bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamical response of the magnetosphere‐ionosphere system to a solar wind dynamic pressure oscillation
Authors: Motoba Tetsuo , Kikuchi Takashi , Okuzawa Takashi , Yumoto Kiyofumi
Published: May 2003

On 28 February 1998, four quasi‐periodic pressure pulses with an amplitude of a few nPa detected by ACE gave rise to periodic compressions of the magnetosphere with period of about 14 min. In concert with periodic compressed and expanded states of the magnetosphere forced directly by the pressure variation, a coherent geomagnetic field fluctuation with the same period appeared on a global scale and was recorded at stations located from polar to equatorial regions. Most ground‐level geomagnetic field signatures on the dayside can be interpreted as the result of a global ionospheric current system, like the global Pc5 event examined by Motoba et al. [2002]. In the afternoon polar ionosphere covered with the dense magnetometer stations, a vortical current structure associated with pressure‐induced field‐aligned currents (FACs) is centered at 72° ± 1° and consists of a counterclockwise (clockwise) vortex in response to positive (negative) changes of solar wind pressure oscillation. Although the vortical current signatures are unclear in the morning sector, each afternoon vortex could pair with the morning one with opposite rotation. During this event, the interplanetary magnetic field (IMF) remained steady with a strong southward orientation (−10 nT or less). In addition to the pressure‐induced FAC system, the steady southward IMF drives the dayside Region 1 (R1) current system, resulting in the familiar large‐scale two‐cell convection pattern in the ionosphere observed by SuperDARN radars. The SuperDARN convection patterns indicated that the ionospheric convection reversal boundary (CRB) in the afternoon was located in the range of 73° ∼ 77°N around 15 MLT. The ionospheric footprint of the pressure‐induced FAC in the afternoon was found to be 1.5° ± 1.1° ∼ 4.0° ± 1.4° equatorward of the CRB. This suggests that the pressure‐induced FAC is started inside the R1 current system originating from the outer magnetospheric boundary layer. We argue that the paired FAC system responsible for the global geomagnetic fluctuations on the ground arises from the oscillatory large‐scale dynamical convection originating well inside the closed field lines in direct response to the quasi‐periodic pressure variations, not from the localized undulations on the magnetopause nor from global eigenmode oscillations of the magnetospheric cavity.
Intense space storms: Critical issues and open disputes
Authors: Daglis I. A. , Kozyra J. U. , Kamide Y. , Vassiliadis D. , Sharma A. S. , Liemohn M. W. , Gonzalez W. D. , Tsurutani B. T. , Lu G.
Published: May 2003

This paper addresses the question of particular characteristics and causes of intense space storms. We focus on several unresolved issues, which are critical to storm research and often nourish open disputes: the extent of interplanetary driving, the role of substorms in storm dynamics through the acceleration of particles to ring current energies, the identity of the lead agent of fast ring current decay right after storm maximum and the cause of the two‐phase recovery of intense storms, the global morphology of the storm‐time ring current, and the predictability of intense storms. Space storm physics has been driven by several more or less successful paradigms during the four decades following the dawn of the space flight era. However, recent suggestions and conclusions resulting from a number of observational and modeling studies have brought significant constraints to several of these paradigms. Thus for example, interplanetary driving through southward oriented magnetic fields is not always by itself sufficient to drive intense space storms because it is conditioned by internal magnetospheric conditions, the MLT distribution of storm‐time magnetic disturbances is often asymmetric during the storm main phase, and charge exchange is not the lead agent of ring current decay at all times.
Lobe cell convection and polar cap precipitation
Authors: Eriksson S. , Peria W. J. , Bonnell J. W. , Su Y.‐J. , Ergun R. E. , Tung Y.‐K. , Parks G. K. , Carlson C. W.
Published: May 2003

The characteristic electric and magnetic field signature of lobe cells as observed by the low‐altitude FAST satellite in 55 dawn‐dusk passes are compared with Polar ultraviolet images of polar cap auroral activity. Initial results from 34 events of UV image coverage suggest that there is an intimate coupling between the sunward convection flow of the lobe cell and transpolar auroral arcs or diffuse polar cap precipitation in ∼62% of these cases. However, in some cases where the field signatures are suggestive of lobe cell convection, there is no detectable particle precipitation either in Polar UVI or the FAST data sets. Moreover, the presence of lobe cells coincide with UV data intensifications in the premidnight 2000–2400 MLT sector and/or the postnoon 1500 MLT region in ∼59% of all cases with UVI coverage. The magnetic local time dependence of the lobe cells and polar cap precipitation on the interplanetary magnetic field (IMF) are examined using the upstream Wind monitor. The relative importance of the IMF By and Bz components are investigated and compared with the predictions of the antiparallel merging model and strongly suggests a connection with the magnetospheric sash, as is further implied by the mapping of magnetic field lines using the Tsyganenko [2002] (T01) model. It was also noted that a majority of lobe cell events occurred during enhanced AE index substorm‐like conditions and that generally stronger AE indices are measured for stronger IMF By magnitudes during these events.
Long‐lasting disturbances in the equatorial ionospheric electric field simulated with a coupled magnetosphere‐ionosphere‐thermosphere model
Authors: Richmond A. D. , Peymirat C. , Roble R. G.
Published: May 2003

