bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of erosion, deposition, and net volumetric change caused by the 1996 Skeiðarársandur jökulhlaup, Iceland, from Synthetic Aperture Radar Interferometry
Authors: Smith Laurence C. , Alsdorf Douglas E. , Magilligan Francis J. , Gomez Basil , Mertes Leal A. K. , Smith Norman D. , Garvin James B.
Published: September 2002

Using repeat‐pass satellite synthetic aperture radar interferometry, we develop a methodology to measure flood‐induced erosion and deposition and apply it to a record 1996 glacier outburst flood (jökulhlaup) on Skeiðarársandur, Iceland. The procedures include (1) coregistration of backscatter intensity images to observe morphological differences; (2) mapping of interferometric phase correlation to identify preserved and modified surfaces; and (3) construction, correction, and differencing of pre‐jökulhlaup and post‐jökulhlaup topography. Procedures 1 and 2 are robust and should be widely applicable to other fluvial environments, while procedure 3 is complicated by uncertainties in phase measurement, baseline estimate, and atmospheric effects. After a correction procedure involving interpolation of digital elevation model elevation differences across low‐correlation areas, we find ∼4 m of elevation change are required to calculate volumes of erosion or deposition. This condition was satisfied for the 40 km2 proglacial zone of Skeiðarársandur, where we estimate +38×106 m3 of net sediment deposition along the ice margin, −2 ×106 m3 of net erosion in channels downstream, and a total net balance of +13 × 106. These estimates are supported by field observations and survey data collected in 1997.
Erosion thresholds and suspended sediment yields, Waipaoa River Basin, New Zealand
Authors: Hicks D. Murray , Gomez Basil , Trustrum Noel A.
Published: September 2002

Characteristics of the suspended sediment load in the Waipaoa River, New Zealand, and in two of its tributaries (Mangatu and Te Arai Rivers) are examined for evidence of different dominant erosion processes in the basins upstream. Different shapes of the suspended sediment concentration‐water discharge relations lead to differences in long‐term average yield, event‐yield magnitude‐frequency relations, and relative importance of large flows and rare events. In the Mangatu River, frequent runoff events are relatively more important to the long‐term yield (11,540 t km−2 yr−1), half of the long‐term average load is transported by events with return periods less than ∼1 year, and there is little evidence of an erosion threshold limitation on sediment supply. This is consistent with the predominance in the Mangatu basin of hillslope erosion processes that involve scour by surface and channelized runoff, particularly gully erosion. This contrasts with the Te Arai River where sediment concentration tends to be much lower at low and moderate flows, frequent runoff events transport less of the long‐term yield (4600 t km−2 yr−1) than do rarer, large‐magnitude flood events, event sediment yields are an order of magnitude lower during events with subannual return periods, and half of the long‐term average load is transported during events with a return period of >2 years. Some of these characteristics appear to result in part from two populations of runoff events in the Te Arai basin; nonetheless, they are consistent with field evidence that most of the sediment supplied to the Te Arai stream network is generated by shallow landslides which are activated once a rainfall threshold is exceeded.
Nonlinear analysis and forecasting of a brackish Karstic spring
Authors: Lambrakis N. , Andreou A. S. , Polydoropoulos P. , Georgopoulos E. , Bountis T.
Published: September 2002

Nonlinear methods and artificial neural network techniques are applied to the study of the regime and the possibility of short‐term forecasting of discharges of the spring of Almyros, Iraklion, Crete. Questions regarding the nonlinearity and chaotic characteristics of the system necessitate the examination of dynamical properties. Toward this objective the time series of daily average discharges is analyzed in detail. First, the dimensionality of the dynamics in the reconstructed phase space is found to be quite low, ∼3–4. Then several tests are applied to examine the nonlinearity and the presence of noise in the data. Using the surrogate time series test, a high degree of nonlinearity and a deterministic nature are revealed, while the differentiation test showed that the presence of high‐frequency noise in the series of the discharge is not dynamically important. These suggest that an attempt to forecast the short‐term future behavior of this time series may turn out to be quite successful. Nonlinear methods, such as Farmer's algorithm and artificial neural networks, were employed and found to exhibit a very satisfactory predictive ability, with neural networks achieving a slightly better performance.
Characterization of mixing and dilution in heterogeneous aquifers by means of local temporal moments
Authors: Cirpka Olaf A. , Kitanidis Peter K.
Published: September 2002

