bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of assimilative mapping and source surface model results for magnetospheric events of January 27 to 28, 1992
Authors: Peroomian Vahé , Lyons Larry R. , Shultz Michael , Pridmore‐Brown D. C.
Published: February 2004

We calculate polar cap boundaries and polar cap equipotentials obtained by using two modifications of the source surface model (SSM) for three intervals specified by the Geospace Environment Modeling (GEM) grand challenge and compare with results from the assimilative mapping of ionospheric electrodynamics (AMIE) code. In the first modification Birkeland currents are self‐consistently added to the SSM. In the second modification the magnetotail radius is increased, and the lobe magnetic field is modified to obtain a more realistic magnetotail. Both modifications of the SSM, although simple, are found to reproduce many of the features seen in the AMIE results. These include a duskward (dawnward) shift of the separatrix in the northern (southern) hemisphere because of negative IMF By effects, and a good agreement for southern hemisphere equipotentials and for the locations or the nightside separatrix between open and closed field lines. However, both versions of our model give northern hemisphere separatrices that are more nearly circular than those observed and show IMF By effects not present in the AMIE results; they also displace the dayside separatrix poleward of the positions indicated by observations.
Electrical resistivity of Fe3O4 to 48 GPa: Compression‐induced changes in electron hopping at mantle pressures
Authors: Morris E. R. , Williams Q.
Published: February 2004

We have measured the electrical resistivity of Fe3O4 to pressures of 48 GPa at temperatures between 258 and 300 K in order to evaluate compression‐induced changes in electron exchange between divalent and trivalent iron ions. At ambient pressures, inverse spinel‐structured magnetite is well known for its electron hopping between divalent and trivalent iron octahedral sites, and our results thus provide constraints on the role of intervalence charge transfer in altering the electrical resistivity of iron‐rich phases. The resistivity of Fe3O4 decreases by more than an order of magnitude between 0 and ∼20 GPa, where it reaches a minimum, subsequently increasing by a factor of 2 between ∼20 and 48 GPa. This finding implies that the electronic exchange between Fe2+ and Fe3+ ions is notably enhanced by the initial 7% of volumetric compression but is marginally impeded at higher compressions. We associate this discontinuity in slope with a phase transition from the inverse spinel structure to a monoclinic structure at ∼20 GPa. Both previous Mössbauer work and the small change in magnitude of the resistivity indicate that electron hopping persists as the mechanism of charge transport through the transition. The resistivity at these pressures is within an order of magnitude of characteristic metallic values, but measurements of the temperature dependence of resistivity demonstrate that the high‐pressure phase remains semiconducting. However, the resistivity of Fe3O4 is 1–2 orders of magnitude less than that of Fe0.94O at 48 GPa, illustrating the profound effect of intervalence charge transfer (particularly at high pressures) on the electrical properties of iron‐rich oxides. By measuring electrical resistivity at pressures between 48 and 12 GPa, at temperatures between 258 and 300 K, we also constrain the pressure dependence of the activation enthalpy for conduction of the high‐pressure phase of Fe3O4. We demonstrate that the activation volume of conduction is pressure dependent for the low‐pressure (magnetite) phase. Such pressure dependent behavior implies that simulations of mantle resistivity that use a constant activation volume may underestimate the resistivity of the lower mantle. We also evaluate the shifts in the iron‐wüstite, wüstite‐magnetite, magnetite‐hematite, and wüstite‐hematite oxygen fugacity buffers with depth in order to assess the oxygen fugacity conditions that could generate mixed valence compounds in the lower mantle.
The crustal structure of the Canary Basin: Accretion processes at slow spreading centers
Authors: Ranero César R. , Banda Enric , Buhl Peter
Published: February 2004

Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower‐crustal reflectivity is almost absent and reflections from the crust‐mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust‐mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid‐Atlantic Ridge.
Rise of starting plumes through mantle of temperature‐, pressure‐, and stress‐dependent viscosity
Authors: Weinberg Roberto F.
Published: February 2004

The evolution of size and excess temperature of starting plumes is calculated as a function of (1) Newtonian and power law (stress‐dependent) mantle rheology, and (2) buoyancy flux into the plume through a feeding conduit in mantle of temperature‐ and pressure‐dependent viscosity. The initial size and excess temperature that allow plumes starting at the core‐mantle boundary to reach the base of the lithosphere 200 K or more warmer than the surroundings are determined for a range of conditions. The results suggest that starting plumes can be divided into “slow” and “fast” plumes. Slow plumes become large and cool through entrainment of the surroundings during ascent, so that they are likely to have little impact on the evolution of the lithosphere. Fast plumes, on the other hand, rise so rapidly that cooling and entrainment become unimportant. They reach the lithosphere while they are still hot (often with an excess temperature similar to that at the source) and are likely to strongly perturb the lithosphere. The change from slow to fast plumes occurs over a small increase in plume buoyancy at the source, particularly in power law mantle. Small buoyancy flux through the conduit affects only “slow” plumes, whereas high flux causes plumes of different initial radius to reach the lithosphere with similar radius and excess temperature. The results suggest that the study of important starting plumes (those that will have considerable geological consequences) can be simplified to that of (1) nonentraining (constant radius) thermals (plume heads isolated from the source) for small buoyancy flux and (2) nonentraining plumes for high flux. In all cases, most entrainment occurs in the lower mantle. Comparison between power law and Newtonian mantle viscosities suggests that the upper mantle will respond as a power law fluid because it yields lower effective viscosity when submitted to the buoyancy stress of plumes than does a corresponding Newtonian mantle. The power law response of the upper mantle is unlikely to considerably influence ascent time of plumes, but it will considerably change the way plumes interact with the lithosphere.
Electron microscopy of iron oxides and implications for the origin of magnetizations and rock magnetic properties of Banded Series rocks of the Stillwater Complex, Montana
Authors: Xu Weixin , Geissman John W. , Van der Voo Rob , Peacor Donald R.
Published: February 2004

The origins of multiple magnetizations of the Archean Stillwater Complex have been investigated through scanning electron microscopy and scanning transmission electron microscopy observations of mineralogical relations, using representative samples from nine sites in mafic Banded series rocks. On the basis of directional grouping and demagnetization behavior, three magnetizations (here labeled A, B, and C) have been recognized. The natural remanent magnetizaton (NRM) is lypically dominated by only one of these magnetizations and multicomponent behavior in individual specimens is rare. The A remanence resides in magnetic grains of high median destructive fields and high, discrete laboratory unblocking temperatures and is inferred to be a primary thermoremanent magnetization, of circa 2.71 Ga age. The B and C magnetizations, of lower median destructive fields and more distributed unblocking temperatures, are inferred to be secondary and related to alteration, including serpentinization, involving limited, moderate to low‐temperature fluid interaction, perhaps in response to thermotectonic events (e.g., mafic dike emplacement). Samples with NRM dominated by the A magnetization contain titanium‐free magnetite needles (width <1 μm and maximum dimension between about 2 and 50 µm ) with preferred orientations in cumulus plagioclase crystals; these oxides are interpreted to have formed during initial crystallization of cumulus phases. In samples containing only B or C magnetizations, such needles are absent in cumulus plagioclase, whereas secondary Fe‐bearing silicates, particularly zoisite and chlorite, are conspicuous. The samples dominated by the B or C magnetizations contain anhedral low‐chromian magnetite (FeCr2‐xFexO4, 1.5<x<2.0) along fractures in both cumulus plagioclase and pyroxene and in zones of more pervasive alteration of pyroxenes. The paragenesis of these oxides is consistent with an origin related to hydrothermal alteration. Hysteresis data show that plagioclase crystals containing magnetite needles have higher remanent coercivities (Hcr=60 to 100 mT), whereas plagioclase crystals with anhedral chromian magnetite have remanent coercivities less than 60 mT, consistent with their alternating field response. Because oxygen isotope data show no indication of appreciable alteration of host plagioclase, we infer that the absence of magnetite needles and their associated A magnetization are caused by selective alteration of the magnetite needles to nonmagnetic phases in the remagnetized gabbros. In this case, rock magnetic and paleomagnetic data appear to be higher resolution indicators of fluid/rock interaction than the stable isotope data. Anhedral chromian magnetite precipitated during hydrothermal alteration and therefore carries secondary magnetizations.
A faulting model for the 1992 Petrolia earthquake: Can extreme ground acceleration be a source effect?
Authors: Oglesby David D. , Archuleta Ralph J.
Published: February 2004

The 1992 Petrolia earthquake (M=7.0) was quite remarkable in that an accelerometer at Cape Mendocino (CAP) recorded a high‐frequency pulse of almost 2 g, whereas the nearby (roughly 6 km away) Petrolia station (PET) recorded a maximum of only 0.6 g. In our model for this event, we have investigated the possibility that extreme ground acceleration with high spatial variability can be caused by a source effect, specifically the rupture of a barrier/asperity. This type of rupture feature can produce extreme ground acceleration provided that certain geometrical constraints are met. In the present study, we derive a combined high‐ and low‐frequency faulting model for the 1992 Petrolia earthquake that is consistent with the slip distribution on the fault and that produces the high‐frequency pulse at CAP. Our model consists of four parts: (1) A low‐ frequency (0.1–0.6 Hz) inversion that determines the slip, rupture time, and risetime distributions on the fault; (2) an interpolation and perturbation of the above distributions that produces synthetic accelerograms between 0.6 and 3.0 Hz; (3) random phase signal that is scaled to the spectral amplitude level of the data between 3.0 and 12.5 Hz; and (4) the rupture of a barrier‐like asperity that produces greatly amplified radiation at CAP but not at other stations. We find that our model produces a good fit to the near‐source records of the 1992 Petrolia event. The results emphasize the interaction between fault geometry and rupture propagation to produce ground motion, as well as the hazard associated with locations above the hanging walls of dipping faults, where the geometry permits the production of extreme ground acceleration.
Structure and sharpness of phase transitions and mantle discontinuities
Authors: Stixrude Lars
Published: February 2004

