bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of atmospheric azimuthal asymmetry on the analysis of space geodetic data
Authors: Chen G. , Herring T. A.
Published: February 2004

We develop a formalism for parameterizing and evaluating the effects of lateral variations in the properties of Earth's atmosphere on the propagation of microwave signals. A parametric form is incorporated into our analysis of very long baseline interferometry (VLBI) data, and the estimated atmospheric delay gradients are compared with those calculated from three‐dimensional weather analysis fields from the National Center for Environmental Prediction (NCEP). For a 12‐day series of experiments in January 1994, the VLBI and the NCEP analyses show common atmospheric gradient delays with amplitudes of up to 30 mm at 10° elevation angle. Comparison of the characteristics over a longer period of time reveals common mean north‐south gradients with amplitudes up to ≈10 mm at 10° elevation at midlatitudes. No discernible mean east‐west gradients were found in either data set. The root‐mean‐square (RMS) variations of the gradient effects, determined from the NCEP analysis, are similar in the north‐south and east‐west directions, with a typical RMS scatter of 6–10 mm at 10° elevation. After accounting for gradients, detailed analysis of the January 1994 VLBI data shows clearly that the residual station height variations of ≈10 mm at Westford, Massachusetts, are almost totally explained by the effects of atmospheric pressure loading.
Effect of crystal defects and internal stress on the domain structure and magnetic properties of magnetite
Authors: Özdemir Özden , Dunlop David J.
Published: February 2004

Domain structures in magnetite are very sensitive to crystal imperfections, which play a major role in hysteresis and remanence by hindering the motion of domain walls. Using the Bitter colloid technique, we have observed spike and closure domains of the style predicted by Néel [1944] around nonmagnetic inclusions, chemically altered regions, and grain boundaries in a natural single crystal of magnetite. Isolated inclusions within body domains have pairs of attached Néel spikes which reduce magnetostatic energy by diluting magnetic poles. In one example we calculated that spikes reduced the energy by a factor of 6–7. In some cases 71 °, 109° and 180° domain walls are pinned to defects either through spikes or via chains of subsidiary closure domains. One example of pinning by a spike gave a calculated microcoercivity of 0.54 mT, similar to the bulk coercive force of 0.5 mT for the crystal. “Colloid gaps” in 180° and other walls form lines parallel to a <111> easy axis and are evidence of underlying line defects, for example, dislocations, whose stress fields deflect the spins locally, weakening the magnetic field gradient above the walls. We have also observed bending of 180° walls anchored at pinning sites on the grain boundary, the first direct experimental evidence of the effect of internal stresses on the domain structure of magnetite. We determined internal stress magnitudes in the range 7–34 MPa from the observed linear dimension and transverse displacement of each bowed wall. Finally, we measured hysteresis curves on a companion magnetite crystal at temperatures T from ambient to the Curie point of 585°C. Observed coercivity Hc varies with T as λ111W0.5/Ms, in agreement with theoretical predictions of impedance of a wall of width w by dislocation stress fields. We therefore propose that the stability of remanence in multidomain magnetite is mainly due to pinning of domain walls by crystal defects.
Time dependent inversion of geodetic data
Authors: Segall Paul , Matthews Mark
Published: February 2004

The recent expansion of permanent Global Positioning System (GPS) networks provides crustal deformation data that are dense in both space and time. While considerable effort has been directed toward using these data for the determination of average crustal velocities, little attention has been given to detecting and estimating transient deformation signals. We introduce here a Network Inversion Filter for estimating the distribution of fault slip in space and time using data from such dense, frequently sampled geodetic networks. Fault slip is expanded in a spatial basis set ßk(x) in which the coefficients are time varying, s(x, t) = ∑k=1M ck(t)ßk(x) The temporal variation in fault slip is estimated nonparameterically by taking slip accelerations to be random Gaussian increments, so that fault slip is a sum of steady state and integrated random walk components. A state space model for the full geodetic network is formulated, and Kalman filtering methods are used for estimation. Variance parameters, including measurement errors, local benchmark motions, and temporal and spatial smoothing parameters, are estimated by maximum likelihood, which is computed by recursive filtering. Numerical simulations demonstrate that the Network Inversion Filter is capable of imaging fault slip transients, including propagating slip events. The Network Inversion Filter leads naturally to automated methods for detecting anomalous departures from steady state deformation.
Chemical remagnetization and burial diagenesis: Testing the hypothesis in the Pennsylvanian Belden Formation, Colorado
Authors: Banerjee Sanjay , Elmore R. Douglas , Engel M. H.
Published: February 2004

