Bulletin of the Seismological Society of America, Vol. 83, No. 1, pp. 25-37, February 1993

THE VELOCITY STRUCTURE OF THE AUSTRALIAN SHIELD FROM SEISMIC TRAVEL TIMES

BY J. ROGER BOWMAN* AND B. L. N. KENNETT

ABSTRACT

Travel times between sources and stations in the Australian shield are used to develop P- and S-wave velocity models for the Precambrian shield of western and central Australia. Errors in travel times are minimized by using sources for which locations and origin times are constrained by local data or fault scarps. Apparent velocities between 1500 and 2200 km for P (8.8 km / sec) and S (4.8 km/sec) require a high-velocity lid, and the maximum range of observation of this branch (2200 km) requires that the lid extend to a depth of 205 _+ 20 km. Because of the distribution of sources and receivers, there are no observations from the shield at ranges greater than 2300 km, so we use times from stations in Phanerozoic eastern Australia and published global summary times as a guide for interpretation at greater ranges. Allowing for known differences in teleseismic travel-time residuals between the Precam- brian shield and eastern Australian, the P times are best fit by a model that has a small velocity inversion at the base of the lid, and the S times are best fit by a model with a more substantial reduction in velocities. However, both P- and S-wave travel times can be satisfied with models that have velocities below 210 km that are concordant with global averages such as IASP91. Thus, the velocities inferred beneath the lid are low relative to those of the lid but not necessarily low relative to global averages.

INTRODUCTION

There is a consensus that the oceanic lithosphere consists of a crust of about 6-km thickness on top of a relatively high-velocity lid that overlies a zone of lower velocity. Also, it is thought that the thickness of the lid increases as a function of distance from the mid-ocean ridge and of the age of the lithosphere (Watts et al., 1980). On the other hand, there is substantial variation and uncertainty regarding the structure of the continental lithosphere. This uncer- tainty is the result of a longer history of formation and deformation for the continents, of lateral variations in composition and structure that are more abrupt than found in the ocean, and also of the complexity of the continental crust, which serves to mask the deeper s tructure of the continental lithosphere.

This paper investigates the structure of the Austral ian shield using absolute travel times. The Austral ian continent is composed of an amalgamation of crustal blocks that can be broadly grouped into the Precambrian western and central crat0ns and the Phanerozoic eastern province (Fig. 1 and Plumb, 1979). Structural differences between the Precambrian shield and eastern Austral ia are inferred from surface-wave dispersion (cf. Muirhead and Drummond, 1991) and teleseismic travel-time residuals (Drummond et al., 1989), possibly extend- ing below 200 km depth (Finlayson, 1982a). Here we focus on the Precambrian cratons because of the location of suitable seismic sources within the shield.

This work complements studies of waveforms from Indonesian ear thquakes

* Present address: Aus t ra l ian Seismological Centre, Aust ra l ian Geological Survey Organisation, GPO Box 378, Canberra , ACT 2601, Austra l ia

25

26 J. R. BOWMAN AND B. L. N. KENNETT

recorded in Austral ia (Hales et al . , 1980; Leven, 1985; Bowman and Kennett , 1990) by adding strong constraints on absolute travel t imes in the upper 200 km of the mantle. The focus on sources and stations within the Austral ian shield avoids possible contamination by lateral variations between the continent and island arc structures. We find that a thick (205 + 20 km) lid overlying a zone of lower velocities is required to match the travel times. This style of velocity structure is similar to tha t inferred beneath northwestern Australia from modeling near teleseismic waveforms of Indonesian ear thquakes (e.g., Bowman and Kennett , 1990). In the distance range from 1500 to 2200 kin, the velocity model derived in this paper reduces the mean residual between observed and calculated P travel t imes from - 7 . 8 sec for the global JB model (Jeffreys and Bullen, 1940) to 0.0 sec. Similarly, for S waves the mean residual is reduced from - 1 7 . 7 to - 0 . 7 sec. Because of the sparse distribution of seismograph stations in Australia, an accurate representat ion of travel times in this dis- tance range is critical for determining accurate locations of moderate-size earthquakes.

