One‐dimensional physical reference models for the upper mantle and transition zone: Combining seismic and mineral physics constraints

Abstract

One‐dimensional seismic reference models are known to be a nonunique solution to global seismic data, hampering an interpretation in terms of physical structure. Here we test the compatibility of the simplest hypothesis of a mantle convecting as a whole, with a constant pyrolitic composition with phase transitions, directly against the kind of seismic data that went into global seismic reference models, focusing on upper mantle structure down to 800 km depth. By randomly varying the elastic and anelastic parameters of the main mantle minerals within their uncertainty bounds, we generate a set of 100,000 adiabatic, pyrolitic models. A small number of these models (<0.1%) give a fit to far‐regional P and S travel times (reprocessed ISC catalog, Δ = 18.5°–26°) and fundamental spheroidal and toroidal modes (reference Earth model Web page, l > 60) that is as satisfactory as the fit of preliminary reference Earth model (PREM) or AK135(‐F). Although the accepted models have widely different combinations of the mineral parameters, there is a preference for a relatively high olivine shear modulus and its pressure and temperature derivatives, relatively low wadsleyite bulk and shear parameters, and relatively high ringwoodite bulk modulus derivatives. The resulting seismic profiles are very similar and, compared to PREM or AK135, have lower velocities above 400 km, larger jumps near “410,” lower transition zone gradients, lower jumps around “660,” and stronger gradients directly below. Such physical models that fit seismic data well enough to be useful as a seismic reference model can significantly facilitate physical interpretation of seismic structures.