bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulations of radiation belt formation during storm sudden commencements
Authors: Hudson M. K. , Elkington S. R. , Lyon J. G. , Marchenko V. A. , Roth I. , Temerin M. , Blake J. B. , Gussenhoven M. S. , Wygant J. R.
Published: February 2004

MHD fields from a global three‐dimensional simulation of the great March 24, 1991, storm sudden commencement (SSC) are used to follow the trajectories of particles in a guiding center test particle simulation of radiation belt formation during this event. Modeling of less intense events during the lifetime of the CRRES satellite, with similar morphology but less radial transport and energization, is also presented. In all cases analyzed, a solar proton event was followed by an SSC, leading to the formation of a new proton belt earthward of solar proton penetration. The effect on particle energization of varying solar wind and model pulse parameters is investigated. Both a seed population of solar protons and the SSC shock‐induced compression of the magnetosphere are necessary conditions for the formation of a new proton belt. The outer boundary of the inner zone protons can be affected by an SSC and a newly formed belt can be affected by the ensuing or a subsequent storm, which may occur in rapid succession, as was the case in June and July 1991. The acceleration process is effective for both northward and southward IMF, with more energization and inward radial transport for the southward case for otherwise comparable solar wind parameters, because of the initially more compressed magnetopause in the southward case. The inner boundary and stability of the newly formed belt depends on the magnitude of radial transport at the time of formation and subsequent ring current perturbation of adiabatic trapping.
Fault propagation fold growth during the 1994 Northridge, California, earthquake?
Authors: Souter B. J. , Hager B. H.
Published: February 2004

Geological models of buried thrust faults indicate that fault propagation folds (FPF) form and grow with a geometry that depends on that of the fault (Suppe, 1985; Suppe and Medwedeff, 1990). The displacement gradient fields for faults and kink folds are very similar, and both can be modeled using dislocations. In this paper we test a geological model of the FPF associated with the January 17, 1994 Northridge, California, earthquake to determine whether folding along axial planes inferred from geologic models accounts in part for the coseismic surface displacements measured with Global Positioning System (GPS). We test for coseismic deformation on both the main rupture plane and active axial planes of related folds by inverting for the displacements on dislocation planes in an elastic half‐space. A model incorporating two axial planes is preferred to a model with a single rupture plane in a normalized root mean square (NRMS) sense; however, the distribution of axial plane displacements does not correlate with the displacements on the main rupture plane in the way expected for a fault propagation fold. Our results indicate that the deformation associated with folding is too distributed to be resolved on a discrete plane, that the deformation occurs interseismically, or that one or both of the kink bands does not exist. A model of a single elevated plane, which is parallel to, but not coplanar with, the aftershocks, is better in a NRMS sense than the FPF model, indicating that anelastic deformation in the hanging wall may be distributed.
Variations of P wave speeds in the mantle transition zone beneath the northern Philippine Sea
Authors: Brudzinski Michael R. , Chen Wang‐Ping , Nowack Robert L. , Huang Bor‐Shouh
Published: February 2004

Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move‐out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern corner of the Philippine Sea, within a region of approximately 500×500 km2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660‐km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in Vp across the 660‐km discontinuity from approximately 6% to 3%. Such a configuration is consistent with the interpretation that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660‐km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel—Mg‐Fe‐perovskite transition.
Shear wave attenuation in the lithosphere beneath Italy and surrounding regions: Tectonic implications
Authors: Mele Giuliana , Rovelli Antonio , Seber Dogan , Barazangi Muawia
Published: February 2004

More than 700 waveforms produced by 51 shallow earthquakes and recorded at regional distances (250–1400 km) by the Italian seismic network have been analyzed to characterize the amplitude and frequency contents of the crustal and uppermost mantle shear waves Lg and Sn, respectively. The crustal phase Lg efficiently propagates through the relatively stable Adriatic continental crust, while it is not observed along propagation paths crossing major physiographic features, like the Apennine chain and the basinal domain of the Tyrrhenian and Ionian Seas. Similar to Lg, efficient Sn propagation is observed in the uppermost mantle beneath the Po plain and the Adriatic Sea. Efficient Sn transmission is also observed across the northern Ionian Sea and Sicily and in the area between Sardinia and the northern coasts of Africa. Sn are efficiently transmitted across the Sicily Channel, and rather efficient Sn propagate beneath the Ligurian Sea. On the contrary, inefficient Sn transmission characterizes the uppermost mantle beneath the Apennines, the western margin of the Italian peninsula, and the southern Tyrrhenian Sea. Shear wave attenuation suggests the presence of asthenospheric material in the uppermost mantle, probably related to the present‐day extension along the Apennine chain and in the Tyrrhenian basin. This interpretation is consistent with the presence of extensive Neogene and Quaternary volcanic activity in these areas and related high heat flow. Proposed lithospheric delaminatioh processes beneath the Apennines and subduction beneath the Tyrrhenian Sea can reasonably explain the observed high‐attenuation zones in the uppermost mantle. In contrast, a high‐strength mantle lid is inferred to underlay the Po plain, the Adriatic Sea, and the northern Ionian Sea. The available waveforms also indicate that a continuous mantle lid is present beneath Sicily and the extensional domain of the Sicily Channel, as well as in the marine area south of Sardinia.
Evolution of a brine‐saturated layer at the base of a ridge‐crest hydrothermal system
Authors: Lowell R. P. , Germanovich L. N.
Published: February 2004

