HR: 1340h
AN: T33A-1333    [Abstracts]
TI: Fresh Insights into Surface Wave Tomography - Applications to the Australian Region
AU: Fishwick, S
EM: stewart@rses.anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: Procko, K
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: * Kennett, B L
EM: Brian.Kennett@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AB: Strong variations in shear wavespeed in the Australian region are apparent from both global and regional studies and pose particular difficulties for successful surface wave tomography in capturing both the larger and smaller scale variations. A single pass inversion with model-norm damping will result in an underestimate of the small-scale features if the knee of the trade-off curve is used. However, a multi-scale inversion scheme is very effective in the construction of a 3-D shear wavespeed model from the suite of path specific 1-D models, viewed as averages along the path. With a smooth B-spline representation of the velocity variation at each depth the knot spacing can be adapted to allow refinement of scales in the inversion. We have found that an initial pass to retrieve large scale structure with an 8 degree knot spacing, and then a second pass with 2-degree knot spacing provides the best recovery of amplitude anomalies. The multi-scale nature of the wavespeed variations means that care needs to be taken with assessment of resolution, since a conventional chequer-board approach can only focus on one scale at a time. This limitation can be overcome by using a superposition of scale elements in the models constructed for resolution tests with smaller scale systematic variations superimposed on larger block features to provide a direct test of the influence of path coverage in model recovery. Inversion for azimuthal anisotropy is best performed at a moderate scale, e.g., with a 4-degree knot spacing. The observed changes in the direction of anisotropy beneath the continent between 100 and 200 km are consistent with a change from frozen to dynamically induced anisotropy. Surface wave tomography has concentrated on the recovery of shear wavespeeds, with simplifying assumptions about the influence of density. We are employing three-dimensional simulation using the SpeCFEM code of Komatitsch & Tromp as a means of investigating the characteristics of the interaction of seismic surface waves with realistic structures, and the way in which different assumptions in the tomographic inversion will affect the resultant images.
DE: 8180 Tomography
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting