bluuebunny/crossref_metadata_2025 huggingface dataset:
Baylike and continuous variations of the relative level of the late coda during 24 years of observation on Kamchatka
Authors: Gusev Alexander A.
Published: February 2004

The relative amplitude levels of backscattered shear waves (coda) of local earthquakes as expressed in coda magnitude residuals were studied for nine Kamchatka seismograph stations. Coda magnitudes were determined from coda amplitudes measured on three‐component photograph records of 1.2‐s period displacement‐recording (between 1 and 10 Hz) instruments. A coda magnitude scale was specially designed for this study. It is rather precise, with a standard deviation of about 0.15 for log amplitudes measured at any single station. At each station, for each event we compute the station magnitude residual, i.e., station magnitude (averaged over three components) minus network average magnitude. Station corrections for each station and component were determined and included in the procedure. Time series of coda magnitude residuals including 500–1000 events for each station for the 24‐year period of observations (1967–1990) show moderate but statistically significant oscillations around a constant level. Superposed on this background, two prominent anomalies are revealed. One, at station KBG, of 3 years duration, preceded two M ≈ 8 shallow earthquakes within 100 km from the station. Another, at station APH, of 1.5 years duration, preceded a major (volume of 2.5 km3) fissure volcanic eruption within a 70‐km distance from the station. No other comparable shallow earthquakes or eruptions took place on Kamchatka during this period. The sign and the amplitude of the anomalies indicate a possible 30% increase of S ‐wave attenuation in the lithosphere under the stations.
Observation of high‐frequency teleseismic Pn on the long‐range Quartz profile across northern Eurasia
Authors: Ryberg T. , Fuchs K. , Egorkin A. V. , Solodilov L.
Published: February 2004

Short‐period, three‐component recordings of the seismic wave field of Peaceful Nuclear Explosions (PNEs) on long‐range profiles are used to determine the fine structure of the mantle lithosphere. By analyzing the frequency content of the recorded phases and applying different band‐pass filters to the data, it is possible to divide the wave field into two distinctly different constituents: low‐frequency body waves traveling along Fermat paths (first arrivals) and the high‐frequency teleseismic (or long‐range) Pn phase traveling with a group velocity of 8.1 km/s and accompanied by a long, incoherent coda. This high‐frequency teleseismic Pn phase is observable from about 750 km, where it separates from the faster first arrival, to the maximum recording distance of 3145 km. It is recorded from shots at different locations and appears to be almost unaffected by the major tectonic feature along the profile, the Urals. The frequency spectrum of this Pn phase contains more high‐frequency energy (up to 12 Hz) than first arrivals that penetrate deeper into the upper mantle. The teleseismic high‐frequency Pn arrival has a remarkable coda, which is incoherent between closely spaced stations. The coda duration is dependent on the component of motion, being shortest on the vertical and longest on the transverse component. We propose a velocity model that is characterized by a zone extending from the crust‐mantle boundary to a depth of about 100 km. This zone has randomly distributed, spatially anisotropic velocity fluctuations. We propose that these velocity heterogeneities are “stretched” in the horizontal direction. This zone forms a scattering waveguide that confines the high‐frequency teleseismic Pn. There are indications that below this Pn waveguide, either the scale of the velocity fluctuations or the Q factor changes. This is expressed in a separation of the teleseismic Pn phase from the phase diving deeper into the mantle lithosphere.
Multiple late Holocene earthquakes along the Reelfoot fault, central New Madrid seismic zone
Authors: Kelson Keith I. , Simpson Gary D. , VanArsdale Roy B. , Haraden Colleen C. , Lettis William R.
Published: February 2004

The Reelfoot fault is an east vergent, reverse fault underlying the Lake County uplift, a low‐amplitude, late Holocene anticline bordered on the east by the 32‐km‐long Reelfoot scarp. Fluvial deposits across the scarp define an 8‐m‐high, east facing monocline. Most near‐surface deformation along the scarp is accommodated via folding rather than faulting. We interpret the scarp as a fault‐propagation fold developed over a west dipping reverse fault interpreted from shallow seismic reflection data. Trench exposures provide evidence for three episodes of deformation along the Reelfoot fault within the past approximately 2400 years, between A.D. 780 and 1000, between A.D. 1260 and 1650, and during A.D. 1812. Our best estimate of the average recurrence interval for deformation along the scarp is 400–500 years. Each episode of deformation had a slightly different style. The third most recent event produced a small graben a few tens of centimeters deep in the hanging wall of the reverse fault. The second most recent earthquake produced about 1.3 m of throw in the graben, as well as folding along the updip projection of the reverse fault and development of the scarp. These relations suggest that graben development increased through time concomitant with growth of the monocline or that the events are of different magnitude. The 1811–1812 episode of deformation produced abundant liquefaction, prominent folding of fluvial strata along the scarp, and minor faulting in the graben.
Throughgoing crustal faults along the northern margin of the South China Sea and their role in crustal extension
Authors: Hayes Dennis E. , Nissen Susan Spangler , Buhl Peter , Diebold John , Bochu Yao , Zeng Weijun , Chen Yongqin
Published: February 2004

A study of the rifled northern margin of the South China Sea, based on new single‐ship and two‐ship multichannel seismic data, has revealed the presence of highly reflective, steeply dipping normal faults which cut through the entire crust. These faults, with dips of ∼25°–40°, are associated with localized crustal thinning of ∼5–15 km and represent horizontal extension of the order of tens of kilometers. A range of ∼5–20 km of horizontal extension has been measured along individual faults. Although the relative amount of horizontal extension taken up by these faults is modest compared to the total extension across the margin (only about 15% or less), the amount of localized crustal thinning which takes place along these fault surfaces is quite significant. At one fault location, the crustal thickness is reduced by ∼40%. Although the faults described have been recognized at three separate locations along the margin, their trends appear to differ at each. In two examples, the faults are oblique to the inferred, general north‐south extension direction of the margin. This characteristic, taken with the fact that some of the crustal faults also appear to serve as abrupt boundaries between broad regions of differing crustal thickness and reflectivity of the lower crust, suggests that at least some of these faults were present prior to rifting. Such faults are postulated to have been former thrust faults, perhaps associated with a Triassic suture zone and/or Cretaceous subduction margin, which have been reactivated as normal faults during subsequent rifting. Alternatively, some of the throughgoing faults may represent crustal detachment surfaces that first formed as the result of a simple shear response to crustal extension.
Glacial isostatic adjustment and Earth rotation: Refined constraints on the viscosity of the deepest mantle
Authors: Peltier W. R. , Jiang Xianhua
Published: February 2004

We explore the extent to which two properties of Earth's present‐day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle. The analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660‐km seismic discontinuity appears to be allowed by these data. Such observations do not constrain the viscosity of the mantle below about 1400 km depth, however, and here we estimate the average viscosity in this deepest region by invoking observations of the present‐day rate and direction of polar wander and the present‐day magnitude of the nontidal acceleration of the rate of planetary rotation. Since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation, this response is expected to provide the most specific sampling possible of deep mantle properties. Our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self‐consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial‐interglacial transition, namely, the ICE‐4G model. This analysis shows that when Earth rotation observations are combined with relative sea level constraints, the viscosity of the deepest mantle is required to be ∼1 order of magnitude higher than the viscosity of the upper part of the lower mantle. Such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid. The analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations.
The elastic coefficients of double‐porosity models for fluid transport in jointed rock
Authors: Berryman James G. , Wang Herbert F.
Published: February 2004

Phenomenological equations (with coefficients to be determined by specified experiments) for the poroelastic behavior of a dual porosity medium are formulated, and the coefficients in these linear equations are identified. The generalization from the single‐porosity case increases the number of independent coefficients for volume deformation from three to six for an isotropic applied stress. The physical interpretations are based upon considerations of different temporal and spatial scales. For very short times, both matrix and fractures behave in an undrained fashion. For very long times, the double‐porosity medium behaves like an equivalent single‐porosity medium. At the macroscopic spatial level, the pertinent parameters (such as the total compressibility) may be determined by appropriate field tests. At an intermediate or mesoscopic scale, pertinent parameters of the rock matrix can be determined directly through laboratory measurements on core, and the compressibility can be measured for a single fracture. All six coefficients are determined from the three poroelastic matrix coefficients and the fracture compressibility from the single assumption that the solid grain modulus of the composite is approximately the same as that of the matrix for a small fracture porosity. Under this assumption, the total compressibility and three‐dimensional storage coefficient of the composite are the volume averages of the matrix and fracture contributions.
Elevated fluid pressure and fault zone dilation inferred from seismic models of the northern Barbados Ridge decollement
Authors: Bangs Nathan L. , Shipley Thomas H. , Moore Gregory F.
Published: February 2004

In 1992, a large volume of three‐dimensional seismic reflection data were acquired in a 5 × 25 km area across the toe of the Barbados accretionary complex that covers the Deep Sea Drilling Project leg 78A and Ocean Drilling Program legs 110 and 156 drilling sites. These data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone. Seismic models have been constructed across a 6‐km region of the decollement, where it has been mapped as a moderate to bright polarity‐reversed reflection. The models show that this segment of the decollement reflection is caused by a low‐velocity interval, usually 12–16 m thick. The top of the low‐velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1.8 km/s to between 1.7 and 1.65 km/s, with some localized bright reflections with an even lower velocity of 1.6 km/s. The base of the low‐velocity layer is less certain from modeling. The base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer, or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10‐m‐thick interval. Comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement, fluid pressure is at or near lithostatic. Furthermore, the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required. The dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5‐km segment of the decollement. It is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength.
A bound on the rheology of continental lithosphere using very long baseline interferometry: The velocity of south China with respect to Eurasia
Authors: Molnar Peter , Gipson John M.
Published: February 2004