Vent fluids with chlorinities significantly greater than seawater are indicative of mixing between hydrothermal seawater and brines that have formed as a result of supercritical phase separation. We relate the evolution of a brine‐saturated layer to magmatic and tectonic processes at the base of a ridge crest hydrothermal system. A conductive heat balance indicates that under steady state conditions such a brine‐saturated layer is typically ∼1–10 m thick and contains 107 to 108 kg of brine with 6–10 times the chlorinity of seawater. Simple mixing and fractionation models indicate that the depletion and replenishment of brine‐saturated layers may occur on a timescale of years to centuries. A timescale of years is consistent with temporal changes in chlorinity of fluids at vents in areas of recent, known dike injection. In these systems, brines may be undergoing depletion by mixing with liquid‐phase seawater. Vents stably emitting high‐chlorinity fluids on timescales of decades may represent older systems in which brines are being depleted by diffusion into an overlying liquid‐phase hydrothermal system.
Long dormancy, low slip rate, and similar slip‐per‐event for the Emerson fault, eastern California shear zone
Authors: Rubin Charles M. , Sieh Kerry
Published: February 2004

Excavations in a playa along the 1992 rupture of the Emerson fault reveal evidence of two paleoseismic events, with only one large prehistoric rupture in the past 15 millennia. Accelerator mass spectrometer radiocarbon ages of charcoal from playa sediments and from fault‐scarp colluvium directly beneath the playa beds indicate that the last large prehistoric slip event occurred about 9000 ka. Trench‐wall exposures revealed clear evidence of at least one pre‐9 ka rupture at the playa site. The event horizon of this earthquake is between two pedogenic carbonate layers that have radiocarbon ages of 14.8 ka and 24.1 ka, implying that the earthquake occurred about 20,000 years B.P. The actual bracketing ages for this rupture are likely a few thousand years older because of the mean residence time for the pedogenic carbonate and calibration of the 14C ages by 230Th dating. Despite the large uncertainties, a dormant period of at least 6 kyr to as much as 13 kyr separates the older event from the 9 ka event. Because the scarp formed by the penultimate event is similar in height to the scarp formed by the 1992 Mw 7.3 Landers earthquake, the penultimate rupture was, at least locally, similar in size to the most recent rupture. This similarity supports the concept of characteristic slip for the Emerson fault. Preliminary results from paleoseismic studies at other sites on the 1992 rupture suggest that large ruptures occurred on other nearby faults within a few hundred years of this penultimate event on the Emerson fault. The interseismic period that preceded the 1992 earthquake on the Emerson fault was about 40 times longer than the average interval between large events on the nearby San Andreas fault. Therefore, in comparison to events on the San Andreas fault, the 1992 Landers earthquake was an exceedingly rare event.
Petrological systematics of the Mid‐Atlantic Ridge south of Kane: Implications for ocean crust formation
Authors: Reynolds Jennifer R. , Langmuir Charles H.
Published: February 2004

Models of ridge segmentation, mantle flow and melt focusing predict how the chemical compositions of mantle melts should vary along a mid‐ocean ridge axis. The compositions of basaltic lavas can be compared to these predictions to test the models. Such tests have been carried out using basalts from the neovolcanic zone south of the Kane fracture zone (the MARK area), where there are both a large transform and nontransform offsets. Before evaluating mantle models, the effects of differentiation must be accounted for. Fractional crystallization at low pressures (constrained by new melting experiments on these samples) does not account for the data. High pressure or in situ crystallization better account for the differentiation trends; however, these two processes imply different relationships between magmatic differentiation and position within a segment. Irrespective of the differentiation model, significant differences exist among parental magmas. Magmas near the transform have much lower levels of highly incompatible trace elements but higher levels of moderately incompatible trace elements, suggesting both lower extents of melting and a more depleted source. These two characteristics may be natural consequences of the truncation of a melting regime by a large‐offset transform: depleted mantle from across the transform may contribute to the melting regime, while the cooler thermal environment produces less melt. Quantitative modeling of these geochemical characteristics produces thin crust near the transform, consistent with seismic and gravity studies. In contrast, thin crust adjacent to nontransform offsets is associated with no reduction in extent of mantle melting. These results, along with data from other regions, suggest that nontransform offsets overlie a continuous melting regime, and melt focusing creates the variations in crustal thickness. Focused flow may also lead to incompatible element enrichment at segment centers, and relative depletion at segment margins. Only offsets that truncate the melting regime, such as large transforms, are associated with diminished extents of melting within the mantle. Petrological evidence obtained thus far is not consistent with active upwelling to explain crustal thickness variations along nontransform offset bounded segments.
Formation and accumulation of gas hydrate in porous media
Authors: Rempel A. W. , Buffett B. A.
Published: February 2004

