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Ramesh Chandra Agrawal 《Geophysical Journal International》1978,53(3):459-465
summary . Approximately 1000 PKP observations in the range 110° < Δ < 170° have been statistically studied to examine the existence of Bolt's GH branch of the PKP travel-time curve. Data give firm evidence for the existence of this branch from 130 to 153° but the possibility of its extending further cannot be ruled out. Observations for this branch fit the form:
Where a =(1150.3254±0.5328)s, b = (16.4907±0.3803)s, c = (−0.1316±0.0630)s.
Times for this branch are always greater by 2 s than the corresponding Gogna times from Pacific earthquakes. 相似文献
Where a =(1150.3254±0.5328)s, b = (16.4907±0.3803)s, c = (−0.1316±0.0630)s.
Times for this branch are always greater by 2 s than the corresponding Gogna times from Pacific earthquakes. 相似文献
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We invert ISC PcP and PKP absolute and differential traveltimes in an attempt to infer the long-wavelength topography of the core-mantle boundary (CMB). The data selection and processing methods are described and evaluated. These travel-time data are very noisy and the geographic distribution of the data is highly non-uniform, inhibiting reliable inference of CMB topography. Spatial averaging enhances the coherent component of the residual variance (related to heterogeneity), however, the random component of the variance is much larger than the coherent component. We show that for PcP data the coherent signal due to mantle heterogeneity overshadows that arising from the CMB, and that the effects of mantle heterogeneity are mapped into our inferred CMB solutions. The PcP data are not correlated across the spatial averaging bins and seem to have a strong bias due to small-scale structure and/or noise. The non-uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the CMB. Spatial patterns of CMB models inferred from different phases do not agree. Amplitudes of seismically inferred CMB undulations vary greatly. The sensitivity of inferred CMB models to the processing, spatial averaging procedure, and inversion techniques are investigated. Topographic amplitudes increase strongly with increasing input residual variance. The power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher. Based on this work, we conclude that reliable inference of long-wavelength CMB topography is not likely with the current ISC data set or with a spherical harmonic expansion truncated to degree and order 6. 相似文献
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Lateral heterogeneity at the base of the mantle revealed by observations of amplitudes of PKP phases
Summary. Several studies have suggested that there are lateral heterogeneities in the velocity structure at or near the base of the mantle. Such heterogeneities can be studied through the analysis of amplitude ratios of core phases PKPAB and PKPDF for given earthquake-station pairs. In the epicentral distance range 155–175° these arrivals are well separated in time, and PKPAB has near-grazing incidence at the mantle—core boundary 80 that it is highly sensitive to lateral changes in velocity structure in that region, while PKPDF with its near-normal incidence is less sensitive. The observed amplitude ratios vary by well over an order of magnitude, but the size of the ratio is found to be correlated with the region of the core—mantle boundary sampled by the PKPAB rays: beneath the eastern Pacific and western Atlantic, for which abundant data are available, well-defined, contiguous regions (millions of square kilometres in extent) are found which correlated with predominantly larger or smaller amplitudes. We interpret this result as indicating differences in the degree to which the velocity structure of the lowermost mantle is heterogeneous. 相似文献
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