Shear wave splitting analysis beneath Australia:
Influence of the Tasman Line on the formation of the continent
M. Heintz
Measurements of shear wave splitting have been previously performed by
Clitheroe and Van der Hilst (1997) [1] for data recorded at some stations
deployed within the framework of the SKIPPY project, and also at the
permanent stations located in Australia (see also [2] for CAN). The
anisotropy pattern under Australia appears to be very complex by comparison
with other continents. There is significant variation in the magnitude
of splitting (and hence, anisotropy) moving north-to-south along the strike
of subduction.
Moreover, although shear wave splitting measurements performed on SK(K)S
phases at the permanent stations CAN, WRAB, TAU, NWAO and CTAO reveal apparent
isotropy, measurements performed on S phases reveal the presence of anisotropy.
Despite the extensive deployments of portable stations during the
successive SKIPPY, KIMBA, QUOLL, WACRATON, TIGGER and the current
Tasman Line experiments, no systematic study of shear wave splitting
using SK(K)S, PK(K)S and direct S phases
has yet been carried out for the full continent.
Despite the rather good distribution of seismic sources surrounding the
Australian continent, most sources are quite close to
the continent and rather few events are in the distance range
between 85° and 150°, required for shear wave splitting measurements on
SK(K)S phases.
Therefore, direct S phases will be also taken into account (fig. 1),
although the interpretation of the measurements performed on such phases
is not as easy as the one performed on SK(K)S phases.
The variation of the splitting parameters with respect to the backazimuth
will be systematically checked, seeking for the presence of two or more
layers of anisotropy or inclined symmetry axis, and the results of the
anisotropic tomography performed on the continent [3, 4] could be used as
constrain to model systems with several layers of anisotropy.
(a)
(b)
(c)
(d)
Figure 1: Example of shear wave splitting measurement for direct S.
Event: 1993 day 145 13.49S 167.12E Depth 190 km
Station SA06, Distance 20.2°, Azimuth 239.7°
(a) Original seismograms,
(b) Effect of correction for anisotropy on the radial and transverse
components of the seismograms,
(c) Change in polarisation by correcting for shear wave splitting
(d) Contour plot of coherence as a function of phi and dt
*b.c. before correction for anisotropy, °a.c. after correction for anisotropy.
The correction for anisotropy consists of evaluating the energy on the
transverse component associated with the phase of interest, e.g. S,
on the radial component. this evaulation is carried out for many
candidate values of phi and dt (incerements of 1° and 0.05 s),
in order to find the phi and dt pair that best removes the splitting
induced by anisotropy.
[1] G. Clitheroe and R.D. Van der Hilst, Complex anisotropy in the
Australian lithosphere from shear-wave splitting in broad-band SKS records,
AGU Monograph Series, 1997.
[2] G. Barruol and R. Hoffmann, Upper mantle anisotropy beneath Geoscope
stations, J. Geophys. Res. 104, 10757-10773, 1999.
[3] E. Debayle and B.L.N. Kennett, Anisotropy in the Australian upper
mantle from waveform inversion, Annales Geophysicae 16, 37, 1998.
[4] E. Debayle, SV-wave azimuthal anisotropy in the Australian upper
mantle : preliminary results from automated Rayleigh waveform inversion,
Geophys. J. Int. 137(3), 747-754, 1999.
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Questions about this topic to Maggie Heintz:
maggie@rses.anu.edu.au
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