bluuebunny/crossref_metadata_2025 huggingface dataset:
Global distribution of seamounts inferred from gridded Geosat/ERS‐1 altimetry
Authors: Wessel Paul
Published: February 2004

Using a forward modeling approach based on an axisymmetric Gaussian seamount, I characterize the global seamount distribution by locating circular maxima in the gridded vertical gravity gradient field derived from altimetry collected by the Geosat and ERS‐I satellite missions. The global seamount distribution is long‐tailed and resembles a power law distribution for seamounts in the height range 2–7 km. Smaller seamounts are not well isolated by my technique nor are they well resolved in the gridded data. Several factors are likely to influence the height of volcanic seamounts, such as melt availability, magma driving pressure, and plate thickness. The observed relationship between seamount gravimetric amplitudes and the age of the underlying seafloor implies that there is an upper limit on seamount heights. Whether a seamount will reach that height depends most likely on supply‐side factors, such as melt availability and magma driving pressure, but the limiting height itself seems more likely to be controlled by the strength of the oceanic plate. Specifically, compressional stresses directly beneath the seamount as a consequence of the lithosphere's flexural response to loading may eventually exceed the magma driving pressure and prevent magma from reaching the surface, thus limiting the growth of the seamount. Because oceanic plate strength primarily is controlled by plate age, the limit on seamount height is inferred to be a simple function of plate age at the time of seamount emplacement. Using analytical solutions, I present a simple flexural model that predicts the observed global height‐age relationship.
Low‐temperature magnetic properties of magnetite
Authors: King James G. , Williams Wyn
Published: February 2004

Although several studies have recommended removal of secondary components of magnetic remanence by zero‐field cycling from room temperature to a temperature much lower than the low temperature transition for magnetite (about 120 K), the method has not become a standard routine technique. This is partly due to the poor understanding of the behavior of magnetite particles at the low‐temperature transition zone. Previous experiments by other researchers have used magnetite powders. In such powders it is always possible to attribute any discrepancy between the results observed and theory to possible existence of magnetostatic interaction effects or existence of elongated particles in samples presumed to contain only equant particles. Such factors need to be eliminated in order to have a better understanding of the low temperature behavior of magnetite particles. Low‐temperature magnetic properties of lithographically produced arrays of both interacting and noninteracting cubic magnetite particles as well as those from powder particles have been measured as part of this study. A gradual increase in the amount of saturation isothermal rémanent magnetization (SIRM) lost at the Verwey transition Tv with increasing particle size in the pseudo‐single‐domain size range has been observed. This behavior is consistent with the vortex state domain structure. The grain size dependence of the amount of SIRM lost at Tv is most probably what previous researchers reported as a magnetic memory particle‐size‐dependent trend. Magnetic memory measured during the cooling and warming process is shown to be a stress‐related phenomenon. Such measurements could be useful in assessing the nature of stress in a magnetite sample.
Micromechanical modeling of cracking and failure in brittle rocks
Authors: Hazzard James F. , Young R. Paul , Maxwell S. C.
Published: February 2004

Dynamic micromechanical models are used to analyze crack nucleation and propagation in brittle rock. Models of rock are created by bonding together thousands of individual particles at points of contact. The feasibility of using these bonded particle models to reproduce rock mechanical behavior is explored by comparing model behavior to results from actual laboratory tests on different rock types. The behavior of two granite models are examined in detail to study cracking and failure patterns that occur during compressional loading. Because discontinuum models are being used, the rock models are free to crack and break apart under stress, such that the micromechanics of cracking can be examined. Stress waves are allowed to propagate outward from each crack, and it is shown that these dynamic waves significantly affect the rock behavior. As the peak stress in the modeled rock is approached and many of the bonds are close to breaking, a passing wave from a nearby crack is sufficient to break more bonds. This causes clusters of cracks to be created, and then eventual macroscopic shear failure occurs as these clusters connect to bisect the sample. The failure patterns observed in the granite models are similar to those observed in actual laboratory tests.
Failure mode and weakening effect of water on sandstone
Authors: Baud Patrick , Zhu Wenlu , Wong Teng‐fong
Published: February 2004