TRAVEL-TIME DATA FROM AUSTRALIAN EARTHQUAKES AND STATIONS

Study of travel t imes within continental shields is generally hampered by the absence of suitable ear thquake sources and by the sparse distribution of seismo- graph stations. However, in the past five years there have been enough large ear thquakes in the Austral ian shield to motivate a s tudy of regional travel t imes and structure.

Only sources whose location and origin time are well determined are consid- ered in this s tudy (Fig. 1), and all have focal depths less than 8 km. The majority of the data (Figs. 2 and 3) are from three large, surface-faulting ear thquakes near Tennant Creek, Northern Territory, and their aftershocks (Bowman et al . , 1990). Seventy-five ear thquakes that were relocated by Bow- man and Dewey (1991) using the method of joint hypocenter determination and that had epicentral errors of 8 km or less (semi-major axes of the 90% confi- dence ellipse) are included here. Combining teleseismic arrival times with times from the nearby Warramunga (WRA) array and times from portable seismo- graphs ensures the accuracy of the origin time est imates to less than 0.5 sec.

Additional travel t imes are provided by two moderate ear thquakes and two large chemical explosions. The epicenter of an rn b 5.8 ear thquake at Marryat Creek, South Australia, in 1986 (McCue et al . , 1987) is assumed to lie in the center of the rupture plane defined by the two arms of its fault scarp, which has a maximum dimension of 8 km. Arrival t imes from an ear thquake ( M L 5.5) near Meckering, Western Australia, in 1990, with epicentral uncertainty of several kilometers (Dent, 1990), are also used. The origin times for these events were adjusted to align travel t imes at distances of 100 to 300 km with those for the IASP91 model (Kennett and Engdahl, 1991) and have est imated errors of I sec. Two 500,000-kg explosions at Ord River, Western Australia (Fig. 1), afford precise and accurate travel times to stations in Western and Central Australia, with est imated picking error generally 0.1 to 1.0 sec (Denham et al. , 1972).

Most arrival t imes were taken from the Ear thquake Data Report of the U.S. Geological Survey, the Bulletin of the International Seismological Centre, or the bulletin of the Austral ian Seismological Centre. Reading errors are est imated to be 1 and 2 sec for P and S waves, respectively. Additional local and near- regional t imes were read from portable stations, with errors of 0.1 to 1.0 sec.

STRUCTURE OF THE AUSTRALIAN SHIELD 27

,.E

KNA

I I ,' ~ ' ~ . M B L °'[ +

,.NANU .... ~ + ,0

+ 1 "'~. + •WAI~B +

• MEKA ~ MC ~ I +

MRWA I

• I~AL ~ KLGA •FIORR \M ~.KLB "COOL

i - - , i

TC~ WRA ISQI + + +

j -I* +1

+ ~I.ASPA J

+

/ . . . .

@

" r •

o %

FIG. 1. Map of the seismic sources (circles), permanent stations (triangles), and some of the portable stations (pluses) providing travel-time data. Sources include Tennant Creek (TC), Marryat Creek (MC), Ord River (Ord), and Meckering (M). Stations are grouped into simple western, central, and eastern Australian regions as shown by the north-south (dashed lines). Solid line shows the boundary between the Precambrian cratons to the west and the younger provinces (Plumb, 1979).

F igures 2 and 3 show P- and S-wave t rave l t imes for the Aus t r a l i an shield, correc ted to surface focus us ing model IASP91. The symbol type des ignates the rece iver region and a ba r ad jacen t to the symbol des ignates the source. Sym- bols wi th no bars r e p r e s e n t T e n n a n t Creek sources, those wi th bars below are f rom Ord River, ba rs above f rom M a r r y a t Creek, and bars to the r igh t f rom Meckering.

Also shown in F igures 2 and 3 are t r ave l t imes ca lcula ted for the JB model (dashed line; Je f f reys and Bullen, 1940), which are commonly used for locat ing e a r t h q u a k e s wi th global or regional data , and t imes for the IASP91 model (solid line), which provide a be t t e r fit to average global t r ave l t imes (K en n e t t and Engdahl , 1991). The observed Aus t r a l i an t rave l t imes are i n d e p e n d e n t of any global model, because local da t a were used to cons t ra in the locations and origin t imes of the sources. Trave l - t ime da t a are mos t a b u n d a n t for r anges less t h a n 2200 kin, because of the d is t r ibu t ion of sources and stat ions.

The T e n n a n t Creek e a r t h q u a k e s form a source a r r a y sp read over about 40-km range and genera l ly show a sca t t e r of a r r iva l t imes of + i sec for P and ± 2 sec for S. This sca t t e r is a t t r i bu t ed to e r rors in focal dep th and in picking onsets.

28 J. R. BOWMAN AND B. L. N. K E N N E T T

30

v O 2O Ob

i

I--

10

P

/ / / / / ~

+ western Australia * central Australia

eastern Australia o outside Australia * Tennant Creek

Marryat Creek - Meckering

Ord River

\ \ \ (a)

\\ \ \ \

0 0 1000 2000 3000

Distance (km)

,,• ~ ~ (b) 1 8

l ~, / / \

\o\ \k \ \

"~ "+~we2tern Australia ~ ~ ,, ~ \ S ;ra'i,a

30

O 20 o~

I- ~ J " eastern Australia 10 ~- ~ - o outside Australia -J

~- "// ~ Tennant Creek [-// Marryat Creek ~// . Meckering / , Ord River

o l / I i I I i I I I i I I I I I I I I I L i I i I i I I I I 0 1000 2000 3000

Distance (km)

FIG. 2. P-wave travel times for the Australian sources and stations shown in Figure 1. The symbol shape represents the receiver region: western ( + ), central (x), eastern (triangle), and outside (circle) Australia. The bar adjacent to the symbols identifies the source: Tennant Creek (no bar), Marryat Creek (bar above symbol), Meckering (bar to right), or Ord River (bar under). Calculated P travel times are shown for the JB (dashed line) and IASP91 (solid line) models. (a) Stations in western and central Australia. (b) Stations in eastern Australia and outside Australia.

STRUCTURE OF THE A U S T R A L I A N SHIELD 29

80

70

60

5 0

O

~ 4o

I

F-- 30

20

10

8 0

S (a)

x

(

/ /

/

/ J

/ /

/ /

/ f \ \

\

\

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" + • western Australia • central Australia . eastern Australia o outside Australia . Tennant Creek , M a r r y a t C r e e k . Meckering

. , Ord River

1000

Distance (km) 2000 3000

70

60

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O

~ 4o

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20

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F 7 / r/7 / / ~

/ . \ / / ~ * c e n t r a l Australia \ ~ ' f P " ~ eastern Australia

o outside Australia ~f/ * Tennant Creek /, , Marryat Creek

Meckering , Ord River

I I I I I I I I I I I I I I I I I I I [ I I I i I r I I 1 0 0 0 2 0 0 0 3 0 0 0

Distance (km) FIG. 3. S-wave travel times with symbols as in Figure 2. Also shown are S times calculated from

the JB (dashed) and IASP91 (solid) models. (a) Western and central Australia. (b) Eastern Australia and outside Australia.

Several stations, most notably CTA at 1300 km, have scatter of up to + 2.5 sec for P, possibly as a consequence of misidentification as a first arrival of a later phase from an upper-mantle triplication. Data from sources at Ord River, Marryat Creek, and Meckering are generally consistent with those from

30 J. R. BOWMAN AND B. L. N. KENNETT

T e n n a n t Creek. P a n d S t r a v e l t imes to s t a t ions in the A u s t r a l i a n shield (Figs. 2a and 3a) a re gene ra l l y seve ra l seconds f a s t e r t h a n to s t a t ions in e a s t e r n A u s t r a l i a or to s t a t ions to the no r th in I n d o n e s i a and P a p u a N e w G u i n e a (Figs. 2b and 3b). The obse rved P t imes are 8 and 5 sec f a s t e r t h a n J B or IASP91 t imes , respect ive ly , a t d i s t ances of 1500 to 2200 km. This r e su l t is not su rpr i s - ing since our d a t a a re t a k e n en t i r e ly f rom the s tab le A u s t r a l i a n cont inent , w h e r e a s the J B a n d IASP91 models were cons t ruc ted to m a t c h global t r ave l t imes , which include sources a n d rece ivers in a v a r i e t y of tectonic env i ronmen t s .

