bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in rotation induced by Pleistocene ice masses with stratified analytical Earth models
Authors: Vermeersen L. L. A. , Fournier A. , Sabadini R.
Published: February 2004

The rotational response of the Earth to Pleistocene deglaciation is studied by means of a multilayered, viscoelastic Earth model based on the preliminary reference Earth model (PREM). Incompressible viscoelastic deformation is evaluated from a self‐compressed initial state. The novelty of our approach stands on the application of a fully analytical normal mode theory to the response of the Earth to surface loads and variations in the centrifugal potential for large numbers of viscoelastic layers, as requested by PREM. Assuming that both present‐day true polar wander (TPW) and changes in the second‐degree component of the geopotential
are solely due to Pleistocene postglacial rebound, we obtain for a two‐layer viscosity model that the upper mantle viscosity must be lowered to about 1–5×1020 Pa s with respect to the classical value of 1021 Pa s. This upper mantle viscosity is accompanied by an increase of the lower mantle viscosity by a factor of 25, in agreement with some recent relative sealevel (RSL) data analyses and convectively supported long‐wavelength geoid anomalies. When the viscosity contrast is located at 1470 km depth, TPW and
require a viscosity of 1021 Pa s in the upper part of the mantle (above 1470 km depth), with a moderate viscosity increase in the lowermost portion of the mantle. This result indicates that a viscosity of 1021 Pa s is appropriate for a wider portion of the mantle than the upper mantle, in agreement with Haskell's [1935] estimate that was not limited to the seismically inferred 670 km boundary.
Determination of long‐term changes in the Earth's gravity field from satellite laser ranging observations
Authors: Cheng M. K. , Shum C. K. , Tapley B. D.
Published: February 2004

Temporal changes in the Earth's gravity field have been determined by analyzing satellite laser ranging (SLR) observations of eight geodetic satellites using data spanning an interval of over 20 years. The satellites used in the analysis include Starlette, LAGEOS 1 and 2, Ajisai, Etalon 1 and 2, Stella, and BE‐C. Geophysical parameters, related to both secular and long‐period variations in the Earth's gravity field, including the geopotential zonal rates (
,
,
,
, and
) and the 18.6‐year tide parameter, were estimated. The estimated values for these parameters are
;
;
;
;
;
(centimeters) and S18.6+20 = −0.1±0.2 (centimeters). The amplitude and phase for the 18.6‐year tide are in general agreement with the effects predicted by the Earth's mantle anelasticity. The solution accuracy was evaluated by considering the effects of errors in various non‐estimated dynamical model parameters and by varying the data span and data sets used in the solution. Estimates for
from individual LAGEOS 1 and Starlette SLR data sets are in good agreement. The lumped sum values for
and
are very different for LAGEOS 1 and Starlette. The zonal rate determination is limited to degree 6 with the current SLR data sets. Analysis of the sensitivity of the solution for the zonal rates to the satellite tracking data span suggests that the temporal extension of the current SLR data sets will enhance the solution of zonal rates beyond degree 6.
Tomographic images of the El Asnam fault zone and the evolution of a seismogenic thrust‐related fold
Authors: Chiarabba C. , Amato A. , Meghraoui M.
Published: February 2004

The P wave velocity structure of the upper 9 km of the El Asnam, Algeria, fault zone, the site of the Ms7.3 earthquake (October 10, 1980), is obtained by inverting arrival times from 310 selected aftershocks. Computed crustal structure and relocation of aftershocks yield new insight on the structural geology of the area, and help to clarify the evolution of fault‐related folding and the complexity of the rupture episodes during the El Asnam earthquake. The correlation between tomograms and geologic structures observed at the surface and inferred at depth delineates local heterogeneities of the upper crust. There is evidence for distinct large‐scale structural trends, consisting of two main thrusts with antithetic reverse faults. Seismic tomography provides reasonable constraints on the El Asnam fault‐related fold geometry, pointing out a complex preexisting structure along which the rupture evolution is controlled by the heterogeneous crust. We propose that the main shock originated along an inverted normal fault bordering the pre‐Neogene basement. Lateral lithologic heterogeneities obtained by seismic tomography reflect the fault segmentation and geometrical discontinuities along strike, in agreement with the tectonic pattern.
Influences of recurrence times and fault zone temperatures on the age‐rate dependence of subduction zone seismicity
Authors: McCaffrey Robert
Published: February 2004