Very long baseline interferometric (VLBI) measurements using the antenna near Shanghai, China, between 1987 and 1994 show that this site moves at 8 ± 0.5 mm/yr (1σ) in the direction N116.5°E (±4. 1°) with respect to VLBI sites in Europe on the Eurasia plate. Independent measurements of the site at Shanghai with respect to sites on nearly all major plates (Eurasia, North America, Pacific, Australia, and Africa) determine this velocity. South China translates east‐southeast relative to Eurasia more slowly than 20% of India's roughly 50 mm/yr convergence rate with Eurasia and hence accommodates only a small fraction of India's penetration into Eurasia. Apparently, crustal thickening absorbs most of India's convergence. The relatively low speed of south China supports treatments of continental lithosphere as a thin viscous sheet whose strength is governed primarily by power law creep in the upper mantle, and not by friction in the crust.
Fault model for preseismic deformation at Parkfield, California
Authors: Stuart William D. , Tullis Terry E.
Published: February 2004

We construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate (M 5.5–6) earthquake on the San Andreas fault near Parkfield, California. The quasi‐static model simulates fault slip, fault shear stress, and ground deformation for all stages of repeated earthquake cycles. Unstable slip, which is the analog of a moderate Parkfield mainshock, is caused by overall failure of a postulated strong area or patch of the fault zone. The brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law. Stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth. Amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at Parkfield, starting just after the 1966 mainshock, in two ways. In the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations, moments, and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock. In the second way we assign values of most parameters from laboratory friction experiments and a temperature‐depth profile inferred from heat flow data. Overall agreement between model results and observations is comparable for the two methods. In all simulations, preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability. Preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements. However, for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to detect.
Noble gases, K, U, Th, and Pb in native gold
Authors: Engster O. , Niedermann S. , Thalmann C. , Frei R. , Kramers J. , Krähenbühl U. , Liu Y. Z. , Hofmann B. , Boer R. H. , Reimold W. U. , Bruno L.
Published: February 2004

We present determinations of the noble gas and Pb isotopic abundances and of K, Th, and U concentrations of native gold. Our results demonstrate that gold is an excellent carrier for crustal volatiles, but direct dating of gold using the U, Th‐4He, 40K–40Ar, and U fission Xe methods was not successful for various reasons. The main significance of this work is the great sensitivity of gold for trapped gases as well as for gases that were produced in situ which gives the prospects of using gold and its fluid and solid inclusions for the study of paleogas composition. Numerous nuclear effects characterize the noble gas inventory of placer gold from Switzerland and Italy, vein gold from Italy, South Africa, and Venezuela, and lode gold from South Africa. The degassing patterns obtained by mass spectrometry show a low‐temperature release of volatiles around 500°C from fluid inclusions mainly in vein gold and a high‐temperature release from solid inclusions and the gold itself. The low‐temperature volatiles represent species that were trapped when the gold crystallized. We investigated the following trapped species: the isotopes of He, Ne, Ar, Kr, Xe, and Pb, and the abundances of K, U, Th, H2O, and CO2. The crustal gases trapped by gold comprise 3He from 6Li(n,α)3H → β− → 3He, 4He and 40Ar from the U, Th, and K decay, and Xe from 238U fission. We observe 4He/40Ar = 3.9 for the radiogenic trapped gases of tertiary gold and a ratio of 1.4 for Archean gold. These ratios are consistent with the production ratios from U and K at the respective times and demonstrate that gold can be used as a sampler of ancient atmospheric gases. The concentrations of U and Th range from a few parts per billion to a few parts per million, and those of K and Pb range up to some tens of parts per million. The antiquity of trapped Pb is indicated by the Pb‐Pb model age of about 3000 Ma for the lead extracted from vein gold and quartz of the Lily gold mine (South Africa). Gold also contains noble gases produced in situ by U, Th, and K decay. We obtained 4He up to the extremely high concentration of 6.8×10−2 cm3 STP/g (STP: atmospheric pressure and 0°C) for gold separated from a specimen of the Witwatersrand West Rand gold fields, indicating that this gold has a large U content.
Velocity gradients in the northern Indian Ocean inferred from earthquake moment tensors and relative plate velocities
Authors: Tinnon M. J. , Holt W. E. , Haines A. J.
Published: February 2004