The Magnetosphere‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric‐wind‐dynamo influences on low‐latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time‐varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region‐2 field‐aligned currents; and (3) disturbance winds driven by high‐latitude heating and ion‐drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance‐wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low‐latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night‐side east‐west electric fields. The simulations partly support the theories of the so‐called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long‐lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance‐wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind‐produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region‐2 currents.
CRRES electric field study of the radial mode structure of Pi2 pulsations
Authors: Takahashi Kazue , Lee Dong‐Hun , Nosé Masahito , Anderson Roger R. , Hughes W. Jeffrey
Published: May 2003

The radial mode structure of Pi2 pulsations in the inner magnetosphere (L < 7) and its relation to the plasmapause are studied using data acquired by the Combined Release and Radiation Effects Satellite (CRRES) between August 1990 and September 1991. Low‐latitude Pi2 pulsations detected on the ground at Kakioka (L = 1.25) are used as the reference signal to determine the relative amplitude and phase of the electric field oscillations detected at CRRES. The plasmapause is identified using electron density inferred from the plasma wave spectra observed on CRRES. Pi2 events at CRRES are defined to be 10‐min intervals of high coherence between oscillations in the Kakioka horizontal northward magnetic field (H) and CRRES dusk‐to‐dawn electric field (Eφ) components within the Pi2 band (6–25 mHz). The Eφ component represents the poloidal oscillation of the geomagnetic field lines for satellite local times near midnight. Fifty‐five high‐coherence Eφ‐H Pi2 events occurred when both CRRES and Kakioka were within 3 hours of magnetic midnight. For these events CRRES was on L shells ranging from 2 to 6.5 and was either in the plasmasphere or in the close vicinity of the plasmapause, providing evidence for the plasmaspheric origin of low‐latitude Pi2 pulsations. The amplitude of Eφ varied significantly but there is an indication of a maximum near L = 4. The phase of Eφ (relative to Kakioka H) remained near −90° at all distances. These properties are consistent with the radial structure of the fundamental cavity mode oscillations confined in the plasmasphere. For some events observed at L > 3.5 it was also possible to determine the amplitude and phase of the compressional component Bz at CRRES. In contrast to Eφ, the phase of Bz (relative to H) was clustered both at ∼180° and ∼0 for events occurring near the plasmapause. This observation still is consistent with the cavity mode according to a numerical simulation using a dipole magnetic field and a realistic plasmapause plasma density structure, which indicates that the node of Bz is located near the plasmapause. Depending on the satellite position relative to the node, the phase can be either –180° or 0. A negative correlation is found between the Pi2 frequency and the distance of the plasmapause, which is additional support for the cavity mode origin of low‐latitude Pi2 pulsations.
Climatology and latitudinal gradients of quiet time thermospheric neutral winds over Millstone Hill from Fabry‐Perot interferometer measurements
Authors: Emmert J. T. , Fejer B. G. , Sipler D. P.
Published: May 2003

Midlatitude nighttime thermospheric neutral winds are strongly dependent on season, solar activity, and latitude. We use an extensive database of wind measurements made during 1989–2001 by the Millstone Hill Fabry‐Perot interferometer to study the detailed climatology of quiet time neutral winds near an altitude of 250 km. To facilitate the analysis of these data, we develop a local time, day‐of‐year, solar flux, and latitude‐dependent empirical model, with the latitude dependence obtained by considering north looking and south looking observations separately. Our results show that the zonal winds are predominantly eastward after dusk and westward before dawn, with the strongest eastward winds occurring in the winter and with an east‐to‐west transition that occurs earliest in the summer. The zonal winds exhibit weak‐to‐moderate latitudinal gradients, with more westward values to the north. The zonal wind magnitudes decrease with increasing solar flux; the strongest trends occur during winter. The meridional winds are predominantly equatorward in all cases and exhibit strong latitudinal gradients, with larger values to the north. The maximum nighttime equatorward winds decrease with increasing solar flux, except during summer, when there is no significant solar activity variation. They are largest during the summer, except at solar minimum when a semiannual variation is observed and the peak winds occur during the equinoxes. Earlier studies of midlatitude wind measurements are generally consistent with our data, with our results providing a considerably more detailed description of the nighttime wind climatology at midlatitudes.
A numerical simulation of Earth's electromagnetic cavity with the Transmission Line Matrix method: Schumann resonances
Authors: Morente Juan A. , Molina‐Cuberos Gregorio J. , Portí Jorge A. , Besser Bruno P. , Salinas Alfonso , Schwingenschuch Konrad , Lichtenegger Herbert
Published: May 2003