Previous studies have shown that brittle strength of a rock is generally reduced in the presence of water. However, for siliciclastic rocks, there is a paucity of data on the waterweakening behavior in the cataclastic flow regime. To compare the weakening effect of water in the brittle faulting and cataclastic flow regime, triaxial compression experiments were conducted on the Berea, Boise, Darley Dale, and Gosford sandstones (with nominal porosities ranging from 11% to 35%) under nominally dry and saturated conditions at room temperature. Inelastic behavior and failure mode of the nominal dry samples were qualitatively similar to those of water‐saturated samples. At elevated pressures, shear localization was inhibited, and all the samples failed by strain hardening. The compactive yield strengths (associated with the onset of shear‐enhanced compaction) in the saturated samples were lower than those in the dry samples deformed under comparable pressure conditions by 20% to 70%. The reductions of brittle strength in the presence of water ranged from 5% to 17%. The water‐weakening effects were most and least significant in the Gosford and Berea sandstones, respectively. The relation between water weakening and failure mode is consistently explained by micromechanical models formulated on the basis that the specific surface energy in the presence of water is lowered than that in vacuum by the ratio λ. In accordance with the Hertzian fracture model the initial yield stress in the compactive cataclastic flow regime scales with the grain‐crushing pressure, which is proportional to λ3/2. In the brittle faulting regime, damage mechanics models predict that the uniaxial compressive strength scales with λ1/2. In the presence of water the confined brittle strength is lower due to reductions of both the specific surface energy and friction coefficient.
Continuous sampling of hydrothermal fluids from Loihi Seamount after the 1996 event
Authors: Wheat C. Geoffrey , Jannasch Hans W. , Plant Josh N. , Moyer Craig L. , Sansone Francis J. , McMurtry Gary M.
Published: February 2004

For at least 9 years prior to July 1996, hydrothermal fluids flowed from Pele's Vents on Loihi Seamount, Hawaii. In July–August 1996 a tectonic‐volcanic event occurred that destroyed Pele's Vents, creating a pit crater (Pele's Pit) and several sites with hydrothermal venting. In October 1996 we deployed two new continuous water samplers (OsmoSamplers) at two of these hydrothermal sites and collected fluids using traditional sampling techniques to monitor the evolution of crustal and hydrothermal conditions after the event. The samplers were recovered in September 1997, and additional discrete vent fluid samples were collected. The OsmoSampler located along the south rift at Naha Vents captured a change in composition from a low‐chlorinity, high‐K fluid (relative to bottom seawater) to a high‐chlorinity, low‐K fluid. These changes are consistent with the fluid cooling during ascent and being derived from several different sources, which include high‐ (>330°C) and low‐ (<150°C) temperature components and a source of magmatic volatiles. The other OsmoSampler, which was deployed on the rim of Pele's Pit at Lohiau Vents, captured warm hydrothermal discharge that originated from a high‐temperature source (>330°C) into which magmatic volatiles were added. During the deployment, thermal and fluid fluxes decreased. At Naha the transport of heat and chemicals was decoupled. The chemical and thermal evolution of hydrothermal fluids after the event on Loihi is consistent with previous models based on events that have occurred along mid‐ocean ridges. The event at Loihi clearly had an effect on the local hydrography; however, the integrated effect of chemical fluxes to global budgets from similar events is uncertain. Chemical fluxes from similar events may have a global impact, if ratios of chemical (e.g., CO2, Fe/Mn, Mg, sulfate, and K) to thermal anomalies greatly exceed, or are in the opposite direction to, fluxes from mid‐ocean ridge hydrothermal systems.
Global Positioning System measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation
Authors: Chen Z. , Burchfiel B. C. , Liu Y. , King R. W. , Royden L. H. , Tang W. , Wang E. , Zhao J. , Zhang X.
Published: February 2004

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1991–1998 for a network encompassing the eastern part of the Tibetan Plateau and its foreland. Relative to a Eurasian frame defined by minimizing the velocities of 16 GPS stations in Europe, central Asia, and Siberia, stations within all parts of the plateau foreland in south China move 6–10 mm/yr east‐southeast, indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland. North of the plateau, foreland stations move northeastward at ∼10 mm/yr, indicating that the northern boundary of the deformation zone lies north of the plateau. With this realization of a Eurasian frame, the velocity of the GPS station at Bangalore in southern India implies that the northward motion of India is 5–12 mm/yr slower than that predicted from the NUVEL‐1A plate reconstruction. Viewed relative to the South China Block, stations of the northeast plateau, bounded on the north by the Qilian Shan and the Altyn Tagh fault, move NNE to NE with velocities ranging from 19 mm/yr within the plateau to 5–11 mm/yr in its foreland. The Altyn Tagh fault shows left‐lateral slip of ∼10 mm/yr at 95°E and shortening across the fault of <5 mm/yr. Stations south and west of the Xianshuihe/Xiaojiang fault system define a crustal fragment rotating clockwise at ∼10 mm/yr relative to the South China Block around the eastern Himalayan syntaxis. The GPS measurements indicate no significant shortening (< 3 mm/yr) within the Longmen Shan of the central eastern plateau and its adjacent foreland, although the Longmen Shan rise over 6 km in <100 km horizontal distance. Geological studies indicate that the deformational field was established diachronously in late Miocene to Pliocene time, was characterized by no east‐west shortening of Tibetan crust, and has an inhomogeneous style of deformation resulting from a balance of different tectonic processes.
Paleomagnetism of the Late Triassic Petrified Forest Formation, Chinle Group, western United States: Further evidence of “large” rotation of the Colorado Plateau
Authors: Steiner Maureen B. , Lucas Spencer G.
Published: February 2004