The region in the northern Indian Ocean between the Central Indian Ridge, the Sumatra Trench, and 15°S to 9°N exhibits high seismicity, along with basement and sediment deformation. This area represents a diffuse plate boundary between the distinct and separate portions of the Indian and Australian plates. We invert the spatial distribution of horizontal strain rates in this diffuse plate boundary zone to obtain the horizontal velocity field associated with the moment release from 61 earthquakes that have occurred over the past 80 years. In the inversion, the three‐dimensional rotation vector function W is determined at all corner points (i.e., knot points) of a grid such that the predicted strain rate values match the observed strain rates within the grid areas in a least squares sense. The rotation vector of Australia relative to India, defined by the earthquake data alone, is 10.1°S±9.1°, 81.5°E±5.4°, 0.1°±0.06°/m.y. (1 sigma error), which is slightly different in location from the expected vector estimated using seafloor magnetic anomalies and transform fault trends (3.2°S, 75.1°E, 0.288°/m.y.) [DeMets et al, 1994a, b]. By minimizing the rate of work within a continuous viscous medium that accommodates Australia's motion relative to India [DeMets et al, 1994a], we determine the complete horizontal velocity gradient tensor within the zone of diffuse deformation. The viscosity tensor within all of the deforming areas can be specified. When all the regions are free to strain in any direction and have the same viscosity (uniform isotropic viscosity), the predicted strain pattern is either pure extension or pure compression, which is in contrast to the style of earthquake deformation east of Chagos where both thrust and strike‐slip mechanisms are observed. However, when the isotropic viscosity of regions within the Wharton Basin is increased relative to the isotropic viscosity levels of areas along the Ninetyeast Ridge, the plate motion is accommodated by combined thrust and strike‐slip styles of strain along the Ninetyeast Ridge and within the eastern Central Indian Basin. This predicted strain that accommodates plate motion is in agreement, both in direction and style, with the earthquake‐related strains in these regions. Although most of the observed style and direction of strain can be explained by lithospheric strength differences between the Wharton Basin and Ninetyeast Ridge areas alone, the presence of strike‐slip faulting within the Wharton Basin requires a preexisting fabric that favors strike‐slip faulting there as a strain mechanism that accommodates, in part, the motion of Australia relative to India. To first order, the direction of principal strain rates, the style of strain, and the spatial distribution of strain rates from the earthquakes are in accord with the accommodation of the expected plate motion, whereas across the plate boundary zone the rates from 80 years of seismicity average out to at most 60% of the total strain rate.
Irregular recurrence of large earthquakes: An analysis of historic and paleoseismic catalogs
Authors: Goes Saskia D. B.
Published: February 2004

Assessment of a region's seismic hazard requires knowledge of the repeat times and the regularity of earthquake recurrence. For most areas, the earthquake catalog is too short to determine recurrence parameters reliably. Using a large set of events for many regions, however, it should be possible to constrain the average characteristics of earthquake recurrence behavior. This study compiles a set of the most complete historic and paleoseismic series of M ≥ 7 earthquakes from the Middle American Trench, Alaska, Chile, Japan and the San Andreas Fault. The 52 recurrence series show a large average aperiodicity, that is, variations in recurrence time are larger than 40% of the average recurrence interval. Aperiodicity may decrease with increasing magnitude. Neither the use of different magnitude thresholds, the exclusion of foreshocks and aftershocks, the type of statistics (normal or lognormal) used, nor the choice of segmentation results in a significantly smaller aperiodicity. The irregular recurrence behavior exhibited by the data implies that earthquakes are more unpredictable than often assumed in hazard analyses.
Three‐dimensional P and S wave velocity structure of Redoubt Volcano, Alaska
Authors: Benz H. M. , Chouet B. A. , Dawson P. B. , Lahr J. C. , Page R. A. , Hole J. A.
Published: February 2004

The three‐dimensional P and S wave structure of Redoubt Volcano, Alaska, and the underlying crust to depths of 7–8 km is determined from 6219 P wave and 4008 S wave first‐arrival times recorded by a 30‐station seismograph network deployed on and around the volcano. First‐arrival times are calculated using a finite‐difference technique, which allows for flexible parameterization of the slowness model and easy inclusion of topography and source‐receiver geometry. The three‐dimensional P wave velocity structure and hypocenters are determined simultaneously, while the three‐dimensional S wave velocity model is determined using the relocated seismicity and an initial S wave velocity model derived from the P wave velocity model assuming an average Vp/Vs ratio of 1.78. Convergence is steady with approximately 73% and 52% reduction in P and S wave arrival time RMS, respectively, after 10 iterations. The most prominent feature observed in the three‐dimensional velocity models derived for both P and S waves is a relative low‐velocity, near‐vertical, pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level. This feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit. The velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations. High velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates. No large low‐velocity body suggestive of a magma chamber is resolved in the upper 7–8 km of the crust.
Buoyancy‐driven propagation of isolated fluid‐filled fractures: Implications for fluid transport in Gulf of Mexico geopressured sediments
Authors: Nunn Jeffrey A.
Published: February 2004

