bluuebunny/crossref_metadata_2025 huggingface dataset:
Prehistorical earthquakes on the Alpine fault, New Zealand
Authors: Bull William B.
Published: February 2004

Studies of synchronous prehistorical landslides in rugged plate boundary mountain ranges can be used to estimate times of earthquakes, to locate the earthquake‐generating fault zones, and to describe patterns of seismic shaking. Lichenometry can date regional rockfall events caused by large earthquakes in the Southern Alps of New Zealand with a 95% confidence level precision of ≤10 years. Spatial variation of rockfall abundance (an index of seismic shaking) is a function of the intensity and duration of seismic shaking and hillslope sensitivity to earthquakes. Four of the earthquakes of the past 1000 years appear to have caused exceptionally widespread regional rockfall events that were largest opposite the 350‐km‐long Cook segment of the Alpine fault. Lichenometric age estimates (±10 years) of these Cook segment earthquakes at 1748, 1489, 1226, and 967 A.D. are supported by radiocarbon and weathering rind age estimates of times of coseismic geomorphic processes and by times of prehistorical forest‐disturbance events. A mean earthquake recurrence interval of 260 years ± 15 years and similar patterns of seismic shaking imply constant rupture length and slip per event in accord with the characteristic earthquake model.
High‐frequency precursors to P wave arrivals in New Zealand: Implications for slab structure
Authors: van der Hilst Rob , Snieder Roel
Published: February 2004

This report revisits the very early high‐frequency slab phases from earthquakes in the Kermadec slab (between −25°S and −37°S) that arrive as a precursor to the P wave onset at stations in New Zealand. The analysis of short‐period digital records for station SNZO (South Karori New Zealand) for the time period between 1980 and 1986 involved 4 times as many data as in previous studies. We confirm most of the earlier observations, in particular the relationship between source region and observed waveform. Numerical waveform modeling using two‐dimensional slab models confirm that the high‐frequency precursor can be explained by P wave propagation in a thin (8–10 km) high‐velocity layer (HVL). The sensitivity of the high‐frequency phase to source position in the slab is investigated, and we demonstrate that the slab phase does not survive interruptions or strong lateral variations of the HVL. Assuming the shape of the slab from tomographic images and continuity of the HVL, we infer that the large negative travel time residuals can be explained if P wave propagation is 4 to 5% faster than in the ambient mantle for depths to 300 km and about 2% faster for depths between 300 and 600 km. Rather than resorting to regional differences in structure and composition of the HVL, we argue that specific source‐receiver configurations and the shape of the slab explain the observed regional variation in the character of the high‐frequency slab phases.
Impact of seafloor sediment permability and thickness on off‐axis hydrothermal circulation: Juan de Fuca Ridge eastern flank
Authors: Snelgrove Stephen H. , Forster Craig B.
Published: February 2004

Sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust. Interlayered silt‐rich and clay‐rich sediments almost completely bury basement on the eastern flank of the Juan de Fuca Ridge. Laboratory measurements indicate that silt‐rich sediment permeability is 1 to 2 orders of magnitude greater than clay‐rich sediment permeability at similar depths. Numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor. The layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data. Two distinct patterns of hydrothermal circulation are obtained, depending on whether silt‐rich or clay‐rich sediments compose the sediment layer above a permeable upper basement aquifer. A clay‐rich sediment column nearly isolates circulation within the basement aquifer, resulting in closed convection. Computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the Juan de Fuca Ridge. Computed vertical fluid fluxes across the sediment column are small, in agreement with fluid fluxes estimated from sediment geochemical profiles. When the sediments are silt‐rich, 17% of the flow within a convection cell crosses the sediment column. Open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt. Each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow. A transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt‐rich column. Thus the addition of only a small amount of clay to an otherwise silt‐rich sediment column may effectively isolate the crustal aquifer from the ocean.
Growth of correlated pore‐scale structures in sedimentary rocks: A dynamical model
Authors: Aharonov E. , Rothman D. H.
Published: February 2004

