A recent investigation by seismologists at Geoscience Australia has unearthed new evidence with which to modify the epicentral location of the 7 June 1918 Queensland earthquake. This event, and its relocation, has been described in detail by Martin et al. (2023) in a Geoscience Australia Record (currently in review) with key results summarised here. This event is the largest earthquake in eastern Australia in at least the past ~150 years (at the time of writing). It was extensively felt in eastern Queensland and in parts of northern New South Wales. The GA study has documented reports of felt shaking from 225 individual locations for this event.
Almost all previous studies (e.g., Hedley, 1925; Gutenberg and Richter, 1954) repeat the offshore epicentral location (-24o S, 154o E) proposed by the Riverview Observatory in 1918. The currently accepted location ( 23.5o S, 152.5o E) for the 1918 event in the GA hazard catalogue (Allen et al., 2018) is from Everingham et al. (1987).
The attention of early investigations was largely focused on the minor damage sustained by masonry structures in larger coastal towns such as Gladstone and Rockhampton. This is similar to the 1954 Adelaide earthquake, wherein, as pointed out by Martin et al. (2022), focus was largely on damage within the City of Adelaide with poor attention paid to damage in the thinly populated Adelaide Hills. The coastal damage observations from the 1918 Queensland earthquake supported an offshore source. However, the newly found evidence also points to severe shaking at inland locations to the south-west of Gladstone. The strongest evidence comes from the owner of a cattle station at Camboon who described severe shaking; in his words “I could scarcely keep my feet, the place was rocking so”. Minor damage was reported from a few locations such as Banana (Hedley, 1925) but owing to the low-rise, wood-frame, Queenslander-type buildings that made up the building stock in these remote areas no major damage is known of.
Not only did the new GA study by Martin et al. (2023) document these shaking effects, but they also documented a long sequence of felt aftershocks that lasted as long as a year at Camboon. Two large aftershocks have been identified previously (Rynn et al., 1987) that were felt in the Bundaberg-Gladstone region within hours of the mainshock. However, none of the subsequent shocks were perceived anywhere else except in the region of Camboon.
Neither the newly identified zone of aftershocks, nor the region of strongest shaking are consistent with an offshore epicentre. Unfortunately, as with the 1954 Adelaide earthquake (Bolt, 1955-56), instrumental observations are few. The best instrumental evidence for the 1918 earthquake comes from the Riverview observatory in Sydney where the shock was recorded on two different seismographs. The time difference between the primary (P) and secondary (S) seismic waves (multiplied by some constant) can be used as a crude measure of distance to an earthquake’s epicenter. The arrival times of these waves were picked by observatory staff, and these were subsequently published in the Riverview Seismological Station bulletin. The S-P arrival times derived from this publication have supported an offshore source, at a distance of ~1250km from Sydney. But over the course of the investigations by GA, written correspondence was found between Walter Bryan, a renowned Queensland geologist, who questioned this offshore source. In the communication that ensued between him and the Riverview Observatory in the 1930s, it became clear that the arrival picks at Riverview could be erroneous. Surprisingly, this evidence does not appear to have been known to most subsequent investigators.
In this latest investigation, seismologists at GA double-checked these arrivals by consulting the original hardcopy seismograms for the event preserved at GA. They repicked the arrivals which yielded much shorter S-P times. These correspond to a distance of ~990kms which now corresponds well with the distance from Sydney to both the region of high intensities onshore, and the area with documented aftershocks also onshore.
These different lines of evidence now offer convincing evidence that the source of the 1918 Queensland earthquake was onshore in the region of Camboon, and not offshore, as previously assumed. This study also estimates a reviewed magnitude in the range of M6.0-M6.2 based on limited instrumental data that exists for the earthquake from Australia.
References:
- Allen, T. I., M. Leonard, H. Ghasemi, and G. Gibson (2018). The 2018 National Seismic Hazard Assessment for Australia: earthquake epicentre catalogue, Geoscience Australia Record 2018/30, Canberra, 51 pp, doi: 10.11636/Record.2018.030.
- Bolt., B. (1955-1956). The epicentre of the Adelaide earthquakes of 1954 March 1, Journal and Proceedings of the Royal Society of New South Wales, Vol 89-90, pp 40-43.
- Everingham, I.B., D. Denham and S.A. Greenhalgh (1987), Surface-wave magnitudes of some early Australian earthquakes, BMG Journal of Australian Geology and Geophysics, Vol. 10, pp 253-259.
- Gutenberg, B. and C.F. Richter (1954). Seismicity of the earth and associated phenomena, Princeton University Press.
- Hedley, C. (1925). The Queensland earthquake of 1918, Transactions of the Royal Geographic Society of Australia, Vol 1, pp 151 – 156.
- Martin, S.S., P.R. Cummins, J.D. Griffin, D. Clark, T.I. Allen (2022). Reviewing the 1st March 1954 Adelaide Earthquake, South Australia, Australian Earthquake Engineering Society 2022 conference, Mount Macedon, 11 pp (https://openresearch-repository.anu.edu.au/handle/1885/282619).
- Martin, S.S., P.R. Cummins, T.I. Allen, J. Griffin, D. Clark, N. Peljo (2023). The 1918 Queensland Earthquake: Resolving an uncertain epicentre and magnitude, Geoscience Australia Record, Canberra (in review).
- Rynn, J., D. Denham, S. Greenhalgh, T. Jones, P.J. Gregson, K.F. McCue and R.S. Smith (1987). Atlas of isoseismal maps of Australian earthquakes, Bureau of Mineral Resources, Geology & Geophysics Bulletin, Vol 222, 169 p., Canberra.
