bluuebunny/crossref_metadata_2025_split huggingface dataset:
Directional site resonances and the influence of near‐surface geology on ground motion
Authors: Bonamassa Ornella , Vidale John E. , Houston Heidi , Schwartz Susan Y. 
Published: February 2008

We examine the horizontal motions at close stations from earthquakes in the Loma Prieta and Whittier Narrows sequences to study the shear wave polarizations. We use a dense, six station array recording 10 aftershocks for the former, and use two events and 11 stations across the Los Angeles area for the latter.We compute the average azimuth of strongest shaking in the shear wave as a function of frequency from 1 to 18 Hz for each record of each earthquake. The direction of shaking at a given frequency often correlates much better with an empirical site resonance direction than with the direction of shaking expected from the focal mechanism of the earthquake. The effect tends to be greatest at the frequencies that are the most amplified. This phenomenon can complicate determination of the earthquake source at frequencies higher than 1 Hz.Further, since sites only 25 meters apart show different preferred directions, very near‐surface geology is probably responsible. Estimation of directional site resonances may prove useful for seismic design.
Aeromagnetic anomalies and discordant lineations beneath the Niger Delta: Implication for new fracture zones and multiple sea‐floor spreading directions in the “meso‐Atlantic” Gulf of Guinea cul‐de‐sac
Authors: Babalola Olufemi Oladapo , Gipson Mack 
Published: February 2008

An aeromagnetic contour map compiled over shallow water and onshore portions of the Nigerian continental margin, shows several elongate, long‐wavelength anomaly closures with some alternating polarity, separated by steep gradient, NE lineations. The lineations are interpreted as new fracture zones or extensions of previously mapped ones.The NE trend in the western delta region is concordant with Ee fracture zone trends of the deeper Gulf of Guinea. Aeromagnetic lineations of the SE Niger Delta Basin however, discordantly trend ENE. Their termination against the former, is interpreted as evidence of early sea‐floor spreading in a ENE‐WSW direction in addition to the well documented NE‐SW spreading of the Gulf of Guinea and the rest of the “meso‐Atlantic” sea‐floor.The geophysical crustal structure indicate the existence of two Early Cretaceous triple junctions beneath the Niger Delta Basin. The two triple‐junctions further support the hypothesis that the African continent was a multi‐plate system (in the Niger Delta region) during the early opening of the Atlantic.
Calculations of the radial seismic velocity/density ratio for MgO and MgSiO3‐perovskite at high pressure
Authors: Parker S. C. , Wall A. 
Published: February 2008

The experimental values of the ratio of the radial seismic velocity perturbation with respect to the radial density perturbation, (R)T, for MgO at pressures of up to about 20 GPa show a conflicting pressure dependence. However, (R)T, calculated using free energy minimisation at 1000 K and pressures of up to 160 GPa for the lower mantle minerals MgO and MgSiO3‐perovskite, is predicted to decrease with increasing pressure. Previous calculations of the elastic constants of MgO using the Potential Induced Breathing model also predict that (R)T decreases as the pressure increases.The ratio (dlnνs/dlnνp)T for both MgO and MgSiO3−perovskite is also predicted to decrease with increasing pressure.
Nd‐Sr isotopic and geochemical characteristics of the southern Adamello (northern Italy) intrusives: Implications for crustal versus mantle origin
Authors: Kagami Hiroo , Ulmer Peter , Hansmann Werner , Dietrich Volker , Steiger Rudolf H. 
Published: February 2008

Among the Periadriatic Tertiary calc‐alkaline intrusions in northern Italy, the Adamello batholith displays the most convincing evidence for a mafic parentage. This is particularly the case for the southern Adamello, where minor ultramafic to gabbroic bodies form the margins of several distinct and dominantly tonalitic plutons. Isotopic analyses are presented for the Val Fredda and Blumone ultramafic‐gabbroic to tonalitic‐leucoquartzdioritic suites, for the more felsic rock types of the spatially predominant Re di Castello group, as well as for a lamprophyre. This dike of picrobasaltic composition is thought to represent the parental magma. Field relations and a large body of petrographic, geochemical and mineral chemical data point to a cogenetic origin for all of these rocks and to magmatic differentiation with fractional crystallization and accumulation as the main evolutionary mechanism. This is compatible with the isotopic results for the Blumone series and the mafic rocks of Val Fredda with εNd = +3.8 to +1.5 and εSr = −9.1 to −4.6. A common parental magma similar to the lamprophyre (εNd = +3.1, εSr = −4.4) also seems plausible. The isotopically homogeneous felsic lithologies of Val Fredda (εNd = +0.1 to −0.3, εSr = +2.9 to +0.7), structurally intimately associated with their mafic counterpart, indicate crustal contamination incurred prior to their differentiation. For the Re di Castello group, progressing crustal affinity of isotopic compositions (εNd of −1.9 to −4.9 and εSr of +16.5 to +40.9) with chemical differentiation implies large‐scale interaction with a granitoid crustal component. Attempts at quantifying this process reveal individual characteristics and evolution for each pluton. The data indicate extraction of this component from a source in the lower crust with time‐integrated low Rb/Sr, εSr approximating +50 and εN close to −6 corresponding to a 1000 Ma crustal residence age. If a two‐stage evolution is assumed, formation of a gabbroic reservoir from the above parental melt would require uptake of some 4–12 wt % of granitoid component by combined assimilation/fractional crystallization, whereas subsequent evolution to tonalitic compositions would necessitate admixing of 40–70 wt % of granitoid partial melt. Assimilation thus mainly occurs during evolution from gabbroic to tonalitic compositions, when the mafic magma is ponded near the mantle‐crust boundary and where homogenization prior to resuming buoyant ascent is assured.Precise locations and more detailed petrography of the samples is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document 91B‐002; $2.50. Payment must accompany order.
Hydrothermal 3He and manganese plumes at 19°29′S on the Central Indian Ridge
Authors: Jean‐Baptiste Philippe , Mantisi Frédérick , Pauwells Hélène , Grimaud Daniel , Patriat Philippe 
Published: February 2008

We report the discovery of a hydrothermal helium‐3 plume north of the Indian Triple Junction, on the Central Indian Ridge at 19°29′S. Water column δ3He anomalies show a spike of 33.8% at 2460 meter depth, i.e. the highest δ3He anomaly recorded so far in the Indian ocean.The helium‐3 data are correlated with manganese, giving a Mn/3He ratio of 700 mol/cm3 (3.8×104 g/cm3STP). The CH4/3He ratio deduced from available CH4/Mn data in the same area is around 7×106. These values are close to what is observed on water column plumes on the East Pacific Rise.On a regional scale, the 3He distribution over the Indian Triple Junction exhibits a strong δ3He increase at the Ridge crest, which appears to spread eastwards through the deep Indian ocean.
Seismic transmission survey: Mapping in two‐layered media
Authors: Katsumata Kei 
Published: February 2008

I propose a new concept of a seismic survey which takes advantage of mode‐converted transmission waves. This method furnishes us with valuable information independent of any other survey which have been used to explore the Earth's structure. In the seismic reflection survey, both not mode‐converted and mode‐converted reflection waves are used in order to obtain the section of the reflectivity. In the seismic transmission survey, refraction waves with mode changes from P to SV or from SV to Pin propagating through the interfaces are used in order to obtain the section of the transmissivity.The model consists of only a horizontally layered elastic medium. To correct the normal move‐out of the transmission waves, I apply the method which have been developed to process the mode‐converted reflection waves. The approximation error is estimated by comparing with the exact solution in the case of a horizontally two‐layered medium. Numerical simulations prove that this method is available for the two‐layered medium with the discontinuity which has the inclination of 30 degrees.
The effect of two—dimesional terrain with point source on resistivity surveys
Authors: Shi‐zhe Xu 
Published: February 2008