Breakthrough curves of a conservative tracer in a heterogeneous two‐dimensional aquifer are analyzed by means of their temporal moments. The average velocity and the longitudinal macrodispersion coefficient of the equivalent one‐dimensional aquifer obtained through cross‐sectional averaging of concentration can be defined from the first and second central moments of a breakthrough curve integrated over the outflow boundary of the domain. On the basis of an integrated breakthrough curve, one cannot distinguish between actual solute dilution, which involves concentration reduction, and variability of arrival times among parts of the plume at different cross‐sectional positions. Analyzing the temporal moments of breakthrough curves at a “point” within the domain gives additional information about the dilution of the tracer. From these local first and second central moments an apparent seepage velocity υa and an apparent dispersivity of mixing αa can be derived. For short travel distances, αa equals the local‐scale longitudinal dispersivity. It increases with the travel distance but much more slowly than the macrodispersivity. At the large‐distance limit, αa may eventually reach the level of macrodispersivity. Lenses of high conductivity where groundwater flow converges are identified as regions of preferential enhanced mixing. The spatial distribution of these regions causes a high degree of variability of αa within a domain, indicating a high degree of uncertainty in the quantification of dilution at early stages. In an accompanying paper [Cirpka and Kitanidis, this issue] the results of conservative tracer transport are utilized for the study of mixing‐controlled reactive transport.
Inverse chemical modeling and radiocarbon dating of palaeogroundwaters: The Tertiary Ledo‐Paniselian Aquifer in Flanders, Belgium
Authors: van der Kemp W. J. M. , Appelo C. A. J. , Walraevens K.
Published: September 2002

Groundwater samples from the Ledo‐Paniselian aquifer have been interpreted for chemical reaction patterns, 14C age, and recharge conditions. This confined Tertiary aquifer dips NNE from its outcrop in Belgium toward the North Sea over a length of ∼50 km. Conventional 14C ages of the water samples range from 3 to over 40 ka. Inverse chemical modeling was done to correct the 14C ages for the chemical reactions in the aquifer, while accounting for changes in the recharge water quality during the Holocene and late Pleistocene. The aquifer shows a zonal pattern with (going upstream) Na‐, K‐, NH4‐, Mg‐, and Ca‐HCO3 water types. The pattern is a result of freshening: Ca displaces the saline cations Na, K, NH4, and Mg from the aquifer's cation exchange complex in a Chromatographic sequence. The loss of Ca2+ from solution by cation exchange is by far the most important reaction for dissolution of calcite, which increases the apparent 14C age of the water samples. The 14C age furthermore depends on open/closed conditions of calcite dissolution and CO2 gas exchange and CO2 pressure in the recharge area. It is shown that δ13C and CO2 pressure in a soil are interrelated and that the changes in CO2 pressure can be included in an inverse model which considers variations in infiltration water quality. The overall correction for 14C age is obtained by inverse modeling of water quality and δ13C, with optimization on CO2 pressure in recharge water using PHREEQC [Parkhurst, 1995]. The optimized CO2 pressure for the recharge area varies with age and is generally lower in the water samples with an age above 13 ka. The lower CO2 pressure is corroborated by lower δ18O values of the water.
On the propagation of Jovian electrons in the heliosphere: Transport modelling in 4‐D phase space
Authors: Fichtner Horst , Potgieter Marius , Ferreira Stefan , Burger Adri
Published: September 2002

For about 25 years the Jovian magnetosphere is known to be a strong source of electrons with energies up to ∼30 MeV. Many in‐situ observations have not only confirmed this finding originally made with the Pioneer 10 spacecraft, but have also revealed that the Jovian electrons appear to be present, at least close to the ecliptic plane, in the distance range from 0.5 to 25–30 AU. The growth of the observational basis was not accompanied by any comparable progress with the modelling of the propagation of Jovian electrons. The few existing models suffer from the restriction to be valid either only very close to or far away from the source region. They are, therefore, not exploitable for a detailed study of the observations, in particular of those made with the Ulysses spacecraft during the last nine years. Motivated by this situation we have developed a new model of the propagation of both Jovian and galactic electrons. We present first simulation results and comment on their potential value for distinguishing between Jovian and galactic electrons, for bracketing the range of the possible interstellar electron fluxes at low energies, and for a determination of the diffusion tensor of low‐energy electrons (>1 MeV) in the heliosphere.
Context and heliographic dependence of heliospheric planar magnetic structures
Authors: Jones G. H. , Balogh A.
Published: September 2002

An analysis is presented of planar magnetic structures (PMSs) detected in the solar wind by the Ulysses spacecraft between its launch in October 1990 and the end of 1998. In all, 667 such structures were found, with durations between 6 hours (the minimum in the search algorithm) and 66 hours. The total amount of magnetometer data found to represent planar periods accounted for 9% of all data returned from the instrument during 1990–1998. PMSs are found to be a common occurrence within corotating interaction regions. Around one fifth of PMSs can probably be accounted for by the passage of shocks through the solar wind, leading to alignment and amplification of preexisting features. Heliospheric current sheet crossings were found to be close to about 50% of PMSs. The results thus favor several previously proposed PMS formation mechanisms. After other causes of PMSs such as draping around the interplanetary counterparts of coronal mass ejecta are taken into account, there still remains a sizeable minority of events that have no obvious association with other solar wind features. It is suggested that some of these unexplained PMSs may have been caused by compressional processes such as draping around transient features and had remained in the solar wind when the signatures of the features responsible were no longer recognizable.
Magnetohydrodynamic wave mode conversion in the Earth's magnetotail
Authors: De Keyser J.
Published: September 2002

