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THE ESK, SOUTH EAST QUEENSLAND, EARTHQUAKE OF 26 APRIL 1978
b y J.M.W. Rynn and J.P. Webb
(with 4 Text-figures, 1 Table)
ABSTRACT. At 1153 hours UTC (9.53 p.m. local time) on 26 April, 1978, an earthquake of magnitude ML ^ 3 .0 occurred in the Brisbane Valley. The epicentre was at 20°13.26'S, 152°23.21'E and the depth of focus 22 km. This places the event about 5 km west of the township of Esk and about 60 km WNW of Brisbane. No aftershocks were associated with the event. The effect of the earthquake was felt over an area of about 4000 sq km with a maximum intensity of MM = IV reported from Esk.
INTRODUCTION
The southeast Queensland region is one which possesses a low level of seismic activity compared with other seismic zones in Australia (Denham 1976). Since seismic recording began in eastern Australia early this century, only five earthquakes have been located in the Brisbane Valley and surround ing area (a region approximately within a 100 km radius of Brisbane): 1913, near Kilcoy (magnitude 'v 4); 1955, near Nanango (magnitude 'v 3); 1956, off the coast near Brisbane (magnitude 'v 3); 1960, about 25 km northwest of Brisbane (magnitude ^ 5.1); 1976, off the Gold Coast (magnitude ^ 4) (Jones 1958; Bauer 1972). Details of these events are given in Table 1 with epicentres plotted in Text-fig. 1. Several additional earthquakes have occurred, however, within about 100 km of Brisbane. Such events were recorded only on the University of Queensland seismograph stations in the Brisbane area (University George Street campus 1973 - 1951; University St. Lucia campus 1952 - 1963; Mt. Nebo (code BRS) 1963 -- present), and thus cannot be located. These events have been characterised by S-P times (that is the difference in the P- and S- wave arrival times) of less than 10 sec. and it is possible that some may have occurred in the Brisbane Valley region. Since March 1977 a series of six microearthquake sub-arrays have been operated in the Brisbane Valley in connection with the Wivenhoe Dam Seismic Surveillance Project for the Queensland Government Co-ordinator General’s Department. Although over 70 microearthquakes (magnitudes in the range -0.1 to 2.5) have been located in the region during the five year period to April 1982, none have occurred in the vicinity of the Esk earth quake. Details of this project are given by Rynn, Webb, Cuthbertson, Lynam and Doherty (1983).
Pap. Dep. Geol. Univ. Qd. 10(4): 197-205, March 1984 198
27°S
28°
Text-figure 1 Map of Brisbane Valley region, southeast Queensland showing location of Esk earthquake (star) and previously located earthquakes (solid circles). Location of the seismograph station BRS at Mt. Nebo, near Brisbane, is shown by the solid triangle and the location of the Wivenhoe Dam Complex by the cross within a circle. The inset map shows the relative positions of BRS and COO (Cooney Observatory) (solid triangles) in relation to the epicentre (star). 199
Text-figure 2 Map of the epicentral region of Esk earthquake (star denotes epicentre) and seismograph stations (solid triangles) which recorded the event; WCME - Mt. Esk; WCMC - Middle Ck; WCBG-Byron Gully; WCSC- Scrub Creek; BRS-Mt. Nebo. Generalised structured geology boundaries shown by dotted lines with structural features indicated by lower case letters in circles; et-EskTrough;yb-Yarraman Block; mb-Moreton Basin; db-D’Aguilar Block. Faults are shown by the solid lines with the major faults indicated by: WBF-Westem Border Fault; EBF-Eastern Border Fault; NPF-North Pine Fault. The position of the Wivenhoe and Split- Yard Creek Dams are shown by the crosses in squares with notations WD and SCD, respectively. Townships are indicated by crosshatching with letters: T-Toogoolawah, SD-Somerset Dam; E-Esk; C-Coominya; L-Lowood; F-Femvale. 200
GEOLOGY AND TECTONICS
The epicentral zone lies at the southern end of the Esk Trough in the region where the sediments of the Moreton Basin unconformably overlie those of the trough. The Esk Trough is a graben-like feature bounded on the east by the Eastern Border Fault (EBF) and on the west by the Western Border Fault System (WBF) and lies between the upper Palaeozoic Yarraman and D’Aguilar Blocks. Structural details of the Esk Trough, and particularly in the vicinity of the Esk earthquake epicentre, are not well-known due 1o the extremely weathered nature of the rocks and poor outcrop. In general terms, Leven (1977) concluded that the most probable explanation for the structural style within the trough is related to wrench-faulting. The generalised geological structures are shown in Text-fig. 2 with more complete geological details discussed elsewhere (for example: Day, Cranfield and Schwarzbok 1974; Hill 1960, and references cited therein).
