bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional wideband electromagnetic modeling on massively parallel computers
Authors: Alumbaugh David L. , Newman Gregory A. , Prevost Lydie , Shadid John N.
Published: February 2004

A method is presented for modeling the wideband, frequency domain electromagnetic (EM) response of a three‐dimensional (3‐D) earth to dipole sources operating at frequencies where EM diffusion dominates the response (less than 100 kHz) up into the range where propagation dominates (greater than 10 MHz). The scheme employs the modified form of the vector Helmholtz equation for the scattered electric fields to model variations in electrical conductivity, dielectric permitivity and magnetic permeability. The use of the modified form of the Helmholtz equation allows for perfectly matched layer ( PML) absorbing boundary conditions to be employed through the use of complex grid stretching. Applying the finite difference operator to the modified Helmholtz equation produces a linear system of equations for which the matrix is sparse and complex symmetrical. The solution is obtained using either the biconjugate gradient (BICG) or quasi‐minimum residual (QMR) methods with preconditioning; in general we employ the QMR method with Jacobi scaling preconditioning due to stability. In order to simulate larger, more realistic models than has been previously possible, the scheme has been modified to run on massively parallel (MP) computer architectures. Execution on the 1840‐processor Intel Paragon has indicated a maximum model size of 280 × 260 × 200 cells with a maximum flop rate of 14.7 Gflops. Three different geologic models are simulated to demonstrate the use of the code for frequencies ranging from 100 Hz to 30 MHz and for different source types and polarizations. The simulations show that the scheme is correctly able to model the air‐earth interface and the jump in the electric and magnetic fields normal to discontinuities. For frequencies greater than 10 MHz, complex grid stretching must be employed to incorporate absorbing boundaries while below this normal (real) grid stretching can be employed.
On the radiophysics and geophysics of ionogram spread F
Authors: Wright J. W. , Argo P. E. , Pitteway M. L. V.
Published: February 2004

We consider the interpretation of “spread F” ionograms, particularly those echoes that determine most of the radio bandwidth of the phenomenon. We compare expectations based on total internal reflection with theoretical descriptions based on underdense scattering. We conclude that “Rayleigh,” “Bragg,” and “diffuse multiple‐refractive” radio scattering theories are not consistent with observed properties of these echoes. The total‐reflection interpretation is shown to be consistent with rocket and satellite data. Evidence for multiple refractive scatter is found, but it neither greatly nor subtly extends the observed radio bandwidth. Ionograms therefore indicate comprehensively the range of plasma densities within view of the ionosonde, even in conditions of spread F. Using digital ionosonde observations, much can be learned of the spatial structures of plasma density responsible for spread F at auroral and equatorial latitudes. As an example, we suggest how contemporary theory and simulations of the equatorial “bubble” phenomenon may be reconciled with the distinctive equatorial spread F pattern.
The electromagnetic field emitted by a pulsed current traveling along finite, straight wires above the interface of a two‐media configuration
Authors: Kooij B. J.
Published: February 2004

The electromagnetic radiation due to a pulsed traveling wave current along a polygon wire structure located above a lossless half‐space is investigated. The Cagniard‐de Hoop technique with polar variables is used in order to derive closed form expressions for the electric field in the two‐media configuration. Numerical results for the electric field above a lossless half‐space are presented for a four wire polygon structure in which a known traveling wave current is present.
Wavelet‐based methods for the nonlinear inverse scattering problem using the extended Born approximation
Authors: Miller Eric L. , Willsky Alan S.
Published: February 2004

In this paper, we present an approach to the nonlinear inverse scattering problem using the extended Born approximation (EBA) on the basis of methods from the fields of multiscale and statistical signal processing. By posing the problem directly in the wavelet transform domain, regularization is provided through the use of a multiscale prior statistical model. Using the maximum a posteriori (MAP) framework, we introduce the relative Cramér‐Rao bound (RCRB) as a tool for analyzing the level of detail in a reconstruction supported by a data set as a function of the physics, the source‐receiver geometry, and the nature of our prior information. The MAP estimate is determined using a novel implementation of the Levenberg‐Marquardt algorithm in which the RCRB is used to achieve a substantial reduction in the effective dimensionality of the inversion problem with minimal degradation in performance. Additional reduction in complexity is achieved by taking advantage of the sparse structure of the matrices defining the EBA in scale space. An inverse electrical conductivity problem arising in geophysical prospecting applications provides the vehicle for demonstrating the analysis and algorithmic techniques developed in this paper.
A generalized uniform geometrical theory of diffraction ray solution for the diffraction by a wedge with convex faces
Authors: Liang M. C. , Chuang C. W. , Pathak P. H.
Published: February 2004

