bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear wave splitting and upper mantle deformation in French Polynesia: Evidence for small‐scale heterogeneity related to the Society hotspot
Authors: Russo R. M. , Okal E. A.
Published: February 2004

We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (TPT) on Rangiroa in the Tuamotu archipelago; Papeete (PPT) on Tahiti in the Society Islands; Tubuai (TBI) in the Cook‐Austral island chain; and Rikitea (RKT) on Mangareva in the Gambier Islands. We also examined splitting at Pitcairn (PTCN) on Pitcairn Island; because of the short time of operation of PTCN, our results there are preliminary. We find substantial differences in splitting, most likely caused by variable upper mantle deformation beneath the five stations. At TPT the fast split shear wave (ϕ) direction is N66°W±4°, parallel to the current Pacific‐hotspots relative motion (APM) vector; the delay time between fast and slow waves is 1.3±0.2 s. At PPT, on Tahiti, we could detect no splitting despite many clear SKS observations. At TBI, on Tubuai we detected splitting with a delay time of 1.1±0.1 s and a ϕ direction midway between the local APM direction and the fossil spreading direction (N86°W±2°), as locally indicated by the nearby Austral Fracture Zone. At RKT in the Gambier Islands, ϕ trends N53°W±6°, 16° clockwise of the local APM azimuth, and delay time at RKT is 1.1±0.1 s. Results at PTCN include ϕ near N38°W±9° and a delay time of 1.1±0.3 s. These different results imply variable upper mantle deformation beneath the five sites. We interpret splitting at TPT and, possibly, RKT as indicative of asthenospheric flow or shear in the APM direction beneath the stations. At PPT, azimuthal isotropy indicates deformed upper mantle with a vertical symmetry axis, or absence of strong or consistently oriented mantle deformation fabric beneath Tahiti. Either effect could be related to recent hotspot magmatism on Tahiti. At TBI, splitting may be complicated by juxtaposition of different lithospheric thicknesses along the nearby Austral Fracture Zone, resulting in perturbation of asthenospheric flow. The absence of splitting related to fossil spreading in French Polynesia indicates that upper mantle deformation processes postdating lithosphere formation are important at all four sites within that region. The ϕ azimuth at PTCN does not align with either the fossil direction or the APM direction, but our best individual determination of splitting parameters at this station lies within 10° of the local APM at Pitcairn Island.
Gravity anomalies over the Ferghana Valley (central Asia) and intracontinental deformation
Authors: Burov Evgene B. , Molnar Peter
Published: February 2004

Gravity anomalies over the Ferghana Valley, a deep sedimentary basin in the western Tien Shan, differ from those expected for local Airy isostatic equilibrium by 120 mGal and imply a deficit of mass beneath the basin equivalent to a Moho deeper by 10 km than prevails where isostasy holds. Although the Ferghana Valley is underlain by thick Mesozoic and Cenozoic sediment, measurements of density show that this material cannot account for the deficit of mass. Gravity anomalies over both the surrounding mountain belts and the Kazakh Platform farther north imply that local Airy isostatic equilibrium is approached; flexure of a relatively thin, effectively elastic, plate (thickness ∼10–15 km) loaded by the present topography can account for the range of Bouguer anomalies of −50 to −450 mGal over these regions. We infer that approximately north‐south shortening of the relatively thin lithosphere has created mountains north and south of the basin, has warped the basement of the immediate surroundings of the basin up by folding the mantle lithosphere, and has forced the basin floor down beneath the Ferghana Valley. It appears that the preexisting thermal structure and variations in crustal thickness have dictated the styles and distribution of deformation in this region.
Facies architecture and origin of a submarine rhyolitic lava flow‐dome complex, Ponza, Italy
Authors: Scutter C. R. , Cas R. A. F. , Moore C. L. , de Rita D.
Published: February 2004

The Islands of Ponza and Gavi (western Pontine Archipelago, Italy) preserve parts of a subaqueously emplaced, high‐silica (75–77% SiO2) rhyolitic lava flow that overlies the margins of three older domes. Exposures of the lava alone cover 24 km2, and with a thickness of 150 m exposed in the cliffs, the preserved volume of the lava flow is 3.6 km3, and the original volume is likely to have exceeded 7 km3. The lava flow consists of coherent obsidian and several hyaloclastite facies, including in situ and clast‐rotated breccias and sandstones, which all exhibit gradational relationships. The coherent obsidian occurs as separate domains and is often flow banded. Although flow folds occur, the banding is generally subhorizontal, dipping gently to the NNE. The hyaloclastite, representing 90% of the lava by volume, has a diffuse layering varying from 0.5 to 2 m in thickness. This layering also dips gently to the NNE and is marked in places by alternation of some coherent horizons, coarse and finely fragmented in situ hyaloclastite, pumiceous hyaloclastite, and clast‐rotated hyaloclastite. The layering is laterally continuous and consistent in orientation over most of the 13 km length of Ponza: the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow. The resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones. The lava flow has been intruded and crosscut by rhyolitic bodies of various ages, including discordant, coherent to internally fractured contemporaneous rhyolite bodies, and late, penecontemporaneous dikes with glassy margins and alteration haloes.
Waveform inversion of very long period impulsive signals associated with magmatic injection beneath Kilauea volcano, Hawaii
Authors: Ohminato Takao , Chouet Bernard A. , Dawson Phillip , Kedar Sharon
Published: February 2004