We study magnetohydrodynamic wave mode conversion as a mechanism contributing to the observed low‐frequency electromagnetic fluctuation level in the plasma sheet. Low‐frequency waves originating in the magnetosheath can transport energy across the tail flank magnetopause and through the tail lobes toward the plasma sheet. In the plasma sheet boundary layer and in the central plasma sheet, local conditions permit mode conversion to occur in resonant sheets. Resonant coupling to both Alfvén and slow‐mode waves is possible. Energy is fed to the plasma in these resonant sheets, which may help to explain the acceleration and heating of particles observed in the plasma sheet boundary layer and the central plasma sheet.
Dynamic cusp aurora and associated pulsed reverse convection during northward interplanetary magnetic field
Authors: Sandholt P. E. , Farrugia C. J. , Cowley S. W. H. , Lester M. , Denig W. F. , Cerisier J.‐C. , Milan S. E. , Moen J. , Trondsen E. , Lybekk B.
Published: September 2002

We report a study of ionospheric signatures of plasma entry and momentum transfer at the dayside magnetopause during northward oriented interplanetary magnetic field (IMF), combining ground observations of the dayside aurora and ionospheric ion drift (CUTLASS HF radar) with simultaneous particle precipitation data obtained from three overflights by the Defence Meteorological Satellite Program (DMSP) F12, F13 and F14 spacecraft. The observations were taken during a 37‐min long interval of strongly northward IMF (Bz=7 nT; clock angle ∼10°–15°) after a rapid northward turning. The meridan scanning photometer at the ground station recorded a long stepwise poleward retraction and latitudinal widening of the band of auroral emission in the cusp region. Thus the activity includes a series of episodes which are characterized by an initial 1–2 min poleward “step” of the auroral poleward boundary, followed by a ∼3–4 min period of relatively steady auroral latitude. The auroral events were accompanied by bursts of “reverse” two‐cell convection characterized by equatorward flow across the cusp poleward boundary. The three DMSP spacecraft, which traversed the poleward boundary of the cusp aurora from north to south, entered into a region of auroral precipitation where electrons and ions of magnetosheath origin were present, together with equatorward convection. The observations are found to be consistent with a theoretical description of a sequence of bursts of lobe reconnection involving both hemispheres. This process results in the capture of magnetosheath flux tubes and thereby closed flux is added to the dayside magnetosphere.
High‐pressure‐high‐temperature seismic velocity structure of the midcrustal and lower crustal rocks of the Ivrea‐Verbano zone and Serie dei Laghi, NW Italy
Authors: Khazanehdari J. , Rutter E. H. , Brodie K. H.
Published: September 2002

The Ivrea‐Verbano zone and the adjacent Serie dei Laghi, in the inner arc of the western Alps (NW Italy), display an upended cross section through much of the thickness of the continental crust as it existed in that region in Triassic/Jurassic time. We collected a suite of oriented rock samples that represent most of the volume of the rocks of the region. P and S wave velocity measurements were made in three orthogonal directions, related to the mesoscopic fabric of the rocks, at room temperature and up to 550 MPa confining pressure. Combined high‐temperature and high‐pressure measurements were made up to 700°C on a subset of the samples. The lithologic units were divided into 23 different principal rock types, and we present velocity data averaged in terms of these groups. Vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section. These show that Vp and Vs both increase and anisotropy decreases systematically with depth of burial, reflecting variations in rock type and metamorphic grade with depth. Vp and Vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding, with the slowest direction tending to be normal to the foliation or banding. The Vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust. These correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution.
Interaction of rifting and hot horizontal plume sheets at volcanic margins
Authors: Keen C. E. , Boutilier R. R.
Published: September 2002

We have explored the idea that observations of large igneous crustal thickness at volcanic rifted continental margins may be explained by the interaction of rift‐driven flow in the lithosphere with an underlying, sublithospheric hot plume sheet. This concept is assessed by numerical modeling of the viscous flow caused by divergent plate motions, using a viscous, temperature‐ and pressure‐dependent, nonlinear fluid. The plume sheet consists of an initially hot layer just below the lithosphere which responds to the overlying lithospheric motions. For reasonable values of input parameters we can successfully predict the observed thickness of igneous crust at the margin, its formation time, inferred average mantle melt fractions, and, importantly, the eventual transition to normal oceanic crustal thicknesses. Observations are selected from the North Atlantic region, with an emphasis on the recent results from the east Greenland volcanic margin. The best fitting models suggest that relatively little small‐scale convection is needed after the initial strong pulse of mantle upflow due to the presence of moderately hot (∼1450°C) buoyant plume material under the rift. The results favor a thin (∼50 km) plume sheet layer. This hot mantle reservoir is soon depleted and replaced by normal temperature mantle, unless the rift lies directly over the deep source of plume material.
Three‐dimensional seismic model of the Sierra Nevada arc, California, and its implications for crustal and upper mantle composition
Authors: Fliedner Moritz M. , Klemperer Simon L. , Christensen Nikolas I.
Published: September 2002

