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PSEUDO-ISOSEISMAL EFFECTS ON THE CITY OF BRISBANE OF A QUARRY BLAST ON 4 DECEMBER 1978

by J.M.W. Rynn and J.P. Webb

(with 3 Text-figures, 1 Table)

ABSTRACT. At 4.43 pm (0643 Hours UTC) on 4 December 1978, a 1000 sq km area of southeast Queensland experienced an event initially presumed to be a local earthquake. Investigation showed that such effects were the result of a large surface explosion at the Split-Yard Creek Dam quarry in the Brisbane Valley. The maximum intensity, equivalent to earthquake intensities based on the Modified Mercalli Scale of 1931, was MM = V reported from the vicinity of the Wivenhoe Dam complex and several western suburbs of Brisbane. The felt area covered the whole of the city of Brisbane and caused alarm in many cases. One possible cause for such intensities can be related to the fact that a large surface explosion was detonated during a period of thunderstorm activity where the cloud was extremely thick and its base at a very low level.

INTRODUCTION

The “felt” reports associated with earthquakes are well-known and constitute an important study in seismology, viz. isoseismal studies, where intensity values, based on the Modified Mercalli Intensity Scale of 1931 (Richter 1958), are assigned to reports of the effects of such earthquakes on people and property. From such studies, isoseismal maps are produced and these play an important part in the assessment of seismic risk and the development of building codes.

There are, however, other situations, both man-made and natural, which produce similar effects on buildings and people. These include explosions related to the extractive materials and minerals production industries (at open-cut mines and quarries), sonic booms from aircraft and meteorological phenomena (such as thunderstorms). In most of these cases the “felt’ reports are either isolated or come from very small localised areas and identification of the source is very obvious.

Cases where man-made phenomena produce both wide-spread reports and frightening effects are quite rare. It is one of these cases that is reported in this paper.

LOCATION OF EVENT

This event was recorded on a microearthquake network in the Brisbane Valley at stations WDLR, WDPM and WDPK of the Wivenhoe Dam Seismic Surveillance Project (Rynn, Webb, Cuthbertson, Lynam and DohertyPap. Dep. Geol. Univ. Qd. 10(4): 216-223, March 1984

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152*E 153*

Text-figure 1 Map o f south-east Queensland showing location o f blast o f 4 December 1978 at Split-Yard Creek Dam construction site (star), seismograph stations (solid triangles): BRS, and Wivenhoe Dam seismic surveillance network microearthquake sub-array D stations W DLR (Low ood Range) WDPM (Pine M t.) and WDPK (Perry’s Knob) and the pseudo-isoseismal map for the affected region. Roman numerals show zones o f various intensities (values based on the Modified Mercalli Scale o f Intensity o f 1931). Open circles indicate towns in the region which did not experience any effects o f the blast.

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Text-figure 2 Pseudo-isoseismal map for Brisbane and suburbs. Location of this area shown in Text-fig. 1.

Roman numerals show zones of various intensities (as for Text-fig. 1). Opencircles indicate places which experienced some effect of the blast, those with numbers beside indicating higher or lower intensity values than for the particular isoseismal zone. (Map drawn by the Bureau of Mineral Resources, Canberra, Drafting Section.)

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TABLE 1

SEISMIC PARAMETERS FOR SPLIT-YARD CREEK BLAST

Parameter Computed Actual

Origin Time 06:42.50.0 UT approx. 06:42:50 UT (04:42:50 pm EST)

Epicentre 27.372°S, 152.636°E 27.375°S, 152.637°E

Depth 0 km surface

Equivalent magnitude ML (WIV network) 3.5 yield = 2.72 tonne

Explosive: Two blasts fired simultaneously, using ammonium nitrate and dieseleneexplosive (known as ANFO).

Production blast: to extract quarry rock 1.72 tonnes ANFO

Secondary blast: to split large rocks 1.0 tonne ANFO.

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1983) and the University of Queensland Seismograph Station BRS at Mt. Nebo near Brisbane (Text-fig. 1). Analysis of the seismograms in terms of the character of the recorded signals, the time difference between the P- and S- wave arrivals on each seismogram and the epicentral location using the P-wave arrivals from the above four stations, placed the event in the vicinity of the Wivenhoe Dam construction complex. These hypocentral calculations were made using the HYP071 location program (Lee and Lahr 1975) with a simple crustal model (5.85 km/sec 0-10 km; 6.67 km/sec 10-30 km; 7.95 km/ sec greater than 30 km). Subsequent enquiries showed that a man-made explosion occurred at the Split-Yard Creek Dam Quarry (Text-fig. 1). The two sets of location parameters are listed in Table 1.

PSEUDO-ISOSEISMAL STUDIES

Because of the extensive area of felt reports for this event and the initially assumed probability of it being a local earthquake, an intensity study was immediately initiated. With the assistance of Brisbane and Ipswich television and radio stations, reports began coming into the University at 5 pm on the day of the event (4 December 1978). Additional reports were collected over the next two days. A total of 231 reports were obtained from residents in the Brisbane, Ipswich, Gold Coast, Sunshine Coast and Lamington Plateau regions. Although the event was definitely identified as a man-made explosion from the seismograms, it was considered of sufficient public importance to complete the study by compiling an isoseismal map. This map is thus a “pseudo-isoseismal” map as it does not refer to an earthquake although an intensity value based on the Modified Mercalli Intensity Scale of 1931 was assigned to each felt report.

