Structure of the crust and upper mantle beneath Australia from Rayleigh- and Love-wave observations

Physics of the Earth and Planetary Interiors, 38 (1985) 224—234 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

224

Structure of the crust and upper mantle beneath Australia from Rayleigh- and Love-wave observations R.M. Ellis

*

Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, A CT, 2601 (Australia)

D. Denham Bureau ofMineral Resources, Geology and Geophysics, P.O. Box 378, Canberra City, A CT 2601 (Australia) (Received September 5, 1984; revision accepted October 31, 1984)

Ellis, R.M. and Denham, D., 1985. Structure of the crust and upper mantle beneath Australia from Rayleigh- and Love-wave observations. Phys. Earth Planet. Inter., 38: 224—234. Fundamental and first higher modes of the Rayleigh- and Love-wave group velocities along seven paths in Australia were jointly inverted by a controlled Monte Carlo procedure to obtain regional shear-wave velocity structures of the crust and upper mantle. Our data support the results of Goncz and Cleary which show an S-wave low velocity zone centred near 110 km depth in eastern Australia. However, the thickness—velocity contrast of the low velocity zone is significantly smaller. The crustal models for eastern Australia are characterized by upper crusts which are both thicker and have lower velocities than those in western Australia and have a less sharp crust—upper mantle boundary. The 1) throughout the continent, with no obvious S-wave velocities forage theofupper mantle appear to bethickness. similar (— 4.55 km s dependence on the cratonization or crustal

1. Introduction In this paper fundamental and first higher mode Rayleigh- and Love-wave group velocities obtamed from recordings of regional earthquakes are jointly inverted by a controlled Monte Carlo procedure to obtain regional S-wave velocity structures of the crust and and upper mantle beneath the Australian continent. Several earlier surface-wave studies have been undertaken to obtain the S-wave structure. Bolt and Niazi (1964) analysed Rayleigh waves along paths of continental dimensions, to infer gross features of the Australian crust. Subsequently, *

On leave from Department of Geophysics and Astronomy, University of British Columbia, Vancouver, B.C. V6T 1W5, Canada.

0031-9201/85/$03.30

© 1985 Elsevier Science Publishers B.V.

Thomas (1969) obtained Rayleigh-wave phase velocities along selected paths and used forward modelling to obtain one and two layer S-wave crustal models. Goncz and Cleary (1976) used Monte Carlo inversion of Rayleigh waves to 100 s period to infer differences between the upper mantle of eastern and western Australia. In the most detailed study, Mills and Fitch (1977) used a controlled Monte Carlo technique to jointly invert both fundamental and first higher mode Rayleighand Love-wave group velocities to obtain S-wave models within the Tasman fold belt. With the exception of the latter study the resolution at crustal depths has been limited by the lack of reliable data at shorter periods due to the use of distant earthquakes or the waves crossing several geological provinces. In this paper group velocities along four paths

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226

in western Australia, two in eastern Australia (Fig. 1), and one in central Australia are analysed. Periods of the fundamental modes ranged from 8—60 s. Joint inversion of the fundamental and first higher modes of both Rayleigh and Love waves was performed using the controlled Monte Carlo inversion procedure of Mills and Fitch (1977). Our objective was to obtain well-constrained shear-wave models for the crust, to compare these with P-wave models where they exist, and where the data are adequate to investigate the lithospheric thickness.

2. Dispersion analysis Initially eighteen three component data sets were selected for analysis. In each case the earthquakes had ML> 5 and the paths were confined to continental Australia. The records were digitized over time intervals corresponding to a group-velocity range 2.5—5.0 km s~,converted to a 0.5 or 1.0 s sampling interval, and the spectra calculated. At this stage instrumental phase shift corrections were applied; for WWSSN stations the formula of Hagiwara (1958) was used and for ASRO and SRO stations the formulae from Ganse and Hutt (1982) were applied. For station ASP only an amplitude calibration was available. In this case the phase correction was determined by applying the Hilbert transform algorithm of Bolduc et al. (1972). The group velocity versus period was then determined by the Gaussian band-pass filtering procedure of Herrmann (1973) and the results displayed as contour plots. The editing and processing routines, except for the Hubert transform algorithm, were adapted from Herrmann (1978). At this stage, or after preliminary model-