The structure of phase transitions expected from equilibrium thermodynamics is examined. We show that in binary systems, the shape of the coexistence region (phase loop) is controlled primarily by the partition coefficient K. We derive a general and simple one‐parameter expression for the pressure dependence of the yield of the high‐pressure phase and show that this function can be highly nonlinear: most of the transition occurs in a narrow interval near the boundary of the phase loop. Estimates of the effective width of binary phase transitions are less than half the total width of the coexistence region even for relatively mild partitioning (K<1/4). We generalize these results to multiphase and multicomponent transitions. We show that the presence of nontransforming phases can affect the width of the transition substantially. We predict that the width of the olivine to wadsleyite transition in the presence of pyroxene and garnet is approximately half that of the binary phase loop at typical transition zone temperatures. The estimated effective width of this transition in the mantle (4–8 km) is marginally consistent with observations of high‐frequency (0.5–1.0 Hz) P wave reflections from the 410 km discontinuity. We show that the effective width of the garnet to perovskite transition is sufficiently narrow to reflect S wave energy in the frequency range of ScS reverberations (10–40 mHz) and that this transition can account for the observed properties of the 710 km discontinuity.
Dissolution and precipitation via forced‐flux injection in a porous medium with spatially variable permeability: Kinetic control in two dimensions
Authors: Bolton Edward W. , Lasaga Antonio C. , Rye Danny M.
Published: February 2004

We model kinetically controlled dissolution and precipitation in a porous medium. Using quartz as the reactive mineral, our specific focus is on the spatial distribution of flow velocities and on changes in solute concentration and porosity introduced by heterogeneities in the initial permeability field and by differing the solute concentration of the infiltrating fluid. Upon a background permeability we impose isolated “crack‐like” zones of high permeability and low surface area to fluid volume ratios. Our two‐dimensional modeling of forced‐flux injection of oversaturated and undersaturated fluid reveals features inaccessible to previous homogeneous permeability studies. Although realistic velocities can give rise to narrow boundary layers in the solute concentration, disequilibrium is favored in the high‐permeability zones yielding plume‐like distributions of solute concentration. Aside from the rapid changes in porosity near the injection level, the other regions of rapid porosity change occur just downstream from the crack‐like zones, where fluid furthest from equilibrium with respect to quartz encounters regions of higher surface area to fluid volume ratios. Flow rates are significantly enhanced between isolated high‐permeability zones, an effect which is even more dramatic for both closer crack spacing and higher permeability contrasts. Undersaturated injection leads to permeability homogenization along the flow direction due to dissolution at the trailing edges of the cracks. Oversaturated injection tends to increase permeability heterogeneities along the flow direction, due to precipitation at the crack trailing edges. We also discuss the scaling properties of flow lines and reaction rates.
Structure and origin of the 85°E ridge
Authors: Ramana M. V. , Subrahmanyam V. , Chaubey A. K. , Ramprasad T. , Sarma K. V. L. N. S. , Krishna K. S. , Desa Maria , Murty G. P. S. , Subrahmanyam C.
Published: February 2004

The submerged 85°E Ridge in the Bay of Bengal trends approximately N‐S between 19°N and 6°N latitudes. Off the southeast coast of Sri Lanka it takes an arcuate shape and seems to terminate with the northward extension of the Afanasy Nikitin seamounts situated around 2°S latitude. The ridge is characterized by positive magnetic (100–400 nT) and negative free‐air gravity (<−60 mGal) anomalies with variable widths of 100–180 km. Magnetic model studies revealed that the rocks of the 85°E Ridge are magnetized with reversed polarity. The well‐defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin. Two alternative processes for the ridge emplacement have been suggested. Ridge emplacement may be (1) due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early Cretaceous crust in the Bay of Bengal, more precisely at M0 isochron or during the middle Albian reversals within the Cretaceous long normal polarity (K‐T superchron) epoch when the Earth's magnetic polarity changed from normal to reversed polarity, and/or (2) due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin. However, more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model. Further, the Rajmahal traps (normally polarized) and the 85°E Ridge (reversely polarized) appear to be associated with two different episodes of eruption that might have been triggered by the Kerguelen mantle plume. The 85°E Ridge seems to extend into the onshore West Bengal Basin as a subsurface ridge and merges with the reported NNE‐SSW trending zone of strong geophysical anomalies east of Rajmahal traps up to 25°N latitude.
Tectonic significance of the South Iceland Seismic Transform Zone
Authors: Luxey Pascal , Blondel Philippe , Parson Lindsay M.
Published: February 2004

