Towards A Next Generation High Resolution Global Elastic 3D Model of the Whole Mantle: Constraining Vp/Vs and Density/Vs Scaling Factors

Abstract:

Global seismic tomography models have reached impressive levels of resolution and yet, several shortcomings remain. In particular, assumptions about how elastic moduli and density relate are often invoked through fixed scaling factors (R). In doing so, an inherent assumption is implied in the cause of heterogeneity. In this new generation model, we aim to address this by removing such assumptions and exploring the scaling factors themselves: a key factor in determining the source of heterogeneity when combined with insights from mineral physics. In order to construct a next generation global whole mantle model, using the global whole mantle radially anisotropic shear velocity model SEMUCB_WM1 (French and Romanowicz, 2014, 2015) as our starting model, we investigate the depth dependence relationship between seismic wave velocity (Vs and Vp) and density (r). The dataset used in the construction of SEMUCB_WM1 includes long period (>60 s) first and second orbit fundamental and overtone surface wavetrains and long-period (>30 s) body waveforms from 273 globally distributed events, sensitive primarily to Vs and x=(Vsh/Vsv)². We have complemented this database with new recent events, and shorter period body waveform data (> 18 s), in order to increase sensitivity to Vp, particularly in the lower mantle. We also consider existing catalogs of normal mode splitting functions, which have sensitivity to Vp and also to r, as well as the long-wavelength geoid. Forward modeling of waveforms is performed using the Spectral Element Method, and NACT (Non-linear Asymptotic Coupling Theory, Li and Romanowicz, 1995) for sensitivity kernel computation. Splitting functions are modeled in the framework of first order perturbation theory. Successive iterations of the model to constrain the depth variation of the scaling factor Rp = dlnVp/dlnVs, and then update Vs and x are performed using a mini-batch approach (Herwaarden et al., 2020). Meanwhile, various depth profiles for the scaling factor Rr = dln/dlnVs are considered, using constraints from the literature, so as to best fit the splitting function and geoid data. A preliminary updated whole mantle 3D model of Vs and x will be presented along with depth profiles of Rp and Rr, and discussed in the light of previous studies.