ling, the data set was reduced to the seven earthquake-station paths indicated in Fig. 1. The hypocentral parameters are listed in Table I. The remainder were rejected on the basis of limited bandwidth, instrumental problems, or group-velocity curves which were clearly distorted by refracted arrivals or lateral heterogeneities. The dispersion curves with error bars are displayed in Fig. 1(a)—1(g). The estimates of standard errors are based on the sharpness of the group velocity maximum determined from the contours of groupvelocity amplitudes in the group velocity—period plane, the amplitudes of the maxima, and the smoothness of the dispersion curves. In several cases (e.g., CD—ASP), a good data set exists over a wide period range while in others (e.g., SD—ADE) the data set is much more restricted and thus limits the modelling process. Significant differences exist between the groupvelocity plots. For example, the data for the SD—MUN path across western Australia (Fig. 2a) show that the first higher mode Rayleigh- and the fundamental mode Love-wave velocities are higher by 0.2—0.4 km s~ at all periods than those observed from the SD—CTA path (Fig. 21) across eastern Australia. There is therefore a marked difference in the S-wave velocity structure between the western and eastern halves of the continent.

3. Velocity modelling S-wave structure along each of the travel paths was obtained using the controlled Monte Carlo inversion procedure described by Mills and Fitch (1977). Briefly the algorithm operates as follows: (1) starting models are randomly generated within predetermined velocity bounds;

TABLE I List of earthquakes Identification

Year

Month

Day

H

Mm

S

Lat. (5)

Long. (E)

ML

SD Simpson Desert SR Scott Reef CDCadoux BRBroome WGWonnangatta

1972 1979 1979 1979 1982

08 04 06 07 11

28 23 07 14 26

02 05 06 09 11

18 45 45 40 34

52 10 14 51 19

—24.74 — 16.535 —30.769 —18.134 —37.205

136.92 120.175 117.206 122.469 146.956

6.2 7.3 5.5 5.8 5.4

227

(2) when predictions from a particular model are acceptably close to observations, in this case the group velocities, that model is refined; (3) models are refined by applying random perturbations which produce smaller reduced statistics for all models tested; and (4) the procedure is terminated when x2 < 1.0 for all modes tested or after a specified number of iterations,

(a)

SD-MUN

The procedure was applied by deriving initial upper and lower S-wave velocity boundaries for the crust from generalized P-wave models (Finlayson et al., 1979; Finlayson, 1982) by allowing Poisson’s ratio to vary from 0.23 to 0.28. For the mantle, bounds of 4.3—4.8 km s1 were used. Crustal boundaries were fixed at 5 km intervals except in the depth range 0.5—3.0 km where the boundary was allowed to vary. At mantle depths

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Fig. 2. (a) Observed and theoretical Rayleigh and Love wave group velocities for path 1: SD—MUN. Fundamental mode observations are crosses and the first higher mode are solid circles. Assigned observational errors are indicated by bars. Lack of error bars indicate errors smaller than symbol sizes. Curves are the theoretical fits. (b) Observed and theoretical Rayleigh and Love-wave group velocities for path 2: CD—ASP. (c) Observed and theoretical Rayleigh- and Love-wave group velocities for path 3: SR—MUN. The fundamental mode Love-wave group velocity cannot fit a horizontally layered model and was excluded from the computations. (d) Observed and theoretical Rayleigh- and Love-wave group velocities for path 4: BR—ASP. (e) Observed and theoretical Rayleigh- and Love-wave group velocities for path 5: SD—ADE. (f) Observed and theoretical Rayleigh- and Love-wave group velocities for path 6: WG—ASP. (g) Observed and theoretical Rayleigh- and Love-wave group velocities for path 7: SD—CTA.

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the boundaries were fixed at 20 km intervals. Densities were assigned using the velocity—density relationship of Ludwig et al. (1970). For initial tests only the fundamental modes were used, If these runs indicated adjacent layers with similar velocities, these would be concatenated to reduce the number of layers. The starting bounds were then narrowed for the subsequent computations in which the higher modes were also used to constrain the models. Adjustments were also made to the P-wave and density models as suggested by these initial runs. Modelling is considered to be successful when x2 < LO for all modes considered.

