bluuebunny/crossref_metadata_2025 huggingface dataset:
Capture of magnetosheath plasma by the magnetosphere during northward IMF
Authors: Sandholt P. E. , Farrugia C. J. , Cowley S. W. H. , Denig W. F. , Lester M. , Moen J. , Lybekk B.
Published: December 2003

We interpret combined auroral and simultaneous particle data from an overflight of the ground station at Svalbard by the F13 DMSP spacecraft in terms of pulsed “capture” of northward‐directed magnetosheath flux tubes by the magnetosphere, due to sequential lobe reconnection in both the southern and northern hemispheres. The event refers to a ∼40‐min long interval characterized by strongly northward interplanetary magnetic field (IMF) (Bz=7 nT; clock angle ∼ 10°‐15°). The meridan scanning photometer at the ground station records a long stepwise poleward retraction of the band of auroral emission in the cusp region. Each step is marked by an initial, brief poleward leap, followed by a ∼5 min period of relatively steady auroral latitude. In the one event where simultaneous particle data are available (at 1100 MLT), the spacecraft traverses a region of auroral precipitation where electrons and ions of magnetosheath origin are present, together with equatorward convection. With a large sunward IMF tilt (Bx=6‐7 nT) in the winter hemisphere, we suppose that the process starts with reconnection poleward of the southern cusp followed by overdraped lobe flux which reconnects with magnetospheric field lines in the northern hemisphere.
A three‐dimensional hybrid code simulation of the December 1984 solar wind AMPTE release
Authors: Delamere P. A. , Swift D. W. , Stenbaek‐Nielsen H. C.
Published: December 2003

A three‐dimensional hybrid code has been developed to study the interaction between small dense plasma clouds and an ambient plasma. The primary advantage of this code is a seamless interface between kinetic particles (plasma cloud) and an MHD fluid (ambient plasma). This interface provides momentum coupling between two distinct ion populations. As a preliminary test of our code, we have simulated the first 3 minutes of the December 1984 AMPTE artificial comet. The results show good agreement with observations of the magnetic field distribution as well as the lateral motion of the comet head. Examples of other applications for this code include comets, coronal mass ejections, Io's plasma torus, and plasma injection experiments.
High resolution bistatic HF radar observations of ULF waves in artificially generated backscatter
Authors: Wright Darren M. , Yeoman Timothy K.
Published: December 2003

High resolution HF radar observations of ULF waves in the ionosphere are possible by artificially generating irregularities using the EISCAT heater at Tromsø, Norway. The line‐of‐sight velocities from the CUTLASS radars have been combined for the first time to form bistatic flow vectors during a ULF wave, generating ionospheric electric field measurements with a higher accuracy than ever previously recorded by any instrument. A high‐m wave observed in the ionosphere exhibits a frequency which is a harmonic of that of a low‐m field line resonance which is observed simultaneously in the ionosphere and at the ground. The high‐m pulsation resembles a class of particle driven wave previously recorded on VHF radars. These results are consistent with recent HF radar observations of small scale waves possessing similar characteristics to field line resonances and with the theory that the low‐m wave may be driving the high‐m wave through a non‐linear Kelvin‐Helmholtz instability.
Investigating possible causes of the observed diurnal variability in Antarctic NOy
Authors: Weller R. , Minikin A. , König‐Langlo G. , Schrems O. , Jones A. E. , Wolff E. W. , Anderson P. S.
Published: December 2003

The occurence of a diurnal variability in measured total oxidised nitrogen (NOy) was observed at Neumayer, Antarctica, (70°39′S, 8°15′W) during a recent summer measurement campaign. Minima and maxima occurred in the early morning/early evening respectively, with the amplitude of the cycle around 40% of the daily mean NOy values. Given that this campaign was the first to attempt NOy measurements on the Antarctic continent, it is not presently clear whether this is an Antarctic‐wide phenomenon, or local to Neumayer. A similar cycle was observed for HNO3, although HNO3 concentrations and fluctuations are too small to account for all of the NOy variability. In this paper we investigate possible mechanisms that might cause such diurnal signals, focussing on the influence of local meteorology and also of the snow‐pack. Exchange processes at the air/snow interface appear to dominate the observed NOy variability, although an influence from the changing surface inversion strength exists. These findings have important implications in understanding and hence correctly interpreting ice core nitrate data.
Carbon monoxide in the U.S. mid‐Atlantic troposphere: Evidence for a decreasing trend
Authors: Hallock‐Waters Kristen A. , Doddridge Bruce G. , Dickerson Russell R. , Spitzer Shane , Ray John D.
Published: December 2003