Lower Pennsylvanian Belden Formation carbonate rocks from Colorado were subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of two components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM‐250°C) remanence that is interpreted to be a modern, viscous magnetization. An intermediate unblocking temperature (250–400°C) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a thermoviscous remanent magnetization. Many limestones also contain a high unblocking temperature (400–570°C) remanence component which is interpreted to be a CRM. Fold tests from different parts of the basin indicate that the CRM was acquired either before or during Laramide folding. This CRM is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite. Hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the CRM is carried by single‐domain/pseudo single‐domain magnetite. Although elevated 87Sr/86Sr values indicate passage of radiogenic fluids through the limestones, the results of contact vein tests do not support the hypothesis that these fluids were responsible for the CRM. The time of CRM acquisition, which varies from late Paleozoic to Cretaceous, coincides with the modeled time of organic matter maturation in different parts of the basin. This suggests that diagenetic reactions, that were triggered by low to moderate burial temperatures, may have caused the magnetite authigenesis and probably gave rise to the CRM.
Effective media theory with spatial correlation for flow in a fracture
Authors: Walsh Joseph B. , Brown Stephen R. , Durham William B.
Published: February 2004

Standard effective media theory, real‐space renormalization, and first‐order perturbation theory all predict that a percolation threshold, where the electrical and hydraulic conductivity become zero, is reached when the area of contact is half of the total area. However, calculations of the flow properties of joints, whether simulated by continuous fields or discretely as a network of resistors, show that flow is maintained at appreciably larger contact areas. This behavior has been termed “channeling” in the literature. We have modified the standard effective media theory by introducing a rudimentary sort of nonrandomness in an attempt to simulate features like topographic hills and valleys. Using this new model, we calculate overestimates and underestimates of electrical and hydraulic conductivity as a function of the separation between the fracture surfaces. The underestimate is found to be very close to the standard effective media theory, real‐space renormalization, and first‐order perturbation theory results. The mean of the overestimate and underestimate is a good approximation to the published values found from calculations of electrical and hydraulic conductivities for simulations of real joints.
Sequential vein growth with fault displacement: An example from the Austin Chalk Formation, Texas
Authors: Lee Young‐Joon , Wiltschko David V. , Grossman Ethan L. , Morse John W. , Lamb William M.
Published: February 2004

To determine the opening and precipitation history and characteristics of vein‐forming fluids, analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack‐seal calcite veins in the Austin Chalk Formation near San Antonio, Texas. The veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development. They are composed of alternating millimeter‐ to submillimeter‐thick calcite veinlets and host lithons, occasionally crosscut by coarse, equant‐grained secondary calcite veins. Regular changes in δ18O (e.g., −2.6 to −5.6‰, Pee Dee belemnite (PDB)) of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed. A systematic δ18O decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the Austin Chalk. Relatively constant δ13C (approximately +1.4±0.4‰, PDB) for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk. There is no spatial variation in trace element composition of the calcite veinlets along the length of veins. Low Sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of Sr in calcite during vein formation. Normal faults in the Cretaceous Austin Chalk were conduits to upwardly mobile vein‐forming fluids.
A plate model for the simulation of trace element fractionation during partial melting and magma transport in the Earth's upper mantle
Authors: Vernières Jacques , Godard Marguerite , Bodinier Jean‐Louis
Published: February 2004

We propose a new plate model for the simulation of trace element transfer during magmatic and metasomatic processes taking place in the Earth's upper mantle. As in previously published plate models, porous flow is accounted for by propagation of fluid batches through macrovolumes of mantle rocks. Being released from spatiotemporal constraints, the plate model allows much more freedom than the one‐dimensional porous‐flow models for the simulation of fluid‐rock interactions. Hence this approach may account for a wide range of mantle processes, including melt extraction during compaction of molten peridotites, porous flow associated with Chromatographic effects, or fluid‐rock reactions occurring upon melt infiltration at the base of the conductive mantle. The applications presented in this study show several results consistent with published trace element data for mantle rocks and basaltic volcanism. In particular, the proposed models may provide simple explanations for (1) the ultra‐rare‐earth‐element‐depleted composition of peridotites and interstitial melts residual after mid‐ocean ridge basalt extraction, (2) the negative correlation between light rare earth element / heavy rare earth element (LREE/HREE) ratio and refractory character of peridotites, as observed in several suites of mantle rocks, and (3) the origin of ultra‐LREE‐enriched meta‐somatic fluids infiltrated in the lithospheric mantle.
Source array analysis of coda waves near the 1989 Loma Prieta, California, mainshock: Implications for the mechanism of coseismic velocity changes
Authors: Dodge Douglas A. , Beroza Gregory C.
Published: February 2004

