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  • 1
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We use new and pre-existing seismological data to study the neotectonics and the state of stress in the seismic zones of eastern Papua New Guinea and the western Solomon Sea regions. Earthquake hypocentres for the period 1964–90 were relocated, using the Joint Hypocentre Determination (JHD) technique. We show that the T axes of earthquakes in the 100–250 km depth range beneath eastern Papua New Guinea are aligned parallel to the seismic zone, with no seismicity below ≈250 km. This intermediate-depth seismic zone forms an inverted U-shaped zone, with limbs dipping to the north and south, and is interpreted to be seismicity along the subducted Solomon Sea plate. The northern and southern limbs of the seismic zone are connected by a flattened zone of seismicity which is marked by predominantly strike-slip, and a few normal, fault mechanisms. We suggest that the Solomon Sea plate at depth beneath the Finisterre mountains is no longer influenced by the tectonic forces acting at the surface, but breaking up and sinking under its own gravitational forces.A significant north-dipping seismic zone is imaged above the deeper seismic zone. This feature, characterized by thrust mechanisms, extrapolates to the surface along the Ramu Markham Valley and is attributed to the Ramu Markham Fault (RMF). The RMF has an average dip of about 40° to the north-east but its detailed geometry would appear to be complex. The RMF is interpreted to ramp steeply in the uppermost 10–20 km, flattening out sharply to a dip of ≈15° at a depth of about 20 km. The RMF marks the suture between the Finisterre Terrane and the Australia-New Guinea plate and may extend to a depth of 90 km beneath the western limit of the Finisterre mountains. There is a clear gap in seismicity between the RMF and the deeper Solomon Sea plate seismic zone beneath the Finisterre mountains, which, together with the alignment of the T axes of earthquakes along the seismic zone at depths 〉100 km, indicates that these are indeed two distinct features.The seismicity in the depth range 50–80 km, along the New Britain Arc, follows the trend of the 2000 m bathymetric contour of the New Britain Trench and the Finsch Deep far more closely than it does the trend of the surface plate boundary, indicating that the increase in the distance between the Solomon Sea/South Bismarck plate boundary and the active volcanos along the New Britain Arc, west of (6.8°S, 150.0°E) is only a shallow phenomenon, and does not reflect the geometry of the Solomon Sea plate at depths greater than 50 km.
    Type of Medium: Electronic Resource
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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: To date, most shear-velocity heterogeneity models in the lower mantle have been derived using long-period data. Comparatively little use has been made of the vast International Seismological Centre (ISC) data base of shear-wave arrival times. the aim of this study is to use the ISC P and S arrival times to construct global models of P and S heterogeneity in the lower mantle, and then to compare them in order to investigate whether, within the limitations of the data distributions, they might be proportional. the advantages of constructing both compressional- and shear-wave models from the one data set is that they share similar resolution properties, and hence are most reliable in the same areas.We use data from over 21000 events to derive a data set of P and S summary rays whose residuals we invert jointly along with hypocentral parameters of over 600 summary events. Particular attention is paid to data weighting so that outliers are not given undue influence. Furthermore, summary rays with high internal variances are downweighted.In order to diminish the effect of model parametrization on our conclusion, we derive three sets of P and S models expanded in terms of Legendre polynomials for their depth variation, and spherical harmonics for their lateral dependence. Comprehensive resolution and error analysis is performed.Correlation coefficients between our P and S models are highly significant, averaging approximately 0.7 for our lowest parametrization (245 model coefficients), and 0.5 for our more highly parametrized models. Visual comparisons show strong similarities in areas where resolution is high and error is small.We also conduct an experiment in which we derive compressional- and shear-wave heterogeneity models from data sets which contain P and S arrivals from the same seismograms. These data sets sample the mantle almost identically. the resultant models compare well but correlation drops towards the core-mantle boundary, indicating that there are genuine physical differences in the lowermost mantle.Our models indicate that the ratio of relative S to P heterogeneities is close to 2. This value is based on both our complete and restricted data set models and hence is not highly dependent on data weighting.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Splitting functions retrieved from spectra of the free oscillations are sensitive to the lateral variations in P velocity (α), S velocity (β), and density (ρ) simultaneously. In this study they are used to constrain the values of the ratios d ln α/d ln β and d ln ρ/d ln α for the lower mantle. Assuming that the upper mantle structure is obtainable from model M84A (this is not a crucial assumption as experiments indicate), the optimal value of d ln α/d ln β inferred from the modal data is 0.44 and d ln α/d ln β lies in the interval (0.39, 0.60) with 75 per cent confidence, strongly discriminating against the value (0.8) often used. The constraints on density structure of the current data are insufficient to yield new definitive results. The analysis demonstrates, however, that the value of d ln ρ/d ln α could be estimated from a larger set of modal data.
    Type of Medium: Electronic Resource
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