bluuebunny/crossref_metadata_2025 huggingface dataset:
Lateral viscosity variations and post‐glacial rebound: Effects on present‐day VLBI baseline deformations
Authors: Giunchi C. , Spada G. , Sabadini R.
Published: February 2004

By means of a finite element algorithm we investigate the effects of lateral viscosity variations upon the horizontal motions currently detected by VLBI techniques in Europe. Our axisymmetric flat models, appropriate to describe the rebound due to the melting of the Fennoscandia ice‐sheet, are characterized by a layered compressible mantle with linear Maxwell rheology. We have first computed the time‐evolution of theoretical baselines characterized by a simple geometry. In agreement with previous results, the rates of horizontal deformation have been found to be greatly sensitive to both lateral viscosity variations and deep mantle stratification, especially for baselines located in the vicinity of the ice sheet margin. To complete our study, we have compared our numerical results with the observed time‐evolutions of relevant European baselines. Onsala‐Wettzell and Onsala‐Eflsberg are fitted by a laterally varying asthenosphere and sharp viscosity increase in the lower mantle; longer baselines, connecting these sites to Medicina (northern Italy), indicate that post glacial rebound is responsible only for a fraction of the VLBI observations, the residual being thus attributable to continental collision in the Mediterranean Sea.
Low crustal velocities and mantle lithospheric variations in southern Tibet from regional Pnl waveforms
Authors: Rodgers Arthur J. , Schwartz Susan Y.
Published: February 2004

We report low average crustal P‐wave velocities (5.9–6.1 km/s, Poisson's ratio 0.23‐0.27, thickness 68–76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991–1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus‐Tsangpo Suture and the Lhasa Terrane is shield‐like (Pn velocity 8.20–8.25 km/s, lid thickness 80–140 km, positive velocity gradient 0.0015–0.0025 s−1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong‐Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50–60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.
Variations in the frequency‐magnitude distribution with depth in two volcanic areas: Mount St. Helens, Washington, and Mt. Spurr, Alaska
Authors: Wiemer Stefan , McNutt Stephen R.
Published: February 2004

The frequency‐magnitude distribution of earthquakes, characterized using the b‐value, is examined as a function of space beneath Mount St. Helens (1988–1996), and Mt. Spurr (1991–1995). At Mount St. Helens, two volumes of anomalously high b (b > 1.3) can be observed at depths of 2.6–3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b‐value (b ≥ 1.3) at a depth of about 2.3–4.5 km below the crater floor of the active vent Crater Peak. We propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b‐value at shallow depths. Alternatively, interaction of magma with groundwater may have increased pore pressure and lowered the effective stress. The deeper anomaly at Mount St. Helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber. Our results indicate that detailed mapping of the frequency‐magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers.
Direct comparison of meteosat water vapor channel data and general circulation model results
Authors: Roca Rémy , Picon Laurence , Desbois Michel , Le Treut Hervé , Morcrette Jean‐Jacques
Published: February 2004

Following a model to satellite approach, this study points out the ability of the general circulation model (GCM) of the Laboratoire de Météorologie Dynamique to reproduce the observed relationship between tropical convection and subtropical moisture in the upper troposphere. Those parameters are characterized from Meteosat water vapor equivalent brightness temperatures (WVEBT) over a monthly scale. The simulated WVEBT field closely resembles to the observed distribution. The pure water vapor features and the convective areas are well located and their seasonal variations are captured by the model. A dry (moist) bias is found over convective (subsiding) areas, whereas the model globally best acts over Atlantic ocean than over Africa. The observed and simulated seasonal variations show that an extension of the ITCZ is correlated to a moistening of the upper troposphere in subtropical areas. Those results imply a positive large scale relationship between convective and subsiding areas in both observation and simulation, and suggest the relevance of our approach for further climatic studies.
Relationship between crater size and ejecta volume of recent magmatic and phreato‐magmatic eruptions: Implications for energy partitioning
Authors: Sato Hiroaki , Taniguchi Hiromitsu
Published: February 2004

Relationships between crater diameter (D in meters) and ejecta volume (V in cubic meters) of recent magmatic and phreato‐magmatic eruptions are expressed as D=0.11 V0.42, and D=0.97 V0.36, respectively. Crater diameters of phreato‐magmatic eruptions are ca. 2–5 times larger than those of magmatic eruptions of similar ejecta volume, suggesting that magma‐water interaction in phreato‐magmatic eruptions generated mechanical energies of crater formation 1–2 orders of magnitude larger than equivalent‐volume magmatic eruptions. The conversion ratio of thermal energy to kinetic energy in phreato‐magmatic eruptions is estimated to be 0.7–10 percent, which is in accord with the presently available data of laboratory experiments on melt‐water interaction [Wohletz and McQueen, 1984].
Sandbox model simulation of forearc evolution and noncritical wedges
Authors: Wang Wei‐Hau , Davis Dan M.
Published: February 2004