Coda waves are often considered to be generated by backscattering of primary waves from randomly distributed heterogeneities in the crust; however, modeling and experimental work indicates that much of the coda may be due to scattering near the receiver. Knowing where the coda is generated is important for several reasons. Coda waves have been used to characterize site amplification, scattering and attenuation throughout the crust, and velocity changes associated with large earthquakes; however, a lack of knowledge of where coda waves are generated can lead to ambiguous interpretations. In this study we analyze 26 s of coda waves recorded at up to 78 stations using slowness stacking on two source arrays and find that at nearly all stations, regardless of distance from the source, nearly all the arrivals are within one hemisphere, most are strongly clustered near the direct arrival, and the clusters are, in general, symmetrically disposed about the array axis (fault plane). This finding suggests that the coda consists primarily of waves scattered near the stations rather than waves scattered throughout the crust. This result holds even at stations very close to the sources where the amount of coda examined is about 6 times the direct S travel time, and it suggests that much of the coseismic velocity decrease associated with the 1989 Loma Prieta earthquake [Ellsworth et al., 1992] occurred in the shallow crust near the stations. One possible mechanism for inducing such a change is a decrease in the mean stress from the mainshock slip; however, on the basis of static stress modeling, we find that this mechanism would produce a velocity increase in the region where velocity was observed to decrease. We conclude that the velocity decrease was more likely caused by crack opening or crack coalescence due to strong shaking from the mainshock.
Monitoring resistivity change in Parkfield, California: 1988–1995
Authors: Park Stephen K.
Published: February 2004

Continuous electric field measurements from 1988–1995 with a telluric array straddling the San Andreas fault in Parkfield, California have been analyzed for changes in electrical resistivity associated with the four largest (Mb>4.0) local earthquakes and three larger but more distant California earthquakes (1989 Loma Prieta, 1992 Landers, 1994 Northridge). Treatment of the propagation of errors through the analysis process yields formal estimates of the uncertainties in the results and thus a definition of anomalous variations based on statistical confidence. Statistically significant variations of the telluric coefficients were associated with none of the four local earthquakes and none of the California earthquakes. An obvious, but mistaken, conclusion from this disappointing result is that resistivity changes do not occur prior to earthquakes; previous observations in both the field and laboratory have shown that they do occur. A more likely explanation of the current result is that the telluric array is not sufficiently sensitive to resistivity changes associated with such small earthquakes. The next Parkfield earthquake, while overdue, should be of sufficient magnitude to test this explanation. Results from this experiment have shown the need for continuous, redundant measurements in precursory monitoring studies.
Stress evolution in southern California and triggering of moderate‐, small‐, and micro‐size earthquakes
Authors: Deng Jishu , Sykes Lynn R.
Published: February 2004

We calculate the evolution of stresses in southern California, extending the study of Deng and Sykes [1997] by increasing from 6 to 36 the number of earthquakes for which coseismic changes in stress are computed and by expanding from M≥6 to M≥1.8 the range of magnitudes M of events whose focal mechanism solutions are examined in the context of the evolving stress field. The cumulative stress on a given date is calculated with respect to an arbitrary zero baseline just before the 1812 Wrightwood earthquake. By taking into account the long‐term stress loading associated with 98 fault segments and coseismic stress changes for 36 significant earthquakes, our calculations indicate that more than 85% of M≥5 earthquakes from 1932–1995 occurred in regions of positive change in Coulomb failure function (ΔCFF). Most of the remaining about 15% earthquakes that occurred in areas of negative ΔCFF fall very close to boundaries between positive and negative ΔCFF, some of which are sensitive to the less well controlled slip distributions of the earliest historic events. Calculations also show that from 1981 until just before the 1992 Landers earthquake more than 85% of small‐ (M≥3) and micro‐size (M≥1.8) shocks in the Seeber and Armbruster [1995] catalog with mechanisms involving either NW trending right‐lateral or NE trending left‐lateral strike‐slip faulting occurred in regions of positive ΔCFF. The ratio of encouraged to all small‐ and micro‐size events reaches a high value of about 88% if an apparent coefficient of friction μ between 0.0 and 0.6 is used. The highest percentage of earthquakes occurred in areas where stress is about 1 MPa above the 1812 baseline. Most (66%) events occurred in regions of ΔCFF between 0.0 and 2.0 MPa. The upper limit indicates that the approximate range of stress variation in the earthquake cycle is of the order of 2.0 MPa. The fact that the locations of most moderate‐, small‐, and micro‐size earthquakes are still related to stress changes remaining from large historical events might be used to constrain slip distribution of some of those earthquakes and to constrain the locations of future significant events.
Ages of Late Holocene earthquakes on the central Garlock fault near El Paso Peaks, California
Authors: McGill Sally , Rockwell Tom
Published: February 2004