Recent laboratory measurements have shown that pore surfaces of most sedimentary rocks have a fractal dimension ranging mostly between 2.6 and 2.8. The lower and upper cutoffs for fractal behavior are 10−2 and 102 μm, respectively. Moreover, qualitative observations indicate that the fractal dimension increases with diagenetic alteration. To explain these measurements and observations, we construct a physical model of mineral deposition and dissolution on a substrate. We propose that when formation dynamics are reaction controlled, the forming pore‐grain interface can be described by a nonlinear partial differential equation for interface growth. We construct a discrete particle deposition model corresponding to these dynamics. Three‐dimensional computer simulations of the model show that resulting pore‐grain interfaces are fractal, with a fractal dimension that depends on interface growth conditions and varies between D ≈ 2.63 and D ≈ 2.84, in close agreement with observations. Additionally, our model predicts an increase of the amplitude of interface undulations with dissolution and fractal dimension. We conclude that geometrical measures of pore‐grain interfaces, such as the fractal dimension and the roughness amplitude, are an indicator of the diagenetic history of sedimentary rocks.
Evidence from precise earthquake hypocenters for segmentation of the San Andreas Fault in San Gorgonio Pass
Authors: Magistrale Harold , Sanders Christopher
Published: February 2004

We use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike‐slip fault at depth connecting the San Bernardino strand of the San Andreas fault with the Coachella Valley segment of the Banning fault. We inverted 560,000 arrival times from 23,000 earthquakes (1981–1993) for high‐quality hypocenters and three‐dimensional P wave velocity structure in a 1° by 2° area centered on the San Gorgonio Pass. Cross‐sectional plots of relocated earthquakes reveal an abrupt 5 to 7 km high step in the maximum depth of hypocenters. The step riser defines a near‐vertical, locally curved surface that extends westerly more than 60 km from the Coachella Valley segment of the San Andreas fault to the San Jacinto fault. A hypothetical continuous vertical San Andreas fault through San Gorgonio Pass would cross the step at an oblique angle. We suggest that the step is the expression of the contact between different basement rock types juxtaposed by large‐scale right‐slip motion on the ancestral San Andreas fault. South of the step in Peninsular Ranges type basement (intrusives), brittle failure occurs down to about 20‐km depth, while north of the step in San Bernardino type basement (Pelona schist), brittle failure occurs to only about 13‐km depth. The step provides a piercing plane that should be offset about 3 km right laterally by an active, continuous, vertical San Andreas fault. Within the resolution of our mapping the step is not offset in this manner, implying either that there has not been a throughgoing vertical fault at depth, that a throughgoing fault has not experienced enough slip to offset the step, or that a throughgoing fault is not vertical and dips north over the top of the step. Hypocentral patterns and focal mechanisms indicate distributed deformation (thrust, normal, and strike‐slip faulting) over a large volume in the San Gorgonio Pass region; there is no evidence of hypocenter or slip vector alignments that would indicate a throughgoing, continuous, near‐vertical San Andreas fault. In summary, we find no evidence indicating a continuous fault at seismogenic depth connecting the San Bernardino strand and Coachella Valley segment of the San Andreas fault zone. We speculate that this is because the 3 km of right slip on the San Bernardino stand of the San Andreas fault and Coachella Valley segment of the Banning fault has not been sufficient to form a single new structure through the 15‐ to 20‐km gap between the two previously unconnected segments. This implies that large earthquake rupture on the San Andreas fault may be inhibited from propagating through San Gorgonio Pass, thus limiting the maximum magnitudes on the southern San Andreas fault.
Numerical simulation of collapsing volcanic columns with particles of two sizes
Authors: Neri Augusto , Macedonio Giovanni
Published: February 2004

A three‐phase thermofluid‐dynamic model was employed to simulate the behavior of collapsing volcanic columns and related pyroclastic flows. The model accounts for the mechanical and thermal nonequilibrium between a gas phase and two solid phases representative of particles of two different sizes. The gas phase has two components: hot water vapor leaving the vent and atmospheric air. Collisions between particles of the same size were accounted by a solids elasticity modulus, whereas a semiempirical correlation was employed to account for particle‐particle interactions between particles of different sizes. The gas phase turbulence was modeled by a turbulent subgrid scale model. The partial differential equations of conservation of mass, momentum, and energy were solved numerically, by a finite difference scheme, on an axisymmetric physical domain for different granulometric compositions at the vent. Simulations were limited to particles of few hundreds microns, and therefore to dilute flows, in order to mantain a reasonable computational load. Results show the formation of the initial vertical jet, column collapse, building of a pyroclastic fountain followed by the generation of a radially spreading pyroclastic flow, and the development of convective instabilities from the upper layer of the flow which lead to the formation of coignimbritic or phoenix clouds. The analysis of the spatial and temporal distributions of the two solid phases in the different parts of the domain shows nonequilibrium effects between them and allow us to quantify important emplacement processes as pyroclast sedimentation and ash dispersion. In particular, the importance of coupling effects between the two solid phases leads to relevant differences between the behavior of columns with one or two solid phases. A significant influence of the granulometric composition was observed on the pyroclastic flow runout, flow thickness, and particle distribution in the flow and phoenix cloud. The results from simulations appear to be qualitatively in agreement with simple laboratory experiments and field observations.
Application of a simple relation for describing wave velocity as a function of pressure in rocks containing microcracks
Authors: Greenfield Roy J. , Graham E. K.
Published: February 2004