Paleomagnetic poles and magnetostratigraphy were obtained from the late Carnian and early Norian Petrified Forest Formation. The remanent magnetization characteristics of this variegated claystones formation are correlated closely with stratal color; red strata display univectorial magnetizations directed to the origin of orthogonal axes plots between 300° and 630°C, whereas progressively greater overlap in magnetization component stabilities is exhibited by purple, blue, and green strata. Rock‐magnetic, petrographic, and microprobe investigations indicate that detrital titanohematite is the carrier of much of the characteristic magnetization, with a smaller contribution from magnetite in green and portions of the red strata. Eight magnetic polarity intervals were observed in three quarters of the formation (155 m) good correlation is observed with the coeval portion of the Newark Supergroup. Paleopoles from the late Carnian Blue Mesa and the early Norian Painted Desert Members of the Petrified Forest Formation are identical and agree well with other Chinle Group paleopoles from the Colorado Plateau; the tight cluster formed by all poles indicates that little or no apparent polar wander (APW) with respect to North America occurred during the Late Triassic (late Carnian through Rhaetian time). In addition, Chinle Group paleopoles derived from strata located on the North American (NA) craton also are tightly grouped and indicative of a paucity of significant APW through the Late Triassic. With that same observation from the coeval Newark Supergroup, three independent Late Triassic data sets from different tectonic settings in NA indicate minimal or no APW during more than 20 m.y. The combination of the vertebrate fauna and magnetostratigraphy within the Chinle Group provides a high degree of age control, allowing comparisons among coeval Colorado Plateau (CP) and NA craton paleopoles at about a 250–500 kyr level. CP Chinle Group paleopoles are displaced 9±3° clockwise from cratonic paleopoles. The Chinle Group is one of five sets of paleomagnetically sampled Mesozoic strata shown by magnetostratigraphy and/or biostratigraphy to have been deposited coevally on both the CP and the NA craton; paleopoles from the five sets indicate that CP paleopoles have been displaced a total of 10±3° relative to NA cratonic counterparts. Furthermore, the wealth of paleomagnetic and biostratigraphic data now available from both the Chinle and Newark strata demonstrates that nothing is sacred in NA tectonics: Newark Basin paleopoles are displaced 9±3° counterclockwise from western cratonic poles.
Dating of major normal fault systems using thermochronology: An example from the Raft River detachment, Basin and Range, western United States
Authors: Wells Michael L. , Snee Lawrence W. , Blythe Ann E.
Published: February 2004

Application of thermochronological techniques to major normal fault systems can resolve the timing of initiation and duration of extension, rates of motion on detachment faults, timing of ductile mylonite formation and passage of rocks through the crystal‐plastic to brittle transition, and multiple events of extensional unroofing. Here we determine the above for the top‐to‐the‐east Raft River detachment fault and shear zone by study of spatial gradients in40Ar/39Ar and fission track cooling ages of footwall rocks and cooling histories and by comparison of cooling histories with deformation temperatures. Mica40Ar/39Ar cooling ages indicate that extension‐related cooling began at ∼25–20 Ma, and apatite fission track ages show that motion on the Raft River detachment proceeded until ∼7.4 Ma. Collective cooling curves show acceleration of cooling rates during extension, from 5–10°C/m.y. to rates in excess of 70–100°C/m.y. The apparent slip rate along the Raft River detachment, recorded in spatial gradients of apatite fission track ages, is 7 mm/yr between 13.5 and 7.4 Ma and is interpreted to record the rate of migration of a rolling hinge. Microstructural study of footwall mylonite indicates that deformation conditions were no higher than middle greenschist facies and that deformation occurred during cooling to cataclastic conditions. These data show that the shear zone and detachment fault represent a continuum produced by progressive exhumation and shearing during Miocene extension and preclude the possibility of a Mesozoic age for the ductile shear zone. Moderately rapid cooling in middle Eocene time likely records exhumation resulting from an older, oppositely rooted, extensional shear zone along the west side of the Grouse Creek, Raft River, and Albion Mountains.
Aftershocks of the March 9, 1994, Tonga earthquake: The strongest known deep aftershock sequence
Authors: Wiens Douglas A. , McGuire Jeffrey J.
Published: February 2004