A three‐dimensional P wave velocity model of south‐central California from the Coast Ranges to the Sierra Nevada shows that the crust under most of the southern Sierra Nevada batholith has seismic velocities (5.9–6.3 km/s) below the continental average. The crust is not much thicker (on average about 35 km) than in the adjacent Great Valley and Basin and Range province apart from a small, northward thickening, crustal root under the western Sierra Nevada that reaches a depth of 42 km. Crustal velocities above the continental average are observed beneath much of the Great Valley due to a high‐velocity body underlying the sedimentary basin and the Foothills metamorphic belt (6.4–7.0 km/s). Upper mantle velocities are generally low (7.8 km/s) but span a wide range (7.4–8.2 km/s). We display the velocity model in several cross sections and maps of Moho depth and average crustal velocity. The measured velocities in the upper and mid crust of the Sierra Nevada batholith are in good agreement with laboratory measurements on Sierra Nevada tonalités after corrections for density and temperature. Peridotite xenoliths from the eastern Sierra Nevada suggest strong upper mantle anisotropy, which could explain some of the velocity heterogeneity in the Sierra Nevada mantle. By the time Cretaceous subduction‐related magmatism ceased, the Sierra Nevada arc must have had a thick mafic lower crust; yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout. A subcrustal layer with velocities below normal Pn velocities (<7.6 km/s) may indicate the presence of lower crustal material in eclogite facies.
Modeling mountain building and the seismic cycle in the Himalaya of Nepal
Authors: Cattin R. , Avouac J. P.
Published: September 2002

A host of information is now available regarding the geological and thermal structure as well as deformation rate across the Himalaya of central Nepal. These data are reconciled in a two‐dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes. Over geological timescale (5 Ma) the ∼20 mm/yr estimated shortening rate across the range is accommodated by localized thrust faulting along the Main Himalayan Thrust fault (MHT). The MHT reaches the surface along the foothills, where it is called the Main Frontal Thrust fault (MFT). The MHT flattens beneath the Lesser Himalaya and forms a midcrustal ramp at the front of the Higher Himalaya, consistent with the river incision and the anticlinal structure of the Lesser Himalaya. Farther northward the MHT roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector. Aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust, increasing the Coulomb stress beneath the front of the Higher Himalaya, where most of the microseismic activity clusters. Negligible deformation of the hanging wall requires a low apparent friction coefficient (μ) less than ∼0.3 on the flat portion of the MHT. On the ramp, μ might be as high as 0.6. Sensitivity tests show that a rather compliant, quartz‐rich rheology and a high radioactive heat production in the upper crust of ∼2.5 μW/m3 is required. Erosion affects the thermal structure and interplays with crustal deformation. A dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure, topography, and deformation field. Using a linear diffusion model of erosion, we constrain the value of the mass diffusivity coefficient to 0.5–1.6×l04 m2/yr. This study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period.
Compensation of the Ontong Java Plateau by surface and subsurface loading
Authors: Ito Garrett , Taira Asahiko
Published: September 2002

Gravity data over Ontong Java Plateau reveal deep crustal structure supporting the plateau's topography. At long wavelengths (>500 km), new shipboard gravity and bathymetry profiles indicate local isostatic compensation. At intermediate wavelengths (250–500 km), however, Bouguer anomalies have large amplitudes, too high to be explained by models of Airy isostasy or volcanic loading on the surface of an elastic lithospheric plate. This finding is also evident in admittance functions, generated from maps of bathymetry and satellite‐derived Bouguer anomalies, which show high values at intermediate wavelengths. In addition, coherence between two‐dimensional Bouguer gravity and bathymetry spectrums decreases to zero, beginning at intermediate wavelengths. Two possible models can explain the high admittance and low coherence at intermediate wavelengths. The first model explains the high admittance by large‐scale lithospheric folding and explains the low coherence by sedimentation and erosion that is uncorrelated with the igneous crustal structure. Lithospheric folding might result from tectonic compression imposed on Ontong Java by the solomon arc and trench system. The second model involves a multiple‐stage accretion history with surface magmatism on a thin elastic plate and magmatic underplating beneath a thick plate. The thick plate during underplating is required to explain the high admittance values at intermediate wavelengths, while decorrelation between underplating and the surface loaded magmas can explain the low coherence. The thick plate also suggests that the lithosphere cooled appreciably since the first magmatic stage and therefore magmatic underplating occurred at a significantly later time than the initial eruption. Calculations predict that the underplated material could comprise as much as 50% of the total crustal volume, suggesting that the later eruptive stage(s) may have been as, or more voluminous than, the first.
Effects of dissolution‐precipitation processes on the strength and mechanical behavior of quartz gouge at high‐temperature hydrothermal conditions
Authors: Kanagawa Kyuichi , Cox Stephen F. , Zhang Shuqing
Published: September 2002