Vast quantities of clathrate hydrate are found in the Arctic and in marine sediments along continental margins. The clathrate structure traps enormous volumes of methane gas, which is both a possible source of global climate change and a potential energy resource. The growth rate and spatial distribution of gas hydrate in the shallow sediments are influenced by a variety of interacting physical processes. In order to quantify these processes, we develop mathematical models for hydrate formation in porous media. An analytical model is derived for the idealized problem of hydrate growth in a porous half‐space which is cooled on its boundary. Our calculations predict the growth rate of a hydrate layer for a given rate of cooling and show that the volume of hydrate is strongly dependent on the two‐phase equilibrium between hydrate and seawater. For a representative phase diagram we find that the volume of hydrate in the layer is less than 1% of the pore volume. Larger volumes of hydrate observed in some locations demand a sustained supply of gas and a long accumulation time. Numerical calculations are used to investigate situations that are more representative of conditions in marine sediments. A simple theoretical expression is derived for the rate of hydrate accumulation due to advection of methane gas from depth. Using typical estimates of fluid velocities in accretionary environments, we obtain an accumulation rate of 1% of the pore volume in 105 years. The predicted vertical distribution of hydrate is consistent with geophysical inferences from observed hydrate occurrences along the Cascadia margin. Similar distributions can arise from the combined effects of in situ methane production and warming due to ongoing sedimentation. Predicted differences between these two formation models may be detectable in geophysical and geochemical measurements.
Production of nucleogenic neon in the Earth from natural radioactive decay
Authors: Yatsevich Igor , Honda Masahiko
Published: February 2004

Nucleogenic neon isotope production resulting from α decay of naturally occurring U and Th was calculated for typical compositions of the mantle and crust using a simple non‐Monte Carlo code for neutron transport. The (α,n), (α,p), and (n, α) reactions, leading to the production of neon isotopes in the outgoing reaction channel, are taken into account in the calculations. In the first stage of calculation, neon production from (α,n) and (α,p)reactions on 17O, 18O, and 19F is considered, and the neutron energy spectrum from (α,n) reactions on the major elements is derived. This spectrum is further used for calculation of the neon isotope yield from the reactions 24Mg (n,α)21Ne, 25Mg(n,α)22Ne, and 23Na(n,α)20Ne. The calculated production of nucleogenic neon is dominated by (α,n) reactions with oxygen in the Earth's mantle and crust. The present calculations give 21Ne/4He production ratios that are approximately half those of the previous estimates by Rison [1980] and Kyser and Rison [1982]. The present study also has shown that the nucleogenic 21Ne production from the 24Mg(n,α) reaction is only about one‐fifth the previous estimate. The contribution of nucleogenic 21Ne from the 24Mg(n,α) reaction is insignificant compared with nucleogenic 21Ne production from 18O(α,n). Our estimate of the nucleogenic 21Ne/radiogenic 4He production ratio both in the mantle and in the crust is 4.5×10−8. This production ratio is quite consistent with nucleogenic 21Ne/radiogenic 4He ratios actually observed in crustal fluid samples ((4.6±0.8) ×10−8). The error in calculation of the nucleogenic neon production, resulting from a random statistical variation of the entry neutron yields and cross sections, is estimated to be less than 5%.
High‐pressure behavior of anorthite: Compression and amorphization
Authors: Daniel Isabelle , Gillet Philippe , McMillan Paul F. , Wolf George , Verhelst Mary A.
Published: February 2004

We present the results of a detailed study of the high‐pressure behavior of anorthite (CaAl2Si2O8) between ambient pressure and 30 GPa under static conditions, using in situ Raman spectroscopy and energy‐dispersive X ray diffraction. On increasing pressure under hydrostatic conditions, the
transition occurs first at 2.6 GPa. At 10 GPa, a reversible polymorphic transition is observed which transforms the
polymorph into a phase of higher symmetry. With further pressure increase, large changes occur in the Raman and X ray spectra between 14 and 16 GPa, premonitory to the onset of pressure‐induced amorphization. Above 16 GPa, anorthite is fully amorphous. However, only samples pressurized to above 22 GPa remain amorphous on recovery to ambient conditions. The high‐pressure behavior of anorthite is highly sensitive to deviatoric stresses. Under less hydrostatic conditions, the 10‐GPa transition described above occurs below 9 GPa, amorphization begins below 11.2 GPa, and samples decompressed from peak pressures as low as 17 GPa are fully amorphous. We have also carried out a thermodynamic analysis of the pressure‐induced amorphization of anorthite, using a two state model for the equation of state (EOS) of CaAl2Si2O8 glass in order to account for the increase in Al coordination. With this EOS, we calculate that the free energies of the crystalline and amorphous materials are equal at about 30 GPa, or slightly lower when effects of structural relaxation in the glass are taken into account, well above the observed crystalline‐amorphous transition at 16 GPa.
Global plate velocities from the Global Positioning System
Authors: Larson Kristine M. , Freymueller Jeffrey T. , Philipsen Steven
Published: February 2004

