Mathematics – Logic
Scientific paper
Feb 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...103.2633v&link_type=abstract
Journal of Geophysical Research, Volume 103, Issue B2, p. 2633-2672
Mathematics
Logic
60
Seismology: Core And Mantle, Seismology: Body Wave Propagation
Scientific paper
The arrival times of 10 compressional and shear phases (P,S,PP,PcP,SS,ScS,PKPab,PKPbc,PKPdf,SKSac) and two sets of differential times (SS-S410S and SS-S660S) are used to infer the Earth's three-dimensional structure. With the exception of SS-SdS, the raw data are composed of arrival times from the International Seismological Centre (ISC) catalogue. The ISC events have been relocated using P and S phases and the ak135 radial Earth model. A total of 818,655 summary arrival times constrain lateral velocity variations in the crust, mantle, and core. We also determine relocation parameters for all events, stations corrections for each phase, boundary topography (410 km, 660 km, core-mantle boundary, inner core boundary), and inner core anisotropy variations. Our model of P and S velocity variations in the crust and mantle contains the general features observed in other body wave inversions: a low-velocity ring around the Pacific Ocean basin in the depth range 35-400 km, high-velocity anomalies associated with subducting slabs most prominent in the depth range 660-1070 km, and high-velocity anomalies surrounding the Pacific basin just above the core-mantle boundary. In the outer core there is evidence of a latitudinal variation in velocity, approaching 0.6% in magnitude, with lower P velocities at the poles and higher velocities near the equator. This pattern is observed throughout the outer core but is most prominent in the depth range 3671-4071 km and near the inner core boundary. Synthetic tests indicate that outer core features are moderately resolvable and inversions of random arrival times indicates that the anomalies are a factor of 2 above the estimated parameter noise level. Lateral heterogeneity in the inner core trades off with anisotropy resulting in poor resolution of isotropic velocity variations. Anisotropy in the inner core is predominantly subparallel to the Earth's axis and of the order of 3% but this value can decrease if more inner core isotropic lateral heterogeneity is allowed. A comparison of structure above and below the core-mantle boundary (CMB) indicates that velocity heterogeneity is uncorrelated across the CMB, but there is a moderate correlation between CMB topography and outer core heterogeneity. An inversion in which the PKPdf residuals were absent indicates that the latitudinal pattern of outer core velocity variations is not due to the influence of inner core velocity structure.
Johnson Lane R.
Vasco D. W.
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