Seismicity of the Australian continent

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Seismicity of the Australian continentH. A. Doyle a , I. B. Everingham b & D. J. Sutton ca Dept of Geophysics & Geochemistry , Australian NationalUniversity , Canberra, A.C.T., 2600b Bureau of Mineral Resources , Mundaring GeophysicalObservatory , Mundaring, W.A., 6073c Dept of Physics , University of Adelaide , G.P.O. Box 498d,Adelaide, S.A., 5001Published online: 01 Aug 2007.

To cite this article: H. A. Doyle , I. B. Everingham & D. J. Sutton (1968) Seismicity of theAustralian continent, Journal of the Geological Society of Australia: An International GeoscienceJournal of the Geological Society of Australia, 15:2, 295-312, DOI: 10.1080/00167616808728700

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SEISMICITY OF THE AUSTRALIAN CONTINENTBy H. A. DOYLE, I. B. EVERINGHAM & D. J. SUTTON

(With 2 Tables and 5 Text-Figures)

(Received 30 July 1968; read in abstract at Canberra, 15 October 1968)

ABSTRACT

Data on Australian earthquakes up to 1966 are presented. The most active areain the continent is the Adelaide seismic zone. The epicentres follow quite closely thetrend of the Flinders and Mt Lofty Ranges, and appear to be associated with theSouth Australian rift zone. Some seismic zones such as that in southwestern Australiamay be associated with structural boundaries at depth in the crust and upper mantle.On the other hand there is little seismicity along some marked faults such as the1000-km long Darling Fault in Western Australia.

All reliable depth determinations for Australian foci so far place them withinthe crust, mostly the upper crust. Such shallow crustal continental earthquakes shouldperhaps be regarded as a special class.

INTRODUCTIONThe Australian continent is adjacent to some

of the most seismically active regions of theglobe, in particular to the Southwest Pacificbelt from Indonesia and New Guinea to NewZealand. To the south and west of the con-tinent are the moderately active mid-oceanridges of the Southern and Indian Oceans.However, Australia itself has been called thequiet continent and seismically this is true, asonly Antarctica, which is almost completelyaseismic, has less contemporary activity. Thefact that these two southern continents, andthe African continent, are so inactive needs tobe taken into account in any theory of Earthhistory.

Both have large Precambrian shield areas,and Australia also has the lowest relief (aver-aging about 300 m) and is perhaps the leastdeformed of all the continents, with no activevolcanoes and no very high mountains (David,1950). The Australian shield or platformoccupies the western and central parts of thecontinent (Fig. 1), with some intracratonicbasins and geosynclines. In the east are theGreat Artesian and Murray Basins, with theEastern Highlands forming an extended arcalong the east coast. These highlands are withinthe Tasman Geosynclinal Zone, a large irregu-lar geosynclinal area, the southeast portion ofwhich is sometimes referred to as the LachlanGeosyncline. The Tasman Geosynclinal Zone

has a history of persistent tectonism extendingfrom the Cambrian (Hills, 1965). It may ex-tend westward under the Murray Basin to theAdelaide Geosyncline. However, it cannot ex-tend very far off the southeast coast of Aus-tralia, as gravity and seismic data indicate atrue ocean structure beneath the Tasman Sea(Standard, 1961). The crustal structure ofAustralia so far examined is normal continental(e.g. Doyle & Everingham, 1964, found a 40km crustal thickness in southern Australia).

Although Australian seismicity is low it haspractical as well as scientific importance. A fewearthquakes of magnitude m^5i occur eachyear, and small local earthquakes in some areasare detected at the rate of several per monthby short-period seismographs of high magnifi-cation. These tremors are not in the main ofsufficient magnitude to be noticed by nearbyinhabitants, but occasionally damage occurs.An earth tremor* was felt at Port Jackson,New South Wales, within a few months of thearrival of the First Fleet in 1788, and anearthquake was reported from the Lake Georgearea of New South Wales (probably the Gun-ning zone) in 1828 (Anon., 1829). Explorerssuch as Sturt noted earth tremors in westernNew South Wales, the Flinders Ranges and innorthwest Australia (see Clarke, 1869; Cleland,1912).

Melbourne has been disturbed by severaltremors since the first noted in 1841 (Gregory,

* We find the term tremor, or earth tremor, useful to denote a small earthquake, often not felt. (Thepioneer seismologist J. A. Ewing defined it as a small earthquake, imperceptible without instrumentalmeans.)

J.geoI.Soc.Aust., 15(2): pp. 295-312, 1968.

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296 H. A. DOYLE, I. B. EVERINGHAM & D. J. SUTTON

1903). Adelaide experienced the most destruc-tive earthquake in our history on 1 March1954, when some millions of pounds worthof damage was claimed to have occurredto buildings (Kerr Grant, 1956). The largestrecorded earthquake within the continent (ex-cluding New Guinea) was that in sparselyinhabited country about 300 km southeast ofCarnarvon, Western Australia, in 1941; thiswas of magnitude m = 6-8 (Clarke, Prider &Teichert, 1955). A shock of larger magnitude(m = 7-4) occurred outside the continentalshelf, 800 km northwest of Carnarvon in 1906,and was felt along the west coast from Albanyto the Port Hedland region, and also at sea byRMS Omrah at Latitude 21-3°S, Longitude105-5°E.*

Among the first accounts of earthquakes inAustralia were those of Jevons (1859) ontremors in New South Wales, and Clarke(1869) in which the 'father of Australiangeology' referred to a catalogue of earthquakesin Australasia but did not publish it. Ellery(1874) reported a Gippsland earthquake, andreferred to Clarke's catalogue as containing160 events up to 1868, but this included NewZealand earthquakes. Biggs (1885) and Shortt(1886) reported the Tasmanian swarm ofearth tremors of 1883-1886, when over 2,500tremors were felt in northeast Tasmania. LaterDodwell (1910) produced an account of theseismicity of South Australia and what is nowpart of the Northern Territory, and Taylordrew up a map of felt earthquakes in NewSouth Wales based on records kept by weatherobservers (in Jose et al., 1912). Gregory(1903) and Howchin (1910) also discussedVictorian and South Australian tremors.

A seismological committee was formed bythe Australasian Association for the Advance-ment of Science which made available earth-quake report sheets and published some listsfor a time (see Reports, 1892-1913). Thiscommittee also encouraged the installation ofMilne seismographs at State astronomical ob-servatories in Australia and New Zealand.