An approximate asymptotic high‐frequency solution is obtained for the electromagnetic wave diffraction by a wedge with convex faces and a moderate wedge angle. The present solution properly includes the edge‐excited surface ray effect and can predict the transition between the space ray and the surface ray. This solution is uniform across the various ray shadow boundaries even when the transition regions associated with these ray shadow boundaries overlap and is expressed in the simple format of the geometrical theory of diffraction. The ansatz for the present solution is based on a uniform solution for the scattering from a curved screen obtained earlier by C.W. Chuang and M.C. Liang, used in conjunction with an asymptotic matching technique. Numerical results based on the present uniform geometrical theory of diffraction solution are found to be in excellent agreement with those based on an independent moment method solution of the exact integral equation.
The onset of movement on the Philippine Fault in eastern Mindanao: A transition from a collision to a strike‐slip environment
Authors: Quebral Ramon D. , Pubellier Manuel , Rangin Claude
Published: February 2004

The spatial and temporal evolution of eastern Mindanao from a zone of active arc‐arc collision to one of pure strike slip environment is addressed. This is made possible by tracing the Philippine Fault, confined to a well defined and narrow zone of deformation, and by differentiating from it earlier collision‐related compressional features, whose deformation is distributed over a broad zone. A diachronous unconformity marking the end of collision‐related deformation reflects the southward propagating nature of the collision and provides limits on the age of initiation of the Philippine Fault. This diachronism suggests that the initiation of the Philippine Fault is related to the completion of the collision. We show the rapidity at which formation and propagation of the fault follows the locking of the collision zone. We likewise demonstrate that the Philippine Fault reactivates pre‐existing collisional features in cutting across the southern Pacific Cordillera.
Tectonics of the Punta Bianca promontory: Insights for the evolution of the Northern Apennines‐Northern Tyrrhenian Sea basin
Authors: Storti Fabrizio
Published: February 2004

The structural setting of the eastern headland of the La Spezia Gulf (Punta Bianca promontory) provides some constraints on the contractional and extensional framework of the Northern Apennines‐Northern Tyrrhenian Sea basin geodynamic system and allows a detailed reconstruction of a complex evolutionary path. Out‐of‐sequence thrusting is supposed in the inner side of the chain. Two different extensional events are identified; the older (early‐middle Miocene) may be related to the imbrication of the deeper metamorphic units of the Apenninic thrust wedge. Younger extension fits very well into an eastward asymmetrical faulting framework, related to the opening of the Northern Tyrrhenian Sea basin, from the late Miocene to the Pliocene‐Pleistocene (?). Well‐exposed low‐angle normal faults developed during this event. Extensional structures were then overprinted and reactivated by a mainly strike‐slip regime in the Quaternary.
U‐Pb geoehronology and evolution of the English River Subprovince, an Archean low P‐high T metasedimentary belt in the Superior Province
Authors: Corfu F. , Stott G. M. , Breaks F. W.
Published: February 2004

U‐Pb isotopic data for zircon, monazite, and titanite are used to assess the evolution of the late Archean English River Subprovince (ERS) and adjacent domains of the Winnipeg River Subprovince in the northern Superior Province, Ontario. The ERS consists of turbiditic metasedimentary rocks, deposited during the final stages of magmatic and tectonic accretion within the greenstone‐granite Uchi Subprovince to the north at around 2720 to 2710 Ma. The sedimentary rocks were intruded by a suite of calc‐alkalic plutons at 2698 Ma. Major regional deformation, amphibolite to granulite facies metamorphism, anatexis, and emplacement of an extensive peraluminous granitic suite culminated at 2691 Ma. Late episodes of metamorphism, metasomatism, and emplacement of pegmatites occurred locally at circa 2680 and 2669 Ma. In contrast, the Winnipeg River Subprovince to the south consists of widespread tonalitic suites formed mainly between 3200 and 2800 Ma and granodioritic to granitic intrusions formed between 2710 and 2690 Ma. Metamorphism occurred at around 2710 Ma in southern domains, and a distinct granulite facies event at around 2680 Ma affected central and northern domains of the Winnipeg River Subprovince. The rapid sequence of events recorded in the ERS supports the notion that metamorphism was caused by thermal perturbations related to injection of granitic magmas. These melts were generated through partial melting of the lower crust by a thermal anomaly presumably caused by mantle upwelling related to collapse of the inactivated subducting plate. The short‐lived nature of the metamorphic events and their cyclicity follow the behaviour predicted by recent models linking the evolution of strain rates to episodes of segregation and upward magma transfer in the course of prolonged orogenic events.
Paleomagnetism, magnetic anisotropy, and mid‐Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex
Authors: Bogue Scott W. , Gromme Sherman , Hillhouse John W.
Published: February 2004

We report paleomagnetic results from layered igneous rocks that imply substantial post mid‐Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low‐Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well‐developed gravitational layering that was deformed during initial cooling (but below 540°C) into folds that plunge moderately to the west‐southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site‐mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the site‐mean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56°±10°) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (±1300 km) of poleward transport relative to the North American craton. Between mid‐Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes.
Cenozoic motion of the Philippine Sea Plate: Palaeomagnetic evidence from eastern Indonesia
Authors: Hall Robert , Ali Jason R. , Anderson Charles D.
Published: February 2004

The history of motion of the Philippine Sea Plate is poorly known because it is isolated from the oceanic ridge system. Interpretation of palaeomagnetic results from the plate has been controversial because declination data have been obtained only from the eastern margin where subduction‐related tectonic processes may have caused local rather than plate‐wide rotations. New palaeomagnetic data relevant to the problem have been obtained from 34 sites north of the Sorong Fault and 29 sites within the Sorong Fault system. These sites record southward movement during the Eocene and northward movement during the Neogene. Sites within the Sorong Fault system record both counterclockwise and clockwise rotations interpreted as the result of Neogene block movements at the southern boundary of the Philippine Sea Plate. North of the Sorong Fault, all sites record clockwise declinations. Neogene rocks have small deflections consistent with rotation about the present‐day Eurasia‐Philippine Sea Plate pole. Oligocene‐middle Eocene rocks show consistent clockwise declination deflections of ∼40°. Declinations of lower Eocene rocks indicate ∼90° of clockwise rotation. We propose that the entire area north of the Sorong Fault in east Indonesia has always been part of the Philippine Sea Plate and that the whole plate has rotated clockwise in a discontinuous manner by approximately 90° since the early Eocene. The new data from north of the Sorong Fault provide a basis for determining rotation poles which satisfy all the palaeomagnetic data from the Philippine Sea Plate and permit its reconstruction.
Change in fabric of melange in the Shimanto Belt, Japan: Change in relative convergence?
Authors: Onishi Celia Tiemi , Kimura Gaku
Published: February 2004

Systematic changes in fabrics of melanges have been detected in rocks of the Shimanto Belt, Japan. Several shear indicators, including S‐C structures, Riedel shears, and folds, in the melanges indicate consistent sense of shear both in outcrop and microscopic scales. The deformation mechanisms, independent particulate flow and pressure solution, suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening. After lithification, cataclastic deformation occurred locally, but most of the earlier deformation features are preserved. This process appears to have taken place along the decollement beneath the off‐scraped accretionary prism. Sinistral reverse sense of shear is observed in the Campanian melange, whereas a dextral reverse sense of shear is preserved in the early Eocene melange in the Shimanto Belt. This change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates. Three different plate models have been proposed for the Late Cretaceous to Early Tertiary western Pacific margin: (1) subduction of the Kula‐Pacific ridge along the continental margin, (2) subduction of the Pacific Plate since 85 Ma, and (3) subduction of the aborted Kula‐Pacific ridge after 43 Ma. The observed change in fabric of melange is consistent only with the third model, which predicts a change in relative convergence from sinistral to dextral at the appropriate time.
Seismotectonics of the April 25, 1992, Petrolia earthquake and the Mendocino triple junction region
Authors: Tanioka Yuichiro , Satake Kenji , Ruff Larry
Published: February 2004

