bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐pressure phase transition in Al(OH)3: Raman and X‐ray observations
Authors: Huang Eugene , Li Alice , Xu Ji‐an , Chen Rong‐Jer , Yamanaka Takamitsu
Published: February 2004

High‐pressure Raman spectra and X‐ray diffraction patterns of gibbsite have been investigated up to 23.3 GPa and 8.7 GPa, respectively in diamond anvil cells at room temperature. More than 20 Raman‐active modes were observed at the ambient conditions and these modes were predominately distributed in two regions: the low‐wavenumber (LW, 200 to 1200 cm−1) and high‐wavenumber regions (HW, 3000 to 3700 cm−1). The wavenumber of Raman modes in LW region increases linearly with pressure while the wavenumber of those in the HW region decreases with pressure. The later observation indicates the enhancement of hydrogen bonds between octahedral layers in the lattice upon compression.

A significant change in the diffraction patterns and Raman spectra was observed for Al(OH)3 above 3 GPa where a phase transition takes place. The high‐pressure phase of Al(OH)3 is quenchable and is tentatively determined as nordstrandite, a triclinic polymorph of gibbsite, on the basis of its diffraction pattern and Raman spectrum.
Mapping heterogeneity of the uppermost inner core using two pairs of core phases
Authors: Kaneshima Satoshi
Published: February 2004

Two pairs of core phases, PKPcd‐PKPdf and PKPbc‐PKPdf, are analysed to map the P‐wave velocity (Vp) of the uppermost inner core. Waveform analysis of PKPcd‐PKPdf shows that Vp relative to PREM (δVp) within 100 km from the inner core boundary (ICB) is laterally heterogeneous by −0.5% to +0.1% with a wavelength of nearly 1000 km. Differential travel times of PKPbc‐PKPdf relative to PREM (δTbc‐df) are positive (negative) for the regions of positive (negative) δVp near the ICB. This correlation indicates that the lateral Vp variation exists predominantly in the inner core side of the ICB. Absolute values of δTbc‐df for both positive and negative δVp regions are too large to be explained with models in which lateral δVp variation is restricted within the upper 200 km of the inner core. The lateral variation appears to extend at least to 300 km below the ICB.
Resolving magnetic flux patches at the surface of the core
Authors: O'Brien Michael S.
Published: February 2004

The geomagnetic field at a given epoch can be used to partition the surface of the liquid outer core into a finite number of contiguous regions in which the radial component of the magnetic flux density, Br, is of one sign. These flux patches are instrumental in providing detail to surface fluid flows inferred from the changing geomagnetic field and in evaluating the validity of the frozen‐flux approximation on which such inferences rely. Most of the flux patches in models of the modern field are small and enclose little flux compared to the total unsigned flux emanating from the core. To demonstrate that such patches are not required to explain the most spatially complete and accurate data presently available, those from the Magsat mission, I have constructed a smooth core field model that fits the Magsat data but does not possess small flux patches. I conclude that our present knowledge of the geomagnetic field does not allow us to resolve these features reliably at the core‐mantle boundary; thus we possess less information about core flow than previously believed.
Regional trends of surface and tropospheric temperature and evening‐morning temperature difference in northern latitudes: 1973–93
Authors: Ross R. S. , Otterman J. , Starr D. O'C. , Elliott W. P. , Angell J. K. , Susskind J.
Published: February 2004

Trends for the 21‐year period 1973–1993 of temperatures at the surface and in the troposphere were analyzed from radiosonde observations in eight north‐latitude regions. The seasonal consistency of the surface and tropospheric trends of temperature and evening‐morning temperature difference was examined for these regions. In most seasons the surface temperature trends were positive in Eurasia and western N. America but were negative in central N. America and eastern Canada. The regions with sizable and statistically significant surface temperature trends usually had significant tropospheric trends up to 500 mb and sometimes to 300 mb. An exception was central Asia, where the strong warming was confined below 850 mb. The significant tropospheric trends showed a tendency to decrease in magnitude with height. Trends of the evening minus morning temperature difference were also analyzed in two sectors, Canada and Russia, where the radiosonde launches occurred at 5am and pm ±2hrs. Trends were fairly consistent in sign between the surface and lower troposphere in both sectors and up to 300 mb over Canada. Most Canadian trends were significant and represented a decreasing range between evening and morning temperatures. Significant Russian trends were primarily in summer.
Three‐dimensional numerical simulation of the hydrothermal system within TAG‐Like sulfide mounds
Authors: Yang Jianwen , Edwards R. N. , Molson John W. , Sudicky E. A.
Published: February 2004