A brief discussion of the seismicity of thewhole continent was given by Gutenberg &Richter (1949) and they listed only ten earth-quakes of magnitudes (M) 5-3 to 7. This wasfollowed by the more detailed map of Burke-Gaffney (1952), and the redetermination of

some epicentres by Bolt (1959). Since then,and particularly in 1958-1959, there has beena great development of seismological stationsin this country (Doyle & Underwood, 1965).From five or six stations of generally low sen-sitivity, the number has grown to over forty.The latest development is the instalment of a20 km X 20 km seismic array with twenty seis-mometers near Tennant Creek (N.T.) by theU.K. Atomic Energy Authority, and operatedby the Australian National University. Thusdata on Australian seismicity are accumulatingat a much greater rate than before. However,large areas of the continent are still not ade-quately covered by seismic stations, particularlyin northwest Australia.

New Guinea could be considered as part ofthe Australian continent, being within the 100-fathom (180 m) contour. However, the seis-micity of New Guinea is not dealt with here.Brooks (1965) has discussed the seismicity ofthe eastern half of the island (Papua and Aus-tralian New Guinea).

DATAFor the epicentres included in the tables

and in Figure 1, a lower limit of magnitudem = 4-5 (ML = 3-5) has been adopted. Thislimit is rather low because of the need to makeuse of as many data as possible, without, how-ever, causing unacceptable distortion arisingfrom non-uniform station distribution. InFigures 2 to 5 epicentres down to magnitudem = 4 (ML = 3) are included to show theseismic zones more clearly. The low magnitudeepicentres (up to 1966) are mostly listed inthe papers cited.

In the Appendix a discussion is given of thedifferent methods of determining magnitude,about which there is some confusion We havepreferred the P wave magnitude m, and haveconverted magnitude values to this scale wherenecessary, as discussed in the Appendix.

Table I lists the more reliable epicentresprior to 1959 when the large expansion ofAustralian stations occurred. Most of thesehave been discussed before by Burke-Gaffney(1952) and Bolt (1959), but are presentedhere for completeness. Several were determinedfrom felt reports, but appear to have reasonableaccuracy despite lack of instrumental data.

Table II lists the more recent epicentres

* Whilst this paper was in press, an earthquake of magnitude 7 occurred near Meckering, WesternAustralia, at Lat. 31-7°S, Long. 117°E on 14 October 1968, causing severe damage in the town.This shallow focus earthquake was located in the southwestern seismic belt and is now the strongestrecorded in Australia. Surface faulting occurred over a distance of 27 km.

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SEISMICITY OF THE AUSTRALIAN CONTINENT 297

-V

to

E

(1959 to 1966 inclusive), grouped by States.These are all instrumentally located and mostare reliable to ± I degree, and better forepicentres within close networks. Apart fromapproximate depth determinations by the U.S.Coast and Geodetic Survey for larger shocks,depths are given only for well recorded earth-quakes near seismic networks. The symbol Rdenotes an approximate depth assumed by theU.S.C.G.S. from preliminary data.

We will now discuss the data in more detailby States, as' in the Tables.

WESTERN AUSTRALIA

The seismicity of Western Australia hasbeen discussed recently by Everingham(1968a) From 1901 till 1958 there was onlyone seismic station in the whole of this vastState. However, the Bureau of Mineral Re-sources has now installed three stations in the

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TABLE I

Australian Earthquakes Prior to 1959 for which Epicentres areAvailable.

Date

10 May 189719 Sep. 19026 Apr. 1903

19 Nov. 190618 Dec. 1913

6 June 191815 Aug. 1919

8 Feb. 1920

10 Apr. 1922

16 Aug. 192928 Dec. 1929

27 Oct. 1930

2 Sep. 193212 July 193410 Nov. 1934

18 Nov. 1934

21 Nov. 1934

12 Apr. 1935

28 Oct. 193720 Dec. 193724 Mar. 1938

17 Apr. 193826 Mar. 19391 May 1939

5 June 1939

29 Apr. 19414 May 194127 June 194114 Feb. 1942

2 Nov. 194414 Sep. 1946

11 June 19476 Aug. 1948

10 Mar. 19495 Apr. 195030 Dec. 195124 June 19527 Sep. 1952

3 Dec. 195328 Feb. 195419 Sep. 19541 Feb. 19551 Jan. 1958

1 Sept. 1958

1 Dec. 1958

Origin Time(G.M.T.)

1011

0713

1810

05

10

2101

02

181423

12

06

01

092220

080319

12

01220720

1419

1003

2219200105

1518101100

11

10

3052

1854-

1421

24

47

2822

03

222447

58

32

32

343503

565607

20

35075550

0548

0329

3050343441

4309380907

18

38

410

2421

30

39

2253

51

322740

41

06

24

430233

220529

43

41305143

4342

1323

33

52

02

Lat.°S

373539

19-120

2433-5

35

40

17040

34-5

38-314-834-9

34-5

34-5

26

26-025-435-5

25-532-031-4

31-5

26-826-325-729-5

38-040

25-537

34-821-125-825-534-8

24-534-828-526-242-2

36-4

16-5

Long.°E

140137144

111-8147

152150-7

111

147-5

120-9149

149

145112-31500

149-5

149-2

151

136-5136-51460

137-2138-0138

138-5

116-1136-9137-8136

145-9148

152-7137

149-3149-2150-9152-8149-3

151-5138-7148-5151-2146-1

149-3

145-5

Mag.(m)

Reference

(6-6)DodweU(1910)(6-3)Dodwell(1910)(5 • 5) Gregory and

7-4

6 14-7

6-4

5-5

6-45-7

5-7

6-35-3

6 0

5-3

5-7

605-7

6-3604-7

4-7

6-8606-65-3

5-7

6 0

Stover (1966)Gutenberg &

Richter(doubtful)Burke-Gaffney

(1952)Gutenberg &Richter (1954)

Burke-Gaffney(1952)

Bolt (1959)Gutenberg &Richter (1954)

Burke-Gaffney(1952)

Holmes (1933)Stover (1966)Burke-Gaffney(1952)

Burke-Gaffney(1952)

Burke-Gaffney(1952)

Bryan &Whitehouse(1938)

Bolt (1959)Bolt (1959)Burke-Gaffney(1952)

Bolt (1959)Bolt (1959)Burke-Gaffneyviyj^

Burke-Gaffney(1952)

Bolt (1959)Bolt (1959)Bolt (1959)Burke-Gaffney

Gaskin (1945)Gutenberg &Richter (1954)

Jones(1948)Burke-Gaffney

5-3 Joklik(1951)(5-0) Jones (1959)(4-5)Jones(1959)(50)Jones(1959)(5 0)Joklik & Casey

(1952)(4-5)Jones(1959)(6) Bolt (1959)(4-5)Jones(1959)(4-5) Jones (1959)5-7 Green et al.

(1967)4-7 Cleary et al.

