206

THE TENT HILL, LOCKYER VALLEY, SOUTHEAST QUEENSLAND EARTHQUAKES

OF 24 MARCH 1981

by J.M.W. Rynn

(with 4 Text-figures, 1 Table)

ABSTRACT. A series of three earthquakes (foreshock, mainshock and after­shock) occurred at Tent Hill, near Gatton, in the Lockyer Valley of southeast Queensland on the morning of 25 March 1981. The main shock, at 4.33 am EST, had a magnitude ML (COO) = 3.1 and was felt over an area of about 500 sq km. The maximum intensity reported was MM = IV in the epicentral region.

INTRODUCTION

The occurrence of earthquakes in southeast Queensland is not a common event - indeed the notion of people experiencing the effects of an earthquake is even more rare. Apart from this ‘ human” factor, two other aspects of these earthquakes are noteworthy. This sequence, to the best of the author’s knowledge, is the first time the classic foreshock-mainshock- aftershock sequence has been recognised, let alone documented, for this region. These earthquakes are also significant for the Wivenhoe Dam Seismic Surveillance Project (Rynn, Webb, Cuthbertson, Lynam, and Doherty, 1983) in which the level and nature of seismic activity in the pre-impounding stage (that is, natural background seismicity) of the Wivenhoe Dam and associated aspects (presently under construction) are being ascertained.

The area of this study, with particular reference to the Wivenhoe Dam complex and major centres of population, is shown in Text-fig. 1. Also included on this figure are the epicentral zone, isoseismal region and seismo­graph stations which recorded the earthquakes.

HYPOCENTRAL PARAMETERS

All three earthquakes were recorded on the four Wivenhoe Dam network seismographs WFPL (Plainland, WFPM (Pine Mt.), WFTG (Toogoo- lawah) and WFMB (Somerset Dam) and the University station BRS (Mt. Nebo) (Text-fig. 1). The foreshock and mainshock were also recorded at the COO (Cooney Observatory) and BDDM (Boondooma Dam) stations (Inset, Text-fig. 1). Selected seismograms written at WFTG are shown in Text-fig. 2.

In computing the hypocentral parameters, only selected data from seismograms were used (Table 1). This was because the signatures for the fore and mainshocks were too complex to reliably distinguish the individual

Pap. Dep. Geol. Univ. Qd. 10(4): 206-215, March 1984

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Text-figure 1 Location map for Tent Hill earthquakes of 24 March 1981 in relation to the Wivenhoe Dam and reservoir (under construction) and major towns. Epicentral zone (cross-in-circle) and isoseismal region are indicated. Seismograph stations of the Wivenhoe Dam network and BRS shown by triangles. Inset map shows epicentral zone (cross-in­circle) in relation to seismograph stations which recorded the earth­quakes (station codes given in text).

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FORESHOCK and MAINSHOCK

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Text-figure 2 Selected seismograms for the foreshock and mainshock recorded at WFPL and WFTG and the aftershock recorded at WFPL. Seismic arrivals for the P- and S-waves are indicated; note the complexity o f the signa­ture at WFTG with the foreshock S-wave arrival buried in the mainshock P-wave arrival. Offsets on seismograms are at one minute intervals. Corrected time (U.T.) indicated on seismograms.

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P- and S-wave arrivals as these events occurred within about 7 sec of each other (taken as the difference in the P-wave arrival times at the closest station WFPL, epicentral distance about 20 km). In the case of the after­shock, it was not recorded at all stations because of its very small magnitude. Computations were carried out with a simple two layer crustal model CP- wave velocities 5.85 km/sec, 0-10 km; 6.67 km/sec, 10-30 km; 7.95 km/sec greater than 30 km) using the hypocentral location program HYP071 of Lee and Lahr (1975). Results of these calculations are given in Table 1 with the epicentral locations shown in Text-fig. 2.

Although the earthquakes occurred outside the Wivenhoe Dam network, the proximity of station WFPL allowed the epicentres to be well controlled. The focal depth control, however, did not allow precise deter­minations of this parameter. The events occurred in the upper crustal layers at depths of about 10±5 km.

