bluuebunny/crossref_metadata_2025 huggingface dataset:
Brightness measurements of the nighttime O I 8446 Å airglow emission from the Millstone Hill and Arecibo Observatories
Authors: Lancaster Redgie S. , Waldrop Laura S. , Kerr Robert B. , Noto John , Solomon Stanley C. , Tepley Craig A. , Garcia Raul , Friedman Jonathan
Published: September 2002

Ground‐based measurements of the O I 8446 Å airglow brightness are made from the Millstone Hill and Arecibo Observatories. These measurements are made at various times throughout the year, and this paper reports those made between the years of 1993 and 1996. The maximum brightness of the 8446 Å airglow emission is found to be less than about 35 Rayleighs during the nighttime at both these midlatitude sites. During the summer this emission is present for only a short time in the evening and morning. During the winter the emission can persist throughout the night. This observed behavior is in agreement with the calculations of current models that predict similar intensities from excitation by photoelectron impact on atomic oxygen. A close examination of these data, however, reveal that differences exist between the model predictions and the measured brightness. One difference is a persistence of the nighttime 8446 Å brightness to times when the model predicts little or no emission from photoelectron production. A second difference is an occasional enhancement of the morning 8446 Å brightness that is not predicted by the model. The former of these differences implies a shortcoming in current understanding of the 8446 Å production mechanisms and must be resolved before atomic oxygen can be monitored from the ground using this emission. The latter may have important consequences for our understanding of upper atmosphere dynamics.
Modeling the high‐energy proton belt
Authors: Vacaresse A. , Boscher D. , Bourdarie S. , Blanc M. , Sauvaud J. A.
Published: September 2002

We use and extend our previously published physical model of low‐energy particle fluxes in the Earth's radiation belts [Boscher et al., 1998] to study four components of the temporal variations of radiation belt particle fluxes: the secular variation, related to that of the Earth's magnetic field; the solar cycle modulation; the magnetic storm effects; and the solar proton events. Our previous proton model, initially developed for equatorially mirroring low‐energy particles, has been extended to higher energies in order to reproduce (1) the solar cycle influence on very high energy particles (around 100 MeV) at low altitudes and (2) the effects induced by strong active periods on protons with energies between 1 and 50 MeV. The good agreement found between our model results and observations shows that our physical model can be a safe contribution to the future development of engineering models that will be able to reproduce both the long‐term variations of trapped proton fluxes (to define mission specifications) and short‐term fluctuations (to understand in‐flight satellite anomalies).
A thermal mechanism for generation of small‐scale irregularities in the ionospheric E region
Authors: Kagan L. M. , Kelley M. C.
Published: September 2002

We present a mechanism for small‐scale irregularity generation in the E region due to a thermal instability driven by the neutral wind. The polarization electric field induced by the neutral wind causes factional heating due to ion neutral collisions. In plasma‐depleted regions this leads to enhanced plasma pressure and plasma is forced out, enhancing the plasma depletion. When heat advection is included, the threshold for instability is lower than neutral winds commonly observed in the midlatitude E region. Another factor contributing to the instability growth is the dependence of the ion neutral collisional frequency on ion temperature. This also results in a smaller threshold neutral wind due to both a reduced ion cooling rate during ion‐neutral energy exchange and heat advection. In a standard two‐fluid treatment the thermal theory predicts two preferable altitude ranges for small‐scale irregularity generation: between 93 and 102 km and near 110 km altitude. We find that the thermal processes may be responsible for the continuous and “thick” continuous echoes observed by the middle and upper atmosphere radar and by the frequency agile radar during the Sporadic E Experiment over Kyushu (SEEK) campaign for 3.2‐m and 6.1‐m field‐aligned irregularities, respectively. The theory may explain the SEEK campaign observations of 6.1‐m echoes at as high as 150 km altitude if the irregularity elongation is stronger than that due to ambipolar diffusion. The predicted phase velocities are much smaller than the ion acoustic velocity, which allows us to associate the instability with the type 2 radar echoes.
Slow shock downstream structure in the magnetotail
Authors: Hoshino M. , Mukai T. , Shinohara I. , Saito Y. , Kokubun S.
Published: September 2002

In this paper we discuss that the two different plasma regions exist inside the plasma sheet during the magnetic reconnection in the magnetotail. The inner plasma sheet consists of a weak magnetic field and an isotropic plasma, while the outer plasma sheet contains the intermediate magnetic field intensity between the lobe and the inner plasma sheet and an anisotropic plasma. The plasma temperature parallel to the magnetic field is larger than the perpendicular temperature in the outer plasma sheet. On the basis of Geotail data analysis, we find that two different discontinuities are formed in the magnetotail: One is the slow‐mode shock, and the other is the contact discontinuity. The slow shock downstream contains a contact discontinuity which separates the slow shock heated plasmas from the isotropic plasmas in the plasma sheet.
Electron pitch angle diffusion by electrostatic electron cyclotron harmonic waves: The origin of pancake distributions
Authors: Horne Richard B. , Thorne Richard M.
Published: September 2002