A large portion of the sediments within the northern Gulf of Mexico contain pore fluid pressures in excess of hydrostatic. Development of geopressure is generally attributed to compaction disequilibrium caused by rapid deposition of low‐permeability sediments in the Miocene and Plio‐Pleistocene. Numerous studies have examined the formation of overpressures and/or expulsion of geopressured fluids into overlying hydropressured strata. However, very little attention has been given to fluid flow within the geopressured zone itself. Movement of oils from Cretaceous or older source rocks into Plio‐Pleistocene reservoirs in the Gulf Basin requires as much as 10 km of vertical migration in a few million years. Precipitation of cements in some geopressured sediments also implies large‐scale fluid flow. New evidence from a deep well in the Eugene Island area, offshore Louisiana, indicates that geopressured sediments are mechanically very weak with a Poisson's ratio greater than 0.4 and a shear modulus or rigidity less than 1 GPa. In addition, large‐scale fluid flow either through interconnected pores or fractures is not occurring in this location, at least at present. An alternative hypothesis is that upward fluid transport in geopressured sediments is caused by buoyancy‐driven propagation of isolated fluid‐filled fractures. Using linear fracture mechanics, I show that vertical fractures with lengths of a few meters can propagate at velocities of 1000 m/yr. Mass flux rates (∼100 kg/m2/yr) are significant assuming a mechanism for formation of fluid‐filled fractures exists, such as hydrofracturing when fluid pressures exceeded the minimum confining stress. Fracture propagation velocity and mass flux rate are strongly dependent on the shear modulus of geopressured sediments.
Osmium isotope systematics of drilled lavas from Mauna Loa, Hawaii
Authors: Hauri Erik H. , Lassiter John C. , DePaolo Donald J.
Published: February 2004

We have investigated the isotopic compositions of Os, Sr, Nd, and Pb in a suite of primitive Mauna Loa lavas from the upper 280 m of the Hawaii Scientific Drilling Project pilot core drilled near Hilo, Hawaii. These lavas were probably erupted from Mauna Loa's northeast rift. Correlations between Os (hosted by olivine) and other isotopes indicate that olivine crystals in these flows are closely related in space and time to the enclosing lava, despite the presence of deformation features and Fe/Mg disequilibrium in some olivines. The temporal isotopic evolution of the lavas matches published data for basalts from Mauna Loa's southwest rift, indicating that the two rifts (as well as the summit) share a common magma feed zone which is distinct from that of Kilauea. The composition of the lowermost HSDP Mauna Loa sample shows some isotopic similarities to modern Kilauea compositions and in this respect compares well with published data on submarine lavas from Mauna Loa's southwest rift. The good correlations among the isotopic tracers of compatible (Os) and incompatible (Sr, Nd, Pb) elements indicate that a depleted upper mantle component is very minor or nonexistent in Mauna Loa lavas. The Os isotope results definitively rule out equilibrium porous flow as a means of melt transport through the lithosphere. The isotopic variations in shield‐stage lavas are most consistent with partial melting of two distinct sources within the Hawaiian plume followed by partial mixing and rapid transport of melts through the oceanic lithosphere. The passage of the Pacific lithosphere over a heterogeneous Hawaiian plume can account for the systematic differences in the compositions of volcanoes from the “Kea” and “Loa” trends as well as the geochemical evolution of individual shields on the island of Hawaii.
A gravity current model of cooling mantle plume heads with temperature‐dependent buoyancy and viscosity
Authors: Bercovici David , Lin Jian
Published: February 2004

Gravity currents are a ubiquitous fluid dynamical phenomenon which involve the horizontal spreading of fluid masses under their own weight or buoyancy. A theoretical model is developed to account for the effects of bulk cooling on the dynamics and morphology of geological gravity currents, with particular focus on mantle plume heads spreading beneath the lithosphere. As many geological gravity currents (e.g., plume heads and lava flows) spread, they cool and thereby become more viscous and dense. All gravity currents initially spread at the same rate as the isothermal currents predicted by Huppert [1982]. However, currents with temperature‐dependent viscosity and/or buoyancy eventually go to a much slower spreading rate than the isothermal currents. Moreover, unlike the isothermal gravity currents, cooling variable‐viscosity and/or variable‐buoyancy currents do not conserve shape as they spread. Both constant volume and constant volume flux currents with strongly temperature dependent viscosity develop steep‐sided flat‐topped, plateau shapes which become more rounded once the currents lose most of their heat. Currents with temperature‐dependent buoyancy develop inflections or even extensive swelling at their flow fronts. The surface expression of the edge‐steepening effect in mantle plume heads is likely to be filtered by lithospheric flexure but may contribute to the flattened plateau shape inferred by Wessel [1993] for the Hawaiian swell. The frontal inflation effect due to variable buoyancy may contribute to the dual‐lobe structure of the Hawaiian swell gravity anomaly and suggests an alternate physical mechanism for forming the torus‐ or horseshoe‐shaped geochemical patterns in the Galápagos and the Marquesas hotspots. Perhaps most significantly, the gravity current model also predicts the thermal (i.e., degree‐of‐melting) pattern for Galápagos and Marquesas hotspots more readily than the traditional entrainment models.
Hettangian and Sinemurian magnetostratigraphy from Paris Basin
Authors: Yang Zhenyu , Moreau Marie‐Gabrielle , Bucher Hugo , Dommergues Jean‐Louis , Trouiller Alain
Published: February 2004