The April 25, 1992, Petrolia earthquake (M s 7.1) occurred at the southern tip of the Cascadia subduction zone. This is the largest thrust earthquake ever recorded instrumentally in the Cascadia subduction zone. The earthquake was followed by two large strike‐slip aftershocks (both Ms 6.6). Moment release of each of the earthquakes is as follows: 4.0 × 1019 Nm in the first 10 s for the mainshock, 0.7 × 1019 Nm in the first 8 s for the first aftershock, and 0.9 × 1019 Nm in the first 2 s for the second aftershock. These indicate that the mainshock and each of the aftershocks may have different tectonic backgrounds. The best depth estimates of the mainshock and the two aftershocks are 14 km, 18 km, and 24 km, respectively. The slip direction of the mainshock is between N75°E and N80°E. This slip direction is not consistent with either the relative motion of the North American and Juan de Fuca plates (N60°E) or between the North American plate and the Gorda deformation zone (N40°E). It has been suggested that the North American‐Pacific plate motion is accommodated by right‐lateral slip on both the San Andreas and Maacama‐Rodgers Creek‐Hayward fault systems; the intervening block is the Humboldt plate. If we modify the relative motion of the southernmost Gorda deformation zone to conform with the seismicity trends and allow the Humboldt‐Pacific plate motion to be about half the total North American‐Pacific motion, then the Gorda deformation zone‐Humboldt relative motion matches the direction of the Petrolia slip vector. Also, the mixture of focal mechanisms in the two distinct aftershock clusters can be explained by motion between the Gorda deformation zone and Pacific plate and the Humboldt and North American plates. The Gorda deformation zone is subducting beneath the Humboldt plate in the Cape Mendocino area, and the Petrolia earthquake ruptured the entire subduction segment between the Gorda deformation zone and Humboldt plate.
Structural variation and landward vergence at the toe of the Oregon accretionary prism
Authors: MacKay Mary E.
Published: February 2004

The Oregon margin near 45°N spans a regional transition in structural style from seaward vergence in the south to landward vergence in the north. This variation probably reflects a regional change in both sediment type and rate of deposition that affects the potential for overpressure in the sediments. Structural style within the survey area shows a gradual northward transition from seaward to landward vergence and to lower slopes within the landward vergent area, suggesting a northward decrease in basal shear stress. Superimposed on this gradational variation are abrupt changes in structural style that correlate with NW striking strike‐slip faults in the Cascadia Basin. Because sediments thicken toward the east, translation along the strike‐slip faults results in juxtaposition of sediments with different physical properties and loading histories. In addition, the faults themselves may act as fluid conduits, resulting in stepwise changes in pore pressure on the décollement and concomitant change in structural style across the faults. Although the Oregon‐Washington margin is dominated by landward vergence, landward vergence has not been adequately explained by theoretical models or replicated in experimental models because of a simple omission in the boundary conditions of the underlying conceptual model. Landward vergence requires not only low basal shear stress but also an arcward dipping décollement (and to a lesser degree, a relatively strong wedge). In order for landward vergence to predominate, these three factors must combine in such a way that the backward verging thrust planes are favored.
Stress field fluctuations along the Dead Sea rift since the middle Miocene
Authors: Eyal Yehuda
Published: February 2004

The Dead Sea transform (DST) is the suture between the African and the Arabian plates, both of which subduct northward under the Eurasian plate. The collision of these plates with Eurasia and their relative motion are the source for their internal deformation, especially in the vicinity of the Dead Sea transform. Geological observations indicate that two distinct paleostrain regimes operated adjacent to the Dead Sea rift and in the Sinai‐Israel subplate: (1) WNW shortening (less than 10%) and NNE extension, beginning in the Turonian, is associated with the development of the Syrian Arc fold belt and attributed to the Syrian Arc stress field (SAS); and (2) middle Miocene to Recent NNW shortening and ENE extension, associated with the 105‐km sinistral displacement along the Dead Sea transform and the opening of the Red Sea, is attributed to the Dead Sea stress field (DSS). It was originally suggested that the SAS terminated during the middle Miocene with initiation of the DSS. However, trend and age analysis of many structures shows that formation of SAS‐compatible structures continued after the middle Miocene up to the Recent. In some middle Miocene and younger rocks structures, such as faults, dikes, volcanic lineaments, and tectonic stylolites compatible with both stress fields were reported. In a few locations, structures compatible with both stress fields crosscut each other. It is suggested that the movements which resulted in the development of the Syrian Arc and other SAS‐compatible structures are continuing to the Recent and that DSS movements are superimposed upon them. The overlapping of SAS and DSS structures adjacent to the DST results from spatial and temporal fluctuations in the overall stress state. We propose that such variations could be caused by the superposition of large earthquake stress drops, associated with movements along the DST, on a steady background, plate scale, stress regime associated with the SAS. Thus DSS‐compatible structures should form in preseismic activity periods, when DST‐related elastic strain is high. On the other hand, the SAS‐compatible structures should form during interseismic activity times, subsequent to large local stress release along the DST.
Structural framework and denudation history of the flanks of the Kenya and Anza Rifts, East Africa
Authors: Foster David A. , Gleadow Andrew J. W.
Published: February 2004