The occurrence of microcracks and pore space reduces the P and S wave velocities in rocks from those expected for the intrinsic mineral matrix. In the low‐pressure regime, before microcracks close, the elastic properties and wave velocities are characteristically nonlinear. This nonlinear behavior reflects the effects of the crack closure spectra on the P and S wave velocities. Several efforts have been made in the past to describe or model this behavior using phenomenological relations. However, success has been limited to a rather restricted pressure range. In the present study, two simple relations are considered that have a clear physical basis and are convenient to use and interpret. Both are tied to the functional form V2 = A + BP + Ce−P/τ. The fitting parameters A and B can be identified directly as second‐ and third‐order elastic properties of the intrinsic mineral matrix of the rock specimen, and C and τ describe the nature of the microcrack distribution. The results from fitting a large number of published igneous rock data sets indicate that the relations studied yield rms errors that are (1) always within the stated accuracy of the data, (2) generally within the stated precision of the data, (3) comparable in “goodness of fit” to previously published but more complex relations over limited pressure range data sets, and (4) clearly superior over extended pressure ranges. Because of their comparative simplicity, demonstrated accuracy, and clearly defined fitting parameters, they are well suited for systematic studies of large numbers of igneous rock data sets.
Evolution of Mauna Kea volcano: Inferences from lava compositions recovered in the Hawaii Scientific Drilling Project
Authors: Yang H.‐J. , Frey F. A. , Rhodes J. M. , Garcia M. O.
Published: February 2004

The lower 776 m of core recovered during the initial phase of the Hawaii Scientific Drilling Project (HSDP) contains lavas erupted from Mauna Kea volcano. Tholeiitic and alkalic basalts, including an Fe‐Ti rich flow, are intercalated in the upper 58 m of Mauna Kea lavas. Similar basaltic sections are subaerially exposed on the lower east flank of Mauna Kea. The Fe‐Ti rich lavas reflect large amounts of clinopyroxene, plagioclase, and olivine fractionation within the crust and upper mantle, but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet‐bearing source. Based on abundances of incompatible elements, the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas. All flow units in the lower 718 m of the HSDP core are tholeiitic lavas. Their variability in major element compositions reflect variable accumulation of olivine. Incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting. Within the tholeiitic part of the core, lavas from 800 m to 950 m formed by the largest extent of melting, whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting. Inferred melt compositions at 16% MgO show that the ∼200 to 400 ka Mauna Kea lavas from the HSDP core and the <250 ka subaerial exposures define an inverse correlation between SiO2 and FeO contents. Based on experimental studies, this correlation is caused by differing pressures of melt segregation. Furthermore, abundances of Nb and SiO2 are also inversely correlated in these calculated melts. In general, the younger lavas are relatively enriched in FeO and incompatible elements but are depleted in SiO2. These trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age. There are, however, geochemical variations which indicate short‐term reversals in this long‐term trend. Previously, the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at Hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis. The long‐term geochemical trends of tholeiitic lavas in the HSDP core also reflect migration of Mauna Kea away from the Hawaiian plume.
Geomagnetic induction in a heterogenous sphere: Azimuthally symmetric test computations and the response of an undulating 660‐km discontinuity
Authors: Everett M. E. , Schultz A.
Published: February 2004