The magnitude of the mainshock was calculated from the COO seismogram using the Bureau of Mineral Resources ML (local magnitude) formula for non-standard seismographs (R. Smith, personal communication, 1981 ; McGregor and Ripper, 1976)

Ml = log A -lo g A0 -log R .............................. (1)

where A = maximum trace amplitude of S-wave (mean to peak)A0 = trace amplitude of reference earthquake (magnitude

zero) at same distance.R = ratio of magnification of non-standard seismograph to

that of Wood Anderson seismograph at the same period.

For the aftershock, the magnitude was calculated by the formula based on the duration of the signature on the seismogram as used by the Preston Institute of Technology, Victoria (G. Gibson, personal communication, 1979)

Md = Ao + A, (log T)a 2 + A,D .............................. (2)

where T = duration in secondsD = epicentral distance in km

and A0 = -1.06, A, = 1.04, A2 = 1.612, A3 = 0.0028.

In the case of the foreshock, only an appropriate magnitude can be estimated as the signature of this event is masked by the mainshock. Using equation 1 for the S-wave arrival recorded at station WFPL, a value of M^ (WFPL) = 2.0±0.3 was obtained.

INTENSITY INVESTIGATION

On the morning following the event, residents and post offices were canvassed by telephone to establish the extent of the felt area. Following

211

this, earthquake questionnaires were distributed throughout the area and an appeal for information made through the local newspaper, the “Gatton Star” . Fifteen questionnaires were received several of which provided a report for effects felt in small farming districts within the isoseismal zone. The result­ing isoseismal map is shown in Text-fig. 4.

The felt area covered about 500 km2 and was elongated in a north- north-westerly direction from the epicentre. A maximum intensity of MM = IV was reported from the Tent Hill area. Most of the reports indicated that residents were awakened by a sudden jolt and noticed their houses shudder­ing and windows and doors rattling. A fairly loud, sudden but short noise accompanied the event. In all cases the direction of the sound was from the southeast. One isolated report was received from Toowoomba about 30 km northwest of the epicentre (Text-fig. 1).

GEOLOGICAL SETTING

The epicentral zone lies within the Moretón Basin close to the mapped boundary between the upper and lower members of the Jurrasic Marburg Formation (Swindon 1971; Text-fig. 3). Details of the regional geology and structural features have been discussed by Swindon (1971) and Day, Cranfield and Schwarzbok (1974). A detailed geological investigation for an area just to the southeast of the epicentral zone was reported by Robinson (1963).

Within the area of this study, two structural features, the Gatton Arch (Jorgenson and Barton 1966; Swindon 1971) and the Lockyer Creek lineament (Beckman and Stevens 1978) have been identified (Text-fig. 3). The Gatton Arch, which trends roughly north-northwest-south-southeast, is a broad plunging anticline which may reflect a buried ridge. The lineaments, determined from current stream directions, are considered to be indicative of structural trends in the area which influenced the present courses of the creeks. Such lineaments could form the eastern part of the Cobar-Inglewood lineament as postulated by Scheibner (1974) to be a major structural trend in eastern Australia.

DISCUSSION

Apart from these earthquakes exhibiting the classic foreshock- mainshock-aftershock sequence, the significance of their occurrence is of prime importance to the background seismicity of the area in relation to the Wivenhoe Dam Project. As can be seen from Text-fig. 3, the earthquakes occurred on the western limb of the Gatton Arch in a region between this structure and the Lockyet Valley lineaments to the north. With this structural setting, it is possible that some localised stress build-up may have occurred with its release manifested by the earthquake occurrences. The question as to whether a causal relationship is related to these local conditions, or if such earthquakes are a result of overall regional stress release in the crustal rocks of the Tasman Geosyncline, as suggested by Denham (1976), is still under study.

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Text-figure 4

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DATE : 24 MARCH 1981 TIME : 18:34:16 UT MAGNITUDE :3.1ML(BMR) EPICENTRE : 27.68°S 152.24°E DEPTH :10km

▲ EPICENTREIV ZONE INTENSITY DESIGNATION (MM) 4 EARTHQUAKE FELT (MM) o EARTHQUAKE NOT FELT

Isoseismal map for the mainshock in the Tent Hill earthquake sequence of 24 March 1981 (origin time: 18:34:15.93 Hrs. U.T.: magnitude ML (COO)=3.1 ; epicentre denoted by solid triangle). Numbers represent intensity values based on the modified Mercalli scale of 1931 (Wood and Neumann 1931) from felt reports (0 indicates effects not felt). Roman numerals designate the various intensity zones. (Map drawn by Bureau of Mineral Resources, Canberra).

