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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Small earthquakes in the Dead Sea depression, part of the Jordan valley-Dead Sea rift, were recorded and analysed. With 117 identified events of 0.5 ≤ML≤ 4.2, the microseismicity, recorded by a local portable network during a period of about 20 months, shows a normal rate of seismicity for the region with b values around 0.8. The recorded seismicity is mainly confined to the basin and its boundaries. In the southern part of the Dead Sea basin we found a tendency to clustering, which is clearly demonstrated in very similar seismograms of several events recorded at the same station. Two clusters on the eastern fault are separated by an area with no seismicity for at least 5 yr. Relative location of events in one of the clusters exhibits a clear north-south lineament. Two active north-south left-lateral strike-slip faults along the east and west boundaries of the southern section of the Dead Sea basin are distinguished and confirmed by using composite focal mechanism solutions. In four, out of more than 60 events, we found normal faulting, where one has a ML= 4.1. For 34 events with 1.6 ≤ML≤ 4.2 we found seismic moment estimates, M0, of 1.2 × 1019≤M0≤ 2.3 times 1022 dyne cm and Brune stress drop estimates, Δs̀, between 0.6 and 92 bars. For earthquakes of M0 smaller than about 5 times 1021 dyne cm, we found only small variations in corner frequency, f0, resulting in decreasing Δs̀ with decreasing M0. This breakdown of the scaling relation for small earthquakes suggests an fmax slightly lower than 10 Hz for the Dead Sea region. The fmax is confirmed by available accelerometer data.
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The results of the Midland Valley Investigation by Seismology (MAVIS) are described. The experiment consists of three refraction profiles of between 80 and 100 km length across the Upper Palaeozoic basins of the Scottish Midland Valley.Three refractors are recognized, thus defining four upper crustal layers. The top two layers are interpreted as Carboniferous plus Upper Old Red Sandstone, and Lower Old Red Sandstone plus Lower Palaeozoic. The structure of the refractor, an unconformity, separating the two layers mirrors surface structure. Deeper refractors, associated with Midland Valley crystalline basement, show no correlation with surface structure. The MAVIS survey allows the refinement of earlier gross interpretation of upper crustal structure in the Midland Valley, but confirms the existence of anomalously shallow (6.4 km s–1) basement, and shows, in conjunction with results of previous work, that a large area of the central and southern Midland Valley is characterized by one velocity signature, probably indicative of a single terrane.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A method is described for finding the magnetotelluric transfer function that has the least amount of curvature consistent with most of the data and with a 1-D conductivity interpretation over the widest possible frequency range. This could be called an ‘Occam’ transfer function. It is represented by the transfer function for the best fitting 1-D conductivity model times a distortion function. The latter permits smooth departures of the transfer function from the 1-D case if the data are inconsistent with a 1-D interpretation. The transfer function, for single-station or remote reference magnetotelluric data, is found by a method of successive interations that is found to converge within six to eight iterations. The estimate of the transfer functions is made robust by using frequency and time weights that remove the effects of outliers in the time and frequency domain. If the weighted residuals for remote reference data satisfy certain necessary conditions for uncorrelated noise then the contribution to the noise by the electric and magnetic data can be estimated and used to evaluate the least-squares and remote reference estimates.Examples illustrate the application of this method to artificial and real data. The latter consist of hourly cable voltage data from the Florida Straits, 1/256-s remote reference magnetotelluric survey data from the Phillippines and daily magnetic data from Tucson and Honolulu.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Intracratonic sag sedimentary basins occur in the middle of stable continental or cratonic blocks. They are rarely fault bounded, although strike-slip faulting can occur within them. A simple model for the development and evolution of these basins is proposed. The mechanism is driven by mildly-coupled convective down-welling of the asthensophere beneath the lithosphere. Initially, a rapid alteration of the mantle convective system causes a descending plume to develop. A depression, which can be of the order of 600 m, can be formed at the earth's surface; this depression, when loaded with sediment, will form a sedimentary basin of the order of 2.5 km thick. If the convective downwelling remains, a period of thermal cooling of the lithosphere occurs, which is similar to the thermal cooling subsidence phase of passive continental margins. This thermal cooling occurs because of the thermal anomaly (temperature decrease) beneath the lithosphere caused by the convective downwelling. If a change of convective pattern occurs and the descending plume is removed, the basin will undergo uplift and erosion, whereby a significant thickness of the sedimentary basin can be removed. The Ordovician and Silurian tectonic development of the Canning Basin, Western Australia, appears to be well explained by this model.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The spectrum of the Earth's gravitational potential and topography, as represented by spherical harmonic expansions to degree 180, have been computed. Modelling the decay in the form of Al-β, values of A and β for several degree (l) ranges were computed. For degree range 5–180, β was 2.54 for the potential and 2.16 for equivalent rock topography. The potential decay was somewhat slower than that (i.e. β= 3) implied by Kaula's rule. However, at high degree ranges, the β values were larger (3.20 for degrees 101–180) agreeing better with recent determinations from terrestrial gravity data and geoid undulations implied by satellite altimetric data. The values imply that the potential decays faster at higher l values. The values of β for topography were fairly uniform around 2 which agrees with a suggestion made by Vening-Meinesz in 1951. We also found that the β value for the Earth's potential agrees well with the value implied by the topography with Airy isostatic compensation with the depth of compensation equal to 30 km. However, the magnitude of the power implied by the topographic/isostatic potential was approximately one-third of the observed potential.