A numerical modeling of Earth's atmosphere is carried out by means of the Transmission Line Matrix (TLM) numerical method with the aim of calculating the Schumann resonance frequencies. The numerical results obtained are very close to the experimental ones and those obtained with the widely accepted two‐scale‐height ionospheric model, which allows us to affirm that this is a valid numerical tool for predicting the Schumann frequencies in the atmospheres of other planets and moons. The great flexibility of the TLM numerical method also allows the study of slight shifts in the Schumann resonance frequencies due to an increase in electrical conductivity at the Earth's poles, originating from high‐energy particle precipitation emitted from the Sun in conjunction with solar flares. A slight increase in the first Schumann resonance frequency is observed during these events, which is associated with a reduction in the dimensions of the electromagnetic cavity.
Storm‐time distortion of the inner magnetosphere: How severe can it get?
Authors: Tsyganenko N. A. , Singer H. J. , Kasper J. C.
Published: May 2003

First results are presented of an effort to model the storm‐time distortion of the magnetic field in the inner magnetosphere using space magnetometer data. Strong geomagnetic storms are relatively rare events, represented by only a small fraction of the data used in the derivation of existing empirical geomagnetic field models. Hence using those models for the mapping of the storm‐time magnetosphere is at most an extrapolation based on trends, obtained from quiet and moderately disturbed data. To overcome that limitation, a set of data was created, containing only clear‐cut events with Dst ≤ −65 nT, with the goal to derive models of the inner and near geomagnetic field (R < 15 RE), representing strongly disturbed geomagnetic configurations and their evolution during the storm cycle. The final data set included about 143,000 records with 5‐min average B‐vectors, covering 37 major storms between 1996 and 2000. Most of the data came from GOES‐8, ‐9, ‐10, Polar, and Geotail spacecraft, and two storms in February–March of 1998 were also partially covered by the data of Equator‐S. In all cases, only those storms were selected for which concurrent solar wind and IMF data were available for the entire duration of the event. Interplanetary medium data were provided by Wind, ACE, and, to a lesser extent, by IMP 8 and Geotail. The inner magnetospheric field was represented using the newly developed T01 model [Tsyganenko, 2002a, 2002b], with a duskside partial ring current with variable amplitude and scale size, an essential part of the storm‐time current system. The modeling revealed an enormous distortion and dawn‐dusk asymmetry of the inner magnetosphere during the peak of the storm main phase, caused by the combined effect of the symmetric and partial ring currents, cross‐tail current, and Birkeland currents. We found that during storms with Dst < −250 nT the tail‐like deformation of the nightside field penetrates so close to Earth that the quasi‐dipolar approximation breaks down at distances as small as 3–4 RE. This finding yields a quantitative answer to the question of why the auroras expand to unusually low latitudes during extremely strong storms. It also may provide a natural explanation for the observed impulsive injections and energizations of charged particles on the innermost L‐shells. Finally, it questions the validity of using the dipole or quasi‐dipole approximation in numerical simulations of severe storms in the inner magnetosphere.
Interplanetary coronal mass ejections in the near‐Earth solar wind during 1996–2002
Authors: Cane H. V. , Richardson I. G.
Published: May 2003

We summarize the occurrence of interplanetary coronal mass ejections (ICMEs) in the near‐Earth solar wind during 1996–2002, corresponding to the increasing and maximum phases of solar cycle 23. In particular, we give a detailed list of such events. This list, based on in situ observations, is not confined to subsets of ICMEs, such as “magnetic clouds” or those preceded by “halo” coronal mass injections (CMEs) observed by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph, and provides an overview of 214 ICMEs in the near‐Earth solar wind during this period. The ICME rate increases by about an order of magnitude from solar minimum to solar maximum (when the rate is ∼3 ICMEs per solar rotation period). The rate also shows a temporary reduction during 1999 and another brief, deeper reduction in late 2000 to early 2001, which only approximately track variations in the solar 10‐cm flux. In addition, there are occasional periods of several rotations duration when the ICME rate is enhanced in association with high solar activity levels. We find an indication of a periodic variation in the ICME rate, with a prominent period of ∼165 days similar to that previously reported in various solar phenomena. It is found that the fraction of ICMEs that are magnetic clouds has a solar cycle variation, the fraction being larger near solar minimum. For the subset of events that we could associate with a CME at the Sun the transit speeds from the Sun to the Earth were highest after solar maximum.
Remaining error sources in the nutation at the submilliarc second level
Authors: Dehant V. , Feissel‐Vernier M. , de Viron O. , Ma C. , Yseboodt M. , Bizouard C.
Published: May 2003