It has been suggested that highly anisotropic electron pancake distributions are the result of pitch angle diffusion by electrostatic electron cyclotron harmonic (ECH) and whistler mode waves in the equatorial region. Here we present pitch angle diffusion rates for ECH wave spectra centered at different frequencies with respect to the electron gyrofrequency Ωe corresponding to spacecraft observations. The wave spectra are carefully mapped to the correct resonant electron velocities. We show that previous diffusion calculations of ECH waves at 1.5Ωe, driven by the loss cone instability, result in large diffusion rates confined to a small range of pitch angles near the loss cone and therefore cannot account for pancake distributions. However, when the wave spectrum is centered at higher frequencies in the band (> 1.6Ωe), the diffusion rates become very small inside the loss cone, peak just outside, and remain large over a wide range of pitch angles up to 60° or more. When the upper hybrid resonance frequency ωUHR is several times Ωe, ECH waves excited in higher bands also contribute significantly to pitch angle diffusion outside the loss cone up to very large pitch angles. We suggest that ECH waves driven by a loss cone could form pancake distributions as they grow if the wave spectrum extends from the middle to the upper part of the first (and higher) gyroharmonic bands. Alternatively, we suggest that pancake distributions can be formed by outward propagation in a nonhomogeneous medium, so that resonant absorption occurs at higher frequencies between (n + ½) and (n + 1)Ωe in regions where waves are also growing locally at < 1.5Ωe. The calculated diffusion rates suggest that ECH waves with amplitudes of the order of l mV m−1 can form pancake distributions from an initially isotropic distribution on a timescale of a few hours. This is consistent with recent CRRES observations of ECH wave amplitudes following substorm injections near geostationary orbit and the timescales for pancake formation. Persistent but much weaker ECH waves can further intensify and maintain pancake distributions during magnetically quiet periods.
Drift resonance and stability of the Io plasma torus
Authors: Zhan Jie , Hill T. W.
Published: September 2002

The observed local time asymmetry of the Io plasma torus is generally attributed to the presence of a persistent dawn‐to‐dusk electric field in the Jovian magnetosphere. The local time asymmetry is modulated at the System 3 rotation period of Jupiter's magnetic field, suggesting that the dawn‐to‐dusk electric field may be similarly modulated. We argue that such a System 3 modulation would have a profound disruptive effect on the observed torus structure if the torus were to corotate at exactly the System 3 rate: the torus would be a resonantly forced harmonic oscillator, and would disintegrate in a few rotation periods, contrary to observations. This destabilizing effect is independent of, and in addition to, the more familiar effect of the centrifugal interchange instability, which is also capable of disrupting the torus in a few rotation periods in the absence of other effects. We conclude that the observed (few percent) corotation lag of the torus is essential to preserving the observed long‐lived torus structure by detuning the resonant frequency (the torus drift frequency) relative to the forcing frequency (System 3). A possible outcome of this confinement mechanism is a residual radial oscillation of the torus at the beat period (∼10 days) between System 3 and the torus drift period.
Simulation of the Saturnian magnetospheric interaction with Titan
Authors: Brecht Stephen H. , Luhmann Janet G. , Larson David J.
Published: September 2002

Flowing plasma interactions with small bodies in the solar system are frequently complicated by the significant size of the ion gyroradii relative to the scale size of the body and the interaction region. This situation often applies to both the flowing ion populations and the atmospheric pickup ions added to the flow in the vicinity of the body. Titan represents an important example of this type of system and is scheduled to be probed in some detail during the Cassini mission tour of Saturn. Initial results from global hybrid (particle ions, fluid electrons) numerical simulations of Saturn's magnetospheric plasma encountering Titan's ionosphere reveal the complexity of the interaction introduced by the ion kinetics. The scale of the interaction region is dominated by the heavy ion gyroradii of the ambient and pickup ions rather than the size of Titan and is found to depend on the mass loading of the magnetospheric flow encountering Titan. Comparison with data from the Voyager 1 flyby suggests that ion kinetics as well as the relative orientations of external plasma ram and pickup ion production regions affect what was observed.
Self‐consistent quasi‐static radial transport during the substorm growth phase
Authors: Le Contel O. , Pellat R. , Roux A.
Published: September 2002

