摘要:

The correspondence between free oscillations and surface waves of a laterally heterogeneous earth is established. Starting from the Born approximation of Woodhouse, for large angular order, coupling between neighbouring multiplets along the same dispersion branch leads to the apparent distance shift noted by Woodhouse and Dziewonski, which they obtained from a surface wave approach.

ISSN:0094-8276    

DOI:10.1029/GL013i013p01478

年代:1986

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL013i013p01482

年代:1986

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL013i013p01484

年代:1986

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL013i013p01487

年代:1986

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL013i013p01488

年代:1986

数据来源: WILEY

摘要:

Lay and Helmberger and their co‐workers have recently proposed models of Earth structure in which the S‐velocity increases discontinuously with depth by about 3% at the bottom of a postulated region of abnormally low (near zero) gradient between depths of about 2300 and 2600 km in the lower mantle. Such models involve drastic departures from previously proposed models of Earth structure in this region. By use of Kirchhoff wave theory it is shown that the pertinent data may be interpreted alternatively as an effect of wave propagation in an inhomogeneous lower mantle. On this alternative interpretation the S‐velocity need nowhere sharply increase with increasing depth at any level in the lower m

ISSN:0094-8276    

DOI:10.1029/GL013i013p01489

年代:1986

数据来源: WILEY

摘要:

Teleseismic recordings of direct S and sS body wave phases, and their core‐reflected counterparts ScS and sScS, from intermediate and deep focus earthquakes are used to analyze the lowermost mantle shear velocity structure beneath Alaska. A model with a 2.75% shear velocity increase 280 km above the core‐mantle boundary accurately matches waveform complexities in both the S and sS wavetrains. Variations in source depth produce systematic shifts in the timing of the triplication arrivals between the S and sS travel time branches that are readily observed in long‐period WWSSN tangential component recordings. The systematic range‐ and depth‐dependence of the observed shifts are well‐predicted by the discontinuity model, and preclude explanations of the waveform complexity as resulting from multiple ruptures at the source, receiver reverberations, or near‐source scattering from s

ISSN:0094-8276    

DOI:10.1029/GL013i013p01493

年代:1986

数据来源: WILEY

摘要:

Maps of aspherical structure in the vicinity of the core‐mantle boundary have been constructed from a tomographic analysis ofP′DFandP′ABtravel times in the distance range 150°‐180°. These maps reveal long‐wavelength features not previously detected by mantle tomography. The peak‐to‐peak amplitude of these features is on the order of 1 s in vertical travel time, too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core‐mantle boundary. We hypothesize the existence of one or more chemical boundary layers whose large‐scale accumulations on the surface of the core may in some ways be analogous to continents at

ISSN:0094-8276    

DOI:10.1029/GL013i013p01497

年代:1986

数据来源: WILEY

摘要:

Short‐period PcP amplitudes have been observed to vary by a factor of about 20 in the epicentral range 40‐85°. This behavior is due to the reflection coefficient at the core‐mantle boundary, which varies from a maximum of about 0.5 at an angle of incidence of about 65° to zero at an angle of incidence of about 85° (corresponding to an epicentral ranges of about 60‐90°). We present physical model experiments in which we study the effect of corrugations of the core‐mantle boundary on PcP amplitudes. We find that corrugations with 2‐50 km scale lengths would radically alter the amplitude‐range behavior of short‐period PcP, reducing its variation with range. The observed pattern of short‐period PcP amplitudes indicates that the core‐mantle boundary is smooth on these scales, with amplitudes less th

ISSN:0094-8276    

DOI:10.1029/GL013i013p01501

年代:1986

数据来源: WILEY

摘要:

We have mapped shear velocity variations near the base of the mantle with ScS‐S differential travel times. Beneath the North Atlantic, there is a systematic variation in velocity over a distance of 1500 km or more, with a pronounced low velocity anomaly centered roughly beneath Madeira island. Beneath the northern Indian Ocean, there is no similar long‐wavelength anomaly. The average ScS‐S residual beneath the Indian Ocean is 3.6 ± 0 .7 s more negative than the Atlantic residual, indicating faster average velocities near the base of the

ISSN:0094-8276    

DOI:10.1029/GL013i013p01505

年代:1986

数据来源: WILEY