A 3‐D finite element algorithm, with incorporated explicit pipe‐like elements, is developed to solve the coupled time‐dependent heat transport and fluid flow differential equations and yield the internal temperature and fluid velocity fields for TAG‐like mounds. A range of models with permeable and impermeable upper boundaries and different thermal conductivities are investigated. The solutions have three common features: a central high‐temperature zone which coincides with a non‐magnetic zone inferred from magnetic data; fluid upflow along the vertical high‐permeability column, surrounding a central pipe; and a maximum fluid velocity of the order of the observed maximum value at the central black smokers. Thermal conductivity variations strongly influence the temperature distribution but not the fluid velocity pattern.
OClO, NO2 and O3 total column observations over Iceland during the winter 1993/94
Authors: Gil M. , Puentedura O. , Yela M. , Parrondo C. , Jadhav D. B. , Thorkelsson B.
Published: February 2004

Ground‐based observation of OClO, NO2, and O3 columns by differential UV‐Visible spectroscopy at twilight during the fall winter of 1993/94 at the sub‐Arctic station of Reykjavik (64°N, 23°W) are presented. Results show no direct evidence of ozone depletion during the period but significant amounts of OClO were observed in December and January when NO2 abundances were at the annual minimum. NO2 columns are found to be controlled by the hours of light available but highly modulated by the lower stratosphere temperature. OClO was observed outside the vortex as well, but only at times when NO2 was low.
On the atmospheric heating efficiency in the Ozone Triplet Channel
Authors: Koppers G. A. A. , Murtagh D. P.
Published: February 2004

In recent studies on the photolysis of ozone the triplet channel has been given much attention as a possible extra ozone source. The vibrationally excited oxygen molecules produced can either be quenched to heat the atmosphere or photolysed at longer wavelengths than normally possible for O2. The latter potentially lowers the fraction of solar energy that is converted to heat. Lack of experimental data to accurately estimate this fraction and the relatively small contribution to the total heating has been used to justify neglect of this effect. However, recent information on the quantum yield of excited oxygen as function of wavelength from ozone dissociation makes it possible to calculate the atmospheric heating efficiency in the ozone triplet channel in a more quantitative fashion. With the new quantum yield data the atmospheric heating efficiency in the triplet channel deviates from unity only above 80 km where the magnitude of the deviation is strongly dependent on the quantum yield. However, the large yield into the singlet channel masks any deviations from unit in the triplet channel. Therefore the current results indicate that previous work, which assumed unity heating efficiency for the triplet channel in the whole altitude range, is within 4% of a detailed calculation. Below 80 km the current results are consistent with previous work.
Paleomagnetic results from the upper silurian of the Shan‐Thai‐Malay Block, southwest Yunnan, China
Authors: Chen Haihong , Zhong Dalai , Heller Friedrich , Dobson Jon P.
Published: February 2004

77 paleomagnetic samples from the Upper Silurian reddish limestone and marlstone of the Shan‐Thai‐Malay block near Baoshan ( 99.1°E, 24.8°N ), western Yunnan, China, reveal a pre‐folding remnant magnetization, with a mean direction of D=49.6°, I= −3.2°, k=9.7, a95=5.5°, corresponding a paleopole at 207.0°E, 36.8°N. These results, unlike the upper Paleozoic data with steeper inclinations published earlier, suggest that the block was located at equatorial latitude during late Silurian time, probably adjacent to the northern margin of Gondwanaland.
Stratospheric temperature anomalies and mountain waves: A three‐dimensional simulation using a multi‐scale weather prediction model
Authors: Leutbecher Martin , Volkert Hans
Published: February 2004

A non‐hydrostatic weather prediction model with an interactive grid nesting facility (MM5) is used to infer the three‐dimensional mesoscale temperature field over the Scandinavian mountains during 23 January 1991. Six hours after the initialization from operational analyses warm and cold anomalies of mesoscale extent developed on isentropic surfaces at stratospheric heights over the windward slope of the mountains and further downstream. Mean mesoscale temperature fluctuations along isentropes amount to more than 4 K while minima and maxima can differ by more than 30 K, consistent with height variations exceeding 3 km. Taking into account the three‐dimensional motion field cooling rates up to 20 K/h are estimated.
Initial investigations of microscale cellular convection in an equatorial marine atmospheric boundary layer revealed by lidar
Authors: Cooper D. I. , Eichinger W. E. , Ecke R. E. , Kao J. C. Y. , Reisner J. M. , Tellier L. L.
Published: February 2004

During the Combined Sensor Program (CSP) in March of 1996, the Los Alamos National Laboratory (LANL) fielded an advanced scanning Raman lidar. The lidar was part of a larger suite of micrometeorological sensors to quantify processes associated with the ocean‐atmosphere interface, including intermittency and coherent atmospheric features in the “warm pool” of the Tropical Western Pacific (TWP) near Manus Island (2° S. lat, 147° E. long). Initial inspection of the data has revealed excellent information on the microscale vertical and horizontal spatial and temporal structure of the equatorial Marine Atmospheric Boundary Layer (MABL). The data from this experiment have added to the increasing body of measurements on surface layer convection and intermittency including, for the first time, the observation of microscale cellular convective structures such as hexagonal patterns associated with Rayleigh‐Bénard cells.
Introduction to Special Section on Measurements and Monitoring at the TAG Hydrothermal Field, Mid‐Atlantic Ridge 26°N, 45°W
Authors: Rona Peter A. , Von Herzen Richard P.
Published: February 2004