A trench across the central Garlock fault within a small playa adjacent to El Paso Mountains revealed buried fissures and fault scarps that provide strong evidence for five surface rupturing earthquakes within the past 5 kyr. Weaker evidence suggested the possibility of three additional events within this same time period. These observations indicate an average recurrence interval of 700 to 1200 years. Nearly all 28 radiocarbon dates on samples of detrital charcoal, wood and shell are in correct stratigraphic order. These dates constrain the ages of the individual faulting events and indicate that the recurrence interval is not periodic but is quite irregular. Individual recurrence intervals range from 190 to 1545 years (if all eight events are actual earthquakes) or from 190 to 3405 years (if only the five well‐documented events represent earthquakes). The two most recent earthquakes occurred within the past 550 years, with preferred ages of about A.D. 1790 and A.D. 1600. The A.D. 1790 event ruptured a narrower width of the fault zone, and may have been a smaller earthquake than the others. The radiocarbon dates from the trench also indicate that the sedimentation rate on the playa has fluctuated within the past 5 kyr. Low sedimentation rates (0.1 to 0.35 mm/yr) and infrequent floods (≤1 per century) prevailed from about 2000 B.C. to A.D. 100 and from A.D. 500 to 1300. During the remaining intervals within the past 5 kyr, the sedimentation rate averaged 1.0 to 1.7 mm/yr and major floods occurred two to four times per century. The changes in sedimentation rate and flood frequency may reflect climatic changes, or they may simply reflect changes in drainage patterns.
Chemical and isotopic evidence of gas‐influenced flow at a transform plate boundary: Monterey Bay, California
Authors: Martin Jonathan B. , Orange Daniel L. , Lorenson Thomas D. , Kvenvolden Keith A.
Published: February 2004

Chemical and isotopic compositions of pore fluids document upward flow through communities of vesicomyid clams in Monterey Bay, California. Within the clam communities, the sulfate reduction zone is only 10 cm thick, and Ca and Mg concentrations decrease to values as low as 2.2 mM and 34.5 mM, respectively, at depths less than 30 cm below the sediment‐water interface. Less than 5 m outside the communities, the base of the sulfate reduction zone is deeper than the greatest penetration of the cores (∼30 cm), and Ca and Mg exhibit only minor changes from seawater values. The sediment exhibits no significant variation in grain size, mineralogy, organic carbon, nitrogen, or carbonate content throughout the region. The composition of pore fluid within clam communities results from upward flow of altered fluid rather than different diagenetic reactions within and outside the communities. Isotopically light dissolved inorganic carbon (DIC), with δ13C values ranging from −3.2 to −54.1‰, could reflect carbon sources from either oxidized thermogenic methane and/or a mixture of oxidized microbial methane and solid organic carbon. The C1/(C2+C3) ratios (ranging from 34 to 1142) and the hydrogen and carbon isotopic compositions of methane (δD values of −109 to −156‰; δ13C values of −30.6 to −86.6‰) suggest that methane is primarily microbial but that a minor component could be thermally generated. Any thermogenic methane would have migrated from great depths, possibly >2 km. The presence of methane is likely to contribute to fluid flow by reducing the density of the fluids. Past fluid migration and venting are reflected by widespread carbonate mineralization at the sediment‐water interface. This mineralization and the geographic distribution and proportions of microbial and thermogenic methane suggest that vent sites migrate when permeability is reduced during carbonate cementation. These results demonstrate that along with convergent and divergent plate boundaries, transform plate boundaries are characterized by fluid flow and that the flow may be widespread, occurring at sites away from fault zones.
Upper Anisian to Lower Carnian magnetostratigraphy from the Northern Calcareous Alps (Austria)
Authors: Gallet Yves , Krystyn Leopold , Besse Jean
Published: February 2004

We present a magnetostratigraphic study of three pelagic limestone sections (Gamsstein, Mendlingbach and Mayerling) from the eastern part of the Northern Calcareous Alps. Together these sections, which contain a rich conodont fauna, yield a sedimentary record encompassing the uppermost Anisian, the Ladinian and the Lower Carnian. Thermal demagnetization and isothermal rémanent magnetization experiments indicate that the magnetization is essentially carried by a mineral of the magnetite family. The high unblocking temperature components isolated from the three sections provide clear magnetic polarity zonations. Correlations between these results, based on the biostratigraphic data and tephrochronology, allow the construction of a composite magnetic polarity sequence from the Illyrian substage (Upper Anisian) to the Julian 2 zone (Lower Carnian). This sequence contains 17 well‐defined polarity reversals, and eight more poorly defined intervals. Correlations can be suggested between the new data and other magnetostratigraphic results previously obtained from marine sections. We estimate that the magnetic reversal frequency was 2.5 to 3.6 reversals per million years during the Ladinian.
Kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 Northridge, California, earthquake
Authors: Unruh Jeffrey R. , Twiss Robert J. , Hauksson Egill
Published: February 2004