The March 9, 1994 (Mw 7.6, depth 564 km), deep Tonga earthquake produced an exceptionally strong aftershock sequence for a deep earthquake. Using eight temporary broadband stations deployed on the Fiji, Tonga, and Niue islands, we identify 144 aftershocks with mb ranging from 3.2 to 6.0 during the first 41 days following the mainshock. The number and seismic moment of the March 9 aftershocks lie within the range of aftershock productivities of typical shallow earthquakes of similar moment. The aftershocks show a power law decay with time following the mainshock, with an exponent (p value) of −1.0, similar to shallow aftershock sequences. Fifty aftershocks can be accurately located using a hypocentroidal decomposition method; these aftershocks align along a near‐vertical ENE striking plane similar to one of the mainshock nodal planes. Sixteen of the 18 well‐located aftershocks during the first 2.5 hours are located to the NNE of the mainshock epicenter, consistent with NNE rupture propagation derived from waveform analysis. Later aftershocks are located south of the epicenter, suggesting aftershock expansion beyond the region of substantial moment release in the mainshock. Several of the aftershocks are located outside of the normal bounds of the seismically active slab, consistent with evidence that the mainshock rupture terminated outside the active slab. Five possible foreshocks occurred along the preferred fault plane within the 22 days prior to the mainshock. At least three “triggered” aftershocks occurred at distances of 50 to 110 km within the first six hours following the mainshock; triggered aftershocks in adjacent slab material may be common for large deep earthquakes. While there is some variation in the focal mechanisms of the aftershocks, most resemble the mainshock focal mechanism. All the mechanisms show compressional axes dipping to the west, consistent with background seismicity in this region. Overall, most of the aftershock characteristics of the 1994 Tonga event are similar to those found for shallow earthquakes, suggesting similarities between shallow and deep earthquakes in the mechanical processes that produce aftershocks.
Predicted seafloor topography of the New Zealand region: A nonlinear least squares inversion of satellite altimetry data
Authors: Ramillien Guillaume , Wright Ian C.
Published: February 2004

We use a nonlinear least squares inversion to derive predicted seafloor topography (hereinafter referred to as RW99) for the New Zealand region (146°E–165°W, 60°S–25°S), combining altimetry data from ERS‐1 and Geosat Geodetic Missions, as well as available shipborne gravity and echo sounding data. Currently, the lithospheric component of the model is principally applicable to thinly sedimented oceanic basins; however, we have attempted, though with only partial success, to compensate for regional crustal variations. The upper part of the oceanic lithosphere has an elastic behavior related to a half‐space cooling model and flexing under seafloor relief load. Using least squares theory, the topographic solution is derived as a linear combination of altimetry and in situ measurements with adjusted coefficients. These coefficients are iteratively fitted using nonlinear operators between bathymetry and altimetry‐derived gravity anomalies assuming their error distributions are Gaussian. The theory enables sparse in situ data to be included in the inversion, such as depth soundings and marine gravity profiles. In comparison with the global model of Smith and Sandwell [1997] (hereinafter referred to as SS97), the RW99 predicted topography is constrained by over threefold more shipborne soundings data and the inclusion of shipborne gravity data. Three strategies are used to validate the RW99 model. Compared to the root‐mean‐square (rms) error of 310 m of the SS97 model, final residual differences for the RW99 model are within the range of 104–250 m. These rms errors are the result of uncertainties of model parameters, especially the elastic thickness and the relief density, but also the complexity of seafloor topography. In addition, the model inversion does not presently consider gravitational contributions of marine sediments of variable thickness.
Point measurements of mass balance of the Greenland Ice Sheet using precision vertical Global Positioning System (GPS) surveys
Authors: Hamilton Gordon S. , Whillans Ian M.
Published: February 2004

Rates of ice sheet thickening or thinning in Greenland are measured using the coffee‐can technique. This entails computing the difference in the vertical velocity of markers anchored in firn and the long‐term rate of snow accumulation. The velocities are obtained from repeat surveys using the Global Positioning System (GPS). With corrections for densification and along‐slope flow, this difference is the local mass balance. For two sites in western Greenland (Camp Century and a site just south of the EGIG line (Crawford Point)) results show ice sheet thinning. A third west Greenland site (inland from Upernavik) is close to balance. Two sites, Dye‐2 in western Greenland and Summit, are thickening slowly.
Insights into the evolution of the uppermost continental mantle from xenolith localities on and near the Colorado Plateau and regional comparisons
Authors: Smith D.
Published: February 2004

The interplay between the evolution of mantle and crust has been investigated by textural and electron probe analysis of Cr‐diopside spinel peridotite xenoliths from six localities and by comparisons to a database of mineral compositions. The six localities are on a 300‐km transect from the Colorado Plateau into the adjoining Rio Grande rift. Typical xenolith populations from the Colorado Plateau province have magnesian olivine (average Fo90.5) and Ti and Na‐poor clinopyroxene. In contrast, typical xenolith populations from the Rio Grande rift and the Basin and Range are characterized by more Fe‐rich olivine (average Fo89.6) and clinopyroxene with a range of Ti and Na (average 0.44 wt% TiO2). The contrast between the xenoliths from these adjacent provinces documents that the Colorado Plateau has a depleted mantle root that helps to stabilize it. At Mount Taylor, however, a locality high on a composite volcano within the eastern Colorado Plateau, most xenoliths are not depleted, and the Fe‐rich olivine (Fo86–Fo88) in them may characterize mantle affected by silicate melt metasomatism at high melt/rock ratios. Low‐Al orthopyroxene occurs at two of the six transect localities, and part of a large porphyroblast contains only 0.13 wt% Al2O3; because of the unusual compositions and textures, water‐rock reactions are inferred to have been important in parts of the refractory continental mantle. Zonation of Ca and Al in orthopyroxene provides constraints on the temperature histories of the unusual pyroxenes. Equilibration temperatures for xenoliths from Lunar Crater, Nevada, average 1270°C, much higher than the average of 1012°C for others from the Basin and Range; Lunar Crater xenoliths may be samples of a plume or asthenosphere emplaced recently into the uppermost mantle.
A three‐dimensional electromagnetic model of the southern Kenya Rift: Departure from two dimensionality as a possible consequence of a rotating stress field
Authors: Simpson F.
Published: February 2004

