Thirty years of seismic network recording in South Australia: selected results

PHYSICS OFTHE EARTH AND PLANETARY INTERIORS

_________

ELSEVIER

Physics of the Earth and Planetary Interiors 86 (1994) 277—299

Thirty years of seismic network recording in South Australia: selected results S.A. Greenhaigh

a,*,

D. Love

b

C. Sinadinovski

a

a School of Earth Sciences, Flinders Universily of South Australia, GPO Box 2100, Adelaide, LA. 5001, Australia b South Australian Department of Mines and Energy, P0 Box 151, Eastwood, SA 5063, Australia

Received 14 December 1994; accepted 30 March 1994

Abstract The year 1993 marks 30 years of seismic network recording of earthquakes in South Australia. The network currently comprises 17 short-period instruments, of which six use digital recording and five employ triaxial sensors. Approximately 350 earthquakes are located within the State each year using a computerised seismic analysis system developed by the Phillip Institute of Technology. A duration-based magnitude scale, equivalent to the Richter ML scale, has been developed for most stations. The pre-network (historical) record of earthquake activity in South Australia dates back to 1837. Epicentres are available for just 45 earthquakes. Of these, ten are of magnitude five or greater. The instrumental data over the last 30 years show a similar trend in epicentral pattern to the historical earthquakes, with the major zones being the Flinders Ranges, Eyre Peninsula and the Southeast. There have been 40 earthquakes of magnitude four or greater since 1963, the largest earthquake being of magnitude six in the Musgrave Ranges during 1986. Mine and quarry blasts within the State have been useful in calibration of the network, both in terms of hypocentral location and amplitude studies. The explosions have also furnished valuable information on the crustal velocity distribution. Epicentral co-ordinates of mine blasts are generally good to within 3—5 km. The explosions yield Richter magnitudes of one (charge size 1 or 2 tonnes) to 2.5 (charge size 50 tonnes). The crustal S waves, used in earthquake magnitude determinations, exhibit an amplitude—distance decay coefficient of 1.2. The P-wave velocity function of the crust increases non-linearly with depth, from about 5.0 km s~ near the 1 at 20 km depth, where the gradient reduces, with the velocity reaching a value of about 7.4 km surface to 6.4 km s s~ at a depth of 40 km.

1. Introduction The year 1993 marks 30 years of seismograph network recording of earthquakes in South Australia. Prior to 1963, only one seismic station was

_______

*

Corresponding author.

operating in the State. The two instruments, a Mime N—S horizontal pendulum, which was installed at the Adelaide Astronomical Observatory in 1909, and a higher-gain Milne—Shaw E—W seismograph commissioned in 1924, were not well suited to the study of local earthquakes. Their primary purpose was to furnish data to Britain on world-wide seismicity. The nearest stations to Adelaide were at the capital cities of adjoining

0031-9201/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-9201(94)02950-G

278

S.A. Greenhaigh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

States, distances of 600—2000 km away (Doyle and Underwood, 1965). There were few earthquakes large enough to be recorded at these distances to allow accurate locations to be made. Burke-Gaffney (1952), and

Lake~ Eyre

to a lesser extent Bullen and Bolt (1956) and Bolt (1957, 1959), provided lists of instrumental locations for some of the earthquakes reported in the Adelaide Observatory seismological bulletins. When the Observatory was closed down in 1948,

SOUTH AUSTRALIA

~

~HWK

—

—

J /

¶j’\i~Le?gh

\

\Creok

WRG~ ~ \i.ate L~—’i~ Woomero’~)Torrens

~.,

ILake IFrome

~

NEW SOUTH WALES

~PNA

~STK

I

~EDO RPA~JeNBK

//~

-~

Port Pine ~HTT

CLV~~

\

/

Port Li~~~’

—

I I

.J~j~’

Lake Goirdner

t~\

30°S

I

~RKJ~

(y ~

~ .‘

AT~S . ADT/ADE OSTR

•\f.

~

35°S

VICTORIA

I

Kangaroo Is. WKA

I

GEXEII

BFD

MGRI 0

50 100

I

i........i

S.A. ~

200km

I

SEISMOGRAPH

Analogue

~igital

,

Vertical

Mount Gambter....j~ NETWORK component station

3—component station

i3SeE

I

4o•s

140°E

Fig. 1. The stations forming the South Australian seismic network, as at October 1992. The map shows the nearby stations in adjoining States. The network comprises primarily vertical-component, short-period analogue instruments, but a number of stations use three-component digital recording.

S.A. Greenhaigh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

279

Table 1

Seismograph stations used in locating South Australian earthquakes up until October 1992 Station Adelaide I, S.A. Adelaide II, S.A. Adelaide III, S.A.

Code

Latitude

Longitude

—

34.93 34.93

ADE

34.967

(°E) 138.63 — 138.63 — 138.7136

—

(0s)

Elevation (m)

655

Station type a

Dates of Operation

Status

Mime Milne—Shaw

1909—1924? 1924—1954?

WWSSN

July 1958—

C C 0

b

(WWSSN, 1962) May 1974— 0 Oct. 1987— 0 Dec. 1970— 0 Oct. 1972— 0 Oct. 1963— 0 Nov. 1977— Jan. 1985 C Hallett, S.A. HTF 33.4305 138.9217 708 SPZ(A) June 1962— 0 Hawks Nest, SA. HKN 30.012 135.186 209 SPZ(A) Aug. 1975— Apr. 1981 C Hawks Nest C, S.A. HWK 29.9578 135.2035 180 3-C(D) May 1991— 0 Island Lagoon, S.A. ILN 31.3932 136.8697 137 SPZ(A) Mar 1970— May 1973 C Kelly Hill Caves, S.A. KHC 35.9825 136.9111 100 SPZ(A) June 1988— 0 Mirlkata, S.A. MTA 29.8694 135.2425 209 SPZ(A) June 1973— C Aug. 1975 Mt. Gambier, S.A. MGR 37.7283 140.471 190 SPZ(A) Apr. 1980— C May 1985 Mt. Gambler, S.A. MGR2 37.820 140.800 190 SPZ(A) Feb. 1990— 0 Naracoorte, S.A. GEX 37.0735 140.825 1 80 3-C(D) Dec. 1989— 0 Nectar Brook C S.A. NBK 32.701 137.983 180 SPZ(A) May 1979— 0 Oodnadatta, S.A. OOD 27.5622 135.4492 130 SPZ(A) Aug. 1972— Nov. 1972 C Parndana KI, S.A. PDA 35.8059 137.2389 140 3-C(D) Mar. 1990— 0 Partacoona, S.A. PNA 32.0057 138.1647 180 SPZ(A) Sept. 1969— 0 Quilpie, Q. QLP 26.5837 144.2348 210 SPZ(A),BMR Aug. 1989— 0 Roopena, S.A. RPA 32.7250 138.7849 95 SPZ(A) Sept. 1977— 0 Sedan, S.A. SDN 34.5093 139.3374 125 3-C(D) Apr. 1990— 0 Seven Hill, S.A. SNL 33.8872 138.6392 480 SPZ(A) Jan. 1966— Dec. 1971 C Stephens Creek, N.S.W. STK 31.8817 141.5918 213 SPZ(A),BMR Apr. 1974— 0 Strathalbyn, S.A. STR 35.1923 138.8943 215 SPZ(D) Nov. 1986 0 The Heights, S.A. THS 34.7412 138.7733 34 SPZ(A) Nov. 1989 0 Umberatana, S.A. UMB 30.2400 139.1280 610 SPZ(A) June 1967— Feb. 1990 0 Willalooka, S.A. WLK 36.4170 140.3210 40 SPZ(A) Mar. 1979— Dec. 1979 C Willalooka, S.A. WKA 36.4170 140.3210 40 SPZ(A) Jan. 1980 0 Woomera, S.A. WSA 31.1444 136.8047 180 SPZ(A) Apr. 1973 Oct. 1981 C Woomera °‘, S.A. WRG 31.1046 136.7634 168 SPZ(A) June 1981— 0 a WWSSN, World Wide Standardized Seismograph Network station (three-component long period and short period); SPZ,