Geodetic observations of surface deformation associated with the 1994 Northridge, southern California, earthquake generally are reproduced by simple models of a large‐scale elastic dislocation on a blind or buried thrust fault. The smaller‐scale aftershocks of the Northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock. These aftershocks, averaged over volumes that are large relative to their rupture radii, reflect a distributed, permanent deformation that is accommodated by local brittle fracture. We use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain, and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain. Aftershock deformation that occurred at depths below about 6 km, and which is associated with the primary rupture zone, is consistent with slow continuation of the southwest‐side‐up reverse slip on the blind Northridge thrust fault. In contrast, aftershock deformation from the upper 5–7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal NE‐SW shortening and horizontal NW‐SE (i.e., fault‐parallel) extension. This pattern of deformation is similar to that associated with the mainshock, as observed geodetically and as calculated from the elastic dislocation model. We interpret that the aftershock activity in the hanging wall represents the quasi‐ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block. The aftershocks along the mainshock rupture zone are interpreted as resulting from either (1) the time‐dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or (2) the continued slip in the weakened fault zone driven by the deformation of a ductile‐elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust. In either case, the aftershock activity suggests that the crust undergoes quasi‐ductile flow as a brittle‐elastic material, and is not a strictly elastic material.
Present‐day plate motion constraint on mantle rheology and convection
Authors: Wen Lianxing , Anderson Don L.
Published: February 2004

Large‐scale mantle convection, to first order, is a system driven by interior density anomalies, modulated by variable plate thickness and extreme rheology variations at the top of the mantle. The rheological difference between oceanic and continental regions significantly influences the surface velocity. We apply a three‐dimensional Newtonian viscous flow model to explain the large‐scale present‐day plate motions. The density anomalies are derived from seismic tomography and a slab model. With a viscosity difference of a factor of 30–60 between continents and oceans in the upper 90 km of the Earth, we are able to explain both the observed large‐scale poloidal and toroidal plate motions. The viscosity difference between continental and oceanic regions has major control on both the poloidal‐toroidal kinematic energy partitioning and the pattern of toroidal motion. Nonlinear rheology can help establish toroidal motion. Plate motions can be explained by assuming either layered or whole mantle flow. In order to match the amplitude of observed plate motions, the value of the reference viscosity (corresponding to that of between 400 to 670 km depth) is 1.6×1021 Pa s for layered mantle flow and 3.2×1021 Pa s for whole mantle flow. However, the predicted net rotation of the lithosphere, from both layered and whole mantle flow models, is very small and cannot account for the amplitude of the net rotation obtained from the plate tectonic models assuming a fixed hotspot reference frame.
Ductile shear zones beneath strike‐slip faults: Implications for the thermomechanics of the San Andreas Fault Zone
Authors: Thatcher Wayne , England Philip C.
Published: February 2004

We have carried out two‐dimensional (2‐D) numerical experiments on the bulk flow of a layer of fluid that is driven in a strike‐slip sense by constant velocities applied at its boundaries. The fluid has the (linearized) conventional rheology assumed to apply to lower crust/upper mantle rocks. The temperature dependence of the effective viscosity of the fluid and the shear heating that accompanies deformation have been incorporated into the calculations, as has thermal conduction in an overlying crustal layer. Two end‐member boundary conditions have been considered, corresponding to a strong upper crust driving a weaker ductile substrate and a strong ductile layer driving a passive, weak crust. In many cases of practical interest, shear heating is concentrated close to the axial plane of the shear zone for either boundary condition. For these cases, the resulting steady state temperature field is well approximated by a cylindrical heat source embedded in a conductive half‐space at a depth corresponding to the top of the fluid layer. This approximation, along with the application of a theoretical result for one‐dimensional shear zones, permits us to obtain simple analytical approximations to the thermal effects of 2‐D ductile shear zones for a range of assumed rheologies and crustal geotherms, making complex numerical calculations unnecessary. Results are compared with observable effects on heat flux near the San Andreas fault using constraints on the slip distribution across the entire fault system. Ductile shearing in the lower crust or upper mantle can explain the observed increase in surface heat flux southeast of the Mendocino triple junction and match the amplitude of the regional heat flux anomaly in the California Coast Ranges. Because ductile dissipation depends only weakly on slip rate, faults moving only a few millimeters per year can be important heat sources, and the superposition of effects of localized ductile shearing on both currently active and now inactive strands of the San Andreas system can explain the breadth of the heat flux anomaly across central California.
New transfer functions for nutations of a nonrigid Earth
Authors: Dehant V. , Defraigne P.
Published: February 2004