We have analyzed 204 days of Global Positioning System (GPS) data from the global GPS network spanning January 1991 through March 1996. On the basis of these GPS coordinate solutions, we have estimated velocities for 38 sites, mostly located on the interiors of the Africa, Antarctica, Australia, Eurasia, Nazca, North America, Pacific, and South America plates. The uncertainties of the horizontal velocity components range from 1.2 to 5.0 mm/yr. With the exception of sites on the Pacific and Nazca plates, the GPS velocities agree with absolute plate model predictions within 95% confidence. For most of the sites in North America, Antarctica, and Eurasia, the agreement is better than 2 mm/yr. We find no persuasive evidence for significant vertical motions (<3 standard deviations), except at four sites. Three of these four were sites constrained to geodetic reference frame velocities. The GPS velocities were then used to estimate angular velocities for eight tectonic plates. Absolute angular velocities derived from the GPS data agree with the no net rotation (NNR) NUVEL‐1A model within 95% confidence except for the Pacific plate. Our pole of rotation for the Pacific plate lies 11.5° west of the NNR NUVEL‐1A pole, with an angular speed 10% faster. Our relative angular velocities agree with NUVEL‐1A except for some involving the Pacific plate. While our Pacific‐North America angular velocity differs significantly from NUVEL‐1A, our model and NUVEL‐1A predict very small differences in relative motion along the Pacific‐North America plate boundary itself. Our Pacific‐Australia and Pacific‐Eurasia angular velocities are significantly faster than NUVEL‐1A, predicting more rapid convergence at these two plate boundaries. Along the East Pacific Rise, our Pacific‐Nazca angular velocity agrees in both rate and azimuth with NUVEL‐1A.
Quantitative thermal emission spectroscopy of minerals: A laboratory technique for measurement and calibration
Authors: Ruff Steven W. , Christensen Philip R. , Barbera Paul W. , Anderson Donald L.
Published: February 2004

Previous descriptions of thermal emission spectroscopy have presented techniques that vary in accuracy and reproducibility. Contributions of thermal energy from the instrument and environment are major calibration factors that limit accuracy in emissivity determination. Reproducibility is related to the stability of these quantities. Sample temperature determination is also a significant factor in arriving at accurate emissivity. All of the factors which impact the measurement of emissivity using an interferometric spectrometer with an uncooled detector are isolated and examined here. An experimental apparatus is presented along with a description of a simplified measurement and calibration scheme used to arrive at quantitative emissivity of minerals. A detailed error analysis examines the effect of errors in each of the calibration parameters in isolation and as part of multiple error propagation. Sample temperature determination from radiance can create emissivity error, but 95% of published minerals have an emissivity maximum of 0.98 or higher, resulting in emissivity error of 2% or less. With worst‐case systematic and random errors included, emissivity can be determined with an uncertainty of ∼4%. In most cases it is less than 2%. Reproducibility with this technique is better than 1%.
Comparative analysis of direct (core) and indirect (borehole imaging tools) collection of fracture data in the Hot Dry Rock Soultz reservoir (France)
Authors: Genter A. , Castaing C. , Dezayes C. , Tenzer H. , Traineau H. , Villemin T.
Published: February 2004

Attributes of several thousand fractures were collected in three boreholes of 2.2, 3.6, and 3.8 km depth, penetrating the Soultz Hot Dry Rock reservoir (France). The fractures were sampled from cores and from several high‐resolution imaging techniques such as borehole televiewer (BHTV), ultrasonic borehole imager (UBI), formation microscanner (FMS), formation microimager (FMI), and azimuthal resistivity imaging (ARI). A comparison was made between the data collected on cores and those provided by different imaging techniques. The comparison clearly establishes that the different wall‐images are not as exhaustive as the core data and cannot provide a complete characterization of the fracture network. Discrete fractures thinner than 1 mm are not properly detected. This is also the case for discrete fractures closer than 5 mm, which appear only as single traces. The imaging techniques are, nevertheless, very powerful for characterizing altered fracture clusters. Whatever the technique used, the fracture strikes were correctly sampled with the different systems. This comparison allowed us to calibrate the fracture population data obtained from the imaging system in order to correct for the filtering effect introduced by the technique itself and by the alteration of the rock mass.
Genesis of flood basalts from eclogite‐bearing mantle plumes
Authors: Cordery Matthew J. , Davies Geoffrey F. , Campbell Ian H.
Published: February 2004