We use data from broadband seismometers deployed around the summit of Kilauea Volcano to quantify the mechanism associated with a transient in the flow of magma feeding the east rift eruption of the volcano. The transient is marked by rapid inflation of the Kilauea summit peaking at 22 μrad 4.5 hours after the event onset, followed by slow deflation over a period of 3 days. Superimposed on the summit inflation is a series of sawtooth displacement pulses, each characterized by a sudden drop in amplitude lasting 5–10 s followed by an exponential recovery lasting 1–3 min. The sawtooth waveforms display almost identical shapes, suggesting a process involving the repeated activation of a fixed source. The particle motion associated with each sawtooth is almost linear, and its major swing shows compressional motion at all stations. Analyses of semblance and particle motion are consistent with a point source located 1 km beneath the northeast edge of the Halemaumau pit crater. To estimate the source mechanism, we apply a moment tensor inversion to the waveform data, assuming a point source embedded in a homogeneous half‐space with compressional and shear wave velocities representative of the average medium properties at shallow depth under Kilauea. Synthetic waveforms are constructed by a superposition of impulse responses for six moment tensor components and three single force components. The origin times of individual impulses are distributed along the time axis at appropriately small, equal intervals, and their amplitudes are determined by least squares. In this inversion, the source time functions of the six tensor and three force components are determined simultaneously. We confirm the accuracy of the inversion method through a series of numerical tests. The results from the inversion show that the waveform data are well explained by a pulsating transport mechanism operating on a subhorizontal crack linking the summit reservoir to the east rift of Kilauea. The crack acts like a buffer in which a batch of fluid (magma and/or gas) accumulates over a period of 1–3 min before being rapidly injected into a larger reservoir (possibly the east rift) over a timescale of 5–10 s. The seismic moment and volume change associated with a typical batch of fluid are approximately 1014 N m and 3000 m3, respectively. Our results also point to the existence of a single force component with amplitude of 109 N, which may be explained as the drag force generated by the flow of viscous magma through a narrow constriction in the flow path. The total volume of magma associated with the 4.5‐hour‐long activation of the pulsating source is roughly 500,000 m3 in good agreement with the integrated volume flow rate of magma estimated near the eruptive site.
Fault linkage: Three‐dimensional mechanical interaction between echelon normal faults
Authors: Crider Juliet G. , Pollard David D.
Published: February 2004

Field observations of two overlapping normal faults and associated deformation document features common to many normal‐fault relay zones: a topographic ramp between the fault segments, tapering slip on the faults as they enter the overlap zone, and associated fracturing, especially at the top of the ramp. These observations motivate numerical modeling of the development of a relay zone. A three‐dimensional boundary element method numerical model, using simple fault‐plane geometries, material properties, and boundary conditions, reproduces the principal characteristics of the observed fault scarps. The model, with overlapping, semicircular fault segments under orthogonal extension, produces a region of high Coulomb shear stress in the relay zone that would favor fault linkage at the center to upper relay ramp. If the fault height is increased, the magnitude of the stresses in the relay zone increases, but the position of the anticipated linkage does not change. The amount of fault overlap changes the magnitude of the Coulomb stress in the relay zone: the greatest potential for fault linkage occurs with the closest underlapping fault tips. Ultimately, the mechanical interaction between segments of a developing normal‐fault system promote the development of connected, zigzagging fault scarps.
Faulting mechanism of anomalous earthquakes near Bárdarbunga Volcano, Iceland
Authors: Nettles Meredith , Ekström Göran
Published: February 2004

The source characteristics of 10 shallow earthquakes of moderate size (M>5) beneath Bárdarbunga Volcano in Iceland are investigated using long‐period and broadband teleseismic data. The highly non‐double‐couple nature of the focal mechanisms for several of the events previously reported in the Harvard centroid moment tensor (CMT) catalog is confirmed by analysis of an expanded long‐period body wave data set, as well as by an independent inversion of intermediate period surface wave data. Modeling of broadband P waveforms constrains the depths of the events to lie between 3 km and 8 km beneath the volcano. Significant complexity in the P waves observed for some events is best explained by a sequence of subevents of varying geometry, rather than by wave propagation effects caused by shallow structure near the earthquake. Inversion of P wave data for two of the earthquakes using a parameterization with subsources results in sequences of events with primarily thrust motion on planes of varying strike. The seismological results are interpreted as faulting on an outward dipping cone‐shaped ring fault beneath the Bárdarbunga caldera. The association in time and space between the September 29, 1996, earthquake and the September 30 eruption near Bárdarbunga and Grimsvötn Volcanos suggests that inflation of a shallow magma chamber, and the associated stress loading of the deeper ring fault, may be the tectonic process which generates the earthquakes.
Fifty years of ferromagnetic single‐domain theory
Authors: Kittel Charles
Published: February 2004