A finite element numerical method is presented for computing electromagnetic induction in a heterogeneous conducting sphere by external source current excitation. The numerical model has applicability in the problem of determining the three‐dimensional electrical conductivity structure of Earth's mantle. The formulation is in terms of vector and scalar potentials. Boundary value problems are derived for fully three‐dimensional (3‐D) and azimuthally symmetric geometries. To validate the code, we check against a quasi‐analytic solution for an azimuthally symmetric configuration of eccentrically nested spheres and against an integral equation solution for an azimuthally symmetric, buried thin spherical shell model. To illustrate the capability of the code for modeling the electromagnetic response of realistic mantle structural variations, we compute solutions for an azimuthally symmetric electrical model whose lateral heterogeneity corresponds to zonal averages of the topographic relief on the 660‐km seismic discontinuity. The latter is derived from differential shear wave travel times. The anomalous responses are small but either spatially correlated or anticorrelated with the relief, depending on the period. The behavior can be reproduced by simple one‐dimensional (1‐D) modeling. The volumetric heterogeneity associated with recent seismic tomographic models should yield a greater surface anomaly, since electromagnetic data are inherently more sensitive to bulk electrical properties than sharp internal interfaces. The finite element method presented here is general and can account for galvanic, oceanic, and non‐P10 source effects. The present implementation does not include these complications, but the code is designed so that they can be added easily. The major obstacle to their inclusion at present is the scarcity of other 3‐D solutions to validate our calculated results.
Cyclic stressing and seismicity at strongly coupled subduction zones
Authors: Taylor Mark A. J. , Zheng Gutuan , Rice James R. , Stuart William D. , Dmowska Renata
Published: February 2004

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long‐term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench‐normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.
Source characteristics derived from very incompatible trace elements in Mauna Loa and Mauna Kea basalts, Hawaii Scientific Drilling Project
Authors: Hofmann A. W. , Jochum K. P.
Published: February 2004

We report trace element analyses by isotope dilution spark source mass spectrometry of selected Hawaii Scientific Drilling Project samples for Mauna Loa and Mauna Kea basalts and picrites. The trace element abundances have been corrected along olivine control lines to an arbitrarily assumed, common MgO content of 12.0%. The corrected patterns all show remarkable similarities. These include negative Th and U anomalies with primitive‐mantle‐normalized ratios of (Th/Ba)n = 0.60 ± 0.12 and 0.50 ± 0.06 and (Th/La)n = 0.50 ± 0.04 and 0.52 ± 0.04 for Mauna Loa and Mauna Kea, respectively. These results are similar to those previously found for both Mauna Loa and Kilauea lavas [Hémond et al., 1994b; Hofmann et al., 1993], and they show no significant differences between tholeiites and alkali basalts from Mauna Kea. Other trace element ratios are more typical of other ocean island basalts and include (Nb/Th)n = 2.13 ± 0.08 and 2.13 ± 0.14 and (Nb/U)n = 1.86 ± 0.20 and 1.75 ± 0.25 for the two volcanoes, respectively. Six Hawaiian volcanoes for which extensive sets of high‐quality Ba‐Th‐U‐La data exist (Mauna Kea, Mauna Loa, Kilauea, Loihi, Haleakala, and Koolau) are all distinguished by a characteristic depletion of Th and U relative to Ba and La. This distinctive, nonprimitive “fingerprint” is found both in tholeiites and alkali basalts, and it is clearly a property of the present‐day Hawaiian source rather than the product of fractionation during (or after) partial melting. It also occurs on Iceland but is found neither in typical depleted‐mantle (“N‐type” mid‐ocean ridge basalt (MORB) source) compositions nor in typical representatives of EM1, EM2, and HIMU‐type oceanic island basalts as defined by Zindler and Hart [1986]. Thus, contrary to the still frequently iterated opinion that Hawaiian basalts are derived from mixtures of primitive and depleted mantle sources [McKenzie and O'Nions, 1995, 1991], the Hawaiian plume must contain strongly fractionated mantle material, which is quite unlike primitive mantle or the source or residue of MORB (and therefore probably also unlike the oceanic lithospheric mantle). We speculate that the Hawaiian source contains significant amounts of recycled oceanic gabbros.
The 40Ar/39Ar and K/Ar dating of lavas from the Hilo 1‐km core hole, Hawaii Scientific Drilling Project
Authors: Sharp W. D. , Turrin B. D. , Renne P. R. , Lanphere M. A.
Published: February 2004