Nearly continuous measurements of carbon monoxide (CO) were made at Shenandoah National Park‐Big Meadows in rural Virginia, a site considered representative of regional air quality, from December 1994 to November 1997. Similar observations were also made at this location from October 1988 to October 1989. These observations combine to indicate a decreasing trend in CO concentration over the U.S. mid‐Atlantic region of about 5.0 ppbv yr−1, with greater than 95% confidence that the slope is significantly different from zero. The decrease suggests U.S. reductions in anthropogenic CO emissions have been effective in reducing pollutant levels. The observed trend is consistent with the U.S. EPA reported trend in emissions and the decrease in Northern Hemisphere tropospheric background CO mixing ratios observed by other researchers.
Analysis of early phase ring current recovery mechanisms during geomagnetic storms
Authors: Liemohn M. W. , Kozyra J. U. , Jordanova V. K. , Khazanov G. V. , Thomsen M. F. , Cayton T. E.
Published: December 2003

A time‐dependent kinetic model is used to investigate the relative importance of various mechanisms in the early phase decay rate of the ring current. It is found that, for both the solar maximum storm of June 4–7, 1991 and especially the solar minimum storm of September 24–27, 1998, convective drift loss out the dayside magnetopause is the dominant process in removing ring current particles during the initial recovery. During the 1998 storm, dayside outflow losses outpaced charge exchange losses by a factor of ten.
Variations of hot O in the thermosphere
Authors: Oliver W. L. , Schoendorf J.
Published: December 2003

Using measurements of electron temperature and density, a model of neutral density, and a theoretical expression for the O+−O collision cross section, we compute the ion temperature and find it to be lower than the measured value. We compute the amount of hot O at 4000°K necessary to provide this heat deficit. At 400 km altitude very little hot O is needed at solar maximum while 1–2% of total O must be hot at solar maximum. Hot O densities at 400 km altitude are lowest during the day, highest at night; lowest at the equinoxes, highest at the solstices; lowest at solar maximum, highest at solar minimum; lowest for magnetically disturbed periods, highest for quiet periods; lowest for a warm, dense thermosphere, highest for a cool, rare thermosphere. All of these variations are consistent with thermal quenching by ambient O, with larger ambient O densities yielding lower hot O densities. This indicates that hot O should form a layer shape in the atmosphere.
Three‐dimensional models of P wave velocity and P‐to‐S velocity ratio in the southern central Andes by simultaneous inversion of local earthquake data
Authors: Graeber Frank M. , Asch Günter
Published: December 2003

The PISCO'94 (Proyecto de Investigatión Sismológica de la Cordillera Occidental, 1994) seismological network of 31 digital broad band and short‐period three‐component seismometers was deployed in northern Chile between the Coastal Cordillera and the Western Cordillera. More than 5300 local seismic events were observed in a 100 day period. A subset of high‐quality P and S arrival time data was used to invert simultaneously for hypocenters and velocity structure. Additional data from two other networks in the region could be included. The velocity models show a number of prominent anomalies, outlining an extremely thickened crust (about 70 km) beneath the forearc region, an anomalous crustal structure beneath the recent magmatic arc (Western Cordillera) characterized by very low velocities, and a high‐velocity slab. A region of an increased Vp/Vs ratio has been found directly above the Wadati‐Benioff zone, which might be caused by hydration processes. A zone of lower than average velocities and a high Vp/Vs ratio might correspond to the asthenospheric wedge. The upper edge of the Wadati‐Benioff zone is sharply defined by intermediate depth hypocenters, while evidence for a double seismic zone can hardly be seen. Crustal events between the Precordillera and the Western Cordillera have been observed for the first time and are mainly located in the vicinity of the Salar de Atacama down to depths of about 40 km.
Stress sensitivity of fault seismicity: A comparison between limited‐offset oblique and major strike‐slip faults
Authors: Parsons Tom , Stein Ross S. , Simpson Robert W. , Reasenberg Paul A.
Published: December 2003

We present a new three‐dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1989 M = 7.1 Loma Prieta earthquake and to compare fault seismicity rates before and after 1989. The major high‐angle right‐lateral faults exhibit a different response to the stress change than do minor oblique (right‐lateral/thrust) faults. Seismicity on oblique‐slip faults in the southern Santa Clara Valley thrust belt increased where the faults were undamped. The strong dependence of seismicity change on normal stress change implies a high coefficient of static friction. In contrast, we observe that faults with significant offset (>50–100 km) behave differently; microseismicity on the Hayward fault diminished where right‐lateral shear stress was reduced and where it was undamped by the Loma Prieta earthquake. We observe a similar response on the San Andreas fault zone in southern California after the Landers earthquake sequence. Additionally, the offshore San Gregorio fault shows a seismicity rate increase where right‐lateral/oblique shear stress was increased by the Loma Prieta earthquake despite also being clamped by it. These responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup; coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes, whereas in more limited offset faults, fluids could rapidly escape. The difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes, since many of these events occur both inside and outside major fault zones.
Lap shear testing of biotite and phlogopite crystals and the application of interferometric strain/displacement gages to mineralogy
Authors: Noe David C. , LaVan David A. , Veblen David
Published: December 2003

