Amplitude of Sdiff across Asia:: effects of velocity gradient and Qs in the D" region and the asphericity of the mantle

The amplitudes of diffracted SH (S-diff) normalized to SKS, together:with the S-diff-SKS times, were analyzed to constrain the structure of the D " region beneath Asia and the northernmost Indian Ocean. While the Sdiff-SKS residuals (delta t; relative to the Preliminary Reference Earth model, or PREM) are consistently negative from 95 degrees to 120 degrees, the amplitude residuals of S-diff/SKS (delta A) show two trends of distance dependence, corresponding to distinct seismic structures in two adjacent zones in D ". In zone A, delta A 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 delta t's match delta A's to an acceptable degree only if a high-quality factor (Q(s)) is assumed. The preferred model for zone a has a 400-500 km thick negative gradient layer, with a similar to 4% velocity discontinuity at the top and Q(s) = 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 Q(s) explains both observations well. To assess the role of mantle asphericity in delta A, 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 delta A, which, if applied to the model predictions, alleviates the need for an extremely high Q(s) 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 S-diff The slab subducted in the Mesozoic may be responsible for the structure depicted in this study. (C) 1999 Elsevier Science B.V. All rights reserved.