Presentation by Dr. Carlos Moreno Chaves.
Illuminating the Mantle Transition Zone Complexities: Modeling the Influence of Mantle Heterogeneities on Teleseismic Converted Waves Using the Spectral Element Method
Abstract:
The depths of the olivine-to-wadsleyite and ringwoodite-to-bridgmanite phase transitions provide key constraints on the thermochemical structure of the mantle transition zone. In the Preliminary Reference Earth Model (PREM), these transitions are expressed as seismic discontinuities near 400 km and 670 km depth. The depths of these boundaries (d400 and d670) can be estimated from the traveltimes of P-to-S converted phases (P400s and P670s) in receiver function analyses, but reliable estimates require accurate corrections for the upper-mantle velocity structure.In this talk, I will demonstrate potential artifacts in receiver-function imaging of d400 and d670 using spectral-element waveform simulations of earthquakes arranged in a spiral around a rectangular seismometer array resembling USArray. The analysis applies a Common Conversion Point (CCP) stacking approach that mirrors a typical P-wave receiver function study. We explore several tomographic models of mantle vp and vs for traveltime-to-depth conversions. When ray theory is used to predict P400s and P670s traveltimes, the resulting receiver function images of d400 and 660 km show spurious undulations up to 10 km with wavelengths shorter than 200 km. Uncertainties in the tomographic models further amplify these undulations to as much as 20 km, depending on the vp and vs models used for both synthetics and traveltime calculations. Importantly, these artificial undulations are of the same order as the topographic variations of d400 and 660 km commonly reported in the literature. These findings highlight that receiver-function studies of the mantle transition zone must carefully account for the limitations of ray-theoretical traveltimes and the uncertainties in mantle velocity structure.