A generic, three‐dimensional (3‐D) model has been developed which explains the three‐dimensionality exhibited by magnetovariational (MV) and magnetotelluric (MT) data from southern Kenya. In this model, observed variations in electromagnetic strike with period and location, impedance phase splitting, and peaks in tipper magnitude are all understood in terms of two regionally two‐dimensional (2‐D) structures striking NW‐SE and N‐S, respectively. The observed period and location dependence of the electromagnetic strike may arise as an indirect consequence of a rotating stress field, with regional‐scale structures formed at different stages of the stress‐strain history of Kenya being preserved as conductive lineaments. These conductive structures are not all confined to the upper crust. Thus, whereas stress data provide constraints on rifting at the upper crustal scale, the MT impedance tensor data provide constraints at lithospheric scales. The constraints and resolution provided by the MV and MT data have been rigorously investigated using 3‐D forward modeling. Decoupling of the period and site dependence of electromagnetic strike aids resolution and constraint of conductors, rendering attempts to fix an average strike in space and frequency inexpedient. An anomalous apparent “strike” at the center of the Rift Valley reflects neither the N‐S strike of the rift, nor the NW‐SE striking shear fabric, but is shown to be a virtual strike arising as a result of coupling between the respective strike directions. The NW‐SE trending conductivity anomaly straddles the rift and both its flanks, extends to at least middle to lower crustal depths, and appears substantially more electrically anomalous than the rift itself. A hypothesis that melt exists in the mantle directly below the rift at latitude 1.8°S is not supported by the MT data.
Seismic migration processing of P‐SV converted phases for mantle discontinuity structure beneath the Snake River Plain, western United States
Authors: Sheehan Anne F. , Shearer Peter M. , Gilbert Hersh J. , Dueker Kenneth G.
Published: February 2004

We experiment with backprojection migration processing of teleseismic receiver functions from the Snake River Plain (SRP) broadband seismic experiment. Previous analyses of data from this experiment have used a common midpoint (CMP) stacking approach, a method widely applied for analysis of P‐SV converted phases (receiver functions) to obtain high‐resolution imaging of upper mantle discontinuities. The CMP technique assumes that all P‐SV conversions are produced by flat‐lying structures and may not properly image dipping, curved, or laterally discontinuous interfaces. In this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions. We first perform synthetic experiments that illustrate the potential improvement of migration processing over CMP stacks. Application of the migration processing to the SRP data set shows most of the major features as in the original CMP work, but with a weaker 410‐km discontinuity and a more intermittent discontinuity at 250 km apparent depth. Random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images. These tests show that a 20‐km elevation of the 660‐km discontinuity directly beneath the Snake River Plain is robust, but that the variations in 410‐km discontinuity topography that we observe are not stable upon resampling. “Bright spots” near 250 km apparent depth are robust upon resampling, but interpretation of these features is complicated by possible sidelobe artifacts from topside Moho reverberations.
A method of combining ICESat and GRACE satellite data to constrain Antarctic mass balance
Authors: Wahr John , Wingham Duncan , Bentley Charles
Published: February 2004

Measurements from the Geoscience Laser Altimeter System (GLAS) aboard NASA's ICESat satellite (2001 launch) will be used to estimate the secular change in Antarctic ice mass. We have simulated 5 years of GLAS data to infer the likely accuracy of these GLAS mass balance estimates. We conclude that ICESat will be able to determine the linear rate of change in Antarctic ice mass occurring during those 5 years to an accuracy of ∼7 mm/yr equivalent water thickness when averaged over the entire ice sheet. By further including the difference between the typical 5‐year trend and the long‐term (i.e., century‐scale) trend, we estimate that GLAS should be able to provide the long‐term trend in mass to an accuracy of about ±9 mm/yr of equivalent water thickness, corresponding to an accuracy for the Antarctic contribution to the century‐scale global sea level rise of about ±0.3 mm/yr. For both cases the principal error sources are inadequate knowledge of postglacial rebound and of complications caused by interannual and decadal variations in the accumulation rate. We also simulate 5 years of gravity measurements from the NASA and Deutsches Zentrum für Luft‐und Raumfahrt (DLR) satellite mission Gravity Recovery and Climate Experiment (GRACE) (2001 launch). We find that by combining GLAS and GRACE measurements, it should be possible to slightly reduce the postglacial rebound error in the GLAS mass balance estimates. The improvement obtained by adding the gravity data would be substantially greater for multiple, successive altimeter and gravity missions.
A new assessment of long‐wavelength gravitational variations
Authors: Chen J. L. , Wilson C. R. , Eanes R. J. , Tapley B. D.
Published: February 2004

