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The P- and S-wave velocity structure of the D” layer beneath the southwestern Pacific was investigated by using short-period data from 12 deep events in the Tonga-Fiji region recorded by the J-Array and the Hi-net in Japan. A migration method and reflected wave beamforming (RWB) were used in order to extract weak signals originating from small-scale heterogeneities in the lowermost mantle. In order to acquire high resolution, a double array method (DAM) which integrates source array beamforming with receiver array beamforming was applied to the data. A phase-weighted stacking technique, which reduces incoherent noise by employing complex trace analysis, was also applied to the data, amplifying the weak coherent signals from the lowermost mantle. This combination greatly enhances small phases common to the source and receiver beams. The results of the RWB method indicate that seismic energy is reflected at discontinuities near 2520 km and 2650 km, which have a negative P-wave velocity contrast of 1 % at the most. In addition, there is a positive seismic discontinuity at a depth of 2800 km. In the case of the S-wave, reflected energy is produced almost at the same depth (2550 km depth). The different depth (50 km) between the P-wave velocity discontinuity at the depth of 2800 and a further S-wave velocity discontinuity at the depth of 2850 km may indicate that the S-wave velocity reduction in the lowermost mantle is about 2-3 times stronger that that of P wave. A look at a 2D cross section, constructed with the RWB method, suggests that the observed discontinuities can be characterized as intermittent lateral heterogeneities whose lateral extent is a few hundred km, and that the CMB might have undulations on a scale of less than 10 km in amplitude. The migration shows only weak evidence for the existence of scattering objects. Heterogeneous regions in the migration belong to the detected seismic discontinuities. These anomalous structures may represent a part of hot plume generated beneath the southwestern Pacific in the lowermost mantle.
Assuming that liquid iron alloy from the outer core interacts with the solid silicate-rich lower mantle the influence on the core-mantle reflected phase PcP is studied. If the core-mantle boundary is not a sharp discontinuity, this becomes apparent in the waveform and amplitude of PcP. Iron-silicate mixing would lead to regions of partial melting with higher density which in turn reduces the velocity of seismic waves. On the basis of the calculation and interpretation of short-period synthetic seismograms, using the reflectivity and Gauss Beam method, a model space is evaluated for these ultra-low velocity zones (ULVZs). The aim of this thesis is to analyse the behaviour of PcP between 10° and 40° source distance for such models using different velocity and density configurations. Furthermore, the resolution limits of seismic data are discussed. The influence of the assumed layer thickness, dominant source frequency and ULVZ topography are analysed. The Gräfenberg and NORSAR arrays are then used to investigate PcP from deep earthquakes and nuclear explosions. The seismic resolution of an ULVZ is limited both for velocity and density contrasts and layer thicknesses. Even a very thin global core-mantle transition zone (CMTZ), rather than a discrete boundary and also with strong impedance contrasts, seems possible: If no precursor is observable but the PcP_model /PcP_smooth amplitude reduction amounts to more than 10%, a very thin ULVZ of 5 km with a first-order discontinuity may exist. Otherwise, if amplitude reductions of less than 10% are obtained, this could indicate either a moderate, thin ULVZ or a gradient mantle-side CMTZ. Synthetic computations reveal notable amplitude variations as function of the distance and the impedance contrasts. Thereby a primary density effect in the very steep-angle range and a pronounced velocity dependency in the wide-angle region can be predicted. In view of the modelled findings, there is evidence for a 10 to 13.5 km thick ULVZ 600 km south-eastern of Moscow with a NW-SE extension of about 450 km. Here a single specific assumption about the velocity and density anomaly is not possible. This is in agreement with the synthetic results in which several models create similar amplitude-waveform characteristics. For example, a ULVZ model with contrasts of -5% VP , -15% VS and +5% density explain the measured PcP amplitudes. Moreover, below SW Finland and NNW of the Caspian Sea a CMB topography can be assumed. The amplitude measurements indicate a wavelength of 200 km and a height of 1 km topography, previously also shown in the study by Kampfmann and Müller (1989). Better constraints might be provided by a joined analysis of seismological data, mineralogical experiments and geodynamic modelling.