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In this work a spherical harmonic theory of ocean tides is presented. The theory is based on Laplace tide equations modified to include turbulence with constant eddy viscosity, linearized bottom friction, and oceanic loading and self-gravitation. Variable bathymetry is also treated in harmonic terms, and no-flow boundary conditions are applied at continental coastlines. The tide and boundary constraint equations are reduced to matrix form and solved by a weighted least-squares procedure. Five zonal luni–solar tides, ranging in period from 14 days to 18.6 yr, are investigated using the theory; such tides have typically been difficult to compute using traditional numerical approaches. The polar motion and changes in the length of day induced by these long-period tides are calculated. Tidal solutions are compared extensively with results from other tidal theories and from recent satellite and sea-level observations. The greatest limitation to accurate prediction of zonal tides—for any theory—appears to be the marginal failure of all tide theories to conserve mass globally; the use of additional mass constraints may be warranted.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A new technique for computing differential seismograms for crustal and upper mantle response to a teleseismic wave (receiver functions) is developed using the matrix formalism of Kennett. The work was motivated by the difficulty of modelling teleseismic SV-waves, and has also proven useful for modelling teleseismic P-waves. This efficient method for calculating differential seismograms is based on three separate methods for computing synthetic seismograms. Two of the synthetic seismogram methods save intermediate results; then the remaining synthetic seismogram algorithm uses the stored results in an efficient calculation of a new synthetic seismogram for a perturbed velocity model. These developments have led to a faster (for a 30-layer model, a 90 per cent reduction in computation time) and more accurate linearized inversion scheme for the determination of velocity models using teleseismic waves.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Model features may be appraised by computing upper and lower bounds for the average value of the model over a specified region. The bounds are computed by constructing extremal models which maximize and minimize this average. In order to compute the most meaningful bounds, it is important that the allowed models are geophysically realistic. In this paper, the appraisal analysis of Oldenburg (1983) is extended to incorporate a bound on the total variation of the extremal models. Restricting the variation discriminates against highly oscillatory models and, as a consequence, the difference between upper and lower bounds is often considerably reduced. The original presentation of the funnel function bound curves is extended to include the variation of the model as another dimension. The interpreter may make use of any knowledge or insight regarding the variation of the model to generate realistic extremal models and meaningful bounds.The appraisal analysis is extended to non-linear problems by altering the usual linearized equations so that a global norm of the model can be used in the objective function. The method is general, but is applied here specifically to compute bounds for localized conductivity averages of the Earth by inverting magnetotelluric measurements. The variation bound may be formulated in terms of conductivity or log conductivity. The appraisal is illustrated using synthetic data and field measurements from southeastern British Columbia, Canada.Bounding the total variation may be viewed as constraining the flatness of the model. This suggests a new method of calculating (piecewise-constant) l1 flattest models by minimizing the norm of the total variation. Unlike l2 flattest models which vary in a smooth, continuous manner, the l1 minimum-variation model is a least-structure model that resembles a layered earth with structural variations occurring at distinct depths.
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  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We show that there can be lateral structure inside the fluid core caused by gravitational forcing from the mantle, from the inner core, or from topography on the core–mantle or inner core–outer core boundaries. We describe a method for calculating the internal structure, given knowledge of the forcing. We discuss the possible effects of this structure on results for the core–mantle boundary topography inferred from observations of the Earth's forced nutations and diurnal earth tides. We consider the possible implications of a thin, low-density fluid layer at the top of the core.
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  • 10
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The fact that vertical plumes and horizontal boundary layers have different cross-sectional dimensions in idealized models of mantle convection is quantified and then exploited to provide a criterion for the selection of an optimal ratio of horizontal and vertical spatial increments in finite difference solutions to the equations governing mantle convection. The effects of varying the ratio r=Δx/Δz on the computed value of the Nusselt number, Nuc, is assessed from a suite of 21 model solutions of Bénard convection at the same Rayleigh number but with varying grid dimensions. It is shown that: (i) for any constant value of r, Nuc varies linearly with (Δx)2 and may be extrapolated to the limit Δx= 0; (ii) the extrapolated value, Nu0, is independent of the value of r employed; (iii) for r= 1 the discretization error (Nu0–Nuc) introduced when Δx 〉 0 may be parametrized in terms of the Rayleigh number and Δx; (iv) it is possible to choose r such that Nuc equals Nu0, the value at Δx= 0, and is independent of Δx; (v) such solutions may be obtained on surprisingly coarse grids when r 〉 1; and (vi) in general Nuc is much less sensitive to the loss of horizontal resolution than vertical. Implications of these results for future developments in the modelling of convection in the Earth's mantle are discussed.
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