Earth's precession and nutations are mainly generated by the luni‐solar tidal torque. Diurnal retrograde variations in the atmospheric and oceanic angular momenta in an Earth‐fixed reference system induce some additional nutation motions. Observed precession and nutations are derived from very long baseline interferometry (VLBI) data, assuming that the direction of the observed quasars are fixed in space. In this study, we consider the effects of two possible causes for explaining discrepancies between the observed nutations and those modeled in MHB2000 (model adopted by the International Astronomical Union): (1) the time variations in the atmospheric (and potentially oceanic) forcing of the nutations, of the free core nutation (FCN), and of the free inner core nutation (FICN), and (2) the possible contamination of VLBI‐derived nutation amplitudes by apparent changes in the directions of the extragalactic radio sources. The robustness of MHB2000 is tested by perturbing some of the parameters and assessing the validity of the resulting nutation amplitudes against realistic estimations. We show that even allowing for large discrepancies related to atmospheric forcing, the ranges of the possible changes in the FCN and FICN periods and damping factors are small.
A testable model of earthquake probability based on changes in mean event size
Authors: Imoto Masajiro
Published: May 2003

We studied changes in mean event size using data on microearthquakes obtained from a local network in Kanto, central Japan, from a viewpoint that a mean event size tends to increase as the critical point is approached. A parameter describing changes was defined using a simple weighting average procedure. In order to obtain the distribution of the parameter in the background, we surveyed values of the parameter from 1982 to 1999 in a 160 × 160 × 80 km volume. The 16 events of M5.5 or larger in this volume were selected as target events. The conditional distribution of the parameter was estimated from the 16 values, each of which referred to the value immediately prior to each target event. The distribution of the background becomes a function of symmetry, the center of which corresponds to no change in b value. In contrast, the conditional distribution exhibits an asymmetric feature, which tends to decrease the b value. The difference in the distributions between the two groups was significant and provided us a hazard function for estimating earthquake probabilities. Comparing the hazard function with a Poisson process, we obtained an Akaike Information Criterion (AIC) reduction of 24. This reduction agreed closely with the probability gains of a retrospective study in a range of 2–4. A successful example of the proposed model can be seen in the earthquake of 3 June 2000, which is the only event during the period of prospective testing.
Crustal shear velocity structure of the south Indian shield
Authors: Rai S. S. , Priestley Keith , Suryaprakasam K. , Srinagesh D. , Gaur V. K. , Du Z.
Published: May 2003

The south Indian shield is a collage of Precambrian terrains gathered around and in part derived from the Archean‐age Dharwar craton. We operated seven broadband seismographs on the shield along a N‐S corridor from Nanded (NND) to Bangalore (BGL) and used data from these to determine the seismic characteristics of this part of the shield. Surface wave dispersion and receiver function data from these sites and the Geoscope station at Hyderabad (HYB) give the shear wave velocity structure of the crust along this 600 km long transect. Inversion of Rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3.66 km s−1, 12 km thick layer overlying a 3.81 km s−1, 23 km thick lower crust. At all sites, the receiver functions are extremely simple, indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities. Joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the Dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds: crustal thickness (35 ± 2 km), average shear wave speed (3.79 ± 0.09 km s−1), and Vp/Vs ratio (1.746 ± 0.014). There is no evidence for a high velocity basal layer in the receiver function crustal images of the central Dharwar craton, suggesting that there is no seismically distinct layer of mafic cumulates overlying the Moho and implying that the base of the Dharwar crust has remained fairly refractory since its cratonization.
Neutral atmospheric delay in interferometric synthetic aperture radar applications: Statistical description and mitigation
Authors: Emardson T. R. , Simons M. , Webb F. H.
Published: May 2003

Variations in the refractive index of the atmosphere cause variations in satellite‐based interferometric synthetic aperture radar (InSAR) observations. We can mitigate tropospheric effects by averaging N‐independent interferograms. Because the neutral atmosphere is uncorrelated at timescales longer than 1 day, using this technique statistically reduces the variance, σ2, of the noise by a factor of N. Using zenith neutral atmospheric delays from Global Positioning System (GPS) data from the Southern California Integrated GPS Network, we find that the average variance depends on the distance between observations, L, and height difference, H, as σ = c Lα + kH with estimated values for c, α, and k of about 2.5, 0.5, and 4.8, respectively, where σ is in mm and L and H are in km. We expect that the value of α is largely site‐independent but the value of c will depend on the water vapor variability of the area of interest. This model is valid over a range of L between approximately 10 and 800 km. Height differences between 0 and 3 km have been used in this analysis. For distances of 100 and 10 km with negligible height differences, σ is estimated to be approximately 25 and 8 mm, respectively. For a given orbit revisit time and image archive duration, we calculate the number and duration (assumed constant) of interferograms required to achieve a desired sensitivity to deformation rate at a given length scale. Assuming neutral atmosphere is the dominant source of noise, a 30° look angle, and an image revisit time of 7 days, detection of a deformation rate of 1 mm yr−1 over distances of 10 km requires about 2.2 years of continuous observations. Given our results, we suggest a data covariance structure to use when using InSAR data to constrain geophysical models.
Compensation effect for electrical conductivity and its applications to estimate oxygen diffusivity in minerals
Authors: Wu Xiaoping , Zheng Yong‐Fei
Published: May 2003

