bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiocarbon in organic compound classes in particulate organic matter and sediment in the deep northeast Pacific Ocean
Authors: Wang Xu‐Chen , Druffel Ellen R. M. , Lee Cindy
Published: February 2004

Radiocarbon (Δ14C) and stable carbon isotopes (δ13C) were measured for total amino acids, carbohydrates, lipids and acid‐insoluble organic fractions separated from sinking particulate organic matter (POMsink), detrital aggregates, sediment floe and sediments collected from the deep Northeast Pacific Ocean. The results show distinct Δ14C signatures among these organic compound classes. Bomb 14C, produced 3–4 decades ago in the atmosphere, was present in all organic fractions in the POMsink in the deep sea. Δ14C values decrease with depth, from the sediment trap POMsink (3450 m) to the sediment (4100 m), with rapid changes occurring at the sediment‐water interface. Total lipid had much ‘older’ Δ14C values than those of amino acid and carbohydrate fractions in detrital aggregates, sediment floe and sediments. These data demonstrate that two processes may be occurring: 1) preferential decomposition of organic matter at the sediment‐water interface, and 2) incorporation of ‘old’ organic carbon into POM and sediment. The alteration of carbon found in these organic compound classes suggests that differences in decomposition and chemical behavior exist for each of these compound classes in the deep ocean.
Computer simulation of pyroclastic flow movement: An inverse approach
Authors: Rossano S. , Mastrolorenzo G. , De Natale G. , Pingue F.
Published: February 2004

A computer simulation of the Campanian Ignimbrite (Southern Italy) utilizes both forward and inverse techniques. The forward technique uses a given set of parameters (eruptive vent, initial velocity and rheology of the pyroclastic mass) to compute the trajectories and limits of pyroclastic flow propagation. The inverse approach, based on the Bayes theorem, calculates the probability density of the input parameters based on the limits of actual deposits in the field. The inverse method allows definition of the most likely set of eruption parameters consistent with the emplacement of the Campania Ignimbrite. These simulations furnish the following results: a) the Campanian Ignimbrite formation is compatible with a purely gravitative mode of movement; b) the areal distribution is consistent with a radial flow pattern from a limited vent area north of Naples; c) the most probable initial flow velocity is about 160 m/s which corresponds to a possible eruptive column height near 1.3 km.
Magnetopause transects
Authors: Sonnerup B. U. Ö. , Guo M.
Published: February 2004

A novel method is described for reconstruction of two‐dimensional current‐layer structures from measurements taken by a single spacecraft traversing the layer. In its present form, the method is applicable only to 2D magneto‐hydrostatic structures that are passively convected past the observing spacecraft. It is tested on a magnetopause crossing of the tangential‐discontinuity type by the spacecraft AMPTE/IRM. The magnetic structures recovered include a magnetic island located between two X‐type nulls as well as a magnetic ‘worm hole’ through which a bundle of weak magnetic flux appears to connect the magnetosphere and the magnetosheath.
Densities and refractive indices of H2SO4/HNO3/H2O solutions to stratospheric temperatures
Authors: Luo Beiping , Krieger Ulrich K. , Peter Thomas
Published: February 2004

The Lorentz‐Lorenz relation is used to estimate the refractive index of aqueous H2SO4/HNO3 solutions (5–70 wt%) for wavelengths from the near‐ultraviolet to the near‐infrared (0.35–2 µm) and for temperatures from 370 K to stratospheric temperatures (185 K). The molecular refractivities of the involved species are based on data of the binary solutions at room temperature. For the extrapolation to low temperatures, a careful estimate of the density of the ternary solutions has been performed. The accuracy of the model predictions is estimated to be better than 2%.
Detailed structure of an oceanic normal fault: A multiscalar approach at DSDP/ODP site 504
Authors: Pezard P. A. , Ayadi M. , Revil A. , Bronner G. , Wilkens R.
Published: February 2004

Seismic data, downhole measurements, images and core obtained at DSDP/ODP Site 504, in the eastern equatorial Pacific, reveal the presence of a normal fault about 500 m into basement. This north‐dipping structure is sub‐vertical in the sediment, and dips about 45° toward the ridge in the hole. Similar faults are expected from seismic data to cut this deep hole within the sheeted dikes and close to the base of the hole, where the drilling was abruptly terminated.

The combined analysis of geophysical and geological data reveals a 300‐m‐thick fault zone, with modified physical and mineralogical properties. The fault was initially identified from a 4° step in magnetic field inclination at 800 mbsf, and the lowest values of electrical resistivity in basement. The fault also exhibits modified mechanical properties and a high open porosity fraction. While fracturing mapped from electrical images decreases slightly versus depth in basement, the fracture density increases with increasing depth within the fault zone. The basalt magnetic properties are strongly modified near the base of the fault zone (from 900 to 1050 mbsf), possibly in relation with early faulting and the circulation of high‐temperature fluids. To the contrary, present ruptures appear to be located near the top of the interval and suggest a propagation of the deformation toward the hanging wall with time.
Magnetic enhancement paths in Loess sediments from Tajikistan, China and Hungary
Authors: Forster Th. , Heller F.
Published: February 2004

Saturation magnetization and saturation remanence of 139 loess and paleosol samples from Tajikistan, China and Hungary increase linearly with low field susceptibility in all sections investigated. Thus magnetic enhancement is caused by increasing quantities of ferrimagnetic minerals. The observed variations of high field susceptibility χh are attributed to variations in the amount of clay minerals as χh is linearly correlated with the < 2 µm grain size mineral fraction. The magnetic enhancement process is compared with a theoretical mixing model based on two magnetic mineral assemblages with different but invariable coercivities. The two‐component model provides a good approximation to the data from Hungary and the magnetically enhanced paleosol samples in Tajikistan and China. Discrepancies between model calculation and data arise in the behaviour of the loess samples in Tajikistan. Here a three‐component model improves the agreement between data and mixing calculation. The magnetic enhancement in the loess in Tajikistan (and China) is different from the one in the paleosols and the link between paleoclimate and susceptibility is not as straightforward as in the Hungarian loess‐paleosol sequence.
Implications of fluctuations in the distribution functions of interstellar pick‐up ions for the scattering of low rigidity particles
Authors: Fisk L. A. , Schwadron N. A. , Gloeckler G.
Published: February 2004

The distribution functions of interstellar pick‐up hydrogen and helium exhibit fluctuations which are correlated. An analysis of these fluctuations reveals that they may be the result of variations in, and provide additional evidence for, large mean free paths for these low rigidity particles. An analysis of the properties of the distribution function of pick‐up hydrogen reveals that these particles do suffer considerable adiabatic cooling, and that the likely cause for the long mean free path is the inability of the particles to scatter through 90° pitch angle.
Observation of a distinct compressional‐mode interface wave on a single fracture
Authors: Roy Sanjit , Pyrak‐Nolte Laura J.
Published: February 2004

A compressional‐mode interface wave propagating along a fracture has been observed in a fracture in limestone. This compressional‐mode interface wave co‐propagates with the bulk compressional wave, is localized to the region surrounding the fracture, and has particle motions containing both transverse and longitudinal displacements. As stress across the fracture increases, the wave energy shifts to earlier arrival times, and is accompanied by a decrease in the transverse displacement and an increase in the longitudinal displacement of the particle motion.
Multidecadal climate variability in the Greenland Sea and surrounding regions: A coupled model simulation
Authors: Delworth Thomas L. , Manabe Syukuro , Stouffer Ronald J.
Published: February 2004

Pronounced oscillations of ocean temperature and salinity occur in the Greenland Sea in a 2000 year integration of a coupled ocean‐atmosphere model. The oscillations, involving both the surface and subsurface ocean layers, have a timescale of approximately 40–80 years, and are associated with fluctuations in the intensity of the East Greenland Current. The Greenland Sea temperature and salinity variations are preceded by large‐scale changes in near‐surface salinity in the Arctic, which appear to propagate out of the Arctic through the East Greenland Current. These anomalies then propagate around the subpolar gyre into the Labrador Sea and the central North Atlantic. These oscillations are coherent with previously identified multi‐decadal fluctuations in the intensity of the North Atlantic thermohaline circulation. The oscillations in the Greenland Sea are related to atmospheric variability. Negative (cold) anomalies of surface air temperature are associated with negative (cold) sea surface temperature (SST) anomalies in the Greenland Sea, with amplitudes up to 2°C near Greenland declining to several tenths of a degree C over northwestern Europe. The cold SST anomalies and intensified East Greenland Current are also associated with enhanced northerly winds over the Greenland Sea.
Paleoclimatic response of the closing of the Isthmus of Panama in a coupled ocean‐atmosphere model
Authors: Murdock Trevor Q. , Weaver Andrew J. , Fanning Augustus F.
Published: February 2004

The paleoclimatic effects of the closure of the Isthmus of Panama ∼3 Ma are investigated using a coupled atmosphere‐ocean model. Consistent with earlier ocean‐only modelling studies, it is shown that prior to closure there is an absence of deep water formation in the North Atlantic. Hence there is a reduction in oceanic heat transport. This is largely compensated for by the atmosphere such that only small changes in total planetary heat transport occur. The model climate of the North Atlantic is significantly warmer after Isthmus closure. In addition, the regions surrounding the Pacific Ocean and South Atlantic are generally cooler while the Indian Ocean is generally warmer in the model present‐day climate. Finally, possible relationships to glaciation and initiation of northern hemisphere glacial cycles are discussed.
Analysis of merged SMMR‐SSMI time series of Arctic and Antarctic sea ice parameters 1978–1995
Authors: Bjørgo Einar , Johannessen Ola M. , Miles Martin W.
Published: February 2004

The most consistent means of investigating the global sea ice cover is by satellite passive microwave sensors, as these are independent of illumination and cloud cover. The Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) and the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSMI) provide information on the global sea ice cover from 1978 to present. The two instruments flew simultaneously during a 6‐week overlap period in July and August 1987, thus enabling intercomparison of the two sensors. Brightness temperatures are corrected for instrument drift and calibration differences in order to produce continuous time series of monthly averaged Arctic and Antarctic sea ice extent and sea ice area through the use of the NOrwegian Remote Sensing EXperiment (NORSEX) algorithm, which relates brightness temperatures to ice concentration. Statistical analysis on the time series estimates the decreases in Arctic ice extent and ice area to be 4.5% and 5.7%, respectively, during the 16.8‐year observation period. The overall trends established here serve to better define and strengthen earlier assertions of a reduced ice cover, based on analysis of SMMR and SSMI data taken separately. These results are consistent with GCM simulations that suggest retreat of the sea ice cover under global warming scenarios.
Analysis of the South American intraplate stress field
Authors: Coblentz David D. , Richardson Randall M.
Published: February 2004

The first‐order South American intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources. The finite element mesh consisted of 3100 nodes in a network of 5993 equal‐area triangular elements which provided a spatial resolution of about 1° at the equator. An important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the Mid‐Atlantic Ridge (e.g., ridge push), the continental margins along the east coast of Brazil and Argentina, and the elevated continental crust (e.g., the Andean Cordillera). Predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated: pinned collisional boundaries and applied collisional boundary forces. Constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress, SHmax, provided by 217 stress indicators from the World Stress Map Project as well as by SHmax magnitude estimates and torque information from previous investigations. Our modeling results demonstrate that the first‐order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate. The predicted intraplate stress field is characterized by a nearly uniform E‐W SHmax orientation throughout most regions of the plate, with stress magnitudes generally less than 20 MPa averaged over a 100‐km‐thick lithosphere. Significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the Andes. Although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained, we estimate a plausible upper bound on the force per unit length acting along the Peru‐Chile Trench to be about 2.5 × 1012 N m−1. While some of our models are consistent with a driving basal drag to balance the torques acting on the plate, the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force.
A numerical simulation of the evolution of temperature and CO2 stratification in Lake Nyos since the 1986 disaster
Authors: Kantha Lakshmi H. , Freeth Samuel J.
Published: February 2004

A one‐dimensional numerical model has been applied to simulate the evolution of the temperature and CO2 stratification in Lake Nyos since the 1986 disaster. It incorporates a second‐moment turbulence closure scheme for mixing and heating and cooling in the upper layers of the lake. The effects of inflowing stream water and input of the CO2, heat, and dissolved solids at the bottom of the lake are taken into account. The model is initialized by conditions observed immediately after the disastrous outgassing event in 1986 and integrated forward for 10 years, forced by seasonally modulated diurnal heating and nocturnal cooling and heavy summer time precipitation at the surface. Four possible conditions are investigated. The first simulation considers the normal seasonal heating/cooling cycle and steady input of heat, dissolved solids, and CO2 at the bottom. It shows that the upper layers of the lake are quite stable under normal seasonal forcing but the bottom layers reach high CO2 concentrations in less than a decade. The second simulation considers a brief introduction at the bottom of the lake of cooler waters that contain less CO2 and temporarily destabilize the bottom layers. The third and fourth simulations consider anomalous surface forcing conditions that can produce anomalous mixed layer deepening during the cooling seasons, capable of releasing the CO2 stored immediately below the normal mixed layer. These results suggest that a destabilization of the bottom layers is more likely to lead to massive and catastrophic degassing. The results also demonstrate the utility of a numerical model in investigations of caldera/crater lakes such as Lake Nyos.
Influence of F(OH)−1 substitution on the relative mechanical strength of rock‐forming micas
Authors: Dahl Peter S. , Dorais Michael J.
Published: February 2004

Microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite [Wilson and Bell, 1979; Kronenberg et al., 1990; Mares and Kronenberg, 1993]. We propose a crystal‐chemical resolution to this conflict, namely, that (001) dislocation glide in biotite is rate‐limited by its fluorine content. Significant F(OH)−1 substitution, and concomitant removal of hydroxyl H+ directed into the interlayer cavity, potentially increases mechanical strength of biotite in two ways: (1) it eliminates K+‐H+ repulsion, thereby strengthening the interlayer bonds, and (2) it allows K+ to “sink” deeper into the interlayer cavity, the resultant geometry being less favorable to basal slip. In testing this hypothesis we analyzed the naturally deformed biotite studied by Wilson and Bell [1979] and documented its very low F content (XF ≤ 0.02) compared to that of the biotite experimentally deformed by Kronenberg et al. [1990]. Our model and the comparative XF data explain why the biotite of Wilson and Bell [1979] deformed more easily in nature than its coexisting muscovite, whereas the biotite of Kronenberg et al. [1990] was mechanically stronger than muscovite similarly deformed by Mares and Kronenberg [1993]. Our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their F(OH)−1 substitution. Our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families. Further testing of crystal‐chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size.
P wave velocity of Proterozoic upper mantle beneath central and southern Asia
Authors: Nyblade Andrew A. , Vogfjord Kristin S. , Langston Charles A.
Published: February 2004

P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source‐receiver path of one event crosses central Africa and lies outside the African superswell while the source‐receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL‐Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL‐Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s−1, velocities between 8.3 and 8.5 km s−1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s−1, velocities between 7.9 and 8.4 km s−1 at a depth of 130 km, and velocity gradients between −0.001 and 0.001 s−1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150–200 km, whereas for southern Africa they bottom at ∼100–150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not consistent with explanations for the African superswell invoking extensive amounts of lithospheric thinning. If extensive lithospheric thinning had occurred beneath southern Africa, as suggested previously, then upper mantle P wave velocities beneath southern Africa would likely be lower than those in our models.
Geoid anomalies and the structure of continental and oceanic lithospheres
Authors: Doin Marie‐Pierre , Fleitout Luce , McKenzie Dan
Published: February 2004

Geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere, on the nature of upper mantle seismic anomalies, and on the thermal evolution of the oceanic lithosphere. For the shallow lithospheric density anomalies considered here, the geoid anomaly is shown to follow approximately the moment law derived from isostasy. By a procedure of least squares inversion, the geoid over old cratons is found to be equal to that over relatively young (50 Myr) oceanic lithosphere. The oceanic geoid as a function of age follows the expected linear trend with flattening at old ages. These results are independent of the degree at which the spherical harmonics are truncated. If the tectosphere consisted only of a thick thermal boundary layer, marked geoid lows, with an amplitude larger than 20 m, should be present over cratons, in disagreement with the present observation. The predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere. This analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir. The density of these cold roots is similar to that of mantle material elsewhere, because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion. Analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages, and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small‐scale convection.
Geochronology of Galápagos seamounts
Authors: Sinton Christopher W. , Christie David M. , Duncan Robert A.
Published: February 2004

Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of these features. The areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 Ma. Age‐distance relationships are consistent with a decrease in the velocity of the Nazca plate relative to the Galápagos hotspot during the past 9 m.y., most likely at ∼5 Ma when there was a change in Pacific‐hotspot motion. Alternatively, the plate velocity remained constant and volcanism at a single seamount endured up to 7 m.y. Lavas from seamounts along the Wolf‐Darwin lineament to the northwest of the Galápagos Platform are young (<1 Ma), with the youngest measured ages closest to the platform.
Type of faulting and orientation of stress and strain as a function of space and time in Kilauea's south flank, Hawaii
Authors: Gillard Dominique , Wyss Max , Okubo Paul
Published: February 2004

Earthquake focal mechanisms of events occurring between 1972 and 1992 in the south flank of Kilauea volcano, Hawaii, are used to infer the state of stress and strain as a function of time and space. We have determined 870 fault plane solutions from P wave first motion polarities for events with magnitudes ML ≥ 2.5 and depth ranging between 6 and 12 km. Faulting is characterized by a mixture of decollement, reverse, and normal faults. Most large earthquakes with magnitude M < 7 slip on reverse faults striking NE at 40° and dipping SE between 60° and 70°. In Hawaii, the earthquakes with M > 7 rupture the decollement plane, since it is the only surface large enough to generate magnitude 7 or larger earthquakes. The percentage of reverse faulting events is high compared to the decollement and normal faulting mechanisms for the period 1972–1983. The percentage of decollement type focal mechanisms becomes dominant after 1983. This pattern of faulting activity suggests that pressure was building up within Kilauea's rift zone prior to the 1983 Puu'Oo eruption. Overall, a single stress orientation with the maximum compressive stress oriented SE perpendicular to the rift and dipping at 45° is compatible with the coeval existence of decollement, reverse, and normal faults. However, in a crustal volume east of longitude 155°10′W, we find a change of the orientation of σ1 from nearly horizontal to plunging 45° SE occurring in 1979. This stress rotation suggests magma movements within the aseismic part of Kilauea's east rift zone. The strain and stress orientations are coaxial in the south flank except within the volume where the stress rotation is observed. We observe a change in the relationship between stress and strain directions caused either by the shifting of seismic activity from reverse faults to decollements, while stress stays constant, or by a rotation of stress, while strain remains constant. Assuming that the model of a noncohesive Coulomb wedge is appropriate for Kilauea's south flank, we find that high pore pressures are prevalent along the decollement and within the wedge for a coefficient of friction equal to 0.85.
Estimates of the Chandler wobble frequency and Q
Authors: Kuehne John , Wilson Clark R. , Johnson Stuart
Published: February 2004

The Chandler wobble frequency F and quality factor Q are estimated by least squares using polar motion data in combination with proxy excitations derived from atmospheric data. Monte Carlo tests show that this approach can provide useful improvements over traditional maximum likelihood methods, which use only observed polar motion. With less than a decade of good simultaneous polar motion and atmospheric data there is no new information about Q, but 90% confidence intervals for F are near ±0.004 cycles per year (cpy), making them comparable to maximum likelihood results based on nearly a century of historical polar motion data. Using our preferred proxy excitation, we obtain an estimate of F near 0.831 cpy (a period of 439.5 mean solar days), which is significantly different from the maximum likelihood value near 0.843 cpy (433 days). This difference can be understood in light of differences in assumptions underlying the two methods.
Variability in oceanic crustal thickness and structure: Multichannel seismic reflection results from the northern East Pacific Rise
Authors: Barth G. A. , Mutter J. C.
Published: February 2004

Multichannel seismic reflection data acquired between 8°50′ and 9°50′N and between 12°30′ and 13°30′N along the East Pacific Rise provide a three‐dimensional view of the young oceanic crust. Seafloor‐to‐Moho reflection travel times vary by up to 0.9 s within our study areas; the total range of crustal travel times in the 9°N area is 1.55 to 2.45 s; the total range in the 13°N area is 1.60 to 2.05 s. The variation is systematic, indicating thinner crust locally associated with overlapping spreading centers (OSCs) and, in the 9°N area, segment‐scale variation along crustal isochrons. Crustal travel time is found to be a valid proxy for oceanic crustal thickness. Outside of the axial low‐velocity volume, thickness can be calculated from time to ∼500 m. Even in the axial region thickness can be calculated to <1 km, if low‐velocity zone position is known. Crustal thicknesses calculated from travel times vary by 2.6 km in the 9°N area, and by 1.5 km in the 13°N area. The majority of this variation is attributed to seismic layer 3 (the lower crust). Segment‐scale variation of ∼1.8 km (∼5.5 to 7.3 km thickness) is observed in the 9°N area, with thinnest crust formed between ∼9°40′ and 9°50′N and thickest formed between ∼9°15 and 9°20′N. Results imply a three‐dimensional pattern of magma supply to the 9°N segment. The OSC at 9°03′N is associated with major disruptions of the segment‐scale pattern, in the form of local thin areas within the discordant zone; the smaller OSC at 12°54′N is not associated with dramatic changes in thickness of the surrounding crust. In the absence of OSCs, the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment. Thicker crust does not always correlate with shallower ridge bathymetry, broader axial cross section, or more negative mantle Bouguer or subcrustal gravity anomaly. Variable thickness of the crust‐mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result. We hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick Moho transition zone and a subcrustal gravity low. Such a diapiric upwelling center appears to be now located beneath the East Pacific Rise near 9°40′ to 9°50′N.
What can airborne gravimetry contribute to geoid determination?
Authors: Schwarz Klaus‐Peter , Li Ye Cai
Published: February 2004

This paper evaluates the suitability of airborne gravimetry for geoid determination. Spectral analysis of gravity data is used to derive the high‐frequency geoid spectrum and to compare it to the error spectrum of the geoid, as derived from an airborne gravity system. Gravity values in various areas with typical topographic features are used for the spectral analysis. The achievable accuracy of airborne gravimetry in geoid determination is studied through an assessment of inertial navigation system (INS) and global positioning system (GPS) errors in the spectral ranges of interest. The results of these computations are presented in a number of graphs which allow the following conclusions. To determine geoid undulations with an accuracy of 1 cm (rms), the minimum wavelength to be resolved is 14 km in flat terrain and 5 km in mountainous terrain. Using a flight speed of 300 km/h at an altitude of 2.5 km or less above ground, current hardware sensitivity is sufficient to resolve the geoid spectrum for such wavelengths. The total GPS/INS induced geoid undulation error (rms) in this case is less than 1 cm for wavelengths between 5 km and 100 km and less than 10 cm for wavelengths between 5 km and 500 km. The cumulative rms geoid error is less than 30 cm for wavelengths up to 1000 km. The accuracy for relative geoid determination is 0.1 ppm for wavelengths up to 100 km and 0.2 ppm for 500 km. The results were verified by analyzing the data collected during an airborne test. The results indicate that airborne gravimetry has the potential to determine geoid undulation with centimeter‐level accuracy and to fill the gaps in the global gravity map in an efficient manner and with an accuracy superior to current terrestrial methods.
Lumped‐element analysis of subglacial hydraulic circuits
Authors: Clarke Garry K. C.
Published: February 2004

Observations of the subglacial hydrological system reveal a richness of responses that are thought to reflect variations in the water supply, morphology, and connectivity of the system. To contribute insight into the physical controls that result in qualitative variations in system response, a lumped‐element formalism is developed and used to simulate the behavior of simple hydraulic circuits. The circuit elements for an idealized subglacial water system differ from those used to analyze conventional hydraulic flows, but the motivation is similar: lumped systems are governed by ordinary differential equations, whereas continuous systems are governed by partial differential equations. The accompanying examples illustrate the capture property of Röthlisberger channels, the concept of morphological switching, the role of water storage geometry in contributing to system nonlinearity, and the effects of seasonal variations in storage capacity and hydraulic resistance.
Seismic reflection investigations of the Mount Cayley bright spot: A midcrustal reflector beneath the Coast Mountains, British Columbia
Authors: Hammer P. T. C. , Clowes R. M.
Published: February 2004

An unusually high‐amplitude, midcrustal reflector was discovered by a Lithoprobe seismic reflection survey crossing a predominantly plutonic region in southwestern British Columbia, Canada. The “bright spot” was observed adjacent to Mount Cayley, a large Quaternary volcanic center associated with the Cascadia subduction zone magmatic arc. Characterization of the bright spot may advance our understanding of either the midcrustal structure of magmatic supply systems or the presence and storage of saline fluids in the midcrust. The location and three‐dimensional geometry of the reflector were determined using travel time inversion. The resulting suite of models consistently describe a northwest dipping reflector that lies 12.5 to 13 km beneath the Mount Cayley volcanic complex. The areal extent of the reflective surface is 3 km by 1 km and the structure is less than 1.6 km thick. The transition between the body and the country rock must be rapid (<200 m) and exhibit a large impedance contrast in order to generate reflections of such high frequencies (30 Hz) and relative amplitudes (8.0 dB). Our preferred interpretation for the reflector is a fossil sill complex associated with the volcanic development of Mount Cayley. However, the data do not rule out the possibility that the reflective structure represents melt lenses or saline fluid produced by the dewatering of the subducted slab. The Mount Cayley bright spot lies just below the brittle upper crust, as do the midcrustal reflectors detected beneath Japanese subduction arc volcanoes. This correlation suggests that thermal and rheological properties just below the brittle/ductile crustal transition may be favorable for reflector formation.
Seismic structure of the upper mantle in a central Pacific corridor
Authors: Gaherty James B. , Jordan Thomas H. , Gee Lind S.
Published: February 2004

The seismic structure of the Tonga‐Hawaii corridor has been investigated by combining two data sets: Revenaugh and Jordan's reflectivity profile from ScS reverberations, which provides travel times to and impedance contrasts across the major mantle discontinuities, and 1500 new observations of frequency‐dependent phase delays for the three‐component S, SS, and SSS body waves and the R1 and G1 surface waves, which constrain the velocity structure within this layered framework. The shear waves turning in the upper mantle showed significant splitting of the SH and SV components, indicative of shallow polarization anisotropy. The data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric, radially anisotropic structure. The final model, PA5, is characterized by a high‐velocity, anisotropic lid, bounded at 68 km depth by a large (negative) G discontinuity; a low‐velocity, anisotropic layer below G, extending to a small L discontinuity at 166 km; an isotropic, steep‐gradient region between 166 km and 415 km; and transition‐zone discontinuities at 415, 507, and 651 km. The depth of the radial anisotropy in PA5 is shallower than in most previous studies based on surface waves and higher modes. The average value of radial shear anisotropy in the lid, +3.7%, is consistent with the magnitude expected from the spreading‐controlled models of olivine orientation, while anisotropy in the low‐velocity zone, which is required by our data set, could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present‐day plate motions. On the basis of recent work by Hirth and Kohlstedt, we suggest that the G discontinuity is caused by a rapid increase in the water content of mantle minerals with depth, marking the fossilized lower boundary of the melt separation zone active during crust formation. The high‐gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval. The average shear‐velocity gradient in the transition zone is lower than in most previous seismic models, in better agreement with the predictions for a pyrolitic composition.
A full coverage, high‐resolution, topographic model of Greenland computed from a variety of digital elevation data
Authors: Ekholm Simon
Published: February 2004

In this paper, a new high‐resolution elevation model of Greenland, including the ice sheet as well as the ice free regions, is presented. It is the first published full coverage model, computed with an average resolution of 2 km and providing an unprecedented degree of detail. The topography is modeled from a wide selection of data sources, including satellite radar altimetry from Geosat and ERS 1, airborne radar altimetry and airborne laser altimetry over the ice sheet, and photogrammetric and manual map scannings in the ice free region. The ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations. The mean accuracy of the ice sheet elevations is estimated to be 12–13 m, and it is found that on surfaces of a slope between 0.2° and 0.8°, corresponding to approximately 50% of the ice sheet, the model presents a 40% improvement over models based on satellite altimetry alone. On coastal bedrock, the model is compared with stereo triangulated reference points, and it is found that the model accuracy is of the order of 25–35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere.
Seismic tomography and deformation modeling of the junction of the San Andreas and Calaveras faults
Authors: Dorbath Catherine , Oppenheimer David , Amelung Falk , King Geoffrey
Published: February 2004

Local earthquake P traveltime data is inverted to obtain a three‐dimensional tomographic image of the region centered on the junction of the San Andreas and Calaveras faults. The resulting velocity model is then used to relocate more than 17,000 earthquakes and to produce a model of fault structure in the region. These faults serve as the basis for modeling the topography using elastic dislocation methods. The region is of interest because active faults join, it marks the transition zone from creeping to locked fault behavior on the San Andreas fault, it exhibits young topography, and it has a good spatial distribution of seismicity. The tomographic data set is extensive, consisting of 1445 events, 96 stations, and nearly 95,000 travel time readings. Tomographic images are resolvable to depths of 12 km and show significant velocity contrasts across the San Andreas and Calaveras faults, a low‐velocity zone associated with the creeping section of the San Andreas fault, and shallow low‐velocity sediments in the southern Santa Clara valley and northern Salinas valley. Relocated earthquakes only occur where vp > 5 km/s and indicate that portions of the San Andreas and Calaveras faults are non vertical, although we cannot completely exclude the possibility that all or part of this results from ray tracing problems. The new dips are more consistent with geological observations that dipping faults intersect the surface where surface traces have been mapped. The topographic modeling predicts extensive subsidence in regions characterized by shallow low‐velocity material, presumably the result of recent sedimentation. Some details of the topography at the junction of the San Andreas and Calaveras faults are not consistent with the modeling results, suggesting that the current position of this “triple junction” has changed with time. The model also predicts those parts of the fault subject to contraction or extension perpendicular to the fault strike and hence the sense of any dip‐slip component. In each locality the relative vertical motion across the fault is consistent with the fault dips found with the new hypocentral locations.
An improved methodology for Thellier‐type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence
Authors: McClelland E. , Briden J. C.
Published: February 2004

We propose a new methodology for Thellier‐type paleointensity experiments. In standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alteration of magnetic mineralogy begins, and subsequent data have to be discarded. We emphasize that when alteration begins at low temperature and continues throughout the experiment (which then fails in standard analyses), the resulting unblocking temperature (Tub) spectrum of the alteration product does not necessarily overlap with the whole Tub spectrum of natural remanent magnetization (NRM), because Tub depends on physical characteristics of the new grains, not the temperatures at which they were formed. NRM may survive in an uncontaminated higher Tub window. If alteration remanence only has low Tub then partial thermoremanent magnetization (pTRM) checks at temperature step Tj‐1 can be used to correct for alteration that has occurred at Tj, and furthermore, thermal demagnetization of a full TRM acquired in the laboratory at the end of the progressive Thellier‐Thellier experiment should reveal the true, uncontaminated Tub spectrum of the NRM in higher temperature intervals. We propose a new experimental sequence which allows monitoring and correction of alteration and provides two semi‐independent estimates of palaeointensity in the circumstances described above. We illustrate the issues involved through new experimental work on igneous rocks from a Paleozoic charnockitic syenite, where we unravel TRM from multidomain isothermal remanence despite massive alteration during laboratory heating. Finally we point out the value of these experiments for verifying primary TRM in igneous rocks, indicating properties that distinguish TRM from other remanence.
Soft plates and hot spots: Views from Afar
Authors: Ebinger C. J. , Hayward N. J.
Published: February 2004