The international oceanographic community carried out a coordinated series of cooperative cruises and dives with Japanese, Russian and U.S. submersibles between 1993 and 1995 to perform baseline measurements and to establish a seafloor observatory to monitor the active sulfide mound in the TAG hydrothermal field before, during and after drilling by Ocean Drilling Program Leg 158 in October–November 1994. The results of that effort and related work are reported herein as group of 23 papers comprising a special section in this issue of “Geophysical Research Letters”.
Mechanics of inner core super‐rotation
Authors: Aurnou J. M. , Brito D. , Olson P. L.
Published: February 2004

A mechanism is proposed to explain the seismologically‐inferred prograde rotation of the Earth's solid inner core in terms of the structure of convection in the fluid outer core. Numerical calculations of convection and dynamo action in the outer core exhibit excess temperatures inside the tangent cylinder surrounding the inner core. We show that this temperature difference generates a prograde thermal wind and a strong azimuthal magnetic field inside the tangent cylinder. Electromagnetic torques on the inner core derived from induced azimuthal magnetic fields and the ambient poloidal field equilibrate when the inner core angular velocity lags the nearby tangent cylinder fluid angular velocity by approximately 14%. The inferred prograde rotation of the inner core (1.1–3°/year relative to the mantle) can be produced by a very small (⋍ 0.001 K) temperature anomaly within the tangent cylinder and indicates strong toroidal magnetic fields with peak intensities of 24–66 mT in that region of the core.
Detection of an ENSO signal in seasonal length‐of‐day variations
Authors: Gross Richard S. , Marcus Steven L. , Eubanks T. Marshall , Dickey Jean O. , Keppenne Christian L.
Published: February 2004

Conservation of angular momentum dictates that as the wind‐driven axial atmospheric angular momentum changes, so will the length‐of‐day (LOD). In particular, as the strength of the seasonal zonal winds change, so should the strength of the seasonal LOD signals. Here, observed changes in the strengths of the annual and semiannual LOD signals during 1963–1991 are analyzed and shown to be both significantly correlated (at the 99% significance level) with the Southern Oscillation Index (SOI), and to exhibit trends of comparable magnitude but opposite signs. This reported correlation between the SOI and changes in the amplitude of the seasonal LOD signals demonstrates a linkage between seasonal LOD (and hence seasonal zonal wind) variability and the El Niño/Southern Oscillation (ENSO) phenomenon. Furthermore, this study suggests that observed variations in the amplitudes of the seasonal LOD signals can be used to study changes in the strengths of the seasonal atmospheric zonal winds on interannual to decadal and longer time scales.
Relative geomagnetic field intensity and reversals for the last 1.8 My from a central equatorial Pacific Core
Authors: Laj Carlo , Kissel Catherine , Garnier Florence , Herrero‐Bervera Emilio
Published: February 2004

We have made a new analysis of core KK78O30 from the central equatorial Pacific. This core has recorded the last 4 reversals. Mineral‐magnetic analysis establishes that these sediments meet the current criteria for reliable relative paleointensity determinations. Normalisation with ARM, IRM and χ yields very similar results. There is no evidence for a “saw‐tooth” pattern such as the one obtained by Valet and Meynadier [1993] and Meynadier et al. [1994] for the last 4 million years. Rather, the average field intensity record appears to display more or less symmetric peaks and troughs, superposed to a broad high between reversals.
Aircraft exhaust sulfur emissions
Authors: Brown R. C. , Anderson M. R. , Miake‐Lye R. C. , Kolb C. E. , Sorokin A. A. , Buriko Y. Y.
Published: February 2004

The conversion of fuel sulfur to S(VI) (SO3 + H2SO4) in supersonic and subsonic aircraft engines is estimated numerically. Model results indicate between 2% and 10% of the fuel sulfur is emitted as S(VI). It is also shown that, for a high sulfur mass loading, conversion in the turbine is kinetically limited by the level of atomic oxygen. This results in a higher oxidation efficiency at lower sulfur loadings. SO3 is the primary S(VI) oxidation product and calculated H2SO4 emission levels were less than 1% of the total fuel sulfur. This source of S(VI) can exceed the S(VI) source due to gas phase oxidation in the exhaust wake.
An estimation of the dynamical isolation of the tropical lower stratosphere using UARS wind and trace gas observations of the Quasi‐Biennial Oscillation
Authors: Schoeberl M. R. , Roche A. E. , Russell J. M. , Ortland D. , Hays P. B. , Waters J. W.
Published: February 2004