T r e n d s in the t r ave l t imes are more eas i ly seen by s u m m a r i z i n g t imes for T e n n a n t C r e e k sources. The box for each s t a t ion in F igu re s 4 a n d 5 r e p r e s e n t s the m e a n reduced t i m e + the s t a n d a r d e r ro r of the m e a n and the d i s tance r a n g e of observa t ion . The s t a n d a r d e r ro r s of the m e a n are gene ra l ly 0.1 to 0.3 sec. Sca t t e r a m o n g t imes f rom s ta t ions a t s imi l a r r a n g e s ref lects l a t e r a l va r i a - t ion in the s t r u c t u r e of the c rus t a n d l i t hosphe re t h a t canno t be accounted for w i th a s t ra t i f i ed veloci ty model . T rave l t imes f rom sources a t Ord River, Mecker ing , and M a r r y a t C r e e k exhib i t s igni f icant ly more s ca t t e r (4 to 5 sec) because t he sources a re smal le r , which inc reases the u n c e r t a i n t y in p ick ing the onset , and beca us e the origin t imes a re less well de t e rmined .

Global a v e r a g e t r ave l t imes (Dziewonski and Anderson , 1981, 1983, h e r e a f t e r a b b r e v i a t e d DA) are shown in F igu re s 4 a n d 5 by filled circles a t 1 ° i n t e rva l s beg inn ing a t 2200 k m and r e p r e s e n t r a y s t h a t h a v e t u r n e d below the 410-km discont inui ty . P t imes to s t a t ions in e a s t e r n Aus t ra l i a , I n d o n e s i a , a n d P a p u a N e w G u i n e a are gene ra l ly 0.5 to 1 sec l a t e r t h a n the s u m m a r y t imes .

The a p p a r e n t veloci ty for P is 8.8 k m / s e c in the r a n g e 1500 to 2200 km. DA

30 P

+ western Australia * central AuStralia ~ • ~' eastern Australia ~ ~ ~ ~-, ~ • * TennantCreek ~ ~ z~:F ~ ,~ ~ E ~

(D , Marryat Creek .o ~ ~ ..~ ~ ~ ~ ! .

p ( / s ) !

j - , . ,

10

"J 500 ~ '1 ~

/ 0 I I I I I I I I I I I I I I I I I I I I I I I I I I

0 1000 2000 3000

Distance (km) FIG. 4. P-wave travel times as m Figure 2, except travel times for Tennant Creek sources are

shown by boxes representing the mean time (_+ the standard error of the mean) and the distance range for each station. Stations in cratonic western and central Australia are shown by pluses and crosses and stations in eastern Australia beyond 2300 km, by open triangles. Filled circles show DA summary travel times. Calculated travel times are shown for several candidate P models.

STRUCTURE OF THE AUSTRALIAN SHIELD 31

80

70

60

50

o ~ 40

!

I-- so

20

10

Vs (km/s) S 3 4 5

+ western Australia 100 ~ . central Australia • _J , eastem Australia I - ~ i -~" - Tennant Creek \ \ ~ -I 300 ~ o" , Marryat C reek . . . . X1 " \~ i ~ %'- - Mecken'ng .......................... X6SOZ \ L , , -~ ~ ~ - - -S "o . Ord River \ J 500 ~ x

0 I I I I t I I I I I I I I I I I I t I I I I I i I I t I 0 1000 2000 3000

Distance (km) FIG. 5. Same as Figure 4 but for S-wave travel times.