Crystals of biotite and phlogopite were deformed in simple shear to determine the influence of compositional and structural variables on behavior during mechanical deformation. The use of an interferometric strain/displacement gage (ISDG) with modified sample grips allowed the testing of small samples that did not contain significant partings along cleavage planes; these grips also permitted the use of very small compressive forces resolved normal to the basal planes. Overall differences in strength between samples from different localities could not be simply related to compositional differences (such as F content) or with the density of growth spirals perpendicular to the basal planes (which might impede the motion of basal dislocations). Differences in strength among samples from the same series were related to the orientation of the applied shear stress, and these strength differences correlated with F content. These represent the first known experiments with ISDGs in mineralogy and demonstrate that there is significant potential for their application to geological problems.
Flow behavior of fine‐grained synthetic dunite in the presence of 0.5 wt% H2O
Authors: McDonnell R. D. , Peach C. J. , Spiers C. J.
Published: December 2003

The deformation behavior of synthetic dunite (Mg‐forsterite plus 2.5 vol% enstatite) with a grain size of ∼1 μm has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850° to 1000°C, strain rates of 10−7 to 10−5 s−1 and a confining pressure of 600 MPa. Nominally dry material behaved purely elastically. In contrast, samples with 0.5 wt% added water flowed at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Dorn‐type power law to the wet data yielded stress exponents (n) of 1.7±0.4 and apparent activation energies in the range 302±22 kJ mol−1. The wet‐deformed samples showed polygonal grains, no crystallographic preferred orientation, no subgrains, low dislocation densities, and evidence for grain boundary cavitation. Grain growth was minor. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sliding dominated mechanism, though the nature of the accommodation process remains unclear, and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated. Application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere.
Global‐scale analysis of the mantle Pds phases
Authors: Chevrot Sébastien , Vinnik Lev , Montagner Jean‐Paul
Published: December 2003

A global‐scale analysis of travel times of the mantle Pds phases (converted from P to S from the discontinuity at a depth d beneath the receiver) is presented. Most of the data are related to P′410′s and P′660′s, recorded by the broadband stations of Geoscope, Incorporated Research Institutions for Seismology (IRIS) and other networks. In order to provide a good global‐scale coverage, published measurements at 37 stations are complemented by our measurements at 45 stations. Lateral resolution of our data is many times higher than that of SS and its precursors, previously used for similar purposes, but we have fewer data in the oceans. Lateral variations of travel times of the major Pds phases are strongly correlated with the lateral P and S velocity variations in the uppermost mantle, as revealed by tomographic studies. The scaling coefficient between the teleseismic S and P residuals, as found from the variations of travel times of the major Pds phases, is around 3.7 for North America and around 3.3 for the rest of the world. Almost all the differential travel times t(P′660′s) – t(P′410′s) are in the range of ±1 s, relative to the average value of 24.0 s. If the lateral variations of the velocities within the mantle transition zone (MTZ) can be neglected, the corresponding variations of thickness of the MTZ are in the range of ±10 km, relative to the standard depth of 250 km, and do not show significant correlation with either velocities in the upper mantle above the MTZ or with the variations of thickness of the MTZ, as found in some recent studies of precursors to SS. There is a weak but marginally significant positive correlation between the variations of thickness of the MTZ inferred from our data and the S velocity variations in the MTZ, as given by Su et al. [1994]. This correlation suggests that both variables contain temperature‐dependent components. Lateral variability of the MTZ is also manifested by extreme weakness of P′410′s at several locations. This may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity. At some stations there are arrivals that can be interpreted as P′520′s. This interpretation implies that the strengh and/or sharpness of 520 km discontinuity, as well as its depth, are laterally variable in a broad range.
Streaming potential in porous media: 2. Theory and application to geothermal systems
Authors: Revil A. , Schwaeger H. , Cathles L. M. , Manhardt P. D.
Published: December 2003

Self‐potential electric and magnetic anomalies are increasingly being observed associated with hydrothermal fields, volcanic activity, and subsurface water flow. Until now a formal theoretical basis for predicting streaming potential of porous materials has not been available. We develop here a model giving both the macroscopic constitutive equations and the material properties entering these equations. The material properties, like the streaming potential coupling coefficient, depend on pore fluid salinity, temperature, water and gas saturations, mean grain diameter, and porosity. Some aspects of the model are directly tested with success against laboratory data. The streaming potential increases with temperature, grain size, and gas saturation, and decreases with salinity. At the scale of geological structures the model provides an explanation for the presence of kilometer‐scale dipolar self‐potential anomalies in geothermal systems and volcanoes. Positive self‐potential anomalies are associated with fluid discharge areas, whereas negative self‐potential anomalies are associated with fluid recharge areas. Self‐potential anomaly maps determined at the surface of active hydrothermal fields appear to be a powerful way of mapping the fluid recharge and discharge areas. In the case of free convection the vorticities of the convection pattern generate a magnetic field. The greater these vorticities, the greater the associated magnetic field. It follows that hydrothermal systems act as natural geobatteries because of the flow of pore fluids in the subsurface of these systems.
Stress transfer by the 1988–1989 M = 5.3 and 5.4 Lake Elsman foreshocks to the Loma Prieta fault: Unclamping at the site of peak mainshock slip
Authors: Perfettini Hugo , Stein Ross S. , Simpson Robert , Cocco Massimo
Published: December 2003