Upper Atmosphere Research Satellite tropical wind and constituent measurements are used to estimate an upper limit of mid‐latitude to tropical trace gas exchange by examining the phase of the N2O/CH4 ratio with respect to the Quasi‐Biennial Oscillation winds. Assuming a simple diffusive model for tropical ‐ mid‐latitude exchange in the 20–28 km region, the mixing times are estimated to be at least 18 months, and, we estimate an eddy mixing coefficient of 7×108 cm²/sec. This low mixing rate suggests that if significant chemical mixing into the lower tropical stratosphere occurs, as has been suggested by observational data, then this mixing must take place below 20 km.
FR waves: A second fault‐guided mode with implications for fault property studies
Authors: Malin P. E. , Lou M. , Rial J. A.
Published: February 2004

Previous work has shown that earthquake S‐waves can become fault zone guided‐waves. We present observational evidence for a second type of earthquake‐generated, fault‐guided seismic wave, apparently involving coupled P‐ and S‐waves. Our data are from a fault located in igneous rocks in Owens Valley, California. In analogy to standard Love and Rayleigh waves, we suggest that purely S‐type modes be indicated on seismograms by “FL” and the new coupled P‐ and S‐type by “FR”. Since FR waves are sensitive to both the local P‐ and S‐wave velocities, their presence implies unique conditions in and near the fault. Modeling of our seismograms suggests a differential reduction in these velocities, with a larger change in bulk modulus than in shear modulus. We propose that this reduction is due to unsaturated fractures within the core of the fault. We view FR waves as a new tool for detecting and characterizing highly fractured fault zones, providing important information on the relation of earthquakes and subsurface fluid conduits.
Resolving Bouguer anomalies in continents—A new approach
Authors: Subba Rao D. V.
Published: February 2004

Nature of anomalies present in free air gravity and their corresponding anomalies in Bouguer gravity over continents are analysed in light of isostatic compensation of topographic masses. These anomalies are classified into regional, residual and local and criteria are evolved to identify them. The regional anomaly corresponds to regional topography that is compensated. The local anomaly is related to local topographic relief. Residual anomaly is caused by subsurface anomalous source in the form of excess or deficit mass and which is of our interest. A new approach is suggested wherein using zero free air values, regional, residual and local anomalies could be separated. This is illustrated for two gravity profiles where the residuals obtained are compared with isostatic anomalies. Limitations of this method are discussed.
Multifractal analysis of phytoplankton biomass and temperature in the ocean
Authors: Seuront Laurent , Schmitt François , Lagadeuc Yvan , Schertzer Daniel , Lovejoy Shaun , Frontier Serge
Published: February 2004

Many attempts have been made to relate phytoplankton biomass ditributions to their turbulent environments. These studies have not taken the intermittent nature of turbulent processes into account, and hence poorly approximate inhomogeneous patterns. Since these oceanic fields are scaling for a wide range of scales, and scaling processes are believed to generically yield universal multifractal (characterized by three basic exponents), it is natural to analyse temperature and phytoplankton biomass in such a framework. Over the range 0.5s to 11h30', the temperature followed a single scaling regime, whereas the phytoplankton had both a low and high frequency regime (the break occurring at about 100s). We estimated the universal multifractal parameters finding that fluorescence was nearly dynamically passive (i.e. similar to temperature) on smaller scales but biologically active at larger scales.
The dust halo of Io
Authors: Ip W. H.
Published: February 2004

The possible formation of dust comas surrounding the volcanic plumes of Io is considered. The main physical ingredient has to do with the electrostatic charging of the tiny solid SO2 grains from recondensation in the expanding gas plumes. In agreement with previous estimates by [Johnson et al., 1980; Morfill et al., 1980a], the condensed grains with radii ≤ 0.01 µm are found to be capable of escaping from volcanic plumes under a variety of plasma conditions. Because of the pointing direction of the corotating electric field, the negatively charged grains would be mostly ejected into the Jovian system on the planet‐facing side. Under the assumption of electrostatic charging limitation in a dense dusty plasma, the lower limit of the volcanic ash injection rate can be estimated to be about 37 g s−1 if the total dust, mass in Loki or Pele is on the order of 108 kg. A much higher mass injection rate (up to 1 kg s−1) is possible in case the top part of the volcanic plumes has a diffuse structure allowing effective electrostatic charging process. The actual value depends on the local plasma environment and the structure of the volcanic plumes. It is expected that the observations from the imaging experiment on Galileo will shed important new light on the existence of the dust halo and hence the identification of the source of the interplanetary dust streams of Jovian origin.
Phase transition buoyancy contributions to stresses in subducting lithosphere
Authors: Bina Craig R.
Published: February 2004