Mauna Kea lava flows cored in the Hilo hole range in age from <200 ka to about 400 ka based on 40Ar/39Ar incremental heating and K‐Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 40Ar/39Ar age spectra that result from mobility of 40Ar and perhaps K, the presence of excess 40Ar, and redistribution of 39Ar by recoil. Comparison of K‐Ar ages with 40Ar/39Ar integrated ages indicates that some of these samples have also lost 39Ar. Nevertheless, two plateau ages of 391 ± 40 and 400 ± 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has a mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 40Ar/39Ar plateau and isotope correlation ages of 326 ± 23, 241 ± 5, 232 ± 4, and 199 ± 9 ka for depths of −415.7 m to −299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at −280 m must be older than the plateau age of 132 ± 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age‐depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.
Thermoelastic properties of MgSiO3 perovskite determined by in situ X ray observations up to 30 GPa and 2000 K
Authors: Funamori Nobumasa , Yagi Takehiko , Utsumi Wataru , Kondo Tadashi , Uchida Takeyuki , Funamori Miho
Published: February 2004

In situ X ray experiments on MgSiO3 perovskite at pressures of 21–29 GPa and temperatures of 300–2000 K were carried out using an MA8‐type high‐pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. The thermal expansion at 25 GPa up to 2000 K was determined from interpolation of the P‐V‐T data obtained in the present study. The 95% confidence level was estimated by taking all possible errors into account, including statistical error and systematic error caused by uncertainty of pressure scales, etc. The thermal expansivity at 25 GPa is expressed as αT,25 = a25 + b25T − c25T2 with the best‐fit parameters of a25 = 2.11 × 10−5 K−1, b25 = 1.80 × 10−9 K−2, and c25 = 1.93 K. The equation of state of MgSiO3 perovskite has been determined using our new data combined with the lower‐pressure data of Wang et al. [1994] and Utsumi et al. [1995]. The optimal set of parameters of the third‐order Birch‐Murnaghan equation of state, which is expressed as P = (3/2)KT,0[(VT,0/V)7/3 − (VT,0/V)5/3]{1 − (3/4)(4 − KT,0′)[(VT,0/V)2/3 − 1]}, where KT,0 = K300,0 + (∂KT,0/∂T)P(T − 300), KT,0′ = K300,0′, VT,0 = V0exp ∫300TαT,0dT, and αT,0 = a0 + b0T − c0T−2, is K300,0 = 261 GPa, K300,0′ = 4, a0 = 1.982 × 10−5 K−1, b0 = 0.818 × 10−8 K−2, c0 = 0.474 K, and (∂KT,0/∂T)P = −0.0280 GPa/K. The reliability of the result is discussed in detail.
Marine controlled‐source electromagnetic sounding: 1. Modeling and experimental design
Authors: Flosadóttir Á. H. , Constable S.
Published: February 2004

Numerical forward modeling, predicting an observable response given a mathematical representation of the Earth, is an important component of practical exploration work. In addition, derivatives which relate changes in response to changes in the Earth model are useful for experimental design and are a crucial element of linearized inversion techniques. Differentiation of kernels followed by numerical integration using a fast Hankel transform provides an efficient combination of forward and sensitivity modeling for frequency‐domain horizontal electric dipole‐dipole sounding over a layered seafloor. Our code is validated against an independent forward modeling technique using a mode analysis and against central difference derivatives. Efficiency is important in the application of regularized inversion to large data sets; we give an example from the East Pacific Rise, requiring 2000 elements in the Jacobian matrix. We illustrate the use of forward modeling and discrete analogs of the Fréchet kernels to provide aid to physical intuition and experimental design in the context of the electrical conductivity of the oceanic lithosphere. By using the most favorable parts of range‐frequency space, experiments using current technology should be capable of distinguishing a thicker, less resistive, from a thinner, more resistive “lithospheric resistor” layer.
Crustal structure of the Colorado Plateau, Arizona: Application of new long‐offset seismic data analysis techniques
Authors: Parsons Tom , McCarthy Jill , Kohler William M. , Ammon Charles J. , Benz Harley M. , Hole J. A. , Criley Edward E.
Published: February 2004