The subaerial expression of the Mid‐Atlantic Ridge on Iceland comprises two overlapping spreading axes, referred to as the West Volcanic Zone (WVZ) and the East Volcanic Zone (EVZ), respectively. The way the spreading rate is distributed on both volcanic zones has an important impact on the stress pattern in the overlap area. Our field data from the area trace the evolution of the stress direction as recorded by slip motion on fault planes. We found four different strike‐slip stress phases. An early N‐S compression phase (A) preceded a NE‐SW compression phase (B). This phase was followed by a SE‐NW compression phase (C). However, we cannot date an E‐W compression, phase (D), relative to the other phases. Numerical modeling based on the assumption that the WVZ has been permanently active during the last 3 Myr and that the EVZ is propagating southward confirms that stress directions have rotated clockwise by more than 140°. These results fit perfectly with our field analysis, and we propose that phase A corresponds to initial EVZ ridge‐tip propagation, phase B to emplacement of the EVZ southern tip near Torfajökull, and phase C to an extreme southern location near the Surtsey Islands of the EVZ southern tip. Phase D could correspond to an intermediate stage between phases B and C. We suggest that this sequence of tectonics, recorded in a regional overlapping ridge‐tip setting, is directly analogous to smaller‐scale and more common phenomena at second‐order ridge discontinuities throughout the global mid‐ocean ridge system.
Melting experiments on a synthetic olivine lamproite composition up to 8 GPa: Implication to its petrogenesis
Authors: Sato K.
Published: February 2004

We have determined melting phase relationships on a synthetic olivine lamproite composition with H2O contents from 6 to 12 wt % in the pressure range of 3.5 to 8 GPa and at temperatures of 1100 to 1500°C. The composition with 6 wt % H2O is multiply saturated with olivine, orthopyroxene, and garnet at 5.3 GPa and 1370°C, indicating that olivine lamproite can coexist with garnet harzburgite. The Mg # (molar 100Mg/Mg+Fe) of olivine, orthopyroxene, and garnet in experiments that closely bracket the multiple saturation point are 94.3, 93.7, and 88.9, respectively, and the garnet contains 3 wt % CaO. This residual mineral assemblage corresponds to low‐Ca garnet harzburgite. Phlogopite crystallized at a temperature 150°C below the multiple saturation point, indicating that all phlogopite in the source is consumed during partial melting prior to magma segregation. If the conditions of multiple saturation represent those of magma segregation, then the source region of the olivine lamproite exists near the base of continental lithosphere. Metasomatized, depleted harzburgite with veins containing centimeter‐sized phlogopite grains have been reported to occur in mantle xenoliths from kimberlites. We propose that olivine lamproite magma is formed by preferential melting of phlogopite veins, with subsequent equilibration of the magma with surrounding garnet harzburgite.
Late Cretaceous paleomagnetism of the East Ranges island arc complex, Kamchatka: Implications for terrane movements and kinematics of the northwest Pacific
Authors: Levashova Natalia M. , Bazhenov Mikhail L. , Shapiro Mikhail N.
Published: February 2004

A Campanian‐lower Paleocene island arc complex was sampled for paleomagnetic studies at 12 sites in the East Ranges tectonic zone of Kamchatka. After thermal demagnetization, a reversed polarity characteristic remanent magnetization (ChRM) was isolated from most volcanoclastic and basaltic units as well as from lava debris from intraformational conglomerates. The fold and conglomerate tests are positive, and the ChRM in the studied rocks is likely primary. The formation‐mean inclination of 66.3°±3.7° corresponds to a paleolatitude of 48.7°±5.0°N which is about 20° lower than the Late Cretaceous North American reference values. Because northward displacement of the studied terrane is indicated by the paleomagnetic data, we examine several models of intraoceanic transport with the Pacific and/or Kula plates and coastwise transport after terrane accretion, far to the south of the present‐day position of Kamchatka. Our preferred interpretation is that the studied island arc complex accumulated at about 83–79 Ma; the island arc, to which the studied terrane had originally belonged, was active between this time and 65–60 Ma. According to geological data, the docking time nearly coincided with cessation of volcanic activity, and northward movement of the island arc took place simultaneously with the volcanic activity. The absolute motion of a subduction zone should have the same direction as the overriding plate; therefore, the subduction zone related to the East Ranges island arc is inferred to have moved northward with the Kula plate or with the Kula and Pacific plates, successively, consuming either the oceanic periphery of a continental plate or some unknown minor oceanic plate. This process went on until 65–55 Ma when the island arc and related subduction zone approached the continental margin and became extinct. The proposed models also place additional constraints on kinematics of the Kula‐Pacific transform plate boundary.
The elastic constants of an aluminous orthopyroxene to 12.5 GPa
Authors: Chai Mu , Brown J. Michael , Slutsky Leon J.
Published: February 2004

The elastic constants, average bulk and shear moduli, and lattice parameters of Kilbourne Hole orthopyroxene [(Mg1.63Fe0.17Ca0.04Mn0.01)(Al0.12Cr0.01)(Si1.89Al0.11)O6] have been determined to a pressure of 12.5 GPa. The molar ratio Mg/(Mg+Fe) in the sample is 0.91 and 5 wt % of Al2O3 and 1 wt % of CaO are present. The bulk modulus, in gigpascals, expressed as a function of the pressure (with an rms misfit of 0.5%) is K = 115.4 + 7.82P ‐ 0.18P2. The unusually large pressure derivative of bulk modulus at 1 bar decreases to a more typical value (∼4) at high pressure. The isotropic shear modulus is linear within experimental uncertainty (the rms misfit is 0.5%) with G = 77.9 + 1.44P. At 1 bar the isotropic longitudinal and shear velocities 8.15 and 4.86 km/s, respectively are approximately 3% faster than those found in nonaluminous orthopyroxenes. Above 4 GPa the average compressional wave velocities in the orthopyroxene exceed those in olivine. These results call into question the assumption, common in models created to interpret mantle seismic data, that high velocities in garnet are balanced by low velocities in orthopyroxene.
Kinematic constraints on porosity change in the toe of the Cascadia accretionary prism: Evidence for cementation and brittle deformation in the footwall of the frontal thrust
Authors: Morgan Julia K.
Published: February 2004

Kinematic analyses are performed on a seismic profile across the deformed protothrust zone (PTZ) of the Cascadia accretionary prism, off the central coast of Oregon, in order to quantify the deformation in this zone and to explore mechanisms and controls on deformation and dewatering. The analyses rely on seismically defined stratigraphy and seismic interval velocities to constrain distortional and volumetric changes within the PTZ, permitting the calculation of the complete deformation field. Two solutions are offered: the first is based on porosity estimates derived from seismic interval velocities by Cochrane et al. [1994b] which suggest extremely high volume loss in the immediate footwall of the frontal thrust; the second solution employs a modified volume distribution in which lateral shear deformation is minimized. This latter approach is justified by evidence from seismic profiles and in drill cores for subhorizontal maximum principal stresses in many prisms. The two solutions yield total shortening of about 1.46 km (12%), with a general landward increase in horizontal shortening and vertical extension; this appears to be accompanied by increasing brittle deformation and porosity decrease, perhaps enhanced by dewatering along protothrusts. The greatest difference between the solutions lies in estimates for tectonically induced volume loss in the footwall of the frontal thrust, which is significantly reduced in the second solution. Relying on kinematic and mechanical arguments that support this result, we propose that the high seismic velocities observed in the footwall of the thrust may result from the presence of intergranular cement, perhaps preferentially precipitated adjacent to the frontal thrust fault in sediments with relatively high fracture permeability and enhanced fluid flow. A high degree of lithification in this zone is consistent with the inferred increase in brittle deformation in the footwall of the thrust.
Dilation of anisotropic rock salt: Evidence from Mount Sedom diapir
Authors: Hatzor Yossef H. , Heyman Eli P.
Published: February 2004

The mechanical behavior of bedded rock salt is studied using constant strain rate biaxial tests performed on 24 bedded rock salt samples from Mount Sedom diapir. Careful measurements of elastic parameters in unload‐reload loops indicate that they are influenced by confining pressure but are not sensitive to anisotropy. The yield stress is also influenced by confining pressure but not by anisotropy. The stress‐strain curve therefore indicates isotropy up to the yield stress. Initiation of dilation marks the onset of dependence of volumetric strain on anisotropy. Major principal compressive stress at dilation (σ1,d) is at maximum when compression is normal to bedding planes (β=0°), the stress decreases with increasing values of β, and minimum values are measured when compression is parallel to bedding planes (β=90°). Beyond onset of dilation point the relative dilation with respect to stress difference (V = |Δεν/Δ(σ1 ‐ σ3)|; σ1 > σ1,d; MPa−1) increases with decreasing confining pressure and with increasing β; namely, dilation is intensified with decreasing confining pressure and as the major principal compressive stress direction becomes parallel with bedding plane orientation. Current models for compression‐dilation boundary ignore anisotropy and therefore provide site specific solutions but fail to describe general rock salt behaviour. A new empirical model for compression‐dilation boundary in anisotropic rock salt, developed for a data set from Mount Sedom, predicts that stress at onset of dilation decreases with decreasing confining pressure and with increasing value of β. The rate of change of σ1,d with respect to β decreases with decreasing confining pressure, and the rate of change of σ1,d decreases with increasing confining pressure for all values of β.
Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms
Authors: Reyners Martin , Robinson Russell , McGinty Peter
Published: February 2004