This paper uses the boundary element method to solve the problem of the effect of two‐dimensional terrain with point current source on resistivity surveys. First, the 3‐D boundary value problem of the electric field of a point source on 2‐d terrain is converted into a 2‐D boundary value problem using a Fourier transform. Then, the 2‐D boundary value problem is transformed into a boundary integral equation by means of Green's formula. The boundary element method is used to solve the integral equation. Finally, using an inverse Fourier inverse transform, we get the electric potential in 3‐D space.Examples calculated for two terrain models show that the results from this method compare well with those from analytic and finite element method. If the terrain has a clear strike, this method can be used for terrain corrections for resistivity surveys. An example of a terrain correction for electric sounding is given. This method can be run on a personal computer and takes a few minutes to correct a profile of data on an IBM AT—grade computer.
Melting in the Mg2SiO4‐H2O system at 3 to 12 GPa
Authors: Luth Robert W. 
Published: February 2008

The melting of forsterite (Mg2SiO4) in the presence of H2O was studied from 3 to 12 GPa in a multiple‐anvil apparatus to constrain the maximum temperature of the hydrous solidus in peridotitic systems relevant to the Earth's mantle. The solidus has a negative slope to 12 GPa, despite the stabilization of Phase E (a hydrous magnesium silicate) coexisting with forsterite and vapour at the solidus at > 9 GPa. Abundant quench vapour deposits in all the run products indicate extensive solubility of silicate in the vapour; however, there is no direct evidence for incongruent solution in the vapour to 9 GPa. At > 9 GPa, the coexistence of Fo + PhE + V at the solidus requires Mg/Si≠2 in the vapour. Silicate melts and hydrous vapours remain immiscible phases in this system to >12 GPa.Given current estimates of the temperature distribution in the mantle, a hydrous vapour can exist in the asthenospheric mantle only below ∼5 GPa (∼160 km depth). At greater depths, water would be present in a hydrous silicate melt rather than in a free vapour. This depth is a maximum, because solidus temperatures in hydrous peridotite systems would be lower than those in the endmember Mg2SiO4−H2O system. In cooler subducting slabs, a hydrous vapour could persist down to the transition zone.
Stratospheric aerosol change in the early stage of volcanic disturbance by the Pinatubo Eruption observed over Tsukuba, Japan
Authors: Hayashida Sachiko , Sasano Yasuhiro 
Published: February 2008

An increase in the amount of stratospheric aerosol due to the Pinatubo eruption (June 12 –15, 1991, 15.14° N, 120.35° E) was observed from the end of June by a lidar in NIES, Tsukuba (36° N, 140° E). The first arrival of volcanic aerosol layers was observed just above the tropopause on June 28, 1991, only two weeks after the eruption. Aerosol layers higher than 20 km appeared sporadically in July and August, reflecting the inhomogeneity of aerosol distribution.After the change in the wind system from summer easterlies to winter westerlies, the main body of the volcanic aerosol layer made its appearance over Tsukuba. The integrated backscattering coefficient (IBC) increased in winter as a result of transportation of aerosols from the tropical region. The IBC, which can be converted to optical thickness, exceeded the level of the value observed after the El Chichon eruption.
Airborne measurement of absolute sea‐surface heights
Authors: Brozena John M. , Peters Mary F. , Forsberg Rene 
Published: February 2008

Topographic profiles may be measured from an aircraft by differencing precise radar or laser altitudes with ellipsoidal heights obtained by means of kinematic interferometric‐mode GPS. Over the ocean, the difference between the two altitudes is the geoid height plus dynamic oceanographic effects such as tides, currents, eddies, etc. Aircraft might provide, on a local scale, altimetry measurements similar to those from GEOSAT and ERS‐1. Such airborne measurements could be applied to the determination of a local high‐resolution geoid or for oceanographic studies.Several long over‐water altimetry profiles were obtained during the 1991 field campaign of the Greenland Aerogeophysics Project (GAP91). Kinematic interferometric GPS between multiple short or zero‐baseline airborne receivers and several sets of stationary GPS receivers on the ground provided precise three‐dimensional positioning for airborne gravity, magnetic and topographic profiling sensor systems mounted aboard a P‐3 Orion aircraft. The logistics of surveying a large region (long GPS baselines and poor satellite coverage) limit the accuracy of the GPS positioning, but the geoid signal is clearly visible in the over‐water profiles. In spite of GPS baseline lengths of up to 1000 km, differences between modeled geoid heights and measured sea‐surface heights ranged from about 1–2 meters r.m.s. Although the differences are primarily due to errors in the GPS positions, some portion is due to errors in the model geoid, uncompensated tides, sea‐ice contamination of the radar data and other errors in the radar. We believe that this represents the first airborne measurement of absolute sea‐surface heights.
2‐D migration velocity estimation using a genetic algorithm
Authors: Jervis Michael , Stoffa Paul L. , Sen Mrinal K. 
Published: February 2008