The sequence of phase transitions undergone by minerals with increasing depth in Earth's mantle is perturbed within subducting lithospheric slabs by their thermal structure. Such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab. While other factors contribute to overall slab stresses, thermal and phase transition effects largely control the structure of the stress field within the slab. The resulting maximum in down‐dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity. Furthermore, metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity. These observations are independent of the variety of failure mechanisms proposed for deep seismogenesis.
Large‐scale sedimentation on the glacier‐influenced polar North Atlantic Margins: Long‐range side‐scan sonar evidence
Authors: Dowdeswell J. A. , Kenyon N. H. , Elverhøi A. , Laberg J. S. , Hollender F.‐J. , Mienert J. , Siegert M. J.
Published: February 2004

Long‐range side‐scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large‐scale view of sedimentation patterns on this glacier‐influenced continental margin. High‐latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross‐shelf troughs and deep fjords. Where ice was slower‐moving, but still extended to the shelf break, large‐scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large‐scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.
Enhanced atmospheric transport of soil derived organic matter in spring over the high Arctic
Authors: Kawamura Kimitaka , Yanase Ayako , Eguchi Takashi , Mikami Takehiko , Barrie Leonard A.
Published: February 2004

Arctic aerosol samples collected from Alert (82.5°N) have been studied using a capillary GC and GC/MS for long chain α,ω‐dicarboxylic acids that are produced in soils by bacterial ω and ω‐1 oxidations of plant‐derived fatty acids. Here, we report the discovery that one month after sunrise in the Arctic, atmospheric concentrations of the soil derived, long chain dicarboxylic acids are significantly (ca. 10 times) enhanced in the high Arctic. Relative abundances of long chain diacids in aerosol total carbon contents also increased in late April to early May. In March to May during peak concentrations, the relative distribution of C20–C26 diacids in the Arctic aerosols was similar to those of Chinese loess samples, suggesting atmospheric transport of Asian dusts over distances of three to five thousand kilometers into the Arctic. This conclusion is supported by air parcel trajectory analyses which showed stronger transport from southern desert regions during peaks in long chain dicarboxylic acids in the Arctic.
Direct geological evidence for prior earthquakes on the 1981 Corinth Fault (central Greece)
Authors: Pantosti D. , Collier R. , D'Addezio G. , Masana E. , Sakellariou D.
Published: February 2004

We present the preliminary results of geomorphic and trench investigations at the eastern end of a scarp produced by the 1981 Corinth earthquake sequence. At this site, the 1981 rupture produced average vertical displacements of 0.6–0.7 m (maximum 1.3 m) that in part occurred along a cumulative scarp which displaces the fan surface by up to 5 m. Based on this cumulative scarp displacing the Holocene fan surface, we estimate a vertical slip rate of ∼1 mm/yr that translates to 1 mm/yr N‐S extension. We found evidence for two individual pre‐1981 earthquakes which occurred, on the basis of preliminary radiocarbon dating on two paleosols, shortly after A.D. 590 and A.D. 1295. Each of these events produced a vertical displacement ranging between 0.5 and 1.2 m, hence comparable to the 1981 scarp height. This suggests that coseismic slip may have been characteristic at this location over the past three seismic cycles. Following this hypothesis and assuming a periodic strain release, a maximum recurrence interval of ∼700 yr is calculated. These results indicate that the 1981 surface‐breaking faults accommodate only part of the regional extension deduced from the geodetic measurements.
Unusual characteristics of lower thermosphere prevailing winds at South Pole
Authors: Portnyagin Y. I. , Forbes J. M. , Makarov N. A.
Published: February 2004

Meteor radar measurements of meridional winds near 95 km altitude, along four orthogonal azimuth directions approximately 2° from the geographic South Pole, were made between January 19, 1995, and January 26, 1996. Herein we report analyses of the prevailing (daily mean) wind component covering the period from January 19 through June 20, 1995. The observed phenomena include (a) A regular month‐to‐month migration in the phase of the s=1 quasi‐stationary component of the wind field; (b) During April, a 10‐day period of strong net wind divergence from the pole; (c) evidence for upwelling associated with very high magnetic activity. It is speculated that the wind divergence event may be associated with the mesospheric penetration of momentum fluxes known to accompany intense wave 1 activity in the Southern Hemisphere during April [Miles et al., 1994].
Post‐eruptive deformation associated with the 1986–87 and 1989 lava flows of Etna detected by radar interferometry
Authors: Briole P. , Massonnet D. , Delacourt C.
Published: February 2004