s u m m a r y P t imes (filled circles) a t 2300 k m are 3 sec l a t e r t h a n observat ions f rom s ta t ions M U N and NWAO at 2200 km. P t imes f rom s ta t ion RKG at 2300 k m are also 1.5 sec l a t e r t h a n those f rom M U N or NWAO, which is a difference s l ight ly l a rger t h a n the 1 sec sca t t e r among t imes f rom s ta t ions be tween 1300 and 2200 km (excluding NANU). Times f rom s ta t ion N A N U are 2.5 sec la te re la t ive to s ta t ions in s o u t h w e s t e r n Aust ra l ia , possibly as the r e su l t of l a te ra l s t ruc tu ra l differences or as a r e su l t of an e r ro r in s ta t ion coordinates . Unfor tu- na te ly , t he re are no Aus t r a l i an shield observat ions beyond 2300 km, so we m u s t re ly on t imes f rom s ta t ions in Phanerozo ic e a s t e rn Aus t ra l i a and the DA s u m m a r y t imes at these ranges . One i n t e rp r e t a t i o n of the offset in t r ave l t imes be tween 2200 and 2300 k m is the p resence of a low-velocity zone below the high-veloci ty lid.

S -wave t imes have an a p p a r e n t velocity of 4.8 k m / s e c be tween 1500 and 2200 km and have a sca t t e r of about 5 sec among s ta t ions and sources (Fig. 3). However , beyond 2200 km S is seldom repor ted , and S t imes a t 2300 k m dis tance in e a s t e rn Aus t ra l i a are sca t t e red t h ro u g h an in te rva l g r ea t e r t h a n 20 sec. DA s u m m a r y global S t imes are 19 sec l a t e r t h a n observat ions f rom M U N or NWAO at 2200 km. The few repor t ed S t imes f rom RKG are severa l seconds later . This p ronounced offset in the S t rave l t imes n e a r 2300 k m is bes t exp la ined by the p resence of a low-velocity zone.

TRAVEL-TIME MODELING

One of our objectives is to develop a veloci ty model t h a t sat isf ies t r ave l t imes in the Aus t r a l i an shield wi th only those devia t ions f rom a global model t h a t are r equ i r ed by the data , and we the re fo re r e t a in the p a r a m e t e r i z a t i o n of K e n n e t t and Engda h l (1991) whe re possible. Crus ta l s t r u c tu r e r em a in s fixed in the

32 J. R. B O W M A N AND B. L. N. K E N N E T T

modeling and consists of two uniform layers with discontinuities at 20 and 35 km. The assumed crustal structure is a reasonable compromise between models derived from refraction experiments in the Pilbara craton of Western Australia, which has a thinner crust (Drummond, 1983), and the north Austral ian craton near Tennant Creek, which has a thicker crust (Finlayson, 1982b). The struc- ture of the upper mantle is represented by linear velocity gradients between discontinuities, which are allowed at 120, 210, 410, and 660 km, and P- and S-wave times are modeled separately.

Travel times are computed for trial models using the analytic formulae of Azbel and Yanovskaya (1972). Models are evaluated using four principal crite- ria. The first criterion is the fit to absolute travel times at distances between 500 and 2200 km, which is controlled largely by the integrated velocity between 50- and 150-kin depth. Second is the match with apparent velocity in the range 1500 to 2200 km, which is a function of the velocity at the base of the lid. Third is the maximum range at which an arrival from the lid is predicted, which varies with the depth to the base of the lid and with the velocity gradient within the lid. The fourth criterion is the fit to the offset in the travel time curve between 2200 and 2300 km, which is controlled by the product of the decrease in velocity below the lid and the thickness of the region of lower velocities. The modeling emphasizes travel times from Tennant Creek sources, because the data are more numerous and consistent than for the other sources.

There are few times for pure shield paths beyond 2300 kin, but at teleseismic range we expect travel times to be in rough accord with global averages, so we can use the DA summary travel times as a guide at intermediate ranges. Teleseismic P-time residuals at stations in western and eastern Australia differ by about 1.5 sec (Drummond et al., 1989; Toy, 1989). If this difference is attr ibuted to structure above 200 km, for example to low velocities between 120 and 190 km in Phanerozoic eastern Australia (Muirhead and Drummond, 1991), then travel times between shield sources and eastern stations can be used to supplement pure-shield path times. For this purpose, times from mixed paths should be advanced by 2 sec, which is the residual difference scaled by the angle of incidence of rays in the upper 200 km of the mantle. In the figures, however, only uncorrected, observed times are displayed.

