GEOPHYSICAL RESEARCH LETTERS
Supplementary Materials for
”The lithosphere-asthenosphere transition and radial
anisotropy beneath the Australian continent”
K. Yoshizawa1and B. L. N. Kennett
2
1 Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido
University, Sapporo, 060-0810, Japan.
2 Research School of Earth Sciences, Australian National University,
Canberra, ACT 0200, Australia.
S1. Data and Method
Full details of the data and methods for the construction of the 3-D shear wave model for the up-
per mantle beneath Australia, and the extraction of the nature of the lithosphere-asthenosphere
transition (LAT) are given in Yoshizawa [2014]. The three-stage inversion scheme of Yoshizawa
and Kennett [2004] was used, using sources in the earthquake belts surrounding Australia
recorded at permanent stations in the region and transportable seismic stations deployed across
the continent (Figure S1). A fully non-linear waveform fitting scheme [Yoshizawa and Kennett ,
2002a; Yoshizawa and Ekstrom, 2010] extracts path-specific multi-mode phase speeds of sur-
face waves for the frequency band from 5-50 mHz. A suite of multi-mode phase-speed maps
at different frequencies are then built, taking into account the ray-path bending due to lateral
heterogeneity with allowance for the effects of finite frequency of the surface waves [Yoshizawa
and Kennett , 2002b]. The attainable horizontal resolution for radial anisotropy is around 300 km
in the continental lithosphere. From the phase speed maps, the local dispersion characteristics
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X - 2 YOSHIZAWA AND KENNETT: LAT AND RADIAL ANISOTROPY OF AUSTRALIA
are extracted and inverted for local radially-anisotropic models at 1◦ × 1◦ knot points, which
are then assembled into SH and SV wave speed models (Vsh and Vsv). Horizontal smoothing
is incorporated by averaging the wavespeeds at adjacent geographical knot points. Finally we
extract 3-D models of the isotropic (Voigt average) S wavespeed Viso =√(2/3)V 2
sv + (1/3)V 2sh
and radial anisotropy ξ = (Vsh/Vsv)2 (Figure S2).
The character of the transition from the lithosphere to the asthenosphere (LAT) is extracted
from the vertical profiles of the isotropic S wavespeed Viso and its vertical gradient dViso/dz.
Surface waves have limited sensitivity to the details of structural transitions, and so there is no
simple criterion for recognizing the base of the lithosphere. We have used two different styles
of measurement to place shallower and deeper bounds on the depth of the LAT. The upper
(shallow) bound shown in Figure S3(a) is provided by the depth of the negative peak in isotropic
S wavespeed gradient. The lower (deeper) bound displayed in Figure S3(b) comes from the depth
of the slowest absolute shear wavespeed (or zero velocity gradient) beneath the lithosphere. The
thickness of the LAT (Figure S3c) is then estimated from the difference between the upper
and lower bounds. The average gradient in the LAT (Figure S3d) represents how far the shear
wavespeed drops across the transition zone.
In Figure S2 (a,b,d), the estimates of the upper and lower bounds for the LAT are superimposed
on vertical-sections for Viso, ξ and dViso/dz. The estimated upper-bound of the LAT from surface
waves and the depths of discontinuity from an S receiver function study [Ford et al., 2010] are
displayed in Figure S3 (a). The S-wave receiver function results show a clear drop in S-wavespeed
in eastern Australia, whose depth is fairly consistent with our estimates of the shallower bound
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on the LAT. The majority of this eastern region is characterized by a large velocity drop in the
LAT relative to central and western Australia (Figure S3d).
Many studies have associated the base of the lithosphere with a prescribed level of perturbation
of S wavespeed from a reference [e.g., Simons and van der Hilst , 2002]. Here, we prefer to use
the absolute shear wavespeed Viso and its vertical gradient, since the LAT estimates are not then
influenced by the choice of reference model.
From synthetic experiments, Yoshizawa [2014] demonstrated that the long-period character of
the multi-mode surface waves limits the definition of an LAT thinner than 40 km; much smoother
structural transitions, which would be invisible to receiver functions, can be recovered well.
