bluuebunny/crossref_metadata_2025 huggingface dataset:
Heterogeneous distribution of the dynamic source parameters of the 1999 Chi‐Chi, Taiwan, earthquake
Authors: Zhang Wenbo , Iwata Tomotaka , Irikura Kojiro , Sekiguchi Haruko , Bouchon Michel
Published: May 2003

The spatial and temporal distribution of the stress on the fault plane of the 1999 Chi‐Chi, Taiwan, earthquake is calculated from kinematic inversion results using a three‐dimensional finite difference method for solving the elastodynamic equations. We analyze the relations between stress and slip for all grid positions on the fault, and use these relations to infer the friction law for the rupture. The dynamic source parameters were also determined. Our results show that for most of the points on the fault, the relation between stress and slip was consistent with the slip‐weakening law during the rupture process, especially for those points with large slip. However, consistency with the velocity‐weakening law is not clear from the observed relation between stress and slip velocity. The distributions of the dynamic parameters on the fault are very heterogeneous. The peak value of the static stress drop is ∼35 MPa. In general, high stress drop occurred in the areas with large slip. The estimated strength excesses are generally small suggesting that the tectonic shear stress had reached the level of the fault strength before the main shock. The slip‐weakening distanceDcand the fracture energyGcare proportional to the final slip. The aftershock activity correlates with the spatial distribution of dynamic source parameters. Usually, the aftershocks near the fault plane are concentrated in regions with small or negative static stress drop, and there were few aftershocks in regions which had large values of critical slip‐weakening distanceDc.
Is there a single surge mechanism? Contrasts in dynamics between glacier surges in Svalbard and other regions
Authors: Murray Tavi , Strozzi Tazio , Luckman Adrian , Jiskoot Hester , Christakos Panos
Published: May 2003

During the 1990s, Monacobreen, a 40‐km‐long tidewater glacier in Svalbard, underwent a major surge. We mapped the surge dynamics using ERS synthetic aperture radar images, differential dual‐azimuth interferometry and intensity correlation tracking. A series of 11 three‐dimensional (3‐D) velocity maps covering the period 1991–1997 show a months‐long initiation and years‐long termination to the surge, with no indication of a surge front travelling downglacier. During the surge, the front of the glacier advanced ∼2 km, the velocity and derived strain rate increased by more than an order of magnitude, and maximum ice flow rates measured during 1994 were ∼5 m d−1. The spatial pattern of both velocity and strain rate was remarkably consistent and must therefore be controlled by spatially fixed processes operating at the glacier bed. We combine these results with those published in the literature to construct a typical Svalbard glacier surge cycle and compare this to surge dynamics of glaciers from other cluster regions, especially those of Variegated Glacier in Alaska. The strong contrast in dynamics suggests that there exist at least two distinct surge mechanisms.
Paleomagnetism and magnetostratigraphy of the lower Glen Canyon and upper Chinle Groups, Jurassic‐Triassic of northern Arizona and northeast Utah
Authors: Molina‐Garza Roberto S. , Geissman John W. , Lucas Spencer G.
Published: May 2003

Twenty‐eight selected sites (individual beds) in the Moenave Formation at the Echo Cliffs, northern Arizona, strata give a Hettangian paleomagnetic pole at 63.7°N, 59.7°E (dp = 2.6°, dm = 5.1°). The Wingate Sandstone and Rock Point Formation at Comb Ridge, southeast Utah, provide a Rhaetian paleopole at 57.4°N, 56.6°E (N = 16 sites; dp = 3.4, dm = 6.5). High unblocking temperatures (>600°C), high coercivity, and data analyses indicate that the characteristic magnetization is primarily a chemical remanence residing in hematite. The Hettangian and Rhaetian poles are statistically indistinguishable (at 95% confidence), they resemble existing data for the Glen Canyon Group, and they provide further validation to the J1 cusp of the North American apparent pole wander path (APWP). The red siltstone and upper members of the Chinle Group, on the south flank of the Uinta Mountains, northern Utah, define a Rhaetian pole at 51.6°N, 70.9°E (N = 20 sites; dp = 3.5°, dm = 6.9°). The Gartra and upper members of the Chinle Group in the north flank of the Uinta Mountains, give paleopoles at 52.0°N, 100.3°E (N = 6 sites; dp = 5.4°, dm = 10.5°) and 50.9°N, 50.1°E (N = 5 sites; dp = 8.8°, dm = 17.5°), respectively. These data indicate no significant rotation of the Uinta Mountains with respect to the craton. In total, data for the plateau and its bordering region of Cenozoic uplifts support estimates of small rotation of the plateau and provide evidence against the hypothesis of a Late Triassic standstill of the North American APWP. Our magnetostratigraphic results are consistent with lithographic and biostratigraphic data that place the Triassic‐Jurassic boundary within the Dinosaur Canyon Member of the Moenave Formation, not at a regional hiatus.
The 13 January 2001 El Salvador earthquake: A multidata analysis
Authors: Vallée Martin , Bouchon Michel , Schwartz Susan Y.
Published: May 2003