Figure 4 shows first-arriving travel-time branches calculated for four P-wave velocity models tha t differ only between 210- and 410-km depth. The structure of the lid was adjusted to match the times, apparent velocity, and maximum range of observation of Pn. Details of the high velocity "snout" are not well constrained by the times, but the data require very high velocity at the base of the lid, which must be at a depth of 185 km or greater. Although there is a tradeoff between the depth to the base of the lid and the velocity gradient in the lower portion of the lid, we were unable to find any satisfactory model with a lid thinner than 185 km or thicker than 225 km. Models tha t have a lid thinner than 185 km and tha t have sufficiently high velocities to fit the P times at distances less than 2000 km do not predict an arrival from the lid at 2200 km, even if an allowance is made for P propagation 1 ° beyond the end of the branch calculated from geometric ray theory. However, at 2200 km stations MUN and NWAO record impulsive, high-frequency P waves, which are unlikely to be diffracted phases. Models with a lid thicker than 225 km predict an arrival from the lid extending beyond 2300 km, which has not been observed. In the preferred model SOZ-P (Table 1), the base of the lid is assigned to 210-km

S T R U C T U R E OF T H E A U S T R A L I A N S H I E L D 33

depth for consistency with the parameterization of IASP91, but the range of acceptable depths is 185 to 225 km.

There are no travel times for pure shield paths beyond 2300 km, so we use the DA summary times and data from eastern Australia at greater range to guide our interpretat ion of mantle structure beneath the lid. After allowance for station corrections, which are of the order _+ 1 sec for P waves and _+ 2 sec for S (Toy, 1989), travel times at teleseismic range ( > 30 °) are not strongly dependent on region, a fact tha t allows construction of meaningful global travel time tables. This implies tha t travel times from Austral ian shield sources should be in rough agreement with DA summary times at 30 °, which is 300 km off the right side of Figures 4 and 5. Here we assume tha t the regional dependence of rays bot toming below the 660-km discontinuity, which become first arrivals beyond about 2600 km, is no more than 150% of the nominal range of teleseis- mic station corrections.

With the high-velocity lid required to satisfy early P travel times out to 2200 km in the models shown in Figure 4, the conundrum is how to retard times at greater range to approximate DA and eastern Austral ian observations. Model K3 was constructed to match closely the DA summary times beyond 2300 km and has a pronounced low-velocity zone. In contrast, model K5 has no velocity inversion at the base of the lid, but ra ther a linear gradient between 210 and 410 km, and predicts times tha t are 2 to 3 sec fast relative to the DA summary times. Model SOZ-P adopts IASP91 values below 210 km and predicts times tha t are only 1 sec faster than the DA times. Times for SOZ-P agree well with eastern Austral ian times after a 2-sec adjustment for the difference in teleseis- mic residuals between western and eastern Australia. The velocity inversion at 210 km in model SOZ-P can be viewed as a re turn to normal mantle velocities ra ther than as a low-velocity zone per se.

Another class of models tha t can delay P times beyond 2300 km, but tha t does not involve a velocity inversion, is characterized by a thick layer of constant velocity below the lid. However, the constant-velocity layer would need to extend to 320-km depth in order to achieve a delay beyond 2200-km equiva- lent to tha t of model SOZ-P, and would then have a ra ther high gradient above

TABLE 1

UPPER-MANTLE VELOCITY MODELS SOZ-P AND SOZ-S

Depth (km) Vp (km/sec) V s (km/sec)

0 5.80 3.55 20 5.80 3.55 20 6.50 3.85 35 6.50 3.85 35 8.20 4.64

120 8.32 4.70 160 8.53 210 8.56 4.76 210" 8.30 4.52 410 9.O3 4.87 410 9.36 5.07 660 10.20 5.60 660 10.79 5.95 760 11.06 6.21

* IASP91 below this depth.

