Global Upper-Mantle Tomography With the Automated Multimode Inversion of Surface and S Wave Forms

We apply the Automated Multimode Inversion (AMI) to a large global dataset, examine the accuracy of our techniques and assumptions, and compute an Sv-velocity model of the upper mantle (crust--660 km) using 61000 seismograms. Structure of the mantle and crust is constrained by waveform information from 306000 time-frequency windows with the fundamental-mode Rayleigh waves (periods from 20 to 400 s) and from windows with 19600 distinct higher-mode wavepackets (S and multiple S wave arrivals). We implement AMI with a 3D reference model; linear equations obtained from all the seismograms of the dataset are inverted for anomalies relative to the 3D reference, in this study composed of a 3D model of the crust and a 1D depth profile in the mantle. Waveform information is related to S- and P-velocity structure within approximate waveform sensitivity areas. Inverting for isotropic variations in S- and P-wave velocities, we also allow for S-wave azimuthal anisotropy---in order to minimize errors due to mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred km, varying with data sampling. We validate the imaging technique with a novel, "spectral-element" resolution test: inverting a global synthetic data set computed with the spectral-element method (Capdeville et al. 2003) through a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-Sv-velocity anomalies beneath mid-ocean ridges and back-arc basins extend down to ~100 km depth only; this corresponds to estimates of primary melt production depth ranges there. Seismic lithosphere beneath cratons bottoms at depths up to 200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighbored by volcanic areas near cratonic boundaries. The images of these low-velocity zones may be showing hot material---possibly of mantle-plume origin---trapped or spreading beneath the thick cratonic lithosphere.