On 13 January 2001, a large normal faulting intermediate depth event (Mw= 7.7) occurred 40 km off the El Salvadorian coast (Central America). We analyze this earthquake using teleseismic, regional, and local data. We first build a kinematic source model by simultaneously invertingPandSHdisplacement waveforms and source time functions derived from surface waves using an empirical Green's function analysis. In an attempt to discriminate between the two nodal planes (30° trenchward dipping and 60° landward dipping), we perform identical inversions using both possible fault planes. After relocating the hypocentral depth at 54 km, we retrieve the kinematic features of the rupture using a combination of the Neighborhood algorithm ofSambridge[1999]and the Simplex method allowing for variable rupture velocity and slip. We find updip rupture propagation yielding a centroid depth around 47 km for both assumed fault planes with a larger variance reduction obtained using the 60° landward dipping nodal plane. We test the two possible fault models using regional broadband data and near‐field accelerograms provided byUniversidad Centro Americana[2001]. Near‐field data confirm that the steeper landward dipping nodal plane is preferred. Rupture propagated mostly updip and to the northwest, resulting in a main moment release zone of approximately 25 km × 50 km with an average slip of ∼3.5 m. The large slip occurs near the interplate interface at a location where the slab steepens dip significantly. The occurrence of this event is well‐explained by bending of the subducting plate.
Spacing of faults at the scale of the lithosphere and localization instability: 1. Theory
Authors: Montési Laurent G. J. , Zuber Maria T.
Published: May 2003

Large‐scale tectonic structures such as orogens and rifts commonly display regularly spaced faults and/or localized shear zones. To understand how fault sets organize with a characteristic spacing, we present a semianalytical instability analysis of an idealized lithosphere composed of a brittle layer over a ductile half‐space undergoing horizontal shortening or extension. The rheology of the layer is characterized by an effective stress exponent,ne. The layer is pseudoplastic if 1/ne= 0 and forms localized structures if 1/ne< 0. Two instabilities grow simultaneously in this model: the “buckling/necking instability” that produces broad undulations of the brittle layer as a whole, and the “localization instability” that produces a spatially periodic pattern of faulting. The latter appears only if the material in the brittle layer weakens in response to a local perturbation of strain rate, as indicated by 1/ne< 0. Fault spacing scales with the thickness of the brittle layer and depends on the efficiency of localization. Localization is more efficient for more negative 1/ne, leading to more widely spaced faults. The fault spacing is related to the wavelength at which different deformation modes within the layer enter a resonance that exists only if 1/ne< 0. Depth‐dependent viscosity and the model density offset the instability wavelengths by an amountaLthat we determine empirically. The wave number of the localization instability, iskjL= π(j+aL)(−1/ne)−1/2/H, withHthe thickness of the brittle layer,jan integer, and 1/4 <aL< 1/2 if the strength of the layer increases with depth and the strength of the substrate decreases with depth.
Azimuth‐dependent amplification of weak and strong ground motions within a fault zone (Nocera Umbra, central Italy)
Authors: Cultrera Giovanna , Rovelli Antonio , Mele Giuliana , Azzara Riccardo , Caserta Arrigo , Marra Fabrizio
Published: May 2003

During three moderate‐magnitude earthquakes occurred in September–October 1997 in the central Apennines, Italy, accelerations larger than 0.5 g were recorded in the town of Nocera Umbra, 10 to 15 km N‐NW of the epicenters. The accelerograph is sited in a fault zone, close to a N30°E tectonic contact. Six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones: the Colfiorito‐Sellano area, S‐SE of the array, and the Gualdo Tadino area, to the north. The array data reveal large variations in terms of both peak ground motions and spectral amplitudes. Within the fault zone, amplifications show a strong dependence on the source azimuth. At the accelerograph site, the effects are particularly large for events from S‐SE: peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 Hz along the N30°E direction of motion, parallel to the strike of the fault. Nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to M0 = 1.2 × 1025 dyn cm. A particle motion analysis shows that the directional effect is also present in the strongest motions, even though the amplification of peak ground motion decreases when M0 increases. Results from stochastic simulations indicate that such a behavior is not due to nonlinearity: applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily.
Transient convective circulation in a sedimented ridge setting: Implications for applying geochemical data to estimate permeability
Authors: Alt‐Epping Peter , Smith Leslie
Published: May 2003