Convergent plate boundaries exhibit a wide range of behaviors, ranging from sediment accretion and underplating to large‐scale erosion of the forearc. We have used sandbox modeling to simulate the growth of thin‐skinned contractional wedges over a strong backstop at an accretionary margin. The backstop in our model causes the growth of two distinct wedges with quite different growth histories, analogous to the accretionary wedge and the inner deformation belt (retrowedge) found in at least some forearcs. After starting with an initially steep slope, the surface flattens as the accretionary wedge grows by frontal accretion, until the wedge attains and maintains a minimum possible critical taper. The retrowedge, driven by uplift of the outer arc high, increases its surface slope as it grows until a kinematic steady state is attained. It exhibits a stable taper with slope larger than the minimum critical taper but never larger than the theoretical maximum critical taper when a basal décollement is present above the backstop. In models in which the retrowedge has not developed a décollement, it has a taper with a slope at the angle of repose. When a basal décollement is present above the backstop, the main body of the retrowedge acts as a bulldozer or a backstop, pushing the sediments in front of it, which then form an additional wedge with a small (minimum) critical taper. A broad outer arc high stands above the toe of the backstop, where maximum principal stresses rotate to meet the stress conditions in the two contractional belts on either side of it. As these belts grow in opposite directions, the accretionary wedge accretes large quantities of sediment at its front, but the retrowedge does not. Because the retrowedge can exhibit a non‐critical, stable taper in the absence of rapid erosion or accretion, it is misleading to assume that active thin‐skinned wedges are necessarily at maximum or minimum critical taper.
Seismic velocity increase and deep‐sea gas hydrate concentration above a bottom‐simulating reflector on the northern Cascadia continental slope
Authors: Yuan T. , Hyndman R. D. , Spence G. D. , Desmons B.
Published: February 2004

The amount of gas hydrates in the accretionary wedge sediments of the northern Cascadia subduction zone off Vancouver Island has been estimated from multichannel seismic (MCS) and Ocean Drilling Program (ODP) data. Detailed semblance velocity analyses and full waveform inversion of MCS data, combined with previously published ODP Sites 889/890 sonic log and vertical seismic profile (VSP) data, show that sediment velocities increase downward more rapidly than the no‐hydrate/no‐gas reference profile from about 1500 m/s near the seafloor to a maximum of 1900 m/s just above the bottom‐simulating reflector (BSR) at a subbottom depth of 224 m. Immediately below the BSR, the MCS velocities drop to ∼1700 m/s. A low velocity of ∼1500 m/s from the VSP data probably represents a thin layer, ∼20 m, containing free gas. The difference between the reference and observed velocities is used to estimate hydrate concentration, which reaches a maximum of 20–30% of the pore space above the BSR. A simple interpretation of the drill core chlorinity dilution data at the ODP site yields a similar hydrate concentration of 35%. The estimated hydrate concentration with subbottom depth represents about 7 m3 of hydrate per square meter of seafloor. The total methane gas at STP, including hydrate above the BSR and the small amount of free gas below, is about 800 m3 per square meter of seafloor, or 200 TCF (trillion cubic feet) on the Vancouver Island continental slope. Application of the same method in the Blake‐Bahama region of the eastern U.S. margin indicates that the velocity enhancement and inferred hydrate concentration are very similar.
Melting experiments on anhydrous peridotite KLB‐1: Compositions of magmas in the upper mantle and transition zone
Authors: Herzberg Claude , Zhang Jianzhong
Published: February 2004

Electron microprobe results are reported for liquid and crystalline phases that were synthesized at 5–22.5 GPa in multianvil experiments on anhydrous peridotite KLB‐1 [Zhang and Herzberg, 1994]. The results provide information on the partitioning of TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, Na2O, and NiO among liquid and the crystalline phases olivine, modified spinel, garnet, magnesiowustite, and magnesium perovskite. Uncertainties in these partition coefficients stem from quenching problems and from the effects of thermal migration of liquid in a temperature gradient. We have, however, exploited the temperature gradients by determining how the crystalline phase chemistry varies throughout the melting interval from the liquidus to the solidus. This has permitted new constraints to be obtained on the compositions of liquids along the anhydrous peridotite solidus at low melt fractions and at pressures in the 5–18 GPa range. It is demonstrated that the wide range of Al2O3 and CaO/Al2O3 contents in picrites and komatiites can be explained by melt segregation at upper mantle pressures that ranged from 3 to ∼10 GPa. These magmas could have formed by anhydrous melting in plumes with temperatures that were only 100°–200°C higher than ambient mantle below ridges, demonstrating that unusually hot conditions are not required to form komatiites. Primary igneous MgO contents in excess of 26% should be rare, and those that do exist in some komatiites can be explained by advanced melting during adiabatic or superadiabatic ascent, by low Na2O in the source, or by melting in hot plumes from the transition zone and lower mantle. Evidence for deep melting in hot plumes is rather conjectural, but it may be contained in some 2700 Myr komatiites that have high MgO and mantle‐like CaO/Al2O3.
Rank‐ordering statistics of extreme events: Application to the distribution of large earthquakes
Authors: Sornette D. , Knopoff L. , Kagan Y. Y. , Vanneste C.
Published: February 2004