The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low‐density mantle. This paper reports results on long‐offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long‐offset data that employ finite difference travel time calculations: (1) a first‐arrival time inverter to find upper crustal velocity structure and (2) a forward‐modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ∼37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ∼15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.
Semidiurnal switching of stratification in the region of freshwater influence of the Rhine
Authors: Simpson J. H. , Souza A. J.
Published: February 2004

Observations in the Rhine region of freshwater influence (ROFI) system in the North Sea show evidence of large semidiurnal oscillations in stability, superimposed on a mean stratification, occurring throughout the stratified region at times of reduced mixing. The amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide. It is deduced that this semidiurnal variation results primarily from cross‐shore tidal straining which interacts with the density gradient to induce stratification. This conceptual picture of the contributing processes has been tested in a one‐dimensional point model forced by the observed slopes and the local density gradients. The model exhibits the same qualitative behavior as the observations, produces oscillations in stratification of the amplitude observed, and confirms the critical role of cross‐shore tidal straining. The large cross‐shore shear nder stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions. The occurrence of semidiurnal variations in stability in the Rhine ROFI is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring.
Sediment transport by waves and currents
Authors: Sleath John F. A.
Published: February 2004

A bed‐load model, based on a generalization of Bagnold's (1954) constitutive equations together with the assumption of similarity profiles for velocity and concentration, was proposed for sediment transport by waves by Sleath (1994). This model is now extended to wave/current flows and applied to three different situations: (a) oscillatory flows with superimposed currents, (b) net transport produced by progressive waves over a flat horizontal bed, and (c) longshore transport induced by waves approaching a beach. In all three cases the model gives reasonable agreement with experiment. In the case of longshore transport the model produces almost the same formula as that of the Coastal Engineering Research Center (1977), apart from additional terms for density and grain size.
Synoptic bottom pressure variability on the Labrador and Newfoundland Continental Shelves
Authors: de Young Brad , Lu Youyu , Greatbatch Richard
Published: February 2004

A high‐resolution, barotropic model of the North Atlantic is used to investigate bottom pressure variability on the Labrador and Newfoundland Shelves. The model has a free surface and is forced by surface atmospheric pressure and wind stress derived from twice‐daily analyses from the European Centre for Medium‐Range Weather Forecasts. Model output is compared with a yearlong set of bottom pressure data (1985–1986). Coherence squared between model and observations is significant at the 95% confidence level at almost all stations and exceeds 0.6 over most of the inner shelf, with good agreement in the phase. The model shows a tendency to overestimate the autospectral energy at periods less than 10 days but to underestimate the energy at longer periods. The model does less well at the outer shelf, where coherence squared is only marginally significant. The weaker performance of the model near the shelf break may indicate a role for physical processes not accounted for in the model (e.g., eddies, stratification) or, alternatively, might be due to the low signal‐to‐noise ratio in the bottom pressure measurements made at the shelf break. Model/data comparisons show that wind forcing dominates over pressure forcing, except at the northern end of the Labrador Shelf, where both forcings are important. Model experiments run with and without Hudson Bay in the model domain demonstrate that Hudson Bay influences the Labradr Shelf in the 2 to 10‐day period range.
Swash on a gently sloping beach
Authors: Raubenheimer B. , Guza R. T. , Elgar Steve , Kobayashi N.
Published: February 2004

Waves observed in the inner surf and swash zones of a fine grained, gently sloping beach are modeled accurately with the nonlinear shallow water equations. The model is initialized with observations from pressure and current sensors collocated about 50 m from the mean shoreline in about l m depth, model predictions are compared to pressure fluctuations measured at five shoreward locations and to run‐up. Run‐up was measured with a vertical stack of five wires supported parallel to and above the beach face at elevations of 5, 10, 15, 20, and 25 cm. Each 60‐m‐long run‐up wire yields time series of the most shoreward location where the water depth exceeds the wire elevation. As noted previously, run‐up measurements are sensitive to the wire elevation owing to thin run‐up tongues not measured by the more elevated wires. As the wire elevation increases, the measured mean run‐up location moves seaward, low‐frequency (infragravity) energy decreases, and higher‐frequency sea swell energy increases. These trends, as well as the variation of wave spectra and shapes (e.g., wave skewness) across the inner surf zone, are well predicted by the numerical model.
Tropical instability wave kinematics: Observations from the Tropical Instability Wave Experiment
Authors: Qiao L. , Weisberg R. H.
Published: February 2004