The Kenya Rift in central Kenya (between ∼1° and 2° north latitude) is flanked by vast areas of Proterozoic crystalline rock where traditional stratigraphic markers useful for constraining Mesozoic to Recent structures are absent. Apatite fission track age and length data from about 100 samples collected from (1) relief profiles in north‐south trending mountain ranges, and (2) east‐west transects between mountain ranges constrain the structural framework of Cretaceous through Tertiary faulting in these flanking areas. Data from the profiles reveal regionally consistent age/elevation trends characterized by three distinct intervals, of 600 to 1200 m altitude, within which the apatite apparent ages are nearly concordant. The three isochronous age/elevation intervals yield apparent ages of ∼180, ∼115, and ∼65 Ma and record times of cooling starting at ≥220, 140–120, and 70–60 Ma. We interpret the cooling to be related to episodes of relatively rapid denudation separated in time by periods of slower exhumation. The isochronous intervals of crust are regionally consistent and relative offsets between the intervals allow fault displacements and block tilts to be estimated, when used in conjunction with the variation of fission track age and length along the transects. The fault block geometry revealed by the displaced isochronous layers indicates that normal faults related to rifting, with relative displacements >1 km, extend at least 100 km east of the Kenya Rift Valley. The data indicate a consistent direction of block tilting for the segment of the rift studied, which is similar to the asymmetric extension observed for other sections of the East Africa Rift System and other narrow rifts. The regional block faulting in this area probably occurred during early to middle Tertiary time associated with the late stages of extension in the Anza Rift and/or the early stages of extension of the Kenya Rift, in a setting similar to the present morphology of northern Tanzania. Therefore focusing of faulting within a central rift valley in Kenya appears to have followed a phase of regional extension with minor total strain.
Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group
Authors: Bassett Kari N. , Kleinspehn Karen L.
Published: February 2004

The age relations, geochemistry, and sedimentology of the Rocky Ridge Formation of the Skeena Group are used to test competing tectonic reconstructions for the mid‐Cretaceous Canadian Cordillera as well as the timing and location of the accretion of the Insular Superterrane. Pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the Bowser basin in the Early Albian to Early Cenomanian. Single‐crystal fusion and step‐heating 40Ar/39Ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded Middle Cenomanian ages of 94.3 ± 0.4, 95.6 ± 1.6, and 95.0 ± 1.6 Ma. Vesicular basalt flows interbedded with crystal‐rich tuff breccias contain evidence for hot emplacement as pyroclastic flows. Individual eruptive centers are identified by their proximal facies, paleoflow indicators within the lava flows, paleoflow indicators within interbedded volcaniclastic fluvial deposits, geochemical differences, and geographic isolation of volcanic deposits. Major and trace‐element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics. The basalts of the northern Rocky Ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative Nb‐Ta anomalies whereas the basalts of the southern Tahtsa Lake volcanic center show depletion to slight enrichment of light rare earth elements, slight enrichment of large ion lithophile elements with minimal negative Nb‐Ta anomalies. The geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension. The presence of deeper marine facies to the west and the lack of a western sediment source in the Skeena Group indicate that the technically active Insular Superterrane was not west of the study area during mid‐Cretaceous time. Thus we reconsider the Omineca Belt as the main axis of a mid‐Cretaceous continental arc, placing the Intermontane Superterrane in the intraarc to forearc position with the Rocky Ridge volcanics erupted along the forearc side of the Omineca arc. Coeval regional strike‐slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for Rocky Ridge intraarc extension. An electronic supplement of Tables A1–A2 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC03496, Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group, Kari N. Bassett and Karen L. Kleinspehn). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.
Mechanisms of inheritance of rift faulting in the western branch of the East African Rift, Tanzania
Authors: Theunissen K. , Klerkx J. , Melnikov A. , Mruma A.
Published: February 2004

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike‐slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes, during which the complex rift segment formed upon the multiphase Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile‐brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift basins.
Modeling of Neogene crustal block rotation: Case study of southeastern California
Authors: Schelle Holger , Grünthal Gottfried
Published: February 2004