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ACKNOWLEDGEMENTS

This research was carried out under the Wivenhoe Dam Seismic Surveillance Project funded through the Co-ordinator General, Premier’s Department, the State Government of Queensland as a joint project with the Geological Survey of Queensland. The author acknowledges the assistance of J. Pix in collating intensity reports and R. Cuthbertson (Geological Survey of Queensland), and C.J. Lynam (this Department), in reading the siesmograms. The Director, Bureau of Mineral Resources, Canberra, kindly permitted publication of the isoseismal map (drawn by Drafting Section, BMR) and made available the COO seismogram (through R. Smith). Thanks are also extended to several colleagues in the Department who discussed the geological aspects of the area and the Departmental secretarial staff for typing the paper. J.P. Webb, D. Denham, S.J. Hearn and S.H. Hall reviewed the manuscript and offered several suggestions for its improvement.

REFERENCES

BECKMANN, G.G. & STEVENS, N.C. 1978. Geological history of the Brisbane River system, in MONROE, R. & STEVENS, N.C. (Eds.), The Brisbane River Symposium. Proc. R. Soc. Qd., 89, 77-85.

DAY, R.W., CRANFIELD, L.C. & SCHWARZBOK, H. 1974. Stratigraphy and structural setting of Mesozoic basins in southeast Queensland and northern New South Wales, in DENMEAD, A.K., TWEEDALE, G.W. & WILSON, A.F. (Eds.), The Tasman Geosyncline - a Symposium. Geological Society of Australia Incorpor­ated - Queensland Division, 319-362.

DENHAM, D. 1976. Effects of the 1973 Picton and other earthquakes in eastern Austra­lia, in DENHAM, D. (Ed.), Seismicity and Earthquake Risk in Eastern Australia. Bull. Bur. Miner. Resour. Geol. Geophys. Aust. 164, 15-31.

JORGENSON, J.T. & BARTON, R.H. 1966. Regional photogeology of the Ipswich Basin - Esk Trough, Queensland. J. Aust. Pet. Expl. Ass., 1966, 121-125.

LEE, W.H.K. & LAHR, J.C. 1975. HYP071 (Revised): A computer program for deter­mining hypocentre, magnitude and first motion pattern of local earthquakes. U.S. Geol. Survey Open File Report, 75-311,110 pp.

McGREGOR, P.M. & RIPPER, I.D. 1976. Notes on magnitude scales. Rec. Bur. Miner. Resour. Geol. Geophys. Aust., 1976/56 (unpubl.).

ROBINSON, G.P. 1963. Sedimentology, stratigraphy and palynology of the Heifer Creek region, Queensland. Univ. Qd. B.Sc. Hons thesis (unpubl.).

RYNN, J.M.W., WEBB, J.P., CUTHBERTSON, R.J., LYNAM, C.J. & DOHERTY, J.E. 1983. The Wivenhoe Dam Seismic Surveillance Project: A preliminary report. A N COLD Bull, in press.

SCHEIBNER, E. 1974. Fossil fracture zones (transform faults), segmentation, and correlation problems in the Tasman Fold Belt system, in DENMEAD, A.K., TWEEDALE, G.W. & WILSON, A.F. (Eds.), The Tasman Geosyncline - a Symposium. Geological Society of Australia Incorporated — Queensland Division, 65-98.

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SWINDON, V.G. 1971. Moreton district, in PLAYFORD, G. (Ed.), Geological Excursions Handbook, A.N.Z.A.A.S. 43rd Congress, Brisbane, Section 3 - Geology. Geolo­gical Society of Australia Incorporated — Queensland Division, 105-125.

WOOD, H.O. & NEUMANN, F. 1931. Modified Mercalli Intensity Scale of 1931. Bull Seism. Soc. Amer., 21,277-283.

J.M. W. Rynn,Dept, o f Geology & Mineralogy, Seismology Group,University o f Queensland,St. Lucia, Brisbane, 4067.