Adelaide C S.A. Mkaroola, S.A. Alice Springs, N.T. Bellfield, Vic. Cleve, S.A. Endilloe, S.A.

ADT ARK ASPA BFD CLV EDO

34.967 30.276 23.6833 37.1766 33.6911 32.3216

138.7136 133.339 133.9005 142.545 136.4955 138.0483

655 520 600 235 238 300

SPZ(A) SPZ(A) 3-C(D),BMR SPZ(A),BMR SPZ(A) SPZ(A)

short-period seismometer; A, analogue; D, digital; 3-C, three-component; BMR, Bureau of Mineral Resources (now known as

Australian Geological Survey Organisation) network stations (adjoining states of N.S.W., Vie., N.T. and Qid.). b C, closed; 0, open. Telemetered.

280

S.A. Greenhaigh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

the Milne—Shaw instrument was moved to the University of Adelaide, where it was placed under the care of the Department of Physics. In 1958, the late Dr. David Sutton of that Department set up a three-component Benioff short-period recording system at Mt. Bonython, near Adelaide, with the 1954 Adelaide earthquake (ML 5.5) still fresh in mind. This earthquake (see Kerr Grant, 1956) involved 30 000 insurance claims totalling £4 million; it was the most damaging earthquake to have been experienced in Australia up to that time. In 1962, the Mt. Bonython station became incorporated into the World Wide Standardized~Seismograph Network (WWSSN), established to provide a worldwide co-ordinated collection system for large earthquakes and underground nuclear explosions. Two further stations were established by Dr. Sutton over the next 12 months, one at Hallett (HTT) in the mid-north of the State, and one at Cleve (CLV) on Eyre Peninsula. By 1979 the South Australian seismic network had been expanded to 12 vertical-component, short-period analogue stations (see Parham et al., 1988). The stations were distributed over a large area of the State, mainly arranged to provide monitoring of the active Flinders—Mt. Lofty Ranges in the populated area of the State (see Fig. 1). Responsibility for operation of the network passed from the University of Adelaide to the South Australian Department of Mines and Energy (SADME) in 1986. During the transition phase (1985—1986) the network was operated by Flinders University. Since 1986 there have been significant changes to the network. In this paper we document those changes and detail some of the contributions of the network, in commemoration of 30 years of seismograph operation.

2. The seismic network 2.1. Analogue and digital stations Table 1 provides a summary of geographic and other information on stations that have at some stage been part of the network. The locations of the 17 currently operating stations are shown in

Fig. 1. The figure also shows the locations of the Bureau of Mineral Resources stations at Beilfield (BFD) in the Victorian Grampians and Stevens Creek (STK) in New South Wales, which are often used in locating South Australian earthquakes. Since the last published account of the network (Parham et al., 1988) and seismicity (Greenhalgh and Singh., 1988), two of the seismograph stations, Mt. Gambier (MGR) and Urnberatana (UMB), were relocated. The old Mt. Gambier site at The Bluff was too noisy, and telemetry of the signal to the SADME office in the city caused radio interference to other users. A new site at Mt. Gambier has been found. The Umberatna station, which had operated since 1967, was replaced in 1987 by a new station at Arkaroola (ARK), located approximately 50 km to the north-east. An additional six new stations have been constructed around the State since 1986 (see Table 1). Two of these stations on Kangaroo Island—Parndana (PDA) and Kelly Hill Caves (KHC)—and the station at Naracoorte (GEX) extend the network coverage into the Southeast of the State, to monitor seismic activity possibly associated with recent volcanism in the region (Sutton et al., 1977). Previously, only stations at Willalooka (WKA) and Mt. Gambier (MGR) provided earthquake monitoring in this area. Three recently established seismic stations at Sedan (SDN), Strathalbyn (STR) and The Heights (THS) provide much-improved seismic coverage of the Mt. Lofty Ranges and the Adelaide region. Six of the 17 stations use digital recording. Five of these employ triaxial (1 or 2 Hz) sensors. The digital recorder used is the Kelungi, manufactured by the Royal Melbourne Institute of Technology (RMIT), Bundoora (previously The Phillip Institute of Technology) (Gibson, 1986). It is a flexible, rugged, low-power instrument which is used in other networks both within and outside Australia. Its design has allowed considerable flexibility in establishing or moving stations. Timing is kept accurate by regular checking against Omega receivers, built by SADME to a design of the Australian Seismological Centre in Canberra, although radio, portable clock and telephone tone

S.A. Greenhaigh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

synchronisation is possible. The seismic signal sampling rates conmionly uLsed are 50—200 Hz. The Kelungi provides 1 megabyte of solid-state memory for triggered event storage. Four of the digital stations, at Parndana (PDN), Naracoorte (GEX), Sedan, (SDN) and Hawks Nest (HWK), have been permanently connected to the telephone network, so that the recorders can be interrogated from the SADME office in Adelaide at any time. The digital recorders have been undergoing continual improvement since first purchased and their performance has improved considerably. The outstanding contribution of these units is in allowing rapid location of a large event, and in recording large events without saturation. Instrumentation details on the analogue seismograph stations have been given by Parham et al. (1988) and will not be repeated here. Suffice to say that short-period (approximately 1 s) vertical seismometers, namely Benioff SP-Z, Geotech 18300, Kinemetrics Ranger SS-1 and Willmore Mk II are used and writing is by means of pen and ink recording on conventional drum recorders (Kinemetrics YR-lA, Geotech RV3O1, as well as units designed and built at the University of Adelaide). Each station incorporates electronic amplification and filtering. Peak displacement gains range from 140K to 2.4M, with the exception of the WNSSN station (ADE) at Adelaide. Fig. 2 shows the number of earthquakes located per year by the network since 1964. Only those earthquakes that occur within the detection limit of at least three stations can be located. The number of located events has grown, in step-wise fashion, from 50 to between 300 and 400. The steps occur at those years when the number of stations in the network was increased, 2.2. Computerised seismic analysis system From July 1988 there have been very substantial changes to the earthquake data processing system. The ISAS system, described by Parham et al. (1988), has been replaced by the PITSIS systern running on a Macintosh computer. PITSIS, which was developed by Phillip Institute of Technology, has programs to perform the following