Rank‐ordering statistics provide a perspective on the rare, largest elements of a population, whereas the statistics of cumulative distributions are dominated by the more numerous small events. The exponent of a power law distribution can be determined with good accuracy by rank‐ordering statistics from the observation of only a few tens of the largest events. Using analytical results and synthetic tests, we quantify the systematic and the random errors. We also study the case of a distribution defined by two branches, each having a power law distribution, one defined for the largest events and the other for smaller events, with application to the worldwide (Harvard) and southern California earthquake catalogs. In the case of the Harvard moment catalog, we make more precise earlier claims of the existence of a transition of the earthquake magnitude distribution between small and large earthquakes; the b values are b2 = 2.3 ± 0.3 for large shallow earthquakes and b1 = 1.00 ± 0.02 for smaller shallow earthquakes. However, the crossover magnitude between the two distributions is ill defined. The data available at present do not provide a strong constraint on the crossover which has a 50% probability of being between magnitudes 7.1 and 7.6 for shallow earthquakes; this interval may be too conservatively estimated. Thus any influence of a universal geometry of rupture on the distribution of earthquakes worldwide is ill defined at best. We caution that there is no direct evidence to confirm the hypothesis that the large‐moment branch is indeed a power law. In fact, a gamma distribution fits the entire suite of earthquake moments from the smallest to the largest satisfactorily. There is no evidence that the earthquakes of the southern California catalog have a distribution with two branches or that a rolloff in the distribution is needed; for this catalog, b = 1.00 ± 0.02 up to the largest magnitude observed, MW ≃ 7.5; hence we conclude that the thickness of the seismogenic layer has no observable influence whatsoever on the frequency distribution in this region.
Modeling gravity‐driven flows on an inclined plane
Authors: Bruno Barbara C. , Baloga Stephen M. , Taylor G. Jeffrey
Published: February 2004

We develop an exact analytic solution for unconfined flows having an assumed rheology advancing on an inclined plane. We consider the time‐dependent flow movement to be driven by gravitational transport and hydrostatic pressure. We examine how these two forces drive flow movement in the downstream and cross‐stream directions by adopting a volume conservation approach. Simplifying assumptions reduce the governing equation to the dimensionless form ∂/∂x(αhm) = ∂/∂y(αhm∂h/∂y), where x and y are the downstream and cross‐stream directions, respectively; h is the flow depth; and α = α(x) and m are prescribed by the rheology of the fluid. We solve this equation analytically for flows of arbitrary m and α using a similarity transformation. This method involves transforming variables and reducing the governing equation to a nonlinear ordinary differential equation. Our solution determines how flow depth and width change with distance from the source of the flow for different α and m based on known or assumed initial parameters. Consequently, from the traditional geometric dimensions of the deposits, these rhelogical parameters can be inferred. We have applied the model to basaltic lava flows and found m values typically between 1 and 2. This contrasts with Newtonian fluids, for which m = 3. The model of α(x) corresponding to constant viscosity approximates the field data of pahoehoe toes (<5 meters in length), whereas models of α(x) corresponding to linearly increasing and exponentially increasing viscosities better approximate the remote sensing data of longer flows (several kilometers in length).
Isotopic evolution of Mauna Kea volcano: Results from the initial phase of the Hawaii Scientific Drilling Project
Authors: Lassiter J. C. , DePaolo D. J. , Tatsumoto M.
Published: February 2004

We have examined the Sr, Nd, and Pb isotopic compositions of Mauna Kea lavas recovered by the first drilling phase of the Hawaii Scientific Drilling Project. These lavas, which range in age from ∼200 to 400 ka, provide a detailed record of chemical and isotopic changes in basalt composition during the shield/postshield transition and extend our record of Mauna Kea volcanism to a late‐shield period roughly equivalent to the last ∼100 ka of Mauna Loa activity. Stratigraphic variations in isotopic composition reveal a gradual shift over time toward a more depleted source composition (e.g., higher 143Nd/144Nd, lower 87Sr/86Sr, and lower 3He/4He). This gradual evolution is in sharp contrast with the abrupt appearance of alkalic lavas at ∼240 ka recorded by the upper 50 m of Mauna Kea lavas from the core. Intercalated tholeiitic and alkalic lavas from the uppermost Mauna Kea section are isotopically indistinguishable. Combined with major element evidence (e.g., decreasing SiO2 and increasing FeO) that the depth of melt segregation increased during the transition from tholeiitic to alkalic volcanism, the isotopic similarity of tholeiitic and alkalic lavas argues against significant lithosphere involvement during melt generation. Instead, the depleted isotopic signatures found in late shield‐stage lavas are best explained by increasing the proportion of melt generated from a depleted upper mantle component entrained and heated by the rising central plume. Direct comparison of Mauna Kea and Mauna Loa lavas erupted at equivalent stages in these volcanoes' life cycles reveals persistent chemical and isotopic differences independent of the temporal evolution of each volcano. The oldest lavas recovered from the drillcore are similar to modern Kilauea lavas, but are distinct from Mauna Loa lavas. Mauna Kea lavas have higher 143Nd/144Nd and 206Pb/204Pb and lower 87Sr/86Sr. Higher concentrations of incompatible trace elements in primary magmas, lower SiO2, and higher FeO also indicate that Mauna Kea lavas formed through smaller degrees of partial melting at greater depth than Mauna Loa lavas. These chemical and isotopic differences are consistently found between volcanoes along the western “Loa” and eastern “Kea” trends and reflect large‐scale variations in source composition and melting environment. We propose a simple model of a radially zoned plume centered beneath the Loa trend. Loa trend lavas generated from the hot plume axis reflect high degrees of partial melting from a source containing a mixture of enriched plume‐source material and entrained lower mantle. Kea trend lavas, in contrast, are generated from the cooler, peripheral portions of the plume, record lower degrees of partial melting, and tap a source containing a greater proportion of depleted upper mantle.
Measurements of the complex dielectric constant of volcanic ash from 4 to 19 GHz
Authors: Adams R. J. , Perger W. F. , Rose W. I. , Kostinski A.
Published: February 2004

