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  • 1995-1999  (275)
  • 1935-1939
  • 1995  (275)
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  • 1995-1999  (275)
  • 1935-1939
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  • 11
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A four-year record from the superconducting gravimeter at Cantley, Quebec, is examined for non-linear tidal effects. A linear response is first determined from an adaptation of the response method used in the tidal analysis of sea-level. Residuals left by the linear response clearly show anomalies of a few tens of ngals (1 ngal = 10−11 ms−2). These residuals are most evident in the semi-diurnal band, but the diurnal, terdiurnal and quart-diurnal bands are also affected. A subsequent analysis shows that these are most likely the loading effects of shallow-water non-linear tides, principally in the Bay of Fundy and the east coast of North America. The loading effect of global degree-three diurnal and semi-diurnal tides is also interpreted. The amplitude of the latter tides in the open ocean is a few millimetres, so the superconducting gravimeter is sensing ocean tides at a level surpassing that of most standard tide gauges, and of satellite altimetry. The linear response analysis, and the identification and removal of the non-linear tides, provides the clearest look to date at the tide-free spectrum.It is shown that any simple non-linear response that doubles the frequency must be at least 65 dB below the linear response, if the source of the non-linearity is in the solid Earth, and 82 dB below the linear response, if the source of the non-linearity is in the instrument. If the observed non-linear response is entirely due to loading by non-linear ocean tides, as seems likely, then these figures are 76 dB and 93 dB, respectively.
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  • 12
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Using the T-matrix, or extended boundary condition, method, we model seismic wave scattering from earth models with irregular interfaces. This is motivated by numerous observations that surface and subsurface topography is responsible for the amplification and de-amplification of seismic waves and the generation of coda waves. We demonstrate that irregular Moho and free-surface scattering significantly affects teleseismic P-wave receiver function waveforms. Receiver functions are routinely used to obtain detailed crustal and upper mantle structure. For a layer-over-halfspace model, we perform a sensitivity study on the effects of ray parameter, interface roughness and wavelength, receiver position, and incident wave frequency content. The free-surface responses vary significantly with receiver position, interface wavelength and with incident wave frequency content. Using frequency-wavenumber analysis, the coda is shown to contain scattered Rg waves and S waves. Receiver functions are formed from the free-surface responses by deconvolution, and these are then inverted, as synthetic ‘data’, for plane-layered velocity structure, to show how scattered energy maps onto the final velocity models. Spurious structure models result, with the amplitude of the spurious features (for example low-velocity zones) decreasing with interface roughness. The resulting velocity models also vary appreciably with receiver position.
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  • 13
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We present a new palaeomagnetic investigation of the two sites from the Steens Mountain (Oregon) volcanic record of a Miocene polarity reversal which were supposed to record very fast changes of the geomagnetic field or impulses (Mankinen et al. 1985; Prévot et al. 1985a,b). Approximately 130 cores were first drilled from the two initial sites, belonging to sections A and B, in order to obtain at least one detailed and complete vertical sampling of each lava flow. Thermal analyses of natural remanent magnetization, complemented by some alternating field treatments, low-field thermo-magnetic curves, microscopic observations and electron probe analyses of key magnetic phases, lead us to somewhat different conclusions for the first and the second impulses. At site B (first impulse), we find that the dependence of the remanence direction on the sample vertical position in flow B51 does not imply a directional field change during flow cooling, but is better explained by a thermochemical overprinting due to the overlying B50 flow. However, this conclusion does not challenge the existence of the first impulse because this field change seems to be recorded some 25 m away in flow B51 (Coe & Prévot 1989), at a place where it is thick enough for this record not to have been erased by the baking due to B50. Regarding the second impulse, restudied at site A, our new findings are more comprehensively explained by a change in the field direction during cooling of flow A41–2 than by some overprinting. Using a simple model of flow cooling, the angular rate of change of the field is estimated to have been of the order of 2°-3° or 250–350 nT per day during the impulse. This figure is similar to that previously obtained from site D, some 250 m away. However, the directional paths describing the field change are somewhat different at the two sites. New investigations are planned to try to understand the origin of this discrepancy.
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  • 14
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: It is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle, and that this is a relatively small-scale phenomenon compared with anisotropy of the oceanic mantle. During recent years, shear-wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy. The magnitude of the splitting delay times can often rule out a crustal source of anisotropy; however, it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere. It is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon. Here we describe a way to constrain the depth of the anisotropy, by studying the lateral variation of the splitting parameters at neighbouring seismic stations. We use SKS and SKKS phases recorded at the NARS-NL array, a relatively dense network of broad-band stations separated by about 50 km. Fresnel zones at different depths are calculated for these phases. The depth of the anisotropy is constrained by the criterion that Fresnel zones corresponding to different splitting observations should not overlap. Variability in the splitting measurements for events with different directions of approach recorded at one station, and for single events recorded at various stations, provides evidence for a non-homogeneously distributed source of anisotropy, located below the array at different depths. We show that this variability in the measurements cannot be due to effects of crustal scattering. Our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth. This suggests that the deformation in this region is subject to significant lateral variations.
