This page outlines a study of seismic anisotropy in the mid-mantle around the Tonga-Kermadec subduction system, using modelling and data analysis of shear-wave splitting, done as a part of my PhD research.
|Map showing locations of events (red circles), stations (blue triangles) and iasp91 (Kennett and Engdahl, 1991) predicted raypaths in the lower mantle. Also shown is plate motion in the hotspot reference frame (grey arrows) and plate boundaries (Gripp and Gordon, 1990), topography/bathymetry from (Smith and Sandwell, 1997) and depth of seismicity of the Tonga, Kermadec and New Hebrides subduction zones (Gudmundsson and Sambridge, 1998).|
|Example of Silver and Chan (1988) dt/phi grid search to measure shear-wave splitting.|
|Example model of mid-mantle anisotropy due to an upwelling in the uppermost lower-mantle beneath the descending Tonga slab (Gurniss et al, 2000). The colour scale denotes the (vertical) shear wave velocity and the black vectors the symmetry axis of the imposed transverse isotropic media. The white lines are example raypaths.|
|Shear wave splitting analysis of an event at station NWAO, Australia. The analysis window is shown by the vertical dashed lines. This analysis shows a well constrained splitting result with rather convincing correction of transverse energy (panel D) and particle motion (panel C). The best-fitting shear wave splitting parameters were determined to be dt=6.3+/-0.35s and phi=-46+/-4deg. The seismograms were filtered using a bandpass filter with corner frequencies of 0.02 and 0.15Hz.|
|Shear wave splitting time versus epicentral distance for real data and various models of mid-mantle anisotropy.|