Dielectric data in volcanic ash at weather radar wavelengths (centimeter range) are extremely sparse and are crucial for radar sensing of ash clouds and for imaging of volcanic terrains. This study extends previous data to include a wavelength range of 1.5–7.5 cm and volcanic ash compositions of 50–75% silica. The real part of the complex permittivity, ε′, of volcanic ash is 6 ± 0.5 (1σ) for all wavelengths. The imaginary part, ε″, ranges from 0.08 to 0.27. Both ε′ and ε″ show higher values at lower SiO2 concentration. It is safe to assume in any weather radar applications that the reflectivity factor is K = |(ε − 1)/(ε + 2)|2 = 0.39 ± 0.02 (1σ), regardless of composition or wavelength. The results will help quantify radar observations of volcanic clouds.
An 84‐kyr paleomagnetic record from the sediments of Lake Baikal, Siberia
Authors: Peck J. A. , King J. W. , Colman S. M. , Kravchinsky V. A.
Published: February 2004

We have conducted a paleomagnetic study of sediment cores obtained from the Selenga prodelta region of Lake Baikal, Russia. This record, which spans approximately the last 84 kyr, contributes to a better understanding of the nature of geomagnetic field behavior in Siberia and is a useful correlation and dating tool. We demonstrate that the Lake Baikal sediments are recording variations in the geomagnetic field. The directional record displays secular variation behavior with a geomagnetic excursion at 20 ka and additional excursions appearing as large‐amplitude secular variation at 41, 61, and 67 ka. Smoothing of the geomagnetic excursion behavior occurs in Lake Baikal sediments owing to the intermediate sedimentation rate (13 cm kyr−1). The Lake Baikal relative paleointensity record correlates to absolute paleointensity data for the last 10 kyr and to relative paleointensity records from the Mediterranean Sea and Indian Ocean for the last 84 kyr. This correlation suggests a strong global (i.e., dipole) component to these records and further supports the reliability of sediments as recorders of relative geomagnetic paleointensity. We show that a relative geomagnetic intensity stratigraphy has a potential resolution of 7 kyr by correlating continental and marine records. The geomagnetic intensity stratigraphy helps constrain the age of the difficult to date Lake Baikal sediments.
Prolonged history of silicic peralkaline volcanism in the eastern Pacific Ocean
Authors: Bohrson Wendy A. , Reid Mary R. , Grunder Anita L. , Heizler Matthew T. , Harrison T. Mark , Lee Jeffrey
Published: February 2004

Socorro Island, Mexico, is an alkaline and peralkaline volcanic island located in the eastern Pacific Ocean on a mid‐ocean ridge spreading center that was abandoned at ∼3.5 Ma. Silicic peralkaline rocks comprise up to 80% of the surface of the island, rendering Socorro virtually unique in the Pacific Ocean. Precise, replicate 40Ar/39Ar ages of 21 peralkaline trachytes and rhyolites reveal a history of episodic volcanic activity from ∼540 to 370 ka that may have culminated with caldera formation; repose periods between these episodes may have had maximum duration of ∼30 kyr. After up to 200 kyr of quiescence, 40Ar/39Ar ages indicate that postcaldera silicic peralkaline activity commenced by 180 ka, forming the Cerro Evermann Formation. Postcaldera mafic alkaline lavas of the Lomas Coloradas Formation erupted dominantly between 70 and 150 ka based upon relative age relations. The dominant lithology of precaldera and syncaldera silicic peralkaline deposits on Socorro is nonfragmental and nonvesicular and lacks lithic fragments and fiamme; despite this, numerous lines of evidence including welding zonation, presence of a proximal ignimbrite or co‐ignimbrite deposit, association with a caldera, and compositional heterogeneity within eruptive units suggest that they are dominantly ash flow tuffs. A change in eruptive style, from predominantly explosive to predominantly effusive, followed caldera formation and suggests that a change in the efficacy of magma degassing may be linked to caldera formation. On the basis of the presence of a caldera, the magma chamber associated with Socorro Island is shallow and probably resides within the upper oceanic crust or the edifice. This together with a prolonged history of silicic magmatism indicates that intrusion of mafic magma maintained thermal viability of the magmatic plumbing system. The minimum calculated growth rate for the entire volcanic edifice (7 × 10−4 km3/yr) exceeds those of nonhotspot off‐axis volcanoes in the Pacific by almost an order of magnitude. Eruption rates for subaerial phases on Socorro may be several orders of magnitude smaller than this growth rate and are comparable to subaerial eruption rates of isolated ocean islands related to mantle plumes.
Maximum solubility of FeO in (Mg, Fe)SiO3‐perovskite as a function of temperature at 26 GPa: Implication for FeO content in the lower mantle
Authors: Fei Yingwei , Wang Yanbin , Finger Larry W.
Published: February 2004

