Roughness of the mantle transition zone discontinuities revealed by high-resolution wavefield imaging

摘要

We processed two independent data sets: 2,458,973 three-component seismograms from all EarthScope Transportable Array (USArray) stations and 141,080 pairs of high-quality receiver functions from the EarthScope Automated Receiver Survey using the generalized iterative deconvolution method and shaped the output with a 0.1Hz Ricker wavelet. We used these data as input to our 3-D plane wave migration method to produce an image volume of P to S scattering surfaces under all of the lower 48 states. Model simulations show that compared to the widely used common conversion point stacking method, our method is capable of resolving not only dipping discontinuities but also subtler details of amplitude and topography variations at scales near the diffraction limit. Maps of attributes derived from these discontinuities reveal several previously unobserved features of the 410 and 660 discontinuities (d410 and d660). Topography with many tens of kilometers is resolved at a range of scales. Additionally, we observed large variations of amplitude on the radial component and in the radial to transverse amplitude ratio that correlate with inferred variations in discontinuity topography. We argue that this combination of observations can be explained by roughness at a range of scales. The structural fabric we imaged is preferentially aligned orthogonal to the Farallon-North American relative motion. Two regions we infer to have exceptional roughness show spatial correlation with locations where tomography models indicate that the subducted slab is passing through the transition zone. We suggest that these rough regions are indicators of vertical mantle flows through the transition zone.