We study the stress transferred by the June 27, 1988, M = 5.3 and August 8, 1989, M = 5.4 Lake Elsman earthquakes, the largest events to strike within 15 km of the future Loma Prieta rupture zone during 74 years before the 1989 M = 6.9 Loma Prieta earthquake. We find that the first Lake Elsman event brought the rupture plane of the second event 0.3–1.6 bars (0.03–0.16 MPa) closer to Coulomb failure but that the Lake Elsman events did not bring the future Loma Prieta hypocentral zone closer to failure. Instead, the Lake Elsman earthquakes are calculated to have reduced the normal stress on (or “undamped”) the Loma Prieta rupture surface by 0.5–1.0 bar (0.05–0.10 MPa) at the site where the greatest slip subsequently occurred in the Loma Prieta earthquake. This association between the sites of peak unclamping and slip suggests that the Lake Elsman events did indeed influence the Loma Prieta rupture process. Unclamping the fault would have locally lowered the resistance to sliding. Such an effect could have been enhanced if the lowered normal stress permitted fluid infusion into the undamped part of the fault. Although less well recorded, the ML = 5.0 1964 and ML = 5.3 1967 Corralitos events struck within 10 km of the southwest end of the future Loma Prieta rupture. No similar relationship between the normal stress change and subsequent Loma Prieta slip is observed, although the high‐slip patch southwest of the Loma Prieta epicenter corresponds roughly to the site of calculated Coulomb stress increase for a low coefficient of friction. The Lake Elsman‐Loma Prieta result is similar to that for the 1987 M = 6.2 Elmore Ranch and M = 6.7 Superstition Hills earthquakes, suggesting that foreshocks might influence the distribution of mainshock slip rather than the site of mainshock nucleation.
Determination of lattice parameter, oxidation state, and composition of individual titanomagnetite/titanomaghemite grains by transmission electron microscopy
Authors: Zhou Weiming , Peacor Donald R. , Van der Voo Rob , Mansfield John F.
Published: December 2003

A transmission electron microscopy (TEM) method, higher‐order Laue zone (HOLZ) diffraction, has been applied to measurement of accurate lattice parameters of individual titanomagnetite and titanomaghemite grains. Lattice parameters and cation‐composition data are obtained with TEM/analytical electron microscopy (AEM) for individual grains smaller than 0.1 μm in diameter. A suite of Ocean Drilling Program/Deep Sea Drilling Project ocean‐floor basalts containing titanomagnetite‐titanomaghemite with a wide range of oxidation degree (z), were employed to establish the relationship between lattice parameter (a), z and Ti‐content (x) for naturally occurring samples. Standardization was accomplished with measurements on grains sufficiently large to allow the determination of absolute oxygen contents by electron microprobe analyses, which directly yielded z values. The probe data confirm that the maghemitization process in ocean‐floor basalts is one involving loss of iron and indicate a nearly linear relationship between lattice parameters and oxidation degrees for a given x value, except for the smallest and largest values of z. The standardized relation permits determination of z for individual submicrometer‐sized grains using only measured a‐values and cation compositions as determined by HOLZ diffraction and AEM.
Evaluation of SO2 emission from Mount Etna using diurnal and nocturnal multispectral IR and visible imaging spectrometer thermal IR remote sensing images and radiative transfer models
Authors: Teggi S. , Bogliolo M. P. , Buongiorno M. F. , Pugnaghi S. , Sterni A.
Published: December 2003

We describe a model to evaluate the volcanic SO2 emission using images acquired by the airborne multispectral infrared and visible imaging spectrometer (MIVIS) in the thermal infrared spectral region. The estimating procedure consists of mapping of the SO2 columnar content and evaluation of the total SO2 flux emitted by the volcano. All the atmospheric radiative effects, including the SO2 plume emission/absorption, are computed by the MODTRAN 3.5 radiative transfer code. We apply the model to two MIVIS images acquired over Mount Etna, located on the east coast of Sicily, Italy, during the “Sicilia‐94” remote sensing campaign on July 24 and 25, 1994. The June 25 image was acquired during the night; this is the first attempt to estimate the SO2 content in volcanic plume using nocturnal thermal infrared images. The total flux estimates range from 20.9 kg s−1 (1810 t d−1) to 82.2 kg s−1 (7100 t d−1) depending on the plume geometry used in the procedure. These results, except one case (82.2 kg s−1), are in agreement with the estimates derived from correlation spectrometer (COSPEC) measurements collected during the same period. We also evaluate the dependencies of the results on several parameters such as plume geometry, surface emissivity, water vapor content, and wind speed.
Strain accumulation at Yucca Mountain, Nevada, 1983–1998
Authors: Savage J. C. , Svarc J. L. , Prescott W. H.
Published: December 2003