We have explored phase relations in the fundamental ternary system MgO‐FeO‐SiO2 at lower mantle pressure (26 GPa) via synthesis experiments on the join MgSiO3‐FeSiO3. The maximum solubility of FeO in (Mg,Fe)SiO3 perovskite as a function of temperature has been determined in the multianvil apparatus in the temperature range of 1150–1740°C. Pure perovskites with 10 and 15 mol % FeSiO3 can only be synthesized above 1525°C and 2072°C, respectively, at 26 GPa. Synthesis experiments were also conducted in a laser‐heated diamond anvil cell to resolve a previous discrepancy concerning the maximum solubility of FeO in (Mg,Fe)SiO3 perovskite determined by different high‐pressure apparatus. Both Mössbauer spectroscopic and X ray diffraction studies revealed that laser‐heated samples with more than 10 mol % FeSiO3 invariably contained magnesiowüstite. The coexistence of magnesiowüstite with relatively lower FeO content perovskite results from chemical disequilibrium process caused by the large temperature gradient in the laser‐heated samples. Finally, we conclude that the maximum FeO content at the top of the lower mantle is about 12 mol % for a perovskite‐dominated lower mantle, along a 2000‐K lower mantle geotherm.
Viscoplastic modeling of texture development in polycrystalline ice with a self‐consistent approach: Comparison with bound estimates
Authors: Castelnau Olivier , Duval Paul , Lebensohn Ricardo A. , Canova Gilles R.
Published: February 2004

Ice crystals deform easily by dislocation glide on basal planes, which provides only two independent easy slip systems. The necessary slip on other systems limits the strain rate of polycrystalline ice. The preferred c axis orientation of ice from polar ice sheets develops as a result of intracrystalline slip. An anisotropic viscoplastic self‐consistent (VPSC) approach is used for predicting texture development and mechanical behavior of polycrystalline ice. Results are compared with lower and upper bound estimations. It is assumed that ice crystals deform by basal, prismatic, and pyramidal slip. The resistance of each slip system is determined from experimental data on monocrystals and isotropic polycrystals. The VPSC model can predict the behavior of isotropic polycrystalline ice on both the macroscopic and microscopic scale. This is not the case for the lower and upper bounds. Fabrics simulated in uniaxial extension and compression are qualitatively similar for all models. However, large differences in the rate of fabric development are found. This is explained by the different interaction stiffness between grain and matrix. Fabric concentration obtained with the VPSC model for uniaxial deformation is in close agreement with those observed in polar ices. In simple shear, the single maximum fabric found in situ cannot be reproduced without an extensive (and probably unrealistic) activity of nonbasal systems. The preferential growth of grains well oriented for basal glide associated with rotation recrystallization could be at the origin of the discrepancy between model results and natural simple shear fabrics. Distorted grain shape is found to slightly slow down fabric development.
Calculation of the phase boundaries of wüstite at high pressure
Authors: Stølen Svein , Grønvold Fredrik
Published: February 2004

The phase boundaries of wüstite at high pressure are calculated using a thermodynamic data set describing the Fe‐O system. A new thermodynamic description of wüstite is presented, whereas the thermodynamic descriptions of iron, oxygen, magnetite, and hematite are taken from literature. An equation for the Gibbs energy of formation of wüstite at ambient pressure rests on recent calorimetric studies. The description gives reasonable values for the eutectoidal point and the phase boundaries of wüstite at high temperatures and furthermore explains the formation of a metastable intermediate two‐phase mixture of nearly stoichiometric wüstite and magnetite on heating quenched wüstite to around 500 K. Extrapolations to high pressure are made by taking the compositional, temperature, and pressure variation of the molar volume of wüstite into consideration. In view of the spread in the experimental and estimated bulk modulus values for wüstite, three alternatives regarding the compositional variation of the bulk modulus are considered. The assumption of a composition independent bulk modulus (Ko = 180 or 150 GPa) results in stoichiometric FeO being the stable phase at high pressures, whereas the assumption of a composition dependent bulk modulus results in a significant widening of the homogeneity region of wüstite and also indicates that FeO disproportionates to iron and nonstoichiometric wüstite at high pressures. The effect of changes in ∂Ko/∂T and ∂Ko/∂P on the calculated phase boundaries is considered. Earlier experimental studies and theoretical considerations are discussed, as are the implications of phase transitions in wüstite and in the neighboring phases.
Magnitude of velocity anomalies prior to earthquakes
Authors: Le Ravalec Mickaële , Gueguen Yves , Chelidze Tamaz
Published: February 2004