The compensation effect for electrical conductivity is observed for minerals such as calcite, corundum, enstatite, ferrosilite, olivine, leucite, and periclase, which is referred to a linear correlation between the natural logarithm of preexponential factor and activation energy for the electrical conductivity in each mineral. It is shown that the electrical conductivity due to different conducting species tends to converge to a constant at a characteristic temperature in each mineral. On this basis, the preexponential factor for the oxygen diffusion coefficient is calculated from the electrical conductivity by the Nernst‐Einstein equation, while the activation energy for oxygen diffusion is determined by anion porosity. The estimated oxygen diffusion coefficients from the electrical conductivity in minerals obey the compensation law well, and the linear compensation equation is in good agreement with that from the experimentally determined oxygen diffusion data. Experimentally determined cation diffusion coefficients also test the validity of our method. Comparison between the estimated oxygen diffusion coefficients and the determinations by the ionic porosity method shows that the present method provides useful constraints on the other empirical model to predict diffusion coefficient.
Seismic velocity anisotropy and heterogeneity beneath the Mantle Electromagnetic and Tomography Experiment (MELT) region of the East Pacific Rise from analysis of P and S body waves
Authors: Hammond William C. , Toomey Douglas R.
Published: May 2003

We use teleseismic P and S delay times and shear wave splitting measurements to constrain isotropic and anisotropic heterogeneity in the mantle beneath the southern East Pacific Rise (SEPR). The data comprise 462 P and S delay times and 18 shear wave splitting observations recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment. We estimate the mantle melt content (F) and temperature (T) variation from the isotropic velocity variation. Our results indicate that the maximum variation in F beneath our array is between zero and ∼1.2%, and maximum variation in T is between zero and ∼100 K. We favor an explanation having partial contributions from both T and F. We approximate the seismic anisotropy of the upper mantle with hexagonal symmetry, consistent with the assumption of two dimensionality of mantle flow. Our new tomographic technique uses a nonlinear inversion of P and slow S polarization delay times to simultaneously solve for coupled VP and VS heterogeneity throughout the model and for the magnitude of anisotropy within discrete domains. The domain dimensions and the dip of the anisotropy are fixed for each inversion but are varied in a grid search, obtaining the misfit of the models to the body wave delay data and to split times of vertically propagating S waves. The data misfit and the isotropic heterogeneity are sensitive to domain dimensions and dip of anisotropy. In a region centered beneath the SEPR the best average dip of the hexagonal symmetry axis is horizontal or dipping shallowly (<30°) west. Given the resolution of our data, a subaxial region characterized by vertically aligned symmetry axes may exist but is limited to be <80 km deep. We infer that the mantle flow beneath the SEPR is consistent with shallow asthenospheric return flow from the direction of the South Pacific superswell.
SKS splitting measurements in the Apenninic‐Tyrrhenian domain (Italy) and their relation with lithospheric subduction and mantle convection
Authors: Margheriti L. , Lucente F. P. , Pondrelli S.
Published: May 2003

We present a compilation of singular shear wave splitting measurements in the Italian region. We used the method of Silver and Chan [1991] to determine the splitting parameters of fast direction and delay time from broadband SKS waveforms. The stations analyzed were mainly deployed in temporary experiments; a few are permanent MedNet broadband stations. The splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath Italy. The fast polarization directions define two anisotropic domains: a Tyrrhenian domain with E‐W prevalent fast directions and an Apennines‐Adriatic domain with NW‐SE to NNW‐SSE prevalent fast directions. The stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths. This variability is a sign of a complex upper mantle structure (also shown by seismic tomography) but allows us to constrain the depth of the anisotropic layer in the 100–250 km range, at least in the boundary zone between the two domains. Data at station VSL (Sardinia) suggest the presence of depth‐dependent anisotropic structure, where the shallower signature is consistent with the Tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670‐km discontinuity. We argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding Tyrrhenian plate.
Study of harmonic site position variations determined by very long baseline interferometry
Authors: Petrov Leonid , Ma Chopo
Published: May 2003

