Lithospheric structure of Tasmania from a novel form of teleseismic tomography

Abstract

[1] In 2001 and 2002, a temporary array of 72 seismic recorders was deployed across northern Tasmania (SE Australia), with the aim of imaging the underlying crust and upper mantle using three-dimensional (3-D) teleseismic tomography. Using a recently developed adaptive stacking technique, 6520 relative P wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period. A novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method, a grid-based eikonal solver, is used to map the residual patterns as P wave velocity anomalies. The new scheme proves to be both fast and robust, making it well suited to large data sets and the reconstruction of complex anomalies. The resultant tomographic images of Tasmania exhibit significant lateral perturbations in P wave velocity structure ( 5%) from a 1-D reference model. A marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern Tasmania is underlain by dense rocks with an oceanic crustal affinity, contrasting with the continentally derived siliciclastic core of western Tasmania. Significantly, the Tamar Fracture System does not overlie the narrow transition from relatively fast to slow velocities, which suggests that it may be a near-surface feature rather than a manifestation of deeper crustal-scale suturing as previously thought. Farther west, an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group and Arthur Lineament may be related to remnant subduction of oceanic lithosphere associated with the mid-Cambrian Delamerian Orogeny.

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

“…Therefore, the number, size, position and shape of the cells are implicitly controlled by the data, in which the noise is also treated as unknown in the inversion. This provides a parsimonious trade-off between data fit and model complexity (Bodin et al, 2012b), a more sophisticated pursuit than conventional ad hoc regularisation, predominantly utilised by linearised inversion techniques (Rawlinson et al, 2006;Saygin and Kennett, 2010;Pilia et al, 2013). The principal issue with locally linearised problems around a reference model is that, in general, there is no guarantee as to whether we appropriately tune the parameters (usually damping and smoothing), which can ultimately lead to misinterpretation (Bodin et al, 2012b).…”

Constraints on North Anatolian Fault zone width in the crust and upper mantle from S-wave teleseismic tomography

“…In addition, all residuals with a discrepancy between observed and predicted values greater than 0.5 s after an initial inversion, were removed to improve the final model. To perform the tomography, we use the Fast Marching Teleseismic Tomography code (Rawlinson et al [2006]), an iterative method based on subspace inversion (Kennett et al [1988]) and the Fast Marching Method (Sethian [1999]) to compute arrival times through the laterally heterogeneous model volume. Traveltimes from the source to the boundary of the local model volume are based on ak135 predictions.…”

On the use of sensitivity tests in seismic tomography

“…The constant terms (≥0) and η (≥0) are the damping and smoothing parameters respectively, which govern the trade-off between data fit, model smoothness, and model perturbation relative to the starting model. The model perturbation given by a single iteration of the subspace method is defined by (see Rawlinson et al 2006, for more details):…”