A 14‐station, 50‐km aperture geodetic array centered on the proposed radioactive waste disposal site at Yucca Mountain, Nevada, was surveyed in 1983, 1984, 1993, and 1998 to determine the rate of strain accumulation there. The coseismic effects of the 1992 (MS=5.4) Little Skull Mountain earthquake, which occurred within the array, were calculated from a dislocation model and removed from the data. The measured principal strain accumulation rates determined over the 1983–1998 interval are ε1 = 2±12 nanostrain/yr N87°W±12° and ε2 = −22±12 nanostrain/yr N03°E±12° (extension reckoned positive and quoted uncertainties are standard deviations). The N65°W extension rate is −2±12 nanostrain/yr, significantly less than the 1991–1997 N65°W rate of 50±9 nanostrain/yr reported by Wernicke et al. [1998]. The implied maximum right‐lateral engineering‐shear, strain accumulation rate is γ=ε1−ε2 = 23±10 nanostrain/yr, a marginally significant rate. Almost half (ε1 = 6 nanostrain/yr N90°W, ε2 = −6 nanostrain/yr N00°E, and γ = 12 nanostrain/yr ) of the measured strain rate can be attributed to strain accumulation on the Death Valley‐Furnace Creek (50 km distant) and Hunter Mountain‐Panamint Valley (90 km distant) faults. The residual strain rate after the removal of those fault contributions is not significant at the 95% confidence level.
Internal deformation of the Gorda Plate observed by hydroacoustic monitoring
Authors: Fox Christopher G. , Dziak Robert P.
Published: December 2003

On August 29, 1991, the National Oceanic and Atmospheric Administration VENTS Program began monitoring the northeast Pacific Ocean plate boundaries using continuously recorded data from the U.S. Navy's Sound Surveillance System (SOSUS) hydrophone arrays. Routine detection and location of small (2.4 ≤ M ≤ 3.5) earthquakes revealed a NNE‐SSW trending band of seismicity through the center of the Gorda Plate. The majority of the earthquakes in the band were not detected by land‐based seismic networks. This band of microearthquakes was active from the initiation of SOSUS monitoring in August 1991 through July 1992, at a rate of ∼20 events per week. Since August 1992, however, the activity has effectively ceased. Some of the microearthquakes in the band are likely aftershocks to three large (MW > 6) mid Gorda Plate earthquakes that occurred 1–2 months prior to the beginning of SOSUS monitoring. The hydroacoustically detected events in the band, however, do not occur in the proximity of the MW > 6 events, and the activity level increased several months afterward, suggesting the microseismicity band represents a more general pattern of deformation within the Gorda Plate. The cessation of the midplate activity follows the occurrence of a large earthquake sequence in the adjacent Cascadia Subduction Zone in April 1992. It is proposed that the band of microseismicity and the three large midplate events reflect an accumulation of stress in the Gorda Plate, with the eventual termination of this pattern due to stress reduction associated with movement in the adjacent subduction zone. Stress field modeling of the region yields results consistent with this interpretation.
Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington
Authors: Parsons Tom , Wells Ray E. , Fisher Michael A. , Flueh Ernst , ten Brink Uri S.
Published: December 2003

Eocene mafic crust with high seismic velocities underlies much of the Oregon and Washington forearc and acts as a backstop for accretion of marine sedimentary rocks from the obliquely subducting Juan de Fuca slab. Arc‐parallel migration of relatively strong blocks of this terrane, known as Siletzia, focuses upper crustal deformation along block boundaries, which are potential sources of earthquakes. In a three‐dimensional velocity model of coastal Washington, we have combined surface geology, well data, and travel times from earthquakes and controlled source seismic experiments to resolve the major boundaries of the Siletz terrane with the adjacent accreted sedimentary prism and volcanic arc. In southern Washington and northern Oregon the Siletz terrane appears to be a thick block (∼20 km) that extends west of the coastline and makes a high‐angle contact with the offshore accreted sedimentary prism. On its east flank the high‐velocity Siletz terrane boundary coincides with an en echelon zone of seismicity in the arc. In northern Washington the western edge of Siletzia makes a lower‐angled, fault‐bound contact with the accretionary prism. In addition, alternating, east‐west trending uplifts and downwarps of the Siletz terrane centered on the antiformal Olympic Mountains may reflect focusing of north‐south compression in the northern part of the Siletz terrane. This compressional strain may result from northward transport and clockwise rotation of the Siletz terrane into the relatively fixed Canadian Coast Mountains restraining bend along the coast.
Estimating the hazard of rupture using uncertain occurrence times of paleoearthquakes
Authors: Ogata Yosihiko
Published: December 2003

When a renewal process is fitted to paleoearthquake occurrence data, it is more informative to use the data including the two interval ends before the first event to the beginning of observation period and after the last event till the present time because the sample size is usually small. Our main concern is analysis of the data in which some occurrence times of events themselves are uncertain and given by intervals or some distributions. Specifically, we consider a Bayesian inference where each uncertainty is interpreted as a prior distribution associated with likelihood of a renewal process model. Integration of the posterior with respect to the occurrence times and its maximization with respect to the parameters of the renewal process model are implemented in order to estimate the parameters and further to compare the goodness of fit of other competing renewal process models. Thus the posterior of the selected model provides a reasonable estimate of distribution of uncertain occurrence times associated with the fitted renewal process model. Finally, in a case where occurrence time of the last event is uncertain, a practical method for the assessment of current and future hazard of the forthcoming rupture is provided.
Dynamics of the stream‐power river incision model: Implications for height limits of mountain ranges, landscape response timescales, and research needs
Authors: Whipple Kelin X. , Tucker Gregory E.
Published: December 2003