Position variations of 40 very long baseline interferometry (VLBI) stations at 32 tidal frequencies were obtained from analysis of 3 million measurements of group delays from 1980 to 2002. Residual displacements after the removal of the effect of solid Earth tides were studied. The purpose of this study is to investigate a harmonic signal at any frequency where it is expected and to assess quantitatively whether the models of vertical and horizontal site position variations agree with the observations. It was found that the estimates of station displacements are generally in good agreement with the ocean loading computed on the basis of ocean tide models for the main diurnal and semidiurnal tides. Moreover, VLBI results allow us to discriminate between different ocean tide models. However, discrepancies between VLBI results and all models of ocean loading forK1,K2, andS2tides exceed both the errors of the VLBI estimates and the errors of ocean loading displacements based on the reported formal uncertainties of ocean tide models. It was found that there is a significant nontidal signal at diurnal and annual frequencies. Applying a model of hydrological loading reduces the variance of the residual vertical displacements at the annual frequency by 30%. Using an empirical model of harmonic site position variations in VLBI processing provides a better fit and improves the baseline length repeatability.
The Arctic plate boundary
Authors: Engen Øyvind , Eldholm Olav , Bungum Hilmar
Published: May 2003

Earthquakes provide information on the regional segmentation and seismotectonics of the poorly known boundary between the Eurasian and the North American plates from the Knipovich Ridge to the Laptev Sea continental margin. To this end, we have sorted earthquake epicenter locations and focal mechanism solutions from global and regional catalogs, assessed location errors and network detectabilities, and compiled a well‐constrained Arctic Catalog of events north of 72°N for the period 1955–1999. The seismological data are integrated with bathymetry and potential field data to locate ridge and transform segments along the Arctic mid‐oceanic ridge (MOR), and we suggest that the plate boundary is divided into four regional provinces. The Spitsbergen Transform System comprises a series of short ridge and transform segments. The seismicity is constrained to the plate boundary in the south but is diffuse in the north. Gakkel Ridge has clustered and focused seismicity and relatively elongated ridge segments cut by small‐offset transforms. A first‐order change in axis trend at 60°E divides the ridge into two areas of different morphology and geophysical character. The West Gakkel Ridge has an accentuated relief and high‐amplitude magnetic anomalies, whereas the East Gakkel Ridge has a smoother relief and lower magnetic amplitudes. At the Laptev Sea continental slope, the seismicity indicates that the change from ultraslow seafloor spreading to active continental rifting occurs over a less than 60 km wide continent–ocean transition (COT) with a 150–200 km long sheared margin segment.
Dispersion and anisotropy of elastic waves in cracked rocks
Authors: Schubnel Alexandre , Guéguen Yves
Published: May 2003

Rocks in the Earth's crust contain variable amount of cracks, depending on the deviatoric and confining stress levels, pore pressure, and temperature conditions. Crack damage results in effects that have been investigated for a long time, in particular, a decrease in elastic wave velocity and the development of anisotropy. In this paper, we focus on cracked rocks and develop a method to calculate both the elastic wave anisotropy and the dispersion in a fluid‐saturated cracked rock. We show that analytic expressions can be derived for both properties in terms of the crack density tensor components. Two fundamental theoretical tools are used in this purpose. The first one is the effective medium theory and the second one is the anisotropic poroelastic theory. Using both, it is possible from first principles to predict anisotropy and dispersion. Results are shown to be valid up to a total crack density of 0.5. The failure threshold appears close to a total crack density of 1. Between 0.5 and 1, the model still gives reasonably good results. Anisotropy can increase up to 60% and dispersion up to 30%, depending on the crack distribution. These results point out important differences that could exist in fluid‐saturated rocks between laboratory (high frequency) data and field (low frequency) results.
Fault creep along the southern San Andreas from interferometric synthetic aperture radar, permanent scatterers, and stacking
Authors: Lyons Suzanne , Sandwell David
Published: May 2003

Interferometric synthetic aperture radar (InSAR) provides a practical means of mapping creep along major strike‐slip faults. The small amplitude of the creep signal (<10 mm/yr), combined with its short wavelength, makes it difficult to extract from long time span interferograms, especially in agricultural or heavily vegetated areas. We utilize two approaches to extract the fault creep signal from 37 ERS SAR images along the southern San Andreas Fault. First, amplitude stacking is utilized to identify permanent scatterers, which are then used to weight the interferogram prior to spatial filtering. This weighting improves correlation and also provides a mask for poorly correlated areas. Second, the unwrapped phase is stacked to reduce tropospheric and other short‐wavelength noise. This combined processing enables us to recover the near‐field (∼200 m) slip signal across the fault due to shallow creep. Displacement maps from 60 interferograms reveal a diffuse secular strain buildup, punctuated by localized interseismic creep of 4–6 mm/yr line of sight (LOS, 12–18 mm/yr horizontal). With the exception of Durmid Hill, this entire segment of the southern San Andreas experienced right‐lateral triggered slip of up to 10 cm during the 3.5‐year period spanning the 1992 Landers earthquake. The deformation change following the 1999 Hector Mine earthquake was much smaller (<1 cm) and broader than for the Landers event. Profiles across the fault during the interseismic phase show peak‐to‐trough amplitude ranging from 15 to 25 mm/yr (horizontal component) and the minimum misfit models show a range of creeping/locking depth values that fit the data.
The volcano‐electric effect
Authors: Revil A. , Saracco G. , Labazuy P.
Published: May 2003

The formation of a magmatic intrusion at depth is responsible for the formation of various thermohydromechanical (THM) disturbances including the upsurge of shock waves and diffusion of pressure fronts in the volcanic system. We couple electromagnetic theory (Maxwell equations) and thermoporoelasticity (Biot equations) to look at the ground surface electrical signature of these THM disturbances. The nature of this coupling is electrokinetic, i.e., associated with water flow relative to the mineral framework and the drag of the excess of charge located in the vicinity of the pore water/mineral interface (the groundwater flow disturbance being related here to the THM disturbances in drained conditions). A new set of laboratory data shows that the electrokinetic coupling is very substantial in fractured basaltic and volcaniclastic materials, and in scoria with several hundreds of millivolts of electrical potential gradient produced per megapascal of pore fluid pressure variations. Our theoretical analysis predicts the diffusion of electromagnetic disturbances and quasi‐static electrical signals. These signals can be used as precursors of a volcanic eruption. Indeed, electromagnetic phenomena recorded at the ground surface of a volcanic system, once properly filtered to remove external contributions, provide a direct and quasi‐instantaneous insight into the THM disturbances occurring in the heart of the volcanic structure prior and during a volcanic event. Tomography of the quasi‐static electrical field is discussed and applied to self‐potential profiles performed at the Piton de la Fournaise volcano during the preparation phase of the March 1998 eruption.
Three‐dimensional crustal structure of the Vøring Margin (NE Atlantic): A combined seismic and gravity image
Authors: Torne M. , Fernandez M. , Wheeler W. , Karpuz R.
Published: May 2003

The three‐dimensional (3‐D) crustal structure of the Vøring Margin is investigated by integrating results from reflection and wide‐angle seismic data, well log data, and 3‐D gravity modeling. The reduced “crustal” Bouguer anomaly, obtained after computing the gravity effects of the water layer and Cenozoic and Cretaceous sedimentary cover, displays an arcuate‐shaped gravity high that coincides with the Cretaceous subbasins. The Pre‐Cretaceous layer varies in thickness from 7 to 8 km below the Vøring Marginal High to <2 km in the Mesozoic depocenters and is overlying a relative “high‐density” upper‐middle crust (2850 kg m−3). Underlying the upper‐middle crust, there is a “high‐velocity/density” body, which is imaged along the Marginal High and western region of the Vøring Basin. The High Velocity Body shallows to <14 km depth in the western end of the Marginal High and thins out toward the eastern and southeastern regions of the basin. Regions of thick crust (∼24 km) are located in the eastern and southern parts of the basin and northern and central parts of the Marginal High, whereas thinner crust is observed at the southwest region of the study area (∼16 km) and in the Fenris Graben region and Utgard High (∼18 km). Residual gravity anomalies are related to the distribution of interbedded volcanics and to the presence of sedimentary subbasins under the Marginal High. Crustal thinning is not directly related to thickening of the HVB and/or the location of Cretaceous depocentres, indicating a major rejuvenation of the Cretaceous Moho related to the Maastritchian‐Paleocene rifting episode and massive emplacement of igneous material during breakup.
Magma uprising and flank dynamics on Mount Etna volcano, studied using GPS data (1994–1995)
Authors: Bonforte Alessandro , Puglisi Giuseppe
Published: May 2003

Ground deformation occurring on Mount Etna from 1994 to 1995 is analyzed in this paper. This period was characterized by intense volcanic activity at the four summit craters, with frequent strombolian activity, fire fountains, and emplacement of several new lava flows. Four GPS surveys were carried out during this time, two as routinely planned each year and an additional two in 1995 to acquire more data to follow the activity at the NE Crater. The comparisons between GPS surveys are reported in terms of horizontal and vertical displacements of each station and in terms of areal dilatation and principal strain axes. During the period considered in this work, a trend of increasing areal dilatation of the volcano (at a rate of about 5 μstrain/yr) was recognized; it was briefly interrupted by a small contraction (about 2 μstrain), in the autumn of 1995, when volcanic activity at the summit craters began. In detail, the strain distribution of the network is analyzed; it allows the detection of areas showing anomalous behavior, such as the summit zone or the Pernicana fault. Inversions of the ground displacement vectors have been performed by appropriately combining numerical and analytical approaches. Results of the inversions suggest structures defining an eastward and southward sliding of the eastern and southeastern sectors of Mount Etna.
Topographically driven groundwater flow and the San Andreas heat flow paradox revisited
Authors: Saffer Demian M. , Bekins Barbara A. , Hickman Stephen
Published: May 2003