We develop a self‐consistent description of the slowly changing magnetic configuration of the near‐Earth plasma sheet (NEPS) during substorm growth phase. This new approach is valid for quasi‐static fluctuations ω < k‖vA (vA being the Alfvén velocity), with characteristic frequency lower than the bounce frequencies of electrons and ions (ω < ωbi,ωbe), and for spatial scales larger than the ion Larmor radius. The basic equations are obtained from a linearization of the cyclotron and bounce‐averaged Vlasov equation, together with Maxwell equations. The Vlasov‐Maxwell system of equations is solved for the quasi‐dipolar NEPS region. Using a 2‐D dipole for the equilibrium, we calculate analytically the perturbed components of the electromagnetic field as a function of an external forcing current. The quasi‐neutrality condition (QNC) is solved via an expansion in the small parameter Te/Ti (Te/Ti is the ratio between the electronic and ionic temperatures). To the lowest order in Te/Ti, we find that the enforcement of QNC implies the presence of a global electrostatic potential which is constant for a given magnetic field line but varies across the magnetic field. The corresponding electric field shields the effect of the inductive component of the electric field, thereby producing a partial reduction of the motion that would correspond to the inductive electric field. Furthermore, we show that enforcing the QNC implies a field‐aligned potential drop which is computed to the next order in Te/Ti in a companion paper [Le Contel et al., this issue]. In the present paper, we show that the direction of the azimuthal electric field varies along the field line, thus the equatorial electric field cannot be mapped onto the ionosphere. Furthermore during the growth phase, the (total) azimuthal electric field is directed eastward, close to the equator, and westward, off‐equator. Thus large equatorial pitch angle particles drift tailward, whereas small pitch angle particles drift earthward.
Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration
Authors: Olson Peter , Christensen Ulrich , Glatzmaier Gary A.
Published: September 2002

Numerical calculations of fluid dynamos powered by thermal convection in a rotating, electrically conducting spherical shell are analyzed. We find two regimes of nonreversing, strong field dynamos at Ekman number 10−4 and Rayleigh numbers up to 11 times critical. In the strongly columnar regime, convection occurs only in the fluid exterior to the inner core tangent cylinder, in the form of narrow columnar vortices elongated parallel to the spin axis. Columnar convection contains large amounts of negative helicity in the northern hemisphere and positive helicity in the southern hemisphere and results in dynamo action above a certain Rayleigh number, through a macroscopic α2 mechanism. These dynamos equilibrate by generating concentrated magnetic flux bundles that limit the kinetic energy of the convection columns. The dipole‐dominated external field is formed by superposition of several flux bundles at middle and high latitudes. At low latitudes a pattern of reversed flux patches propagates in the retrograde direction, resulting in an apparent westward drift of the field in the equatorial region. At higher Rayleigh number we find a fully developed regime with convection inside the tangent cylinder consisting of polar upwelling and azimuthal thermal wind flows. These motions modify the dynamo by expelling poloidal flux from the poles and generating intense toroidal fields in the polar regions near the inner core. Convective dynamos in the fully developed regime exhibit characteristics that can be compared with the geomagnetic field, including concentrated flux bundles on the core‐mantle boundary, polar minima in field intensity, and episodes of westward drift.
Climate‐driven polar motion
Authors: Celaya Michael A. , Wahr John M. , Bryan Frank O.
Published: September 2002

The output of a coupled climate system model provides a synthetic climate record with temporal and spatial coverage not attainable with observational data, allowing evaluation of climatic excitation of polar motion on timescales of months to decades. Analysis of the geodetically inferred Chandler excitation power shows that it has fluctuated by up to 90% since 1900 and that it has characteristics representative of a stationary Gaussian process. Our model‐predicted climate excitation of the Chandler wobble also exhibits variable power comparable to the observed. Ocean currents and bottom pressure shifts acting together can alone drive the 14‐month wobble. The same is true of the excitation generated by the combined effects of barometric pressure and winds. The oceanic and atmospheric contributions are this large because of a relatively high degree of constructive interference between seafloor pressure and currents and between atmospheric pressure and winds. In contrast, excitation by the redistribution of water on land appears largely insignificant. Not surprisingly, the full climate effect is even more capable of driving the wobble than the effects of the oceans or atmosphere alone are. Our match to the observed annual excitation is also improved, by about 17%, over previous estimates made with historical climate data. Efforts to explain the 30‐year Markowitz wobble meet with less success. Even so, at periods ranging from months to decades, excitation generated by a model of a coupled climate system makes a close approximation to the amplitude of what is geodetically observed.
P wave crustal velocity structure in the greater Mount Rainier area from local earthquake tomography
Authors: Moran Seth C. , Lees Jonathan M. , Malone Stephen D.
Published: September 2002