For an open, sedimented ridge setting we investigate by means of transient numerical simulations hydrologic factors that can influence chemical signatures of mineral alteration, and how these features relate to the permeability of the upper oceanic crust. The transient state we consider involves three stages: (1) starting from a static flow field and a conductive thermal regime, flow in basement rocks is initiated as unstable convection; (2) with the inflow of seawater to the basement, the convection pattern evolves into a stable convection cell linking the recharge and discharge sites on the seafloor; and (3) convection weakens through time as the system offsets the initial thermal condition to attain a steady state. The duration of each stage, and the extent of the region in the basement undergoing fluid exchange with seawater varies with permeability. By the end of stage 2, for basement permeabilities between 1 × 10−14 and 1 × 10−13 m2, most of the basement and the deeper sediments surrounding the discharge zone is hydrologically isolated, but with active recirculation inside this zone. Through‐flow of seawater is restricted to a relatively narrow boundary layer. The unstable flow geometry during stages one and two is reflected in fluctuations of recharge and discharge rates into and out of the basement, and periodic variations in temperature of the discharging fluids. Fluid discharge at steady state is approximately one‐half of the average occurring during stage one. Changing flow patterns through time lead to a strongly heterogeneous evolution of water/rock ratios in the basement. Average water/rock ratios for the basement approach a log‐log linear correlation with basement permeability as steady state is reached.
Response of subglacial sediments to basal freeze‐on 1. Theory and comparison to observations from beneath the West Antarctic Ice Sheet
Authors: Christoffersen Poul , Tulaczyk Slawek
Published: May 2003

We have constructed a high‐resolution numerical model of heat, water, and solute flows in sub‐ice stream till subjected to basal freeze‐on. The model builds on quantitative treatments of frost heave in permafrost soils. The full version of the Clapeyron equation is used. Hence, ice‐water phase transition depends on water pressure, osmotic pressure, and surface tension. The two latter effects can lead to supercooling of the ice base. This supercooling, in turn, induces hydraulic gradients that drive upward flow of pore water, which feeds the growth of segregation ice onto the freezing interface. This interface may progress into the till and form ice lenses if supercooling is sufficiently strong. Hence, the ice segregation process develops a stratified basal ice layer. In our model, a high basal temperature gradient (∼0.054°C m−1) triggers ice stream stoppage, and the loss of basal shear heating leads to relatively high basal freeze‐on rates (∼3–5 mm a−1). In response, the subglacial till experiences comparatively rapid consolidation. Till porosity can decrease from 40% to <25%, and till strength can increase from ∼3 kPa to >120 kPa, approximately within one century. Basal supercooling arising from redistribution of solutes and ice‐water interfacial effects amounts to ca. −0.35°C below the pressure‐melting point. Fine‐grained till is in our model associated with widely spaced, thick ice lenses. Coarse‐grained till yields thinner ice lenses that are more closely spaced. Our model results compare favorably, although not in all details, with available observational evidence from borehole studies of West Antarctic ice streams.
Granular flow and Riedel band formation in water‐rich quartz aggregates experimentally deformed in torsion
Authors: Schmocker M. , Bystricky M. , Kunze K. , Burlini L. , Stünitz H. , Burg J.‐P.
Published: May 2003

We have investigated the influence of aqueous fluid on the deformation behavior of fine‐grained quartz aggregates in the semibrittle field by deforming wet Dover flint to high shear strains in torsion. Tests were carried out under drained conditions at temperatures of 1250 and 1300 K, 350 MPa confining pressure and constant twist rate. Heat treatments prior to deformation were performed at 1300 K for different amounts of time at confining pressures of 150 or 350 MPa, resulting in an initial dilation with an isotropic distribution of water‐filled pores, followed by compaction with continuous reduction in porosity and grain growth. Deformation of the heat‐treated samples reached steady state behavior until a shear strain of γ = 0.2 followed by sudden hardening stages for shear strains up to γ = 2.5. Deformation was mainly accommodated within the samples by granular flow. During steady state deformation, the pores redistributed anisotropically in planes oriented at ±25° to the maximum principal stress direction σ1. Hardening was associated with the interconnection of pores to form continuous bands in a synthetic R1 (Riedel) orientation, with no discernible lateral offset along them. These R1 bands served as fluid pathways enhancing local drainage of their surrounding, thus causing hardening by decrease in pore pressure. Two samples were unloaded at a shear strain of γ = 1.3 and 2.5, respectively, and reloaded after 0.5 hours at the same deformation temperature and confining pressure. Subsequent deformation occurred primarily by displacement along the existing R1 bands.
Multiple scattering of high‐frequency seismic waves in the deep Earth: Modeling and numerical examples
Authors: Margerin Ludovic , Nolet Guust
Published: May 2003

We apply the modern theory of radiative transfer to the modeling of the global propagation of high‐frequency seismic waves in the Earth. This theory stems from an exact statistical treatment of the wave equation and incorporates rigorously the effects of multiple scattering. The statistical mean time between scattering events (the mean free time) and the typical correlation length of the random fluctuations (the scale length) are introduced as the fundamental parameters of the theory. The integro‐differential equation of transport describes statistically the propagation of energy in phase space and can be conveniently solved by means of Monte Carlo simulations. We provide a general description of the method, stressing the important modifications required to adapt it to global propagation. The theory is applied to the modeling of PKP precursors, probably the best documented examples of wave scattering at the global scale. Guided by recent results of Hedlin et al. [1997], we solve the transfer equation in a variety of Earth models presenting exponentially correlated fluctuations of elastic parameters superimposed upon PREM. The validity of Born approximation is tested in a series of random media with mean free time and scale length in the 100–3200 s and 4–24 km ranges, respectively. For errors in coda envelope amplitude bound by 20%, the Born approximation can be safely applied in media with mean free times larger than about 400 s, relatively independent of the scale length. This corresponds to rather moderate (<0.5% RMS) perturbations, thus severely limiting the range of validity of Born approximation.
Paleomagnetic directions and thermoluminescence dating from a bread oven‐floor sequence in Lübeck (Germany): A record of 450 years of geomagnetic secular variation
Authors: Schnepp Elisabeth , Pucher Rudolf , Goedicke Christian , Manzano Ana , Müller Uwe , Lanos Philippe
Published: May 2003