LOCATION OF ESK EARTHQUAKE
The Esk earthquake was recorded by the four network stations, WCME, WCMC, WCBG and WCSC (Rynn et al. 1983), as well as the standard stations BRS (Mt. Nebo, near Brisbane) and COO (Cooney Observatory, near Armidale), Text-fig. 1. The hypocentre was located from the P-wave arrivals at these six stations and the S-wave arrival at BRS using the hypocentral location program, HYP071, of Lee and Lahr (1975). S-wave arrivals could not be read on the four sub-array seismograms as the records were clipped on these instruments (Sprengnether MEQ-800 smoked-paper seismograph systems). For COO, a clear S-wave arrival could not be identified. Selected seismograms are shown in Text-fig. 3. The solution obtained is: Origin time: 26 April 1978 llh53m 14.78s UT (9.53 pm EST) Epicentre: 27.22±0.04°S, 152.39±0.04°E Focal Depth: 22 km RMS Error: 0.19 sec Magnitude: 3.0 (ML)
The formal error in the epicentre is about 5 km. A focal mechanism solution could not be attempted due to lack of data. Only four seismograms (BRS, WCMC, WCBG, WSCS) showed clear first motions and these were all dilatations. No aftershocks are known to be associated with this earthquake.
ISOSEISMAL STUDY
On the morning following the event, earthquake questionnaires were distributed to all townships and many isolated farmhouses in this part of the B RS Z 201
Text-figure 3 Seismograms for Esk earthquake: upper three traces: BRS three-component set vertical (Z) and horizontals (NS,EW); lower trace - WCMC vertical (Z) component. Seismic arrivals indicated by P (compressional) and S (shear). Time marks (offsets of lines) are one minute apart. 202
I52°15' I52°30' 152°45'
0 10 km 1 ■ ■ DATE : 26 APRIL 1978 TIME : 11:53:13.6 UT MAGNITUDE :3.5ML(BRS) EPICENTRE : 27.23°S 152.31°E DEPTH : 22km
A EPICENTRE IV ZONE INTENSITY DESIGNATION (MM) « EARTHQUAKE FELT (MM) o EARTHQUAKE NOT FELT
Text-figure 4 Isoseismal map for Esk earthquake (epicentre denoted by solid triangle). Numbers represent intensity values based on the Modified Mercalli Scale of 1931 (Wood and Neumann, 1931), from felt reports (O indicates effects not felt). Roman numerals designate the various intensity zones: (Map drawn by Bureau of Mineral Resources, Canberra). 203
Brisbane Valley. The spatial distribution for the survey was selected to include those areas in the vicinities of the Wivenhoe Dam complex and Somerset Dam, as well as the epicentral zone. A total of 50 reports were received (26 positive, 24 negative). An intensity value based on the Modified Mercalli Scale of 1931 was assigned to each report. The resulting isoseismal map is shown in Text-fig. 4. The isoseis- mal zone, that is, the region in which the earthquake was felt, is confined to an area of about 4000 sq km centred on Esk about 5 km east of the epicentre. The strongest felt reports, having a maximum assigned intensity of MM = IV, came from the township of Esk. The earthquake was not felt in the vicinities of the Wivenhoe and Somerset Dams. No information for areas to the west and east of the epicentral zone was available as these areas are unpopulated.