A model is presented which allows us to reexamine the interpretation of velocity anomalies observed before some earthquakes occur. This model extends the well‐known dilatancy models by accounting for two scales of heterogeneities: the variations in pore/crack compressibility at a microscopic scale and the variations in the spatial fluid distribution for partially saturated rocks at a much larger scale. This model is relevant for cracked rocks which are partially saturated. Such a situation is likely to develop in impending earthquake source areas. Although previous models have considered cracked rocks and partially saturated rocks, none of the above heterogeneity‐induced effects has been considered previously so that the previous predictions are in fact shown to be inappropriate. At earthquake or exploration seismic frequencies (1–100 Hz), saturation inhomogeneity can play a prominent role, and this results in velocity dispersion so that high‐frequency predictions may not be valid and low‐frequency velocities should be calculated. In order to do that, we assume that crustal rocks are described by a saturated matrix in which spherical undersaturated pockets are embedded. The matrix is itself made of a rock containing an isotropic distribution of cracks. Then, low‐ and high‐frequency velocities are calculated coupling the Biot‐Gassmann‐Domenico theory with the effective medium approach (differential self‐consistent method). If the diffusion‐dilatancy model of Scholz et al. [1973] is superimposed on our theoretical velocity curves, it appears that the low‐frequency predicted velocity anomaly is quite high when saturation heterogeneities are ignored. It is, however, quite small if this effect is taken into account. This result can explain the lack of observations of precursory changes of seismic velocities before many events.
Remote sensing of volcanic ash clouds using special sensor microwave imager data
Authors: Delene David J. , Rose William I. , Grody Norman C.
Published: February 2004

Measurements from the satellite‐based special sensor microwave imager (SSM/I) were used to collect passive microwave radiation (19–85 GHz) for the August 19, 1992 (UT date), Crater Peak/Spurr volcanic cloud. This eruption was also imaged by a ground‐based C‐band radar system at Kenai, Alaska, 80 km away, and by the thermal infrared channels of the polar‐orbiting advanced very high resolution radiometer (AVHRR). The SSM/I sensor detects scattering of Earth‐emitted radiation by millimeter size volcanic ash particles. The size of ash particles in a volcanic ash cloud can be estimated by comparing the scattering at different microwave frequencies. The mass of particles in the volcanic ash cloud can be estimated by using a theoretical method based on Mie theory or by adapting the empirical methods used for estimating rainfall rates and accounting for the different dielectric constants of volcanic ash and raindrops. For the August 19, 1992, Crater Peak/Spurr eruption, the SSM/I‐based estimate of ash fallout mass (1.3 × 109− 3 × 1010kg) was 4%–85% of the mass fallout measured in the field. Like weather radar systems, the SSM/I offers the ability to sense young volcanic ash clouds during and immediately following (within 30 min) actual eruptions. Because most volcanoes are out of range of weather radar systems, the SSM/I may be an important tool for determining the magnitude, initial trajectory, and potential fallout mass of eruptions. The SSM/I may therefore play a role in mitigating volcanic cloud hazards for aircraft, determining masses where ground sampling is not possible, and in issuing fallout warnings for communities downwind of volcanic eruptions.
Faulting process of the August 8, 1993, Guam earthquake: A thrust event in an otherwise weakly coupled subduction zone
Authors: Campos J. , Madariaga R. , Scholz C.
Published: February 2004

We study a large Mw = 7.7 earthquake that occurred on June 8, 1993, slightly offshore and under the island of Guam in the southern Mariana island arc. From a complete study of P and SH body waves, a relocation of the aftershocks, and the subevents of the main shock, we propose a relatively simple model of the rupture process of this event. We propose that this earthquake ruptured a shallow‐dipping thrust fault that corresponds to the subduction interface under Guam. Like many other earthquakes, this event started with a small foreshock and was followed by two large energy release events located to the northeast of the epicenter along the subduction zone. The rupture process had a relatively short duration of about 32 s, with a weak starting phase that lasted about 8 s. Seismic moments estimated from body waves, surface waves, and Global Positioning System (GPS) are very similar of the order of 4.5 × 1020 N m. The displacement field produced by our best model was compared to the GPS measurements of coseismic slip obtained by Beavan et al. [1994]. We find an excellent agreement both in displacement direction and magnitude between the predicted and observed GPS displacements. This appears to be then the largest earthquake to have occurred on a shallow‐dipping thrust fault in the Mariana subduction zone during this century. Its occurrence requires a reassessment of the concept of seismic coupling in this subduction zone.
Slip in the 1868 Hayward earthquake from the analysis of historical triangulation data
Authors: Yu Ellen , Segall Paul
Published: February 2004