There are differences between the observed values of nutation and the computed ones based on the International Astronomical Union (IAU) 1980 adopted nutation series. These differences can be expressed in the frequency domain where they may reach several milliarc seconds, a level that is too large for practical use. This paper aims to resolve part of these differences by computing a new theoretical model accounting for additional geophysical effects. A new transfer function is computed, based on an Earth initially in a nonhydrostatic equilibrium corresponding to the steady state associated with the present mantle convection. The mantle mass anomalies are deduced from seismic tomography data, and the flow‐induced boundary deformations are computed from internal loading for an Earth made up of a viscous inner core, a liquid outer core, a viscous mantle, and a solid lithosphere. In this way, a new core‐mantle boundary (CMB) flattening is obtained, which gives the observed free core nutation (FCN) period. Furthermore, the global Earth dynamical flattening induced by the mass anomalies in the mantle associated with tomography and by the mass anomalies due to the computed boundary deformations, is in agreement with the J2 form factor (or the observed precession constant). In addition to this nonhydrostatic initial state, the rheology of the mantle is considered as inelastic. The transfer function for nutation is then obtained by numerical integration of motion equations from the Earth's center up to the surface to provide a model which is completely self‐consistent. In order to validate our model, the transfer function is convolved with new rigid Earth nutations, ocean corrections are applied and the final results are then compared with the observed nutations or with the International Earth Rotation Service (IERS) nutation series. The residuals between our model and the observation are about 3 times smaller than those between the IAU 1980 adopted model and the observation. However, our model still presents residuals above the observational error; this is particularly true for the out‐of‐phase part of the residuals, while the in‐phase part gives very small residuals (improvement of about 1 order of magnitude). A further step in this study is a refinement of the modeling of geophysical fluids (core, ocean, and atmosphere).
Segmentation of the northern San Jacinto Fault Zone, southern California
Authors: Sanders Christopher , Magistrale Harold
Published: February 2004

Accurate forecasting of large earthquakes in the San Jacinto fault zone depends on the correct determination of fault segmentation. We have searched for discontinuities in the fault zone using as data geological maps of the fault traces, relocated earthquake hypocenters, focal mechanisms, slip rates, and historic large earthquake rupture zones. We identify nine principal discontinuities in the northern San Jacinto fault zone. These are characterized as structural discontinuities (bends, steps, branches, intersections), geodetic discontinuities (slip rate), and lithologic discontinuities. Most of the discontinuities have more than one defining characteristic. We also analyzed the local strain field orientation along the fault zone. Significant lateral discontinuities in the strain‐field orientation are coincident with many of the structural and geodetic discontinuities we identify. Segment lengths defined from the data vary from about 7 to 35 km. This range of segment lengths is consistent with the lengths of large historical earthquake ruptures. However, evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rupture multiple adjacent segments. In these cases, maximum magnitudes may be larger than those observed historically. Because of the short historic record and the sparse paleoseismic record, probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments. Ours and previous delineations of segment lengths have varied considerably, indicating that assumed segment length is a major source of uncertainty in the recurrence calculations. This source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates.
Experimental laboratory determination of the dynamic elastic properties of wet, drained shales
Authors: Hornby Brian E.
Published: February 2004

The ultrasonic anisotropic elastic properties of drained, saturated shales were measured as a function of confining pressure. Two shales were characterized in this study: a Jurassic outcrop shale retrieved under the sea in a saturated state and a Kimmeridge Clay shale cut from a North Sea borehole. Both shales were highly anisotropic, both texturally, as revealed by scanning electron microscopy analysis, and elastically, as measured ultrasonically in the laboratory. The strongest anisotropy was seen in the Kimmeridge Clay shale, where up to 38% compressional wave anisotropy (Thomsen's parameter ε) and up to 58% shear wave anisotropy (Thomsen's parameter γ) were observed. In addition, for both shales, ε and γ were found to decrease as a function of increasing confining pressure, with the pore pressure drained to atmosphere, while the anellipticity (deviation of the slowness surfaces from ellipses) was found to be positive and decreased as a function of increasing confining pressure. Accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure, from 10.5 to 8.5% for the Jurassic shale. The decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid‐filled porosity.
Seismic characteristics of cavity decoupled explosions in limestone: An analysis of Soviet high explosive test data
Authors: Murphy J. R. , Kitov I. O. , Rimer N. , Adushkin V. V. , Barker B. W.
Published: February 2004