Correlations among subduction zone seismicity, convergence rate and subducting plate age are reassessed considering the possible roles of both recurrence times and fault zone temperatures. Distributions of earthquakes with respect to subducting lithosphere age and convergence rate are grossly explained by a recurrence relation when ages and rates at the world's trenches are taken into account. Correlations between maximum earthquake size Mwmax and convergence rate occur because faster subduction lowers the average recurrence time, so that at random within a limited sampling time, faster subduction zones have larger earthquakes. Published empirical slopes of an assumed linear relationship between Mwmax and convergence rate are predicted to within 1 standard deviation by such a recurrence model. Recurrence predicts that Mwmax should be related to the logarithm of convergence rate and revised age‐rate‐ Mwmax data agree with such a relationship. No resolvable global correlation between age and Mwmax is found. Hence mechanical explanations of subduction zone seismicity based on such correlations are not required. Predicted average steady state temperatures, based on age and dip of the subducting lithosphere and convergence rate, at most subduction zone thrust faults are within a small range of values (±50°C). The few warm outliers, that is, Cascadia, Mexico, and southernmost Chile (south of Chile Rise) that subduct very young lithosphere, also have low seismic coupling coefficients suggesting that high temperature may inhibit seismicity. Fault zone temperatures can explain global variations in seismic coupling coefficients as well as the slab anchor model of Scholz and Campos [1995]. Applying this to the Cascadia subduction zone, in contrast to conclusions drawn from mechanical models of subduction and consistent with many other observations, due to its extremely high temperature, Cascadia may be a region where aseismic subduction predominates.
Fracture localization along faults with spatially varying friction
Authors: Cooke Michele L.
Published: February 2004

Opening‐mode splay fractures have been observed within clusters near fault tips. The spatial distribution of splay fractures along faults influences fluid flow and lends insight into the mechanical processes of faulting. Slip gradients along faults produce stress concentrations which promote the development of opening‐mode splay fractures along faults. Since the slip distribution depends on the distribution of frictional strength along faults, spatial variations in the frictional properties may influence fracture localization. Variations in friction coefficient along faults can reduce the stress singularities at fault tips and promote the development of multiple fractures inwards from the fault tips. The conditions that promote splay crack localization are examined using analytical and numerical fault models. Single splay fractures develop at locations of abrupt friction coefficient change and/or at fault tips when the friction coefficient near the tips is less than a critical value. Fault models with linearly increasing friction coefficient toward the fault tip promote the development of multiple splay fractures within broad zones near the tip.
Application of pattern recognition techniques to earthquake catalogs generated by model of segmented fault systems in three‐dimensional elastic solids
Authors: Eneva Mariana , Ben‐Zion Yehuda
Published: February 2004

Techniques and seismicity parameters described by Eneva and Ben‐Zion [1997] are used to examine synthetic earthquake catalogs generated by Ben‐Zion [1996] for precursory patterns of large model events. Different model realizations represent various levels of fault zone disorder. These include models with uniform properties (U), a Parkfield‐type asperity (A), fractal brittle properties (F), and multi‐size‐scale heterogeneities (M). The seismicity parameters used are based on information contained in typical earthquake catalogs reflecting earthquake distribution in space, time, and size. The analysis highlights the complexity of the information content of the synthetic earthquake catalogs. Simple repetitive precursory signals have not been found. However, local extrema in the examined parameters are found to have significant association in time with large events. Thus our techniques and parameters may be useful for intermediate‐term earthquake prediction, especially when parameters are used in combinations. Some analysis results are the same for all model realizations and some depend on the model. Features characterizing all catalogs are as follows: (1) Large model events are statistically predictable on the basis of patterns in the distribution of smaller events. (2) For a given parameter, the type of precursory extrema (maxima or minima) is the same for all models. (3) The interparameter correlation for any parameter pair has the same sign (positive or negative) in all models. (4) The large events are neither slip‐nor time‐predictable based on previous large events. Results that differ from model to model include the following: (1) The degree of predictability of large events correlates with the degree of regularity in the assumed fault properties, following the order U, F, A, and M. (2) There is no one‐to‐one correlation between type of precursory extrema (maxima or minima) and type of precursory trends (increase or decrease); this produces great variations in observable trends for any given parameter, both from model to model and for different events in the same model. (3) The interparameter correlations vary among models, with the highest correlations in model F. Most discussed patterns are in agreement with observations from seismically active zones, laboratory models, and mining‐induced seismicity.
SKS splitting beneath continental rift zones
Authors: Gao S. , Davis P. M. , Liu H. , Slack P. D. , Rigor A. W. , Zorin Y. A. , Mordvinova V. V. , Kozhevnikov V. M. , Logatchev N. A.
Published: February 2004