A record of about 450 years of geomagnetic secular variation is presented from a single archaeological site in Lübeck (Germany) where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today. The age dating of the oven‐floor sequence is based on historical documents,14C‐dating and thermoluminescence dating. It confines the time interval from about 1300 to 1800 A.D. Paleomagnetic directions have been determined from each oven floor by means of 198 oriented hand samples. After alternating field as well as thermal demagnetization experiments, the characteristic remanent magnetization direction was obtained using principal component analysis. The mean directions of 24 oven floors are characterized by high Fisherian precision parameters (>146) and small α95confidence limits (1.2°–4.6°). For obtaining a smooth curve of geomagnetic secular variation for Lübeck, a spherical spline function was fitted to the data using a Bayesian approach, which considers not only the obtained ages, but also stratigraphic order. Correlation with historical magnetic records suggests that the age estimation for the upper 10 layers was too young and must date from the end of the sixteenth to the mid of the eighteenth century. For the lowermost 14 layers, dating is reliable and provides a secular variation curve for Germany. The inclination shows a minimum in the fourteenth century and then increases by more than 10°. Declination shows a local minimum around 1400 A.D. followed by a maximum in the seventeenth century. This is followed by the movement of declination about 30° to western directions.
Reactive geochemical transport simulation to study mineral trapping for CO2 disposal in deep arenaceous formations
Authors: Xu Tianfu , Apps John A. , Pruess Karsten
Published: May 2003

A reactive fluid flow and geochemical transport numerical model for evaluating long‐term CO2 disposal in deep geologic formations has been developed. The numerical model is needed because alteration of the predominant host rock aluminosilicate minerals is very slow and is not amenable to laboratory experiment under ambient deep‐formation conditions. Using this model, we performed numerical simulations for a commonly encountered Gulf Coast sediment under natural and CO2 injection conditions in order to analyze the impact of CO2 immobilization through carbonate precipitation. Under conditions considered in our simulations, CO2 trapping by secondary carbonate minerals such as calcite (CaCO3), dolomite (CaMg(CO3)2), siderite (FeCO3), and dawsonite (NaAlCO3(OH)2) could occur in the presence of high pressure CO2. Variations in the precipitation of secondary carbonate minerals strongly depend on rock mineral composition and their kinetic reaction rates. Using the data presented in this paper, the CO2 mineral‐trapping capability after 10,000 years is comparable to CO2 dissolution in pore waters (2–5 kg CO2 per cubic meter of formation). The addition of CO2 mass as secondary carbonates to the solid matrix decreases porosity. A small porosity decrease can result in a significant decrease in permeability. Despite simplifications in the current model, a comparison between initial model results and field observations for natural diagenesis of Gulf Coast sediments is instructive. Most of the simulated mineral alteration pattern is consistent with the observations. Some inconsistencies are noted, which can help identify issues, limitations, and areas where the conceptual model requires improvement. The numerical simulations described here provide useful insight into potential sequestration processes, and their controlling conditions and parameters.
Permeability change during experimental fault smearing
Authors: Takahashi Miki
Published: May 2003

Smear sourced from shale or clay is thought to provide an across‐fault barrier to fluid flow in sedimentary rocks. Permeability changes of this phenomenon were investigated using a triaxial testing machine. Experimental specimens consisting of interlayered siltstone (low initial permeability, ∼10−16 m2) and sandstone (high initial permeability, ∼10−13 m2) were subjected to 20, 30, and 40 MPa of effective normal stresses on a precut surface, dividing each specimen at a 30° angle to its axis, at axial shortening velocities between 0.14 and 1.41 μm s−1. Permeability was measured by the oscillation method, and permeability changes categorized in three distinct regimes that corresponded to progressively increasing rock deformation: regime 1, rapid reduction due to compaction of the siltstone layer prior to fault movement; regime 2, constant and minimum permeability while the smear developed; and regime 3, permeability recovery caused by smear thinning and loss of smear continuity. The duration of regime 2 and of smear continuity recorded provide measures of the sealing potential produced by the smear. The shale smear factor SSF, defined as (fault throw)/(thickness of low‐permeability layer), shows that there may be a relation between seal potential and effective normal stress. Both factors in SSF can reach higher values when effective normal stress is 40 MPa than when that is ≥30 MPa.
Analysis of the field dependence of remanent magnetization curves
Authors: Egli R.
Published: May 2003

