IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS015 Oral Presentation 2330
Seismic imaging of the lithosphere beneath southeast Australia using data
from multiple array deployments
Dr. Nick Rawlinson
Research School of Earth Sciences Australian National University
Brian L. N. Kennett
Over the past five years, the Research School of Earth Sciences at the Australian National University has
pursued a vigorous campaign of passive seismic array deployments to substantially increase the high
density data coverage of the lithosphere beneath southeast Australia. To date, a total of eight separate
seismic arrays have been deployed for periods ranging between 5-8 months in Tasmania, Victoria,
eastern South Australia and southern New South Wales. When combined, this cluster of adjacent arrays
comprises nearly 350 recorders (mostly short period), with station spacings ranging between 15-50 km.
With the long term goal of covering much of eastern Australia at high density for the purposes of
passive seismic imaging, including teleseismic and ambient noise tomography, this ambitious project has
the potential to make a significant contribution to seismology and the understanding of the Australian
continent.In this presentation, we will focus on results from several recent studies that have been
undertaken using data from the multiple seismic arrays in southeast Australia. The SEAL (2004-2005)
and EVA (2005-2006) experiments, which involved two adjacent deployments in eastern Victoria and
southern New South Wales, yielded high quality teleseismic data that have been mapped as 3-D
variations in P-wavespeed. This was done using a new iterative non-linear tomography scheme which
makes use of a robust grid based wavefront tracking technique to solve the forward problem of data
prediction, and a subspace inversion method to reconcile the predictions with observations by adjusting
the model parameters. Analysis of the tomographic images reveals a distinctly faster upper mantle
beneath the Delamerian Orogen compared to the western Lachlan Orogen which lies to the east. This
suggests that the former is underlain by Proterozoic lithosphere, and the latter by Palaeozoic
lithosphere. One advantage of having high density passive seismic data spanning a large region is that it
has the potential to be combined with more focused active source experiments. For example, the island
of Tasmania, which lies at the southern tip of southeast Australia, hosted a large 3-D crustal refraction
and wide-angle reflection experiment in 1995, which comprised some 44 land-based stations and
36,000 marine airgun shots. Recently, this dataset was combined with teleseismic arrival time data from
the 2002 TIGGER experiment of Tasmania in a simultaneous inversion for 3-D lithospheric P-wavespeed
and Moho geometry. In contrast to previous results from separate inversions of the active and passive
source datasets, the new images reveal a zone of elevated wavespeed beneath the Cambrian Mt. Read
Volcanics, and indicate that both crustal thinning and elevated wavespeeds occur beneath northeast
Tasmania, which supports the case for the existence of a prior passive margin.
Keywords: seismic tomography, australia, lithosphere