References
Ford, H. A., K. M. Fischer, D. L. Abt, C. A. Rychert, and L. T. Elkins-Tanton (2010), The
lithosphere–asthenosphere boundary and cratonic lithospheric layering beneath Australia from
Sp wave imaging, Earth Planet. Sci. Lett., 300 (3-4), 299–310, doi:10.1016/j.epsl.2010.10.007.
Simons, F. J., and R. D. van der Hilst (2002), Age-dependent seismic thickness and mechan-
ical strength of the Australian lithosphere, Geophys. Res. Lett., 29 (11), 24–1–24–4, doi:
10.1029/2002GL014962.
Yoshizawa, K. (2014), Radially anisotropic 3-D shear wave structure of the Australian lithosphere
and asthenosphere from multi-mode surface waves, Phys. Earth Planet. Inter., 235, 33–48, doi:
10.1016/j.pepi.2014.07.008.
Yoshizawa, K., and G. Ekstrom (2010), Automated multimode phase speed measurements for
high-resolution regional-scale tomography: application to North America, Geophys. J. Int.,
183 (3), 1538–1558, doi:10.1111/j.1365-246X.2010.04814.x.
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Yoshizawa, K., and B. L. N. Kennett (2002a), Non-linear waveform inversion for surface waves
with a neighbourhood algorithm—application to multimode dispersion measurements, Geo-
phys. J. Int., 149 (1), 118–133, doi:10.1046/j.1365-246X.2002.01634.x.
Yoshizawa, K., and B. L. N. Kennett (2002b), Determination of the influence zone for surface
wave paths, Geophys. J. Int., 149 (2), 440–453, doi:10.1046/j.1365-246X.2002.01659.x.
Yoshizawa, K., and B. L. N. Kennett (2004), Multimode surface wave tomography for the Aus-
tralian region using a three-stage approach incorporating finite frequency effects, J. Geophys.
Res., 109 (B2), B02,310, doi:10.1029/2002JB002254.
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Figure S1. (a) Ray path distribution for fundamental mode Rayleigh waves at 71.4 s period
for the data set of Yoshizawa [2014]. Yellow triangles indicate the temporary broad-band stations
deployed by ANU, blue triangles the FDSN stations, and red circles the employed seismic events.
(b) Checker board resolution test for phase speed maps with a 6-degree cell pattern.
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Figure S2. Shear wavespeed and radial anisotropy maps beneath the Australian continent
from the model of Yoshizawa [2014]. (a) Isotropic S wave speed and (b) radial anisotropy maps at
depths of 70, 120 and 200 km, together with vertical cross sections across the center of continent;
(A) EW cross section at 25◦S and (B) NS cross section at 132◦E. Red dashed lines indicate
the cratonic boundaries; NAC: North Australian Craton, WAC: West Australian Craton, SAC:
South Australian Craton, Mu: Musgrave Province in the suture zone. (c) A reference isotropic
S wave speed profile (Viso) used to plot the maps in (a) and (b), and average SV and SH wave
speed profiles (Vsv and Vsh) as well as radial anisotropy ξ of the Australasian region compared to
anisotropic PREM (modified by removing the 220 km discontinuity) indicated with a dashed line.
(d) Vertical gradient of isotropic S wave speeds used to estimate the upper and lower bounds of
LAT.
Figure S3. Estimated parameters for the lithosphere-asthenosphere transition (LAT) from
the 3-D isotropic S wave speed model of Yoshizawa [2014]: (a) upper bound of LAT plotted with
S receiver function results by Ford et al. [2010] (circles with the depth labels), (b) lower bound
of LAT, (c) LAT thickness from the difference between (a) and (b), and (d) velocity gradient
between the upper and lower bounds of LAT. The black dashed curve dividing eastern and
western Australia is taken from the receiver function analysis of Ford et al. [2010], indicating the
region where S-to-P converted signal from the bottom of the lithosphere can be found (eastern-
side) or not (western-side). White dashed lines are cratonic boundaries. NAC: North Australian
Craton, WAC: West Australian Craton, SAC: South Australian Craton, Cp: Capricorn Orogen,
Mu: Musgrave Province, Pi: Pilbara Craton, Yi: Yilgarn Craton.