We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift‐orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift‐parallel again, although a small number of measurements are still rift‐orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N‐S trending rift. All three rifts are underlain by low‐velocity upper mantle as determined from teleseismic tomography. Rift‐related mantle flow provides a plausible interpretation for the rift‐orthogonal fast directions. The rift‐parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small‐scale mantle convection with rift‐parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift‐parallel fast directions are caused by oriented magmatic cracks.
Dislocation recovery kinetics in single‐crystal calcite
Authors: Liu Ming , Evans Brian
Published: February 2004

We investigated static recovery of dislocations in calcite by annealing a deformed single crystal at 873–1073 K in a CO2 atmosphere (pCO2=0.1 MPa) for different times. The sample used for the annealing experiments was shortened ∼5% parallel to
at 873 K and at a strain rate of ∼10−5 s−1. Slip occurred primarily on
and
with minor contributions from
. Intensively jogged dislocations and sinusoidal, curved dislocations with short wavelengths of∼0.15 μm are common in both the as‐deformed sample and the annealed samples. There is no observable decrease in the average apparent diameter (∼0.1 μm) of dislocation loops in the annealed samples. The activation energy for dislocation recovery is 184±22 kJ mol−1. The recovery kinetics and dislocation substructures suggest that dislocation recovery may be rate controlled by dislocation climb, which is, in turn, controlled by vacancy core diffusion. The dimensions of the bowed‐out dislocation segments and prismatic loops suggest that the source‐sink distance for vacancies along dislocation lines is smaller than the mean free path of core vacancies and may allow core diffusion to be dominant.
Lg coda Q variation across Eurasia and its relation to crustal evolution
Authors: Mitchell Brian J. , Pan Yu , Xie Jiakang , Cong Lianli
Published: February 2004

An extensive set of broadband and short‐period seismograms from regional seismic events has been used to derive a tomographic image of broad‐scale variations of Lg coda Q for most of Eurasia. The image indicates that Lg coda Q, at a frequency of 1 Hz, varies between about 200 and 1000. Lowest Q values (<350) all lie within the Tethysides orogenic belt that was formed from the collision of the African/Arabian and Indian plates with the Eurasian plate. Relatively low Q values are found in the Arabian Peninsula (350–500), a region of recent uplift and volcanism, and in the eastern portion of the Altaid belt (∼450), a region covered by thick sediments. High Lg coda Q values (>800) are found in four regions, the East European shield, the Siberian shield, the Indian shield, and the southwestern (and oldest) portion of the Altaid belt. The pattern of Lg coda Q variation across Eurasia suggests that crustal Q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large‐scale compressional tectonic activity or upper mantle heating that occurred there. We interpret the low Q as resulting largely from hydrothermal fluids, generated by tectonic activity or heating, that now reside in cracks and permeable portions of the Eurasian crust. Crustal Q increases with time, after that activity, as fluids are lost to the surface or absorbed by retrograde metamorphism.
Seismic anisotropy of oceanic upper mantle: Shear wave splitting methodologies and observations
Authors: Wolfe Cecily J. , Silver Paul G.
Published: February 2004

We develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy. To obtain high‐quality estimates of receiver splitting at island stations, we devise a stacking method that finds the optimum splitting parameters and 95% error region for teleseismic shear‐wave phases from a suite of earthquake events. To obtain additional measurements in oceanic regions, we develop techniques to measure splitting parameters and errors for SS phases that sample upper mantle anisotropy at their bounce points. However, we find the data are often of low resolution, and anomalous characteristics are sometimes found that make splitting difficult to interpret. Ten second splitting of SS is observed across the BANJO seismic array, but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce. The receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the Pacific region, with fast polarization azimuth (ϕ) in the lithosphere oriented in the fossil spreading direction and ϕ in the asthenosphere oriented in the absolute plate motion direction. This model has been proposed to explain surface wave data in the Pacific Ocean, but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted. While the island splitting measurements could reflect the influence of individual hotspot upwellings, hotspot effects do not appear to be universally dominant. We propose that splitting observations alternatively indicate broad‐scale differences in the underlying character of oceanic upper mantle anisotropy, associated with coherant patters of lithospheric structure and asthenospheric flow. In particular, splitting, surface wave models, and regional studies all support a model where lithospheric anisotropy throughout the South Pacific has been erased or reoriented toward the absolute plate motion direction, whereas more limited observations in the North Pacific indicate that the fossil lithospheric signature appears to be preserved.
Diagenesis and fault valve seismicity of crustal faults
Authors: Hacker Bradley R.
Published: February 2004