Subduction of the Pacific plate in the northern South Island and southernmost North Island of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25° clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low‐angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low‐angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.
Mapping active magma chambers by b values beneath the off‐Ito volcano, Japan
Authors: Wyss Max , Shimazaki Kunihiko , Wiemer Stefan
Published: February 2004

The b value of the frequency‐magnitude relation, and thus the mean magnitude, in the off‐Ito volcanic area is not uniform based on detailed b value tomography using about 10,000 events in an area of 10 km radius and in the upper 15 km of the crust. A high b value anomaly (b=15) below 7 km depth and with a radius of about 2 km, located below the coast southeast of Ito, contrasts with lower values of typically b=0.7 north of it and at shallower depths. On the basis of surface deformations, a tensile crack dislocation source was located above 7 km depth. Thus we surmise that the magma chamber in the off‐Ito area was located below 7 km depth. The correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies. The magma body may be described as consisting of two parts: a lower, well‐established chamber below 11 km depth and a shallower part (11–7 km depth). The b value in the volume surrounding the magma chamber increased from 0.7 during the 1980s to 1.5 during the earthquake swarms of the 1990s. This may reflect a trend of increasing crack density due to the intrusion activity.
Quantifying lateral heterogeneities in fluvio‐deltaic sediments using three‐dimensional reflection seismic data: Offshore Gulf of Mexico
Authors: Deshpande Anil , Flemings Peter B. , Huang Jie
Published: February 2004

Scale‐invariant statistics and strong anisotropy in rock properties are documented from well log and three‐dimensional seismic data in Pleistocene fluvial and deltaic deposits in the Eugene Island 330 field, Gulf of Mexico. Systematic quantification of lateral lithologic heterogeneity within a stratigraphic framework, using reflection seismic data, reveals that fluvial and deltaic depositional systems exhibit statistical behavior related to stratigraphic fabric. Well log and seismic data profiles show a decay in power spectra with wavenumber, k, according to k−β with β between 1 and 2.3. Vertical and horizontal well log data exhibit power law scaling behavior with β between 1 and 2. Horizontal seismic horizon slices were extracted from within fluvial and deltaic deposits. Seismic data from fluvial and deltaic deposits exhibit distinct differences in variance, and each depositional system reveals directional anisotropy in β. One‐dimensional seismic data profiles from the fluvial deposit reveal anisotropy along depositional strike (β=1.1) and dip (β=2.2). The higher correlation observed in the direction of stratigraphic dip for the fluvial deposit may reflect the depositional fabric associated with channel systems. Correlation parameters (β) obtained in specific depositional environments can potentially be used to quantify rock fabric and provide constraints in the modeling of sedimentary processes.
Estimation of crustal stochastic parameters from seismic exploration data
Authors: Pullammanappallil Satish , Levander Alan , Larkin Steven P.
Published: February 2004

Stochastic models for the crystalline crust produce synthetic seismograms that compare well with recorded data for a variety of crustal ages and tectonic environments. In this paper, we explore the parameter space describing such stochastic models as a basis for formulating the inverse problem; that is, we wish to estimate the parameters which define a stochastic model from the recorded backscattered wave field. We base the estimation on approximate relations between the primary reflectivity series, which is the ideal wave field response of a medium, and various seismic gathers. A two‐dimensional lateral correlation method is used to investigate the sensitivity of synthetic wave fields to horizontal characteristic length. A derived empirical relationship relates the scale length to the half width of the correlation coefficient. A horizontal wavenumber‐time domain spectral analysis successfully estimates the horizontal characteristic length (ax) and the fractal dimension (D) of the stochastic medium from which the wave field was backscattered. We employ a Monte Carlo based optimization scheme to fit the observed spectral values to a one‐dimensional von Kármán function and simultaneously estimate values for ax and the Hurst number ν. The method yields consistent results when tested on synthetic data. Application of this technique to data sorted into different domains shows that zero‐offset sections yield the best estimate. We also present results of our estimates using this method on a Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) dynamite source shot gather and a vibroseis‐source Consortium for Continental Reflection Profiling (COCORP) data set recorded in the Basin and Range Province.
On the variability of the Chandler frequency
Authors: Vicente Raimundo O. , Wilson Clark R.
Published: February 2004