On the basis of the geochemistry of flood basalts, we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core‐mantle boundary. We present finite element models of the melting in starting plumes (plume heads) in both continental and oceanic regions. Starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt, or any melt at all, for reasonable plume head temperatures (1400–1600°C) or lithosphere ages (6.25–100 m.y.). This result is in contrast to the results of Farnetani and Richards [1994] whose models require higher plume head temperatures (1600–1700°C) and a weak, mobile lithosphere in order to generate volumes of melt similar to flood basalts. Plumes composed of 15 wt % eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings. The maximum extent of melting in these models is about 10–30%. This agrees with the model of Hofmann and White [1982], who argued that ocean island basalts are derived from subducted oceanic crust. If the embedded eclogite thermally reequilibrates with the surrounding plume material, then melt volumes and total extents of melting increase by a additional factor of 2–3. A simple two‐dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume‐related melting occurs over 50–60 m.y., a much longer period than for flood basalts.
Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas
Authors: Sinton Christopher W. , Duncan Robert A. , Denyer Percy
Published: February 2004

The pre‐Tertiary oceanic crust exposed on the west coast of Costa Rica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya Peninsula, Playa Jacó, and the Quepos Peninsula using 40Ar‐39Ar radiometric dating and to assess the petrologic relationships between the different localities using major element, trace element, and Sr, Nd, Pb isotopic data. Radiometric ages of basalts and diabases from the Nicoya Peninsula are 88–90 Ma (with a weighted mean of 88.5 Ma), and those of two intrusive rocks (a gabbro and plagiogranite) are both 83–84 Ma. The combined geochemical data indicate that the sampled Nicoya Peninsula rocks belong to a single suite related by fractional crystallization of similar parental magmas. Nd and Pb isotopic ratios indicate a common mantle source distinct from that of mid‐ocean ridge basalts. Both the age and composition of the Nicoya rocks are consistent with the idea that they are a part of the Caribbean Cretaceous oceanic plateau [Donnelly, 1994]. The Jacó lavas are geochemically similar to the Nicoya Peninsula suite, and a single age of 84 Ma is identical to the age of the Nicoya Peninsula intrusives. The one analyzed Quepos basalt has a radiometric age of ∼64 Ma, and it is enriched in incompatible elements relative to the Nicoya rocks. Similarities in Nd and Pb isotopic ratios indicate that the Quepos and Nicoya/Jacó lavas were derived from a similar mantle source to that which produced the Nicoya rocks, possibly the Galapagos plume.
Mechanical effect of fluid migration on the complexity of seismicity
Authors: Yamashita Teruo
Published: February 2004

Spatio‐temporal variation of earthquake activity is modeled assuming fluid migration in a narrow porous fault zone whose boundaries are impermeable. The duration of earthquake sequence is assumed to be much shorter than the recurrence period of characteristic events on the fault. Principle of the effective stress coupled to the Coulomb failure criterion introduces mechanical coupling between fault slip and pore fluid pressure. A linear relation is assumed in our simulations between the accumulated slip and fault zone width on the basis of laboratory and field observations. High complexity is observed in the rupture activity so long as an inhomogeneity is introduced in the spatial distribution of initial strength, which is defined as the fracture threshold stress before the intrusion of the fluid. Frequency‐magnitude statistics of intermediate‐size events obeys the Gutenberg‐Richter relation for all the models in which spatial heterogeneity is introduced for the initial strength. The behavior of larger‐size events seems to be rather model dependent. It is also observed that the rupture occurrence tends to be inactivated immediately before the occurrence of the largest event in a sequence. This never happens if a brittle rupture is assumed in an elastic medium with no mechanical effect of fluid. This inactivation will occur because it takes much time to build up fluid pressure to break a fault segment having high initial strength, whose rupture triggers the largest event in a sequence. Our calculations also show that a single predominant principal event cannot be observed in a sequence when both the variance and average value of the distributed initial strengths are large. This may explain a feature observed for earthquake swarm.
The 1994–1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in a zone of horizontal shear stress
Authors: Sigmundsson Freysteinn , Einarsson Páll , Rögnvaldsson Sigurdur Th. , Foulger G. R. , Hodgkinson K. M. , Thorbergsson Gunnar
Published: February 2004

Since July 1994 an unusually persistent swarm of earthquakes (M<4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hrómundartindur volcanic system. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at 6.5±3 km depth beneath the center of the volcanic system. The system is within the stress field of the south Iceland transform zone, and the majority of the recorded earthquakes represent strike‐slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modeled as a point source in an elastic half‐space, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 μstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.
Source and path effects in the wave fields of tremor and explosions at Stromboli Volcano, Italy
Authors: Chouet Bernard , Saccorotti Gilberto , Martini Marcello , Dawson Phillip , De Luca Gaetano , Milana Giuliano , Scarpa Roberto
Published: February 2004