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  • 15
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An approach is presented for interpolating a property of the Earth (for example temperature or seismic velocity) specified at a series of ‘reference’ points with arbitrary distribution in two or three dimensions. The method makes use of some powerful algorithms from the field of computational geometry to efficiently partition the medium into ‘Delaunay’ triangles (in 2-D) or tetrahedra (in 3-D) constructed around the irregularly spaced reference points. The field can then be smoothly interpolated anywhere in the medium using a method known as natural-neighbour interpolation. This method has the following useful properties: (1) the original function values are recovered exactly at the reference points; (2) the interpolation is entirely local (every point is only influenced by its natural-neighbour nodes); and (3) the derivatives of the interpolated function are continuous everywhere except at the reference points. In addition, the ability to handle highly irregular distributions of nodes means that large variations in the scale-lengths of the interpolated function can be represented easily. These properties make the procedure ideally suited for ‘gridding’ of irregularly spaced geophysical data, or as the basis of parametrization in inverse problems such as seismic tomography.We have extended the theory to produce expressions for the derivatives of the interpolated function. These may be calculated efficiently by modifying an existing algorithm which calculates the interpolated function using only local information. Full details of the theory and numerical algorithms are given. The new theory for function and derivative interpolation has applications to a range of geophysical interpolation and parametrization problems. In addition, it shows much promise when used as the basis of a finite-element procedure for numerical solution of partial differential equations.
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  • 16
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Patras area lies in the western part of central Greece. It is an area characterized by high seismicity and complex neotectonics. Several devastating earthquakes have occurred in the region since 600 BC. Contemporary crustal deformation is examined in this area using microearthquake data recorded over a lengthy period, during 1983–84, by the Patras Seismic Network, principally, and to a lesser extent by the Volos Seismic Network.The microseismicity (1.8–3.9 ML) defines a zone deepening to the NE, which justifies a possible extension of the Gulf of Corinth major graben towards the Trikhonis Lake to the NW.Spectra of 108 well-located microearthquakes are estimated, using P-waves obtained by selective windowing designed to include only the P-phase; seismic moments in the range 0.3–45.7 × 1012 Nm are obtained, accompanied by estimates of seismotectonic source parameters including source radii, average stress drop and average coseismic slip.Poor correlation is found between seismic moment and magnitude, and the likely reason is the complex nature of the neotectonic regime existing in the area.Two zones differing in crustal deformation characteristics are observed. The Corinth-Trikhonis zone reveals two sets of characteristic faults. The first set is represented by microearthquakes showing distinctive and relatively higher seismic moments in conjunction with lower stress drops and seismic slips. This set of faults shows greater source radius than the second set, and therefore the faults are longer. The second set is characterized by an almost constant source radius within the range of uncertainty, and a wide range of seismic moments, stress drops and seismic slips. The Rio zone is characterized by low seismic slip, stress drop and fault radii, with the exception of the locality south of the city of Patras, where relatively higher seismic slip, stress drop and fault radius are observed.
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  • 17
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper demonstrates how magnetic overprints of a geological series can provide information over a long time period, which can be interpreted in terms of geotectonic evolution. According to these new results, the Late Carboniferous-Early Permian rhyolites from the northern Vosges have recorded the magnetic field over a major part of the Permian. Recent radiometric dating assigns a Late Carboniferous-Early Permian age (298 Ma) to the previously ‘Middle Permian’ rhyolitic volcanism of the northern Vosges. A palaeomagnetic study was undertaken on rhyolites of the Nideck-Donon massif and on neighbouring Permian basins (Villé, Saint-Dié) in order to obtain a new pole for the Early Permian and to detect overprints. Standard palaeomagnetic and rock magnetic experiments demonstrate that over 90 per cent of the characteristic directions of magnetizations are of secondary origin. The high-temperature magnetizations carried by secondary haematite and haematized titano-magnetite fail the fold test, and display directions consistent with the Saxonian-Thuringian directions from Variscan Europe (Nideck-Donon: 186°/−17°, k = 167 for 26 sites, VGP: 50°N/178°E; S.aint-Dié basin: 184°/−20.5° for two sites, VGP: 53°N/180°E; Villé basin: 191.5°/−25° for two sites, VGP: 53°N/168 E; Vosges dikes: 197.5°/−26.5° for two sites, VGP: 53°N/158°E). Part of the magnetizations with intermediate unblocking temperatures of magnetite consists also of post-tectonic overprints. Two consistent mean directions with lower inclinations, obtained in the Nideck-Donon rhyolites (182°/−7°, k= 459 for 13 sites, VGP: 45°N/184°E) and pyroclastic deposits from the Villébasin (178°/−9, k = 519 for three sites, VGP: 46° N/190° E), were probably acquired during Late Autunian-Early Saxonian times. In rhyolites the pre- and post-tectonic characteristic mag netizations are carried by magnetite, while in the ash-fall deposits the carrier is allogenic haematite. A few older magnetizations (191/−3, two sites, VGP: 42°N/173°E), consistent with magnetizations carried by magnetite from rhyolites of the central Black Forest, and results from coeval volcanics as well as from Autunian sediments have been observed in tuffs from the Villi basin (188°/1.5°, two sites, VGP: 40°N/177°E. The oldest magnetizations (207°/10°, k = 329, VGP: 32°N/155°E), presumably of Stephanian age, were identified in two rhyolitic stocks within the granitic basement from the central Vosges and central Black Forest. The distribution of the palaeomagnetic directions suggests that, from the Late Autunian to the Late Thuringian, overprinting due to low-temperature alteration of titano-magnetite and crystallization of secondary haematite was more or less continuous.The apparent polar wander path (APWP) computed with the new results and the published poles shows a hairpin, which implies a drastic change of the European plate motion during the Permian. The clockwise rotation of Europe initiated in the Late Visean-Namurian stopped in the Late Autunian-Early Saxonian. This event corresponds to the end of the Variscan convergence and of the Appalachian orogeny. In the northern Vosges, the hinge of the APWP is also associated with the tectonic phase responsible for the tilting of the volcanic layers. The motion of the European plate was then converted into a counterclockwise rotation and a northward drift until the Late Triassic.