34 J. R. BOWMAN AND B. L. N. KENNETT

the 410 km discontinuity (model K9, Fig. 4). This class of model is similar to model NWB-1 (Fig. 6) tha t was derived for northwestern Australia using waveform data (Bowman and Kennett, 1990), yet the relative amplitudes of first and later arrivals in tha t study were best matched by a model with a negative velocity gradient below 230 km. Therefore, we prefer a model such as SOZ-P tha t has a small velocity inversion below 210 km.

There are no observed S times for pure shield paths beyond 2300 km and few for stations in eastern Australia or in the neighboring island arcs (Fig. 3), which suggests tha t these stations lie in the shadow of a low-velocity zone, so we must rely on the DA summary travel times as a guide for interpretation. S times calculated for models with a high-velocity lid, parameterized as a single linear gradient between 35- and 210-km depth, fit the observed times to 2200 km quite well (Fig. 5). Travel times for three models with different structures below the lid are distinctive beyond 2200 km. Model X1 was constructed to fit closely the S-wave summary times and has very low velocities below 210 km. Model X6, on the other hand, has no velocity inversion and predicts times tha t are 8 sec earlier than the summary times and predicts arrivals beyond 2300 km on the Sn branch, which are not observed. Model SOZ-S is a compromise structure tha t has IASP91 velocities below 210 km and predicts an S arrival from the lid out to 2200 km followed by a 12-sec offset in the S times. The predicted arrival at 3000 km is about 5 sec earlier than the summary times, which is on the order of the maximum difference in S station residuals between western and eastern Australia (Toy, 1989).

DISCUSSION

Comparison with Australian Waveform Observations

The SOZ models for P and S waves presented above are characterized by a high-velocity lid and a deep velocity inversion (Table 1, Fig. 6). These models were developed principally to satisfy seismic travel times within the Australian shield, and the main constraints on the presence and depth of the velocity inversion are the high apparent velocity of both P and S waves from 1500 to 2200 km and the discontinuity in travel times beyond 2300 km. Several models have been constructed previously to satisfy a different class of data, P wave- forms from earthquakes in the Indonesian arc recorded at arrays in northern Australia (Hales et al., 1980; Leven, 1985; Bowman and Kennett, 1990). The velocity models based on waveforms are generally similar to SOZ-P in this depth range, but predict times tha t are 2 to 3 sec later than times for model SOZ-P in the distance range from 1500 to 2000 km, probably owing to lower velocities near the sources used in the waveform studies, which are in a tectonic region.

The principal constraint on the velocity inversion in the models based on waveforms is the low amplitude of the first arrival from shallow earthquakes in the range 1600 to 2200 km (Bowman and Kennett, 1990). Broadband seismo- grams from the Warramunga array for earthquakes in Indonesia provide addi- tional independent evidence for a deep velocity inversion; for both S H and S V polarizations, there is a dramatic loss of high frequencies in the first arrivals beyond 2000 km range (Goody and Kennet t , 1991). Thus, travel times, ampli- tudes, and frequency content all point toward a zone of lower velocities below 210 km.

STRUCTURE OF THE AUSTRALIAN SHIELD

4 :_½' ---l/

I

T

10 0

- - s o z - s

SNA L -~1

I

4 5

Veloc i ty (km/s ) 5 6 7 8 9

I ~ I I I

!

S O Z - P ~

. . . . . . $25 .8 ,

I I

7 8 9

100

35

C9 200 o

300 3

400

1 ~)500 3 6

FIG. 6. (a) Comparison of P velocity models for Australia (SOZ-P; this study), for northwestern Eurasia (K8; Given and Helmberger, 1980), and shield North America (S25; LeFevre and Helm- berger, 1989). (b) Comparison of S models for the Australian shield (SOZ-S; this study), the Russian platform, and the Canadian shield (SNA; Grand and Helmberger, 1984, 1985).

