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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: 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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  • 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: In conjunction with airgun profiles on Lake Vänern, 48-channel seismic recordings were made along a 7 km long profile on the northern peninsula, Värmlandsnäs. This peninsula lies in the Mylonite Zone which runs through SW Sweden. Four airgun profiles were recorded, two on the eastern side of the Mylonite Zone and two on the western side, each profile being on the order of 20-30 km long. These profiles have been processed using the method of common data point (CDP) stacking which resulted in a total of approximately 50 km of sub-surface coverage. We have interpreted the data in the context of the regional geology and results from other geophysical studies in the area. Important results are the imaging of lower crustal reflectors and Moho at near-vertical incidence angles. The dips and depths of these reflectors are in fair agreement with other studies. The upper crust on the eastern side of the Mylonite Zone appears to be considerably more reflective. This may be due to the presence of mafic intrusions or to the reworking of the crust during the Sveconorwegian orogeny. A brief comparison with results from the GLIMPCE profiles over the Grenville province is also made.
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  • 4
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: It is well known that the Cauchy integral formula may be used in the study of integral transforms, leading to an elegant formulation in 2-D. We show that the quaternionic analysis of Fueter, and in particular the quaternionic analogue of the Cauchy formula, may be applied to derive integral transforms of vector functions in 3-D in an elegant manner. In particular we show that this result contains as special cases results of interest in the study of integral transforms in geophysics.
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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 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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  • 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
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In this paper we consider two fundamentally different processes that can be responsible for the organization of energy in seismic coda into discrete time-independent frequency bands. One process involves the resonance of energy in low velocity horizons and the other requires the interaction of time offset wavefields produced by subevents within multiple-event mine explosions (ripple-fired quarry blasts). We examined data collected by high frequency seismometers in Kazakhstan, USSR, and observed regular time-independent spectral modulations in coda resulting from events strongly suspected to be Soviet quarry blasts, but not in the coda from single event calibration explosions detonated at similar ranges. We conclude these modulations are a source effect and due to ripple-firing. This modulation is independent of the source-receiver azimuth and we infer that the spatial array of subshots in each event must be small. We demonstrate that simple linear superposition theory can be used to reproduce effectively the spectral modulation observed in real quarry blasts. On the basis of these observations we attempt to discriminate between the two types of events using a spectral pattern-based algorithm that seeks time-independent features. We consider the detrimental effect that resonant energy in low velocity horizons can have on the successful application of our algorithm.
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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: Tectonic deformation in the NE Basin and Range Province of the western United States is concentrated in two zones, the N-S trending northern part of the Intermountain seismic belt in northern Utah, SE Idaho and SW Wyoming and the E-W trending Central Idaho seismic zone, which converge at Yellowstone in NW Wyoming. Accepting the popular view that the Yellowstone volcanic field is above an upwelling plume from the deep mantle, I explore the possibility that deformation of the brittle upper crust in these two seismic zones may be caused by return flow from this plume. This return flow is assumed sheared SW in the direction of motion of the North American plate relative to Yellowstone. The Snake River Plain (SRP), a linear outcrop of Neogene and Quaternary basalt that trends SW from Yellowstone for more than 400 km, bisecting the angle between the two seismic zones, may be a trail formed as the plate moves SW across the upwelling plume.Beyond ∼150 km from Yellowstone, slip vectors on active normal faults in both deforming zones are oriented inwards at ∼45° towards the SW azimuth along the SRP. Normal Slip rates on these faults are typically 1 mm yr−1, implying extensional strain rate ∼10−15 S−1. Proportions of normal slip and strike slip suggest faults in the Intermountain seismic belt are rotating anticlockwise at ∼2° Myr−1 around vertical axes, and faults in the central Idaho seismic zone are rotating clockwise at similar rates. Within ∼100 km of Yellowstone, observed sense of rotation around vertical axes in both zones appears reversed. Tilt rates of hanging wall beds suggest most faults in both zones are rotating at ∼2° Myr−1 (also ∼10−15 S−1) around horizontal axes oriented NW.The relative size of rotation rate around vertical axes and horizontal extensional strain rate is shown to imply that velocity in the underlying deforming ‘fluid’ is small in the direction perpendicular to the axis of the SRP. In an initial model, this velocity is set to zero, enabling the two-dimensional pattern of flow at the horizontal upper boundary of this deforming fluid to be solved straightforwardly. This approximation enables observed rates and senses of horizontal extension and rotation around a vertical axis to be explained as consequences of the pattern of horizontal gradients of velocity parallel to the axis of the SRP in the deforming fluid beneath the brittle layer.A refinement suggests that velocity perpendicular to the SRP near the upper boundary of the deforming ‘fluid’ is small but inward towards the SRP from both sides, and is accompanied by downwelling beneath the SRP beyond ∼100 km SW of Yellowstone. This pattern of flow will act to move magma in the uppermost mantle beneath the SRP from its sides. It may thus reconcile petrological observations that SRP basalts do not show deep mantle characteristics expected had they been extruded from the Yellowstone plume, with the existence of this plume nearby.Conservation of angular momentum around horizontal axes perpendicular to the SRP for the combined system of the plume interacting with the North American plate leads to the deduction that the plume radius is ∼50 km, consistent with results of other investigations, and its upwelling velocity is ∼60 mm yr−1.