A new method to calculate and analyze coercivity distributions of measured acquisition/demagnetization curves of remanent magnetization is presented. The acquisition/demagnetization curves are linearized by rescaling both the field and the magnetization axes. An appropriate filtering of the linearized curves efficiently removes measurement errors prior to evaluating the coercivity distributions. The filtered coercivity distributions are modeled using a set of generalized probability density functions in order to estimate the contributions of different magnetic components. An error estimation is calculated for these functions with analytical and numerical methods in order to evaluate whether the model is significantly different from the measured data. Three sediment samples from Baldeggersee (Switzerland) and three samples of urban atmospheric particulate matter (PM) have been analyzed using this method. It is found that the coercivity distributions of some of the magnetic components show significant and consistent deviations from a logarithmic Gaussian function. Large deviations are found also in the coercivity distributions of theoretical AF demagnetization curves of single‐domain and multidomain particles. Constraints in the shape of model functions affect the identification and quantification of magnetic components from remanent magnetization curves and should be avoided as far as possible. The generalized probability density function presented in this paper is suitable for appropriate modeling of Gaussian and a large number of non‐Gaussian coercivity distributions.
Origin of ocean island basalts: A new perspective from petrology, geochemistry, and mineral physics considerations
Authors: Niu Yaoling , O'Hara Michael J.
Published: May 2003

Consideration of petrology, geochemistry, and mineral physics suggests that ancient subducted oceanic crusts cannot be the source materials supplying ocean island basalts (OIB). Melting of oceanic crusts cannot produce high‐magnesian OIB lavas. Ancient oceanic crusts (>1 Ga) are isotopically too depleted to meet the required values of most OIB. Subducted oceanic crusts that have passed through subduction zone dehydration are likely to be depleted in water‐soluble incompatible elements (e.g., Ba, Rb, Cs, U, K, Sr, Pb) relative to water‐insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti). Melting of residual crusts with such trace element composition cannot produce OIB. Oceanic crusts, if subducted into the lower mantle, will be >2% denser than the ambient mantle at shallow lower mantle depths. This negative buoyancy will impede return of the subducted oceanic crusts into the upper mantle. If subducted oceanic crusts melt at the base of the mantle, the resultant melts are even denser than the ambient peridotitic mantle, perhaps by as much as ∼15%. Neither in the solid state nor in melt form can bulk oceanic crusts subducted into the lower mantle return to upper mantle source regions of oceanic basalts. Deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting volatiles and incompatible elements as a result of metasomatism taking place at the interface between the low‐velocity zone and the cooling and thickening oceanic lithosphere. These metasomatized and recycled deep portions of oceanic lithosphere are the most likely candidates for OIB sources in terms of petrology, geochemistry and mineral physics.
Transport of water into the lower mantle: Role of stishovite
Authors: Panero Wendy R. , Benedetti Laura Robin , Jeanloz Raymond
Published: May 2003

When subjected to lower‐mantle pressures and temperatures, natural “anhydrous” basalt containing 0.2 wt.% H2O forms a phase assemblage in which SiO2 stishovite is a significant carrier of hydrogen (up to 500 ppm H2O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO3 perovskite appears to be not (upper bound of 50 ppm (wt) H2O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H2O in residual stishovite increases from ∼100 to ∼400 ppm by weight upon partially melting the high‐pressure mineral assemblage at 28–60 GPa. We infer that the trace concentration of Al within residual stishovite increases upon partial melting, thereby increasing the coupled abundance of H in this crystalline phase. The “anhydrous” component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the Earth, with subducted stishovite potentially returning ∼102 times the amount of water present in today's atmosphere.
Moment tensor solutions for small and moderate earthquakes in the Ibero‐Maghreb region
Authors: Stich Daniel , Ammon Charles J. , Morales Jose
Published: May 2003

We applied time domain moment tensor inversion of local and regional waveforms to small and moderate (Mw = 3.5–5.7) shallow earthquakes from the Iberian Peninsula, northern Morocco, and northern Algeria. For the 6+ years period from November 1995 to March 2002 and the previous Network of Autonomously Recording Seismograms (NARS) experiment, moment tensor solutions were obtained for 58 events, considerably increasing the total number of available solutions in the study area. For each event we performed a moment tensor inversion and a double‐couple grid search. For simple faulting events the grid search is valuable as a quality test for its ability to reveal potential ambiguities of the solutions and to assess confidence limits of fault plane parameters or principal axes orientation. The computed mechanisms show regional consistency: A large part of the Iberian Peninsula is characterized by normal faulting mechanisms with SW‐NE oriented T axes. Thrusting and SE‐NW compression is dominant in Algeria. In the Alboran Sea, the westernmost part of the Mediterranean, and the transition between both regimes, strike‐slip mechanisms dominate with approximately N‐S oriented P axes. This pattern suggests a regional anomaly characterized by clockwise rotation of the principal horizontal stress orientations.
Self‐consistency in reference frames, geocenter definition, and surface loading of the solid Earth
Authors: Blewitt Geoffrey
Published: May 2003