Kittel [1946] gave the original presentation of the theory of the single‐domain (SD) structure of fine ferromagnetic particles and their response to an applied magnetic field. The theory predicted that the SD coercive fields would be determined by the crystalline and shape anisotropies of the particle and could be much higher than those for the multidomain specimens used in magnetic recording at that time. The current magnetic recording industry has developed the use of magnetic fine particles. In geophysics, the observational evidence for plate tectonics, continental drift, and reversals rely, in lava flows, on the thermoremanent magnetization acquired on cooling through the Curie temperature. In sediments where physical deposition of magnetic particles occur, the measurements are of the magnetic orientations at times of deposition or soon thereafter of single‐domain grains of inorganic or biogenic origin. To obtain the maximum total magnetic moment in a magnetobacterium, it is counterproductive for magnetosome growth to exceed the single‐domain size limit. If in future extraterrestrial single‐domain magnetic particles are found that are characteristic of magnetotactic bacteria, as in meteorite arrivals from Mars or other planets, they could be cited as evidence for extraterrestrial life.
Origin of the absence of magnetic lineations in the Yamato Basin of the Japan Sea: Magnetic properties of mafic rocks from Ocean Drilling Program Hole 794 D
Authors: Fukuma Koji , Shinjoe Hironao , Hamano Yozo
Published: February 2004

We investigated magnetic properties of mafic rocks recovered from Ocean Drilling Program Hole 794 D in the Yamato Basin of the Japan Sea. In contrast to other back‐arc basins in the western Pacific, lineated magnetic anomaly patterns have not been recognized in the Yamato Basin. Our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks. The dominant magnetic mineral of Hole 794 D is titanomaghemite as commonly observed for typical oceanic basalts, but the grain size is much larger (greater than several tens of microns). Remanence properties, i.e., low natural remanent magnetization intensities (≈1.5 A/m), low Koenigsberger ratios (≈1.0), high viscous remanent magnetization, and mixed magnetic polarities, indicate that the mafic rocks cannot be a source of lineated magnetic anomalies. While Curie temperature, saturation magnetization, and susceptibility values are not significantly different from those of typical oceanic basalts, magnetic properties related to stability are greatly reduced. The large grain size of the mafic rocks from Hole 794 D plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts, and is caused by the formation of sill‐sediment complexes beneath the Yamato Basin. Sill‐sediment complexes, rather than extrusive basalts in typical oceanic crust, are responsible for the absence of lineated magnetic anomalies in the Yamato Basin.
Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy
Authors: Peng Z. X. , Mahoney J. J. , Hooper P. R. , Macdougall J. D. , Krishnamurthy P.
Published: February 2004

The northeastern Deccan Traps include flows with strong isotopic and elemental affinities to lavas in the southwest as far as 900 km away. However, many of the northeastern flows show a negative correlation of206Pb/204Pb with εNd(t) and positive correlation of206Pb/204Pb with (87Sr/86Sr)t, distinct from any of the isotopic fields for the southwestern formations; this array (from εNd(t)=+5, (87Sr/86Sr)t=0.7041,206Pb/204Pb=17.9 to εNd(t)=0, (87Sr/86Sr),=0.7068,206Pb/204Pb=20.5) overlaps the Ambenali Formation (Fm.) field and runs toward that of the “common signature” [Peng et al., 1994] that appears to have been an important magma type in several of the lower formations of the southwest. This array appears to represent mixing between an Ambenali‐type and (1) a common‐signature end‐member or (2) an end‐member with even higher206Pb/204Pb and87Sr/86Sr and lower εNdthan the common signature; such mixing has been postulated, but not documented previously, for the Deccan. Lavas isotopically and chemically indistinguishable from the Ambenali Fm. form the tops of sections near Jabalpur and Chikaldara. Beneath them are flows that chemically resemble the Poladpur Fm. (which sits immediately below the Ambenali in the southwest) and several lavas similar to the Khandala Fm. (beneath the Poladpur and intervening Bushe Fm. in the southwest). Given the general stratigraphic correspondence with the southwestern sequence, many of these northeastern flows may be petrogenetically related to the respective southwestern formations. Lavas with broadly Poladpurand Khandala‐like elemental compositions are also abundant in a thick section south of Mhow; several other Mhow‐area flows resemble the distinctive Bushe Fm. However, nearly all of the Poladpur‐ and Khandala‐like northeastern Deccan lavas analyzed have higher206Pb/204Pb man any southwestern Poladpur or Khandala Fm. basalts. If they indeed are related petrogenetically to these formations, many of the northeastern magmas interacted less with lOw‐206Pb/204Pb continental material and thus must have erupted from different feeder systems.
Introduction to Special Section: Stress Triggers, Stress Shadows, and Implications for Seismic Hazard
Authors: Harris Ruth A.
Published: February 2004