We address the problem of velocity estimation in heterogeneous media using a combination of nonlinear inversion and migration velocity analysis. In velocity estimation, the travel time information in seismic reflection data are nonlinearly related to the velocity perturbations in the subsurface. By taking a functional of seismic traces, the migrated data themselves, we define a misfit criterion which greatly reduces the oscillatory nature of the objective function. Migration is inherently a smoothing process; it collapses diffractions, focuses reflected energy and suppresses random noise. We use the lateral consistency of reflectors after migration as a measure of model misfit. If we compare one migrated shot record with an adjacent record, the misfit will be only slightly affected by large velocity variations even though reflectors may show large errors in depth positioning. The closer the spacing between shot records the smaller the variations in misfit; hence the smoother the objective function.We search for global minima of the objective function thus defined, using a genetic algorithm (GA) and a linearized inversion scheme. We illustrate the techniques and results from the algorithms by applying them to a realistic scale synthetic data set. As expected, the success of a linearized scheme depends strongly on the starting model while GA does not depend on the choice of the initial population of models. GA is computationally intensive but our choice of spline parameterization reduces the number of model parameters significantly and makes the algorithm computationally tractable for real earth problems.
Correction to “Changes in the Earth's rotation by tectonic movements”
Authors: Vermeersen L. L. A. , Vlaar N. J. 
Published: February 2008

Present‐day true polar wander and the secular non‐tidal acceleration of the earth are usually attributed to post‐glacial rebound. In the models which relate this rebound to changes in rotation, the mantle is assumed to relax passively to the melted ice‐loads. The lithosphere is usually modeled as a highly viscous upper layer in these models, having viscosities which exceed mantle viscosities by several orders of magnitude.We propose that lithospheric processes unrelated to post‐glacial rebound and taking place under non‐isostatic conditions are also able to induce non‐negligible influences on the earth's rotation. Examples of such processes are mountain building and erosion, foundering flexure of oceanic basins and lithospheric snapbacking resulting from detachment of subducting slabs. Lithospheric and crustal rheologies and intraplate‐stresses are the dominant factors in these mechanisms, contrary to the mantle rheologies which are assumed to dominate the process of post‐glacial rebound.
Measurement of the gas phase concentration of H2SO4 and methane sulfonic acid and estimates of H2SO4 production and loss in the atmosphere
Authors: Eisele F. L. , Tanner D. J. 
Published: February 2008

Measurements of gas phase sulfuric and methane sulfonic acid (MSA) have been performed using a relatively new atmospheric pressure selected ion chemical ionization mass spectrometric technique at two field sites. Both gas phase acids are photooxidation products, and their concentrations are seen to qualitatively follow solar flux. While sulfuric acid concentrations typically decline in conjunction with declining solar radiation, they sometimes level off in the mid 105–106 molecules cm−3 range after dark, even in relatively clean air. The reason for this quasi‐stable nighttime sulfuric acid concentration is not well understood but may be a result of a steady state exchange of sulfuric acid between particles and the gas phase.Measurements of OH, H2SO4, and SO2 concentrations in conjunction with aerosol number and size distribution also made possible the independent calculation of gas phase sulfuric acid production and loss rates. Calculated production and loss rates are seen to agree well in relatively clean air during the daylight hours. At night, however, the sulfuric acid concentrations and its calculated loss rate often have a nonzero value. In more polluted air masses, calculated gas phase sulfuric acid losses significantly exceed calculated production if H2SO4/aerosol reaction probabilities of 1.0 or 0.5 are assumed.
Crustal‐scale thrusting and origin of the Montreal River monocline‐A 35‐km‐thick cross section of the midcontinent rift in northern Michigan and Wisconsin
Authors: Cannon W. F. , Peterman Z. E. , Sims P. K. 
Published: February 2008

