Toward a Comprehensive Understanding of Transition Zone Seismic Discontinuities: Part I. New Constraints from Receiver Function Forward and Backward Scattering Waves
Abstract:Transition zone discontinuities, among all, hold the key to resolve the mystery of mass and heat transport in the Earth’s mantle and the composition of the Earth’s interior. In previous efforts, the data are limited to either upper mantle triplications, converted waves or mantle reflections (e.g. SS precursors, ScS reverberations). When multiple datasets are jointly analyzed, they are often restrained at relatively long period (~ 0.1 Hz). To complement previous efforts, we advocate a simple and effective strategy to tackle a number of seismic observables altogether. Specifically, we involve broadband direct converted waves (e.g., P410s, P660s) and the topside reflections (the multiples, e.g., PpP410s, PpP660s) in the context of P wave receiver function technique. Such a tactic not only minimizes tradeoffs between velocity and density jumps, but also allows a superior resolution on the sharpness of the boundary and a detailed description of transition zone discontinuities.
Here we summarize our first attempt in the region of stagnant slab beneath Chinese continent. We processed waveforms from 1000 stations of the Chinese seismic array using an automatic scheme to remove noisy waveforms and retained close to ~300,000 high quality receiver functions in the L-Q-T coordinate system. While avoiding interferences from other mantle waves, we perform slowness stacking of direct converted waves and the multiples, respectively, at several discrete frequency bands between 0.05 Hz and 1Hz and obtain amplitude estimates and uncertainties through the bootstrap method. To properly calibrate the amplitudes of receiver functions, we take into account the effect of incoherent stacking due to discontinuity topography and frequency-dependent attenuation. Our findings indicate that the 410 is a sharp boundary with a small density jump (<< 5 km, ΔVs=5-6%, Δρ=1.5-2%), but there is no significant gradient near the sharp transition. While the 660 is best described by a sharp boundary and a moderate density jump (<< 5 km, ΔVs=3.5-4%, Δρ=4-4.5%), there is a substantial gradient near the boundary (> 3x10-3 km/s/km, > 3x10-3 g/cm3/km). We will discuss the implications of these new findings in the context of thermochemical state of the transition zone in a separate presentation (Song et al. S034: Theory of Earth, this meeting).