Laboratory experiments on simulated quartz gouges at a temperature of 927°C, a confining pressure of 300 MPa, and a pore water pressure of 200 MPa demonstrate the interactions between cataclastic and dissolution‐precipitation processes and their effects on the strength and mechanical behavior. Significant porosity reduction and microstructures, including grain interlocking, grain interpenetration, and widespread growth of euhedral‐shaped grains, indicate the operation of dissolution‐precipitation processes. Under conditions which favor the activity of dissolution‐precipitation processes, i.e., for small grain size or at slow displacement rates, deformation is distributed across the whole gouge layer, and strength increases continuously with increasing displacement, so that coefficients of friction reach to 0.7. In contrast, for specimens with large grain size sheared at fast displacement rates, deformation is localized along a gouge‐forcing block interface after a small amount of displacement and results in slip softening and subsequent quasi‐stable sliding with coefficients of friction as low as 0.45. Analysis of our results indicates that where the kinetics of dissolution‐precipitation is sufficiently fast to partly accommodate shear strain, the mechanical behavior and strength of quartz gouge can be characterized by slip hardening and high strength with velocity strengthening; in contrast, where cataclastic processes predominantly accommodate shear strain, the mechanical behavior and strength can be characterized by slip softening and subsequent localized sliding with low strength and velocity strengthening.
Exhumation of the ultrahigh‐pressure continental crust in east central China: Cretaceous and Cenozoic unroofing and the Tan‐Lu fault
Authors: Ratschbacher Lothar , Hacker Bradley R. , Webb Laura E. , McWilliams Michael , Ireland Trevor , Dong Shuwen , Calvert Andrew , Chateigner Daniel , Wenk Hans‐Rudolf
Published: September 2002

The orogenic architecture of the world's largest ultrahigh‐pressure exposure, the Hong'an‐Dabie Mountains of the Triassic Qinling‐Dabie orogenic belt, is dominated by Cretaceous and Cenozoic structures that contributed to its exhumation from ≤30 km depth. Cretaceous magmatic crustal recycling (≥50% for the entire Dabie) and heating (>250° to >700°C) were most prominent in Dabie, and exhumation, magmatism, and cooling were all controlled by Cretaceous transtension. Exhumation was accomplished principally by an asymmetric Cordilleran‐type extensional complex in the northern Dabie (Northern Orthogneiss unit) between 140 and 120 Ma, at rates as fast as 2 mm/yr and average horizontal stretching rates of up to 6 mm/yr. Cretaceous reactivation occurred within a regional transtensional strain field as a result of far‐field collisions and Pacific subduction. The onset of crustal extension was preceded and possibly facilitated by a reheating of the Hong'an‐Dabie crust (∼140 Ma) coeval with the onset of voluminous magmatism in eastern China (∼145 Ma), which resulted from a change in Pacific subduction from highly oblique to orthogonal. The Tan‐Lu continental‐scale fault was a normal fault zone in the mid‐Cretaceous (∼110‐90 Ma) and underwent ≥5.4 km dip slip and ≥4 km throw in the Cenozoic. During the India‐Asia collision the Qinling‐Dabie belt acted as the structural discontinuity between the strike‐slip‐dominated escape tectonics south of the Qilian‐Qinling‐Dabie belt and the rifting‐dominated tectonism north of it. The most prominent Cretaceous and Cenozoic structures of the Hong'an‐Dabie, the Xiaotian‐Mozitang and the Jinzhai fault zones, respectively, reactivated major lithospheric structures of the Triassic orogen, i.e., the Huwan detachment zone and the suture.
Upper mantle seismic wave velocity: Effects of realistic partial melt geometries
Authors: Hammond William C. , Humphreys Eugene D.
Published: September 2002

We investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle. The amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations. The geometries of the inclusions are taken directly from laboratory experiments of mantle melting, with finite element meshes constructed to conform to these shapes. The shear and bulk moduli of the composite material are found for both the unrelaxed (isolated inclusions) and relaxed (pressure equalized inclusions) cases by assigning appropriate material properties to the fluid. Modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse‐shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction. The three‐dimensional response is estimated from two‐dimensional distributions of the melt phase by determining the mode II and mode III components of elastic modulus reduction separately and summing their effects. In general, cuspate and naturally organized melt inclusions cause greater velocity reduction. It is shown that VP and VS reduction per percent partial melt are at least 3.6% and 7.9%, respectively. Even higher values for velocity reduction are possible above 1% melt fraction if melt exists only in tubules below 1% melt fraction. The lower, more conservative values of velocity reduction are ∼70% greater for VP and 84% greater for VS than the analytically determined values for ellipsoidal inclusions. Somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model.
Secondary earthquake fractures generated by a strike‐slip fault in the South Iceland Seismic Zone
Authors: Belardinelli Maria Elina , Bonafede Maurizio , Gudmundsson Agust
Published: September 2002

