under study. There is relatively good agreement between model results and experimental electron concentration data along the Magion 2 satellite orbit. It was found that the best fit results are achieved with an additional plasmaspheric heat source, while the importance of vibrationally excited molecular nitrogen has not yet been firmly established.
ULF waves and the ground magnetic field
Authors: Itonaga M. , Yumoto K.
Published: February 2004

A theory is put forward to account for the horizontal structure and polarization of the magnetic field of low‐frequency ULF waves on the ground at low latitudes. The theory is based on the condition that the component of the rotation of magnetic field normal to the Earth's surface vanishes in the neutral atmosphere and also on an assumption that the phase in the H component of the ground magnetic field is constant latitudinally at low latitudes. When the H component phase is independent of the latitude, the D component amplitude increases monotonically from the magnetic equator to higher latitudes. The D component is 180° out of phase in the northern and southern hemispheres as well as east and west of longitude of the wave source. The sense of rotation of the magnetic perturbation vector in the H–D plane is clockwise and counterclockwise for the eastward and westward propagations, respectively, in the northern hemisphere and vice versa in the southern hemisphere. The major axis of the H–D plane ellipse orients in the northeast and northwest directions west and east of the source longitude, respectively, in the northern hemisphere and vice versa in the southern hemisphere. For compressional MHD disturbances emitted from a distant source region, their wave fronts surround the Earth in the inner plasmasphere owing to the slowdown of the Alfvén speed just inside the plasmapause and wind around it near the Earth. This suggests that the latitudinally constant phase of the H component is a common feature in low‐latitude ULF waves.
CRRES observations of radiation belt protons: 2. Time‐dependent radial diffusion
Authors: Albert J. M. , Ginet G. P.
Published: February 2004

Flux measurements made by CRRES of high‐energy, equatorially mirroring protons in the range L ≤ 3 are observed to differ greatly from diffusive steady state profiles, both before the magnetic storm of March 24, 1991, and during two intervals following it. Time rates of change of the phase space density ƒ are obtained from time series of the observations and compared to those calculated from the time‐dependent radial diffusion equation using profiles from the time averages of the observations. In general, good agreement is obtained for E ≤ 20 MeV, indicating that the outer zone protons were undergoing diffusive relaxation from highly disturbed configurations. Such a configuration is known to have been produced by the March 1991 storm, and we hypothesize that a similarly disruptive event occurred prior to the CRRES launch. For particle energy above 20 MeV or L > 3, limitations in the instrument sensitivity and integration time preclude a quantitative comparison.
Spatial structure of the solar wind and comparisons with solar data and models
Authors: Neugebauer M. , Forsyth R. J. , Galvin A. B. , Harvey K. L. , Hoeksema J. T. , Lazarus A. J. , Lepping R. P. , Linker J. A. , Mikic Z. , Steinberg J. T. , von Steiger R. , Wang Y.‐M. , Wimmer‐Schweingruber R. F.
Published: February 2004

Data obtained by instruments on the Ulysses spacecraft during its rapid sweep through >90° of solar latitude, crossing the solar equator in early 1995, were combined with data obtained near Earth by the Wind spacecraft to study the spatial structure of the solar wind and to compare to different models of the interplanetary magnetic field derived from solar observations. Several different source‐surface models matched the double sinusoidal structure of the heliospheric current sheet (HCS) but with differences in latitude as great as 21°. The source‐surface model that included an interplanetary current sheet gave poorer agreement with observed current‐sheet crossings during this period than did the other source‐surface models or an MHD model. The differences between the calculated and observed locations of the HCS were minimized when 22° of solar rotation was added to the constant‐velocity travel time from the source surface to the spacecraft. The photospheric footpoints of the open field lines calculated from the models generally agreed with observations in the He 10,830 Å line of the locations of coronal holes with the exceptions that (1) in some places, open field lines originated outside the coronal hole boundaries and (2) the models show apparently closed‐field regions just inside some coronal hole boundaries. The patterns of mismatches between coronal hole boundaries and the envelopes of open field lines persisted over at least three solar rotations. The highest‐speed wind came from the polar coronal holes, with the wind originating deeper within the hole being faster than the wind coming from near the hole boundary. Intermediate and slow streams originated in smaller coronal holes at low latitudes and from open field regions just outside coronal hole boundaries. Although the HCS threaded regions of low speed, low helium abundance, high ionization temperature, and a high ratio of magnesium to oxygen densities (a surplus of an element with low first‐ionization potential), there was a great deal of variation in these parameters from one place to another along the HCS. The gradient of speed with latitude varied from 14 to 28 km s−1 deg−1.
Experimental evidence on the role of the large spatial scale electric field in creating the ring current
Authors: Wygant J. , Rowland D. , Singer H. J. , Temerin M. , Mozer F. , Hudson M. K.
Published: February 2004