We have estimated the Chandler frequency from a variety of polar motion time series derived from optical and space geodetic data which span various time periods from 1846 through the early 1990s. Estimates ofFvary, depending upon which time series is employed, but the variation is not significant when associated intervals of confidence are considered; thus there is no evidence that the true Chandler frequency has varied. Using a maximum likelihood method, our preferred estimate ofFis 0.8433 cycles per tropical year (cpy), ±0.003 cpy, or equivalently a period of 433.1 mean solar days (msd), ±1.7 msd. Estimates are given on the condition that the Gaussian statistical model of the excitation process is valid, and new approaches which employ observations of the polar motion excitation process should eventually provide better estimates, which may differ from those determined here.
Seismic‐geodynamic constraints on three‐dimensional structure, vertical flow, and heat transfer in the mantle
Authors: Forte Alessandro M. , Woodward Robert L.
Published: February 2004

Joint inversions of seismic and geodynamic data are carried out in which we simultaneously constrain global‐scale seismic heterogeneity in the mantle as well as the amplitude of vertical mantle flow across the 670 km seismic discontinuity. These inversions reveal the existence of a family of three‐dimensional (3‐D) mantle models that satisfy the data while at the same time yielding predictions of layered mantle flow. The new 3‐D mantle models we obtain demonstrate that the buoyancy forces due to the undulations of the 670 km phase‐change boundary strongly inhibit the vertical flow between the upper and lower mantle. The strong stabilizing effect of the 670 km topography also has an important impact on the predicted dynamic topography of the Earth's solid surface and on the surface gravity anomalies. The new 3‐D models that predict strongly or partially layered mantle flow provide essentially identical fits to the global seismic data as previous models that have, until now, predicted only whole‐mantle flow. The convective vertical transport of heat across the mantle predicted on the basis of the new 3‐D models shows that the heat flow is a minimum at 1000 km depth. This suggests the presence at this depth of a globally defined horizon across which the pattern of lateral heterogeneity changes rapidly.
Differential rotations of north Kyushu Island related to middle Miocene clockwise rotation of SW Japan
Authors: Ishikawa Naoto
Published: February 2004

Paleomagnetic measurements were made on samples of Cretaceous granites and Tertiary sedimentary rocks from north Kyushu Island in order to clarify the kinematics of clockwise (CW) rotation of the SW Japan tectonic block at 15 Ma. A paleomagnetic direction from Oligocene sedimentary rocks in the northern part of north Kyushu Island indicates a significant CW rotation of the region (R=35.9°±11.0°) relative to eastern Asia, while the direction from Oligocene to early Miocene sedimentary rocks in the western part of north Kyushu Island shows no significant tectonic displacement. In situ directions of secondary magnetization from Eocene sedimentary rocks in the southwestern part of north Kyushu Island, and stable remanences from Cretaceous granites, provide CW deflections of declination. These paleomagnetic data, combined with those previously reported, suggest that the main part of north Kyushu Island (Main Kyushu) was rotated CW (36°) between 30 and 14 Ma relative to eastern Asia, whereas no significant tectonic displacement has taken place since 20 Ma in the western part (Western Kyushu). Considering regional geological structures, the paleomagnetic results indicate that the western extent of the SW Japan tectonic block which experienced middle Miocene CW rotation is truncated west of Main Kyushu. The paleomagnetic data also suggest that Main Kyushu rotated counterclockwise (28° ± 13°) relative to the main part of SW Japan between 30 and 14 Ma. This result implies that the westernmost part of the SW Japan block (Main Kyushu) did not experience as much rotation as its main part during CW rotation of the whole block.
Southern California permanent GPS geodetic array: Spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 Landers earthquake
Authors: Wdowinski Shimon , Bock Yehuda , Zhang Jie , Fang Peng , Genrich Joachim
Published: February 2004

The June 28, 1992 (Mw=7.3) Landers, California, earthquake was the first earthquake to be surveyed by a continuously operating Global Positioning System (GPS) array. The coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake. We employ a new spatial filtering technique that removes common‐mode errors from the coordinate time series. This approach provides precise estimates of site‐specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations. All sites indicate significant coseismic horizontal displacements of 5–65 mm with uncertainties of 1–2 mm. Horizontal displacements are in general agreement with elastic dislocation models, in particular for sites closer to the epicenter. Vertical displacements range from −13 to +7 mm with uncertainties of 2–4 mm. The observed vertical displacements in all cases show 5–10 mm more subsidence than expected from geodetic and seismic/geologic models. Significant postseismic horizontal displacements totaling 6±2 mm (10–20% of the coseismic displacement) are detected at the three sites closest to the epicenter. These displacements are modeled as a short‐term exponential relaxation with a decay time of 22±10 days superimposed on a longer‐term linear interseismic trend. Scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the epicenter provides a measure of site strain, which agrees well with the direction and magnitude determined from more precise laser strain meter data. The time series do not show any detectable preseismic displacements.
High‐resolution secular variation of the geomagnetic field in western Europe over the last 4 centuries: Comparison and integration of historical data from Paris and London
Authors: Alexandrescu Mioara , Courtillot Vincent , Le Mouël Jean‐Louis
Published: February 2004