We estimate the degree 2 long‐wavelength gravitational variations C21, S21, and C20 using Earth rotational changes, caused by mass redistribution and movement within the Earth system. These rotation changes are accurately measured by space geodetic techniques. Wind and oceanic current effects are removed from the Earth rotation series using atmospheric and oceanic circulation data‐assimilating models. The results are compared with LAGEOS Satellite Laser Ranging (SLR) determinations and also with geophysical contributions estimated from atmospheric surface pressure, continental water storage, and nonsteric sea level changes. Our conjecture is that using Earth rotational changes to infer long‐wavelength gravitational variations has the potential to be more accurate than satellite‐based techniques in some cases. Consistent with this, we find that C21 and S21 variations from this study are in better agreement with geophysical observations than LAGEOS SLR determinations of C21 and S21. However, the Earth rotation‐derived estimate of C20 variation is probably less accurate than the LAGEOS‐derived result owing to the large atmospheric wind contribution to length‐of‐day variation which must first be removed and the natural sensitivity of LAGEOS to C20 changes via precession of the satellite node.
Geophysical characteristics of the southern Mariana Trough, 11°50′N–13°40′N
Authors: Martínez Fernando , Fryer Patricia , Becker Nathan
Published: February 2004

Despite slow opening rates generally inferred for the Mariana Trough, the southernmost part of the basin has “fast spreading” geophysical and morphologic characteristics that are unlike the features of the basin to the north. A side‐scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11°50′N and 13°40′N and identifies the following characteristics: the ridge across‐axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross‐sectional area variations of 1 to 7 km2; the along‐axis mantle Bouguer gravity gradient is 0.2 mGal/km; axial segmentation occurs as overlapping axes and small deviation in trend; no transform fault offsets exist despite significant changes in the trend of the spreading center. Characteristics of the surrounding basin include shallower overall depth than in the north; no well‐developed frontal arc high in the southernmost trough; the close proximity of submarine arc‐type volcanoes to the spreading center; and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank. These characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin. We propose that these effects are related to (1) the geometry of trench rollback in the southern trough leading to trench‐parallel extension generating inward radiating extensional faults; (2) decoupling of the trench‐parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin; and (3) augmentation of the spreading center's magmatic budget by arc magmatic sources contributing to its fast spreading character. Although these effects may be accentuated in the southern Mariana Trough by the geometry of trench rollback and position of the slab, which here underlies the spreading center, they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid‐ocean ridges.
Pore fluid pressure, apparent friction, and Coulomb failure
Authors: Beeler N. M. , Simpson R. W. , Hickman S. H. , Lockner D. A.
Published: February 2004

Many recent studies of stress‐triggered seismicity rely on a fault failure model with a single free parameter, the apparent coefficient of friction, presumed to be a material constant with possible values 0 ≤ μ′ ≤ 1. These studies may present a misleading view of fault strength and the role of pore fluid pressure in earthquake failure. The parameter μ′ is intended to incorporate the effects of both friction and pore pressure, but is a material constant only if changes in pore fluid pressure induced by changes in stress are proportional to the normal stress change across the potential failure plane. Although specific models of fault zones permit such a relation, neither is it known that fault zones within the Earth behave this way, nor is this behavior expected in all cases. In contrast, for an isotropic homogeneous poroelastic model the pore pressure changes are proportional to changes in mean stress, μ′ is not a material constant, and −∞ ≤ μ′ ≤ +∞. Analysis of the change in Coulomb failure stress for tectonically loaded reverse and strike‐slip faults shows considerable differences between these two pore pressure models, suggesting that such models might be distinguished from one another using observations of triggered seismicity (e.g., aftershocks). We conclude that using the constant apparent friction model exclusively in studies of Coulomb failure stress is unwise and could lead to significant errors in estimated stress change and seismic hazard.
Nucleation and stability of ferromagnetic states
Authors: Newell Andrew J. , Merrill Ronald T.
Published: February 2004

Each ferromagnetic state, such as the single‐domain (SD) state, has critical fields and grain sizes at which it becomes unstable. To determine the properties of these critical points, a three‐dimensional numerical micromagnetic model is combined with nucleation theory. Isothermal hysteresis and grain growth are simulated for cuboids with no internal stress or magnetocrystalline anisotropy. Most jumps in hysteresis loops are turning points at which the susceptibility goes to infinity. The SD state becomes unstable at a pitchfork bifurcation, which has a jump in susceptibility but a continuous change in magnetic moment. This is a generalization of curling mode nucleation. As the grain size increases, there is an increasing gap in field between the curling mode nucleation and the first irreversible jump in magnetization. A similar gap is often seen experimentally between the formation of a small spike domain and the appearance of a full size body domain. For the first time in a micromagnetic simulation, minor branches are traced from the main hysteresis loop. When they occur, the main loop becomes wasp waisted. At any given grain size the lowest‐energy state has SD‐like stability in response to changes in magnetic field. A high‐stability component of remanence is commonly observed in pseudo‐single‐domain grains. It has previously been assumed that the high stability must be due to SD‐like regions in larger grains, but the micromagnetic simulations demonstrate that SD‐like stability does not require a SD‐like mechanism.
Present crustal movement and strain distribution in Central Europe inferred from GPS measurements
Authors: Grenerczy Gyula , Kenyeres Ambrus , Fejes István
Published: February 2004