Despite the importance of the Hayward fault for earthquake hazards in the San Francisco Bay area, neither the rupture length nor the average coseismic slip in the most recent major earthquake, which occurred in 1868, is well known. We estimate slip in the 1868 earthquake by analyzing triangulation data collected between 1853–1860 and 1876–1891. We apply a denuisancing procedure that projects out the dependence of that data on station coordinates, allowing their displacements to be estimated directly. The best fit to the data is obtained with a 52‐km‐long by 10‐km‐deep rupture that extends from Warm Springs to near Berkeley, considerably longer than the 30 km of reported surface breakage. The estimated slip for an earthquake of this length is 1.9 ± 0.4 m, and the seismic moment is M0 = 3.0 (±0.7) × 1019 N m, corresponding to Mw 7.0. The data are inconsistent with regionally high strain rate prior to the 1906 earthquake, as had been previously suggested. The triangulation data record northeastward displacement of station Loma Prieta, presumably as a result of the 1865 earthquake. The fault normal displacement is not easily explained by slip on either the San Andreas fault or the 1989 Loma Prieta rupture surface. This observation is, however, consistent with an MW ∼6 3/4 thrust event northeast of the San Andreas fault in the southern Santa Cruz mountains thrust belt.
Discrete scale invariance, complex fractal dimensions, and log‐periodic fluctuations in seismicity
Authors: Saleur H. , Sammis C. G. , Sornette D.
Published: February 2004

We discuss in detail the concept of discrete scale invariance and show how it leads to complex critical exponents and hence to the log‐periodic corrections to scaling exhibited by various measures of seismic activity close to a large earthquake singularity. Discrete scale invariance is first illustrated on a geometrical fractal, the Sierpinsky gasket, which is shown to be fully described by a complex fractal dimension whose imaginary part is a simple function (inverse of the logarithm) of the discrete scaling factor. Then, a set of simple physical systems (spins and percolation) on hierarchical lattices is analyzed to exemplify the origin of the different terms in the discrete renormalization group formalism introduced to tackle this problem. As a more specific example of rupture relevant for earthquakes, we propose a solution of the hierarchical time‐dependent fiber bundle of Newman et al. [1994] which exhibits explicitly a discrete renormalization group from which log‐periodic corrections follow. We end by pointing out that discrete scale invariance does not necessarily require an underlying geometrical hierarchical structure. A hierarchy may appear “spontaneously” from the physics and/or the dynamics in a Euclidean (nonhierarchical) heterogeneous system. We briefly discuss a simple dynamical model of such mechanism, in terms of a random walk (or diffusion) of the seismic energy in a random heterogeneous system.
Steeply reflected ScSH precursors from the D″ region
Authors: Schimmel Martin , Paulssen Hanneke
Published: February 2004

We present evidence for precursors to the ScS and SScS (SDS and sSDS) phases observed at epicentral distances smaller than 30°. These precursors are intermittently observed in broadband recordings from the six Incorporated Research Institutions for Seismology stations used (western Pacific region and South America). They appear approximately 35–50 s ahead of the ScS arrival on the transverse component and are independent of hypocentral depth. The observed precursors are characterized by relatively large amplitudes in the frequency band from 0.05 to 0.2 Hz. A more detailed analysis is restricted to a subsidiary data set from station SNZO (New Zealand). The coherency, frequency dependence, slowness, polarity, and polarization of the precursors are discussed. Many explanations for SDS can be rejected, and we conclude that a reflector 180 km above the core‐mantle boundary causes these occasionally strong precursors. The large SDS/ScS amplitudes at low frequencies require an unrealistically large impedance contrast for a one‐dimensional model. We test the possibility of focusing the SDS phase by a discontinuity with topography and show that structure with scale lengths of ∼l9°–25° (1200–1600 km, i.e., larger than the Fresnel zone) can account for large intermittently observed SDS amplitudes with their geometrical reflection points within the same Fresnel zone. This is surprising, since it is often assumed that scale lengths smaller than the Fresnel zone must be responsible for variations within a Fresnel zone. The limited data set permits no conclusions about the global properties of this phase or its implied reflector.
Fluid flow in and around a mud volcano field seaward of the Barbados accretionary wedge: Results from Manon cruise
Authors: Henry Pierre , Le Pichon Xavier , Lallemant Siegfried , Lance Sophie , Martin Jonathan B. , Foucher Jean‐Paul , Fiala‐Médioni Aline , Rostek Frauke , Guilhaumou Nicole , Pranal Vincent , Castrec Maryse
Published: February 2004

A field of mud diapirs and mud volcanoes situated in the Barbados trench at 13°50′N and extending along an old oceanic fracture zone (Mercurus) was investigated during the Manon cruise using both surface ship and Nautile submersible sampling and measurements. The entire zone from 13°50′N up to 14°20′N has an anomalously high heat flow which implies that fluids are drained into it from a segment of the accretionary wedge a few hundred kilometers wide. Two structures interpreted as diatremes, Atalante and Cyclops, expell large amounts of water and methane. We propose that they were formed from the release of a light fluid when gas hydrates were dissociated in the sediment as the result of the circulation of warm fluid in the area. However they expell only a small fraction of the incoming fluid, implying that disperse flow is the dominant mode of expulsion in this area. The chemoautotrophic communities on the surface of the structures rely mostly on sulfides. Submersible observations, temperature measurements in the sediment, and the chemistry of the pore fluid indicate that convection of seawater occurs within the first few meters of sediment through high‐permeability channels, such as cemented carbonate conduits. We propose that this convection is driven by the density difference between the pore fluid and seawater, but fresh water released by the dissolution of shallow hydrates may also contribute. This shallow convection may be a frequent process in cold seep environments.
Satellite observations and interpretation of the 1991 Mount Pinatubo eruption plumes
Authors: Holasek R. E. , Self S. , Woods A. W.
Published: February 2004