We present results from a local earthquake tomographic imaging experiment in the greater Mount Rainier area. We inverted P wave arrival times from local earthquakes recorded at permanent and temporary Pacific Northwest Seismograph Network seismographs between 1980 and 1996. We used a method similar to that described by Lees and Crosson [1989], modified to incorporate the parameter separation method for decoupling the hypocenter and velocity problems. In the upper 7 km of the resulting model there is good correlation between velocity anomalies and surface geology. Many focal mechanisms within the St. Helens seismic zone have nodal planes parallel to the epicentral trend as well as to a north‐south trending low‐velocity trough, leading us to speculate that the trough represents a zone of structural weakness in which a moderate (M 6.5–7.0) earthquake could occur. In contrast, the western Rainier seismic zone does not correlate in any simple way with anomaly patterns or focal mechanism fault planes, leading us to infer that it is less likely to experience a moderate earthquake. A ∼10 km‐wide low‐velocity anomaly occurs 5 to 18 km beneath the summit of Mount Rainier, which we interpret to be a signal of a region composed of hot, fractured rock with possible small amounts of melt or fluid. No systematic velocity pattern is observed in association with the southern Washington Cascades conductor. A midcrustal anomaly parallels the Olympic‐Wallowa lineament as well as several other geophysical trends, indicating that it may play an important role in regional tectonics.
Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He‐Pb‐Nd‐Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges
Authors: Schilling J.‐G. , Kingsley R. , Fontignie D. , Poreda R. , Xue S.
Published: September 2002

He‐Pb‐Nd‐Sr isotope systematics in basalts from the Mid‐Atlantic Ridge (MAR) from 65°N to 78°N are reported for mapping the zone of influence of the Jan Mayen and Iceland mantle plumes in the Arctic. The geographical variation and the two distinct trends observed in the He‐Pb‐Nd‐Sr isotope space clearly indicate that the boundary between the zone of influence of the “low3He/4He” Jan Mayen plume and the “high3He/4He” Iceland mantle plume is in the vicinity of the Spar FZ. Modeling indicates that the dispersion of the Jan Mayen plume is not purely radial, but extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen fracture zone (FZ) due to the large denivellation (change of level) of the base of the lithosphere caused by the large age offset (∼20 Myr). The incompatible parent and daughter (PD) element concentrations and their ratios for the basalt population from the Jan Mayen plume are highly coherent with the Pb‐Nd‐Sr isotope ratios and show essentially the same geographical pattern in spite of large variations in the mean degree of fusion  and extent of fractional crystallization. In contrast, over the southern Kolbeinsey Ridge, unusual decouplings are observed between He‐Pb‐Nd‐Sr isotopic systematics and incompatible element variations, as well as inferred melting conditions. These decouplings are best explained by a modified version of the dispersion model of the Iceland plume byMertz et al.[1991] which was based on more limited isotopic data, and the fluid dynamic models ofIto et al.[1999]. In addition to binary mixing of the Iceland plume with the depleted asthenosphere a combination of (1) defluidization of the Iceland mantle plume occurring deeper than the dominant zone of dry decompression melting for enhancing He and heat transport along the southern Kolbeinsey Ridge and (2) fractional melting accompanying the northward dispersion and decompression of the Iceland mantle plume is required in order to explain the difference in wavelength between the He gradient and the Pb‐Nd‐Sr isotope gradients observed in mapping the dispersion of the Iceland plume along the MAR. Note that the Tjörnes TZ does not act as a dam against the northward dispersion of the Iceland plume, contrary to what the rare earth element variation, by itself, previously suggested. Finally, over the Knipovich Ridge the large scatter and lack of any systematics between the Pb‐Nd‐Sr isotope and related parent/daughter ratios along this immature, discontinuous, and shear‐dominated ridge, running parallel and close to the Svalbard continental break (74°–79°N), suggest the involvement in the melting of randomly distributed continental mantle lithosphere schlierens present in the depleted upper mantle source of the Knipovich Ridge basalts.
Seismic reflectivity anomalies in sediments at the eastern flank of the Juan de Fuca Ridge: Evidence for fluid migration?
Authors: Zühlsdorff Lars , Spieß Volkhard , Hübscher Christian , Breitzke Monika
Published: September 2002