Many aspects of earthquake mechanics remain an enigma as we enter the closing years of the twentieth century. One potential bright spot is the realization that simple calculations of stress changes may explain some earthquake interactions, just as previous and on going studies of stress changes have begun to explain human‐induced seismicity. This paper, which introduces the special section “Stress Triggers, Stress Shadows, and Implications for Seismic Hazard,” reviews many published works and presents a compilation of quantitative earthquake interaction studies from a stress change perspective. This synthesis supplies some clues about certain aspects of earthquake mechanics. It also demonstrates that much work remains before we can understand the complete story of how earthquakes work.
Volcanic spreading at Kilauea, 1976–1996
Authors: Delaney Paul T. , Denlinger Roger P. , Lisowski Michael , Miklius Asta , Okubo Paul G. , Okamura Arnold T. , Sako Maurice K.
Published: February 2004

The rift system traversing about 80 km of the subaerial surface of Kilauea volcano has extended continuously since the M 7.2 flank earthquake of November 1975. Widening across the summit has amounted to more than 250 cm, decelerating after 1975 from about 25 to 4 cm yr−1 since 1983. Concurrently, the summit has subsided more than 200 cm, even as the adjacent south flank has risen more than 50 cm. The axes of the upper zones, about 10 km from the summit, subsided before 1983 at average rates of 9 and 4 cm yr−1, respectively, and at rates of 4 and 3 cm yr−1 since. The middle southwest rift zone is also subsiding and, at the other end of Kilauea's subaerial rift system, subsidence along the lower east rift zone has averaged 1–2 cm yr−1. Deformation of Kilauea's south flank has been continuous, although subject as well to displacements caused by major rift zone seismic swarms. Whereas horizontal strains across the subaerial south flank seem to have been generally compressive after 1975, they have been extensional since about 1980 or 1981, interrupted only by the east rift zone dike intrusion of 1983. Because the magnitudes of these contractions and extensions are much less than the extension across the rift system, the subaerial south flank is apparently sliding seaward on its basal decollement more than it is accumulating horizontal strains within the overlying volcanic pile. Kilauea suffers from gravitational spreading made even more unstable by accumulation of magma along the rift system at depths in excess of about 4–5 km in the presence hot rock incapable of withstanding deviatoric stresses. This seismicly quiescent zone decouples the south flank from the rest of Hawaii's volcanic edifice; the rift zones at lesser depths exhibit a more brittle and, therefore, sporadic extensional behavior. Judging from the modern extension record of the summit, which both predates the M 7.2 earthquake of 1975 and has outlived its 10‐year period of aftershocks, Kilauea will continue to spread along its rift system as its south flank slips seaward to accommodate the accretion of magma and its relatively dense olivine‐rich differentiate.
Time‐dependent changes in failure stress following thrust earthquakes
Authors: Freed Andrew M. , Lin Jian
Published: February 2004

Two‐dimensional (2‐D) viscoelastic finite element models were used to calculate the time‐dependent changes in Coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension. Results suggest that thrust earthquakes cause a coseismic increase in Coulomb stress along antithetic lobes normal to the slip plane. Following a quake, creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust. Under certain conditions, transfer of stress may lead to a further buildup of high Coulomb stress along the base of the upper crust, potentially shortening the time to failure of other faults in the region. The conditions under which an antithetic lobe of high Coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following: the lower crust or upper mantle has an effective viscosity not greater than 1019 Pa s; the thrust fault has a moderate dip angle (40°–50°); the brittle/ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion; and the crust has a low apparent coefficient of friction (<0.2). Postseismic increases in Coulomb stress within the upper crust may also be caused by aseismic creep on the fault. Stress changes due to this mechanism are maximized with a high apparent coefficient of friction. Analysis of experimentally determined non‐Newtonian flow laws suggests that wet granitic, quartz, and feldspar aggregates may have a viscosity of the order of 1019 Pa s. The calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate. Results of this 2‐D analysis suggest a potentially important role of viscous flow in controlling time‐dependent postseismic stress changes that warrant further investigation using 3‐D viscoelastic analysis.
Small‐scale deformations associated with the 1992 Landers, California, earthquake mapped by synthetic aperture radar interferometry phase gradients
Authors: Price Evelyn J. , Sandwell David T.
Published: February 2004