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  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Fault plane solutions of earthquakes within and on the margins of the Tibetan Plateau show diverse styles of faulting and deformation, with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike-slip faulting resulting in roughly east-west crustal extension within the plateau. The direction of overthrusting of the Himalaya onto the Indian Shield is radially outward, varying from southwest in the western Himalaya to south-southeast in the east. Assuming that the Indian Shield behaves rigidly, this requires a west-northwest divergence of western Tibet from southeastern Tibet at a rate of 18 ± 9 mm yr−1, comparable with the rate of convergence at the Himalaya. Fault plane solutions of earthquakes in the southern portion of the Tibet Plateau consistently show large components of normal faulting on roughly north-striking planes and corroborate such extension. Within the high plateau, where elevations exceed 5000 m, normal and strike-slip faulting occur so that an overall east-southeast-west-northwest extension of the region (at about 10 mm yr−1) is partitioned into roughly equal parts of crustal thinning and north-northeast-south-southwest crustal shortening (about 5 mm yr−1). In general, strike-slip faulting characterizes solutions for earthquakes within eastern Tibet, where mean elevations drop below 4500–5000 m, but the orientations of the strike-slip faults vary across the region. In central Tibet, left-lateral slip occurs on planes trending roughly northeast, but for earthquakes farther east, the orientations of that plane become progressively east-west and then southeast. This variation in orientation implies a rotation of material along curved left-lateral shear zones. Thus, the eastward extrusion of Tibet appears to be facilitated not only by rapid left-lateral shear, but also by large clockwise rotations of the material in eastern Tibet. The rate of eastward extrusion of material in eastern Tibet, relative to the Tarim Basin to its north, is roughly 30–40 mm yr−1. Fault plane solutions of earthquakes in the northern and eastern margins of Tibet show large components of thrust faulting, with the P-axes, oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau. The orientation of this crustal shortening is northeast-southwest on the northeastern margin, east-west on the eastern margin, and northwest-southeast in the Longmenshan on the southeastern margin. Thus, at least some of the extrusion of eastern Tibet out of India's northward path into Asia is absorbed by crustal shortening on the margins of the plateau. The variation from normal faulting in the high Tibetan Plateau, where elevations exceed 5000 m, to dominantly strike-slip faulting farther east where elevations are lower, and then to thrust faulting on the margins of the plateau, where elevations drop below 3000 m, surely results, at least in part, from a decrease in the value of the vertical stress: the magnitude of the east-west compressive stress need not vary across the plateau.
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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: We present a model for the propagation of seismic fracture on a heterogeneous fault on which the strength and applied stress vary locally along the fault plane in a random fashion. The propagation of fracture is controlled by three macroscopic parameters: the average of the difference between the applied stress and fault strength, a measure of fault heterogeneity and the average coupling between elementary faults. Evolution of fracture can then be represented by an epidemics model, whose domains of slip are obtained from percolation theory. The geometry of these domains contains the principal features of both barrier and asperity models.
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