A high‐resolution seismic survey was carried out in the vicinity of Ocean Drilling Program (ODP) Leg 168 drill sites at the eastern flank of the Juan de Fuca Ridge. Three seismic systems with different source frequencies up to 4000 Hz were used simultaneously along each seismic line. The data sets were combined to provide the best possible resolution at any given depth level. An integrated interpretation of narrow zones of low reflection amplitudes is presented, which are related to basement highs but are independent of seafloor topography. The effect is most pronounced above a buried basement ridge in the vicinity of ODP Sites 1030 and 1031, where higher porosity values are present. Higher porosities can reduce seismic impedance contrasts and may therefore cause the observed low reflection amplitudes. In addition, Biot‐Stoll's [Biot, 1956a, b; Stoll, 1989] model suggests that attenuation coefficients are porosity dependent and reflection amplitudes are further decreased at higher seismic frequencies when porosity increases. Since zones of higher porosities are potential pathways for fluids, they may be associated with hydrothermally driven fluid discharge, which was previously predicted above the buried basement ridge. It is therefore proposed that the observed seismic amplitude reduction in the vicinity of ODP Sites 1030 and 1031 indicates zones of upward fluid migration.
Contribution of radar interferometry to a two‐step inversion of the kinematic process of the 1992 Landers earthquake
Authors: Hernandez Bruno , Cotton Fabrice , Campillo Michel
Published: September 2002

We study the rupture process of the 1992 Landers earthquake. To limit the trade‐off between slip amplitude and rupture time that affects solutions using only seismological data, we adopt a two‐step approach. We first constrain the slip distribution and its uncertainty by independent geodetic data to recover in the second step the temporal details of the rupture propagation. The first step consists of an inversion of interferometric data and Global Positioning System measurements, both independently and together, to constrain slip distribution on a three‐segment fault model along both strike and dip direction. We use a genetic algorithm to test the uniqueness of the solution and a least squares formulation to find the model which best fits the data. We conclude from the results of these geodetic inversions that interferometric data are rich enough to access the slip distribution in the case of the Landers earthquake. Since the surface deformations are more sensitive to shallow slip in our configuration, the slip amplitude is better resolved near the surface than at depth. The resulting slip distribution is in agreement with geological observations at the surface and confirms the heterogeneous nature of the Landers earthquake. Most of the slip occurs at shallow depths, on the Homestead Valley fault (second segment), with a maximum value of around 7 m. Another high slip zone is observed on the Johnson Valley fault (first segment) at 8 km depth. In the second step, we invert strong motion data with the a priori final slip amplitude and its uncertainty deduced from geodetic data to constrain the time history of the rupture process. This second step emphasizes a strong variation of the temporal development of the earthquake. Fast rupture front velocities appear within high slip zones, and the rupture slows when it encounters a resistance along the fault. On average, the rupture front propagates with velocities close to theSwave velocity and terminates about 20 s after initiation. The large variations in both slip amplitude and rupture velocity suggest that the rupture process is better described by successively breaking asperities than by a pulse propagating with constant velocity.
Insights into mid‐ocean ridge basalt petrogenesis: U‐series disequilibria from the Siqueiros Transform, Lamont Seamounts, and East Pacific Rise
Authors: Lundstrom C. C. , Sampson D. E. , Perfit M. R. , Gill J. , Williams Q.
Published: September 2002

Parent‐daughter disequilibria between (230Th)/(238U), (231Pa)/(235U) and (226Ra)/(230Th) (parentheses refer to activities) have been measured by thermal ionization mass spectrometry and inductively coupled plasma‐mass spectrometry in basalts from three tectonomagmatic settings of the East Pacific Rise (EPR) at 8°20′‐10°N. Mid‐ocean ridge basalts (MORB) from the Siqueiros Transform, the Lamont Seamounts, and the EPR ridge crest span a large compositional range from primitive, high‐MgO basalts with strong incompatible element depletions (DMORB) to typical normal MORB (NMORB) to rare incompatible element enriched basalts (EMORB) derived from a more enriched source isotopically. Concentrations of U vary from <13 ppb in DMORB to >400 ppb in EMORB while Th/U ranges from 2 in DMORB up to 3 in EMORB. The young‐looking high‐MgO basalts have (226Ra)/(230Th) that ranges from 3.2 to 4.2, while EMORB appear old being near secular equilibrium. Initial (231Pa)(235U) are very high (>2.5) in all of the Siqueiros basalts. Three basalts from the Lamont Seamounts have low incompatible element concentrations and low Th/U and are in secular equilibrium for (226Ra)/(230Th) while the sample located closest to the ridge axis has significant 226Ra and 231Pa excesses and minor 230Th excess. DMORB lack 230Th excess, have high excesses of226Ra and 231Pa, and resemble experimentally determined melts of peridotite at 1 GPa, implying derivation from relatively shallow level melting of spinel lherzolite at low residual porosity. Disequilibria for all three parent‐daughter pairs are consistent with typical axial NMORB resulting from mixing of melts derived from heterogeneous sources, specifically 90–95% DMORB with 5–10% EMORB. The observation that all samples, regardless of tectonomagmatic setting, lie on the same mixing trend suggests that melting beneath seamounts and transforms is similar to melting beneath the ridge axis. Variations in 230Th excess over short spatial scales imply that garnet‐bearing mafic veins create all of the 230Th excess observed in typical NMORB.
A method for evaluating basement exhumation histories from closure age distributions of detrital minerals
Authors: Lovera Oscar M. , Grove Marty , Kimbrough David L. , Abbott Patrick L.
Published: September 2002