Most large earthquakes in the South Iceland Seismic Zone occur on N‐S trending dextral strike‐slip faults. The resulting rupture zones display complex en echelon patterns of secondary structures including NNE trending arrays of (mostly) NE trending open fractures and hillocks. Three spatial scales characterize the surface faulting pattern: the length of the main fault (MF ∼104m), the arrays of fractures, here interpreted as surface evidence of subordinate faults (SF ∼102m), and the individual open fractures (OF ∼10 m). In order to improve our understanding of the genetic relationship between the OF and the MF we computed the stress field induced by slip on a buried MF using a dislocation model in a layered half‐space where the fault surface is assumed to be embedded in the basement rock, topped by a softer, tens of meters thick near‐surface layer. The OF were modeled as secondary fractures of either of the following types: (1) pure mode I cracks opening in the near‐surface layer in the direction of the tensile principal stress or (2) mixed‐mode cracks, slipping at depth as shear cracks and opening near the surface due to low confining pressure. Assuming that both types of secondary fractures occur in the SISZ, we can explain their first‐order characteristics by reasoning that they are the end product of a hierarchy of deformation produced by the main fault. The Coulomb failure function after the earthquake (obtained by summing the MF stress change and the lithostatic stress) suggests that subordinate faulting (SF) must be expected to occur in response to the main rupture below the upper soft layer, extending to a depth of ∼100 m. The total stress change induced by the MF and the SF is shown to yield quantitative explanations of the spatial extent, the “double en echelon” arrangement and the orientation of surface fractures observed in the field in terms of a simple frictional model of rupture.
Chemical composition of basement fluids within an oceanic ridge flank: Implications for along‐strike and across‐strike hydrothermal circulation
Authors: Wheat C. Geoffrey , Elderfield Henry , Mottl Michael J. , Monnin Christophe
Published: September 2002

Compositions of basement fluids are presented for four sites along a 3.5‐m.y.‐ old, partly buried basement ridge on the eastern flank of the Juan de Fuca Ridge. This ridge is roughly parallel to the active ridge axis of the Endeavor Segment ∼100 km to the west. From south to north these sites are Baby Bare Outcrop, Ocean Drilling Program (ODP) Site 1026, and the southern and northern sides of Mama Bare Outcrop. The composition of basement fluids is determined or estimated from analyses of pore water samples that were extracted from sediments at each of these sites, spring waters from Baby Bare, and basement fluids that vented from the open ODP Hole 1026B. Chemical trends in basement fluids along this transect show increasing alteration from south to north. A similar trend was observed along an ODP transect perpendicular to the ridge axis with increasing fluid alteration from west to east. Much of the increase in fluid alteration along the ODP transect is explained by greater water‐rock exchange with increasing basement temperature to the east. In contrast, the trend along the 3.5‐m.y.‐old ridge is best explained by diffusive exchange with the overlying sediment. The rate of this exchange is used to constrain hydrologie properties within basaltic basement. Flow within the 3.5‐m.y.‐old ridge is inferred to occur from south to north and lacks significant exchange with basement fluids from the active ridge crest to the west. Thus the two flow systems are hydrologically distinct, and flow paths are likely influenced by the complex distribution of permeability in basement, the pattern of seafloor morphology, and the type and rate of sedimentation.
Three‐dimensional seismic velocity structure of the San Francisco Bay area
Authors: Hole J. A. , Brocher T. M. , Klemperer S. L. , Parsons T. , Benz H. M. , Furlong K. P.
Published: September 2002

Seismic travel times from the northern California earthquake catalogue and from the 1991 Bay Area Seismic Imaging Experiment (BASIX) refraction survey were used to obtain a three‐dimensional model of the seismic velocity structure of the San Francisco Bay area. Nonlinear tomography was used to simultaneously invert for both velocity and hypocenters. The new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm. Numerous inversions were performed with different parameters to test the reliability of the resulting velocity model. Most hypocenters were relocated <2 km from their catalogue locations. Large lateral velocity variations at shallow (<4 km) depth correlate with known surface geology, including low‐velocity Cenozoic sedimentary basins, high‐velocity Cenozoic volcanic rocks, and outcrop patterns of the major Mesozoic geologic terranes. Salinian arc rocks have higher velocities than the Franciscan melange, which in turn are faster than Great Valley Sequence forearc rocks. The thickess of low‐velocity sediment is defined, including >12 km under the Sacramento River Delta, 6 km beneath Livermore Valley, 5 km beneath the Santa Clara Valley, and 4 km beneath eastern San Pablo Bay. The Great Valley Sequence east of San Francisco Bay is 4–6 km thick. A relatively high velocity body exists in the upper 10 km beneath the Sonoma volcanic field, but no evidence for a large intrusion or magma chamber exists in the crust under The Geysers or the Clear Lake volcanic center. Lateral velocity contrasts indicate that the major strike‐slip faults extend sub vertically beneath their surface locations through most of the crust. Strong lateral velocity contrasts of 0.3–0.6 km/s are observed across the San Andreas Fault in the middle crust and across the Hayward, Rogers Creek, Calaveras, and Greenville Faults at shallow depth. Weaker velocity contrasts (0.1–0.3 km/s) exist across the San Andreas, Hayward, and Rogers Creek Faults at all other depths. Low spatial resolution evidence in the lower crust suggests that the top of high‐velocity mafic rocks gets deeper from west to east and may be offset under the major faults. The data suggest that the major strike‐slip faults extend sub vertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike‐slip motion. The extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity, strong ground motion, and regional stress.
Radiocarbon in the Arabian Sea water column: Temporal variations in bomb14C inventory since the GEOSECS and CO2air‐sea exchange rates
Authors: Bhushan R. , Somayajulu B. L. K. , Chakraborty S. , Krishnaswami S.
Published: September 2002