We demonstrate the use of hourly Geostationary Meteorological Satellite (GMS) and NOAA polar‐orbiting advanced very high resolution radiometer (AVHRR) satellite images of the June 1991 Mount Pinatubo volcanic eruption plumes in providing details of the timing, changing eruptive style, and umbrella plume movement and dynamics. A chronology of explosive events, which culminated in a >9‐hour‐long climactic phase on June 15, has been determined from satellite data and favorably compared to ground‐based and other ancillary information. Maximum eruption column altitudes of 40 km and column‐top undercooling in excess of 55°C below the ambient atmosphere occurred during the climactic phase. Phases dominated by Plinian versus coignimbrite eruptive plumes can be distinguished on the images. Analysis of the spreading umbrella plume during the climactic phase suggests that over the first 4–5 hours the plume spread laterally as a gravitational intrusion before becoming advected in the ambient winds. We compare the rate of plume growth to a model of a radially symmetric, continuously fed intrusion to describe the motion. The rate of plume growth shows good agreement with this simple model for the first 4–5 hours. From column‐top heights we determine eruption rates and obtain an independent estimate of the total volume of magma erupted from June 12–16 of ∼5.5 km3 dense rock equivalent (DRE), in good agreement with an independent estimate of the total eruptive volume made from deposits on land and submarine ash fall of up to 5.3 km3 DRE. We also show that secondary explosions in the Pinatubo ignimbrite deposits produced ash plumes reaching altitudes as high as 19 km in September 1991. This study indicates that realistic interpretation and monitoring of major eruption plumes can be accomplished by analysis of high temporal resolution satellite data.
Geodesy using the Swedish permanent GPS network: Effects of signal scattering on estimates of relative site positions
Authors: Jaldehag R. T. K. , Johansson J. M. , Rönnäng B. O. , Elósegui P. , Davis J. L. , Shapiro I. I. , Niell A. E.
Published: February 2004

This paper presents results from a study of elevation‐angle‐dependent systematic effects on estimates of relative site positions within the Swedish permanent Global Positioning System (GPS) network. Two months of data from 16 sites have been analyzed with three different elevation cutoff angles, namely, 10°, 15°, and 20°. We present offsets between these solutions and demonstrate that estimates of the vertical component of several baselines strongly depend on the minimum elevation angle (elevation cutoff angle) of the data analyzed. Offsets of 22.3 ± 1.6 mm in the vertical component are evident when the elevation cutoff angle is changed from 10° to 20°. We investigate these offsets and conclude that a significant part is due to differential phase errors caused by scattering from structures associated with the mounting of the antenna to the pillar and with the pillar itself. The horizontal components of baseline are less affected. We found, however, that the offsets in the horizontal components increase with baseline length. For the longest baselines (∼1500 km) offsets of more than 5 mm are evident in the north component when the elevation cutoff angle is changed from 10° to 20°. These offsets are most likely due to differential phase errors caused by nonuniform antenna phase patterns; an effect that presumably increases with baseline length and which also might increase because of scattering from the pillars and the antenna mounts. We identify the scattering structure and reduce associated errors in the vertical component of baseline to a significant degree on one of the sites by using microwave‐absorbing material. The results presented are of importance for those analyzing data from existing networks and for those who intend to establish permanent GPS geodetic networks.
Thermal state of the Taranaki Basin, New Zealand
Authors: Funnell Rob , Chapman David , Allis Rick , Armstrong Phil
Published: February 2004

The Taranaki Basin is an active‐margin basin that has been significantly affected by Miocene subduction tectonics along the Pacific‐Australian plate boundary. We have analyzed its present‐day thermal state using 354 bottom‐hole temperatures (BHTs) from 115 wells distributed throughout the basin. The measured temperatures were corrected using an exact solution to Bullard's equation rather than the Horner approximation, thereby allowing for recovery dependence on well diameter and correction for some BHTs at early time after circulation had ceased. Thermal conductivity measurements were completed on 256 samples from eight wells, and matrix conductivities were determined for six end‐member lithologies by inversion. Formation conductivities are based on the conductivity and relative proportion of each end‐member component. Corrected BHTs, in situ thermal conductivity, and estimates of sediment heat production were combined to compute the present‐day, steady state heat flow. The average heat flow is 60 mW m−2, but important geographic variations are present: heat flow on the Western Platform is remarkably consistent at 53–60 mW m−2, attesting to its relative stability since the Late Cretaceous; heat flow in the southern part of the basin is 65–70 mW m−2 due to as much as 3 km of late Miocene erosion; on the southern onshore and to the south of the peninsula, heat flow is 50 ± 3 mW m−2, possibly due to the heat sink effects of crustal thickening; heat flow is highest at 74 mW m−2 on the northern peninsula adjacent to the Taranaki volcanic zone, suggesting a causal relationship between Quaternary volcanism and high heat flow.