The wave fields generated by Strombolian activity are investigated using data from small‐aperture seismic arrays deployed on the north flank of Stromboli and data from seismic and pressure transducers set up near the summit crater. Measurements of slowness and azimuth as a function of time clearly indicate that the sources of tremor and explosions are located beneath the summit crater at depths shallower than 200 m with occasional bursts of energy originating from sources extending to a depth of 3 km. Slowness, azimuth, and particle motion measurements reveal a complex composition of body and surface waves associated with topography, structure, and source properties. Body waves originating at depths shallower than 200 m dominate the wave field at frequencies of 0.5–2.5 Hz, and surface waves generated by the surficial part of the source and by scattering sources distributed around the island dominate at frequencies above 2.5 Hz. The records of tremor and explosions are both dominated by SH motion. Far‐field records from explosions start with radial motion, and near‐field records from those events show dominantly horizontal motion and often start with a low‐frequency (1–2 Hz) precursor characterized by elliptical particle motion, followed within a few seconds by a high‐frequency radial phase (1–10 Hz) accompanying the eruption of pyroclastics. The dominant component of the near‐ and far‐field particle motions from explosions, and the timing of air and body wave phases observed in the near field, are consistent with a gas‐piston mechanism operating on a shallow (<200 m deep), vertical crack‐like conduit. Models of a degassing fluid column suggest that noise emissions originating in the collective oscillations of bubbles ascending in the magma conduit may provide an adequate self‐excitation mechanism for sustained tremor generation at Stromboli.
Laboratory measurements anomalous 0.1–0.5 Hz streaming potential under geochemical changes: Implications for electrotelluric precursors to earthquakes
Authors: Jouniaux Laurence , Pozzi Jean‐Pierre
Published: February 2004

Streaming potentials resulting from flow of various salt solutions in rock were measured on saturated sediments (Fontainebleau sandstones). The streaming potential ΔV was found to be proportional to the driving pore pressure ΔP. Pulses of amplitude 15–40 mV in the frequency range of 0.1 to 0.5 Hz were observed when the conductivity of the injected water was decreased and the fluid flow rate was relatively low, corresponding to a Darcian velocity of 17 to 30 cm/h. The amplitudes of these pulses are 47% to 133% of the corresponding steady components of the ΔV values. Such geochemically induced effects may possibly be responsible for the frequency signals from 0.1 to 0.5 Hz that were sometimes observed before an earthquake.
Dynamics of crustal compensation and its influences on crustal isostasy
Authors: Zhong Shijie
Published: February 2004

Deviation from isostasy is commonly believed to be caused by the strength of the Earth's lithosphere. An analysis of crustal compensation dynamics suggests that the deviation may have a dynamic origin. The analysis is based on analytic models that assume that (1) the medium is incompressible and has a layered and linear viscoelastic rheology and (2) the amplitude of topography is small compared with its wavelength. The models can describe topographic relaxation of different density interfaces at both small (e.g., postglacial rebound) and large time‐scales. The models show that for a simple crust‐mantle system with topography at the Earth's surface and Moho representing the only mass anomalies, while the crust always approaches the isostatic state at long wavelengths (>800 km), crustal isostasy may not be an asymptotic limit at short wavelengths, depending on crustal and lithospheric rheology. For a crust with viscosity smaller than lithospheric viscosity, at wavelengths comparable with widths of orogenic belts (i.e., <300 km), the crust tends to approach a state with significant overcompensation (i.e., excess topography at the Moho) within a timescale of about 107 years, and this characteristic time depends on wavelengths and crustal viscosity. This overcompensation is greater for weaker crust and stronger lithosphere. A thicker crust or lithosphere also enhances this overcompensation. If crustal and lithospheric viscosities are both large and comparable, the asymptotic state for the crust displays a slight undercompensation. For an elastic and rigid upper crust, the crust eventually becomes undercompensated after a characteristic decay time of topography at the Moho. The characteristic time is dependent on viscosity and thickness of the lower crust. The deviation from isostasy arises because these viscosity structures result in a ratio of vertical velocity at the surface to vertical velocity at the Moho which in the asymptotic state for short wavelengths differs from the ratio of density contrast at the Moho to that at the surface.
Thermoremanence, anhysteretic remanence and susceptibility of submicron magnetites: Nonlinear field dependence and variation with grain size
Authors: Dunlop David J. , Argyle Kenneth S.
Published: February 2004