Evidence for a weak San Andreas Fault includes (1) borehole heat flow measurements that show no evidence for a frictionally generated heat flow anomaly and (2) the inferred orientation of σ1 nearly perpendicular to the fault trace. Interpretations of the stress orientation data remain controversial, at least in close proximity to the fault, leading some researchers to hypothesize that the San Andreas Fault is, in fact, strong and that its thermal signature may be removed or redistributed by topographically driven groundwater flow in areas of rugged topography, such as typify the San Andreas Fault system. To evaluate this scenario, we use a steady state, two‐dimensional model of coupled heat and fluid flow within cross sections oriented perpendicular to the fault and to the primary regional topography. Our results show that existing heat flow data near Parkfield, California, do not readily discriminate between the expected thermal signature of a strong fault and that of a weak fault. In contrast, for a wide range of groundwater flow scenarios in the Mojave Desert, models that include frictional heat generation along a strong fault are inconsistent with existing heat flow data, suggesting that the San Andreas Fault at this location is indeed weak. In both areas, comparison of modeling results and heat flow data suggest that advective redistribution of heat is minimal. The robust results for the Mojave region demonstrate that topographically driven groundwater flow, at least in two dimensions, is inadequate to obscure the frictionally generated heat flow anomaly from a strong fault. However, our results do not preclude the possibility of transient advective heat transport associated with earthquakes.
The North Anatolian fault in the Sea of Marmara
Authors: Le Pichon X. , Chamot‐Rooke N. , Rangin C. , Sengör A. M. C.
Published: May 2003

Using the detailed geometry and tectonics of the Main Marmara fault that connects the Gulf of Izmit fault with the Ganos fault established by Le Pichon et al. [2001] and the new GPS data set recently analyzed by Meade et al. [2002], we confirm the existence of a Marmara block delimited to the north by the northern branch of the North Anatolian fault (the Marmara branch) and to the south by the southern branch. The kinematics of this block indicate that the Marmara branch that crosses the Sea of Marmara is close to pure dextral strike slip at a rate of 23 mm/yr on its whole length. The short Cinarcik section is the only one where a significant extensional component is predicted. A simple kinematic model accounts for slip partitioning there, with 8–10 mm/yr of extension to the south of the basin and 23 mm/yr of dextral strike slip along the northern Cinarcik margin. The new GPS data demonstrate the existence of important asymmetric elastic loading along the Main Marmara fault that may have significant implications for the seismotectonics of the area. The expected future large earthquake may rupture the whole main Marmara fault. The partitioning in the Cinarcik basin may produce magnitude 7 normal faulting earthquakes if their repetition time is about 250 years. The 1896 earthquake may have been one of these normal fault earthquakes. The Marmara block is less than 1 Ma. Prior to its formation, it was part of the Anatolian block. At this time the Ganos fault was already a dextral strike‐slip fault, but the fault system in the Sea of Marmara would have been affected by an extensional component increasing toward the east that accounts for the increasing size of the sedimentary basins eastward.
Geomorphic interpretation of low‐temperature thermochronologic data: Insights from two‐dimensional thermal modeling
Authors: Safran Elizabeth B.
Published: May 2003

Low‐temperature thermochronologic data are useful for estimating mountain erosion rates because late stage cooling in mountain ranges often reflects exhumation via geomorphic erosion processes. Traditional interpretations of thermochronologic data assume steady, uniform erosion, conditions often violated in real mountain ranges. Here I examine whether unsteady or nonuniform erosion histories are detectable in patterns of erosion rate estimates made by traditional means. To do so, I numerically model the subsurface temperature field in a two‐dimensional section of Earth's crust under various illustrative geomorphic scenarios. I generate simulated cooling ages within two isotopic systems, apatite fission track (FT) and (U‐Th)/He, (He) at two landscape locations, ridge tops and valley bottoms. From these ages I make “field estimates” of erosion rates using the commonly employed elevation dependence and mineral pair methods. Calculations show that if errors on erosion rate estimates are kept small by replicate or transect sampling, many unsteady or nonuniform erosional histories do produce distinctive patterns of erosion rate estimates. For example, temporally increasing erosion rates produce poor agreement between erosion rate estimates made with a single method, while decreasing erosion rate produces good agreement between estimates made with a single method. Nonuniform erosion that decreases relief can produce erroneously high erosion rate estimates based on the elevation dependence method but also accurate and relatively precise mineral pair‐based estimates. A history of decreasing relief can be distinguished from one of steady, uniform erosion by (1) close agreement between mineral pair estimates and poor agreement between elevation dependence estimates and (2) the magnitude of the difference between FT and He ages. The insights gleaned from these analyses can guide forward modeling of topographic and thermal evolution.