In 1992, an international collaboration was initiated to study the present‐day geodynamics of Central Europe. The primary tool of this research is the Global Positioning System (GPS) therefore the Central European GPS Geodynamic Reference Network was established in 1994. The network extends from the south in the Eurasian‐African plate boundary zone to the East European Pre‐Cambrian Platform in the northeast. During the first phase of the project four GPS campaigns were organized between 1994 and 1997. Analysis of GPS data provided RMS repeatabilities around 2 mm in the horizontal and 5–6 mm in the vertical component for each campaign. The velocities of GPS sites situated on the Eurasian plate were compared with those predicted by the NNR‐NUVEL‐1A plate velocity model. The intraplate velocity field shows that the northern part of Central Europe seems generally stable with velocities under 2 mm/yr. However, the southern sites close to the plate boundary zone show higher values and abrupt changes of velocities indicating the existence of narrow deformation zones. The primary feature revealed by the strain calculation in the Alpine‐Pannonian‐Dinaric system is the dominant north‐south contraction resulting from the collision of the African and Eurasian plates transmitted by the Adriatic microplate. However the direction of contraction in the Alpine‐Pannonian‐Dinaric junction zone changes from NW‐SE at the northeastern boundary of the Adriatic microplate to NE‐SW toward the Bohemian Massif. The magnitudes of these strain rates are similar, −8.0±5.3 ppb/yr and −8.6±2.5 ppb/yr, respectively over ∼200×350 km subnetworks. We observe E‐W oriented (93°±13°) right‐lateral shear (12±5 ppb/yr), between the Alpine‐North Pannonian unit and the southernmost sites in the Dinarides and Southern Alps, along with left‐lateral shear (97°±7°; 10±2 ppb/yr) relative to the stable Bohemian Massif in the north. Both the strain field and the intraplate velocities presented show that the Alpine‐North Pannonian unit is presently crushed out to the east with 2 mm/yr velocity on average along two main fault zones as a consequence of the compression caused by the Adria‐Alpine collision.
Aftershock asymmetry/rupture directivity among central San Andreas fault microearthquakes
Authors: Rubin Allan M. , Gillard Dominique
Published: February 2004

Using a waveform cross‐correlation technique, we have obtained precise relative locations for nearly 75% of the Northern California Seismic Network catalog (4300 earthquakes) occurring between 1984 and 1997 along 50 km of the San Andreas fault. Errors in relative location are meters to tens of meters for events separated by tens to hundreds of meters. We find that consecutive earthquakes in the relocated catalog occur no closer than a distance approximately equal to the radius of the first rupture, as estimated from the moment‐magnitude relationship of Abercrombie [1996] assuming a 10‐MPa stress drop. When the relative position vectors between consecutive events are normalized by this distance and projected onto the fault surface, they define a hole whose shape suggests that typical microearthquakes are elongate in the mode II (slip‐parallel) direction by several tens of percent. Moreover, of the 100 immediate aftershocks occurring closest to the mode II edges of the prior rupture, more than twice as many occur to the northwest than to the southeast. We interpret this asymmetry as resulting from the large contrast in material properties across the fault. Models of dynamic rupture between dissimilar media predict that ruptures in this region may run preferentially to the southeast, in the direction of motion of the lower‐velocity material. If so, then the barriers that stop rupture fronts moving to the southeast should initially be farther from failure, on average, than the barriers that stop rupture fronts moving to the northwest. Once the rupture stops, the induced stress change is more symmetric but the fault remains farther from failure (on average) to the southeast. This interpretation receives some support from pulse width measurements on a localized set of 72 magnitude 0.6 to 3.6 earthquakes.
Large river offsets and Plio‐Quaternary dextral slip rate on the Red River fault (Yunnan, China)
Authors: Replumaz A. , Lacassin R. , Tapponnier P. , Leloup P. H.
Published: February 2004

Using multispectral SPOT images and 1/100,000 topographic data, we present an improved map of the active Red River fault zone between Midu (Yunnan, China) and Hanoi (Vietnam). The fault zone is composed of parallel strands, one of which, the Yuanjiang fault was previously undetected. There also appears to be a component of extension all along the fault zone. Such extension increases toward the SE, from Yunnan to the south China sea coast, and the vector describing the motion of south China relative to Indochina points within the N45°–135°E quadrant. We attempt to assess the Plio‐Quaternary dextral slip rate on the Red River fault (RRF) by restoring large river offsets and searching for the largest, plausible one. Across much of Yunnan, the fault is perpendicular to local catchments that drain into the Red River. From precise mapping of the river courses on SPOT satellite images and on 1/100,000 topographic maps, numerous multiple offsets along the fault can be detected and reconstructed. The lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred. We thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m, up to an offset of 50 km. For each increment we measure the misfits (root mean squares, RMS) between the upstream and downstream channels. The best fit and smallest RMS are obtained for an offset of 25±0.5 km that we interpret to represent the clearest, large right‐lateral displacement recorded in the geomorphology along the active Red River fault. Since dextral motion is likely to have started around 5 Myr, the most probable average Plio‐Quaternary slip rate on the fault is of order of 5 mm/yr. We attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes. Our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets.
Remagnetization of Lower Cretaceous limestones from the southern Pyrenees and relation to the Iberian plate geodynamic evolution
Authors: Dinarès‐Turell Jaume , Garcia‐Senz Jesús
Published: February 2004