This map is shown in Text-figs. 1 and 2 and is presented in two parts owing to the density of data obtained from the city of Brisbane. The maximum observed effect was equivalent to an intensity of MM = V (felt strongly outdoors, trees disturbed, objects on shelves displaced) reported from the vicinity of the Wivenhoe Dam complex and two isolated cases in the western Brisbane suburbs of Jindalee and Jamboree Heights. As shown in Text-fig. 1, the general trend of felt reports were in the northeast and south­east directions with isolated reports from the Gold Coast (particularly the Southport area) and Maroochydore. For the city of Brisbane, the pseudo- isoseismals trend northwest-southeast with the pseudo-intensities decreasing towards the eastern suburbs (Text-fig. 2). Throughout the Brisbane and Ipswich areas, the average equivalent intensities were MM = IV (generally felt by everyone; vibrations like a heavy truck passing by; a heavy object striking the house; severe rattling of windows, doors and crockery) and MM = III (vibrations like passing light trucks; felt indoors).

CAUSAL RELATIONSHIP WITH METEOROLOGICAL CONDITIONS

The phenomena of atmospheric and surface blasting with the resulting effects of sound and vibrations on populated areas are well-known to the meteorological, extractive material and engineering communities. Several

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Text-figure 3 Seismograms of blast of 4 December, 1978; from the top; WDLR, WDPM, WDPK and BRS. The seismic arrivals are denoted by P (compressional), S (shear) and A (air wave). Timing marks (offsets of horizontal lines) are one minute apart.

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case studies have been reported ; for example, presumed atmospheric nuclear explosions (Savino and Rynn 1972) and small scale local blasting operations (Delaney 1982). In such cases, the effects on populated areas are considered to be strongly influenced by the prevailing atmospheric conditions at the time of the blast. Causal relationships have been attributed to such meteoro­logical phenomena as the jet stream, temperature inversions, prevailing winds and adverse weather conditions. The resulting sound wave in the air is considered to be a type of channel wave propagated between some atmo­spheric layer and the surface of the earth.

For the effects of this blast to be reported over the large area, as shown in Text-fig. 1, one would expect the transmitted seismic energy (in the earth) to be very high and consequently be related to a very large yield of explosive. This was not the case since the total explosive used was not unusual for the operations connected with the Split-Yard Creek Dam project and the record of the signatures on the seismograms was not abnormally large. It was noted however that a very large air wave was associated with this event (Text-fig. 3). When the mechanics of the blast became known, namely, a large blast on the surface, it was apparent that the effects felt by the populace were related to energy associated with the air wave.

In such a case where a large amount of energy was transmitted into the atmosphere, it was thought that adverse meteorological conditions, for example, in the form of thick cloud and rain, could provide a casual relation­ship through near-surface reverberations. This was indeed the case. On the afternoon of 4 December 1978, an airmass storm which contained thunder­storm activity covered the whole Brisbane, Ipswich and coastal regions. The cloud base was at about 800 m with cloud thickness up to 10000 m and 8/8 cloud cover. Rain had begun to fall at about 2 pm EST and continued through the time of the blast. The wind was blowing from the NNE-NW direction with a speed of a few kph.

These meteorological conditions of thick cloud cover and prevailing wind direction are considered to be the most probable explanation to account for the pseudo-isoseismal effects reported. Such would be the result of reverberations of sound energy between the cloud base and the ground for this large surface explosion.

ACKNOWLEDGEMENTS

The authors wish to thank the Co-ordinator General, Premier’s Department, State Government of Queensland for providing funding for the Wivenhoe Dam Seismic Surveillance Project. Assistance with the field work and analysis of the data for this event was given by Mr C.J. Lynam (this Department) and Mr J. Doherty (formally with the Geological Survey of Queensland). Meteorological information was kindly supplied by the Brisbane Weather Bureau. Text-fig. 2 drawn by Bureau of Mineral Resources, Canberra, Drafting Section and published with kind permission of the Director. Our thanks are also extended to Drs S.H. Hall, D. Denham and Mr P. Jones for critically reviewing the manuscript.

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REFERENCES

DELANEY, W. 1982. Meteorological forecasting of blast noise from the Del Park mine- site; in Papers Presented at the Australasian Institute of Mining and Metallurgy Conference, Melbourne, 1982. Aust. Inst. Min. Metall., Series 11, 221-231.

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.

RICHTER, C.F. 1958. Elementary Seismology, W.H. Freeman and Co., San Francisco, 768 p.

RYNN, J.M.W., WEBB, JP ., CUTHBERTSON, R.J., LYNAM, C.J. & DOHERTY, J.E. 1983. The Wivenhoe Dam Seismic Surveillance Project — A Preliminary Report. ANCOLD Bull., in press.

SAVINO, J.M. & RYNN, J.M.W. 1972. Quasi-static loading of the earth by propagation, air waves./. Geophys. Res. 77, 5033-5041.

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