Results of measurements of14C in the upper 1500 m water column in 10 stations of the Arabian Sea are presented. The study carried out during 1994–1995, which includes reoccupation of three Geochemical Ocean Sections Study (GEOSECS) stations (1977–1978), provides an opportunity to assess the temporal variations in the distribution and inventory of bomb radiocarbon over a period of ∼2 decades. The Δ14C values of surface waters (5 m) range between 31 and 58‰, ∼20‰ less than those measured during 1978 at the three GEOSECS stations. The higher Δ14C values measured during the present (as well as GEOSECS) study occur in the southern Arabian Sea, which can be attributed to the influx of low‐salinity and high‐Δ14C waters from the Bay of Bengal. The Δ14C in the Persian Gulf Water (PGW) and the Red Sea Water (RSW) are in the range of −34 to −59‰ and −89 to −113‰, respectively, and do not show any significant differences from those measured during the GEOSECS. The bomb14C inventories at the three GEOSECS stations during 1977 and 1995 show that at station 417 in the central Arabian Sea it has increased by ∼30%, whereas at the other two stations, 416 and 418 in the north and south central Arabian Sea, it is nearly the same. In general, the bomb14C inventory in the region ranges between 3.5 and 7.7×109atoms cm−2. The CO2air‐sea exchange rates, derived from the bomb14C inventory, yield values of 7–16 mol m−2yr−1. These exchange rates coupled with reportedpCO2measurements in surface Arabian Sea waters yield CO2evasion rates in the range of 50–180 Tg‐C yr−1for the Arabian Sea.
Iron and manganese in the Ross Sea, Antarctica: Seasonal iron limitation in Antarctic shelf waters
Authors: Sedwick Peter N. , DiTullio Giacomo R. , Mackey Denis J.
Published: September 2002

Dissolved iron and manganese and total dissolvable iron were measured in water column samples collected from the polynya region of the southern Ross Sea during cruises in November–December 1994 (spring 1994) and December 1995 to January 1996 (summer 1995). Iron and manganese addition bottle incubation experiments were also performed during these cruises in order to assess the nutritional sufficiency of ambient iron and manganese concentrations for growth of the phytoplankton community. Generally high dissolved iron concentrations (>0.5 nM) and relatively complex iron and manganese vertical profiles were obtained during the spring 1994 cruise, compared with the summer 1995 data. Dissolved iron concentrations in the upper water column averaged 1.0 nM during spring 1994 and 0.23 nM in summer 1995, excluding two stations where concentrations exceeding 1 nM are attributed to inputs from melting sea ice. The observed differences in the distribution of iron and manganese between spring 1994 and summer 1995 are attributed to seasonal decreases in the up welling of bottom waters and melting of sea ice, which supply these metals into the upper water column, combined with the cumulative removal of iron and manganese from the water column throughout the spring and summer, due to biological uptake, vertical export and scavenging by suspended and sinking particles. Results of the metal addition bottle incubation experiments indicate that ambient dissolved iron concentrations are adequate for phytoplankton growth requirements during the spring and early summer, when algal production is highest and Phaeocystis antarctica dominates the algal community, whereas low dissolved iron concentrations limit algal community growth later in the summer, except in the stratified, iron‐enriched waters near melting sea ice, where diatoms are able to bloom. Our observations and the inferred seasonal distributions of P. antarctica and diatoms in these waters suggest that iron availability and vertical mixing (i.e., irradiance) exert the primary controls on phytoplankton growth and community structure in the southern Ross Sea during the spring and summer.
Seasonal variability of sea surface height in the South China Sea observed with TOPEX/Poseidon altimeter data
Authors: Ho Chung‐Ru , Zheng Quanan , Soong Yin S. , Kuo Nan‐Jung , Hu Jian‐Hua
Published: September 2002

TOPEX/Poseidon altimeter measurements along 16 ground tracks in the South China Sea from November 1992 to October 1997 were used to observe the seasonal variability of sea surface height (SSH). In winter (November, December, January, and February) the SSH images are generally characterized by a sea surface tilting downward toward the east. High sea levels on the western side of the sea have a little monthly change, and a low (minus) sea level peak is centered in the northeastern deep basin. In spring (March and April), a single high sea level peak (HP) centered at 14°N, 114°E becomes a dominant feature. In summer (May, June, July, and August) the SSH images show a sea surface tilting downward toward the west. The HP moves from 14°N, 114°E in May to the northwest of Luzon Island in July. Low sea levels are centered in the offshore of the northern and southern Vietnamese coast. In fall (September and October) the appearance of an eastward low sea level jet in the western basin between 12° and 14°N constitutes a major feature of the SSH images. These results are comparable with previous results and drifter measurements.
Estimating horizontal diffusivity in the East Sea (Sea of Japan) and the northwest Pacific from satellite‐tracked drifter data
Authors: Oh Im Sang , Zhurbas Victor , Park WonSun
Published: September 2002