The longitudinal profiles of bedrock channels are a major component of the relief structure of mountainous drainage basins and therefore limit the elevation of peaks and ridges. Further, bedrock channels communicate tectonic and climatic signals across the landscape, thus dictating, to first order, the dynamic response of mountainous landscapes to external forcings. We review and explore the stream‐power erosion model in an effort to (1) elucidate its consequences in terms of large‐scale topographic (fluvial) relief and its sensitivity to tectonic and climatic forcing, (2) derive a relationship for system response time to tectonic perturbations, (3) determine the sensitivity of model behavior to various model parameters, and (4) integrate the above to suggest useful guidelines for further study of bedrock channel systems and for future refinement of the streampower erosion law. Dimensional analysis reveals that the dynamic behavior of the stream‐power erosion model is governed by a single nondimensional group that we term the uplift‐erosion number, greatly reducing the number of variables that need to be considered in the sensitivity analysis. The degree of nonlinearity in the relationship between stream incision rate and channel gradient (slope exponent n) emerges as a fundamental unknown. The physics of the active erosion processes directly influence this nonlinearity, which is shown to dictate the relationship between the uplift‐erosion number, the equilibrium stream channel gradient, and the total fluvial relief of mountain ranges. Similarly, the predicted response time to changes in rock uplift rate is shown to depend on climate, rock strength, and the magnitude of tectonic perturbation, with the slope exponent n controlling the degree of dependence on these various factors. For typical drainage basin geometries the response time is relatively insensitive to the size of the system. Work on the physics of bedrock erosion processes, their sensitivity to extreme floods, their transient responses to sudden changes in climate or uplift rate, and the scaling of local rock erosion studies to reach‐scale modeling studies are most sorely needed.
Phase separation and fluid mixing in subseafloor back arc hydrothermal systems: A microthermometric and oxygen isotope study of fluid inclusions in the barite‐sulfide chimneys of the Lau Basin
Authors: Lécuyer Christophe , Dubois Michel , Marignac Christian , Gruau Gérard , Fouquet Yves , Ramboz Claire
Published: December 2003

Fluid inclusions in barite and sulfide in chimneys (both active and inactive) from three hydrothermal sites of the back arc Lau basin were studied with microthermometric and isotopic methods to determine the chemistry and evolution of hydrothermal aqueous fluids. Strontium isotope compositions of sulfides from the Lau basin reflect the presence of anhydrite and barite inclusions. The 87Sr/86Sr ratios of these two sulfate minerals vary from 0.7045 to 0.7078 and are interpreted as the result of mixing between various proportions of the hydrothermal end‐member and pure seawater. The microthermometric study of fluid inclusions reveals that mixing with seawater involved different kinds of aqueous fluid end‐members. A high‐temperature Mg‐depleted end‐member of high salinity (≥5.5 wt % eq NaCl) was found at the Vai Lili site. A uncommon low‐temperature Mg‐rich end‐member was also identified at the Hine Hina site in association with barite deposition. At the Vai Lili site, very low salinity fluids were produced in addition to a very saline brine (≥30 wt % NaCl) that was trapped inside anhydrite precipitated from an active vent at a temperature of 342°C. Oxygen isotope ratios of water inclusions range from 2‰ to 4.4‰ for chalcopyrite, barite, and sphalerite minerals. The 18O enrichment and the high salinities of many fluids from the Lau basin are accounted for by the specificity of the back arc setting. This non mid‐ocean ridge setting is characterized by the shallow depth of the hydrothermal systems that allows frequent unmixing of high‐temperature liquids. The abundance of silicic magmas also provided magmatic fluids (including brines) that mixed with seawater‐derived aqueous fluids.
Regional heat flow variations across the sedimented Juan de Fuca Ridge eastern flank: Constraints on lithospheric cooling and lateral hydrothermal heat transport
Authors: Davis E. E. , Chapman D. S. , Wang K. , Villinger H. , Fisher A. T. , Robinson S. W. , Grigel J. , Pribnow D. , Stein J. , Becker K.
Published: December 2003