Dehydration and hydration reactions can significantly change permeability and fluid pressure in fault zones. Conversely, sudden drops in fluid pressure caused by faulting, or gradual increases in fluid pressure caused by fault sealing, can cause subsequent long‐term dehydration or hydration of minerals in fault zones. Reactions with gentle Clapeyron slopes are most susceptible to changes in fluid pressure and produce nonintuitive results—dehydration reactions reduce fluid pressure, whereas hydration reactions increase fluid pressure. Dehydration and hydration reactions also influence fault‐valve behavior. The strength of the effect depends on the rate of reaction relative to the rate of fault sealing. Reactions with steep PT slopes operate together with sealing processes to decrease PH2O, but they oppose the decrease in porosity caused by sealing. Reactions with gentle PT slopes work in concert with sealing to change PH2O and porosity. Regardless of the type of reaction, where reaction rates exceed sealing rates, PH2O can be maintained near the equilibrium pressure of the reaction until the reactant minerals or fluids are exhausted, sharply modifying the timescale for earthquake recurrence.
Velocity distribution in snow avalanches
Authors: Nishimura K. , Ito Y.
Published: February 2004

In order to investigate the detailed structure of snow avalanches, we have made snow flow experiments at the Miyanomori ski jump in Sapporo and systematic observations in the Shiai‐dani, Kurobe Canyon. In the winter of 1995–1996, a new device to measure static pressures was used to estimate velocities in the snow cloud that develops above the flowing layer of avalanches. Measurements during a large avalanche in the Shiai‐dani which damaged and destroyed some instruments indicate velocities increased rapidly to more than 50 m/s soon after the front. Velocities decreased gradually in the following 10 s. Velocities of the lower flowing layer were also calculated by differencing measurement of impact pressure. Both recordings in the snow cloud and in the flowing layer changed with a similar trend and suggest a close interaction between the two layers. In addition, the velocity showed a periodic change. Power spectrum analysis of the impact pressure and the static pressure depression showed a strong peak at a frequency between 4 and 6 Hz, which might imply the existence of either ordered structure or a series of surges in the flow.
CRUST 5.1: A global crustal model at 5° × 5°
Authors: Mooney Walter D. , Laske Gabi , Masters T. Guy
Published: February 2004

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948–1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5° × 5° tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a standard database (ETOPO‐5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical Vp‐ Vs and Vp‐density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5.1. We find that the application of crustal corrections to long‐period (167 s) Rayleigh waves significantly increases the variance in the phase velocity maps and strengthens the upper mantle velocity anomalies beneath stable continental regions. A simple calculation of crustal isostacy indicates significant lateral variations in upper mantle density. The model CRUST 5.1 provides a complete description of the physical properties of the Earth's crust at a scale of 5° × 5° and can be used for a wide range of seismological and nonseismological problems.
Thermal fluxes associated with the 1993 diking event on the CoAxial segment, Juan de Fuca Ridge: A model for the convective cooling of a dike
Authors: Cherkaoui Abdellah S. M. , Wilcock William S. D. , Baker Edward T.
Published: February 2004

The 1993 diking event on the CoAxial segment, northern Juan de Fuca Ridge, generated at least three hydrothermal event plumes high in the water column and lower chronic plumes along ∼40 km of the ridge axis. A 2‐year time series of temperature and light attenuation measurements within the water column shows a rapid decay of the maximum rise height of the chronic plumes over the first 3 months following the eruption. We use these measurements to estimate the hydrothermal heat fluxes into the chronic plumes at two different sites. We hypothesize that the chronic plumes resulted from the convective cooling of a dike and construct a simple two‐dimensional, numerical model of this process for a vertical dike intruded into a cold, saturated porous layer. The width of the dike and the permeability and thickness of the porous layer control the transfer of heat from the dike to the seafloor. We investigate the effects of these parameters on the temporal evolution of the surficial heat fluxes. The results show that simple convective cooling of the dike can reproduce the observed temporal evolution of the heat fluxes for permeabilities in the range 10−11‐10−12 m2 and a 3‐ to 5‐m‐wide dike. The high permeabilities obtained are consistent with flow through highly permeable extrusives or with the creation of a zone of high permeability near the dike walls due to fracturing. The width of the dike may well be overestimated by our model because of the limitations of the continuum approximation for the permeability in the narrow upflow zone adjacent to the dike.
From collision to collapse: Complex strain permutations in the hinterland of the Scandinavian Caledonides
Authors: Hartz Ebbe H. , Andresen Arild
Published: February 2004