40C I

281

F

s.A. SEISMIC NETWORK

200

0 100

Z

0 64 F

68

72

76

80

84

88

92

Year Fig. 2. Frequency polygon showing the number of earthquakes per year located by the network, since 1964. The increases in apparent activity which occurred in the late 1970s and again in the late 1980s correspond to periods of expansion of the network.

three major functions: (1) display and analysis (e.g. filtering and spectral analysis) of seismograms from single- or three- component digital instruments; (2) earthquake location and magnitude computation from input arrival times, amplitudes and coda duration (various P and S phases, including Moho reflections, are used in the location algorithm, which can be set up for any multilayer velocity model); (3) catalogue and retrieval of earthquake data with output to disk, printer and plotter; a great variety of selection parameters are available for rapid searching of the catalogue. Various other utility programs are available in the PITSIS system. New subroutines can be easily linked as well. Almost identical earthquake processing systems are operated by the Bureau of Mineral Resources in Mundaring, W.A., and Canberra, and by RMIT (Bundoora), and are connected to the telephone network, so that it is easy to trans-

S.A. Greenhalgh etal. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

282

fer data between States. Similar systems are also installed in Queensland, at Brisbane and Rockhampton.

and Singh (1986). The scale is given by M (SA) = 0.7 + log A + 1.10 log ~ L

iO

wa

10

+0.0013~+C1

2.3. Duration-based magnitude scale Magnitudes have been reported for South Australian local earthquakes since 1963. A van-

ety of scales have been developed (see Greenhalgh and Parham (1986) for a history, and a discussion of some of the problems). A revised magnitude scale ML (SA), equivalent to the Richter ML scale but incorporating the local attenuation function, was developed by Greenhalgh

C

(1) where Awa is the equivalent Wood—Anderson trace amplitude (in millimetres, zero-to-peak), ~ is the epicentral distance in kilometres, and C, is a station correction which takes account of the local geology. The quantity AWA is obtained by first computing the ground displacement corresponding to the peak trace deflection at each station, via the instrument response curve for that

station, and then multiplying by the Wood— Anderson gain function at the measured period.

6

I

ADT

CLV

10

100

Duration

-10

,t

1000

10

100

Duration -c (secs)

100

1000

Duration-c (secs)

(secs)

1000

10

100

1000

Duration ,r (secs)

Fig. 3. Richter magnitude vs. event duration for four representative stations—ADT, CLV, RPA and HTT. (Note the curvilinear trend on the log—linear scale.)

S.A. Greenhalgh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

A magnitude scale which is not as sensitive to source radiation anisotropy and geological inhomogeneity, and at the same time does not place great demands on measurement accuracy, may be expected to produce more consistent results. A duration-based scale (Herrmann, 1975; Real and Teng, 1983; Bakun, 1984) is the most promising in this regard. It is also more convenient, as maximum amplitude is unreadable on analogue instruments for large or close events, owing to signal clipping, The basic parameter in all formulae for calculating duration magnitude is the base-lU logarithm of the signal duration T. The definition of total signal duration differs with different workers but is here defined as the time interval from the onset of the first arrival to the point on the seismogram at which the signal falls and remains below the background noise level. Scattergrams of iogT versus an otherwise determined local magnitude ML for four representative stations of the State network are shown in Fig. 3. The data base spans the time interval 1978—1989. The independent (amplitude-based) local magnitude was the average Richter magnitude ML (SA) for all stations recording each

event. All plots exhibit a generally high correlation. The deviation from a linear trend is significant. The following duration magnitude formula, which accommodates the curvature, was fitted to the data: MD

=

ADE ADT ARK CLV EDO ELO HKN HTT HWK MGR NBK PNA RPA UMB WKA WLK WRG WSA

No. of observations

a0

49 334 169 478 734 112 42 1265 296 29 1752 2096 1783 573 207 11 503 177

0.878 ±0.101 0.508 ±0.006 —0.861 ±0.009 0.032 ±0.044 —0.791 ±0.034 —0.448 ±0.075 —2.402 ±0.335 — 1.730 ±0.044 3.780 ±0.069 0.569 ±0.154 —0.589 ±0.022 — 1.098 ± 0.032 —0.907 ±0.025 — 1.649 ±0.080 —0.136 ±0.077 0.342 ±0.221 — 1.540 ± 0.064 — 1.303 ± 0.162

a0

+ a1(log107-) + a2A (2) where a0, a1, a2 and n are constants to be determined. In addition to the duration term, this formula also involves an epicentral distance (~) term. Several workers (Real and Teng, 1973; Herrmann, 1975; Bakun and Lindh, 1977) have reported a ‘knee’ in the scattergrams such as those of Fig. 3, in support of a curvilinear function such as (2), whereas others (Aki, 1969; Aki and Chouet, 1975) have investigated the theoretical basis for the observed properties of duration-based magnitude scales. The coda are generally explained in terms of scattered body waves and surface waves and also as a diffusion process. The non-linearity observed in scattergrams such as Fig. 3 has been attributed to the source spectrum corner frequency passing through the peak instrumental response (Herrmann, 1975; Bakun and Lindh, 1977), although Herrmann (1980) has suggested

Table 2 MD coefficients in duration magnitude formula M8dD = a1 + a1 (log10TY’ Station

283

+

a2

a1

a2

n

(x 10~) 0.671 ±0.046 0.454 ±0.019 0.615 ±0.023 0.857 ±0.020 0.929 ±0.014 0.736 ±0.027 1.774 ±0.118 1.844 ±0.025 0.263 ±0.008 0.301 ±0.025 0.903 ±0.010 0.954 ±0.013 0.991 ±0.011 1.410 ±0.036 0.453 ±0.018 0.489 ±0.045 1.389 ±0.03 1 1.405 ±0.072

4.76 ±3.00 8.68 ±1.58 12.00 ±1.70 7.40 ±0.84 16.92 ±0.97 14.55 ±1.90 3.70 ±2.44 12.18 ±0.71 9.00 ±1.35 —0.39 ±2.37 11.73 ±0.53 15.09 ±0.56 11.89 ±0.52 10.00 ±1.15 6.69 ±1.21 9.49 ±3.85 10.49 ±1.19 14.33 ±3.04

1.54 1.91 1.85 1.43 1.51 1.71 1.26 1.00 2.64 2.72 1.48 1.48 1.45 1.25 2.19 2.16 1.30 1.20

284

S.A. Greenhaigh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

an alternative explanation in terms of changes in the coda shape owing to dispersion by the Earth’s Q filter. The coefficients in Eq. (2) were determined for each station separately and in such a way as to align the resulting duration magnitude scale with the ML (SA) scale. This ensures that MD corresponds in absolute terms to ML as far as

~

.