Seafloor heat flow has been estimated continuously along a transect on the sedimented eastern flank of the Juan de Fuca Ridge to provide constraints on the scale and rate of lateral heat and fluid transport in the upper igneous crust, and on the total flux from the young lithosphere beneath. The profile extends from 20 km east of the ridge axis, where turbidite sediments lap onto the oceanic crust at an age of less than 1 Ma, and ends 110 km east of the axis over 3.6 Ma crust. Estimated heat flow is derived from the combination of the depth to igneous basement determined from seismic reflection profiles, and basement temperatures measured in nine Ocean Drilling Program boreholes along the transect. The strategy is based on the knowledge that in this young area upper basement temperatures are locally homogenized by vigorous hydrothermal circulation, and hence vary smoothly between drilling sites. At the young end of the transect near the region of extensive outcrop, heat flow is only 15% of that predicted for lithosphere of this age. Heat flow increases systematically to the east, reaching the full predicted level about 20 km from the point of sediment/basement onlap. The anomaly can be accounted for by mixing and/or eastward flow of cool water in the upper igneous crust beneath the sediment at a Darcian rate of the order of 1 m yr−1. A geochemical signature of “fresh” seawater in basement is observed many tens of kilometers farther to the east, although the thermal signature is no longer present; along the remainder of the transect the heat flow conforms very well to that predicted by simple boundary layer cooling theory once the thermal effects of rapid sedimentation on this ridge flank are accounted for.
Conditions for earthquake surface rupture along the San Andreas Fault System, California
Authors: Streit Jürgen E.
Published: December 2003

Earthquake ruptures that nucleate along faults at depth can only propagate to the Earth's surface if the shear stresses during faulting exceed the yield strength of the uppermost crust, where fluid pressures are usually hydrostatic. Suprahydrostatic fluid pressures may be required at depth for the seismic reactivation of misoriented faults, such as the San Andreas fault system. Based on fracture criteria for the wall and fault rock, slip events that can cause ground breakage are estimated to occur at seismogenic depth along fault planes that form angles of less than 65° on average with the maximum principal stress direction. Such slip events require minimum shear stresses of about 30 MPa at hypocentral depth. For faults with long interseismic periods, and thus, inferred high cohesive strength, the predicted reactivation angle is ≤55°, suggesting that several segments of the San Andreas fault system in southern California, including the San Bernardino area, parts of the Elsinore fault zone and the San Jacinto fault, provide the most probable hypocentral sites for future large earthquakes that disrupt the surface. To provide further constraints on locations for such earthquakes, we urgently need to investigate frictional properties of fault rocks and the state of stress at depth by employing laboratory experiments, focal mechanism studies, and by drilling into seismically active faults such as the San Andreas fault system.
Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust
Authors: Bangs Nathan L. B. , Shipley Thomas H. , Moore J. Casey , Moore Gregory F.
Published: December 2003

A volume of three‐dimensional seismic reflection data, acquired in 1992, imaged the decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 × 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protodecollement and decollement at five sites with a logging‐while‐drilling (LWD) tool to log density and other physical properties of the decollement. We construct a regional map of density, and inferred porosity, within the decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protodecollement forms in a radiolarian‐rich Miocene mudstone with an anomalously high porosity (70–75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10‐km‐long, 1‐ to 2‐km‐wide, NE‐SW zone in which porosity is 70%, as high as it is in the protodecollement. This zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out‐of‐sequence thrusts above the decollement.
An estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks
Authors: Alexandrescu Mioara Mandea , Gibert Dominique , Le Mouël Jean‐Louis , Hulot Gauthier , Saracco Ginette
Published: December 2003

It has recently been proposed that geomagnetic jerks observed at the Earth's surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1.5 when wavelet analyzed. Such a signal should have suffered some distortion when diffusing from the core‐mantle boundary (CMB) through the conducting mantle. Assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle, we compute the distortion that a pure singularity introduced at the CMB suffers as it traverses the mantle. We compute this distortion through its effects on the so‐called ridge functions extracted from the wavelet transform of the signal. This distortion is very similar to the small but significant one that we observe in real data. We therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones. Assuming, for example, a thickness of 2000 km for a uniform lower conducting mantle, we find an electrical conductivity smaller than 10 S m−1. This value is in reasonable agreement with values derived from high‐pressure experiments for a silicate mantle.
Apophyllite 40Ar/39Ar and Rb‐Sr geochronology: Potential utility and application to the timing of secondary mineralization of the Kirkpatrick Basalt, Antarctica
Authors: Fleming Thomas H. , Foland Kenneth A. , Elliot David H.
Published: December 2003

Apophyllite (KCa4Si8O20(F, OH)·8H2O), which formed as a low‐temperature secondary mineral in the Kirkpatrick Basalt, has been examined to evaluate its potential for geochronology using the K‐Ar (by the 40Ar/39Ar technique) and Rb‐Sr methods. The 40Ar/39Ar total‐gas ages for apophyllites from the Kirkpatrick Basalt are 114–133 Ma for the central Transantarctic Mountains, 95–114 Ma for south Victoria Land, and 76–100 Ma for north Victoria Land. Within individual hand samples, apophyllite 40Ar/39Ar apparent ages show variations of up to 24 m.y., and within single large (up to ∼2 cm) crystals, differences of up to 14 m.y. are observed. The 40Ar/39Ar incremental‐heating spectra are generally flat but have lowand high‐temperature discordances attributed to 39Ar recoil. Rb‐Sr model ages for the apophyllites range from 94 to 144 Ma and vary from concordant with the 40Ar/39Ar dates to as much as 14 m.y. older. The dates are consistent with other temporal indicators of low‐temperature alteration in the basalts and are interpreted to broadly reflect the time of apophyllite precipitation. They document a geographically widespread early to middle Cretaceous secondary mineralization which is inferred to be caused by a major period of groundwater movement associated with the initiation of uplift and denudation of the Transantarctic Mountains. The results suggest that apophyllite can produce geologically meaningful ages by both Rb‐Sr and K‐Ar methods, although further work is required to fully understand the variations in age that may be attributed to Ar loss, alkali mobility, or an extended history of mineral precipitation.
Loading rate impact on fracturing pattern: Lessons from hydrocarbon recovery, Lacq Gas Field, France
Authors: Lahaie Franz , Grasso Jean Robert
Published: December 2003