A long core (1110 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 600 paleomagnetic samples were obtained from a 148‐m‐thick series of Hettangian and Sinemurian rocks. A composite demagnetization using thermal (up to 300°C) followed by alternating field technique (up to 100 mT) is used to separate the magnetic components. A low unblocking temperature component (<250°C) with an inclination of about 65° is interpreted as a present‐day field overprint. The characteristic remanent component with both normal and reversed antipodal directions is then isolated between 5 and 70 mT. Eighty‐one polarity intervals are recognized in this study. The higher reversal frequency of the late Hettangian/early Sinemurian time interval contrasts with a lower reversal rate observed in the rest of the early Liassic. A rough mean estimate of about 5 reversals/m.y. can be proposed for the earliest Jurassic. These results represent a significant contribution to the magnetic polarity reversal timescale for a time interval hitherto poorly known and add to the magnetic reversal frequency curve of the last 350 m.y.
Bathymetric surface modeling: A machine learning approach
Authors: Dysart Paul S.
Published: February 2004

The objective of this work is to develop a surface modeling system capable of characterizing large gridded bathymetric databases with potential applications to problems such as the extrapolation of survey data to higher resolution, the interpolation of bottom characteristics between surveyed regions, and the correlation of bottom features with other geophysical measurements. The system employs a two‐dimensional stochastic seafloor model to represent blocks of gridded bathymetry by a small number of model parameters that describe the physical features of the ocean bottom. This study focuses on the inversion component which is designed to estimate the model parameters quickly without iteration or starting values. Quality high‐speed inversions are obtained using machine learning techniques, overcoming many practical limitaions imposed by conventional least squares techniques. This approach leads to the approximation of a direct mapping between the moments of the Fourier transformed data and the model parameter variables. Mapping is accomplished by the universal approximation capabilities of multilayer networks. Validation tests performed on synthetic examples spanning the entire model parameter space yield parameter estimation errors less than 5%. The modeling system is applied to a large northeast Pacific data set to examine correlations between bottom characteristics and sediment thickness. Errors in model parameters estimated from the bathymetric data by Monte Carlo simulation are generally less than 10% dependent on data quality and conformance to model assumptions. The major advantages offered by this approach are clearly demonstrated by its error tolerance and the speed of inversion processing.
Hydrogeology of the Hawaii Scientific Drilling Project borehole KP‐1: 2. Groundwater geochemistry and regional flow patterns
Authors: Thomas D. M. , Paillet F. L. , Conrad M. E.
Published: February 2004

A series of downhole and surface water samples were taken from the 1‐km‐deep KP‐1 borehole located on the eastern flank of the island of Hawaii. Early samples from depths of more than 700 m showed salinities nearly equivalent to seawater but having anomalous cation concentrations that are attributed to ion exchange between formation fluids and residual drilling mud clays. Later deep samples found only minor variations from seawater cation chemistry that are consistent with low‐temperature weathering of basalts; δ18O values are equivalent to seawater values and are consistent with this interpretation. Carbon 14 activities of dissolved inorganic carbonate indicate a water age ranging from 5890 to 7170 years B.P. and fluid transport rates of 1.8 to 2.2 m/yr. Fluid samples from perforations at 310 m in the borehole demonstrate that a freshwater aquifer is present at the Mauna Kea/Mauna Loa interface; borehole resistivity logs indicate that it is ∼200 m thick. Although it has not yet been possible to obtain samples of the freshwater zone without contamination from the deep saline fluids, the chloride concentrations of the low‐salinity zone are estimated using a mixing enthalpy calculation to be less than 100 mg/L. Light stable isotope data indicate that the fresh water at 320 m is derived from recharge entering the island at an average elevation of 2000 m. Inferred 14C activities of the dissolved bicarbonate in the freshwater zone indicate an average calibrated age of 2200 years B.P. and an average fluid velocity of at least 14 m/yr. A regional water flow model is proposed that suggests that the fresh water found at the 320‐m depth is derived from rainfall recharge from the middle elevations of Mauna Kea volcano. This rainfall is channeled beneath the Mauna Loa lavas by the thick soil layer separating the two volcanoes. A second shallow fresh‐to‐brackish water zone, derived from Mauna Loa recharge, is also inferred to exist below the carbonate formation that underlies the shallow basal lens. The results of our preliminary study of the groundwater system below the KP‐1 drill site demonstrate that intervolcano and interflow aquicludes can have a substantial impact on water circulation and discharge from young island volcanoes.
Petrography and olivine and glass chemistry of lavas from the Hawaii Scientific Drilling Project
Authors: Garcia Michael O.
Published: February 2004

Many of the lavas from the Hawaii Scientific Drilling Project (HSDP) are olivine‐rich (>10 vol %) and weakly altered. The Mauna Loa lavas from the upper part of the HSDP hole are more olivine‐rich and generally have olivines with higher forsterite contents than the underlying Mauna Kea lavas. Olivine‐rich lavas from these volcanoes contain both euhedral, undeformed phenocrysts and kink‐banded xenocrysts of olivines, unlike what was assumed for typical subaerial Hawaiian tholeiites. The forsterite content of both types of olivine ranges widely (80–90%). Many of the HSDP lavas have olivines with forsterite contents of 89–90%, indicating that they grew in magmas with at least 15 wt % MgO. Most of these lavas contain even higher MgO contents (18 to 28 wt %), which are a result of accumulation of olivine phenocrysts and xenocrysts. The olivine xenocrysts in these lavas are inferred to be derived from disaggregation of deformed dunite cumulates, which are present in many of these lavas. Glasses from pahoehoe crusts on some of the HSDP flows have major element compositions that confirm the subdivision of the core based on whole rock compositions. The moderately evolved compositions of the HSDP glasses indicate quenching temperatures similar to those measured during the current Kilauea eruption.
Constraints on partial melting imposed by rare earth element variations in Mauna Kea basalts
Authors: Feigenson Mark D. , Patino Lina C. , Carr Michael J.
Published: February 2004

Thirty‐one basalts from Mauna Kea collected through the Hawaii Scientific Drilling Project [1994] have been analyzed by inductively coupled plasma (ICP) mass spectrometry for rare earth element (REE) concentrations. The systematic variations in REE abundance provide constraints for theoretical models of mantle partial melting. Lavas are given a first‐order correction for the effects of clinopyroxene and olivine fractionation or accumulation so that all lavas can be considered direct mantle melts. Failure to account for fractionation generates errors in calculated source mineralogies and REE patterns. Incremental models of melt generation (including fractional, aggregated fractional, and continuous melting) are found to reproduce the observed lava compositions only under special conditions: a garnet‐free source, a fractionated heavy rare earth element (HREE) source pattern, and aggregation of melt fractions. These conditions are not consistent with the high‐pressure phase equilibria for Mauna Kea lavas. Equilibrium batch melting provides a significantly better match to the observed lava chemistry and is less sensitive to changes in initial conditions. The equations for batch melting can be inverted to compute initial source REE patterns and partitioning behavior and are internally consistent with the calculated forward models. Successful batch melting models include several with an approximately flat chondrite‐normalized source REE pattern, degrees of melting between about 1 and 18%, and a mantle residue containing garnet and clinopyroxene.
In situ measurements of inelastic light scattering in Monterey Bay using solar Fraunhofer lines
Authors: Ge Yuntao , Voss Kenneth J. , Gordon Howard R.
Published: February 2004

The contribution of inelastic scattering processes (scattering with a change in wavelength) to the light field in the ocean can be measured directly by observing the apparent decrease in absorption (line filling) of the Fraunhofer absorption lines in the solar spectrum. An instrument possessing very high spectral resolution has been developed to directly measure the Fraunhofer line filling in the upwelling and downwelling irradiances as a function of depth. Measurements made in Monterey Bay show line filling similar to that predicted on theoretical grounds when Raman scattering by water itself is assumed to be the only inelastic process; however, the lines are filled more rapidly with depth than predicted. We believe the more rapid filling results from solar‐induced fluorescence of dissolved organic matter. A simple model including both fluorescence and Raman scattering, with the fluorescence parameters (quantum efficiency, excitation spectrum, and emission spectrum) taken from the literature, provides a closer but not precise agreement with the observations. Direct measurement of the fluorescence parameters is likely required to achieve complete agreement with theory.
Observations of nearshore circulation: Rip currents
Authors: Smith Jerome A. , Largier John L.
Published: February 2004

A sector‐scanning Doppler sonar was deployed at the end of the Scripps Institution of Oceanography pier, aimed in toward the surf zone. Estimates of acoustic backscatter intensity and radial velocity were obtained seaward of the break point, over a wedge with an arc of about 45° and a radius of 200 to 400 m. The acoustic intensity is proportional to some combination of the bubble density and the suspended sediment in the water. Well‐defined jets of water extending seaward from the surf zone or “rip currents” were observed. These rip currents occurred episodically, recurred aperiodically one to four times per hour or so, and lasted several minutes each. The water transported offshore within the rips was rich in suspended sediment and bubbles, producing distinctive bright patterns that can be easily recognized in successive images of backscatter intensity from the sonar. Fluid velocities were estimated from the Doppler shift of the sonar signal, approaching 0.7 m/s in some of the events. Simultaneously, a transect of temperature, density and turbidity within the rip current was obtained with conductivity‐temperature‐depth (CTD) and optical backscatter strength (OBS) sensors. These data describe the ejection of warm turbid water from the surf zone and provide a comparison between the acoustic intensity and the optical turbidity of the fluid. The horizontal extent of the sonar measurements and the vertical resolution of the CTD/OBS measurements complement each other in the study of the dynamics of the nearshore environment. Exchange rates are not fully resolved here, since only one seaward jet is contained within the field of view; however, these observations suggest that rip currents could cause significant exchange between the nearshore and offshore waters.
Quantitative skill of quasi‐geostrophic forecasts of a baroclinically unstable Iceland‐Faroe Front
Authors: Miller Arthur J. , Poulain Pierre‐Marie , Robinson Allan R. , Arango Hernan G. , Leslie Wayne G. , Warn‐Varnas Alex
Published: February 2004