The Devonian basins of west Norway are located in the hanging wall of a regionally extensive west dipping extensional shear zone, the Nordfjord‐Sogn Detachment Zone, which separates eclogite‐bearing, lower crustal rocks in the footwall from Caledonian allochthonous rocks in the hanging wall. The shear zone is deformed into a series of west plunging, open synforms and antiforms, with the Devonian basins located in the synforms. Previous tectonic interpretations of the Caledonian orogen invoke NW‐SE contraction, followed by E‐W extension. However, analysis of age relationships between faults and folds below sediments within the Hornelen basin indicates a more complex deformation sequence, including (1) foreland‐directed thrusting; (2) early orogen‐parallel extension, forming orogen‐orthogonal grabens; (3) vertical flattening of the upper crust, forming orthorhombic normal faults; (4) E‐W extension contemporaneous with orogen‐parallel shortening, resulting in orogen‐transverse folds and a steeply dipping conjugate strike‐slip fault pattern; and (5) subhorizontal extension along orogen‐parallel faults. The sequence of structural relationships shows a gradual change in orogen‐orthogonal strain from the initial phase of shortening to final phases of extension, which reflects a relative decrease in forces from the colliding continents and a relative increase in gravitational forces related to crustal thickening. The structural observations show that extension in the upper crust was initiated by orogen‐parallel extension rather than shortening. This is in conflict with recent models suggesting that the collapse of the Caledonides was initiated by the northward collision of Avalonia into Euramerica. Alternatively, we suggest that the strain pattern recorded in western Norway is a direct result of orogenic collapse and that both orogen‐parallel extension and shortening may be a common feature in continental collisional orogens, as intermediate phases between overlapping orogen‐parallel shortening (collision) and collapse (extension).
Drilling‐induced core fractures and in situ stress
Authors: Li Yongyi , Schmitt Douglas R.
Published: February 2004

The relationship between the shapes of drilling‐induced core fractures and the in situ state of stress is developed. The stress concentrations at the well bore bottom are first determined using a complete three‐dimensional finite element analysis. Existing in situ compressional stresses generate large tensions in the immediate vicinity of the bottom hole which are sufficient to rupture the rock. Tensile fracture trajectories within these concentrated stress fields are predicted using a simple model of fracture propagation. These modeled fracture trajectories resemble well the observed shapes of drilling‐induced core disking, petal, and petal‐centerline fractures. Further, this agreement suggests that both the shape of the drilling‐induced fracture and the location at which it initiates depends on the in situ stress state existing in the rock mass prior to drilling; the core fractures contain substantial information on in situ stress conditions. In all faulting regimes the coring‐induced fractures initiate near the bit cut except for most cases under thrust faulting regime where the fracture initiates on the well bore axis. Further, under thrust faulting conditions only disk fractures appear possible. Both petal and disking fractures can be produced in strike‐slip and normal faulting regimes depending upon the relative magnitudes between the least compressive horizontal principal stress and the vertical overburden stress. The predicted fracture shapes are in good qualitative agreement with observations of drilling‐induced fractures described in the literature from laboratory experiments and field programs in which in situ stresses are measured by other means. The relationship of the morphology of coring induced fractures and in situ stresses suggests that the fractures can be used as independent complementary indicators in identifying stress regimes.
Persistent water level changes in a well near Parkfield, California, due to local and distant earthquakes
Authors: Roeloffs Evelyn A.
Published: February 2004

