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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YOSHIZAWA AND KENNETT: LAT AND RADIAL ANISOTROPY OF AUSTRALIA X - 3

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.