We have measured initial susceptibility χ0 and the dependence of anhysteretic rémanent magnetization (ARM) and thermoremanent magnetization (TRM) on applied field for seven samples of magnetite, with mean grain sizes from 40 to 540 nm. TRM acquisition is nonlinear in geomagnetically relevant weak fields, contrary to the usual assumption made in paleointensity determination. Over the 40–540 nm range, χ0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores. ARM is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples. ARM and TRM have similar variations, as d−1 for d≤1 μm, suggesting a common source for this pseudo‐single‐domain (PSD) dependence on grain size. However, TRM is 10–20 times more intense than ARM in grains around 0.2 μm in size. The TRM microstate seems to be a two‐domain structure, whereas the ARM microstate may be a vortex structure, which has never before been convincingly demonstrated by magnetic measurements. Independent evidence comes from theoretical fits to TRM and ARM field dependence data. In moderate and strong fields, TRM in 215–540 nm magnetites is explained by two‐domain theory, but ARM is not. In smaller grains (d≤0.1 μm), a PSD theory predicts that TRM is carried by single‐domain (SD) moments of entire grains but ARM resides in moments (≈1 per grain) 20–40 times weaker. These remanence levels match those of (metastable) SD and vortex states, respectively. We therefore propose that magnetite grains in the 0.1–0.5 μm size range can remain in metastable SD or two‐domain states following acquisition of TRM but revert to a vortex ground state when field‐cycled, for example, in ARM acquisition or alternating field demagnetization.
Signal processing of diurnal and semidiurnal variations in radon and atmospheric pressure: A new tool for accurate in situ measurement of soil gas velocity, pressure gradient, and tortuosity
Authors: Pinault Jean‐Louis , Baubron Jean‐Claude
Published: February 2004

Signal processing of diurnal and semidiurnal variations of both atmospheric pressure and radon concentration in soil gases is shown to be useful for estimating soil gas transport parameters. The two daily‐cycle peaks at 12‐ and 24‐hour periods in the Power Spectral Density (PSD) of atmospheric pressure seem to be present everywhere on Earth's surface, and it is the effect of these regular pressure variations on the radon concentration in soil gases that makes it possible to determine three soil gas transport parameters which can be used to estimate real gas velocity; i.e. tortuosity τ, the ratiok/nbetween intrinsic permeability and effective porosity (that part of porosity involved in gas transport), and the pressure gradient α. The parameterskandncan be determined independently if the gas flux at the surface is measured at the same time. The method is robust, representative, and accurate: since it allows reliable estimation of transport parameters, it can provide relevant information about the depth of the radon source and the time it takes for information to reach the surface when radon bursts occur at depth. Radon is an appropriate soil gas tracer because it exists in all soils. Moreover, the measurement of radon concentration requires only passive sensors that do not hamper the rising gas column. Gas flux data obtained in Andalusia, Spain, in connection with mineral exploration are processed as examples. Determining the complete set of transport parameters helps in the interpretation of recorded radon outbursts, which are found to be correlated with regional seismic activity.
The timing and duration of the Karoo igneous event, southern Gondwana
Authors: Duncan R. A. , Hooper P. R. , Rehacek J. , Marsh J. S. , Duncan A. R.
Published: February 2004

A volcanic event of immense scale occurred within a relatively short period in early Jurassic time over large regions of the contiguous Gondwana supercontinent. In southern Africa, associated remnants of thick volcanic successions of lava flows and extensive dike and sill complexes of similar composition have been grouped together as the Karoo Igneous Province. Correlative volcanic and plutonic rocks occur in Antarctica and Australia as the Ferrar Province. Thirty‐two new 40Ar‐39Ar incremental heating experiments on feldspars and whole rocks from Namibia, South Africa and East Antarctica produce highly resolved ages with a vast majority at 183±1 Ma and a total range of 184 to 179 Ma. These are indistinguishable from recent, high‐resolution 40Ar‐39Ar and U‐Pb age determinations reported from the Antarctic portion of the province. Initial Karoo volcanism (Lesotho‐type compositions) occurred across the entire South African craton. The ubiquitous distribution of a plexus of generally nonoriented feeder dikes and sills intruding Precambrian crystalline rocks and Phanerozoic sediments indicates that these magmas penetrated the craton over a broad region. Lithosphere thinning of the continent followed the main pulse of igneous activity, with volcanism focused in the Lebombo‐Nuanetsi region, near the eventual split between Africa and Antarctica. Seafloor spreading and dispersion of east and west Gondwana followed some 10–20 m.y. afterward. The volume of the combined Karoo‐Ferrar province (∼2.5×106 km3) makes it one of the largest continental flood basalt events. The timing of this event correlates with a moderate mass extinction (Toarcian‐Aalenian), affecting largely marine invertebrates. This extinction event was not as severe as those recorded at the Permian‐Triassic or Cretaceous‐Tertiary boundaries associated with the Siberian and Deccan flood basalts events, respectively. The difference may be due to the high southerly latitude and somewhat lower eruption rates of the Karoo event.
Mapping of a magma reservoir beneath Nikko‐Shirane volcano in northern Kanto, Japan, from travel time and seismogram shape anomalies
Authors: Horiuchi Shigeki , Tsumura Noriko , Hasegawa Akira
Published: February 2004