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Forward computations show that fundamental mode group velocities which are restricted to penods of <50 s are insensitive to low velocity layers in the 80—140 km depth range. Further, the errors in group velocities at the more sensitive longer periods are often large due to the limited energy radiated at these wavelengths by moderately sized regional earthquakes. The first higher modes provide more resolution of the deeper structure (50—150 km) because they spend more time within the deeper layers. However, due to their low amplitudes and rapid variation of velocity with period, larger errors are associated with the observations. Based on these re-

230

suits and the stability of the velocities obtained through the Monte Carlo procedure, only the SD—MUN, CD—ASP, and WG—ASP data sets were examined for possible low velocity layers in the upper mantle,

4. Velocity models Derived velocity models are shown in Fig. 3 and listed in Table II. Theoretical fits are plotted with the experimental data in Fig. 2. The models shown are obtained by averaging five to fifteen successful models, except for the SD—CTA pattern when x2 ~ 1.25. The theoretical curves are for a typical successful model, As should be expected the crustal structure for the SD—MUN and CD—ASP paths are similar, The velocity of the upper crust is in excess of 3.5 km ~ within 1 km of the surface, the lower

crustal velocities are near 3.9 km s’, and the M-discontinuity is in the 45—50 km depth range where the velocity increases over a small depth range to near 4.6 km s~’.The SR—MUN model is similar. In contrast to the western Australia results, those from the WG—ASP and SD—CTA paths in eastern Australia produce significantly different models. In the east the upper 10—15 km of the crust has a velocity <3.4 km s’, velocities in excess of 3.75 km s’ are not reached until 30 km depth, and the crustal velocities increase more gradually to those of the upper mantle. The SD—ADE velocity profile most closely resembles the western models. The upper crustal velocity of 3.47 km is slightly lower but its 18 km thickness, the jump to 3,83 km s’ at 20 km depth, and the sub-Moho velocity of 4.55 km s~ are characteristic of the western profiles. The BR—ASP data set is enigmatic. The group ~

TABLE II S-Wave velocity—depth models SD-MUN (kin)

(km s’)

0.0 1.6 20 30 40 50 80 100 120 140

2.6 3.67 3.81 3.94 4.16 4.67 4.55 4.46 4.45 4.80

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(km s~)

0.0 1.9 20 35 45

*

2.48 3.47 3.83 3.91 4.50

CD-ASP (kin)

(km s’)

SR-MUN (km)

(km s1)

0.0 1.3 15 20 30 45 50 80 100 120 140

2.70 3.53 3.67 3.87 3.94 4.32 4.53(4.56) * 4.55(4.47) 4.50(4.57) 4.52(4.41) 4.67(4.70)

0.0 1.4 15 30 40 45

2.50 3.50 3.82 4.02 4.48 4.56

WG-ASP (kin)

(km s_I)

(kin)

(km s’)

0.0 1.7 10 30 45 50 55 80 100 140

2.58 3.45 3.69 4.06 4.45 4.62 4.71 4.40 4.25 4.66

0.0 1.6 15 30 45 50

2.46 3.35 3.53 3.94 4.32 4.54

BR-ASP (km)

(km s —

0.0 1.6 25 65

2.47 3.53 4.39 4.55

SD-CTA

Unbracketed values determined with both Love and Rayleigh earth flattening corrections applied. Bracketed values for Love-wave corrections only.

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231 TABLE III (a) Shear-wave velocity models of Thomas (1969)

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velocities require an upper crust characteristic of the western models. However, near 25 km a jump near 4.3 km s’ is required. Velocities near 4.6 km s as expected for the upper mantle, are not

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required until 65 km depth, a much deeper M-discontinuity than has been found anywhere in Australia. The only velocity control in the region is from the profiles from the Ord River explosions —

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(1977) (b) Shear-wave velocity model CTA1S1 of Mills and Fitch Depth (kin) (km s_I 0.0 2.28

(Denham et al., 1972; Simpson, 1973) which pass through the centre and the eastern end of the

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tinuity near 40 km but the data were not sufficient surface-wave path. These indicate an M-disconto estimate any crustal velocity models. Thus the surface-wave data suggest considerable lateral heterogeneity in the upper lithosphere along this profile. Examination of the travel path on the Geol. Soc. Australia (1971) tectonic map confirms this hypothesis. The earthquake epicentre lies within the Fitzroy Trough—Broome platform. The path then passes through the Canning basin and into the Arunta Block crossing the Ngalia basin which has been suggested to be as deep as 7 km and to have associated with it Moho topography as large as 10 km over distances of 100 km (Lambeck and Penney, 1984). Clearly the assumption of lateral homogeneity is violated. —

_____________________ 20

40

60

80

Depth(km) 251N114 Fig. 3. Shear-wavevelocity—depth curves for the seven paths. 1: SD—MUN, 2 CD—ASP, 3: RB—MUN, 4: BR—ASP, 5: SD—ADE, 6: WG—ASP, 7: SD—CTA. Dotted sections are only

weakly constrained by the data set. Parameters of the velocitydepth profile are presented in Table II.