(1964)(5-0)Jones(1959)

southern part of the State and the Public WorksDepartment has operated one at Kununurrasince 1966. Only three accurate epicentres hadbeen located in the Western Australian regionup to 1958. Among these was that of 29 April1941, about 300 km southeast of Carnarvonand of magnitude (m) 6-8, the largest earth-quake recorded beneath the Australian con-tinent until the recent Meckering event. It hada radius of perceptibility of about 850 km,

TABLE II

Epicentres of Earthquakes 1959—1966 for which m & 4-5.(a) Western Australia

Epicentres determined by Mundaring Geophysical Observa-tory

(McGregor, 1967; Everingham, 1968 a, b).

Date

3 Oct. 195927 Nov. 195912 June 1961

• 18 June 196130 July 196123 Aug. 196110 Nov. 19611 Jan. 1962

18 Jan. 196326 Feb. 196315 Apr. 196318 Apr. 1963

^ 27 Aug. 1963* 7 Sep. 1963

19 Nov. 1963¥= 23 Mar. 1964• 12 May 1964

18 May 1965=£ 19 May 1965

10 Sep. 196511 Oct. 196523 Feb. 196626 Apr. 196630 Apr. 196629 Aug. 1966

• 13 Nov. 196625 Dec. 1966

OriginTime

(G.M.T.)

120618160718142305140019190817220710021204030303130322

072500132801592949104358154452410817132407405824014134

222251583433145216-819541043350514-444-65247-60127131758104827

Lat.°S

34-525-834-220-136-918-537-5

(34-0)32-222-2

(21-8)32-316-613-23 1 017-811-017-525-018126-937-831-716-93 0 024-131-0

Long.°E

114-5116-2114-5119-3121-0119-0118-4

(126-0)117-1121-1

(120-3)117-2128-6122-1116-3122-81260121-0112-5122-2110-5106-8111-6121-01090111-9116-4

Mag.(m)

5-15-34-55-64-55-45 05-05-85-25 14-85-8

Depth(km)

18J?1SR1SR1318/?18/?18/?18/?1818J?18J?18/?33R

(5-5) (256)4-95-95-65-05-95-25-25-34-85 05-45-64-5

18/?33/?3318/?OR

18/?18/?18/?18/?ISR18/?33/?18/?

White (1968)USCGS epicentre

(b) South Australia and Northern Territory(Epicentres from Sutton & White (1968)).

*

*

•

*

++

0

•

+

0

Date

21 May 19599 Sep. 19592 Nov. 1959

18 Aug. 196030 Aug. 196031 Aug. 196012 Nov. 196010 June 196110 Jan. 19623 Mar. 1962

16 May 19627 July 19626 Sep. 1962

14 Mar. 196328 Mar. 196329 Mar. 19633 Dec. 1963

12 July 196417 Jan. 196525 Jan. 1965

1 Mar. 19652 Mar. 1965

14 Mar. 19654 June 1965

28 Aug. 196528 Aug. 196522 Nov. 196514 Sep. 196617 Oct. 196623 Nov. 19663 Dec. 19667 Dec. 1966

OriginTime

(G.M.T.)

1104011521022315192221041401012104000220131512100000221700200602

2817170423140358360441304857315659394822391847452645152527480649

Sutton & White (1966a)Sutton & White (19666)White (1968)USCGS epicentre.

463057-24810500800252235-5042135-0132044-402-834-954-202-853-942-310-338-841-409-736-704-703-43738-9

Lat.°S

31-432-733-433-834-033-534-634-536-3533-035-531-334-525-715-431-131-934-128-131-930-330-531-932-032-232-433-831-734-434-408-833-2

Long.°E

139-0138-21360136-15136-0136-4135-51350139-8136-0137-7138-6139-0137-4133-3138-5138-4134-7135-8138-5138-1138-2138-5138-6138-3138-2134-3141-4136-0139-3135-5139-0

Mag.(m)

5-45-35-95-35-35-45-45-25-15-25-54-64-55-55-75-14-65-05-25-64-85-85-75-35-34-94-84-75-05-35-34-7

Depth(km)

0«

25

33ROR

9

OR

34

1625

11

19

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SEISMICITY OF THE AUSTRALIAN CONTINENT 299TABLE II (Continued)

(c) Victoria and TasmaniaMost epicentres have been determined by Department of

Geophysics, Australian National University, and have beenre-examined by Underwood (1967).

Date

28 Jan. I9601 June 1960

20 Oct. 196021 Oct. 196024 Dec. 196022 Jan. 196122 Jan. 1961

3 Feb. 196110 Apr. 196111 Apr. 1961

• 1 June 19611 June 1961

12 Sep. 19619 Oct. 1961

10 Oct. 196110 Jan. 196210 Jan. 196227 Mar. 196214 June 19633 Nov. 1963

• 14 Nov. 196417 Nov. 1964

• 18 Mar. 19658 May 1965

• 14 Sep. 196514 Sep. 1965

• 14 Sep. 19653 May 19665 July 1966

17 Oct. 196627 Oct. 196615 Dec. 1966

OriginTime

(G.M.T.)

2305201416141814140009130723130506171912102118061212121922170019

3618224742433937103535191343364012542300531609253444530726122508

5647-604-657-008-555454532-623-435-75109-414-308311722-247-844-30118-53 1 153-236-3571353-139-430-84629-1

Lat.°S

36-836-838-638-93939-5404037-138-338-438-340-739-239-238-538-537-938-643-34 0 038-440-238-738-7

Long.°E

147-1145-4146-5146-5143-5155-5155-5148-5144-1146-5144-2144-1156-6145-9145-9146-5146-5146-5146-4146-0144-3146-4149-6149-2144-2

After shock38-737-039-637-94 0 040-4

144-3147-1145-0146-2154-5155-4

Mag.(m)

(4-7)4-54-74-65-84-74-74-74-74-74-74-94-94-74-74-54-54-55-2

(5 0)5 04-75-54-55-54-56 05-74-84-65-25-8

Depth(km)

15128

529

62

16

27

8

18

• USCGS epicentre.

(d) New South Wales(Most epicentres determined by Department of Geophysics,

Australian National University.)

Date

• 18 May 195912 Oct. 19596 Sep. 19606 Oct. 19609 Dec. 19608 Feb. 1961

13 Feb. 196116 May 1961

+ 21 May 196123 Nov. 196116 Jan. 196229 Sep. 196215 May 19638 Jan. 1964

13 Mar. 196421 Mar. 196429 May 196419 Dec. 196422 Apr. 1966

OriginTime

(G.M.T.)