DISCUSSION
The occurrence of this earthquake is significant to seismological studies in southeastern Queensland and particularly to the Wivenhoe Dam study. Not only was this event strongly felt in a major provincial town, but also it was the largest earthquake to have occurred in the southeast corner of Queensland since the Mt. Glorious earthquake of 1960 (Bauer 1972). The Esk earthquake is probably the most accurately located event of any that has occurred in Queensland owing to the presence of the Wivenhoe microearth quake array. The epicentre for the Esk earthquake lies within the Esk Trough (Text-fig. 2) and does not appear to be associated with any geologically mapped or inferred faults. In particular, it is not associated with either the Eastern or Western Border fault system. The focal depth of 22 km places the hypocentre of this earthquake within the lower crust. This is in contrast to all other microearthquakes in the region for which the depths of focus are less than about 10 km, that is, in the upper crust (Rynn et al. 1983). It is considered that this earthquake is a result of regional stresses in the crust similar to the preferred casual mechanism for most, if not all, eastern Australian earthquakes which occur within the Tasman Geosyncline (Denham 1976).
ACKNOWLEDGEMENTS
This research was carried out jointly with the Geological Survey of Queensland, Geophysics Section under the Wivenhoe Dam Seismic Surveillance Project, funding for which is provided by the Premier’s Department (formerly the Co-ordinator General’s Department) of the State Government of Queensland. We wish to thank Mr. J. Doherty (formerly with the Geological Survey of Queensland) for field assistance in the isoseismal survey and Mr C.J. Lynam (Department of Geology, University of Queensland) for reading the BRS seismograms. The Director, Bureau of Mineral Resources, TABLE 1 u X u S CO J « o w Û os > X ' O u *< H 00 M Z u - > c e o < H < flQ u z D í o D Z z c o o z CA < J - -< CO O o w Q Q 1—1 1*5 -4-> w +2 X . > s a g M V o o w •G Q CO s 3 * X <4—1 ) ,0 a • J - -*-> 4 c GO 5 cd 0) G
No depths are available ; in all cases they are assumed to be crustal. 205
Canberra kindly gave permission to use the seismic data from the Cooney Observatory (through Mr R. Smith) and to publish the isoseismal map (drawn by the Drafting Section, B.M.R.). Drs S.H. Hall and J.H. Leven reviewed the original manuscript and suggested several improvements.
REFERENCES
BAUER, J. 1972. The earthquake of 17 November, 1960, in the Mt. Glorious region, southeast Queensland. Univ. Qd Dep. Geol. Hons Thesis (unpubl.) 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, 319-362, Geol. Soc. Aust., Qd Div. DENHAM, D. 1976. Effects of the 1973 Picton and other earthquakes in eastern Australia. Bull. Bur. Miner. Resour. Geol. Geophys. Aust., 164. HILL, D. 1960. The Upper Brisbane Valley Trough; in HILL, D. & DENMEAD, A.K. (Eds.),/. Geol. Soc. Aust., 7,269-274. JONES, O.A. 1958. Queensland earthquakes and their relation to structural features. Jour. Proc. R. Soc. NSW, 92,176-181. LEE, W.H.K. & LAHR, J.C. 1975. HYP071 (Revised): A computer program for deter mining hypocentre, magnitude, and first motion pattem of local earthquakes. U.S. Geol. Survey Open File Report, 75-311,110 pp. LEVEN, J.H. 1977. A gravity survey of the southern Esk Trough, southeastern Queens land, Pap. Dep. Geol. Univ. Qd, 8,25-36. 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. ANCOLDBull., in press. WOOD, H.O. & NEUMANN, F. 1931. Modified Mercalli Intensity Scale of 1931. Bull. Seism. Soc. Amer., 21, 277-283.
J.M. W. Rynn & J.P. Webb, Dept, o f Geology & Geophysics, Seismology Group, University o f Queensland, St. Lucia, Queensland, 4067.