232

In the models of Fig. 3, no low velocity crustal layers are present. They are not required on any path. However, the data set does not preclude thin (i.e., 5—10 km thick) low velocity zones. The velocity steps at the base of the crust are attributed to three sources: true velocity gradients which are widely observed in Australian P-wave refraction studies (e.g., Finlayson, 1983), variation in crustal thicknesses along the paths, and the computational restriction of fixed boundaries at 5 km intervals, As indicated earlier the constraints provided by the data at depths near 100 km are comparatively weak. However, all successful models for the WG—ASP profile require a low velocity zone (LVZ) in the 80—140 km range with a maximum iW = —0.36 km s1. Further, the higher mode group velocities are an excellent data set and the Love-wave data extend to 60 s. Thus there is strong evidence for a LVZ in this region. All successful SD—MUN solutions also require an LVZ but with a maximum AV= —0.22 km s’. However, for the reversed path (CD—ASP) the data do not require an LVL and the model is ambiguous in the 50—100 km depth range: For computations using higher modes, earth flattening corrections are required to the Earth models before calculating surface-wave dispersion. The corrections to Love waves are exact (Biswas and Knopoff, 1970) but those applied to Rayleigh waves (Alterman et al., 1961) tend to overcorrect and give velocities which are too high (Anderson, 1966). Inversion of the CD—ASP data set without earth flattening yields a low velocity zone in the upper mantle, When only the Love-wave correction is applied, the velocities at these depths in the three 20 km thick layers between 80 and 140 km are 4.47, 4.57 and 4.41 km s a pattern which suggests that this data set only weakly resolves the structure at this depth. When the overcorrecting Rayleigh approximation is also applied, there is no evidence of a LVZ. We also note that where LVZ’s are reported in shield regions (Calcagnile, 1982; Sacks and Snoke, 1983), the depth is > 100 km compared to 80 km suggested here. In view of these uncertainties, we conclude that this data set is inadequate to determine whether a low velocity zone exists near 100 km depth in this region of western Australia. The derived models are there~,

fore shown with broken lines at these depths to indicate low reliability.

5. Comparison with previous surface-wave studies The studies of the 1960’s allow only general comparisons due to their limited resolution. Bolt and Niazi (1964) presented only a general S-wave model for the Australian crust on which they suggested an average crustal thickness of between 30 and 35 km and a thickening of the crust or alternatively a decrease in the S-wave velocity to the northwest of the CTA—Perth line. Our crustal thicknesses are all in excess of 40 km and only if the BR—ASP model is considered reliable would our data set be consistent with their second observation. The paths used and the simplicity of their models limit comparison of our results to two profiles of Thomas (1969), namely his ADE—RIV and RIV—TAU two-layer plus half space structures in eastern Australia. His results are consistent with those from our closest path, WG—ASP, in that the upper crustal velocity is low, 3.4 km s the lower crustal velocity is near 4 km ~ and his Moho is at 40 km as compared to 45 km (Fig. 3). The results of Goncz and Cleary (1976) and Mills and Fitch (1977) are more relevant to the present study. The former performed Monte Carlo inversions of fundamental mode Rayleigh-wave dispersion curves extending from 10 to 90 s. Goncz et al. (1975) obtained these curves by summing cross-correlograms of a large number of earthquakes prior to computation of phase and four distinctly different phase velocity curves for “East” and “West” Australia. Four structural models consisting of two crustal and four subcrustal layers were presented for each region. The average of these models indicates that the “East” compared to the “West” has lower upper crustal velocities 3.52 and 3.58 km sfl’, respectively, and higher sub-Moho velocities of 4.64 and 4.59 km s1. These results are consistent with those of this paper for which the corresponding average values are 3.42 and 3.57 km ~ for our upper crustal section and 4.63 and 4.59 km ~ for our upper mantle. However, their principal result is the pres— ~,