Crustal motion can be described as a vector displacement field, which depends on both the physical deformation and the reference frame. Self‐consistent descriptions of surface kinematics must account for the dynamic relationship between the Earth's surface and the frame origin at some defined center of the Earth, which is governed by the Earth's response to the degree‐one spherical harmonic component of surface loads. Terrestrial reference frames are defined here as “isomorphic” if the computed surface displacements functionally accord with load Love number theory. Isomorphic frames are shown to move relative to each other along the direction of the load's center of mass. The following frames are isomorphic: center of mass of the solid Earth, center of mass of the entire Earth system, no‐net translation of the surface, no‐net horizontal translation of the surface, and no‐net vertical translation of the surface. The theory predicts different degree‐one load Love numbers and geocenter motion for specific isomorphic frames. Under a change in center of mass of surface load in any isomorphic frame, the total surface displacement field consists not only of a geocenter translation in inertial space, but must also be accompanied by surface deformation. Therefore estimation of geocenter displacement should account for this deformation. Even very long baseline interferometry (VLBI) is sensitive to geocenter displacement, as the accompanying deformation changes baseline lengths. The choice of specific isomorphic frame can facilitate scientific interpretation; the theory presented here clarifies how coordinate displacements and horizontal versus vertical motion are critically tied to this choice.
Vertical deformation of the Japanese islands, 1996–1999
Authors: Aoki Yosuke , Scholz Christopher H.
Published: May 2003

The interseismic vertical deformation field of the Japanese islands is derived from the Global Positioning System (GPS) data between 1996 and 1999. Although vertical components of the GPS velocities have rarely been used as important information because of their higher noise level, information on the vertical deformation field will enable us to separate rigid plate motions from deformation due to interplate coupling because horizontal displacements contain both effects, whereas vertical displacements contain only the interplate coupling effect. The results show that (1) the precision of the vertical velocity is generally better than 2 mm/yr, (2) the existence of clear uplift peaks of about 8 mm/yr in the Shikoku and Tokai regions is consistent with strong coupling with a gradual brittleplastic transition zone at the Nankai and the Suruga trough, respectively, (3) clear uplift peaks do not seem to exist in the Tohoku region because the Japan trench plate boundary is too far oceanward from the GPS sites, (4) significant uplift is obtained in the western part of Hokkaido, which may be associated with the long‐term postseismic deformation due to the 1983 Japan Sea and the 1993 Hokkaido‐nansei‐oki (M = 7.6) earthquakes, and (5) uplift in northeastern Japan along the Japan Sea coast is apparently associated with distributed compression due to incipient subduction.
Effect of low‐temperature treatments on pseudo‐Thellier paleointensity determination
Authors: Yu Yongjae , Dunlop David J. , Özdemir Özden
Published: May 2003

In paleointensity determination on multidomain (MD) magnetite, demagnetization is always easier than restoring remanence in companion acquisition steps. As a result, paleointensity results follow a convex‐down curve on the Arai plot. In an attempt to overcome this problem, the effect of systematic low‐temperature demagnetization (LTD) on pseudo‐Thellier results was examined. LTD was applied not only to the initial anhysteretic remanent magnetization (ARM, simulating natural remanent magnetization or NRM) but also after each partial anhysteretic remanent magnetization (pARM) acquisition step. Three practical benefits of systematic LTD were discovered. (1) LTD‐treated pseudo‐Thellier results always yielded more accurate intensity values regardless of grain size and sample lithology. (2) LTD improved the behavior of MD magnetite in paleointensity determination, although not as much as for single‐domain and pseudosingle‐domain grains. (3) An unanticipated bonus was a more even distribution of data points on the Arai plot, resulting from the fact that LTD causes a shift of points toward the upper end of the Arai curves. Constructing a three‐dimensional map of the distributions χ and χLTD was successful in quantitatively explaining behavior on the Arai plots.
Shallow soil CO2 flow along the San Andreas and Calaveras Faults, California
Authors: Lewicki J. L. , Evans W. C. , Hilley G. E. , Sorey M. L. , Rogie J. D. , Brantley S. L.
Published: May 2003

We evaluate a comprehensive soil CO2 survey along the San Andreas fault (SAF) in Parkfield, and the Calaveras fault (CF) in Hollister, California, in the context of spatial and temporal variability, origin, and transport of CO2 in fractured terrain. CO2 efflux was measured within grids with portable instrumentation and continuously with meteorological parameters at a fixed station, in both faulted and unfaulted areas. Spatial and temporal variability of surface CO2 effluxes was observed to be higher at faulted SAF and CF sites, relative to comparable background areas. However, δ13C (−23.3 to −16.4‰) and Δ14C (75.5 to 94.4‰) values of soil CO2 in both faulted and unfaulted areas are indicative of biogenic CO2, even though CO2 effluxes in faulted areas reached values as high as 428 g m−2 d−1. Profiles of soil CO2 concentration as a function of depth were measured at multiple sites within SAF and CF grids and repeatedly at two locations at the SAF grid. Many of these profiles suggest a surprisingly high component of advective CO2 flow. Spectral and correlation analysis of SAF CO2 efflux and meteorological parameter time series indicates that effects of wind speed variations on atmospheric air flow though fractures modulate surface efflux of biogenic CO2. The resulting areal patterns in CO2 effluxes could be erroneously attributed to a deep gas source in the absence of isotopic data, a problem that must be addressed in fault zone soil gas studies.
Effects of prestress state and rupture velocity on dynamic fault branching
Authors: Kame Nobuki , Rice James R. , Dmowska Renata
Published: May 2003

