A degree 8 mantle shear velocity model from normal mode observations below 3 mHz (original) (raw)

1999, Journal of Geophysical Research

We present inversions for a new three-dimensional mantle v• model, MM2_L12D8, using a recently compiled catalogue of • 2300 normal mode structure coefficients for 90 multipiers below 3 mHz. These inversions demonstrate the capabilities and limitations of existing normal mode data and reveal new images of structures in the midmantle (900-1800 km depth), which is poorly resolved by surface wave and body wave data. Our inversions are distinguished both by efforts to maintain consistency with a variety of seismic models, and hence data sets, and by attempts to characterize the sensitivity of our model to the choice of damping, to unspecified structures, and to data errors. We find that sensitivity to damping is the dominant source of model uncertainty, but MM2_L12D8 proves to be a robust model of v• with amplitude uncertainties less than 35% for most depths and degrees. Other characteristics of MM2_L12D8 include X 2 misfit to normal mode structure coefficients which is 58% smaller than that of the best existing models, greater similarity to existing models than they have to each other, perturbations relative to existing vs models that are largest in the midmantle, and amplitudes that are most consistent with existing models that employ global, rather than local, basis functions. MM2_L12D8 also displays definite images of "slabs" and "plumes" in the midmantle and a spectrum of heterogeneity that is more continuous with depth than in most other models. These characteristics suggest that the midmantle participates in a very long wavelength pattern of circulation that involves at least the whole lower mantle. Inversions for Vp and p heterogeneities decorrelated from v• structure demonstrate that there is a significant signal from such structures in the normal mode data, but Vp and p models are much more sensitive to damping than are v• models. The normal mode catalogue must be expanded before normal mode models of Vp and p approach the reliability of the v• structures in MM2_L12D8. (This model, together with our catalogue of structure coefficients, is available at web site phys-geophys.colorado.edu/geophysics/nm.dir.) 1. Introduction 1.1. Motivation This paper describes inversions of a catalogue of normal mode structure coefficients below 3 mHz for a degree 8 model of shear velocity structure in the mantle. There are three motivations for this study. First, there are significant discrepancies among mantle vs models, even at the longest wavelengths. These structural differences include the amplitude and geometry of dynamically important features such as "slabs" and "plumes" in the lower mantle. Second, recent normal mode measurements are particularly sensitive to structures in the midmantle (900-1800 km depth), where the resolution provided by body wave and long-period surface wave data is poorest [Masters et al., 1996] and where discrepancies among existing models are largest. Several new sets of normal mode structure coefficients [Tromp