Using scaling parameters of induced seismicity in size and space domains (seismic b value and spatial correlation dimension D, respectively), we analyze the fracturing pattern of the gas reservoir of Lacq in correlation with the production rate. On the basis of previous observations and a previous model [Segall et al., 1994], we assume that the cause of induced seismicity is a poroelastic stressing, such that the production rate can be viewed as a stress rate for the rocks adjacent to the reservoir. During the 20‐year study period the production rate has decreased dramatically, leading to a change of poroelastic stress rate from 4.5×10−3 to 1.5×10−3 MPa/yr. Accordingly, we observe a significant decrease in the size of activated fractures and a reduction of the spatial clustering of fracturing, both arguing for a stress rate impact on the fracturing pattern in the gas field. We interpret these changes in terms of a change in the deformation mode of stressed rocks from a more brittle to a more ductile behavior as the stress rate was decreased. This hypothesis is supported by independent evidence from a number of experimental, numerical, and field studies. The loading rate thus appears as a possible candidate to drive changes of b and D in time and space. The apparent sensitivity of the Earth's crust fracturing pattern to loading rate variations of much lower frequency than the tidal variations suggests that the Earth's crust deformation mode itself be frequency dependent. Also, our observations may have strong implications in contexts where the loading rate is induced by human intervention (e.g., hydrocarbon recovery, mines), suggesting the possibility of adjusting the loading rate so as to shape the fracturing pattern locally according to human needs.
Heterogeneity of the uppermost mantle beneath Russian Eurasia from the ultra‐long‐range profile quartz
Authors: Morozova Elena A. , Morozov Igor B. , Smithson Scott B. , Solodilov Leonid N.
Published: December 2003

The 3850‐km long Deep Seismic Sounding profile QUARTZ crosses six major geologic provinces in Eurasia and is sourced by 3 nuclear and 48 chemical explosions. We present the first interpretation of the entire data set, using two dimensional (2‐D) ray tracing and inversion, resolution analysis, and 1‐D amplitude modeling. Our interpretation shows a 42‐km‐thick, high‐velocity crust under the Baltic Shield, a 29‐km‐thick crust and high‐velocity upper mantle under the Mezenskaya depression, 52‐km‐thick crust with high‐velocity lower crust and uppermost mantle under the Urals, and 40‐km‐thick crust under the West Siberian basin deepening to 45 km under the Altay‐Sayan fold belt. High‐velocity (8.4 km/s) uppermost mantle is found under the Mezenskaya depression and under the east flank of the Urals. One almost continuous upper mantle boundary occurs at 65‐to 80‐km depth, and another with an approximately 40‐km‐thick LVZ occurs at 120‐to 140‐km depth. The shallow upper mantle blocks and the two extensive interfaces indicate strong upper mantle heterogeneity. Resolution analysis based on direct multivariate model perturbations, artificial neural network and principal component analysis, indicate the depth uncertainty of the 410‐km discontinuity within ±6 km, and also its trade‐off with dip and velocities above and below the discontinuity. Decreased near‐critical amplitudes of reflections from the 410‐km and 660‐km discontinuities indicate that these boundaries are most likely represented by gradient zones about 15–20 km thick. Lithosphere thins, asthenospheric velocity decreases, and the 410‐km discontinuity dips to the SE approaching the Himalayan orogenic belt.
Analytical derivations for the behavior and fabric evolution of a linear orthotropic ice polycrystal
Authors: Gagliardini O. , Meyssonnier J.
Published: December 2003

The fabric of polar ice exhibiting orthotropic symmetry (i.e., three orthogonal planes of reflexional symmetry) is described by a continuous orientation distribution function (ODF) which represents the density of grains whose c axes have a given orientation. For a given ODF, the constitutive law of such anisotropic ice is obtained by a homogenization procedure based on the assumptions that the ice single crystal behaves as a linear transversely isotropic material and that the grains experience a uniform state of stress. Under loading conditions which respect the symmetries of orthotropy, the evolution of the macroscopic behavior of a polycrystal of ice, as well as the evolution of its fabric, are fully determined by analytical expressions. The solution for a transversely isotropic polycrystal is deduced as a reduction from the orthotropic case. In this special configuration, the results from the present model are compared to fabric measurements made by Thorsteinsson et al. [1997] on the GRIP ice core.