Coseismic water level rises in the 30‐m deep Bourdieu Valley (BV) well near Parkfield, California, have occurred in response to three local and five distant earthquakes. Coseismic changes in static strain cannot explain these water level rises because (1) the well is insensitive to strain at tidal periods; (2) for the distant earthquakes, the expected coseismic static strain is extremely small; and (3) the water level response is of the incorrect sign for the local earthquakes. These water level changes must therefore be caused by seismic waves, but unlike seismic water level oscillations, they are monotonic, persist for days or weeks, and seem to be caused by waves with periods of several seconds rather than long‐period surface waves. Other investigators have reported a similar phenomenon in Japan. Certain wells consistently exhibit this type of coseismic water level change, which is always in the same direction, regardless of the earthquake's azimuth or focal mechanism, and approximately proportional to the inverse square of hypocentral distance. To date, the coseismic water level rises in the B V well have never exceeded the seasonal water level maximum, although their sizes are relatively well correlated with earthquake magnitude and distance. The frequency independence of the well's response to barometric pressure in the frequency band 0.1 to 0.7 cpd implies that the aquifer is fairly well confined. High aquifer compressibility, probably due to a gas phase in the pore space, is the most likely reason why the well does not respond to Earth tides. The phase and amplitude relationships between the seasonal water level and precipitation cycles constrain the horizontal hydraulic diffusivity to within a factor of 4.5, bounding hypothetical earthquake‐induced changes in aquifer hydraulic properties. Moreover, changes of hydraulic conductivity and/or diffusivity throughout the aquifer would not be expected to change the water level in the same direction at every time of the year. The first 2.5 days of a typical coseismic water level rise could be caused by a small coseismic discharge decrease at a point several tens of meters from the well. Alternatively, the entire coseismic water level signal could represent diffusion of an abrupt coseismic pore pressure increase within several meters of the well, produced by a mechanism akin to that of liquefaction. The coseismic water level changes in the BV well resemble, and may share a mechanism with, coseismic water level, stream discharge, and groundwater temperature changes at other locations where preearthquake changes have also been reported. No preearthquake changes have been observed at the BV well site, however.
Seismic structure and indicators of magma budget along the Southern East Pacific Rise
Authors: Hooft Emilie E. E. , Detrick Robert S. , Kent Graham M.
Published: February 2004

In this paper we re‐examine the relationship between seismically constrained variations in crustal structure along the southern East Pacific Rise (SEPR) and the segment‐scale variations in axial depth, morphology, basalt geochemistry, and hydrothermal activity that have often been attributed to along‐axis differences in the supply of magma to the mid‐ocean ridge. Along >800 km of the fast spreading SEPR, good correlations exist between axial depth, ridge cross‐sectional area, mantle Bouguer anomaly, and the MgO weight percent of basalts recovered from the rise axis. These correlations indicate along‐axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of ∼100,000 years. In contrast, we show that the depth and width of the midcrustal magma sill, the thickness of seismic layer 2A, and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross‐sectional area. We suggest that the emplacement geometry (width of the intrusion zone and flow lengths), not magma supply, controls extrusive layer (seismic layer 2A) thickness. We hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events (dike intrusion, eruptions, faulting) which occur on much shorter timescales (∼10–100 years) than the longer‐term variations in magma supply reflected in along‐axis changes in the shape and depth of the ridge axis.
Stratigraphic contrasts and tectonic relationships between Carboniferous successions in the Trans‐Alaska Crustal Transect corridor and adjacent areas, northern Alaska
Authors: Dumoulin Julie A. , Watts Keith F. , Harris Anita G.
Published: February 2004

The Carboniferous succession along the Trans‐Alaska Crustal Transect (TACT) corridor in the Atigun Gorge area of the central Brooks Range consists of the Kayak Shale (Kinderhookian) and the Lisburne Group (Kinderhookian through Chesterian). The Kayak Shale is at least 210 m thick; it is chiefly black, noncalcareous shale with several limestone beds of pelmatozoan‐bryozoan packstone and formed in an open‐marine setting. The Lisburne Group is a carbonate rock succession about 650 m thick and consists mainly of skeletal packstone, wackestone, and milestone which contain locally abundant calcispheres, ostracodes, algae, and sponge spicules; it accumulated largely in a shallow water platform environment with restricted circulation. This restriction was probably produced by a coeval belt of skeletal sand shoals recognized 70 km to the west in the Shainin Lake area. Significant and apparently abrupt shifts in the age and lithofacies of Carboniferous strata occur across the central and eastern Brooks Range. These shifts are most marked in a zone roughly coincident with what is interpreted by many workers to be the leading edge of the Endicott Mountains allochthon. Notable lithologie contrasts are also observed, however, between sections in the northern and southern parts of the Endicott Mountains allochthon. This suggests that considerable tectonic shortening has taken place within the allochthon, as well as between it and parautochthonous rocks to the northeast. The Carboniferous section near Mount Doonerak is more similar in age and lithofacies to coeval sections in the central Brooks Range that are considered allochthonous than to parautochthonous sections to the northeast.
Comparison of shear wave splitting and finite strain from the India‐Asia collision zone
Authors: Davis Paul , England Philip , Houseman Gregory
Published: February 2004