06 1221 2302 5819 3406 5718 5619 2306 5221 4001 1706 1501 3118 2223 3008 0702 1809 4616 4403 26

59-2(40)01

114719540 3 0

(25)52353328363933

(16)52-7

Lat.°S

36-23134-43333-535-333-93134-533-532-234-434-934-133-934-234-833-734-2

Long. Mag. Depth°E (m) (km)

148-7151-5150-9148-5151-5149-3149-0147-5150-5149149-2150-0149-3147-9148-2149-2149-2148-2148-2

5-5(5)4-54-74-54-54-5

5-8(4-8)4-94-54-54-54-54-54-54-54-5

17

19

14

• Cleary, Doyle & Moye (1964)+ Cleary & Doyle (1962)

being felt from Port Hedland to Albany andKalgoorlie (Everingham, 1968a). At MeeberrieStation near the epicentre, ground cracks werefound oriented meridionally in a narrow zone35 m long. They were up to 10 cm wide and12-15 cm deep. Whether these were only

slumping effects in soft ground is unknown.Felt reports of earthquakes were kept at thePerth Observatory for many years and thesehave proved helpful.

In Western Australia seismic activity appearsto be mainly confined to zones near the westernand northwestern coastal regions and onlyoccasional earthquakes are known to haveoccurred in the inland parts of the shield.Nevertheless shield areas may be more activethan is sometimes supposed.

Earthquakes have been most frequent (a)in the region around Broome, (b) in theMarble Bar-Port Hedland area, (c) west ofCarnarvon near the continental margin, (d)southeast of Carnarvon near Lat. 26 °S, Long.117°E, and (e) in a zone with its centre nearLat. 32°S, Long. 117°E crossing the south-western part of the shield. Activity to a lesserdegree has occurred in other localities such asthat to the west of Cape Leeuwin and the areaabout 130 km southsoutheast of Wyndham.

TABLE II (Continued)(e) Queensland

(Epicentres and magnitudes determined by University ofQueensland Seismological Station; courtesy of Dr J. P.Webb.)

Date

19 Oct. 196017 Nov. 196028 Dec. 196115 Jan. 19643 Mar. 19643 June 1965

OriginTime

(G.M.T.)

36 3100 2036

553556-3

591359

Lat.°S

21-027-428-518-025-427-8

Long.°E

148-8150-7142-5143-3151-7150-1

Mag.(m)

4-75-1

(5-6)4-94-55-4

pth(km)

+ White (1968)

Epicentres in the northernmost part of theState are shown in more detail along withgeological structures described by Veevers(1967) in Figure 2. In addition U.S.C.G.S.epicentres for 1960-1967 in the southern partof the Banda Arc and provisional epicentresof several 1967-1968 Australian earthquakes(not listed in Table II) are plotted to illustratethe regional seismicity more clearly.

The most northerly towns in the State suchas Wyndham, are occasionally shaken by earth-quakes in the Timor region, but there may alsobe local activity. The Kimberley Block mayprove to be more active than shown here whenseismic stations are set up in the northwest inthe future.

In the region illustrated in Figure 2 the in-tense seismicity associated with the orogeniczone passing through the Timor region pre-dominates and there is comparatively little

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activity south of the 200-m bathymetriccontour marking the edge of the Australiancontinent. Here, the majority of continentalepicentres occur in an arcuate zone aroundthe western end of the Canning Basin wherethree earthquakes (1929, 1961, 1964) havebeen large enough to be well recorded on over-seas seismographs.

the intensified seismicity at its junction withthe Canning Basin axis.

Since the establishment of Mundaring seismicstation, and later Kalgoorlie and other tem-porary stations, by the Bureau of MineralResources, many low magnitude earthquakeshave been located in a minor seismic zoneacross the southwest corner of the State. The

Geological structure128° after Veevers (1967)

Fig. 2. Seismicity and geological' structures, northwestern Australia.Larger circles, m > 4-4; smaller circles, 4-0 < m < 4-4Earthquake epicentres not listed in Table II are shown byopen circles, Australian epicentres being obtained from Mun-daring Observatory annual reports for 1967 and 1968 and theremainder from U.S.C.G.S. Earthquake Data Reports.

Evidence for downwarping seaward of theCanning Basin is given by the tectonically de-pressed nature of the continental shelf marginthere (Fairbridge, 1967), hence seismicactivity could indicate that the warping is con-tinuing within the arc of epicentres. Bouguergravity contours trend perpendicularly to thecoast, suggesting that the basin extends faroffshore. Stresses associated with the CartierFurrow may influence the warping and explain

epicentres are shown in Figure 3, which in-cludes smaller magnitude events not listed inTable II. This seismic zone has been referredto as the Yandanooka-Cape Riche Lineament(Everingham, 1968a) and may have somerelation to deep structure across the south-west corner of the Shield.

Partial coincidence of this zone of seismicitywith landscape zones outlined by Mulcahy(1967) is of interest. The junction of the Salt

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Lake and Sand-plains region with the YoungerLaterites is of significance because it marksthe limit of preservation of the Tertiary drain-age system, and a climatic control for its pre-sent position has been suggested (Mulcahy,1967). However, some parallelism with thezone of seismicity is also apparent, which sug-gests that the formation of the present land-scape may also have been influenced by tectonicprocesses such as differential uplift or a changein tilting across the active lineament. Bouguergravity anomaly contours also trend parallel tothe zone of seismicity and interpretations ofgravity, seismic-refraction (Everingham, 1965),and electromagnetic data (Everett & Hyndman,

1967) indicate that a change in crustal or upper-mantle features could take place across thisseismic zone. Furthermore, the active zone canbe correlated with geological features and datasuggest that the lineament has been a zone oftectonic importance at various times duringand since the Precambrian (Everingham,1968a). The northnorthwesterly projection ofthis zone meets the Darling Fault where itchanges from a northerly to a northwesterlystrike.

Possibly because of the lack of recordeddata, other active regions in the western partof the State do not show clear trends and theirtectonic implications are obscure. Earthquakes

4-0 « rn « 4-4 •

Fault •

m > 4-4 |

• Buried fault

O Kalgoorlie

100 200 300

Kilometres

115- 118' •ay

Fig. 3. Southwestern Australian seismicity. Epicentres of smaller earthquakes obtained from Evering-ham (1968 a, b) and McGregor (1967).

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may be related to upper-mantle effects alongthe west coast region where primarily ten-sional stresses have occurred since the Pre-cambrian to form the Perth and CarnarvonBasin structures. It is interesting to find thatthe central basin regions along the westerncoast are almost aseismic at present, with stressadjustments apparently taking place along eachside of the basins but not exactly at theirmargins.

It is an interesting fact that few epicentresare close to the 1,000-km long Darling Faultwhich marks the western boundary of the shieldand eastern boundary of the Perth Basin. Amajor negative gravity anomaly exists over thisbasin and the region is isostatically unbalanced.There is no geological evidence of movementon the Darling Fault since the Pliocene. How-ever, the 1941 earthquake epicentre was eastof the northern end of the Darling Fault, nearsome parallel faults. Allen et al. (1965), in acomprehensive study of Southern Californiaearthquakes, concluded that small earthquakesdo not necessarily follow large fault zones, butthat infrequent large earthquakes tend to do soif the zone is active. However, the geologicalevidence is against the Darling Fault's beingactive. There is also little evidence so far ofepicentres along other faults of the Perthbasin, or along the Stirling, Bremer and FrazerFaults which are long but ancient structures(see Tectonic Map of Australia, 1961).