Paleomagnetic study (34 sites) of thick Cretaceous ∼5000 m marine carbonate strata from the Organyà Basin (OB) in the southern Pyrenees reveals a uniformly normal polarity characteristic remanence with varying declination (D) (Berriasian‐Barremian, D = 295°; Aptian‐Albian, D = 348°, upper Cretaceous, D = 354°) that resides in magnetite and predates Late Cretaceous/Tertiary folding. The lack of reverse polarity magnetizations in the Berriasian‐Barremian series, together with their distinct hysteresis properties that are typical of remagnetized carbonates elsewhere, are taken as evidence for a secondary overprint in the lower part of the succession. A hypothesized age of remagnetization near the Barremian/Aptian boundary fits with a major tectosedimentary event in the basin that changed deposition from platform to basinal conditions and increased subsidence during the Aptian. The angular difference (∼53°) between declination from the remagnetized strata and the mean for Aptian‐Albian strata indicates that a counterclockwise rotation occurred between the two acquisition times. Part of this rotation is interpreted to be local and due to slip variation along strike of a normal fault that bounded the basin and tilted the pre Aptian series. This Cretaceous extensive phase is framed in the rift evolution of the northern margin of the Iberian plate. The inferred remagnetization age within the OB falls within the range of previously inferred widespread Cretaceous remagnetization events affecting the Iberian plate. In the OB, remagnetization is speculated to have taken place through chemical (burial diagenetic) origin rather than simple thermoviscous resetting.
Anomalous splitting of free oscillations: A reevaluation of possible interpretations
Authors: Romanowicz Barbara , Bréger Ludovic
Published: February 2004

The splitting of normal modes sensitive to structure in the inner core is larger than can be explained by three‐dimensional heterogeneity confined to the earth's mantle. A preferred interpretation of this anomalous splitting involves inner core anisotropy, providing a unified explanation of these data and of observed trends in the travel times of PKP waves that sample the inner core. We reexamine this interpretation, as well as a previously suggested alternative one in terms of outer core structure. Our motivation comes from recent results which indicate that simple, smooth models of inner core anisotropy are in disagreement with some PKP observations. We invert a recently assembled high quality dataset, comprising modes sensitive to mantle and outermost core structure, as well as modes sensitive to mantle, outer core and inner core structure. We compare models parametrized to include either inner core anisotropy or heterogeneity in the outer core. We show the following: (1) Outer core models, with fewer parameters, provide good overall fits to most modes with weak or strong sensitivity in the inner core, except for mode 3S2, the inner core mode with the strongest splitting. (2) Vs and ρ profiles obtained for models with outer core structure using mantle and core modes are in better agreement with those obtained using data sensitive to mantle structure alone, whereas the Vp structure in D′ shows a stronger c20 component when inner core anisotropy is considered. (3) Lateral heterogeneity restricted to the mantle, in particular strong heterogeneity in D′, fails to account consistently for the splitting of all modes. Outer core models thus deserve further investigation and should be weighted against more complex models of inner core anisotropy to explain anomalous splitting of normal modes and PKP travel times. The outer core structure obtained, while arguably unrealistic, is consistent with concentration of light elements, either confined to the Taylor cylinder tangent to the inner core or in polar caps at the top and bottom of the outer core. A small amount of shear within such polar caps might account for the excess splitting of some core modes.
Three‐dimensional characterization of a fractured granite and transport properties
Authors: Gonzalez‐Garcia R. , Huseby O. , Thovert J.‐F. , Ledésert B. , Adler P. M.
Published: February 2004

A three‐dimensional fracture network in a granite block is reconstructed from a series of experimental serial sections. It is visualized and its most important geometrical characteristics are studied, namely the orientation of the fractures, the connectivity of the fractures, the number of cycles, per unit volume, the distributions of surface areas and of the intersection lengths, and the number of finite solid blocks isolated in the solid matrix by the network. Though the network mostly consists of two families of fractures, it is interesting to note that a simple model of randomly oriented, monodisperse hexagons often yields a good order of magnitude for the various geometrical properties, which have been measured on the real block. The flow properties are then studied; examples of velocity field are provided as well as histograms of velocities; the permeability tensor is determined and is found to be in good agreement with Snow's formula. Finally, dispersion is analyzed by means of a random walk method; histograms of arrival times are provided and interpreted in terms of dispersion tensor.