Since ray paths of direct P and S waves are largely distorted in a magma body, a seismic shadow zone is created. However, scattered or diffracted waves with very small amplitudes can arrive in the shadow zone because their ray paths are different from those of the direct waves. They become the first arrival in the shadow zone. More than 100 seismologists from many institutions in Japan joined together to perform an intensified seismic study in the Nikko‐Ashio area, northern Kanto, Japan. It was suggested that there is a magma body beneath Mount Nikko‐Shirane, an active volcano, that causes a seismic shadow zone, because P waves recorded by the joint observation are anomalously attenuated in cases when they propagate through a zone beneath the eastern part of the volcano. We developed a new method to estimate the shape of the anomalous attenuation zone by proposing the use of a new parameter, the energy ratio between directP or S waves and their coda waves, to discriminate whether or not each observation station is located inside or outside the shadow zone. We estimated the shape of the anomalous attenuation zone by dividing the study area into blocks with dimensions of 2 km × 2 km × 1 km and using the energy ratio data of P waves for local events. The obtained result shows the existence of an anomalous attenuation zone in the area east of Nikko‐Shirane volcano with a diameter of about 10 km at depths greater than 3 km. It was also found from plots of deep event seismograms recorded by the same observation that travel times are delayed by about 0.7 s for ray paths crossing the anomalous attenuation zone obtained by the energy ratio data. This value requires the P wave velocity in the anomalous zone to be less than that of the surrounding crustal material by at least about 30%, suggesting the existence of a huge magma reservoir in this area.
Strengths of serpentinite gouges at elevated temperatures
Authors: Moore D. E. , Lockner D. A. , Ma Shengli , Summers R. , Byerlee J. D.
Published: February 2004

Serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones. To test these hypotheses, we have measured the strength of chrysotile‐, lizardite‐, and antigorite‐rich serpentinite gouges under hydrothermal conditions, with emphasis on chrysotile, which has thus far received little attention. At 25°C, the coefficient of friction, μ, of chrysotile gouge is roughly 0.2, whereas the lizardite‐ and antigorite‐rich gouges are at least twice as strong. The very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content. When the adsorbed water is removed, chrysotile is as strong as pure antigorite gouge at room temperature. Heating to ∼200°C causes the frictional strengths of all three gouges to increase. Limited data suggest that different polytypes of a given serpentine mineral have similar strengths; thus deformation‐induced changes in polytype should not affect fault strength. At 25°C, the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities, consistent with previous studies. At temperatures up to ∼200°C, however, chrysotile strength is essentially independent of velocity at low velocities. Overall, chrysotile has a restricted range of velocity‐strengthening behavior that migrates to higher velocities with increasing temperature. Less information on velocity dependence is available for the lizardite and antigorite gouges, but their behavior is consistent with that outlined for chrysotile. The marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth. The velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip, but in the presence of a hydrostatic fluid pressure gradient, all varieties of serpentine are too strong to explain the apparent weakness of faults such as the San Andreas.
Multiple enrichment of the Carpathian‐Pannonian mantle: Pb‐Sr‐Nd isotope and trace element constraints
Authors: Rosenbaum Jeffrey M. , Wilson Marjorie , Downes Hilary
Published: February 2004

Pb isotope compositions of acid‐leached clinopyroxene and amphibole mineral separates from spinel peridotite mantle xenoliths entrained in Tertiary‐Quaternary alkali basalts from the Carpathian‐Pannonian Region of eastern Europe provide important constraints on the processes of metasomatic enrichment of the mantle lithosphere in an extensional tectonic setting associated with recent subduction. Principal component analysis of Pb‐Sr‐Nd isotope and rare earth element compositions of the pyroxenes is used to identify the geochemical characteristics of the original lithospheric mantle protolith and a spectrum of infiltrating metasomatic agents including subduction‐related aqueous fluids and silicate melts derived from a subduction‐modified mantle wedge which contains a St. Helena‐type (HIMU) plume component. The mantle protolith is highly depleted relative to mid‐ocean ridge basalt‐source mantle with Pb‐Nd‐Sr isotope compositions consistent with an ancient depletion event. Silicate melt infiltration into the protolith accounts for the primary variance in the Pb‐Sr‐Nd isotope compositions of the xenoliths and has locally generated metasomatic amphibole. Infiltration of aqueous fluids has introduced radiogenic Pb and Sr without significantly perturbing the rare earth element signature of the protolith. The Pb isotope compositions of the fluid‐modified xenoliths suggest that they reacted with aqueous fluids released from a subduction zone which had equilibrated with sediment derived from an ancient basement terrain. We propose a model for mantle lithosphere evolution consistent with available textural and geochemical data for the xenolith population. The Pb‐Sr‐Nd isotope compositions of both alkaline mafic magmas and rare, subduction‐related, calc‐alkaline basaltic andesites from the region provide important constraints for the nature of the asthenospheric mantle wedge and confirm the presence of a HIMU plume component. These silicate melts contribute to the metasomatism of the mantle lithosphere rather than being derived therefrom.