We analysed 92 interferograms produced using ERS1 SAR images taken on Etna between May 1992 and October 1993. Nineteen show a local subsidence in the eastern flank of the volcano, correlated with the location of the 30 October 1986 – 1 March 1987 and 27 September – 9 October 1989 lava flows. Using fringe unwrapping and data gridding techniques, and assuming that over the sampled time‐window, deformation was a linear function of time, we derive a map of along range rate of motion. The correlation between the deformation field and the area covered by recent lavas suggests that compaction of the lava flows continues several years after the eruptions. The area of maximum subsidence (47 mm/yr.) is localised at the narrowing of the 1989 flow, between 1500 and 1700 m a.s.l‥ We observe that subsidence extends outside the lava flows, accounting for at least 12 mm/yr. Assuming a relaxation process of the substrate in response to loading produced by recent lavas, a simple 1D Maxwell visco‐elastic model predicts a maximum subsidence rate of 25 mm/yr and a relaxation time of about 3.5 years. The relaxation time agrees with those derived from post‐eruptive displacements observed by levelling on Etna and Piton de la Fournaise after several eruptions. We conclude that at the time of our measurements, 25 to 50% of the deformation was related to relaxation of the substrate and the other part due to compaction of the lava flows.
Relationship between tropical upper tropospheric moisture and eastern tropical Pacific Sea surface temperature at seasonal and interannual time scales
Authors: Newell Reginald E. , Zhu Yong , Read William G. , Waters Joe W.
Published: February 2004

Water vapor in the tropical upper troposphere, above the altitudes ordinarily sampled by radiosonde humidity elements, is important in controlling atmospheric infrared energy loss to space, both by its dominance of infrared transmission and its role in cloud formation. Much of the water vapor arrives in the upper troposphere above about 12 km in plumes associated with cumulonimbus complexes; values at 215 and 147 hPa have recently been estimated from the Microwave Limb Sounder on the NASA Upper Atmosphere Research Satellite. In equatorial regions, time changes of upper troposphere moisture over a three year period, 1991–1994, are closely related to sea surface temperature changes in the eastern tropical Pacific, including seasonal and non‐seasonal variations. Maps of the annual and semi‐annual components at 215 and 147 hPa show maxima over the eastern hemisphere land masses and the maritime continent, as well as over Central and South America.
Modeling of summertime meso‐β scale cyclone in the Antalya Bay
Authors: Karaca Mehmet , Dobricic Srdjan
Published: February 2004

The Antalya Bay meso‐β scale cyclone observed on 8 of July, 1993 is simulated to understand the physical mechanisms responsible for its generation using the National Meteorological Center (NMC)/Eta limited area model. Although the NMC/Eta model was developed to forecast synoptic‐scale events, it also simulates well the meso‐scale features. The results of the model indicate that a strong westerly flow, orographic forcing due to the Taurus Mountains and surface temperatures of the warm pool in the Antalya Bay are necessary and sufficient conditions to generate the short‐lived meso‐β scale in the Bay.
Stagnation of Ice Stream C, West Antarctica by water piracy
Authors: Anandakrishnan Sridhar , Alley Richard B.
Published: February 2004

The dynamic behavior of the West Antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea‐level rise. Attention is focused on the fast‐moving and rapidly responding ice streams that drain the ice sheet. One of these, ice stream C, largely stopped about a century ago, and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice‐sheet. Here, new data are presented indicating that the slowdown of the ice stream is restricted to its lower part, and occurred because of loss of lubrication on localized “sticky spots” at the bed of the ice stream. The increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream B, together with slow drawdown of the ice sheet, rather than from any general stabilizing feedbacks.
The rotation of LAGEOS and its long‐term semimajor axis decay: A self‐consistent solution
Authors: Farinella Paolo , Vokrouhlický David , Barlier François
Published: February 2004

We develop a self‐consistent model for the evolution of the spin axis of LAGEOS and the related long‐term semimajor axis perturbations, due to asymmetric emission/reflection of radiation from the satellite's surface. We show that the theory developed by Bertotti and Iess [1991] for the evolution of LAGEOS's rotation under magnetic and gravitational torques, which we have somewhat generalized here, can lead to a successful fit of the observed semimajor axis residuals, provided the correct initial conditions for the direction of the spin axis are chosen. The remaining residuals have an rms dispersion of 0.50 × 10−12 m/s2, comparable to that of previous solutions, based on purely empirical fits of the spin axis direction as a function of time. The spectrum of the residuals indicates that they are probably due to unmodeled radiation forces (e.g., from Earth albedo and/or penumbra passages). Our solution allows us to predict the future evolution of LAGEOS's rotation for about another decade in the future, until the spin rate will become so slow that some basic assumptions of the theory will fail. A similar model can also be used to model/predict the coupled spin‐orbit evolution of the LAGEOS II satellite, launched in late 1992, although the available data still cover a span of time too short for reaching significant quantitative conclusions concerning this satellite.
Moment, energy, stress drop, and source spectra of western United States earthquakes from regional coda envelopes
Authors: Mayeda Kevin , Walter William R.
Published: February 2004