Numerical modeling using a two‐dimensional distinct element method (DEM) was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the Pacific‐North America transform plate boundary. Paleomagnetic and structural investigations have shown that large (30°–50°) horizontal rotations of crustal blocks occurred in the Eastern Transverse Zone and the Northeast Mojave Block (southeastern California) since the end of early Miocene. The main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not. The simulations were performed using different stress boundary conditions and different parameters of the fault strength. Applying an assumed ratio of 2 for the horizontal stress components as boundary condition, block rotations in the order of 40° only occur if (1) the major horizontal stress SH is oriented N20°E with a maximal deviation of 15° to the east or west, and (2) the fault strength described by the friction coefficient is smaller than 0.12, or 14% only of the value 0.85 for intact rocks (Byerlee's law). For a weakening model of the fault strength an initial friction coefficient of 0.85 allows also rotations of about 40° if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0.12 or smaller. For the same amount of rotation (40°) a decreasing stress ratio requires a lowering of the friction coefficient used, whereas an increase of the ratio allows a higher friction coefficient. The simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation, especially the cumulative slip values. Discrepancies in the observations and modeling results for the western parts of the Mojave Desert are the result of a neglect of the three‐dimensional deformation in our two‐dimensional modeling.
Accretion of the southern Banda arc to the Australian plate margin determined by Global Positioning System measurements
Authors: Genrich Joachim F. , Bock Yehuda , McCaffrey Robert , Calais Eric , Stevens Colleen W. , Subarya Cecep
Published: February 2004

Global Positioning System geodetic measurements at thirteen locations in Indonesia and four in Australia reveal that the Australian continent has accreted the Banda island arc to its margin. Small relative velocities of five sites on west Java, south Kalimantan, Bali, and south Sulawesi define a rigid Sunda shelf that moves relative to northern Australia in a manner consistent with pole locations from NUVEL‐1 Australia‐Eurasia but at a rate that is about 7% slower. Block‐like northward motion of the southern Banda arc toward the Sunda shelf at nearly the same rate as Australia suggests that the Timor trough is now inactive as a thrust. Little of the convergence of Australia with Eurasia is accommodated by strain within the Banda arc structure. Most of the convergence appears to occur as northward translation of the rigid arc with shortening on the Flores and Wetar thrusts and possibly on faults within the back arc basin.
A Nonlinear Mixed Integer Program Model for Evaluating Runoff Impoundments for Supplemental Irrigation
Authors: Ziari Houshmand A. , McCarl Bruce A. , Stockle Claudio
Published: February 2004

On‐farm runoff collection through small impoundments (ponds) is a potential irrigation water source. This study evaluates the economic feasibility of such impoundments for supplemental irrigation in the Blacklands region of Texas. This is done using a risk sensitive model which simultaneously considers water supply, irrigation system investment, irrigation scheduling, and crop mix selection. A two‐stage, mixed integer, nonlinear mathematical programming model under uncertainty, was used to formulate the problem and solved with Benders' decomposition. Pond‐based supplemental irrigation is found to be both risk reducing and net income increasing in the study area. The model results also show off‐farm water needs to be worth more than $100 per acre foot to make impoundments undesirable.
Modified Soil Water Retention Functions for All Matric Suctions
Authors: Fayer Michael J. , Simmons C. Steven
Published: February 2004

The Brooks‐Corey and van Genuchten functions were modified to adequately represent retention at all matric suctions. The modification consisted of replacing the residual water content with an adsorption equation (Campbell and Shiozawa, 1992). The modified functions retain the form of the original functions in the wet range and the form of an adsorption equation in the dry range. The modified functions provided excellent fits to data from six soils with textures ranging from sand to silty clay. The modified van Genuchten function can use previously determined parameters to obtain a reasonable representation of the high matric suction range, thus allowing for the use of existing parameter sets. The modified functions, as well as the function proposed by Rossi and Nimmo (1994), were combined with the Mualem conductivity model to generate closed‐form analytical expressions for the calculation of hydraulic conductivity.
Watering up After Clear‐Cutting on Forested Wetlands of the St. Lawrence Lowland
Authors: Dubé Stéphane , Plamondon André P. , Rothwell Richard L.
Published: February 2004

Clear‐cutting on wetlands of the St. Lawrence lowlands raised the water table levels on seven of the eight studied sites encompassing five forest types and four soil subgroups. Water table levels dropped 3 cm after clear‐cutting on the eighth site, which was the wettest site with precut water levels within the top 10 cm. This was explained by evaporation from exposed water surfaces. The magnitude of the water table rises increased with the depth of the precut water table. The seasonal mean and maximum rise were respectively 20 and 52 cm on a poorly drained mineral soil which had the lowest precut water table levels. The smallest rises, with means around 7 cm, were associated with high precut water table on bogs and on fens. The watering up was not reduced on fens where a lateral flow occurs. This study indicated that transition sites between the bogs or fens and the uplands were most susceptible to hydrologic changes after clear‐cutting. Watering up was caused by reduced evapotranspiration, of which the major component was interception. The rise of the water table observed in the clear‐cut and the bordering forest indicates that clear‐cutting in narrow strips is not an effective solution to avoid water table rise. Silvicultural treatments to maintain interception and transpiration by leaving logging debris, small trees, and preestablished regeneration would be more effective.
Fingering of Dense Nonaqueous Phase Liquids in Porous Media: 2. Analysis and Classification
Authors: Held Rudolf J. , Illangasekare Tissa H.
Published: February 2004