Some offshore epicentres near Latitude 37°Scould be related to the Diamantina FractureZone which extends south of Western Australiafrom the Indian Ocean (see Stover, 1966).Earth tremors felt near mining towns (e.g.Kalgoorlie) are sometimes caused by rock-bursts, that is, release of hydrostatic stress indeep mines.

SOUTH AUSTRALIA AND NORTHERN TERRITORY

The most active area of the Australian con-tinent is that of the South Australian seismiczones. The South Australian area has beenmonitored since 1962 by a small network ofstations operated by the University of Adelaide,and the epicentres have been discussed bySutton& White (1968).

The South Australian epicentres occurmainly in two belts (Fig. 4), the major onebeing within the Adelaide Geosyncline andreferred to as the Adelaide seismic zone. Itextends from Kangaroo Island through the MtLofty and Flinders Ranges to Leigh Creek inthe north. The epicentres generally follow theranges and also the western boundary of the

Upper Proterozoic Sequence and the fold trendsin that part of the geosyncline. They curvearound to the east of Lake Torrens. It will beof interest to see if future data to the northshow trends towards the western side ofLake Eyre where Wopfner & Twidale (1967)report earth tremors and mound springsaligned meridionally. Dodwell (1910), fromearly reports of earth tremors, mapped activeareas south and west of Lake Eyre, and farthernorth towards Oodnadatta. Another possibilityis for continuation of the Adelaide zone north-east towards the Cooper Creek Basin wheresome epicentres have been mapped. Laherrere& Drayton (1965) show geophysical evidencefor trends in both these northwesterly andnortheasterly directions from the Leigh Creekarea. A long fault structure has been mappedparalleling Strzelecki Creek to the northeast(Sprigg, 1953). A possible branching alsooccurs near Quorn where some epicentres trendeastwards along the Olary Arc towards BrokenHill. The most active section of the Adelaidezone at present is the Hawker-Quorn area(Sutton & White, 1968).

The other main South Australian seismiczone is on Eyre Peninsula. Some epicentresthere may be associated with the Lincoln Faulton the eastern side of Eyre Peninsula (Glaessner& Parkin, 1958), and other epicentres offshoreon the western side appear to be aligned north-wards. Thus association with the edges of theGawler block is possible.

The Adelaide geosyncline has been describedby Hills (1961) as a rejuvenated Palaeozoicorogenic belt. It originated in the late Protero-zoic, suffered its main deformation in theCambrian, with revived tectonic activity in theCainozoic. There was northerly and north-easterly trending faulting in the Mt LoftyRanges, and movements near the east marginsof Yorke Peninsula and Eyre Peninsula, andat Kangaroo Island. St Vincent and SpencerGulfs are regarded as gr'aben. The Adelaideearthquake of 1954 could be associated withthe Eden Fault which forms part of the scarpof the Adelaide Hills (Kerr Grant, 1956).Minor cracking occurred at the foot of TapleyHill.

The largest known South Australian earth-quake, and one of the largest in Australianhistory was the Kingston-Beachport earthquakeof 1897. This occurred outside the two seismicbelts, damaging many buildings in Kingston,Robe and Beachport in the southeast corner ofthe State (Howchin, 1910). It was felt fromPort Augusta to Melbourne and parts of Tas-

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135° 136° 137" 138° 139° 140°

28° • O -f

29°

30°

o

-h

31°

35°

36°

O 3-5

O 4-5

O5.O-5

) m

>rn< 4-54 ™ < 5

^ m < 5-5

< m < 6

> 6

4-

i

; KANGAROV ISLAND\ \_

FAULTS

•28

-29°

•30°

32"

Olary

33°

-34°

35°

36°

37

_100 km37°

135° 136 138° 139° 140°

Fig. 4. South Australian seismic zones. Epicentres from Sutton & White(1968); with some major faults, by courtesy of Tectonic Map Committee.

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mania, thus having a radius of perceptibilityof about 600 km. Rents were produced in theground 2*5 to 3 m deep which spouted sandand water. A landslide was produced and theshock was felt at sea off Kingston on asteamer which was thought to have struck areef. In 1948 an earthquake centred northwestof Beachport was felt up to 400 km away. Off-shore faulting may be involved. The mainOtway basin ends to the northwest in this area

under eastern Australia as part of the 'granitic'layer detected by seismic methods.

Northern Territory seismicity is known intwo regions; southeast from Darwin, and inthe extreme southeast of the Territory. Dodwell(1910) showed a zone of activity in the DalyWaters-Darwin region, but there has been littleevidence of seismicity there more recently. Theonly listed earthquake in the area was in 1963.The southeast region of the Territory has been

35°S

40°S

145°E 150°E

Fig. 5. Seismicity of southeast Australia. Epicentres largely fromAustralian National University reports.

at the major Lucindale normal fault, trendingeastward (Weeks & Hopkins, 1967).

South Australian seismicity has been asso-ciated by some writers with the proposedeastern margin of the Precambrian shield (e.g.Gutenberg & Richter, 1954). However, it isquite likely that the Precambrian rocks extend

quite active in the past. It is within the GreatArtesian Basin and may be the northern end ofthe Adelaide seismic zone.

VICTORIA AND TASMANIA

For many years there was only one seismicstation in Victoria (Melbourne). In Tasmania,

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for the first time, in 1957 the University ofTasmania began setting up a four-station tele-metered network, and in 1963 the AustralianNational University extended its network intoVictoria (Doyle & Underwood, 1965).

Underwood (1967) has recently discussedVictorian seismicity. It is greater in the easternhalf of the State. The main areas of activityare the South Gippsland Hills and farthernorth, and off the Otway coast.

The former area is marked by a group ofepicentres extending north from Wilson Pro-montory (Fig. 5). This area includes the SouthGippsland Hills which are faulted horsts, andthe Gippsland plains, a sunkland traversed byeastnortheasterly-trending faults (Hills, 1951).Western Port Bay and Port Phillip also appearto be sunklands. The earthquakes near theOtway coast and ranges may be related to north-easterly-trending faulting along these struc-tures (see Weeks & Hopkins, 1967). Moredetailed studies are needed to check this, pre-ferably using stations in western Victoria. Oneof the largest Victorian earthquakes known wasone of a series near Cape Otway in 1903.Sand and mud was ejected near the mouth ofthe Hopkins River. It is of interest to note thatpresent seismicity has no obvious relation tothe many Holocene volcanic centres in westernVictoria (Oilier & Joyce, 1964).

There is a possible alignment of epicentresin a northwesterly direction in eastern Vic-toria. The Highlands, however, appear to befaulted down on the northwest side withnortheasterly-trending faults (Beavis, 1961).Lake Omeo is believed to have been producedby fault damming of a stream (Hills, 1951).