We have developed a two‐dimensional, thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement. Using this model, we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata. At relatively high mean denudation rates of 0.5 km m.y.−1sustained over millions of years, magmatic heating events have minimal influence upon the age distributions of detrital minerals such as K‐feldspar that are moderately retentive of radiogenic Ar. At lower rates, however, the effects of batholith emplacement may be substantial. We have applied the approach to detrital K‐feldspars from forearc strata derived from the deeply denuded Peninsular Ranges batholith (PRB). Agreement of the denudation history deduced from the detrital K‐feldspar data with thermochronologic constraints from exposed PRB basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known.
A probabilistic treatment of fluvial entrainment of cohesionless particles
Authors: Hunt A. G.
Published: September 2002

In the traditional treatment of sediment entrainment in fluvial hydrology, equating a turbulent fluid force arising from shear stress with a particle weight (adjusted for buoyancy) gives the condition on critical shear for particle entrainment. This stipulation leads to the entrainment condition that the volume d3 of the largest particles entrained is proportional to the sixth power of the friction velocity. An alternate approach is to consider streams as reservoirs of kinetic energy, analogous to heat baths, from which energy can be removed to lift a particle over a potential energy barrier. Following this analogy, the probability of transferring an energy, E, from the fluid to a particle, is proportional to the Boltzmann factor, exp(‐E/H), where H is the appropriate analog to kT, a unit of thermal energy associated with the heat bath. By making the analogy H = (1/2)ρw(ν*2)d3, where ν* is the friction velocity and ρw is the fluid density, the entrainment condition is recovered in a probabilistic sense, i.e. the probability of entrainment of particles larger than d drops rapidly to zero. The new condition may be interpreted in terms of the likelihood that eddies (similar in size to the particle concerned) transfer their energy to a particle. The advantage of this approach is that it can treat particles of arbitrary size on a bed of arbitrary geometry. Extension of the treatment to such collections of particles allows derivation of a self‐consistent integral equation for the distribution of particle sizes on a bed.
Redistribution of dynamic stress during coseismic ruptures: Evidence for fault interaction and earthquake triggering
Authors: Belardinelli Maria Elina , Cocco Massimo , Coutant Olivier , Cotton Fabrice
Published: September 2002

We investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault. The dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods. After a transient phase, the stress time history evolves to the final static stress value. We compare the static stress changes resulting from this model with those computed from a static dislocation model. We have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 (MS6.9) Irpinia earthquake. These two subevents are separated in time by nearly 20 s, while the third (and last) subevent occurred 40 s after the rupture onset. We compute the dynamic stress changes caused by the rupture of the first subevent. Our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault. On the average the static stress level on the second subevent (20 s) fault plane is reached nearly after 14 s. The dynamic rupture did not jump from a rupturing segment to the adjacent one immediately, but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event. The dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3.4 km/s. The delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults. In particular, rate‐ and state‐dependent frictional law can explain a delayed instability after a sudden change in stress. Using the estimated values of the subevent triggering delay and the shear stress change, we attempt to constrain the parameter Aσ on the 20 s fault. The values here inferred agree well with those resulting from previous studies.
Mechanics of low‐stress forearcs: Nankai and Cascadia
Authors: Wang Kelin , He Jiangheng
Published: September 2002

The Nankai and Cascadia subduction zones have low‐stress forearcs, where the margin‐normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms. In the absence of active back‐arc spreading, the forearc margin‐normal stress is controlled mainly by the total shear force along the subduction fault (or the plate coupling force) and the gravitational force. The plate coupling force creates compression, and the gravitational force, in the presence of margin topography, creates lateral tension. We give a simple expression that relates the forearc stress to topography and average fault strength, with the latter represented by an effective coefficient of friction μ′. We also use a finite element model of two converging plates in frictional contact to model the forearc stresses. The analyses indicate that a low‐stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth. For Nankai and Cascadia, where the coupled fault areas are narrow and shallow because of the young and hot subducting plates, μ′ is about 0.05 or less and in any case less than 0.09. The low fault strength is probably not a unique property of low‐stress forearcs, since the same low μ′ values combined with wider and deeper coupled areas can be shown to create large margin‐normal compression. In addition, there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces. Great earthquakes occur at subduction zones having low‐stress forearcs such as Nankai and Cascadia.
Delayed magnetization of the deeper kilometer of oceanic crust at Ocean Drilling Project Site 504
Authors: Hall James M. , Muzzatti Angelina
Published: September 2002

