P Wave Tomography of Western Australia

Agus Abdulah, Cvetan Sinadinovski and B.L.N. Kennett

Geologically, Western Australia is characterized by four main geologic features: Pilbara, Bangamall, Capricorn and Yilgarn Cratons. Pilbara in the north and Yilgarn in the south are Achaean Cratons. These two cratons are separated by two younger sediments, they are Bangamall ‘Grenvillean' basin and Capricorn (early proterozoic orogen) [Betts et al., 2002]. This research is carried out to image the 3D structure of these geological features by using P-wave tomography technique.

There are about 45 seismic stations across Western Australia (Figure 1) which record local and broadband earthquakes with magnitudes between 3.0 and 7.0mb and depth between 1.0 and 600 km. This station-event pairs produce nearly 600 raypaths.

600 P wave traveltimes were hand picked and store it as data input for our tomography work. The tomography routine and the Fast Marching Method of Rawlinson et al. (2004 are used to trace seismic path in 1D model of ak135. Our area of study was discretised using grid node configuration with number of propagation grids are 30 x 46 x 55 for depth, latitude and longitude respectively.

The preliminary result of the tomography image at -15km of Western Australia is shown in Figure 2. The color represents P wave perturbation in percent relative to the ak135 model. Green represents high seismic speed and brown represents low seismic speed. The Achaean Cratons of Pilbara in the north and Yilgarn in the south are associated with high seismic speed anomaly, while the Bangamall and Capricorn are associated with low seismic speed.

Reference:

Betts P. G., D. Giles, G.S. Lister and L. R. Frick. 2002. Evolution of the Australian lithosphere, Australian Journal of Earth Sciences, 49 , 661–695

Rawlinson, N., and Sambridge, M., 2004. Wavefront evolution in strongly heterogeneous layered media using the fast marching method, Geophysical Journal International, 156, 631-647.