The uplifted Ethiopian plateau region encompasses amagmatic rift basins and basins with nascent seafloor spreading. Rift segments in the Main Ethiopian rift and southern Afar show a distinct structural segmentation, with a south to north reduction in the length, width, and spacing of fault zones; rift segments in northern Afar, where extension exceeds 100%, show a magmatic segmentation. The objectives of remote sensing, gravity, and modeling studies of the Ethiopian plateau area are (1) to summarize morphological patterns along the length of the rift system and (2) to relate variations in effective elastic thickness of the lithosphere within the Ethiopian plateau region to tectonics. Inverse models of new and existing Bouguer gravity data from the rifted regions of the uplifted plateau constrained by seismic data are used to relate the variations in the geometry of the along‐axis structural and magmatic segmentation to variations in flexural rigidity. We use the wavelength dependence of the coherence between gravity and topography to estimate the flexural rigidity, or, equivalently, the effective elastic plate thickness (Te), of the lithosphere. Estimates of Te range from 17 ± 2 km to 5 ± 3/−2 km within the 300‐km‐wide Afar Depression. These estimates of Te within the rift are considerably less than values found beneath the uplifted but largely unfaulted plateau to the west (Te ≥ 56 km). These results show that the transition from a “continental segmentation” to “oceanic segmentation” corresponds to a decrease in rift basin segment length and the separation of faults and magmatic centers, an increase in magmatic construction, and a marked decrease in effective elastic thickness. We suggest that the length scales of extensional segments prior to the onset of seafloor spreading are controlled primarily by plate strength, at least in rift areas affected by mantle plumes.
A dissipation‐based analysis of an earthquake fault model
Authors: Schmittbuhl Jean , Vilotte Jean‐Pierre , Roux Stéphane
Published: February 2004

We analyze the dynamics of a discrete dynamical model of earthquake faulting, derived from the Burridge‐Knopoff model. The system is shown to exhibit a characteristic event size (L* = 2v2f l2) that separates two distinct regimes in the statistical distribution of event sizes and magnitudes. The dynamics of the system exhibits scaling laws that are in agreement with observed seismic laws. Influence of the frictional rate of dissipation, of the elastic stiffness coupling, and of the system size is investigated. The dynamics of the system is rather insensitive to the numerical treatment of the nonsmooth friction. In contrast, the exact form of the velocity weakening friction law is shown to have a major effect on the dynamics. For friction laws allowing local reversal backslipping, the distribution of the magnitudes does not exhibit a Gutenberg and Richter (GR) distribution, while by precluding backslipping a GR distribution is observed. Two populations of events can be characterized based on dissipation: weakly dissipative events that allow the mechanical energy of the system to increase; and strongly dissipative events that release a large fraction of the elastic potential energy of the system and introduce large stress heterogeneities. A coarse grain analysis in terms of the stored elastic energy and the magnitude of the disorder provides new interesting insights on the dynamics of the model. Weakly dissipative events, which reproduce the seismic laws, are shown to follow a deterministic evolution. A statistical criterion for the initiation of big dissipative events is proposed.
Evaluating hysteresis in earth materials under dynamic resonance
Authors: Kadish Abraham , Johnson Paul A. , Zinszner Bernard
Published: February 2004

A lumped parameter model is derived for studying hysteretic effects in resonant bar experiments on rock. The model uses equations of state obtained by approximating closed hysteresis loops in the stress‐strain plane by parallelograms. The associated approximate nonlinear state relations have a sound speed (modulus) that takes two values. Assuming hysteresis and discrete memory to be the primary nonlinear mechanisms, periodic solutions corresponding to these equations of state are obtained analytically for single‐frequency continuous wave drivers, and their frequency spectral densities are analyzed. In this simple approximation, if hysteretic contributions to the signal speed are a correction to the linear elastic signal speed (i.e., the parallelogram is narrow), the model predicts that the spectral density at even multiples of the source frequency is zero, and an approximate “pairing” of amplitudes is predicted for odd harmonic multiples. Comparison of the model spectrum with experimental data shows the model to be qualitatively correct. We conclude that hysteresis is an important mechanism in rocks. We consider the model to be a prototype.