The Fe‐Ti magnetic carriers of the dike‐rich second kilometer of oceanic crust at Ocean Drilling Program Site 504 are shown to be secondary in origin, that is, their formation is likely to considerably postdate initial igneous crustal formation. An important consequence of the delay in formation of carriers is that the polarity of their magnetization has a significant probability of opposing that of the overlying, little‐altered extrusives. If this is the case, the linear magnetic anomalies in the area have sources crust, which, at least locally, is magnetized with successive layers of opposite polarities. The magnitude of the contribution of deeper layers to anomalies will depend both on their thickness and average magnetization, which are not as yet well known. Two types of secondary magnetite, SMI and SMII, separated spatially and by the steps leading to their formation, are recognized in the dikes of the second kilometer of the crust at Site 504. Both were formed in conditions of hydrothermal greenschist metamorphism. Since all newly formed oceanic crust is thought to experience this type of alteration at an early stage, polarity reversal with depth as a result of the delayed formation of magnetic carriers is likely to be widespread.
A model for growth of steep‐sided vent structures on the Endeavour Segment of the Juan de Fuca Ridge: Results of a petrologic and geochemical study
Authors: Tivey Margaret Kingston , Stakes Debra S. , Cook Terri L. , Hannington Mark D. , Petersen Sven
Published: September 2002

Petrologic and geochemical studies of vent solids from the Main Endeavour Field (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate that the steep‐sided vent structures characteristic of these sites form dominantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical calculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of vent fluid decreases, ammonia‐ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia‐poor fluids present at most other unsedimented mid‐ocean ridge vent sites. Deposition of silica stabilizes flanges and allows structures to attain large size. It also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones, thus decreasing entrainment of seawater. Most inactive vent samples recovered from areas peripheral to the HRF and MEF are similar to barite + silica rich samples from the Explorer Ridge and Axial Seamount and likely formed from precipitation of silica and barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF, are pyrrhotite rich and likely formed from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans‐Atlantic geotraverse (TAG) and Cleft Segment white smoker chimneys.
Tectonic evolution of the Tancheng‐Lujiang (Tan‐Lu) fault via Middle Triassic to Early Cenozoic paleomagnetic data
Authors: Gilder Stuart A. , Leloup P. Hervé , Courtillot Vincent , Chen Yan , Coe Robert S. , Zhao Xixi , Xiao Wenjiao , Halim Nadir , Cogné Jean‐Pascal , Zhu Rixiang
Published: September 2002

The north‐striking Tancheng‐Lujiang (Tan‐Lu) fault is a conspicuous and controversial feature of the eastern Asian landscape. Near the southeast extremity of the fault in Anhui Province, we collected paleomagnetic samples at 17 Middle Triassic (T2) and 10 Upper Cretaceous (K2) to lower Cenozoic (E1) sites. T2 remanent magnetizations are interpreted as primary in two of three areas. The three areas are rotated 37° to 137° counterclockwise with respect to the South China Block (SCB) reference direction. K2‐E1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas. Counterclockwise rotation of T2 strata therefore ended before K2 and is attributed to left lateral shear acting along Tan‐Lu during the North China Block (NCB)‐SCB collision. In Shandong Province, 700 km north of the Anhui sites, four areas containing 33 Upper Jurassic (J3) and Cretaceous sites have negligible declination differences, except for one which has dispersed directions. The fold test is inconclusive for this latter area and positive for the other three. Regional concordance of the J3‐E1 paleomagnetic data (including paleolatitudes) together with observed deformation patterns suggest that an extensional regime prevailed in the Late Cretaceous and Cenozoic. Euler pole positions that constrain the North‐South China collision and account for Tan‐Lu motion suggest at least 500 km of sinistral shear took place along the fault, and either (1) subduction and related ultrahigh pressure (UHP) metamorphism occurred near the present location of the Qinling‐Dabieshan and Sulu UHP belts while Tan‐Lu acted as a transform fault that connected the two subduction zones, or (2) Tan‐Lu and Sulu were parts of the same transform fault system and no UHP rocks formed in situ at Sulu. In either case, UHP rocks originally exhumed near Dabieshan could have been transported by plate capture toward Sulu along Tan‐Lu. After North and South China impacted near Dabieshan, the Tan‐Lu fault grew within the SCB as the Dabieshan corner indented the SCB, causing folds in SCB cover rocks to conform to the NCB margin. Late Cretaceous to Cenozoic reactivation of Tan‐Lu, with both right lateral strike‐slip and normal fault motion, occurred as the SCB extruded east relative to the NCB under the influence of the India‐Asia collision.
Nature and origin of upper crustal seismic velocity fluctuations and associated scaling properties: Combined stochastic analyses of KTB velocity and lithology logs
Authors: Goff John A. , Holliger Klaus
Published: September 2002