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  • 18
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Magnetotelluric (MT) field data are often distorted by small-scale, near-surface conductivity anomalies. In this paper, the Occam regularized inversion technique is expanded to solve simultaneously for the galvanic distortion matrix, regional strike, and resistivity parameters that correspond to the smoothest regional 2-D resistivity model fitting the data. This involves fitting all four complex components of the MT impedance tensor. By solving for smooth structure along with the strike and decomposition parameters, rather than the usual two-step decomposition/inversion process, we avoid inferring conductivity features that result only from an erroneous choice of strike and decomposition parameters. Furthermore, we ensure that the recovered regional impedance tensor is physically realistic. A method is presented for determining subsets of the data corresponding to the assumed physical model, that of 2-D structure at inductive scale-lengths, overlain by galvanic scatterers. This method is based on the linear dependence of the elements of the distorted MT tensor. Tests of the extended Occam algorithm are presented for both synthetic data and a subset of the BC87 data. Results show that, for appropriate subsets of the data, the determination of strike and decomposition parameters can be fully automated with the extended Occam algorithm.
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  • 19
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Lunar daily geomagnetic variations (L) exhibit distinctive behaviour at many sites near coasts. In this study, data from an array of three-component magnetometers that operated for a four-month summer period at 22 sites throughout New Zealand are used to examine L. The Chapman-Miller method is used to determine the four main luni-solar harmonics of L at each site. Analysis of 3.5 years of data from the geomagnetic observatory at Eyrewell indicates that the array data produce representative values of L. Separation of the dominant lunar semidiurnal harmonic (L2) into parts of oceanic and ionospheric origin indicates near-uniform ionospheric contributions to the X and Y field components at all sites, with peak values occurring within 3 hours of lunar transit. The ionospheric contribution to the Z field component is smaller, and suggests secondary electric currents flowing in the deep water to the north-east and south-west of New Zealand. The oceanic contribution to L2 tends not to be significant for X and Y. Its amplitude for Z is greatest in the eastern North Island and in the south-western South Island, and its phase angle for Z is locally consistent throughout the North Island and in the south-western South Island. The difference between the phase angles that characterize these northern and southern regions is about 180°. This pattern suggests near-parallel, in-phase electric currents flowing in the deep water to the north-east and south-west of New Zealand in response to tidal flows. This is in agreement with both the general pattern seen in the United Kingdom and lunar daily geomagnetic variations observed under the Tasman Sea and in eastern Australia, where the magnetic effects of tidal flows in shallow coastal waters are secondary to those from more distant, deep waters. The oceanic contribution to L2 might be used to estimate the poorly known tidal flows in the deep water off New Zealand.
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  • 20
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We compute synthetic seismograms of SH waves that are multiply scattered by randomly distributed cracks. All the cracks are assumed to have the same length and strike direction; the crack surfaces are assumed to be stress-free, or to undergo viscous friction. We analyse the deterministic wave equation, and rigorously treat multiple crack interactions. We first calculate the wavefield in the wavenumber domain, and then we obtain the time-domain solution by its Fourier transform.A plane wave whose time dependence is described by the Ricker wavelet is assumed to be incident upon the region of crack distribution. The scattered waves are efficiently excited when the half-wavelength of the incident wave is close to or shorter than the crack length. High-wavenumber components are shown to be more abundant in the scattered waves when the crack distribution is denser. The time delay of the arrival of the primary wave, due to crack scattering, is shown to be prominent when the wavelength of the incident wave is much longer than the crack length. When the crack surfaces are subject to viscous friction, both the amplitudes of the scattered waves and the time delay of the primary-wave arrivals are smaller than those for the case of stress-free crack surfaces. When the crack distribution is statistically homogeneous, the calculated attenuation coefficient Q−1 and phase velocity v of the primary wave are generally consistent with those obtained by a stochastic analysis based on Foldy's approximation. A short analysis on the effect of inhomogeneous crack distribution shows that the wavenumber at which Q−1 is at its peak value is smaller than that expected from the stochastic analysis for homogeneous crack distribution.
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