We present a new method to estimate stable seismic source parameters, such as energy, moment, and Orowan stress drop, using regional coda envelopes from as few as one broadband station. We use the method to compute path‐ and site‐corrected seismic moment‐rate spectra for 117 recent western United States earthquakes. Empirical Green's function corrections were applied to our surface‐ and body‐wave coda envelope measurements to generate S‐wave source spectra. These source spectra provide stable, single‐station estimates of radiated seismic energy Es and seismic moment Mo that for common events are in excellent agreement with network‐averaged estimates obtained using local and regional data. Teleseismic moment estimates are compatible with our regional results, but teleseismic energy estimates appear to be nearly an order of magnitude low. We estimated the seismic moment of events ranging between Mw 2.2 and 7.3, and energy estimates for which we had measured at least 70% of the total energy, generally events above Mw 3.3. We use these estimates to examine the behavior of derived parameters such as the Orowan stress drop (Δσ = 2μEs/Mo). While the earthquakes we studied have a small range in Orowan stress drop, generally between 0.1 and 20 MPa, they show a strong tendency for Orowan stress drop to increase with moment, approximately as Mo0.25. We believe this is a source effect and is not due to inadequate bandwidth or attenuation correction, and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern California. Many of the large high stress drop earthquakes show complexity in their moment‐rate spectra near the corner frequency and cannot be fit by a simple ω‐square model. Instead, above the first corner frequency, the spectral decay ranges between f−1.0 and f−1.5. This leads to larger estimates of radiated energy than predicted with a simple ω‐square model and has implications for seismic hazard estimation. Coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy: (1) Coda amplitudes vary little with geology and source‐radiation anisotropy and allow accurate single‐station applications; (2) path‐corrected coda amplitude measurements can be applied to very large regions, allowing a comparison of source parameters throughout the western United States using a common methodology and stations; (3) because long‐period coda can last for hours for large local and regional events, it allows the analysis of seismograms with clipped early arrivals.
Combinations of Earth orientation measurements: SPACE94, COMB94, and POLE94
Authors: Gross Richard S.
Published: February 2004

A Kalman filter has been used to combine independent measurements of the Earth's orientation taken by the space‐geodetic observing techniques of lunar laser ranging, satellite laser ranging, very long baseline interferometry, and the Global Positioning System. Prior to their combination, the data series were adjusted to have the same bias and rate, the stated uncertainties of the measurements were adjusted, and data points considered to be outliers were deleted. The resulting combination, SPACE94, consists of smoothed, interpolated polar motion and UT1‐UTC values spanning October 6.0, 1976, to January 27.0, 1995, at 1‐day intervals. The Kalman filter was then used to combine the space‐geodetic series comprising SPACE94 with two different, independent series of Earth orientation measurements taken by the technique of optical astrometry. Prior to their combination with SPACE94, the bias, rate and annual term of the optical astrometric series were corrected, the stated uncertainties of the measurements were adjusted, and data points considered to be outliers were deleted. The adjusted optical astrometric series were then combined with SPACE94 in two steps: (1) the Bureau International de l'Heure (BIH) optical astrometric series was combined with SPACE94 to form COMB94, a combined series of smoothed, interpolated polar motion and UT1‐UTC values spanning January 20.0, 1962, to January 27.0, 1995, at 5‐day intervals, and (2) the International Latitude Service (ILS) optical astrometric series was combined with COMB94 to form POLE94, a combined series of smoothed, interpolated polar motion values spanning January 20, 1900, to January 21, 1995, at 30.4375‐day intervals.
Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust
Authors: Nicolas Adolphe , Boudier Françoise , Ildefonse Benoît
Published: February 2004

From 32 detailed cross sections through the gabbro unit of the Oman ophiolite, it is concluded that the thickness of this unit is on average 3.6 km. The lower layered gabbros represent two thirds, and the upper homogeneous foliated gabbros represent one third the gabbro unit. Assuming that the overlying basaltic lid (sheeted dikes and extrusives) is 1.5–2 km thick, the average crustal section in the Oman ophiolite is 0.5–1 km thinner than the standard 6‐km‐thick oceanic crust usually considered to be produced at fast spreading ridges, a point which is discussed. Variations in gabbro thickness between 5.4 km and 1.5 km are recorded. There is a general correlation throughout the ophiolite belt, particularly in the southeastern massifs, such that the thinnest gabbro units (2.2–2.5 km thick) overlay the thickest (300–700 m) transition zones which separate them from the mantle harzburgites and the thickest gabbro units (3.6–3.9 km thick) overlay the thinnest (5–100 m) transition zones. The combination of thinner gabbro units and thicker transition zones is observed above mantle diapirs or in domains which, following our structural models, were accreted above diapirs and have drifted in the spreading direction. If it is assumed that the extrusive basalt and the sheeted dike complex units have a constant thickness, such large variations indicate similar variations in the Moho level below the ridge of origin; in particular, the Moho above mantle diapirs should be some 1–1.5 km shallower than away from diapirs. As the Oman ophiolite is considered to derive from a fast spreading paleoridge, this doming should be detected in actual fast spreading ridges, as suggested by Barth and Mutter [this issue] and Wang et al. [this issue].
The effects of spreading rate, the magma budget, and the geometry of magma emplacement on the axial heat flux at mid‐ocean ridges
Authors: Chen Yongshun John , Morgan Jason Phipps
Published: February 2004

