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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: Palaeomagnetic results are reported from 16 sites within volcanic and volcano-sedimentary rocks of Cretaceous and Eocene age from the central Pontides in northern Turkey. The sampled area is bounded by the North Anatolian Fault (NAF) to the south and the Black Sea to the north. The palaeomagnetic data collected demonstrate the presence of both non-rotated and anticlockwise rotated regions; (i) declination directions including Lower–Upper Cretaceous and Eocene sites do not indicate any significant rotations north of where the NAF exhibits an arcuate shape, (ii) 20° anticlockwise rotation was observed in the Sinop peninsula, the northernmost extremity of the central Pontides, (iii) in the west of the central Pontides, two sites with normal and reversed antipodal directions indicate a 20° anticlockwise rotation and (iv) another region lies to the east of the study area and shows a rotation in an anticlockwise sense of about 5°–30°. The Upper Cretaceous and Eocene directions reported in this paper and other palaeomagnetic studies show that there is not much difference between the inclination values of the Upper Cretaceous (I= 40.1°, α95= 7.2°) and the Eocene sites (I= 39.6°, α95= 8.1°). Furthermore, the values of the inclination available from both sedimentary and volcanic Upper Jurassic–Lower Cretaceous formations are shallower than the Upper Cretaceous ones. This suggests that the entire Pontides have undergone a northward drift of over 2000 km since the Upper Jurassic.
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  • 2
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper describes a new method that can be used to estimate the geomagnetic K-index values automatically with a computer. The underlying philosophy of the method together with a short description of the mathematics involved are discussed. It is shown how existing ideas in the relatively new field of data-adaptive filtering can be modified and extended to develop a powerful Linear-phase Robust Non-linear Smoothing method (LRNS method). The properties of this method are ideally suited for the computer K-index estimation problem.The method was applied to Hermanus digital geomagnetic data which extend to nearly one decade. Even with this large amount of data a 99 per cent agreement with handscaled values was still obtained when differences of ±1 in the K-values were neglected. A 70-80 per cent total agreement was determined with the majority of the differences occurring during quiet days. This is the result of the very small dynamic range of small K-indices (0 and 1) at Hermanus resulting in the handscaler frequently giving a value of 0 where the computer can detect this small variation and gives a value of 1.What is more important, is that the performance of the method stays virtually the same irrespective of the day or month of the year, or the year or years used in the comparisons. This proves the method's adaptiveness to any changes in the SR pattern irrespective of day-to-day, seasonal or solar activity variations. This is achieved without any changes in the original input parameters. Because of its adaptiveness it is believed that the method will also adapt to data from different geographical locations thus giving us a possible global method.
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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 simple model of superficial hydrostatic equilibrium on a rotating isotropic self-gravitating sphere is employed to derive a planetary oblateness formula. The relation obtained describes oblateness on the basis of spin rate, polar radius and the mass of the planet only. An application to the oblate planets is made and the consistency with the most basic and known oblateness laws of geodesy is verified. By making the further hypothesis that the Earth volume and mass has stayed constant, one derives that the size of the terrestrial oblateness adaptation which has taken place during the last half a billion years is a contraction of 2 km at the equator an uplift of 4.1 km at the pole. For the more remote history it appears very likely that the central condensation of the Earth has decreased systematically over the past 3.5 billion years.
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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: The 3-D velocity structure beneath the Euro-Mediterranean domain is investigated down to 1200 km by using teleseismic P-arrival times collected by the Euro-Mediterranean Seismological Centre (EMSC) in Strasbourg. The Simultaneous Iterative Reconstruction Technique (SIRT) is used to invert the data, while the direct problem is formulated by using a spherical block partition of the model space and an analytical ray tracing so that the whole ray path lies in the investigated 3-D model box. Only events located within epicentral distances of 30°-70° are considered in this study.After reviewing the SIRT method, a synthetic test has shown the reliability of the results for structures beneath the lithosphere, while for shallower parts of the model and depths larger than 900 km, limited reliability is achieved. For the upper mantle, the model shows clear correlations with major known surface tectonic features, in particular a well marked sub-vertical slab in the Calabrian Arc and the presence of a roughly North orientated slab in the Aegean area. At greater depths, our model shows slow velocities around 600 km beneath the two main hercynian massifs in Europe.
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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: Observations of underside P-wave reflections from discontinuities deep within the upper mantle which precede the P'P’ phase have provided important constraints on the existence and physical properties of these dynamically critical features. In this study, we extend earlier work by examining for the first time, broad-band recordings of those earthquakes for which one would expect to observe P'P’ and its associated precursors P'dP’. An exhaustive search of 20 yr of Carnegie data and 11 yr of Gräfenberg data uncovered a number of fine P‘P’ observations, but in only one or perhaps two instances are there evidence of a precursor from near 670 km depth. These null results are consistent with what has been reported in earlier studies and provide us with an opportunity to estimate the variability of P velocity near 670 km or, alternatively, the scale length of discontinuity topography which acts to defocus the precursor. This has been done by using the reflectivity and Gaussian beam methods to compute synthetic seismograms for a suite of 1-D and 2-D earth models which vary slightly from PREM. We find that the null data may be satisfied either by (i) reducing the PREM impedance jump at 670 km from 8.5 to 4-5 per cent; (ii) smoothing this 8.5 per cent jump linearly over a zone 15-30 km thick or (iii) imposing deformations in the 670 km discontinuity as small as 10 km in amplitude and 300 km in wavelength. For the one case of an observed precursor, the relative size of the underside reflection and main P‘P’ phase suggests that, in this instance, the precursor's amplitude is correctly predicted by PREM.
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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: 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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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The features caused by residual stresses in carboniferous sandstone drill cores from 5300 m depth were investigated by various methods: strain redistribution after secondary overcoring, anisotropy of ultrasonic wave velocity and its pressure dependence, as well as fracture tests. This was complemented by microscopic inspection of thin sections. Comparison of the different methods applied revealed a remarkably good correlation between principal stress directions. All effects observed can be explained by one common feature of stress redistribution–the formation of microcracks. The crack-closing pressure derived from acoustic velocity measurements, as well as the tensile strength resulting from the Brazilian test, gave an estimate of the magnitude of residual stresses.
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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: 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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  • 9
    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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  • 10
    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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