The main borehole of the German Continental Deep Drilling Program (KTB) extends over 9000 m into a crystalline upper crust consisting primarily of interlayered gneiss and metabasite. We present a joint analysis of the velocity and lithology logs in an effort to extract the lithology component of the velocity log. Covariance analysis of lithology log, approximated as a binary series, indicates that it may originate from the superposition of two Brownian stochastic processes (fractal dimension 1.5) with characteristic scales of ∼2800 m and ∼150 m, respectively. Covariance analysis of the velocity fluctuations provides evidence for the superposition of four stochastic process with distinct characteristic scales. The largest two scales are identical to those derived from the lithology, confirming that these scales of velocity heterogeneity are caused by lithology variations. The third characteristic scale, ∼20 m, also a Brownian process, is probably related to fracturing based on correlation with the resistivity log. The superposition of these three Brownian processes closely mimics the commonly observed 1/k decay (fractal dimension 2.0) of the velocity power spectrum. The smallest scale process (characteristic scale ∼1.7 m) requires a low fractal dimension, ∼1.0, and accounts for ∼60% of the total rms velocity variation. A comparison of successive logs from 6900–7140 m depth indicates that such variations are not repeatable and thus probably do not represent true velocity variations in the crust. The results of this study resolve disparity between the differing published estimates of seismic heterogeneity based on the KTB sonic logs, and bridge the gap between estimates of crustal heterogeneity from geologic maps and borehole logs.
Effects of fO2 and H2O on andesite phase relations between 2 and 4 kbar
Authors: Martel Caroline , Pichavant Michel , Holtz Francois , Scaillet Bruno , Bourdier Jean‐Louis , Traineau Hervé
Published: September 2002

Experimental phase equilibria have been investigated on three medium‐K silicic andesite (60–61 wt % SiO2) samples from Mount Pelée at 2–4 kbar, 850–1040°C, under both vapor‐saturated CO2‐free and vapor‐saturated CO2‐bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal‐ and rapid quench experiments were performed. Two ranges of oxygen fugacity (fO2) were investigated: NNO (Ni‐NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low‐Ca pyroxene (opx); these three phases coexist over a large temperature (T)‐H2O range (875–950°C and 5–7 wt % H2O in melt). Amphibole (am) is stable under near vapor‐saturated CO2‐free conditions at 876°C. At 900°C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor‐saturated CO2‐free conditions, pl + mt is replaced by am + mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pl and opx, except in the most H2O‐rich part of the diagram at 930°C, where opx is replaced by Ca‐rich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changingfO2 Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pl + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For fO2 conditions typical of calk‐alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near‐liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents.
Paleomagnetic study of upper Miocene rocks from northern Chile: Implications for the origin of Late Miocene‐Recent tectonic rotations in the southern Central Andes
Authors: Somoza Rubén , Singer Silvia , Tomlinson Andrew
Published: September 2002

Paleomagnetic studies in the southern Central Andes have shown the widespread presence of clockwise vertical‐axis rotations. Rock units sampled in these studies, however, are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern Chile being from Mesozoic and lower Tertiary rocks, whereas most data in the southern Altiplano, Puna, and Cordillera Oriental are from upper Tertiary rocks. In this paper we present the results of a paleomagnetic study on upper Miocene sedimentary rocks and ignimbrites from the Precordillera of northern Chile (at 22°S). These rocks are coeval with the initiation of crustal shortening in the eastern foreland fold‐thrust belt which some tectonic models relate to oroclinal rotation of northern Chile. Primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites, suggesting the study area has acted as a single coherent block with respect to vertical‐axis rotational deformation. Although minor inadequate sampling of paleosecular variation can affect our data set, its time‐averaged paleomagnetic direction is indistinguishable from the expected late Miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 Ma. This suggests that late Miocene‐Recent oroclinal rotation of the northern Chilean forearc, if present, must be either very low or nonuniform. A direct implication of this result is that much of the unquestionable tectonic rotations detected from upper Miocene rocks in the southern Central Andes is of local origin. Available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the Andes.