We explore two‐dimensional, steady state, flow and thermal models of oceanic spreading center structure. These models include the effects of hydrothermal heat transport and crustal accretion for two basic accretion geometries: (1) a lens model where all crust below the sheeted dike layer passes through a magma lens at the base of the dike layer before subsiding and flowing to deeper levels; and (2) a dike model where crust is emplaced in a vertical slot from the surface to Moho and subsequently moves horizontally (rigidly) away from the axis of accretion. The axial heat flux resulting from these two end‐member accretion geometries is compared, and an assessment is made of the feasibility of using various observations to differentiate between these two accretion geometries. We find that the steady state axial heat flux is predominantly influenced by three factors: the spreading rate, the magmatic budget (crustal thickness) and the efficiency of hydrothermal cooling. The total steady state heat flux from the neovolcanic zone (2 km wide) increases almost linearly with the spreading rate for both the dike and lens accretion geometries. The major difference between the dike and the lens models is nonthermal: they predict different accumulated strain distributions within off‐axis crust. Crustal flow due to crustal accretion within a crustal‐height “dike” leads to little accumulated strain, while intense crustal strain results from crustal subsidence and flow below a steady state magma lens. Ophiolite and marine seismic observations of crustal layering appear to be the strongest observational tests to discriminate between these two accretion geometries; they currently favor a lens‐like model of lower crustal accretion.
Frictional constraints on crustal faulting
Authors: Boatwright John , Cocco Massimo
Published: February 2004

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate‐ and state‐dependent friction law, where we differentiate velocity‐weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity‐strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two “intermediate” fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This “mixed” frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.
Horizontal viscoelastic‐gravitational displacement due to a rectangular dipping thrust fault in a layered Earth model
Authors: Fernández José , Yu Ting‐To , Rundle John B.
Published: February 2004

Calculations of horizontal displacements due to a rectangular finite thrust fault in a viscoelastic‐gravitational layered Earth model are presented. The Earth model consists of a single elastic‐gravitational layer overlying a viscoelastic‐gravitational half‐space. A review of the full three‐dimensional theoretical solutions is presented along with the explicit solutions for horizontal displacements. Several examples of computations for dipping faults with various angles, and located at different depths, are shown. The results indicate that viscoelasticity introduces a long‐wavelength component into the interseismic deformation field which is not present in published elastic techniques and also that a proper consideration of gravity is necessary only for near‐field computations at longer periods of time. A pattern is found in the cumulative displacement of cycled earthquakes, which indicates that the viscoelastic displacements are visible for longer recurrence time events and that these may serve as a time index for the various stages between cycles.
Earthquake nucleation on model faults with rate‐ and state‐dependent friction: Effects of inertia
Authors: Roy Mousumi , Marone Chris
Published: February 2004

Laboratory studies suggest that earthquake nucleation involves a transition from quasi‐static slip when inertial effects are negligible to inertia‐driven, dynamic motion. This transition occurs via quasi‐dynamic motion, during which the effects of inertia become increasingly important. The characteristics of this transition, which depend on frictional properties of the fault, determine the observability of earthquake nucleation by seismic, geodetic, or other means. By investigating the role of inertia during nucleation, we obtain a quantitative definition of the limiting velocity Vin, which marks the end of quasi‐dynamic motion and the onset of instability. For reasonable friction parameters and fault widths, we obtain estimates of Vin for crustal faults. To study the roles of inertia, stiffness, and friction parameters on preseismic motion, we simulate triggered instabilities in a one‐dimensional model of a homogeneous fault, with rate and state variable friction. In most of our simulations, triggering is achieved by applying a stress perturbation to an initially creeping fault under steady state friction. We also investigate triggering on faults which are initially locked and overstressed compared to their nominal frictional strength, due to time‐dependent healing. We study the amount, Up, and duration, Tp, of preseismic slip as a function of system mass m and other model parameters. For crustal faults, we interpret the relevant mass as a product of density and fault width and find that wider fault zones result in smaller Up and larger Tp. Both Up and Tp are proportional to the system stiffness K, the characteristic slip distance Dc, and the friction constitutive parameter a and inversely proportional to the size of the triggering event. We find greater Up and Tp for constitutive laws which allow strengthening at zero velocity, compared to laws that require slip or a combination of slip and aging for state evolution. In contrast to quasi‐static modeling, our simulations suggest a minimum stress perturbation criterion for instability, which may be interpreted in terms of a strain threshold for triggered seismicity.