This paper presents the first simultaneous in situ measurements of the large‐scale convection electric field and the ring current induced magnetic field perturbations in the equatorial plane of the inner magnetosphere and compares them to the evolution of major geomagnetic storms as characterized by Dst. The measurements were obtained from the University of California, Berkeley double‐probe electric field experiment and the Air Force Geophysics Laboratory fluxgate magnetometer on the CRRES spacecraft. This spacecraft had an apogee near geosynchronous orbit and a perigee near 300 km altitude. We focus on the major geomagnetic storm on March 24, 1991, for which the maximum negative excursion of Dst was about −300 nT. During the main phase of the storm, the large‐scale electric field repeatedly penetrated earthward, maximizing between L = 2 and L = 4 with magnitudes of 6 mV/m. These magnitudes were larger than quiet time values of the electric field by a factor of 60 or more. Electric potential drops across the dusk region from L = 2 to L = 4 ranged up to 50–70 kV in concert with increases in Kp up to 9 and dDst/dt (an indicator of the net ring current injection rate) which ranged up to −50 nT/hr. These electric fields lasted for time periods of the order of an hour or more and were capable of injecting ring current ions from L = 8 to L = 2.4 and energizing particles from initial plasma sheet energies of 1–5 keV up to 300 keV through conservation of the first adiabatic invariant. The data obtained during the recovery phase of this storm provide the first direct experimental evidence in the equatorial plane that the electric field is systematically diminished or shielded earthward of the inner edge of the ring current during this phase of the geomagnetic storm. Also observed during the 2‐week recovery phase were episodic enhancements in the electric field which coincided and were colocated with enhancements of in situ ring current intensity and which also coincided with decreases in Dst. These enhancements in the electric field and in the ring current magnetic field perturbation occurred at progressively larger radial positions as the recovery phase continued. Evidence for regions of reversed convection near midnight during the recovery phase is provided. An unexpected and important feature of this data set, during both main and recovery phases, near 1800–2100 MLT, is that electric fields are often much stronger earthward of L = 4 or L = 5 than at positions more distant than L = 6. This suggests important features of the interaction between the hot ring current plasma and the large‐scale electric field in the inner magnetosphere are not yet understood.
Electrostatic discharge in Martian dust storms
Authors: Melnik Olga , Parrot Michel
Published: February 2004

Although the Martian atmosphere does not satisfy general requirements for lightning generation, there is a possibility of electrical discharge in the case of strong surface winds and a resulting extremely large dust mass loading in the course of large dust storms occurring on this planet. On Earth, negative potential gradients of many thousands of volts per meter have been measured during dust storms when winds are sufficiently strong. However, owing to a lower pressure in the Martian atmosphere, the required voltage for electrical breakdown is lower than on Earth. After a brief review of the observations concerning Earth's atmosphere and the laboratory experiments performed to understand these phenomena, the results of numerical simulation of the electrification in Martian dust storms are presented. Known characteristics of Martian dust grains and the Martian atmosphere are considered, and the different forces applied to the dust particles are taken into account. The electrostatic potential and the evolution of the charge density are calculated using Poisson's equation and the continuity equation. Two different wind configurations in the Martian atmosphere are studied: a wind vortex and a wind parallel to the ground encountering an obstacle (a small hill). It is shown that electric discharge occurs under specific conditions.