233

ence in the “East” of a low velocity channel in the upper mantle which is absent in the “West”. This channel, at depths > 90 km, has a thickness-velocity difference product in excess of 35 km2 s’. The best data of Mills and Fitch (1977) were for a path from southeast Australia to CTA. For comparison, their model CTA1S1 is presented in Table I. Although they constrained the velocity boundaries on the basis of one P-wave refraction study (Connelly and Collins, 1973) rather than artificially at 5 km intervals as in this study, the essentials of their crustal model are similar to the WG—ASP and SD—CTA structures. Details do vary near the surface but the three models all contain large sections with velocities near 3.6 and 4.0 km s’ with the boundary between these layers at 30 km depth. In our study the Moho is 5 km deeper and substantial disagreements exist in the upper mantle. Their subcrustal shear velocities are only 4.20—4.32 km s’, which corresponds to a high Poisson’s ratio of 0.30—0.32; whereas our velocities in the same depth range are close to 4.64 km s on both profiles 6 and 7. Further, no S-wave LVZ is observed in the upper mantle. In contrast, our results WG—ASP path of eastem Australia substantiate those of Goncz and Cleary (1976) in that a low velocity layer is found in the upper mantle albeit with a thickness-velocity difference contrast of 20 km2 s1. Our data for western Australia are inadequate to either confirm or contradict their upper mantle model. —

—

6. Discussion Sacks and Snoke (1983) recently summarized a number of studies to determine the base of the lithosphere and they found that rift valleys, basins and some aseismic platforms have lithospheric thicknesses of 45 to 140 km as defined by a well-developed LVZ i.e., ~V —0.3 to —0.4 km ~ In contrast, results for Precambrian shield regions indicate that if a LVZ exists in the upper mantle it is at most a moderate one starting at depths > 100 km. Our results are generally consistent with these observations. For the southeast Australian path, WG—ASP, principally across an aseismic platform, the data indicate a well-defined —

LVZ starting at a depth of 80 km. In contrast the paths across the Australian shield from central to southwest Australia indicate that if a LVZ exists, the thickness—velocity difference product must be small. Our data do suggest that if it exists it may start at depths as shallow as 80 km. The scale of this study is quite different from those of P-wave refraction programs but there are marked similarities in the results. Generalized models for the crust in eastern Australia (Finlayson et al., 1979; Finlayson, 1983) exhibit thick (25—30 km) upper crustal sections wih characteristic velocities near 6.0 ~ ~ The P-wave velocity of the upper sub-basement section, typically the 2—15 km depth range, is 5.9 km s1 compared to 3.4 km s1 for the S-wave models which yields a Poisson’s ratio of 0.25. Further the velocity increase from the lower crust to upper mantle takes place over a depth range of 5—10 km similar to that found in the surface-wave study. The detailed explosion data for western Australia are limited. However, the crustal model of Drummond (1983) for the Pilbara Block starts with a near surface velocity of 6.0 km s’, has a crust with no distinct boundaries and a moderate velocity gradient to within 5 km of the Moho where there is a rapid velocity increase from 6.3 to 8.1 km s’. The western Australia S-wave models are similar, particularly for paths SD—MUN and CD—ASP. One major difference between our S-wave models and the P-wave studies from large explosions and earthquakes (see Finlayson, 1983) is the lack of variation in the S-wave velocities in the upper mantle across the continent, compared to the equivalent P-wave observations. For P waves the velocities vary from 8.2 km ~ in the west (Yilgarn and Pilbara Archaean cratons) to 7.9 or 8.0 km s1 beneath the Palaeozoic Tasman fold belt. However, for the S waves, the velocities at say 55 km are similar (—4.55 km s~) throughout the continent. —

—

Acknowledgements We thank Dr. R.B. Herrmann who provided computer programs to process the data set and obtain the group velocities and Todd Bostwick