We use data of satellite‐tracked, mixed‐layer drifters collected for the World Ocean Circulation Experiment/Tropical Ocean‐Global Atmosphere (WOCE/TOGA) Surface Velocity Program in the East Sea (Sea of Japan) and the northwest Pacific Ocean during 1991–1997 to obtain reliable estimates of geographically varying horizontal diffusivity, integral timescale and space scale. For the diffusivity we suggest calculation of both the minor principal component of the diffusivity tensor in a definition by Davis [1991] and the half growth rate of the minor principal component of the displacement variance tensor. Numerical simulation of particle motion is used to prove that the minor principal component estimates of diffusivity, in contrast to regular estimates, are insensitive to ensemble averaging over particles taken from a finite area in a shear mean flow. In the East Sea, typical values of the diffusivity, timescale, and space scale are (1.7–5.2)×107 cm2 s−1, 0.7–1.7 days, and 10–25 km, respectively. In the northwest Pacific Ocean, typical estimates of the diffusivity, timescale, and space scale in mean are a factor 2–3 larger, namely, (2.3–15.1)×107 cm2 s−1, 1.7–3.7 days, and 18–62 km, respectively. It was found that the space scale L and the first mode internal Rossby radius Ri are related as L ≈ Ri, which suggests a parameterization of lateral diffusivity k∞ of the form k∞ = u Ri, where u is the rms current velocity fluctuation.
Simulated low‐frequency modes of circulation in the Arctic Ocean
Authors: Häkkinen Sirpa , Geiger Cathleen A.
Published: September 2002

The variability of the Arctic circulation is investigated for a 43 year period (1951–1993) from a coupled ice‐ocean model. Empirical orthogonal function (EOF) analysis shows that the variability of the sea surface height (SSH) and vertically integrated transport is organized so that in the leading mode the whole Arctic operates as a single gyre. The mode is associated with the Arctic Oscillation (AO) [Thompson and Wallace, 1998], and it explains over 70% of the variance in the vertically integrated transport and 25% of the SSH variability. The physical interpretation of this mode is derived to arise from its close connection to the Atlantic inflow to the Arctic. The mode shows a major shift toward cyclonic circulation in the end of the 1980s which is associated with a large multiyear pulse of Atlantic water to the Arctic. Thus this event appears as the likely initiation of the Atlantic layer warming observed during the recent years [Carmack et al., 1995]. Overall, the first mode shows strong decadal variability as reported by Proshutinsky and Johnson [1997]. The second mode of the oceanic circulation, which explains 9% of the variance in the transport, contains two gyres with opposing cyclonicity in the Eurasia and Canada basins. It projects onto the North Atlantic Oscillation (NAO) pattern and displays a 14 year cycle which is known to exist in the midlatitude North Atlantic surface temperatures [Deser and Blackmon, 1993]. A further examination reveals that this mode describes the variability of the flow through the Barents Sea, which is modulated by the water mass modification due to the local heat flux variability. The apparent NAO connection is provided by a simultaneous correlation between the time series of this second mode and the leading heat flux mode in the North Atlantic which is associated with NAO.
An upwelling case study on Florida's west coast
Authors: Weisberg Robert H. , Black Bryan D. , Li Zhenjiang
Published: September 2002

Using a combination of satellite sea surface temperature imagery; in situ currents, temperature, and sea level data; and a numerical circulation model, we describe an upwelling event observed along Florida's west coast. Although ubiquitous features of continental shelves, descriptions of coastal upwellings (or downwellings) are generally complicated by the continuum of processes in which they occur. For the case study presented the winds were light, and the baroclinic background currents were steady for several days prior to the event. These conditions allow the evolution of a specific winddriven upwelling event to be viewed as an initial value problem. The data and the numerical model analysis show that the route of adjustment by the sea level and currents is a classical Ekman‐geostrophic one. Sea level drops due to an offshore Ekman transport, causing a geostrophic alongshore jet, and the associated onshore flow within the bottom Ekman layer accounts for the appearance of cold water at the surface. This surface manifestation occurs as an elongated feature, centered about the 25 m isobath, that spans roughly half of the Florida peninsula from just north of Tampa Bay to Key West. The coldest water is observed offshore from Tampa Bay, and we explain this region of locally maximum upwelling on the basis of the coastline and isobath geometries.
Response of the South China Sea to Tropical Cyclone Ernie 1996
Authors: Chu Peter C. , Veneziano Joseph M. , Fan Chenwu , Carron Michael J. , Liu W. Timothy
Published: September 2002

A moving tropical cyclone is an intense localized source of surface wind stress and wind stress curl that produces a significant response in the ocean environment, especially in the ocean thermal structure, the upper ocean currents, and the sea surface elevation. Such a response has been well identified in the open‐ocean region, but not in the coastal ocean region. In this study we use the Princeton Ocean Model with 20 km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography to identify the response of the South China Sea to Tropical Cyclone Ernie 1996. Results show strong similarities in the responses between open ocean and coastal regions, including near‐surface strong asymmetric response such as divergent currents with near‐inertial oscillations, significant sea surface temperature cooling, biase to the right of the storm track, sea surface depressions in the wake of the storm, and subsurface intense upwelling and cooling at the base of the mixed layer to the right of the storm track. The unique features of the SCS response to Ernie are also discussed.