The Landers earthquake (Mw 7.3) occurred on June 28, 1992, and ruptured nearly 100 km of previously mapped and unmapped faults in the Mojave Desert. We use synthetic aperture radar interferometry (InSAR) to examine the cumulative surface deformation between April 24 and August 7, 1992, in a 100 × 100 km region surrounding the northern portion of the earthquake rupture. Also, we introduce a technique for manipulating SAR interferograms to extract short‐wavelength displacement information. This technique involves computation and subsequent combination of interferometric phase gradient maps. The InSAR results show significant deformation signatures associated with faults, fractures, dry lake beds, and mountainous regions within 75–100 km of the main rupture. Using the phase gradient method, we are able to extract small‐scale deformation patterns near the main rupture. Many of the preexisting, mapped faults within 50 km of the main rupture experienced triggered slip; these include the Old Woman, Lenwood, Johnson Valley, West Calico, and Calico Faults. The InSAR results also indicate right‐lateral offsets along secondary fractures trending N‐NE within the left‐lateral zone of shear between the main rupture and the Johnson Valley Fault. Additionally, there are interesting interferogram fringe signatures surrounding Troy Dry Lake and Coyote Dry Lake that are related to deformation of dry lake beds.
Bubble collapse as the source of tremor at Old Faithful Geyser
Authors: Kedar Sharon , Kanamori Hiroo , Sturtevant Bradford
Published: February 2004

Old Faithful Geyser, Yellowstone, was used as a natural laboratory for fluid‐flow‐induced seismic activity. Pressure measurements within the geyser's water column, obtained simultaneously with seismic measurements on the surface, demonstrated that the tremor observed at Old Faithful results from impulsive events in the geyser. Tremor intensity is controlled by the rate of occurrence of these impulsive events. There is no resonance observed within the water column. The impulsive events are modeled by a collapse of a spherical bubble, including the effects of residual non‐condensible gas and damping. The pressure data can be explained by a collapse of a ∼5 cm radius bubble driven by a pressure difference of ΔP = 0.3×105 Pa and effective viscosity νE = 0.04 m2/s. Using a quasi‐static geyser model, we treat the individual bubble collapses as cooling events that occur when the water column reaches a critical temperature. Their rate of occurrence is controlled by the heating rate dT/dt of the water column. As a result, the intensity of the hydrothermal and seismic activities is determined by the heat and mass input rate into the geyser. It is demonstrated that a sharp widening of the conduit can cause the number of events per unit time to drop (as observed) while the water level is still rising and heat is being input, and thus the tremor intensity can be modulated by variations in the conduit shape.
Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget
Authors: Saffer Demian M. , Bekins Barbara A.
Published: February 2004

Down‐hole geochemical anomalies encountered in active accretionary systems can be used to constrain the timing, rates, and localization of fluid flow. Here we combine a coupled flow and solute transport model with a kinetic model for smectite dehydration to better understand and quantify fluid flow in the Nankai accretionary complex offshore of Japan. Compaction of sediments and clay dehydration provide fluid sources which drive the model flow system. We explicitly include the consolidation rate of underthrust sediments in our calculations to evaluate the impact that variations in this unknown quantity have on pressure and chloride distribution. Sensitivity analysis of steady state pressure solutions constrains bulk and flow conduit permeabilities. Steady state simulations with 30% smectite in the incoming sedimentary sequence result in minimum chloride concentrations at site 808 of 550 mM, but measured chlorinity is as low as 447 mM. We simulate the transient effects of hydrofracture or a strain event by assuming an instantaneous permeability increase of 3–4 orders of magnitude along a flow conduit (in this case the décollement), using steady state results as initial conditions. Transient results with an increase in décollement permeability from 10−16 m2 to 10−13 m2 and 20% smectite reproduce the observed chloride profile at site 808 after 80–160 kyr. Modeled chloride concentrations are highly sensitive to the consolidation rate of underthrust sediments, such that rapid compaction of underthrust material leads to increased freshening. Pressures within the décollement during transient simulations rise rapidly to a significant fraction of lithostatic and remain high for at least 160 kyr, providing a mechanism for maintaining high permeability. Flow rates at the deformation front for transient simulations are in good agreement with direct measurements, but steady state flow rates are 2–3 orders of magnitude smaller than observed. Fluid budget calculations indicate that nearly 71% of the incoming water in the sediments leaves the accretionary wedge via diffuse flow out the seafloor, 0–5% escapes by focused flow along the décollement, and roughly 1% is subducted.
Convective fluid flow through heterogeneous country rocks during contact metamorphism
Authors: Gerdes Martha L. , Baumgartner Lukas P. , Person Mark
Published: February 2004

