Physics
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999e%26psl.173..101k&link_type=abstract
Earth and Planetary Science Letters, Volume 173, Issue 1-2, p. 101-112.
Physics
4
Scientific paper
The amplitudes of diffracted SH (Sdiff) normalized to SKS, together with the Sdiff-SKS times, were analyzed to constrain the structure of the D'' region beneath Asia and the northernmost Indian Ocean. While the Sdiff-SKS residuals (/δt relative to the Preliminary Reference Earth model, or PREM) are consistently negative from 95° to 120°, the amplitude residuals of Sdiff/SKS (/δA) show two trends of distance dependence, corresponding to distinct seismic structures in two adjacent zones in D''. In zone A, /δA increases significantly with distance, suggesting the presence of a negative velocity gradient in the base of the mantle. The travel time residuals independently require that the average velocity of zone A be faster than that of PREM. One-dimensional structures that reconcile both sets of constraints were sought through systematic forwarding modeling. Models with negative gradients that satisfy /δt's match /δA's to an acceptable degree only if a high-quality factor (Qs) is assumed. The preferred model for zone A has a 400-500 km thick negative gradient layer, with a ~4% velocity discontinuity at the top and Qs = 1000, an about three-fold increase from the PREM value. In zone B, the amplitude-distance curve is virtually flat, and a 200-300 km thick high-velocity layer with PREM-like gradient and Qs explains both observations well. To assess the role of mantle asphericity in /δA, we estimate the strength of focusing of the S waves into the Fresnel zone at the onset of diffraction in vertical cross-sections of 3-D tomographic models SAW12D and SKS12WM13. Both models predict stronger focusing in zone A than in zone B. The focusing effect is translated to a positive base-line shift in /δA, which, if applied to the model predictions, alleviates the need for an extremely high Qs in zone A. The simple 2-D experiment suggests that velocity gradient and the anelastic attenuation of the D'' layer as well as the mantle heterogeneity all probably contribute to the decay characteristics and the level of amplitude of Sdiff. The slab subducted in the Mesozoic may be responsible for the structure depicted in this study.
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