During the summer of 1960, Soviet scientists conducted a series of high‐explosive, cavity decoupling tests in a mine in Kirghizia. These decoupled tests were carried out in a variety of mined cavities in limestone, including spherical cavities with diameters ranging from 3.5 to 10 m as well as nonspherical cavities with volumes of about 25 m3. The experiments of this test series consisted of 10 tamped and 12 decoupled explosions having yields of 0.1, 1.0, and 6.0 t, and seismic data were recorded at locations in the mine over a distance range extending from about 10 to 250 m from the sources. These data provide valuable new insight into the dependence of decoupling effectiveness on variables such as cavity volume, cavity shape, and charge emplacement geometry. In particular, analyses indicate that chemical explosions at a depth of 290 m in limestone are essentially fully decoupled in spherical cavities with scaled cavity radii larger than about 27 m/kt⅓ and that the low‐frequency decoupling effectiveness under such conditions is approximately independent of cavity shape for roughly cylindrical cavities with length‐to‐width ratios of as much as 6–12. These results suggest that the possibility of cavity decoupling in nonspherical cavities in hard rock media will have to be carefully evaluated in establishing the seismic verification regime for the Comprehensive Test Ban Treaty.
Effects of slip, slip rate, and shear heating on the friction of granite
Authors: Blanpied Michael L. , Tullis Terry E. , Weeks John D.
Published: February 2004

The stability of fault slip is sensitive to the way in which frictional strength responds to changes in slip rate and in particular to the effective velocity dependence of steady state friction Δμss/Δ ln V. This quantity can vary substantially with displacement, temperature and slip rate. To investigate the physical basis for this behavior and the possible influence of shear heating, we slid initially bare granite surfaces in unconfined rotary shear to displacements of hundreds of millimeters at normal stresses, σn of 10 and 25 MPa and at room temperature. We imposed step changes in slip rate within the range 10−2 to 103.5 μm/s and also monitored frictional heating with thermistors embedded in the granite. The transient response of μ to slip rate steps was fit to a rate‐ and state‐dependent friction law using two state variables to estimate the values of several parameters in the constitutive law. The first 20 mm of slip shows rising friction and falling Δμss/Δ ln V; further slip shows roughly constant friction, Δμss/Δ ln V and parameter values, suggesting that a steady state condition is reached on the fault surface. At V ≤ 10 μm/s, Δμss/Δ ln V = −0.004 ± 0.001. At higher rates the response is sensitive to normal stress: At σn = 25 MPa granite shows a transition to effective velocity strengthening (Δμss/Δ ln V = 0.008 ± 0.004) at the highest slip rates tested. At 10 MPa granite shows a less dramatic change to Δμss/Δ ln V ≈ 0 at the highest rates. The maximum temperature measured in the granite is ∼60°C at 25 MPa and 103.5 μm/s. Temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogeneous frictional heating over the sliding surface. The simplest interpretation of our measurements of Δμss/Δ ln V is that the granite is inherently velocity weakening (∂μss/∂ ln V < 0) and temperature strengthening (∂μss/∂T−1 < 0) at all velocities. At high slip rates the response of μ to changes in temperature from shear heating may outweigh the response to changing velocity, such that the net effect Δμss/Δ ln V > 0 mimics velocity strengthening. These results have implications for the frictional behavior of faults during earthquakes. High slip rates may cause a switch to effective velocity strengthening which could limit peak coseismic slip rate and stress drop. For fluid‐saturated faults, strengthening by this mechanism may be partly or fully offset by weakening due to thermal pressurization of a poorly drained pore fluid.
Growth fault/rollover systems: Birth, growth, and decay
Authors: Mauduit Thomas , Brun Jean Pierre
Published: February 2004

Growth fault/rollover systems are extremely common structures of thin‐skinned extensional systems resulting from gravity gliding above salt. Their development is studied using laboratory experiments based on brittle‐ductile models where the sedimentary cover and synkinematic sediments are simulated using brittle sand and the underlying decollement layer by viscous silicone putty. The experiments are compared to natural examples from seismic lines. The experimental results are used to present the evolution of growth fault rollover systems, these being the result of a finite amplitude instability with three distinct stages: birth, growth, and decay. Each of these stages is identified with typical structures which can be recognized on seismic sections. A discussion is presented on the interactions in time and space between synthetic and antithetic listric faults parallel and transverse to the direction of displacement.
Crustal structure of the Mid‐Atlantic Ridge south of the Kane Fracture Zone from seafloor and sea surface gravity data
Authors: Ballu Valérie , Dubois Jacques , Deplus Christine , Diament Michel , Bonvalot Sylvain
Published: February 2004

