Joseph S. Resovsky

New and refined constraints on three‐dimensional Earth structure from normal modes below 3 mHz

We present the results of generalized spectral fitting (GSF) regressions which estimate normal mode structure coefficients for the observable spheroidal and toroidal free oscillation multiplets below 3 mHz. The size, accuracy, and precision of our new catalogue of modal constraints make it a powerful new tool for assessing and refining three-dimensional Earth models. The estimates include more than 3100 coefficients for 90 multiplets and 25 pairs of coupled multiplets, including several deep mantle overtones previously obscured by fundamental modes. The coefficients constrain mantle structures of both even and odd spherical harmonic degrees, through degree 12 in some cases. Improvements in accuracy and precision have been achieved with three innovations: the development of GSF, an enhancement of the established spectral fitting technique which incorporates both Coriolis and structural coupling between multiplets; the application of GSF to an edited, high signal-to-noise and geographically diverse data set of more than 4500 seismograms from 33 high moment earthquakes; and the assignment of coefficient uncertainties using a Monte Carlo method to simulate the effects of seismic noise, theoretical errors, and coefficient covariances. The results of GSF are assessed by examining the internal consistency of estimated coefficients and through comparisons with recent mantle models. The new catalogue of structure coefficients and uncertainties is available as an electronic supplement to this paper and through the University of Colorado internet site.

Using probabilistic seismic tomography to test mantle velocity–density relationships

We use a neighborhood algorithm to explore the fit to long period seismic data of a wide variety of long wavelength mantle models. This approach to the global tomographic inverse problem yields probability distributions for seismic velocities, density, and related properties as functions of depth. Such distributions can be robust even when individual models are not, and allow us to test several assumptions about the Earth that have long been enforced a priori in inversions. In particular, we are able to test the paradigm ofdeep mantle heterogeneity that is dominantly thermal in origin, producing velocity and density perturbations that are well correlated and have relative amplitudes given by Nlnb/Nlnvs 6 0.5. Our distributions show that such relationships are unlikely, and even though the results are consistent with recent best fitting models from damped seismic inversions, they demonstrate that many specific properties ofsuch models are not robust. The data clearly f density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger (yielding Nlnb/Nlnvs s 1.0) than damped inversions allow. These characteristics are most pronounced in the upper mantle transition zone and the base ofthe lower mantle, suggesting layered convection. The negative density^velocity correlations f at these depths imply dominantly chemical heterogeneity, while the likelihood ofrelatively high amplitude density variations suggests that variable iron content is an important component ofthis heterogeneity. These results, which we show to be consistent with independent gravity constraints, represent a profound change in the interpretation of seismic constraints. In addition, the distributions show that even though best fitting density models from recent inversions or our sampling are consistent with the data, most specific properties of such models are not robust. This implies that it is more appropriate to use seismic model distributions, rather than individual models, to make geodynamic and geochemical inferences.

The feasibility of normal mode constraints on higher degree structures

Case studies of four multiplets (υs-sensitive 1S7 and 1S8 and υp-sensitive 5S5 and 5S6) are considered to test the accuracy of normal mode constraints on aspherical structure at degrees above 2 (4,6,8). Analyses of misfit, along-branch consistency, and consistency with existing mantle models argue for the feasibility of constraints on higher structural degrees. Preliminary interpretation indicates a relative insensitivity of data misfits to dlnυp/dlnυs at degrees 6 and 8 in the lower mantle (perhaps due to inaccuracies in current δυs models), and provides weak evidence of some decorrelation between δυs and δυp in the middle and lower mantle.