A structurally simple, 35‐km‐thick, north facing stratigraphic succession of Late Archean to Middle Proterozoic rocks is exposed near the Montreal River, which forms the border between northern Wisconsin and Michigan. This structure, the Montreal River monocline, is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the Keweenawan Supergroup (Middle Proterozoic) along the south limb of the Midcontinent rift, and disconformably underlying sedimentary rocks of the Marquette Range Supergroup (Early Proterozoic). These rocks lie on an Archean granite‐greenstone complex, about 10 km of which is included in the monocline. This remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition.Tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension. The faults responsible for the monocline are newly recognized components of a well‐known regional fault system that partly closed and inverted the Midcontinent rift system. Resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 +/−20 Ma and followed very shortly after extension that formed the Midcontinent rift system.
Experimental determination of mean Doppler‐reflectivity relationship with a vertically pointing Doppler radar
Authors: Pasqualucci F. 
Published: February 2008

A pseudo noise coded continuous wave Doppler radar, operated at a frequency of 35 GHz, was used to measure drop‐size distributions and the mean Doppler for vertical velocities
. The data have been used to determine a relationship between,
and the radar reflectivity factor, Z. 95% of the
experimental data fall within ±0.3 m sec−1 of the
function
= 2.67 Z0.095 (
m sec−1).The spread of the data may be attributed to a small contribution of unknown vertical air velocity in the low altitude (15 m to 25 m) scattering region.
The magnetic field on the magnetospheric boundary from laboratory simulation data
Authors: Podgorny I. M. , Dubinin E. M. , Potanin Yu. N. 
Published: February 2008

A simulation experiment has studied the magnetospheric boundary for high Alfven Mach numbers. The artificial magnetosphere was produced as a result of interacting a supersonic and super‐Alfvénic flow of collisionless plasma with the magnetic field of a dipole. In the presence of southward magnetic field components, a "visor" forms at the day side in the plasma flow, preventing reconnection at the front side of the magnetosphere. Field lines of the plasma flow envelop the polar cap, forming the mantle. In the presence of northward field components the convection in the polar cap is sunward, while with southward components convection is antisunward.Simultaneously with the convection caused by reconnection, convection due to viscous interaction is observed along the boundary, and clearly in the region of closed field lines.
Magma mixing at mid‐ocean ridges: Evidence from legs 45 and 46‐DSDP
Authors: Dungan Michael A. , Long Philip E. , Rhodes J. M. 
Published: February 2008

An integrated petrologic and geochemical study of basalts recovered in Legs 45 and 46 (DSDP) has yielded evidence that these moderately evolved basalts are mixtures of primitive, mantle‐derived tholeiites with more evolved magmas. The hybrid nature of these rocks is recognized on the basis of disequilibrium mineralogy. Plagioclase phenocrysts exhibit substantial diversity in composition and zoning patterns including both normally and reversely zoned grains and abundant textural evidence of resorption. Many olivine and plagioclase phenocrysts are too refractory (e.g., Fo90−88) to be in equilibrium with liquids of the host basalt composition and are in fact of a composition consistent with crystallization from a primitive, mantle‐derived basalt liquid. An estimate of this primitive melt, presumed to be parental to the observed lavas, has been derived from melt inclusions trapped in the olivine phenocrysts. These are characterized by high Ca/Al and low TiO2, two chemical features typical of primitive ocean floor basalts recovered elsewhere in the Atlantic.We suggest that subvolcanic magma chambers beneath mid‐ocean ridges receive periodic injections of this primitive melt and its attendant phenocrysts which mix with fractionated chamber‐bound magmas, resulting in observed moderately evolved lavas. Chemical modeling of ocean floor basalt differentiation assuming a combination of magma mixing and crystal fractionation alleviates some apparent anomalies encountered in previous studies (Rhodes et al., 1978).