Seafloor and sea surface gravity data are inverted together to construct a model for the near‐axis crustal structure of a slow spreading ridge. The seafloor data set offers two main advantages: it allows us to recover shorter‐wavelengths signal and to constrain the value of a potential field at two different levels. The model we propose here would not have been derived from sea surface data alone. It is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the Mid‐Atlantic Ridge at Kane (MARK) area, during the Hydrosnake (1988) and Gravinaute (1993) cruises. The primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge. First, a forward two‐dimensional (2‐D) model of the crustal structure across the axis is fit to observed gravity anomalies, using constraints from geological and structural observations. Bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling. This forward model is the starting point of a 2‐D Monte Carlo inversion of seafloor and sea surface data. In addition to the crustal thickness variations along‐axis, our data document the amplitude variations of the crustal thickness and/or its density in the across‐axis direction. The model resulting from our inversion exhibits several features of the crustal structure in the MARK area: (1) The presence of a low‐density (Δρ =−300±50 kg/m3) body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber, or more probably to a highly hydrothermally fissured zone. (2) Both long‐ and short‐wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain: the mean value and the amplitude of Bouguer anomalies are higher on the western part. This difference suggests that axial processes, in this area, are very asymmetric. (3) Abyssal hills are not associated with a single gravity signature: for instance, on the west side of the axis, one of the explored hills has no Bouguer anomaly and is interpreted as a neovolcanic ridge, whereas the others are associated with a shifted Bouguer anomaly high and are interpreted as having thinner magmatic crust. (4) The last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops. In the MARK area, we find that “Pink Hill,” a topographic high where serpentinized peridotites are outcropping, is much more serpentinized on its east flank, toward the axial valley, than on its west flank. Alteration occurring mainly by fluid circulation through faulted zones, the asymmetric serpentinization suggests that deep‐origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism.
Magnetization of axial lavas from the southern East Pacific Rise (14°–23°S): Geochemical controls on magnetic properties
Authors: Gee Jeff , Kent Dennis V.
Published: February 2004

Although the spatial association of iron‐rich lavas and high‐amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8–16% FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0–6 ka) of axial lavas from this ultrafast spreading ridge (∼150 mm/yr full rate) effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low‐temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size‐related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulvöspinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron‐rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R=0.63). Because the range of iron contents in mid‐ocean ridge basalts is limited, the best fit slope (4.44 A/m per %FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (∼10 A/m at 8.5% FeO* and ∼50 A/m at ∼16% FeO*). Finally, we demonstrate that along‐axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.
Oxide‐metal equilibria to 2500°C and 25 GPa: Implications for core formation and the light component in the Earth's core
Authors: O'Neill Hugh St. C. , Canil Dante , Rubie David C.
Published: February 2004

The solubility of O in Fe‐rich liquid metal in equilibrium with magnesiowüstite (Mg,Fe)O was investigated at pressures to 25 GPa and temperatures to 2500°C in a 6–8 type multianvil apparatus. The experiments were designed to produce near millimetre sized pools of quenched liquid metal free of other phases, so that the equilibrium composition of the liquid metal could be recovered, taking into account the effects of quench modification. The amounts of O found in the metal are relatively small (<1 wt %) and decrease with increasing pressure at constant temperature and Mg/(Mg+Fe) ratio of the coexisting magnesiowüstite. The solubility of Si varies inversely with O and does not increase substantially with pressure when in equilibrium with mantle compositions. Experiments at 25 GPa in which silica activity is buffered by coexisting MgSiC>3‐perovskite produce only small amounts of Si (∼1 wt %) dissolved in Fe‐rich metal. These results, in conjunction with cosmochemical constraints on the bulk composition of the Earth (the depletion of the Earth in cosmochemically volatile elements), leave the identity of the putative “light component” in the Earth's core as an enigma. It is difficult to account for the light component if it constitutes more than just a few percent of the core. The experiments also measure the partitioning of Fe, Ni, Co, Cr, and Ti between the Fe‐rich liquid metal and magnesiowüstite phases. Ti is not significantly siderophile under the investigated conditions. When normalized to a constant metal/oxide partition coefficient for Fe, both Ni and Co become less siderophile with increasing pressure and Cr becomes more siderophile. The effect of temperature on Ni and Co partitioning is small at high temperatures, but important for Cr. The presently observed mantle abundances of Fe, Ni, Co, and Cr cannot be explained by equilibrium partitioning into the metal of the Earth's core under the pressure temperature conditions covered by the present experiments.