We investigate whether observations of shear wave splitting in Asia may be related to distributed strain of the lithosphere or to strain caused by motion of the Asian lithosphere over the deep mantle. The HS2‐NUVEL angular velocity for Eurasia, and that of Minster and Jordan [1978], predict “absolute” plate motions that are a factor of at least 4 slower than the NNE relative motion of India with respect to stable Eurasia. Shear of the upper mantle beneath the south Asian lithosphere is therefore likely to be dominated by the velocity field caused by the internal deformation of Asia, rather than by plate motion. If that shear causes SKS splitting, fast polarization directions in tectonic Asia should be aligned approximately in the direction of the motion of Asian lithosphere relative to the deep mantle, which for southern Asia is NNE. In general, however, fast polarization directions are not aligned with this direction, and in some cases are nearly orthogonal to it. We calculate finite deformation of the Asian lithosphere, in response to its indentation by India and, from the deformation, predict the orientations of fast polarization directions aligned with the principal axes of elongation. These predictions agree with the gross features of the anisotropy in Tibet and the Tien Shan. Orientations of anisotropy in the Baikal region of Siberia are consistent with the orientations of the local principal axes of active strain. Orientations of anisotropy in Mongolia, however, are inconsistent both with the azimuth of the velocity of this region with respect to the deeper mantle and with the orientation of the calculated maximum elongation of the region. We conclude that SKS splitting observations from Asia agree better with those predicted by calculations of internal lithospheric deformation due to indentation by India than with strain predicted from motion of the lithosphere over the deeper mantle. These results place constraints on the depth range where shear wave splitting occurs, which we propose lies predominantly within the lithospheric mantle.
Estimates of the effective elastic thickness of the continental lithosphere from Bouguer and free air gravity anomalies
Authors: McKenzie Dan , Fairhead Derek
Published: February 2004

In oceanic regions the effective elastic thickness Te of the lithosphere is generally estimated from the free air gravity field and approximately corresponds to the depth of the 450°C isotherm. In continental regions the Bouguer anomaly is commonly used and gives thicknesses for shields as great as 130 km, where geotherms reach temperatures of 800–1000°C. If this result were correct, it would require the continental lithosphere to be able to support elastic stresses at considerably higher temperatures than the oceanic lithosphere can. However, detailed examination of the free air and Bouguer anomalies over continents shows that values of Te obtained from Bouguer anomalies over shields are often upper bounds, rather than estimates, because most of the short‐wavelength topography has been removed by erosion, and what remains is incoherent with both the free air and Bouguer gravity anomalies. The present study uses free air anomalies, and topography as a load whose geometry is known. The main importance of subsurface density contrasts and their associated gravity anomalies is that they reduce the signal to noise ratio, without affecting the estimated value of Te. No estimates of Te from free air gravity and topographic data from North America, East Africa, Australia, the Indian peninsula, and the former Soviet Union exceed 25 km, and all are smaller than the seismogenic thickness. The largest estimate, of 42 km, comes from profiles across the Himalayan foredeep. The gravity in this region may be affected by dynamic effects. There is therefore no evidence that the continental lithosphere can support elastic stresses for geological times at temperatures above about 350°C.
Uplift of the Transantarctic Mountains and the bedrock beneath the East Antarctic ice sheet
Authors: ten Brink Uri S. , Hackney Ron I. , Bannister Stephen , Stern Tim A. , Makovsky Yizhaq
Published: February 2004

In recent years the Transantarctic Mountains (TAM), the largest noncontractional mountain belt in the world, have become the focus of modelers who explained their uplift by a variety of isostatic and thermal mechanisms. A problem with these models is a lack of available data to compare with model predictions. We report here the results of a 312‐km‐long geophysical traverse conducted in 1993/1994 in the hinterland of the TAM. Using detailed subglacial topography and gravity measurements, we confirm the origin of the TAM as a flexural uplift of the edge of East Antarctica. Using an elastic model with a free edge, we can jointly fit the topography and the gravity with a plate having an elastic thickness of 85±15 km and a preuplift elevation of 700±50 m for East Antarctica. Using a variety of evidence, we argue that the uplift is coincident with a relatively minor tectonic event of transtensional motion between East and West Antarctica during the Eocene rather than the Late Cretaceous rifting event that created the Ross Embayment. We suggest that this transtensional motion caused the continuous plate to break, which created an escarpment that significantly increased the rates of erosion and exhumation. Results from the geophysical traverse also extend our knowledge of the bedrock geology from the exposures within the TAM to the ice covered interior. Our interpretation suggests that the Ferrar flood basalts extend at least 100 km westward under the ice. The Beacon Supergroup of Paleozoic and Mesozoic sediments thins gradually under the ice and its reconstructed thickness is reminiscent of profiles of foreland basins. Finally, there is no indication in the gravity field for an incomplete rebound due to significant melting of the East Antarctic ice sheet since the last glacial period.
Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge
Authors: Chadwick William W. , Embley Robert W.
Published: February 2004