/ M

0 STUDY

~ 3000

/ ~ -~

2000

/

~ ioao

MD is its reputedly greater consistency between station estimates of a given event owing to the fact that the surface waves comprising the coda, being of longer period are attenuated more slowly and less erratically by variations in local geology. The coefficients were found by stepwise multiplc linear regression so as to obtain the best estimates of ML from the values of T and z~for each event. Unlike some networks which use a single network formula, in South Australia the large aperture of the network, with attendant variations in local geology, and the range of equipment in use at the various stations has meant that individual station coefficients are more appropriate. The standard deviation amongst station estimates of MD for an event, averaged over 3182

1200/ ~ 1000~ 800

MD STUDY

-

//

Z

0 0

I

I

200

1.00

Epicentral

I

I

I

I

600

800

I

I

1000

distance (km)

Fig. 5. Histogram of epicentral distances for data used in the MD study.

earthquakes, was found to be 0.11 magnitude units. By contrast, the amplitude-based scale, ML, produced a larger standard deviation of 0.16 units. The magnitude coefficients are shown in Table 2. Apart from HWK, MGR, WKA and WLK, which involve the fewest observations, the exponent n values lie between 1.0 and 1.9. The a1 coefficients are of the same order for most stations. The distance coefficients a2 are generally small and could be set to zero and would still be within one standard error for earthquakes out to distances of 1°. The magnitude and distance ranges used in establishing the scale represented by Eq. (2) and Table 2 can be seen in Figs. 4 and 5, respectively. As is usual for South Australian events, the magnitudes are generally small (ML 0.5—4). The number of observations beyond 500 km in Fig. 5 drops off markedly. To avoid regression extrapolation errors, MD should be regarded as being limited to the ranges implied by Figs. 4 and 5. =

o 600~ 1+00Z

2

200 0~’” 0

3. Seismicity

1

I

I

2

3

1+

I

I

5

6

3.1. Historical

Richter magnitude. ML Fig. 4. Histogram of event magnitudes (averaged over all stations) for data used in the MD study.

The earliest reported earthquake in South Australia occurred in Adelaide on 28 July 1837,

S.A. Greenhalgh eta!. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

only 6 months after proclamation of the new colony. Blackett (1907) described the effects of this earthquake and the concern it caused amongst the new settlers. The Rev. Julian Ed-

mund Wood reported a further severe shock felt in Adelaide in June 1856, and one at Lake Bonney in the Southeast during December 1861 (Howchin, 1910; Kerr Grant, 1956).

Table 3 Historical (Pre-Network) Earthquakes in South Australia (1883—1962 inclusive) UT 1 2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

285

Date and time

Latitude (°S)

Longitude (°E)

Depth (lun)

Magnitude (ML)

Place

1883-07-07 13:58 1887-04-16 13:10 1887-04-1622:10 1889-02- 12 06:45 1893-08-13 02:10 1896-08-2202:56 1896-08-22 06:30 1896-08-23 11:30 1897-05-10 05:26 1898-04-10 21: 10 1899-05-0203:30 1902-05-0705:10 1902-05-13 18:50 1902-06-05 22:35 1902-09-1821:00 1902-09-19 10:30 1902-09-20 09:35 1902-09-21 04: 10 1903-08-1421:10 1905-08-21 18:35 1908-04-09 16.25 1911-10-24 12:00 1911-10-26 10:00 1914-05-28 13 :21 1921-04-23 19:00 1937-10-28 09:34 1939-03-26 03:56 1939-05-01 19:07 1939-06-05 12:20 1941-05-04 22:07 1942-02-14 22:50 1948-08-06 03 :29 1954-02-28 18:09 1954-03-02 20: 15 1959-03-02 12:22 1959-05-21 11:28 1959-09-09 04: 17 1959-11-0201:17 1960-08-18 15:04 1960-08-3021:23 1960-08-31 02:14 1960-11-12 23:03 1960-06-10 15:58 1962-03-03 22:04 1962-05-16 21 :41

35.100 33.500 34.300 34.000 34.333 33.750 33.750 33.750 37.300 37.300 37.300 32.750 34.150 33.166 32.500 35.000 35.000 32.500 33.917 34.200 33.917 33.750 33.667 34.950 33.267 26.100 31.100 31.400 31.500 26.300 29.500 37.360 34.930 34.930 34.980 3 1.400 32.700 33.360 33.800 34.000 33.350 34.600 34.500 33.000 35.5 10

138.700 139.000 135.800 139.000 139.000 138.917 138.9 17 138.917 139.750 139.750 139.750 138.500 138.880 138.350 138.500 138.000 138.000 138.500 138.500 138.800 138.617 136.500 136.417 138.700 138.833 136.500 138.300 138.000 138.500 136.900 136.000 139.680 138.690 138.690 138.730 139.000 138.200 135.980 136.150 136.000 136.400 135.500 135.000 136.000 137.660

14.0 14.0 14.0 0.0 0.0 0.0 0.0 0.0 14.0 14.0 0.0 0.0 0.0 0.0 0.0 4.0 4.0 0.0 0.0 14.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 18.0 4.0 4.0 4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

4.5 4.4 5.4 4.6 3.6 4.3 4.0 4.2 6.3 4.9 5.0 4.8 3.5 3.1 4.4 5.9 4.8 3.9 4.1 4.6 4.7 4.8 5.5 3.9 5.1 5.5 5.7 3.9 3.9 5.1 4.3 5.6 5.4 3.2 2.6 4.4 4.1 4.9 4.1 4.1 4.3 4.3 4.0 4.0 4.4

Mt. Barker Mt. Bryan Cummins Robertstown Kapunda Burrs Burra—aftershock Burrs—aftershock Beachport Beachport Robe Mid-north Marrahel Caltowie Spalding Warooka Warooka—aftershock Spalding—aftershock Clare Riverton Peterborough Cleve Eyre Peninsula Adelaide Jamestown Simpson Desert Parachilna Lake Torrens Hawker Simpson Desert Margaret Creek Robe Adelaide Adelaide—aftershock Adelaide Wilpena Melrose Mamblin Ungarra Ungarra Ungarra Coffin Bay Coffin Bay Kimba Investigator Straight

-

286

S.A. Greenhalgh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

The Government Astronomer, Sir Charles Todd, collected reports from 1887 to 1908. His successor, G. Dodwell, published a summary of seismic activity for this period (Dodwell, 1910). A partial tabulation of South Australian earthquakes which were felt over the period 1840—1917 was given by Hunt (1918), based on records kept by rainfall observers for the Bureau of Meteorology. This publication includes many of Todd’s earthquake reports, prepared for the Australasian Association for Advancement of Science. It was this Association which was later instrumental in procuring the Milne Seismograph for the Observatory. From recent inspection of archival material held at the SA Public Records Office, it appears that the Observatory continued to collect intensity information and felt reports of earthquakes after establishment of the seismograph in June 1909, but the practice was discontinued sometime in the 1920s. Much of the information has been lost. The late Dr. D. Sutton compiled a scrap-book of South Australian earthquakes beginning in 1932, using information extracted from local newspapers. The scrap-book is held on file at the SA Department of Mines and Energy. In common with the Dodwell (1910) and Hunt (1918) compilations, the greatest concentration of felt reports has come from the vicinity of the Flinders Ranges, from such places as Quorn, Hawker, 40

FLINDERS RANGES SEISMIC ZONE

-

.

-

.

30

-

.

1

20

-

I

10

-

~_______

Record

Inc~mpLet~ 0

______

1680

-

1900

-

I

-

1920

-

1940

1960

Blinman and Beltana. Fig. 6 shows the number of earthquake reports per 10 year period felt (intensity mainly MM Il—MM V) in the region since 1880. The information has been gleaned from all of the above sources. Unfortunately, the record is incomplete between the First and Second World Wars. From the short duration and lack of simultaneous reports at nearby towns, it may be coneluded that the majority of these earthquakes were of low magnitude (ML < 3). The uneven population distribution throughout the Flinders Ranges over this time interval, and a comparison with recent instrumental data, would strongly suggest that many perceptible earthquakes have been missed. Isoseismal maps for thirteen historical (prenetwork) South Australian earthquakes have been published by Everingham et al. (1982), Rynn et al. (1987) and McCue (1993). Included here are the three most damaging earthquakes in the State’s history—Beachport (1897), Warooka (1902) and Adelaide (1954). Detailed accounts of the macroseismic effects of these earthquakes have been given by Dyster (1979). Additional information on the seismological aspects has been provided by McCue (1975). An exhaustive search of old newspapers and other documents relating to historical South Australian earthquakes was recently carried out by Malpas (1991). From the information uncovered she was able to draw isoseismal maps for a further 13 important historical earthquakes. Table 3 is a complete listing of all pre-1963 South Australian earthquakes for which epicentres and magnitudes are available. For most events since 1937, instrumental magnitudes have been determined (see Sutton and White, 1968; Everingham et al., 1987). The magnitudes for the earlier earthquakes were based on isoseismal characteristics, such as felt radius, using magnitude—inten(1989a). sity relationships derived by Greenhalgh et al. The epicentres for all of the earthquakes listed

-

1960

Time (years)

Fig. 6. Number of felt earthquakes per decade in the Flinders—Mt. Lofty Ranges seismic zone since 1880.

in Table 3 are plotted in Fig. 7. The numbers appearing beside each epicentre refer to the entry numbers in Table 3. The size of each star is proportional to the magnitude. The epicentres group into four seismic zones—Adelaide Geosyn-

S.A. Greenhaigh et a!. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

dine, Eyre Peninsula (west of Spencer Gulf), the Southeast, and Simpson Desert (near the Northem Territory border in the north of the State)— which also form four separate geological provinces.

287

1968 (Sutton and White, 1966; 1968; Stewart, 1971, 1984; Stewart and Sutton, 1971; Stewart et al., 1973; McCue, 1975; Sutton et al., 1977; McCue and Sutton, 1979; Greenhalgh and Singh, 1988; Greenhalgh and McDougall, 1990; Greenhalgh et al., 1994). Tectonic interpretations of the seismicity have also been undertaken (Doyle et al., 1968; Stewart and Mount, 1972; Stewart, 1976; Sutton et al., 1977; Greenhalgh and Singh, 1988). Crustal structure studies from earthquakes in

3.2. Network data Network (instrumental) studies of the seismicity of South Australia have been carried out since

i

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Fig. 7. Historical earthquake epicentres for all known events of magnitude three or greater (1883—1962 inclusive). The numbers refer to the entries in Table 3.

288

S.A. Greenhalgh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

South Australia have been reported by Sutton and White (1966), White (1969), Thomas (1969), Stewart (1972) and Greenhalgh et al. (1989b). Fig. 8 is an epicentral plot for all earthquakes of magnitude 4 or greater recorded since the network commenced operation in 1963. The mdividual earthquakes, identified by their numbers, are listed in Table 4. The majority of events are located in the hilly country (Flinders Ranges) east of Lake Torrens. The plot reiterates the historical

25°c

pattern (Fig. 7), but with two added events (25 and 26) in the Musgrave Ranges, wheme there had been no previous activity. Within the Geosyncline the bias of historical data toward the south is partly due to the longer period of more intensive settlement. Fig. 9 shows the earthquake epicentres south of latitude 30°S for the period from 1969 to mid-1991 when at least seven stations of the network were operating. Only events of magni-

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Fig. 8. Earthquake epicentres for all events of magnitude four or greater, located by the South Australian seismic network (1963—1992 inclusive). The numbers alongside each epicentre correspond to the entries in Table 4.

S.A. Greenhaigh et a!. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

tude greater than or equal to ML 3 are plotted. The diagram also depicts the main geological provinces, and several mapped faults. Again, the major earthquake zones are clearly seen. It is obvious that most of the activity occurs in the Adelaide Geosyncline Zone from Copley in the north to Kangaroo Island in the south. The remaining activity occurs on Eyre Peninsula (particularly the eastern side) and the Southeast corner of the State (around Mt. Gambier). Focal depths are shallow, being mainly confined to the upper 10 km of the crust (Stewart, 1972; Greenhaigh et al., 1986). There is no direct correlation between seismicity and known faults.

289

The focal mechanisms for six earthquakes occurring within the Flinders Ranges have been determined by McCue and Sutton (1979) and Greenhalgh and Singh (1988) from the distribution of P-wave first motions. They are shown in Fig. 9. The arrows drawn about each fault plane solution depict the direction of principal stress. The predominant stress appears to be NE—SW horizontal compression. This is consistent with the computed stress pattern for the northerly moving Indo-Australian plate (Cloetingh and Wortel, 1986). The sense of motion is mainly strike slip on steeply dipping faults. The same compressional process, which is responsible for

Table 4 South Australian earthquakes of magnitude ML ~ 4 located by the network, 1963—1992 UT

Date and time

Latitude (°S)

Longitude (°E)

Depth (kin)

Magnitude (ML)

Place

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

1965-01-17 02:48 1965-01-25 20:22 1965-03-02 15: 18 1965-03-14 12:47 1965-06-04 10:45 1965-08-2800:26 1966-11-23 20:48 1969-01-29 15:03 1971-01-0623:54 1971-07-2607:50 1972-04-18 22:20 1972-04-27 11:50 1974-02-2701:57 1975-01-03 02:18 1975-11-22 19:32 1978-03-2622:37 1979-10-22 11:13 1980-04-15 00:38 1980-08-30 14:23 1980-11-13 08:56 1981-10-02 17:35 1982-07-15 01:44 1982-11-10 09:31 1983-12-29 17:42 1986-03-3008:54 1986-07-11 07:18 1986-12-16 04:29 1990-02-08 08:23 1990-12-01 22:35 1992-08-22 10:54

