The long-wavelength (harmonic degree, l < 6) pattern of mantle transition zone thickness (WTZ) is well constrained globally by travel-time variations of stacked SS-precursors (underside reflections off the discontinuities at approximately 410 & 660 km depth). Other techniques (e.g. receiver functions) can image short-wavelength (< 500 km) variations in WTZ, but only for limited regions where data are available. The amplitudes of the discontinuity phases are low (often below the noise level), so stacking is necessary to obtain robust travel times and associated interface depths. Stacking effectively smooths/averages over short-wavelength variations. The large lateral extent of SS-precursor sensitivity kernels tends to further reduce the sensitivity to small-scale structure for traditional stacking methods.

In this study we apply an adaptive stacking method that maximizes resolution where data are available. In addition to stacking the SS-precursor waveforms, we stack the associated finite-frequency Frechet kernels with sensitivity to discontinuity topography [Dahlen, 2005]. We then invert the stacked kernels with the travel times of the stacked waves for transition zone thickness. The method may be thought of as an additive algorithm analogous to the differencing algorithm of multi-channel cross-correlation. Data stability is ensured by the bootstrap error estimation. While the technique is computationally expensive relative to traditional stacking and inversion techniques, the results yield finer-scale resolution in regions with good data coverage.

The resultant transition zone thickness models possess narrow curvilinear regions of anomalously thick WTZ down-dip of subduction zones and various different sized regions of thinned WTZ beneath hotspots. The curvilinear regions near subduction zones are larger in vertical extent and narrower in horizontal extent than the equivalent long-wavelength anomalies imaged by previous stacking methods. The observation of horizontally smaller subduction-related transition zone thickening effectively decreases the upper volumetric limit of cool oceanic lithosphere currently stored within the transition zone.