The kinematics of planetary waves originating from instability of the nearsurface equatorial currents are reported on using velocity measurements from an array of acoustic Doppler current profilers deployed in the equatorial Pacific during the Tropical Instability Wave Experiment. A distinctive wave season was observed from August to December 1990, with wave energy confined primarily above the core of the Equatorial Undercurrent. Particle motions in the horizontal plane are described by eccentric ellipses oriented toward the north, but tilting into the cyclonic shear of the South Equatorial Current. The tilt is maximum near the surface just north of the equator and decreases to the south and with depth. The distribution of wave variance is narrowband in both frequency and zonal wavenumber, with central period, zonal wavelength, and westward directed phase propagation estimated to be 500 hours, 1060 km, and 59 cm s−1, respectively. Neither the meridional nor the vertical wavenumber component is statistically different from zero. These results generally agree with previous findings on tropical instability waves from the Atlantic and Pacific Oceans, and, in the undersampled arena of geophysical measurements, they provide an example where statistical inference is supported by an ensemble of independent measurements.
The role of the interface in exchange through the Strait of Gibraltar
Authors: Bray N. A. , Ochoa J. , Kinder T. H.
Published: February 2004

Five cross‐strait hydrographic sections repeated several times during the Gibraltar Experiment in 1985–1986 are used to examine the structure of the interface layer between the inflowing Atlantic waters and outflowing Mediterranean waters in the Strait of Gibraltar. The interface is 60–100 m thick, with a strong vertical salinity gradient identified by fitting individual salinity profiles to a piecewise‐linear, three‐layer model. The interface is deeper, thicker, fresher, and colder on the west end of the strait than in the Narrows, where there is a minimum in thickness and a maximum in salinity gradient. Farther east, the interface thickens again and continues to get saltier, warmer, and shallower. Property variations in all three layers are also cast in terms of the three principal water types involved in the exchange. The traditional Knudsen model of exchange is extended to three layers, assuming that the interface is a transport‐carrying third layer with uniform vertical shear. As much as half of the inflowing or outflowing transport occurs in the interface layer. Transport converges in both the upper and lower layers, implying, over the length of the strait, vertical exchange between layers that is comparable to about half the horizontal exchange. The richness of structure and complexity of interaction between the interface and the upper and lower layers argues against the use of two‐layer models to characterize exchange through the Strait of Gibraltar.
The lognormal distribution as a model for bio‐optical variability in the sea
Authors: Campbell Janet W.
Published: February 2004

The lognormal distribution is presented as a useful model for bio‐optical variability at a variety of spatial and temporal scales. A parametric statistical framework is presented for using the lognormal model to assess the effects of heterogeneity and scale on closure. Variability at small scales affects but is unresolved by large‐scale measurements. An assumed lognormal distribution allows one to integrate over small‐scale variability to predict large‐scale measurements. Examples are presented to demonstrate how knowledge of the variance can be incorporated into models to relate measurements made at different scales.
Variability in the chlorophyll‐specific absorption coefficients of natural phytoplankton: Analysis and parameterization
Authors: Bricaud Annick , Babin Marcel , Morel André , Claustre Hervé
Published: February 2004

Variability in the chlorophyll (chl) a‐specific absorption coefficients of living phytoplankton aph*(λ) was analyzed using a data set including 815 spectra determined with the wet filter technique in different regions of the world ocean (covering the chlorophyll concentration range 0.02–25 mg m−3). The aph* values were observed to decrease rather regularly from oligotrophic to eutrophic waters, spanning over more than 1 order of magnitude (0.18 to 0.01 m2 mg−1) at the blue absorption maximum. The observed covariation between aph*(λ) and the field chl a concentration (chl) can be explained considering (1) the level or pigment packaging and (2) the contribution of accessory pigments to absorption. Empirical relationships between aph*(λ) and 〈chl〉 were derived by least squares fitting to power functions. These relationships can be used to produce aph* spectra as a function of 〈chl〉. Such a simple parameterization, if confirmed with further data, can be used, e.g., for refining estimates of the carbon fixation rate at global or regional scales, such as those obtained by combining satellite pigment concentration maps with primary production models based on physiological parameters, among which aph* is an important one.
Observation of internal tides in the East China Sea with an underwater sliding vehicle
Authors: Kuroda Yoshifumi , Mitsudera Humio
Published: February 2004