Grabens have been mapped immediately adjacent to recently erupted submarine lava flows on the Cleft and CoAxial segments of the Juan de Fuca Ridge (JdFR). The grabens are 10–100 m wide and 5–15 m deep and are located uprift and/or downrift of the eruptive vents that fed the flows. We interpret that these structures formed (or were reactivated) directly over the dike that fed the eruptions as it was intruding toward the surface. These graben structures are primary conduits for diffuse hydrothermal venting on the seafloor during the cooling of newly intruded dikes. The axial summit “graben” or “caldera” on the East Pacific Rise (EPR) in some locations has similar dimensions to the grabens observed on the JdFR, and we interpret that it too may be a dike‐induced graben structure (although often buried by subsequent eruptions where it is narrowest). These grabens on the JdFR and the EPR are distinctly narrower and deeper than grabens that formed during well‐documented dike intrusions on slow spreading rifts on land (Iceland and Afar). Mechanical modeling suggests that narrow grabens would form when a dike is at shallow depth where it imposes a high stress perturbation on the ambient stress state to cause faulting. Therefore the narrow grabens on the JdFR and EPR imply that a relatively high level of horizontal compressive stress typically exists perpendicular to the ridges, and this must be overcome by high dike‐induced perturbations to cause faulting. This is probably because gradual plate spreading rarely gets enough time to lower the compressive stress significantly due to the high frequency of dike intrusion events relative to the plate spreading rate. The dikes that do intrude in this environment must have relatively high internal magma pressure. Therefore the size and character of dike‐induced grabens that form at the surface on intermediate to fast spreading ridges reflect the fact that volcanism dominates over tectonism in regulating the local stress state where a robust magma supply is available.
Effect of the variance of pore size distribution on the transport properties of heterogeneous networks
Authors: Bernabé Yves , Bruderer Céline
Published: February 2004

The goal of this paper is to evaluate the effect of the variance of pore size distribution on the transport properties of rocks. Several heterogeneous network realizations with very broad, uniform, or log uniform pore size distributions were constructed. A series of networks were then derived deterministically from these initial networks by repeatedly applying a shrinking operator to the pores of the original realizations. This operator was devised in such a way as to maintain the mean pore size constant while changing the variance of the pore size distribution, therefore allowing its effect on the transport properties to be isolated. We thus assessed the validity of several permeability models from the literature as a function of the variance of the pore size distribution. We found that the product of the permeability by the electrical formation factor was proportional to the square of the critical radius as proposed in the Katz‐Thompson model [Katz and Thompson, 1986]. However, we observed that the dramatic flow localization occurring at high pore size variance was not restricted to the backbone of the critical subnetwork (or critical path) as assumed in the Katz‐Thompson model. We propose that a better justification of the relation mentioned above arises from the underlying percolation problem of the viscous‐inertial transition observed in harmonic flow as a function of frequency. In addition, we appraised the stochastic Bernabé‐Revil model [Bernabé and Revil, 1995]. We found that this model was more and more difficult to implement as the pore size variance was increased. A possible interpretation could be that at high levels of pore‐scale heterogeneity, very large pore size fluctuations occur and the flow pattern is so strongly and determimstically related to these extreme fluctuations that a stochastic description becomes inadequate.
Aftershock series of event February 18, 1996: An interpretation in terms of self‐organized criticality
Authors: Correig Antoni M. , Urquizú Mercè , Vila Josep , Manrubia Susanna C.
Published: February 2004

An aftershock interevent time series, initiated on February 18, 1996, in the eastern Pyrenees was analyzed. The threshold detection magnitude was set at 1.9, and the series was assumed to be complete for an interval of 77 days. The original time series does not fit Omori's law, probably because of sudden changes in the rate of occurrence, interpreted as an increase in the production rate. When the recorded interevent time series is classified in terms of leading aftershocks (those that satisfy a relaxation process) and cascades (those occurred at a nearly constant rate), the new time series of the leading aftershocks fits Omori's law quite well, with p=0.94. Interpreted in terms of Dietrich's model, the series of leading aftershocks correctly predicts a return time for the main shock of the order of 50 years. To interpret the series of cascades, a minimalist, self‐organized critical model was used. Although it is very simple, the model correctly reproduces the two‐level structure in the observed time series, that is, the sequence of leading aftershocks and a cascade sequence emerging from each aftershock. This model may be given physical justification in terms of the Cochard and Madariaga [1996] nucleation model.