27.968 31.928 30.525 31.949 32.000 32.225 34.347 31.797 33.459 31.375 31.578 31.263 28.454 31.243 37.977 32.389 33.312 33.263 31.762 33.739 32.476 31.048 37.945 30.794 26.285 26.205 36.118 27.891 26.582 27.452

135.655 138.495 138.222 138.569 138.479 138.297 139.303 139.115 138.560 138.756 138.619 138.891 136.819 138.716 140.207 138.923 136.955 137.030 139.361 138.825 138.878 138.339 137.437 138.405 133.019 132.875 136.577 137.342 131.325 128.791

0.0 0.0 10.0 0.0 3.7 16.1 13.4 20.3 11.9 18.6 11.9 12.5 6.0 6.4 17.9 16.4 34.3 31.0 8.0 18.4 6.8 17.7 17.6 20.4 19.5 0.0 7.5 9.0 21.6 5.0

4.0 4.5 4.8 4.6 4.1 4.1 4.1 4.1 4.5 4.9 5.4 4.0 4.4 4.0 4.2 4.1 4.1 4.5 4.5 4.1 4.0 4.0 4.2 4.3 6.0 5.5 4.4 4.5 4.1 4.0

Warrina Wilson Leigh Creek Wilson Cradock Willochra Truro Baratta Spalding Wilpena Wilpena Blinman Douglas Creek Blinman Cape Banks Yalpara Pine Hill Kimba Round Hill Clare Carrieton Nilpena Southern Ocean Beltana Marryat Creek Marryat Creek Cape de Coudie KI Lake Eyre North Mt. Woodroffe WA. Border

290

S.A. Greenhalgh et aL/Physics of theEarth and Planetary Interiors 86 (1994) 277—299

uplift of the Flinders and Mt. Lofty Ranges along a previous structural feature—the Adelaide Rift Zone—is also responsible for the earthquakes.

earthquakes. The main blast sources are from the Middleback Iron Range (Iron Baron, Iron Knob and Iron Monarch) and the Leigh Creek Coalfield. In previous years, explosions at the Mt. Gunson and Kanmantoo copper mines were also recorded on the seismograph stations. The locations of the blast sources, in relation to the network, are shown in Fig. 10. Numerous stone quarries operate throughout

4. Mine and quarry blast studies The South Australian seismic network regularly records large mine blasts in addition to

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Fig. 9. Epicentral plot of South Australian earthquakes for the period January 1969—October 1992, for all events of magnitude three and larger. The map shows the seismicity in relation to the tectonic provinces and the known faults. Focal mechanisms are also shown for six earthquakes in the Adelaide Geosyncline.

S.A. Greenhalgh et aL/Physics of the Earth and Planetary Interiors 86 (1994) 277—299

the Adelaide Hills. Blasting at these quarries is picked up on the Mt. Bonython (ADE—ADT) station and on the other recently commissioned permanent network stations in the Adelaide region (see Fig. 1). The seven portable seismograph stations which were temporarily set up some years ago in connection with the Adelaide—Mt. Lofty Ranges microearthquake survey (Parham, 1981) also recorded many blasts for the quarries shown in Fig. 11. Such controlled-source explosions, of

known location and approximate origin time, are useful for network calibration purposes as well as for crustal structure studies. 4.1. Hypocentral errors The statistical uncertainty in earthquake epicentre locations in South Australia are typically between 2 and 5 km. At least these are the random errors which are returned by the least-

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AUSTRALIA

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50 100

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Fig. 10. Location map of mine and quarry blasts in relation to the State seismic network.

292

S.A. Greenhalgh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

MS

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Fig. 11. Quarry blast sites in the Adelaide Hills and locations of temporary seismic stations used in the 1978 Adelaide Hills microearthquake survey.

squares location procedure. The actual errors may be larger than this, as a result not only of errors in time picks and phase identifications, but also of systematic errors in the crustal velocity model. The seismic travel times to the various seismograph stations from numerous blasts have been used in a source location study, in an attempt to quantify the errors in focal co-ordinates. For each

of the mines at Iron Baron, Iron Duke, Iron Monarch, Mount Gunson and Leigh Creek (see Fig. 10), between five and ten separate explosions during 1988, well recorded on the network, were selected for hypocentre determination. Locations were performed using multiple P and S phases, as discussed in an earlier section. The number of stations used in the analysis ranged from four to

S.A. Greenhaigh et aL/Physics of the Earth and Planetary Interiors 86 (1994) 277—299

293

Table 5 Hypocentral location errors for mine blasts recorded on SA seismic network Mine

No. of blasts

No. of stations used U

Iron Baron 5 4—8 (6) Iron Duke 10 7—10 (8) Iron Monarch 5 5—7 (6) Leigh Creek 8 6—9 (7) Mt. Gunson 6 4—7 (5) U Figures in parentheses are mean values.

ten. The Leigh Creek blasts provide a good test of earthquake location accuracy in the Northern Flinders Ranges. The Iron Triangle blasts simulate an earthquake on Eyre Peninsula, another active seismic zone. The actual source co-ordinates for each blast were provided by the mine operators. These coordinates are only good to within 0.1 km, taking into account the finite area involved in a mine blast. By comparing the computed latitude, longitude and focal depth with the known epicentre and surface focus, we were able to calculate errors in each of the hypocentral co-ordinates. These errors averaged over all blasts at each mine, are given in Table 5. The results are encouraging. Computed epicentres are generally accurate to within 2—6 km. The focal depths appear accurate to within 1—3 km. Some of the individual blasts are better than this; others are worse. It is only the mean values and their standard errors which are given in Table 5. Two of the Leigh Creek blasts, which were excluded from the averaging process, had latitude and longitude errors of 18—25 km. Apart from these, the worst case was a latitude error of 10 km and a corresponding longitude error of 13 km, for an Iron Duke blast. The source—receiver distances involved in the above analysis were generally in the range 30—300 km (see Fig. 10). We would expect the location capability to improve with a more closely spaced, smaller-aperture network, such as that used in a microearthquake survey. Table 6 lists the mean errors in latitude, longitude and focal depth found in locating quarry blasts with the seven-station Adelaide Hills mi-

Mean errors in hypocentre (km) Latitude Longitude

Focal depth

(kin)

(h~i)

(km)

4.5 ±0.9 4.6 ±0.9 1.1 ±0.4 2.7 ±0.4 3.0 ±0.6

3.4 ±1.3 4.0 ±1.7 2.2 ±0.8 3.5 ±0.5 3.8 ±1.3

1.9 ±0.7 1.9 ±0.3 2.2 ±1.3 1.6 ±0.5 2.7 ±0.6

croearthquake network (Fig. 11). Propagation path distances varied from a few kilometres to a few tens of kilometres. A single-layer velocity model, appropriate to the upper crust and different from that for the longer-range mine blasts, was used in the location procedure. In fact, the P and S velocities for the upper crust are simultaneously solved for along with the hypocentre. It is assumed that each arrival follows a direct path between earthquake focus and the recording station. The average P and S velocities, taken over all blasts, are: V~, 5 .09( + 0.08) km/s and =