Detailed structures of internal tides in the East China Sea were observed during autumn from 1986 to 1989, using an underwater sliding vehicle that can measure temperature, conductivity, and depth continuously, sliding up and down along a faired cable. A variety of structures were observed and are classified with internal Froude numbers of the lowest two modes, which depend on the barotropic tidal current velocities and the phase speeds of internal waves (i.e., stratification). In the central East China Sea, the second‐mode waves are most strongly generated at the shelf break and then propagate shoreward onto the East China Sea continental shelf. Since the barotropic tidal current is faster than the phase speed of the second‐mode waves on the shelf, the generation mechanism is consistent with conventional models. On the other hand, in the southern East China Sea the lowest‐mode response is dominant, although the tidal current velocity is slower than the lowest‐mode phase speed. A nonlinear model is proposed for this response.
Sea surface topography fields of the tropical Pacific from data assimilation
Authors: Miller Robert N. , Busalacchi Antonio J. , Hackert Eric C.
Published: February 2004

Time series of maps of monthly tropical Pacific dynamic topography anomalies from 1979 through 1985 were constructed by means of assimilation of tide gauge and expendable bathythermograph (XBT) data into a linear model driven by observed winds. Estimates of error statistics were calculated and compared to actual differences between hindcasts and observations. Four experiments were performed as follows: one with no assimilation, one with assimiation of sea level anomaly data from eight selected island tide gauge stations, one with assimilation of dynamic height anomalies derived from XBT data, and one with both XBT and tide gauge data assimilated. Data from seven additional tide gauge stations were withheld from the assimilation process and used for verification in all four experiments. Statistical objective maps based on data alone were also constructed for comparison purposes. The dynamic response of the model without assimilation was, in general, weaker than the observed response. Assimilation resulted in enhanced signal amplitude in all three assimilation experiments. RMS amplitudes of statistical objective maps were only strong near observing points. In large data‐void regions these maps show amplitudes even weaker than the wind‐driven model without assimilation. With few exceptions the error estimates generated by the Kaiman filter appeared quite reasonable. Since the error processes cannot be assumed to be white or stationary, we could find no straightforward way to test the formal statistical hypothesis that the time series of differences between the filter output and the actual observations were drawn from a population with statistics given by the Kaiman filter estimates. The autocovariance of the innovation sequence, i.e., the sequence of differences between forecasts before assimilation and observations, has long been used as an indicator of how close a filter is to optimality. We found that the best filter we could devise was still short of the goal of producing a white innovation sequence. In this and earlier studies, little sensitivity has been found to the parameters under our direct control. Extensive changes in the assumed error statistics make only marginal differences. The same is true for long time and space scale behavior of different models with richer physics and finer resolution. Better data assimilation results will probably require relaxation of the assumptions of stationarity and serial independence of the errors. Formulation of such detailed noise models will require longer time series, with the attendant problems of matching very different data sets.
A neural network as a nonlinear transfer function model for retrieving surface wind speeds from the special sensor microwave imager
Authors: Krasnopolsky V. M. , Breaker L. C. , Gemmill W. H.
Published: February 2004

A single, extended‐range neural network (SER NN) has been developed to model the transfer function for special sensor microwave imager (SSM/I) surface wind speed retrievals. Applied to data sets used in previous SSM/I wind speed retrieval studies, this algorithm yields a bias of 0.05 m/s and an rms difference of 1.65 m/s, compared to buoy observations. The accuracy of the SER NN for clear (low moisture) and cloudy (higher moisture/light rain) conditions equals the accuracy of NNs trained separately for each of these cases. A new moisture retrieval criterion based on a single, physically interpretable parameter, cloud liquid water, is proposed in conjunction with the SER NN. Using this retrieval criterion, (1) a moisture retrieval threshold for cloud liquid water of 0.5 kg/m2 was estimated, and (2) 40% of the data rejected by previous rain flags could be recovered. When the SER NN was trained using this retrieval criterion, a bias of 0.03 m/s and an rms value of 1.58 m/s were obtained and only 2% of the data were rejected. Also, a slight improvement in retrieval accuracy for cloudy conditions was achieved (∼10%) by including SSM/I brightness temperatures at 85 GHz. Finally, the limitations of NN algorithms are discussed in light of the present application.