Geochemical and petrologic data from contact aureoles consistently document fluid focusing through small‐scale permeable structures. We use stochastic representations of permeability in a series of transient numerical simulations to assess how such small‐scale rock heterogeneities influence kilometer‐scale fluid convection around a shallow crustal pluton. The sensitivity study considers different permeability scenarios by varying statistical characteristics of the permeability distribution (mean, variance, and spatial correlation). Large‐scale convective flow patterns in heterogeneous contact aureoles are shown to deviate significantly from equivalent homogeneous aureoles in ways that cannot be predicted without detailed mapping of the permeability field. Fluid focused through high‐permeability zones results in cumulative fluid fluxes as high as 1×108kg m−2after 2×105years, while nearby low‐permeability regions experience time‐integrated fluxes ≪105kg m−2. Spatially variable flow patterns preclude extrapolation of flow directions or fluxes determined from one portion of the aureole to another. Several simulations show high‐permeability zones developing isolated long‐lived convection cells far from the intrusion. Reduced permeability contrasts (low variance) or anisotropic permeabilities inhibit development of such hydrologically isolated regions. When average aureole permeability is 10−16m2, maximum metamorphic temperatures deviate by ≤30° from conductive cooling models despite locally high fluxes. Advective temperature displacements as large as 170° occur only when average permeability is 10−15m2and can be detected using isotopic or petrologic thermometry. These temperature deviations reflect local permeability heterogeneities and could be useful in identifying high‐permeability paleochannels. These results also imply, however, that temperature profiles taken from only one portion of a heterogeneous contact aureole may not be regionally representative or used to infer the overall advective‐conductive thermal structure of the large‐scale hydrothermal system.
Time functions of deep earthquakes from broadband and short‐period stacks
Authors: Houston Heidi , Benz Harley M. , Vidale John E.
Published: February 2004

To constrain dynamic source properties of deep earthquakes, we have systematically constructed broadband time functions of deep earthquakes by stacking and scaling teleseismic P waves from U.S. National Seismic Network, TERRAscope, and Berkeley Digital Seismic Network broadband stations. We examined 42 earthquakes with depths from 100 to 660 km that occurred between July 1, 1992 and July 31, 1995. To directly compare time functions, or to group them by size, depth, or region, it is essential to scale them to remove the effect of moment, which varies by more than 3 orders of magnitude for these events. For each event we also computed short‐period stacks of P waves recorded by west coast regional arrays. The comparison of broadband with short‐period stacks yields a considerable advantage, enabling more reliable measurement of event duration. A more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment, producing scaled time functions with both correct timing and amplitude. We find only subtle differences in the broadband time‐function shape with moment, indicating successful scaling and minimal effects of attenuation at the periods considered here. The average shape of the envelopes of the short‐period stacks is very similar to the average broadband time function. The main variations seen with depth are (1) a mild decrease in duration with increasing depth, (2) greater asymmetry in the time functions of intermediate events compared to deep ones, and (3) unexpected complexity and late moment release for events between 350 and 550 km, with seven of the eight events in that depth interval displaying markedly more complicated time functions with more moment release late in the rupture than most events above or below. The first two results are broadly consistent with our previous studies, while the third is reported here for the first time. The greater complexity between 350 and 550 km suggests greater heterogeneity in the failure process in that depth range.
Some characteristics of the stress field of the 1995 Hyogo‐ken Nanbu (Kobe) earthquake
Authors: Bouchon Michel , Sekiguchi Haruko , Irikura Kojiro , Iwata Tomotaka
Published: February 2004

We investigate the characteristics of the stress field associated with the 1995 Hyogo‐ken Nanbu (Kobe) earthquake. We use for the study a tomographic model of the fault slip inferred from the numerous near‐field recordings, and we calculate the space and time evolution of shear stress on the fault during the earthquake. We show that the spatial distribution of stress drop is very heterogeneous. The apparent strength of the fault at the onset of the earthquake is low (of the order of 1 MPa or less), which indicates that the pre‐earthquake tectonic shear stress was close to the static friction over most of the fault. At many locations the stress drop rotates significantly during sliding. As was originally proposed bySpudich[1992], we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake. The pre‐earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 MPa. Average values of the initial and final shear stresses over the fault are 3.3 and 1.6 MPa, respectively, indicating that about half of the pre‐earthquake tectonic stress was released during the earthquake. There is a relatively strong correlation between the initial and final stress distributions, which suggests that intrinsic fault properties, not modified by the earthquake, control the spatial distribution of tectonic stress over the fault. We show that on average over the fault the dynamic friction coefficient is equal to about 40% of the static friction coefficient. Diagrams depicting the evolution of shear stress as a function of slip are consistent with slip‐weakening behavior, but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion.
The northern limit of the subducted Juan de Fuca Plate system
Authors: Cassidy John F. , Ellis Robert M. , Karavas Costas , Rogers Garry C.
Published: February 2004