—

V~=3.08(+0.07)km/s —

With the exception of Riverview and Noarlunga blasts, the epicentral location errors for each quarry are less than 1 km. Expressed as a fractional error of the average distances involved, this is a larger error in epicentre locations than with those found using the State network. The Table 6 Location errors for quarry blasts using the Adelaide Hills microearthquake network Quarry No. of Mean errors in hypocentre (km) bO asts Latitude Longitude Focal (kin) (kin) depth (km) Horsnell Gully Stonyfell Greenhill Riverview Linwood Broadview Noarlunga

7 2 4 3 5 3 2

0.22+ 0.29 0.66±0.28 0.28 ±0.21 4.44±2.78 0.87±0.75 1.08±0.72 0.50 ±0.49

0.78+0.20 1.89±1.18 0.08 ±0.17 0.04±1.67 0.40±1.14 0.15±0.92 5.78 ±3.06

3.5+ 1.0 3.1±1.2 3.2 ±1.0 6.3±1.4 3.1±1.3 1.6±1.6 0.0 ±0.0

294

S.A. Greenha!gh et aL /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

focal depth errors of Table 6 are actually worse than those of TableS. This is not surprising given the neglect of depth-sensitive phases in mi croearthquake locations. By contrast, such phases are an integral part of hypocentre determination

100

Source : STONYFELL Station

•

:

•

‘-

with the State-wide network. The quarry blast location errors of Tables 5 and 6 can be taken as an approximate guide to the expected accuracy of actual earthquake epicentres and corresponding focal depths. They lend support to the validity of the statistical measures of uncertainty in hypocentral co-ordinates given with each location.

I I

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4.2. Amplitude—charge size relations

~ 30

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-1 of Magnitudes several kilometres. The seismograms part on thehundred maximum of local amplitude earthquakes of the are crustal based Sg in phase, which dominates the coda out to distances written by mine and quarry blasts have a similar, albeit recognisable difference to earthquake records. We have computed the magnitudes for a selection of blasts. In Fig. 12, these magnitudes are plotted against the charge size for each explosion, The data are coded according to source. The

________________________________________

-

SA. QUARRY BLASTS

x Mt. Gunson 0 Iron Knob

o ~°

A Iron Boron

x~

• Leigh Creek

is

-

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xx

1

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~,, E

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1 I

1

3

I

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5 10 C tonnes) 30 Charge size

50

100

Fig. 13. Peak amplitude of the erustal S wave vs. charge size for explosions at Stonyfell quarry and Kanmantoo mine, recorded on the WWSSN station at Adelaide.

charge size vs. magnitude data can be crudely fitted by the relationship ML 0.7 + log~0W (3) =

0 Adelaide hills

-

.1

/

here W -

~

S

.

is

charge weight, in tonnes of explosive.

W

-

~

a

We see that the magnitudes fOr the Leigh Creek and Iron Baron blasts, which are as large as 50 tonnes, have magnitudes as great as ML 2.5. 1 or 2 tonnes, yield magnitude smaller than ML —1. TheWe smaller have taken Adelaide the analysis Hills blasts, one step which further, are only to determine the exponent m in the amplitude scaling law

0 0

I

0.1

0.3

ID

0.5

A=KWm I

1

3

Charge weight

5

10

30

50

100

W (tonnes)

Fig. 12. Magnitude—charge size plot for various quarry blasts in South Australia.

(4)

where A is the maximum ground amplitude of the S wave. This was achieved by plotting logA vs. logW for blasts at each of the mines Kanman-

S.A. Greenhalgh et aL /Physics of the Earth and Planetary Interiors 86 (1994) 277—299 I Source : KANMANTOO Station : HALLETT

30

I

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Table7 m for mine and Amplitude-charge size relationship A = KW quarry blasts

.__~__,_~_~.~‘T Quarry/mine Stonyfell Kanmantoo

10

mo.~

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Mt. Gunson I 3

I

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5 10 Charge size I tonnes)

1

Iron Baron

I

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Source : KANMANTOO Station: SEVENHILL

•

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~30

Station ADE ADE HTT SVH HWK NBK

observations No. of 53 39 20 21 7 11

m Exponent 0.62 0.78 0.44 0.43 0.28 0.30

SE 0.12 0.12 0.10 0.14 0.11 0.11

UMB WRG I-ITT NBK PNA UMB

11 9 8 7 8 44

0.31 0.48 0.76 0.55 0.40 0.94

0.09 0.07 0.30 0.16 0.50 0.70

ISL PNA

18 7

0.77 0.27

0.37 0.16

those repor

~

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295

~10-

I

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1

3 5 10 Charge size I tonnes)

30

Fig. 14. Amplitude—charge size relationship for Kanmantoo blasts recorded at the Stations Hallett and Seven Hill.

too, Mt. Gunson, Iron Baron and Leigh Creek, as well as for the Stonyfell quarry. The slope of each graph yields the factor m. Representative plots for the Adelaide station are shown in Fig. 13. The diagram is for small blasts from Stonyfell and large blasts from Kanmantoo. Fig. 14 shows another set of Kanmantoo blast results, recorded at the distant stations Sevenhill and Hallett. The m values obtained for all source—station combinations are given in Table 7. They range from 0.3 to 0.8. Such exponent values bracket

ted in studies elsewhere (see Bath, 1974, p. 374). Our unweighted, global average value for the exponent is 0.52. 4.3. S-Wave attenuation The seismic attenuation coefficient n in the relationship A k/~° (5) can be determined by plotting the ground amplitude A against source—receiver distance for vanous explosions. This has been carried out for up to 20 blasts at each of five quarries. The amplitude parameter A chosen was the peak particle velocity. Results are summarised in Table 8. The table gives mean exponent values n along with stand=

Table 8 Attenuation coefficient n in relationship A = k/N1 for S waves from mine and quarry blasts Quarry/mine

No. of blasts

Adelaide Hills 16 Iron Baron 8 Iron Knob 10 Mt. Gunson 16 Leigh Creek 20 0 Values in parentheses are average values.