234

whose routine to yield contour plots of the group velocity aided the evaluation of data. R.M. Ellis received a Travel Grant and an International Collaborative Research Grant from the Natural Sciences and Engineering Research Council of Canada and supplementary funding from the Bureau of Mineral Resources, Geology and Geophysics. Pauline Greig typed the text and Mike Steele prepared the diagrams. D. Denham publishes with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics. References Alterman, W., Jarosch, M. and Pekeris, C.L., 1961. Propagation of Rayleigh waves in the Earth. Geophys. J.R. Astron. Soc., 4: 219—241. Anderson, D.L., 1966. Recent evidence concerning the structure and composition of the Earth’s mantle. In: L.M. Ahrens, F. Press, S.K. Runcorn and H.C. Urey (Editors), Physics and Chemistry of the Earth. Pergamon Press, New York, 6: 1—131. Biswas, N.N. and Knopoff, L., 1970. Exact earth flattening for Love waves. Bull. Seismol. Soc. Am., 66: 1123—1137. Bolduc, P.M., Ellis, R.M. and Russell, R.D., 1972. Determination of the seismograph phase response from the amplitude response. Bull. Seismol. Soc. Am., 62: 1665—1672. Bolt, B.A. and Niazi, M., 1964. Dispersion of Rayleigh waves across Australia. Geophys. J.R. Astron. Soc., 9: 21—35. Calcagnile, G., 1982. The lithosphere—asthenosphere system in Fennoscandia. Tectonophysics, 90: 19—35. Connelly, J.B. and Collins, C.D.N., 1973. Bowen Basin seismic refraction survey, May—June, 1973: operational report, Bur. Mi Resour. Aust. Rec., 1973/312 (unpublished). Denham, D., Simpson, D.W., Gregson, P.J. and Sutton, D.J., 1972. Travel times and amplitudes from explosions in northern Australia. Geophys. J.R. Astron. Soc., 28: 225—235. Drummond, B.J., 1983. Detailed seismic velocity/depth models of the upper lithosphere of the Pilbara Craton, northwest Australia. BMR J. Aust. Geol. Geophys., 8: 35—51. Finlayson, D.M., 1982. Geophysical differences in the lithosphere between Phanerozoic and Precambrian Australia. Tectonophysics, 84: 287—312.

Finlayson, D.M., 1983. The mid-crustal horizon under the Eromanga Basin, eastern Australia. Tectonophysics, 100: 199-214. Finlayson, D.M., Prodehl, C. and Collins, C.D.N., 1979. Explosion seismic profiles, and implications for crustal evolution, in southeastern Australia. BMR J. Aust. Geol. Geophys., 4: 243-252. Ganse, R. and Hutt, C.R., 1982. Directory of World Digital Seismic Stations. Rept SE-32, World Data Centre A, 439 Geol Soc. Australia, 1971. Tectonic map of Australia and New Guinea 1:5 000000, Sydney. Goncz, J.H. and Cleary, J.R., 1976. Variations in the structure of the upper mantle beneath Australia, from Rayleigh wave observations. Geophys. J.R. Astron. Soc., 44: 507—516. Goncz, J.H., Hales, A.L. and Muirhead, K.J., 1975. Analysis of Rayleigh and Love wave dispersion across Australia. Geophys. J.R. Astron. Soc., 41: 81—105. Hagiwara, T., 1958. A note on the theory of the electromagnetic seismograph. Bull. Earthq. Res. Inst., 36: 139—164. Herrmann, R.B., 1973. Some aspects of band-pass filtering of surface waves. Bull. Seismol. Soc. Am., 63: 663—671. Herrmann, R.B. (Editor), 1978. Computer Programs in Earthquake Seismology. 2V, Dept. of Earth and Atmospheric Sciences, St. Louis Univ., 450 pp. Lambeck, K. and Penney, C., 1984. Teleseismic travel time anomalies and crustal structure in central Australia. Phys. Earth Planet. Inter., 34: 46—56. Ludwig, W.J., Nafe, J.E. and Drake, C.L., 1970. Seismic refraction. In: A.E. Maxwell (Editor), The Sea. Wiley-Interscience, 4, Pt 1, 1970, pp. 53—84. Mills, J.M. and Fitch, T.J., 1977. Thrust faulting and crust—upper mantle structure in East Australia. Geophys. J.R. Asoron. Soc., 48: 351—384. Plumb, K.A., 1979. The tectonic evolution of Australia, Earth Sci. Rev., 14: 205—249. Sacks, I.S. and Snoke, J.A., 1983. Seismological determinations of the subcrustal continental lithosphere, Annu. Rep. Director, Dept. Terrestrial Magnetism, Carnegie Institution, Washington, pp. 465—471. Simpson, D.W., 1973. P wave velocity structure in the upper mantle in the Australian region. Ph.D. thesis, Australian National Univ., pp. 212. Thomas, L., 1969. Rayleigh wave dispersion in Australia. Bull. Seismol. Soc. Am., 54: 167—182.