Fingering of dense nonaqueous phase liquids (DNAPLs) as seen in three‐dimensional experirments with saturated, homogeneous porous media was analyzed. A consistent geometrical quantification of finger configurations was obtained using concepts of fractal and multifractal scaling. Fractal patterns that determine the probabilistic distribution of the DNAPL were found to be representative for every experimental combination of sand and DNAPL. These patterns could be attributed to either capillary or viscous fingering regimes. With multifractal formalisms we were able to give a description of the underlying process kinetics. The generalized dimension Dq relates results to diffusion‐limited aggregation (DLA) or invasion percolation type models. The spectrum of singularities ƒ(α) is invariable for cross sections of an experiment and in turn can be used for a classification of the displacement system. The width of the ƒ(α) curve in the range of positive moments quantifies displacement instability. Phase transitions are indicated for the more stable displacement systems.
On Characterization of Anomalous Dispersion in Porous and Fractured Media
Authors: Berkowitz Brian , Scher Harvey
Published: February 2004

A key characterization of dispersion in aquifers and other porous media has been to map the effects of inhomogeneous velocity fields onto a Fickian dispersion term (D) within the context of the conventional advection‐dispersion equation (ADE). Recent compilations of data have revealed, however, that the effective D coefficient is not constant but varies systematically with the length or timescale over which transport occurs. A natural strategy to encompass this “anomalous” behavior into the context of the conventional ADE is to make D time dependent. This approach, to use D(t) to handle the same anomalous dispersion phenomena, has also been common in the field of electronic transport in disordered materials. In this paper we discuss the intrinsic inadequacy of considering a time‐dependent dispersivity in the conventional ADE context, and show that the D = D(t) generalization leads to quantifiably incorrect solutions. In the course of proving this result we discuss the nature of anomalous dispersion and provide physical insight into this important problem in hydrogeology via analysis of a class of kinetic approaches. Particular emphasis is placed on the effects of a distribution of solute “delay times” with a diverging mean time, which we relate to configurations of preferential pathways in heterogeneous media.
Multiphase Nonisothermal Transport of Systems of Reacting Chemicals
Authors: White S. P.
Published: February 2004

A mathematical model of multiphase transport of heat, mass, and reacting chemical species in a porous media is developed. The model is appropriate for describing many of the reactions that take place in a geothermal reservoir. This model leads to the separation of transport terms from reaction terms in the equations for chemical species concentration. A numerical method for the solution of the transport equations is developed based on the integrated finite difference method. Four example problems representative of those of interest in geothermal reservoirs are solved. These examples consider up to 14 chemical species and nine chemical reactions among these species.
Stochastic Analysis of the Velocity Covariance and the Displacement Covariance Tensors in Partially Saturated Heterogeneous Anisotropic Porous Formations
Authors: Russo David
Published: February 2004

Considering steady state, unsaturated flow in an unbounded domain of heterogeneous formation with three‐dimensional, anisotropic structure, first‐order approximations of the principal and off‐diagonal components of the velocity covariance tensor were derived for the general case in which the separation vector is inclined to the mean gradient vector, J, at arbitrary angles, taking into account the texture of the soil materials constituting the heterogeneous formation. The derived velocity covariance tensors were employed for modeling transport in the vadose zone for the general case in which J is inclined to the principal axes of the formation heterogeneity at arbitrary angles, by the use of a Lagrangian formulation. Results of this study suggest that under unsaturated flow, for given statistics of the formation properties and given water saturation θ, macrodispersion will diminish in formations of coarser texture when J does not coincide with the principal axes of the heterogeneous formation. In this case and in formations of coarser texture, the principal axes associated with the principal components of the dimensionless macrodispersion tensor, D′ij may be deflected in a direction opposite to that of J relative to the principal axes of the formation heterogeneity. Coarser texture may compensate for the enhanced lateral dispersion and the enhanced deflection of the principal axes of the principal components of D′ij, due to an increase in the statistical anisotropy of the heterogeneous formation. For a formation of given statistics and texture and for a given θ, the longitudinal component of D′ij decreases while its transverse components increase when J is more inclined to the longitudinal axis of the formation heterogeneity. The latter situation is pertinent to periods of redistribution between successive water applications, in which increasing capillary pressure gradients combined with spatial heterogeneity in the formation properties may increase the transverse components of J.
Cost‐Effective Abatement of Multiple Production Externalities
Authors: Connor Jeffery D. , Perry Gregory M. , Adams Richard M.
Published: February 2004

This article explores the issue of multiple externalities through a case study of irrigated agricultural production in eastern Oregon. A mathematical programming model is used to demonstrate the manner in which a policy directed at one externality (soil erosion) may influence the incidence of another externality (groundwater pollution). A key determinant of multiple externality outcomes is interdependence in the processes producing agricultural commodities and externalities. Potential benefits from coordinating a policy to address multiple environmental objectives are discussed.
A Distributed Slope Stability Model for Steep Forested Basins
Authors: Wu Weimin , Sidle Roy C.
Published: February 2004

A distributed, physically based slope stability model (dSLAM), based on an infinite slope model, a kinematic wave groundwater model, and a continuous change vegetation root strength model, is presented. It is integrated with a contour line‐based topographic analysis and a geographic information system (GIS) for spatial data extraction and display. The model can be run with either individual rainfall events or long‐term sequences of storms. These inputs can be either actual storm records or synthesized random events based on Monte Carlo simulation. The model is designed to analyze rapid, shallow landslides and the spatial distribution of safety factor (FS) in steep, forested areas. It can investigate the slope stability problem in both temporal and spatial dimensions, for example, the impact of timber harvesting on slope stability either at a given time or through an extended management period, the probability of landslide occurrence for a given year, and the delivery of landslide sediments to headwater streams. The dSLAM model was applied in a steep, forested drainage of Cedar Creek in the Oregon Coast Ranges using actual spatial patterns of timber harvesting and measured rainfall during a major storm which triggered widespread landslides in that area in 1975. Simulated volume and number of failures were 733 m3 and 4, respectively. These values agreed closely with field measurements following the 1975 storm. However, the effect of parameter uncertainty may complicate this comparison. For example, when soil cohesion values of 2.0 and 3.0 kPa were used, the failure volume changed by factors of 2.04 and 0.41, respectively, compared with the average condition of 2.5 kPa used in the simulation. For soil depths 30% higher and lower than the standard condition, the failure volume changed by factors of 2.0 and 0.27, respectively. When maximum root cohesion changed from 12.5 kPa (average condition) to 10 kPa, the failure volume increased 1.73‐fold; for the case of 15 kPa, the failure volume changed by a factor of 0.55. The simulated failures caused by the storm were mostly in hollows. The simulations show that the spatial distribution of FS is controlled mainly by topography and timber‐harvesting patterns and is greatly affected by groundwater flow patterns during major rainstorms. Most areas with FS < 3.0 corresponded with the distribution of blocks clear‐cut in 1968, and all elements with FS < 2.0 were in areas clear‐cut in 1968. Areas with low FS (1.0–1.6) expanded dramatically during the rainstorm and decreased at a slow rate after the storm. Factors of safety in hollows declined sharply during the storm.
Errors in Streamflow Drought Statistics Reconstructed From Tree Ring Data
Authors: Brockway Charles G. , Bradley A. Allen
Published: February 2004

Hydrologic variables such as streamflow may be reconstructed for years prior to the observational record through the use of historical tree ring data. Reconstructed records provide additional information for the estimation of statistics relating to rare events such as large and persistent water deficits. However, the reconstruction process can introduce bias and inflate the errors of estimated statistics. The biases and standard errors of streamflow drought statistics derived from such reconstructed records are evaluated for idealized conditions by employing a Monte Carlo simulation method. Both the bias and the standard error depend to some degree on the goodness of fit of the reconstruction model, the calibration sample length, the reconstruction sample length, and the autocorrelation of the reconstructed variable. The bias is most dependent on the goodness of fit of the reconstruction model. Standard errors are often strongly dependent on several of these factors. For a given drought statistic the sample length of an observed record which yields the same standard error as the longer reconstruction is called the effective sample length. Effective sample lengths are found to be less than 50% of the total reconstructed record length in many cases.
A Comparison of Unbiased and Plotting‐Position Estimators ofLMoments
Authors: Hosking J. R. M. , Wallis J. R.
Published: February 2004

Plotting‐position estimators ofLmoments andLmoment ratios have several disadvantages compared with the “unbiased” estimators. For general use, the “unbiased” estimators should be preferred. Plotting‐position estimators may still be useful for estimating extreme upper tail quantiles in regional frequency analysis.