HR: 17:30h
AN: S32E-07    [PDF]
TI: Source and Receiver Effects on Rayleigh Waves
AU: * Ferreira, A M
EM: Ana.Ferreira@earth.ox.ac.uk
AF: Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Woodhouse, J H
EM: John.Woodhouse@earth.ox.ac.uk
AF: Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR United Kingdom
AB: Recent studies have greatly advanced knowledge of the three-dimensional structure of the upper mantle, but, up to now, the influence of source and receiver effects on the observed seismic waveforms has received little attention. In addition to the path, surface waves are also affected by source and receiver anomalous local structures and by uncertainties in the source mechanism and hypocenter coordinates. Separating each of these contributions to the seismic wavefield is a major challenge in seismic waveform tomography. We use full surface wave ray theory upon 3-D mantle model S20RTS combined with global crust model CRUST 2.0 to investigate these effects on Rayleigh waves. Since full ray theory allows one to express the seismic wavefield in terms of the product of a source term, a term due to propagation and a receiver term, it is a useful tool to study the relative influence of each of these terms. Besides, we use it in an iterative inversion procedure similar to the Harvard-CMT method to determine earthquake source parameters. We estimate that the influence of the local structure at the receiver on the vertical-component waveforms is negligible compared with the other terms. Likewise, the resulting phase perturbation due to local effects, both at the source and at the receiver, is small. However, theoretical amplitude anomalies due to the local structure at the source and to the deviation in takeoff azimuth of the ray leaving the source are more important than previously thought. These two types of anomalies have similar magnitudes and the resulting perturbation is as important as that produced by focusing and defocusing effects along the propagation path. Moreover, we investigate the impact of the ray theoretical forward modelling in characterising the seismic source of large, global events. Including the overall source anomaly into the modelling improves the fits of the theoretical seismograms to the data, and, hence, our ability to extract additional information from the available data. These effects should be taken into account in the next generations of tomography models.
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: 2003 Fall Meeting