Comparison with Global Observations

The SOZ models for Australia are compared in Figure 6 to models derived from waveform modeling for the Canadian shield and northwestern Eurasia. These models are all characterized by a high-velocity lid over a zone of lower velocities. Canadian shield S model SNA (Grand and Helmberger, 1984) and P model S25 (LeFevre and Helmberger, 1989) have higher velocity than the SOZ models at depths above 160 km and a shallower, gradational velocity inversion. As a consequence, SNA and $25 predict earlier arrivals and a shorter maximum range for lid phases than allowed by Austral ian travel times. Model K8 from Eurasia (Given and Helmberger, 1980) has mantle P velocities similar to SOZ-P above 140 km depth, but below 150 km, K8 has a thin low-velocity zone in a depth range at which high velocities are required by the Austral ian data. Impulsive, high-frequency seismograms are recorded out to 2200 km in the Austral ian shield, so these arrivals are unlikely to be diffractions from beneath the lid.

The lid of the Kaapvaal craton in South Africa is est imated to be less than 130 km thick from analysis of surface waves (Bloch et al., 1969; Cichowicz and Green, 1992). The depth of the velocity inversion in SOZ was influenced by the choice of model parameterization used in IASP91, but nevertheless the travel times would not be consistent with the base of the lid at a depth shallower than 185 km. Thus, the lid beneath the Austral ian shield is thicker than under the Canadian, Eurasian, or South African shields. On the other hand, beneath the Siberian platform, results from deep seismic sounding profiles indicate tha t the base of the lithosphere is defined by a significant low-velocity zone at 200 km depth (Egorkin et al., 1987), which is consistent with the Austral ian result.

Recent global tomographic images show high velocities in the mantle beneath the Austral ian continent to depths of 400 km or greater (e.g., Woodward et al.

36 J. R. BOWMAN AND B. L. N. K E N N E T T

1992). The travel t imes presented in this paper, on the other hand, require a high-velocity lid above about 210 km but can be satisfied below 210 km by velocities near the global average, such as those of model IASP91 (Kennett and Engdahl, 1991). Because of their potential to resolve vertical structure, regional studies of short-period data remain a valuable complement to tomographic inversions.

Comparison with Models of Anisotropy

Leven et al. (1981) at tr ibute the high P-wave velocity (8.6 km//sec) at 200-km depth beneath northern Australia to velocity anisotropy. They argue that the velocity on the nor th-south paths sampled in their s tudy is higher than predicted for reasonable petrological models of the mantle and suggest that the northward drift of the Austral ian continent may have oriented anisotropic minerals in a zone of shear decoupling at the base of the lithosphere. Unfortu- nately, they were unable to test this hypothesis because of the restricted distribution of ear thquake sources.

The P-wave travel t imes in Figures 2 and 4 sample a different range of azimuths than the data of Leven et al. (1981). The apparent velocity of P waves from Tennant Creek ear thquakes is 8.8 km//sec across stations in southwestern Australia (MEKA, KLGA, KLB, MRWA, BAL, NWAO, and MUN) along an azimuth of 225 °, 45 ° different from that of Leven et al. (1981). Nevertheless, this velocity is comparable to that reported by Leven et al. (1981) and thus weakens the evidence for anisotropy as the cause of high lid velocities. Only two stations on the shield, MBL and NANU, are si tuated at the appropriate range along an eas t -wes t azimuth from Tennant Creek. The low apparent velocity between these stations provides only ambiguous support for azimuthal anisotropy be- cause of unknown crustal s tructure effects at these stations.

Both the P and S velocity structures (Fig. 6) are characterized by high-veloc- ity lids over deep velocity inversions. The general concordance of P and S structures is indirect evidence that the high P velocities are not primarily a consequence of anisotropy. Rather, the exaggerated decrease in S velocity below 210 km, relative to that of P , suggests tha t thermal structure plays a dominant role.

ACKNOWLEDGMENTS

We thank Peter Gregson and Ed Paull for data from the Mundaring Geophysical Observatory. Oli Gudmundson, Anton Hales and Ken Muirhead provided useful comments on the manuscript. Constructive reviews by John Vidale and Steve Grand are appreciated. Bob Woodward kindly provided preprints and figures in advance of publication.

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RESEARCH SCHOOL OF EARTH SCIENCES AUSTRALIAN NATIONAL UNIVERSITY GPO BOX 4 CANBERRA, ACT 2601 AUSTRALIA

Manuscript received 16 January 1992