University of Connecticut, Department of Geology and Geophysics
Storrs, CT
06269-2045
+1-860-486-1930;
cormier@geol.uconn.edu
poster/oral:
Several models of D" heterogeneity are investigated by forward modeling their effects on high frequency precursors of PKIKP scattered by the heterogeneities. Two anisotropic distributions of scale lengths do not agree with observations: (1) longer horizontal scale lengths (200 km) than vertical scale lengths (20 km) and (2) longer vertical scale lengths (200 km) than horizontal scale lengths (20 km). Model (1) is suggested by intermittent wide angle reflections by apparent discontinuities above the CMB and by waveguiding of short-period P far beyond the core shadow inferred from long period P, but it predicts too low a frequency content to PKIKP precursors. Model (2) is suggested by anisotropy sometimes observed in long period S waves bottoming in D", but it under predicts the late precursor coda close to the PKP-B caustic. An isotropic distribution of scale lengths better predicts the observed precursor coda, but it still under predicts the late coda at distances greater than 140 deg unless 0.5 to 1 % heterogeneity with 10-50 km scale lengths extends at least 600 km above the core-mantle boundary.
(1) University of Connecticut, Department of Geology and Geophysics, Storrs, CT
06269-2045
+1-860-486-1930; cormier@geol.uconn.edu
(2) U S Geological
Survey, DFC, Box 25046, Stop 967,
Denver, CO 80225, USA
+1-303-273-8424;
choy@NEIS.CR.USGS.GOV
poster/oral:
Investigations of inner core anelasticity have thus far been frustrated by sparse results having high scatter , leading to simple frequency independent, average Q models for the whole of the inner core that are often difficult to reconcile with observations in the frequency band of free oscillations. New polar paths of PKIKP at long range and an increase in high-quality broadband digital waveforms makes it again worthwhile to investigate inner core anelasticity . Broadband displacement waveforms of PKIKP in the distance range 155 to 180 deg are being inverted for inner core anelasticity. PKP-AB is used as the reference waveform in this analysis, with a range of a priori constraints placed on possible D" anelasticity, which may more strongly affect the PKP-AB waveform. A grid search is made for the best fitting linear, visco-elastic, operator that converts inverse Hilbert tranformed PKP-AB to PKIKP. An observation first noted by Doornbos, that a relaxation spectrum centered in the body wave band predicts too long of a PKIKP rise-time, is confirmed by some broadband waveforms. Two possible models are being considered to fit these observations: (1) a relaxation spectrum in which the low frequency corner lies near or slightly above the frequency band of high frequency body waves, and (2) pulse broadening by stratigraphic attenuation possibly created by aligned lenses of partial melt. In either mechanism, the high scatter in real or apparent Q measurements of the inner core may be an effect of lateral heterogeneity in homologous temperature (T/Tm). If outer core convection in cylindrical rolls aligned near the axis of rotation leads to less efficient heat transport out of the inner core in the polar regions, then higher T/Tm in these regions may account for observations of increased attenuation along polar paths relative to equatorial paths.