Using initialization and validation conditions from hydrographie surveys of the Iceland‐Faroe Front in August 1993, shipboard quasi‐geostrophic model forecasts executed in real time are evaluated for quantitative skill in terms of anomaly correlation coefficient and rms error. The prototype dynamical forecasts are synoptically initialized from two observed initial states and validated against observations obtained 3–4 days later. The forecasts correlate with the validating observations, yielding anomaly correlation coefficients of 0.75–0.80, which beat persistence‐of‐day‐zero forecasts by 0.07–0.30, depending on the region of interest and the initial state. Thus this quasi‐geostrophic model is able to forecast the rapidly evolving currents of this front with quantitative skill. The forecast fields of stream function are also used to diagnose the physical processes of the frontal current variations. Energetic diagnostics clearly reveal that the frontal current evolution is controlled by baroclinic instability processes. Baroclinic wave disturbances at middepth serve to transfer available gravitational energy to kinetic energy, which then is transferred through the water column, resulting in the observed (and modeled) rapid changes in the frontal current.
Disposition of solar radiation in sea ice and the upper ocean
Authors: Ebert E. E. , Schramm J. L. , Curry J. A.
Published: February 2004

A one‐dimensional sea ice model with an ice thickness distribution is presented to examine the disposition of the incoming surface shortwave radiation within the sea ice and the upper ocean. The sea ice model consists of 15 different ice thickness categories and an open water (leads) category. Ice growth, melting on horizontal and vertical surfaces, meltwater pond growth and drainage, and snow accumulation evolve independently for each of the ice categories. Leads, melt ponds, and thinner ice categories are of particular interest, as these features account for nearly all of the solar energy that is transmitted into the upper ocean beneath the ice pack. Also examined is the surface reflection and absorption, internal ice absorption, and lateral melting for ponded and pond‐free ice. Area‐averaged results show that 69% of the total annual solar energy is reflected, 15% is absorbed by the snow cover, 12% is absorbed by the ice, and 4% is transmitted to the ocean mixed layer through thin ice and leads.
The temporal and spatial characteristics of TOPEX/POSEIDON radial orbit error
Authors: Marshall J. A. , Zelensky N. R. , Klosko S. M. , Chinn D. S. , Luthcke S. B. , Rachlin K. E. , Williamson R. G.
Published: February 2004

Satellite orbit error has long been the bane of oceanographers who analyze altimetry data. However, radial orbit error on TOPEX/POSEIDON (T/P) has been reduced to the 3 to 4‐cm root‐mean‐square (rms) level over a 10‐day repeat cycle, which represents an order of magnitude improvement over earlier altimetry missions such as Geosat. Consequently, oceanographers are now able to directly evaluate the absolute ocean topography to unprecedented accuracy levels. While significantly reduced, the T/P orbit error still requires quantification. This study examines the spatial and temporal characteristics of the T/P radial orbit error, as assessed through the analysis of laser tracking residuals and orbit comparisons with independently generated trajectories. Spectral analyses of the orbit differences between the orbits determined from satellite laser ranging and Doppler Orbitography and Radiopositioning Integrated by Satellite data and the independently determined reduced dynamic Global Positioning System (GPS) ephemerides indicate that the predominant power is at the once‐per‐orbital revolution frequency with 2‐ to 3‐cm peaks. When the orbit differences are colinearly aligned to a fixed geographic grid and spectral analysis is performed at each geographic grid point, a nearly 60‐day period is found with maximum amplitudes in the 2‐ to 4‐cm range. The contribution of both conservative and nonconservative force and measurement mismodeling to this error signal are assessed. We demonstrate that the ∼60‐day error period seen at fixed geographic locations arises from weaknesses in the dynamic ocean tidal models used in the orbit calculations. New tidal models have been developed which significantly reduce this error. Second‐generation orbits incorporating many model improvements have been computed and demonstrate a significant reduction in the radial orbit error signals. Some orbit error still exists, and methods for further model improvements and the possibility of achieving 1‐cm radial rms orbit accuracy in T/P are discussed.