Open AccessGlobal Earth structure: inference and assessment
Author(s) -
Vasco D. W.,
Johnson Lane R.,
Marques Osni
Publication year - 1999
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1046/j.1365-246x.1999.00823.x
Subject(s) - geology , classification of discontinuities , mantle (geology) , geodesy , lanczos resampling , seismology , geophysics , mathematics , physics , mathematical analysis , eigenvalues and eigenvectors , quantum mechanics
Summary A suite of compressional and shear phase arrival times ( P , S , PP , PcP , SS , ScS , PKPab , PKPbc , PKPdf , SKSac ) and two sets of differential times ( SS − S 410 S , SS − S 660 S ) are used to infer lateral variations in P and S velocity structure in the entire Earth (crust, mantle and core). Volumetric heterogeneity is parametrized by 22 depth layers, each of which is subdivided into a grid of cells. In addition to volumetric hetero‐geneity, we determine topography on the major internal velocity discontinuities, station corrections, event relocation parameters and inner core anisotropy, a total of 96 300 unknowns. The model parameters are estimated using an iterative Lanczos algorithm to obtain a partial singular value decomposition. A total of 5000 Lanczos values and vectors are used to construct model parameter estimates as well as measures of model parameter resolution and covariance. The Lanczos resolution estimates provide lower bounds on conventional SVD‐based measures. Throughout most of the mantle we find that regions beneath the continents, particularly in the Northern Hemisphere, are well resolved. With the exception of Africa, velocity variations are moderately well resolved in that portion of the outer core lying beneath the continents. Velocity heterogeneity is poorly resolved in the inner core where cell dimensions are small and deviations in velocity heterogeneity trade off with variations in anisotropy. Variations in the uppermost mantle (2–3 per cent) are correlated with surface tectonics, while at greater depths high‐velocity variations coincide with regions of past and present subduction. The high‐velocity anomalies extend into the lower mantle, although with a reduced amplitude (0.5–1.0 per cent) until the lowest mantle (2.5 per cent). At the base of the outer core a large‐scale pattern of heterogeneity (0.8 per cent) is observed, slower at the poles. In the inner core we infer a peak anisotropy of 2–3 per cent which varies laterally and with depth. However, the magnitude and distribution of the anisotropy trades off strongly with velocity heterogeneity.