The Tasmanian data are sparse despite theswarm of earth tremors felt in northeast Tas-mania in 1883-1886. Four of these reachedan intensity of about VI (MM), and one wasfelt in Victoria and New South Wales as wellas over all Tasmania. The epicentres appearedto be offshore.

Recent epicentres have been plotted byGreen et al. (1967), mostly in western Tas-mania, but we have plotted very few becauseof the low magnitudes. However, the epi-centres near Flinders Island in the TasmanSea near Longitude 156°E (Fig. 1) areintriguing. Bass Strait is believed to be a sunk-land (Weeks & Hopkins, 1967). In Tasmaniamajor structures were produced late in theCretaceous or early Cainozoic, when the Mid-lands and Derwent graben were initiated.There are four main directions of Cainozoicfaulting, between north and northwest.

NEW SOUTH WALES

Quite a large number of epicentres areplotted for New South Wales, and southeasternAustralian generally (Fig. 5), partly becauseof the higher station-density and population.The region includes the highest portion of thecontinent, the Snowy Mountains in the South-east Highlands, which were produced by Caino-zoic movements culminating in the Kosciusko'Epoch'. Since 1958 most epicentres in south-eastern Australia have been located using theseismic networks of the Australian NationalUniversity, the Sydney Water Board and theSnowy Mountains Hydro-Electric Authority(e.g. Cleary, Doyle & Moye, 1964).

There are at least four minor seismic zonesin New South Wales:

(a) Cunning Area. This is in central NewSouth Wales, about 50 km northnortheast ofCanberra. Most epicentres trend southeastwardand northwestward from the north end ofLake George to Gunning and Dalton, and mayextend beyond Young (see Cleary, 1967a).Bouguer gravity contours also trend north-westwards in the Gunning area (A. Flavelle,pers. cotnm.), so there may be a deepstructure with this orientation. Seismic activityhas been felt in this area since the early settle-ments, and damage to homes has occurred onseveral occasions (cf. Joklik, 1951). As wellas the main northwesterly alignment of epi-centres, there is some indication of a meri-dional alignment of epicentres, which could beassociated with the meridional Cullarin scarpalong the western shore of Lake George. Somegeologists believe this scarp to be a recentfault. However, few epicentres have beenlocated directly west of Lake George. Cleary(1967fl) postulates compressive northwest-southeast forces that have produced large-scalewedging outlined by these two epicentral direc-tions.

(b) Robertson-Bowral Area. An earthquakeof magnitude 5i occurred on 22 May 1961 inthis area, 100 km southwest of Sydney. It hadshown little activity previously (Cleary &Doyle, 1962; Doyle, Cleary & Gray [in prep.]).The earthquake was one of the most widelyever felt in New South Wales and caused con-siderable damage to old stone buildings in theepicentral region, and also a landslide. It wasfollowed by an aftershock sequence of over100 earth tremors which were aligned in anorthwesterly direction. Direction of motionstudies, however, suggested that the fault ofthe main shock was a northeasterly-striking

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reverse fault. The aftershocks may delineatesecondary faulting, orthogonal to the mainfault.

(c) Snowy Mountains and Monaro Plains.This area in the Southeastern Highlands hasa scattering of epicentres mainly on and to theeast of the main range (Cleary, Doyle &Moye, 1964). An earthquake of magnitude 5occurred north of Berridale in 1959, anddirection of motion studies suggested a primary,northeasterly-trending reverse fault here also,parallel to the Crackenback-Thredbo valley.After-shocks were aligned northwestward alongpossible wrench faults. These may be relatedto the large wrench fault of Lambert & White(1965).

(d) New England Area. Earthquakes are feltin this area occasionally and a few epicentreshave been located, but seismic stations arerequired in northern New South Wales to de-lineate the seismicity more accurately there.

Thus northwesterly trends are apparent insome minor seismic zones in New South Wales.This was somewhat unexpected as the struc-tural trends are mainly meridional in southernNew South Wales and much of Victoria, forexample, the Palaeozoic granitic batholiths.However, near the border there are somenortheasterly- and northwesterly-trending struc-tures. The northwesterly epicentre alignmentsapparently represent contemporary tectonicmovement in the crust not always apparent insurface geology.

The continental shelf and coast of NewSouth Wales have not shown much seismicity.The coast has a narrow continental shelf andthe tablelands step down rather abruptly to thecoastal plains. Jaeger & Browne (1958) sug-gested the possibility of seismicity being asso-ciated with coastal and off-shore faulting, Thereis a little evidence of this along the centralcoast of New South Wales according to Doyle,Cleary & Gray (1968), who report small mag-nitude earth tremors in a study of the SydneyBasin seismicity. Epicentres also tend to followthe western margin of the Sydney Basin.

QUEENSLAND

This State appears to have quite a low levelof seismicity. Queensland epicentres for 1950-1958 were given by Jones (1959), and Dr J. P.Webb has kindly made available data on somerecent ones. There was no seismological stationin the State until 1937 when a Milne-Shawinstrument was set up at Brisbane, and ChartersTowers station did not begin recording till

1957, so that the data may be biased to thesoutheast of the State.

The great majority of epicentres are withinthe highlands and coastal areas, the mainareas of known highly folded and faulted rocks.Detailed correlation with faults and otherstructures cannot be made on present data.However, it may be noted that earthquakeshave occurred in the Maryborough Basin andto the west of this structure. These include theGayndah earthquake of 1935 which may havebeen associated with known northnorthwesterly-trending faults through the region (Bryan &Whitehouse, 1938). On the day following thisearthquake, subsidence and a crack 35 m longparallel to the faulting were found on a railwayembankment near Abercorn.

Queensland's largest known earthquake wasthat of 6 June 1918 (m = 6*1). Its position isuncertain, but we have taken Gutenberg &Richter's position near the coast. Hedley(1925) reported that it was felt from StLawrence to the New South Wales border, andwest to Roma.

The St George tremor of 1954 was close toone of the deepest parts of the Great ArtesianBasin, in the Surat sub-basin. Eighteen tremorswere felt at Georgetown in January-February1964; the main earthquake only is listed here.

Queensland, like Australia as a whole, ismade up of three basic structural elements; aPrecambrian block which is in the northwestand north; sedimentary basins, dominated hereby the Mesozoic and Cainozoic Great ArtesianBasin in the centre; and a geosynclinal element,the Palaeozoic Tasman Geosyncline in theeast. Within the last are the Eastern (or North-eastern) Highlands and the coastal plains (Hill& Denmead, 1960). The Highlands are a seriesof ranges and plateaux with scarps to the eastand trend northnorthwestwards, roughlyparallel to the coast.