No. of stations 4—5 (4) 4—7 (6) 4—8 (6) 4 4—9 (6)

Distances a

Exponent n

SE

(~)a 8— 20(12) 28—213 (114) 25—185 (126) 25—182 (113) 73—399(221)

1.92 1.16 1.24 0.88 1.21

0.25 0.38 0.93 0.11 -0.50

296

S.A. Greenhalgh et al. /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

ard errors for each source. The distance ranges and number of stations used in the analysis are listed in Table 8. The n factor incorporates both the geometric spreading g and the anelastic absorption a, which are normally separated in an equation of the form log10A=C—g log00A—a~=C—nlog10~ (6) For example, Greenhalgh and Singh (1986), in a study of 718 South Australian earthquakes, obtamed a g value of 1.1 and an a value of 0.0013 (see Eq. (1)). Considering the Leigh Creek and Iron range blasts only, the average composite attenuation value of n 1.2 (see Table 8) corresponds to an absorption factor of a 0.0015. This assumes an average distance of 150 km. The agreement with the earthquake result is very good. The geometric spreading term of g 1.2 corresponds to almost the spherical spreading of the S wave. The higher value of the attenuation coefficient (n 1.9) obtained at short range for the Adelaide Hills quarry blasts can be attributed to either or both of two factors. First, the spreading loss is more severe than spherical; for example, if the phase is a head wave we expect n 2. It is possibleinthat the arrival has beenthecritically refracted the upper crust. Second, absorption a would be much larger for the higher-frequency waves, which are recorded at shorter distances, than for the mine blasts. The S-wave attenuation value found for South Australia (n 0.9—1.9) is comparable with determinations made for similar tectonic provinces elsewhere in the world. For example, Bath (1974) obtained values of n 0.65—1.6 for Sweden, and Hays (1969) reported values in the range n 1.5— 3 for central North America. =

8.5

=

=

=

=

=

4.4. Crustal velocity—depth function The velocity structure of the Earth’s crust in South Australia has been investigated by a number of workers over recent decades, using both earthquake and explosion (refraction) techniques. A review has given by Greenhalgh et al. (1989b,c). Regrettably, no deep reflection profiles have been recorded in the State. From the seismic work that

-

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50

100

150

I

I

I

200

250

300

Distance X 1km) Fig. 15. Apparent-velocity vs. average epicentral distance for crustal diving waves from mine and quarry blasts in South Australia. - The data can be fitted by an approximate linear relationship.

has been done, a simple average model of uniform crustal thickness of 38 kin and P-wave upper veloc1, overlying a uniform ity of 6.32 km s 8.05 km s has been extracted mantle of velocity for use in earthquake locations. The average crustal velocity and thickness for South Australia is similar to that found in other Precambrian shield areas, such as Canada (Barr, 1971) and Fennoscandia (Sellevoll and Warrick, 1971). However, no compelling evidence exists for a mid-crustal discontinuity in South Australia. Such a boundary has been mapped for the adjacent areas of Western Australia (Mathur, 1974) and Northeastern Australia (Finlayson et al., 1984). In this section, we present some new information on the crustal velocity distribution, obtained by inversion of mine and quarry blast data. Fig. 15 is a plot of apparent velocity vs. distance in South Australia for various explosions recorded on the permanent as well as the portable (microearthquake survey) stations of the network. The graph also includes previously unpublished data from several student projects, which entailed the recording of refraction profiles north of Ade—‘

SA. Greenhalgh etaL /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

Velocity ( km Is) 5.0

5.5 I

10

-

20

-

30

-

6.0 I

6.5 I

7.0 I

7.5 I

8.0 I

8.5 I

297

crust, and decreases smoothly with depth. The depth-averaged value for the entire crust (to 40 km) of 6.35 km s~ is only slightly higher than the value of 6.32 km s~ derived from earthquakes and routinely applied in hypocentre determinations. The South Australian crustal velocity function is similar to that obtained for Minnesota (Greenhalgh, 1981), although the latter shows a lesser velocity gradient in the upper crust.

-c

5. Conclusions

0 50

-

60

-

70 Fig. 16.

The South Australian seismic network has ex-

Crustal velocity—depth

function deduced by

Wiechert—Herglotz inversion of the data of Fig. 15.

-

laide. Each data point in Fig. 15 is the average apparent velocity from a number of blasts. The apparent velocities refer to the prominent arrival (principally ‘first breaks’). They were computed from the known co-ordinates of the source and the various stations recording each blast. It was necessary to restrict the analysis to those stations (distance ranges) for which there was no confusion that the same phase had been recorded. The distances in Fig. 15 are mean values for each blast recording. The quasi-linear increase of apparent velocity with range implies a velocity gradient in the crust. The V—X data can be inverted via the Wiechert—Herglotz integral to obtain the crustal velocity function V—Z (see Greenhaigh and King, 1980) Z(V)

1

=

~

—~[‘cosh

(V \ dX

-

~

dV

dV

(7)

The relationship J’~ a + bX3 suggested by Fig. 15, can be integrated in closed form to yield the velocity—depth function of Fig. 16. The P-wave velocity increases smoothly from approximately 5.1 km ~ at the surface (neglecting the sedimentary section) to 7.2 km s at a depth of 38 km. The velocity gradient is greatest in the upper =

—

-

with only 40 earthquakes of magnitude four or .

.......)

panded from its original three stations in 1963 to its present configuration of 17 short-period seismographs, of which six use digital recording and five use three-component sensors. In the last 7 years, responsibility for operation of the network has transferred from The University of Adelaide to Flinders University and finally to the SA Department of Mines and Energy. The computerised seismic analysis system now being used is that developed and marketed by the Phillip Institute of Technology in Melbourne. It provides complete information on earthquake locations, including data archiving, retrieval and display. Various statistical analyses of earthquake occurrence can also be undertaken. A special duration-based magnitude scale has been developed for each station, and tied in to Richter magnitude. The seismic network has allowed the overall pattern of seismicity within South Australia to be determined. The main zones of activity are the Flinders—Mt. Lofty Ranges, Eyre Peninsula and the Southeast. About 400 earthquakes are located each year. The level of activity is fairly low, greater occurring in the last 30 years. The historical (pre-network) catalogue of South Australia earthquakes includes only ten entries for magnitude greater than or equal to five, the largest being the Beachport earthquake of 1897 (ML, 6.3). There are only six earthquakes for which focal mechanisms have been determined. The predominant stress appears to be northeast— southwest horizontal compression.

298

S.A. Greenhalgh et aL /Physics of the Earth and Planetary Interiors 86 (1994) 277—299

Mine and quarry blasts are regularly recorded on the State network. The blasts can generally be located with an accuracy of a few kilometres which is reassuring in terms of earthquake hypocentral co-ordinates. The blasts have furnished valuable additional information on amplitude—charge-size relationships and seismic-wave attenuation, which has importance in magnitude calibration. The velocity function within the crust has been deduced by Wiechert—Herglotz inversion of the travel times from the various explosions. The P-wave speed increases from about 5.2 km ~ at the surface to 6.5 km s at 20 km depth. The velocity gradient decreases in the lower crust. At 40 km depth the velocity is about 7.2 km s~. The seismic stations of the South Australian network are gradually being converted to threecomponent, digital instruments. There are also plans to add several new stations, especially in the Adelaide Hills region, to better monitor the major population centre of the State. With the improvements in instrumentation and coverage we can expect a subsequent improvement in location capability and the mapping of active faults. —

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