We consider a mode II rupture which propagates along a planar main fault and encounters an intersection with a branching fault. Using an elastodynamic boundary integral equation formulation, allowing the failure path to be dynamically self‐chosen, we study the following questions: Does the rupture initiate along the branch? Does it continue? Is the extensional or compressional side most favored for branching? Does rupture continue on the main fault too? Failure is described by a slip‐weakening law for which the strength at any amount of slip is proportional to normal stress. Our results show that dynamic stresses around the rupture tip, which increase with rupture velocity at locations off the main fault plane relative to those on it, could initiate rupture on a branching fault. As suggested by prior work, whether branched rupture can be continued to a larger scale depends on principal stress directions in the prestress state and on rupture velocity. The most favored side for branching rupture switches from the extensional to the compressional side as we consider progressively shallower angles of the direction of maximum compressive prestress with the main fault. Simultaneous rupturing on both faults can be activated when the branching angle is wide but is usually difficult for a narrow branching angle due to strong stress interactions between faults. However, it can be also be activated by enhanced dynamic stressing when the rupture velocity is very near the Rayleigh velocity. Natural examples seem consistent with the simulations that we present.
Correlation of microseismic and chemical properties of brittle deformation in Locharbriggs sandstone
Authors: Ojala Ira O. , Ngwenya Bryne T. , Main Ian G. , Elphick Stephen C.
Published: May 2003

The time‐dependent properties of ceramic materials such as rocks depend both on preexisting cracks and chemical properties acting at their tips. We have examined the direct effect of chemical processes on the growth of a crack population by carrying out triaxial flow‐through compression tests on Locharbriggs sandstone. The tests were carried out at temperatures of 25–80°C and at strain rates ranging from 10−5 to 10−8 s−1 under constant stress rate loading. The exit pore fluid was analyzed after the tests for the concentration of dissolved ions and acoustic emission was monitored in real time throughout the tests. The exit pore fluid silica concentration and microcrack damage derived from the acoustic emission (AE) data both exhibited an exponential increase during the strain hardening phase of deformation. Damage parameters inferred from the AE data predict the stress‐strain curves adequately, or at least up to the point of strong microcrack coalescence. The damage parameters and silica signal were strongly correlated by a power law relationship. The observed environment and strain rate dependence of mechanical properties can hence be attributed uniquely to time‐dependent crack growth by the stress corrosion mechanism.
Ocean currents at Axial Volcano, a northeastern Pacific seamount
Authors: Lavelle J. W. , Baker E. T. , Cannon G. A.
Published: May 2003

Axial Volcano is a magmatically active seamount that straddles the Juan de Fuca Ridge (JdFR). With a summit depth of ∼1400 m, the volcano intercepts deep ocean flows at a height well above that of the linear ridge adjoining it to the north and south. Multiyear, yearlong observations of currents at Axial show that mean flow circles the seamount in an anticyclonic sense, consistent with dynamical constraints, with means as large as 6.6 cm s−1. Spectra of currents indicate prominent tidal, inertial, and in some cases weather‐band period (3‐ to 7‐day) peaks, as has been seen elsewhere along the JdFR ridge. For K1, O1, and S2 tidal currents at the seamount, amplitudes are several times larger than the corresponding background amplitudes and rotation around the tidal current ellipses is anticyclonic. M2 tidal currents show none of the same properties. Energy at the inertial frequency decreases by more than an order of magnitude from 100 m above to 400 m below the volcano's summit. In contrast, weather‐band (WB) period energy increases tenfold over the same depth range, with WB energy all but unobservable at summit depth. The presence of WB oscillations makes Axial unlike other seamounts where long observations have been made. The current meter observations also provide a picture of seasonal trends. As expected, during the period Summer 1996 to Spring 1997, largest amounts of WB energy are found during autumn and winter; in contrast, WB motion at Axial Volcano during Winter 2000 was summer‐like in intensity. A linear, baroclinic model of forced damped azimuthal mode‐one oscillatory flow around an idealized axisymmetric seamount shows that amplification of diurnal flow should occur principally at the seamount summit while amplification of the WB should occur, as the observations show, on the seamount flanks. Model and measured phase relationships for temperature and current time series pairs circumscribing Axial support the view that azimuthal mode‐one oscillations are prominent. Over the 0.8‐ to 6‐day range for forcing period (TC) the model indicates that the principal topographic resonance, as indicated by kinetic energy (KE) amplification, occurs for a TC greater than 1 and less than 2 days with only mild dependence in that range on background stratification and seamount shape. Additional but weaker topographic resonances can occur at longer periods in model results, but as TC becomes slightly larger than the frictional timescale, topographic resonances no longer appear. High dissipation and thus short frictional timescales reported by others in regions of rough, sloping topography make topographic resonances at Axial at periods greater than a few days somewhat unlikely. The relatively broad WB spectral peaks at some Axial sites are consequently more likely the result of spectral peaks in forcing than of topographic resonance.
Observations of oceanic whitecaps in the north polar waters of the Atlantic
Authors: Stramska Malgorzata , Petelski Tomasz
Published: May 2003