De Keyser (1963) recently reported evi-dence for a major fault in Queensland, 1,000km long and forming the western boundary ofpart of the Tasman Geosynclinal Zone. Naobvious correlation can be seen yet betweenthis fault and seismicity.

DISCUSSIONSeismic activity on the Australian continent

may be divided into three general regions:1. Near the coastal areas of Western Aus-

tralia, plus a minor seismic zone across thesouthwest corner of the State.

2. The South Australian seismic belts near

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the rift zone area, with possible extensions tothe north.

3. The Eastern Highlands within the Tas-man Geosyncline, with some particular seismiczones such as the Gunning zone in New SouthWales.

Future improved data may reveal otheractive regions in northwest Australia and theinterior. The Kimberley block, which is bor-dered by mobile zones, may show significantseismicity. Permanent seismic stations are de-sirable in northwest Australia, and also centralAustralia, to check this.

The most obvious correlation is that of theAdelaide seismic zone with the Mt Lofty andFlinders Ranges and the nearby rifts. The focioccur mainly beneath the ranges rather thanbelow the rift or graben structures. The riftsinclude Lakes Torrens and Eyre, the latter thelowest area on the continent, 16 m below sealevel. This region is often thought of as theeastern edge of the Western Australian shield.

There is appreciable seismic activity alsoin the highest portion of the continent, theSnowy Mountains, but to a lesser degree.Richter (1958) pointed out that seismicity doesnot necessarily match structures in detail. Forexample, the seismicity of the Rocky Moun-tains is much lower than that of the CaliforniaCoast Ranges.

The Gunning seismic zone in New SouthWales is not associated with marked topo-graphy, but it does cut across a lower portionof the Great Divide between the northern endof the Southeastern Highlands and the BlueMountains area. In southwest Australia theYandanooka-Cape Riche zone coincides withmajor fold directions in the metamorphic rocks.There is some evidence of changes in crustaland upper mantle structure across the zone(Everingham, 1965; Everett & Hyndman,1967). Thus it may be argued that these seismiczones are associated with structural boundariesin the crust and perhaps the upper mantle.Kanasewich (1965) reported greater seismicityalong geological boundaries in the Canadianshield.

There is good evidence of significant differ-ences between shield regions and areas ofyounger uplift. Heat-flow values are twice ashigh in southeast Australia as on the WesternAustralian shield (Howard & Sass, 1964); andCleary (1967A) has found delays in teleseismicP travel times to southeast Australia relativeto those of the shield. Travel-time delays cor-relate with high heat flows and appear to be

caused by different structures and higher tem-peratures in the upper mantle (Hales & Doyle,1967).

Tasmania and Victoria are notable forgraben structures and extensive volcanics, andsouth of Tasmania a ridge-like structure ex-tends without quite meeting the Antarctic mid-ocean ridge. This South Tasmania ridge hasshown very little seismic activity, and couldhave some similarities to the Kerguelen-Gaussberg 'aseismic' ridge in the southernIndian Ocean. It suggests the possibility thatthe high heat flows of Tasmania and southeastAustralia were produced by a formerly activeoceanic ridge, extending under the continent.Heirtzler et al. (1968) show two meridionalfracture zones in the Southern Ocean, onetrending towards Tasmania, and the other to-wards the Adelaide rift zone. They are asso-ciated with displacements of the mid-oceanridge south of Australia. The Australian struc-tures have also been associated by some geolo-gists with the Trans-Antarctic Mountains andother features of the Antarctic continent.

The Tasman Sea basin appears to be aseis-mic, apart from some epicentres grouped nearLatitude 40 °S, Longitude 156°E. These epi-centres may have some relation to activitynear Flinders Island farther west (Underwood,1967). Thus the question arises as to whetherthere is a westnorthwesterly striking seismiczone across Bass Strait from Longitude 156°Eto Flinders Island and farther westwards ornorthwestwards.

There are some epicentres near the edges ofthe Canning and Sydney basins (e.g. Doyle,Cleary & Gray, 1968) but correlation withbasin structures should not be over-emphasised.There is surprisingly little seismicity alongsome extensive known faults. This is of par-ticular interest in the case of the 1,000-kmlong Darling Fault in Western Australia. Thisis one of the longest fault structures in theworld and marks the western edge of theWestern Australian shield. There is also littleseismic evidence so far for the proposedDarling lineament in New South Wales of Hills(1956). However, three epicentres in Queens-land are near the northeastern extension of thelineament. Rudd (1961) has suggested its con-tinuation into the Surat Basin, and it may alsobe related to the inferred Redan fault nearBroken Hill.

All reliable depth determinations for Aus-tralian epicentres place the foci within thecrust, mostly the upper crust. No certainexamples of foci within the mantle have yet

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occurred. Australian seismicity is similar inthis and other ways to that of much of NorthAmerica, Africa and Central Asia. Accordingto Solov'ev (1961), 'in the zone of block-faulted mountains (of Central Asia), not asingle creditable case is known where thefocus would have been located fully below theEarth's crust'. Such shallow crustal continentalearthquakes may have quite different mech-anisms from those of deep earthquakes andfrom those of ocean-ridge earthquakes, andshould perhaps be placed in a special class ofearth movement.

Another feature is that Australian earth-quakes have a much greater radius of percep-tibility for a given magnitude than given byGutenberg & Richter (1956) for Californiaearthquakes. Richter (1958) also noted agreater area of perceptibility for shocks ineastern North America. It is possible thatunusual structure reduces the felt area inCalifornia.

Richter has likened the Eastern Highlands tothe American Appalachians and Ozarks whichhave similar minor seismicity and no contem-porary volcanism. They appear similar also tothe Ural Mountains of Russia. The SouthAustralian rift zone is akin to that of the StLawrence area of North America. We havetherefore in Australia and North America largeshield areas with minor seismic activity, riftzones bordering part of the shield, and Palaeo-zoic mountain chains beyond with minoractivity.

Such earthquakes in the upper crust (depthsnormally less than 20 km) can be explainedby the classic Reid model of strain release byfracture. This model has serious objections fordeep earthquakes, but explains many featuresof shallow earthquakes (Mogi, 1967), such asthe occurrence of aftershocks at shallowerdepths than the main shock as observed byCleary & Doyle (1962) for the Robertsonearthquakes.

The sources of stress are usually thought tobe thermal imbalance in the upper mantle,perhaps with convective motion producingtraction on the overlying cooler and morebrittle layers, which cannot adjust fast enough

by creep. Of great interest is the possible re-lation of seismic zones to mantle convection.Runcorn (1967) has attempted to derive theglobal stress-patterns exerted by convectioncurrents on the crust from satellite gravitydata. He shows northnortheasterly-orientedstresses over the Australian continent producedlargely by convection from the Antarctic mid-ocean ridge south of Australia. Carey (1958)and others believe that northwesterly tensionalfractures have been important in eastern Aus-tralia. On the other hand Cleary (1967a) andUnderwood (1967) have argued for north-westerly-directed compressive forces at presentin southeastern Australia from direction ofmotion studies.

