HR: 0830h
AN: S31E-0800    [PDF]
TI: Inversion of Surface and S Wave Forms for Seismic-Velocity Perturbations Relative to a Three-Dimensional Reference Model
AU: * Lebedev, S
EM: sergei@geo.uu.nl
AF: Utrecht University, Earth Sciences, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: van der Hilst, R D
EM: hilst@mit.edu
AF: MIT MIT, EAPS, Cambridge, MA 02139 United States
AB: Waveform inversions of surface and S waves can be formulated so as to directly constrain average seismic-velocity anomalies along the path. In the Automated Multimode Inversion scheme of Lebedev and Nolet (2003), waveform phase information is extracted from time-frequency windows uncontaminated by scattered waves and put in the form of independent linear constraints (with uncorrelated uncertainties) on seismic-velocity anomalies in the Earth. In this and similar schemes, waveforms are inverted for path-averaged perturbations, implying that lateral variations of the (Frechet) derivatives of phase velocities with respect to seismic velocities have to be assumed negligible within the waves' sensitivity volumes. This assumption is only valid within certain limits (generally different for every source-station path). For example, high heterogeneity in the Earth's crust will normally prohibit using signal at frequencies too high or modelling structure too shallow. Here we re-formulate the averaging scheme so as to expand the field of validity of the approximation, then map the limits of that field, and then use the new scheme in order to compute an upper-mantle model of Asia and surroundings. Our initial reference model is the 3-D crustal model CRUST2 (Laske, 2001) atop a laterally homogeneous mantle model. For every point of the model grid on the surface we compute phase velocities of surface-wave modes and their Frechet derivatives. For each source-station path, we then compute the reference phase velocities and the average derivatives as integrals over an estimated sensitivity area, the latter averaged over the frequency band of the inversion. Waveform inversions of tens of thousand seismograms produce linear equations which are inverted together for a 3-D model of the crust and upper mantle. Taking this 3-D model as a new reference, we repeat the whole procedure. More iterations follow and continue until convergence to a final 3-D model. We shall discuss selected features of the new model in the presentation.
DE: 7218 Lithosphere and upper mantle
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting