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Seismic attenuation of the inner core: Viscoelastic or stratigraphic?
Author(s) -
Cormier Ver F.,
Xu Li,
Choy George L.
Publication year - 1998
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/1998gl900074
Subject(s) - attenuation , scattering , perpendicular , viscoelasticity , physics , anisotropy , rotation (mathematics) , optics , oscillation (cell signaling) , inner core , shear waves , computational physics , geology , shear (geology) , geometry , geophysics , chemistry , thermodynamics , petrology , biochemistry , mathematics
Broadband velocity waveforms of PKIKP in the distance range 150° to 180° are inverted for inner core attenuation. A mean Q α of 244 is determined at 1 Hz from 8 polar and 9 equatorial paths. The scatter in measured Q −1 exceeds individual error estimates, suggesting significant variation in attenuation with path. These results are interpreted by (1) viscoelasticity, in which the relaxation spectrum has a low‐frequency corner near or slightly above the frequency band of short‐period body waves, and by (2) stratigraphic (scattering) attenuation, in which attenuation and pulse broadening are caused by the interference of scattered multiples in a velocity structure having rapid fluctuations along a PKIKP path. In the scattering interpretation, PKIKP attenuation is only weakly affected by the intrinsic shear attenuation measured in the free‐oscillation band. Instead, its frequency dependence, path variations, and fluctuations are all explained by scattering attenuation in a heterogeneous fabric resulting from solidification texturing of intrinsically anisotropic iron. The requisite fabric may consist of either single or ordered groups of crystals with P velocity differences of at least 5% and as much as 12% between two crystallographic axes at scale lengths of 0.5 to 2 km in the direction parallel to the axis of rotation and longer in the cylindrically radial direction, perpendicular to the axis of rotation.