Some analyses of the magnitude frequencyrelations for Australian earthquakes appear toshow large regional differences in the parameterb of the equation

where N is annual frequency and M magnitude.Everingham (1968a) obtained a value for

b close to 1*0 for the southwestern Australiaseismic zone, and Sutton & White (1968)obtained a similar value for the South Aus-tralian seismic zones, a value found in manyparts of the world. However, Underwood(1967) obtained a high value of 1-4 for theGunning zone, and 0*6 for Victorian earth-quakes. According to Scholz (1968), high bvalues may be related to more ductile rock.The low value for Victoria could also be dueto insufficient data, this is certainly the case forthe b value of 0-4 given by Miyamura for allAustralia, and quoted by Allen et al. (1965),as only early Australian data were used.

Finally, it needs to be remarked that despitethe comparatively low level of Australianseismicity, moderate sized earthquakes(m = 5-6) occur frequently enough to be apotential danger, and this should be borne inmind in the design of large structures, par-ticularly in the Adelaide area.

ACKNOWLEDGMENTSOne of us (I.E.) thanks the Director of the

Bureau of Mineral Resources, Geology andGeophysics for permission to take part in thispublication.

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BATH, M., 1966: Earthquake Energy and Magni-tude, in Physics and Chemistry of the Earth,7, pp. 117-165. Pergamon, Oxford.

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CLARKE, E. DE C , PRIDER, R. T., & TEICHERT, C.,1955: Elements of Geology for Western Aus-tralian Students. (3rd Ed.) Univ. of West.Aust. Press, Perth.

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CLEARY, J. R., 1967a: The Seismicity of theGunning and Surrounding Areas, 1958-61. J.geol. Soc. Aust., 14, pp. 23-29.

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, & DOYLE, H. A., 1962: Application of aSeismograph Network and Electronic Com-puter in Near Earthquake Studies. Bull. seism.Soc. Am., 52, pp. 673-682.

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Adelaide, S.A. 5001.

APPENDIX

Magnitude DeterminationMagnitude is a quantity that is intended to rate

earthquakes according to their 'size'. It is deter-mined from the trace amplitude on a seismogramand ideally, of course, all such determinationsfrom seismograms obtained at different locationsshould yield the same value of magnitude for aparticular earthquake. Intensity, on the other hand,'refers to the degree of shaking at a specifiedplace. This is not based on measurement but is arating assigned by an experienced observer usinga descriptive scale' (Richter, 1958). It is the latterfigure that is usually of interest to insuranceassessors and others concerned with earthquakedamage.

The procedure for determining magnitudes isfar from straightforward and the complexitiesinvolved are clearly indicated by Richter (1958),Bath (1966), and others. Chiefly because of thedifferent characteristics of seismograms with differ-ing epicentral distance and depth of focus, threemain types of magnitude scale have emerged—local magnitude scales, surface wave magnitudescale (A/), and body wave magnitude scale (m).

The original local magnitude scale (ML) wasdevised by Richter for earthquakes in SouthernCalifornia using standard Wood-Anderson seismo-graphs. With the installation of many short period,high-magnification, electromagnetic seismographs(e.g. Benioff instruments), it has been possible insome regions to obtain consistent Richter localmagnitudes from the records of such instrumentsafter allowing for the different magnification andassuming the same amplitude-distance relation

found by Richter for Southern California. This isthe procedure adopted for small local WesternAustralian earthquakes by Everingham (1968a)who uses the mean maximum amplitude of thehorizontal components of the 5 phase recorded byshort-period Benioff seismographs at Mundaring.Magnitudes determined by the Department of Geo-physics at the Australian National Universityassume the same amplitude-distance relation asRichter but are based on peak-to-peak amplitudeson seismograms from Benioff short period verticalinstruments in the A.N.U. network. Although theoriginal Wood-Anderson seismometers are hori-zontal instruments, this method gives internallyconsistent results and gives values consistent withthe Richter scale (ML). Richter gives the relation-ship between the local magnitude scale (ML)and the body wave magnitude scale (/n) asm = 1-7 + 0-8 ML - 0-01 M^ which is closelyapproximated by

m = 1-8 + 0-73 ML . . . (1)in the ML range from 1 to 6 (White, 1968). Themagnitudes given in Table II(c) and II(d) arevalues of m obtained from measured ML valuesusing this equation.

In general it is difficult to extend a local magni-tude scale beyond about 600 km, and a magnitudescale has been developed, initially devised byGutenberg and Richter, which uses the computedground amplitude of surface waves with periodsnear 20 seconds. This scale (designated M) was par-ticularly useful when few stations were equippedwith high-magnification, short-period instruments.According to Richter (1958) this scale is related

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to the body wave scale (m) by the equationm = 2-5 + 0-63 M

Most of the magnitudes in Table I were originallydetermined as surface wave magnitudes and havebeen converted to body wave magnitudes (m) bythis equation.

Since deep focus earthquakes do not registersurface waves of appreciable amplitude withperiods near 20 seconds, it is obvious that a mag-nitude scale for such earthquakes must be foundedon records of body waves. Such a scale (m)already mentioned above, was devised by Guten-berg, and requires a knowledge obtained fromseismograms of log (A/T) where A is theamplitude of the ground motion and T the cor-responding period. The value of m is determinedfrom the equation

m = Ao + log (A/T)where Ao is an empirical constant which is afunction of distance and must be determined.Everingham (1968a) has adopted this procedurefor larger earthquakes in Western Australia re-corded at Mundaring and uses modified values ofthe constant Ag which are applicable to paths thatcross the Western Australian shield. By comparingvalues of the Richter local magnitude scale (ML)

with the body wave magnitudes obtained from theMundaring records ( f ^ p y ) for a number of earth-quakes at about 100 km distance, and using Equation(1), he finds the relation m = mMnlf + 0-4. Themagnitudes (m) in Table II(a) were obtained bysubstituting determined values of mMUS in thisequation.

In South Australia, an attempt was madeinitially to measure magnitudes according to theRichter local magnitude scale but it was foundthat such a procedure resulted in magnitudes thatincreased with distance. As a result it was necessaryto set up a modified scale (White, 1968) whichis also based on the maximum peak-to-peakamplitude obtained from Benioff short-periodvertical records. This scale is in fact based on thesimple law that the maximum peak-to-peakground velocity is inversely proportional to theepicentral distance. According to White, the re-lationship between the South Australian magnitude(mL) and the body wave magnitude (m) is con-veniently given by m = mL + 1 - 0 . Magnitudes forthe listed South Australian earthquakes are valuesof m obtained from measured values of mL andthe equation above.

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Documents

Seismicity of the Australian continent