Analysis of data recorded at an array of three‐component broadband seismograph stations deployed on northern Vancouver Island and the adjacent British Columbia mainland, at the northern end of the Cascadia subduction zone, provides the first constraints on the S wave velocity structure of this region and permits us to define the northern limit of the subducted Juan de Fuca plate system. During a 2‐year period, more than 80 teleseisms were recorded at our five stations. The method of receiver function analysis was used to constrain the S velocity structure to upper mantle depths. Beneath the northern three stations, a relatively simple continental crust is interpreted with a well‐defined Moho near 37–39 km depth. An upper crustal S velocity discontinuity at these stations is interpreted as the top of the high‐velocity rocks of the Wrangellia terrane. In contrast, more complicated structure dominated by pronounced low‐velocity zones dipping to the NE are interpreted beneath our southern two stations. The shallower low‐velocity zone is 6–8 km thick, has an S velocity contrast of 0.6–1.1 km/s, and lies within the continental crust. This feature is similar to a pronounced low‐velocity layer (the E zone) imaged beneath southern Vancouver Island. The deeper low‐velocity zone is interpreted as the subducted oceanic crust. We interpret the pronounced change in S velocity structure that we observe as the northern limit of the subducted oceanic plate beneath Vancouver Island. This change coincides with significant changes in topography, heat flow, gravity, and geochemistry.
Precipitation sealing and diagenesis: 1. Experimental results
Authors: Tenthorey Eric , Scholz Christopher H. , Aharonov Einat , Léger Albert
Published: February 2004

During burial and diagenesis of granular aggregates, significant permeability reduction may be induced by the formation of low‐temperature, authigenic minerals. To quantitatively assess the importance of this process, we have conducted a series of hydrothermal flow‐through experiments using deionized water and labradorite/quartz sand. All experiments were conducted in a modified triaxial apparatus, configured to allow continuous permeability measurements. Under most of the conditions tested, significant permeability reduction is observed with no concurrent decrease in porosity. The overall permeability reduction sometimes exceeds 1 order of magnitude over 4 days and is positively correlated to temperature and deviatoric stress. Scanning electron microscope observations together with data from additional experiments show that the observed permeability reduction is entirely a result of secondary mineral growth. Si and Al concentrations in the postexperiment fluids are also correlated to temperature and stress, confirming the link between the chemical state of the system and permeability behavior. In all experiments, permeability reduction is fastest early and levels off in the late stages. To explain the permeability behavior as a function of time, a conceptual model is developed in which precipitation of authigenic minerals is rapid at early times while dissolution of quartz and labradorite is most active. As the system approaches equilibrium, the components necessary for secondary mineral formation are liberated at a lower rate, thereby causing precipitation to slow. Although authigenic mineral formation does not reduce total pore space in these experiments, there is a reduction in effective porosity, which results in permeability reduction.
Location of the southern edge of the Gorda slab and evidence for an adjacent asthenospheric window: Results from seismic profiling and gravity
Authors: Beaudoin Bruce C. , Hole John A. , Klemperer Simon L. , Tréhu Anne M.
Published: February 2004

As the Mendocino Triple Junction migrates northward along the California margin it is widely presumed to leave a “slab‐free” or “asthenospheric” window in its wake. A 250‐km‐long south‐north seismic refraction‐reflection profile crossing the transition from transform to subduction regimes allows us to compare and contrast crust and upper mantle of the North American margin before and after it is modified by passage of the Mendocino Triple Junction. From the seismic data we have determined that (1) the crust is laterally homogeneous in velocity to a depth of 20 km (interpreted by us as Franciscan complex), (2) below 20 km depth the crust is characterized by velocities of ≥7.0 km/s for the southern half of the profile and by velocities of ≤7.0 km/s for the northern half, (3) regions of high reflectivity in the crust occur below ∼20 km depth throughout the profile, and (4) the North American crust is thickest (∼35 km) in the center of the profile and thins to ∼25 km at either end. From the gravity data we have determined that (1) asthenospheric densities (3.2 g/cm3) occur subjacent to the North American crust in the center of the profile, and (2) a wedge of lithospheric mantle density material (≥3.2 g/cm3) is required on the southern end of the profile. We interpret these combined results to indicate that our profile crosses the southern edge of the Gorda plate and that directly adjacent to this edge is an asthenospheric window with overlying mafic rocks in the crust. These mafic rocks and a reforming lithospheric mantle increase in thickness southward.
Present‐day tectonic plate motions and crustal deformations from the DORIS space system
Authors: Crétaux Jean‐François , Soudarin Laurent , Cazenave Anny , Bouillé Florence
Published: February 2004

Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) data acquired between January 1993 and December 1996 from the SPOT‐2, SPOT‐3, and TOPEX/Poseidon satellites have been analyzed to determine velocities for 45 sites on eight major tectonic plates. For 28 sites far from deformation zones, the velocity estimates agree with plate model predictions. Least squares computation of poles of rotation, which model the plate motions, shows that for Eurasia, Africa, Pacific, and South America plates, the agreement is better with NUVEL‐1, while for Australia, Antarctica, Nazca, and North America plates the DORIS Euler vectors are closer to NTJVEL‐1A. In general, DORIS results do not differ significantly from other space geodetic techniques determinations but provide better estimates for plates poorly or inhomogeneously covered by Global Positioning System (GPS), Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) networks, such as Africa. The DORIS coverage of this plate allows discussion of intraplate deformations due to the motion of the eastern Africa part which constitues the Somalia plate. Sites located in deformation zones, such as western Eurasia boundaries, central Asia, southwestern America coast, South East Asia, show motion with respect to their own plates. Comparisons with other geodetic measurements for colocated stations, or with regional geodynamical models, show the interest of DORIS in active zones where global plate models are not valid.
Congruent melting kinetics of albite: Theory and experiment
Authors: Greenwood James P. , Hess Paul C.
Published: February 2004

Congruent melting of albite, NaAlSi3O8, is found to be a surface‐initiated process, with melt nucleating at external surfaces and internal cracks and cleavage planes. Melting rates are found to be within a factor of 5 of rates calculated by a normal growth model. The melting point of albite is shown to be in the 1100° to 1120°C range. Melting rates are found to be proportional to the amount of superheat and inversely proportional to the viscosity of the melt phase. The strong dependence of melting rates on viscosity is related to the configurational entropy by the Adam and Gibbs [1965] theory. The normal growth model is shown to be appropriate for describing the congruent melting kinetics of silicates and oxides at moderate amounts of superheat. Homogeneous melting models are probably predicting the temperature of mechanical instability of the solid, rather than the true melting point. Surface premelting is discussed and shown to not be related to the premelting effects described by Richet et al. [1994]. Surface premelting is predicted for albite.
Redox control of sulfur degassing in silicic magmas
Authors: Scaillet Bruno , Clemente Béatrice , Evans Bernard W. , Pichavant Michel
Published: February 2004

Explosive eruptions involve mainly silicic magmas in which sulfur solubility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silicic magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO2 in recent explosive silicic eruptions in arc settings, which invariably have displayed an excess of sulfur relative to that calculated from melt degassing. If this excess sulfur is stored in a fluid phase, it implies a strong preference of sulfur for the fluid over the melt under oxidized conditions, with fluid/melt partition coefficients varying between 50 and 2612, depending on melt composition. Experimentally determined sulfur partition coefficients for a dacite bulk composition confirm this trend and show that in volcanic eruptions displaying excess gaseous sulfur, the magmas were probably fluid‐saturated at depth. The experiments show that in more reduced silicic magmas, those coexisting only with pyrrhotite, the partition coefficient decreases dramatically to values around 1, because pyrrhotite locks up nearly all the sulfur of the magma. Reevaluation of the sulfur yields of some major historical eruptions in the light of these results shows that for oxidized magmas, the presence of 1–5 wt % fluid may indeed account for the differences observed between the petrologic estimate of the sulfur yield and that constrained from ice core data. Explosive eruptions of very large magnitude but involving reduced and cool silicic magmas, such as the Toba or the Bishop events, release only minor amounts of sulfur and could have consequently negligible long‐term (years to centuries) atmospherical effects. This redox control on sulfur release diminishes as the melt composition becomes less silicic and as temperature increases, because both factors favor more efficient melt sulfur degassing owing to the increased diffusivity of sulfur in silicate melts under such conditions.
Rigorous connection between physical properties of porous rocks
Authors: Gibiansky Leonid , Torquato Salvatore
Published: February 2004

Rigorous cross‐property bounds that connect the effective thermal conductivity k* (or the electrical conductivity σ*) and the effective bulk modulus K* of any isotropic, two‐phase composite were recently derived by the authors. Here we reformulate these bounds and apply them to porous rocks with dry or fluid‐filled pores. It is shown that knowledge of the effective conductivity can yield sharp estimates of the effective bulk modulus (and vice versa), even in cases where there is a wide disparity in the phase properties. The bounds yield, in particular, relations between the formation factor and the bulk modulus of the porous medium. By using the same approach we obtain new relations between the bulk moduli of a dry porous material and the bulk modulus of the same material with fluid‐filled pores that are more general than the traditional Gassmann equation. The Gassmann formula for the bulk modulus of the fluid‐saturated porous medium is shown to correspond to a lower bound on this quantity. Limiting cases that we consider include cracked materials with dry and fluid‐saturated pores. Theoretical results are tested against experimental measurements of the effective bulk modulus of dry and water‐saturated Westerly granite and sandstone samples. We found good agreement between our cross‐property bounds and the experimental data, even when the experimental data depart from the Gassmann formula. Our results add new insight to understanding of the properties of the porous media. They show that the Gassmann approximation works well for rocks with high porosity but needs to be corrected for rocks with high crack‐type porosity.