Digital photographs of the sea surface were analyzed for the fraction of aerial coverage by whitecaps (stage A and B) in the north polar region of the Atlantic. Photography was accompanied by measurements of wind velocity, air temperature and humidity, sea surface temperature, and observations of significant wave height. Whitecap coverage increased significantly with an increase in wind speed (or wind friction velocity). Our data exhibit lower values of the average whitecap coverage at low and moderate wind speeds than previous estimates from literature. In addition, our results indicate that the prediction of whitecap coverage can be improved if the state of the development of surface waves is taken into account. Changes in sea surface temperature (2 to 13°C) and near‐water air stability showed no discernible effect on whitecap coverage at any given wind speed within our data set.
Relationships among tracer ages
Authors: Waugh Darryn W. , Hall Timothy M. , Haine Thomas W. N.
Published: May 2003

Measurements of chemical tracers whose spatial gradients are primarily due to the time dependence of sources and/or sinks are often used to define “tracer ages” in an effort to diagnose transport. However, a major problem with interpreting these tracer ages is that different tracers can yield different ages, and at present, it is not clear what aspects of the transport are measured by the different tracers. We use the concept of a distribution of transit times to compare the timescales derived from different tracers, including CFCs, tritium‐helium, and radioactive tracers. By performing a systematic study over a range of transit time distributions we examine under what conditions two tracers yield similar or different ages. It is shown that there can be significant differences in tracer ages and that in general, tracer ages are not fundamental timescales of the flow. Furthermore, even if ages from two tracers are similar these ages can be very different from the mean (ideal) age or the age of a third tracer. It is also shown that significant temporal variations in tracer ages can occur for steady transport and that these changes are of similar magnitude to the changes in CFC and tritium‐helium ages observed in the North Atlantic and North Pacific over the 1980s and 1990s. Accounting for the changes in tracer ages caused by steady transport is necessary before attributing changes in tracer ages to changes in transport. The possibility of using the differences in ages from different tracers to infer information about the transit time distribution is also examined. It is shown that two tracer ages can constrain the first two moments (mean age and width) of the distribution, but how tightly these are constrained depends on the tracers used, the certainty of the age calculations, and the flow characteristics.
Effect of environmental conditions on observed, modeled, and assimilated sea ice motion errors
Authors: Meier Walter N. , Maslanik James A.
Published: May 2003

Previous evaluations of ice motion have primarily consisted of comparisons with in situ observations and general sensitivity studies with wind speed and direction. An assimilation framework allows the quality of ice motions to be studied as an explicit function of model and model forcing parameters. Here we investigate the effects of local conditions on remotely sensed, modeled, and assimilated motions. We use local conditions, as given by the ice model, to examine ice motion errors based on (1) proximity to the coast, (2) ice thickness, and (3) wind‐forcing. In comparison with buoy observations, both modeled and remotely sensed ice motions have lowest errors in moderately thick ice, in low winds, and away from coastal regions. Wind speed plays the largest role in the errors, and higher winds speeds lead to substantially higher motion error standard deviation. However, higher wind speeds also increase the correlation of modeled and remotely sensed motions with buoy observations. An optimal interpolation assimilation methodology, which led to substantial improvements in ice motion quality on a regional and seasonal basis, was employed in this study. Assimilation improved the quality of the ice motions (lower error, higher correlation) throughout most ranges of wind speed and ice thickness and both near the coast and in the central ice pack. This study suggests that adjusting assimilation weights based on local model conditions could efficiently yield even higher quality assimilated motion fields.
North Atlantic Deep Water and Antarctic Bottom Water: Their interaction and influence on the variability of the global ocean circulation
Authors: Brix H. , Gerdes R.
Published: May 2003

Interhemispheric signal transmission in the Atlantic Ocean connects the deep water production regions of both hemispheres. The nature of these interactions and large‐scale responses to perturbations on timescales of years to millennia have been investigated using a global three‐dimensional ocean general circulation model coupled to a dynamic‐thermodynamic sea ice model. The coupled model reproduces many aspects of today's oceanic circulation. A set of experiments shows the sensitivity to changes in different surface boundary conditions. Buoyancy changes in the Weddell and Labrador Seas exert a direct effect on the overturning cells of the respective hemisphere. They influence the density structure of the deep ocean and thereby lead to alterations in the strength of the ACC. Changing the wind stress south of 30°S influences the magnitude of the deep water production of both hemispheres. The interhemispheric effect in these experiments cannot be explained solely by advective mechanisms. Switching off the wind stress over the latitude band of the Drake Passage leads to a slow gradual decrease of the water mass transport in the ACC, resulting in an almost complete cessation. The model results prove the necessity to continue integrations over thousands of years until new equilibria are established.