Declination (D) and inclination (I) data from the Paris and London areas have been critically compiled by Alexandrescu et al. [1996a] and Malin and Bullard [1981], respectively. Declination data begin in the mid‐sixteenth century, and inclination data begin in the late sixteenth century. The data are remarkably well correlated between the two sites and can be used to construct two more complete and accurate synthetic series, in which many gaps or times with scattered or uncertain data can be amended. These synthetic series of D and I, believed to be valid for much of western Europe, are next filtered for external variations by applying an 11‐year smoothing. Secular variation (the first time derivative) is then computed. Secular variation of annual means of D shows the well‐known jerks of the twentieth century (1901, 1925, 1969, and 1978). Detailed analysis of monthly mean values confirms these events and further suggests jerks in 1913 and 1991, though it is still too early to be sure of the latter. Interestingly, the data confirm the existence of a large jerk in 1870. Analysis of the annual means from three other observatories supports the significance and extent of this event. From 1680 to 1870 there is no evidence of major jerks. Though more noisy and much shorter, the inclination data seem to confirm this general pattern and correlate rather well with those of D. In conclusion, pending discovery of other archives, we suggest that the two centuries from 1680 to 1870 may have been a period of smooth, parabolic secular variation devoid of jerks (in Western Europe). This “quiet” period was interrupted in 1870, ushering in a century of repeated jerks with a rather characteristic sawtooth pattern.
Initiation of antiplane shear instability under slip dependent friction
Authors: Campillo Michel , Ionescu Ioan R.
Published: February 2004

We study the initiation of an unstable antiplane elastodynamic shear process under slip‐weakening friction. We give an analytical expression of the slip that we intrepret using an eigenvalue analysis. Considering only the part of the solution associated with positive eigenvalues, we define a “dominant part” characterized by an exponential growth with time. An explicit formula is given for the dominant part that controls the development of the instability after the application of an initial perturbation on the surface or inside the elastic body. It shows that in response to a small initial perturbation the instability will develop in a limited spectral domain. The limiting wavenumber (or reciprocal critical length) is a function of the parameters of the friction law and the elastic properties. The part of the solution associated with negative eigenvalues (the “wave part”) becomes rapidly negligible when the instability develops. We found that in the initiation phase the displacement field in the elastic body has a simple exponential dependence on the coordinate perpendicular to the fault. Using the expression of the dominant part, we estimate the duration of the initiation phase. We show the accuracy of the theoretical analysis by comparison with numerical tests computed with an independent technique. Finally, we show how the initiation phase determines the evolution toward the dynamic rupture propagation. We introduce the critical patch length in a natural way. The transition between the initiation and the propagation stages is characterized by an apparent supersonic velocity of the rupture front.
Deformation of the Kenai Peninsula, Alaska
Authors: Cohen Steven C. , Freymueller Jeffrey T.
Published: February 2004

Crustal deformation on the Kenai Peninsula in southern Alaska has been studied using data obtained from Global Positioning System (GPS) measurements in 1993 and 1995 and leveling observations in 1964, immediately after the Prince William Sound earthquake. This analysis shows that the Kenai Peninsula has experienced as much as ∼900 mm uplift during the past 3 decades and that the uplift forms an ∼125 km wide elongate dome with its major axis trending southwest to northeast following the trend of the major tectonic features of the region. The averaged uplift rate between 1964 and 1995 is as high as 30 mm yr−1, although the current uplift rate may be substantially lower. The GPS measurements cast further doubt on previously suspect tide gauge data for Nikiski, Alaska, which indicated rapid postseismic uplift at this site located in northwest Kenai Peninsula adjacent to Cook Inlet. Examination of the three‐dimensional GPS data indicates that the eastern Kenai Peninsula is currently undergoing significant SSE to NNW contraction in response to North American‐Pacific Plate convergence. The horizontal velocities are consistent with the predictions of an elastic half‐space model for the interseismic deformation. This result, taken in combination with the small changes in uplift between 1993 and 1995, suggests that most of the present deformation is due to steady plate convergence rather than transient postseismic rebound.