Onshore Relocation of the 1918 Bundaberg Earthquake
- Briefing Note 502
Southeastern Queensland was struck by a strong earthquake at 04:14 a.m. local time on 7 June 1918. As shown on the right side of Figure 1, previous studies have suggested that the epicentre of this earthquake lies off the coast of Bundaberg. The epicentral location shown by the orange star marked RIV was based upon instrumental observations from the Riverview College observatory in Sydney. However, this epicentre lies about 250 km to the northeast of an inland region that experienced both the strongest shaking effects and numerous felt aftershocks, as shown on the left side of Figure 1. Martin et al. (2024) reviewed available macroseismic data from 224 geographic locations and seismograms of the earthquake. They found that the most likely epicentral location was inland at about 24.93° S and 150.88° E in the Banana Shire and North Burnett region, as shown by the red stars in Figure 1. The revised magnitude of Mw 6.0 ± 0.3 indicates that it was one of the largest onshore earthquakes in eastern Australia in the past century.
Locations of the strongest mainshock intensities and aftershocks
Martin et al. (2024) carefully reviewed intensity observations and concluded that the strongest mainshock intensities occurred in Banana Shire and North Burnett Shire, whose locations are shown on the right side of Figure 1. This region coincides with the red ellipse in Figure 2, which encompasses the region where aftershocks were reported in the days and weeks following the mainshock, as identified by Martin et al. (2024). The revised mainshock epicentral estimate (red star) and the locations of the 1935 Gayndah and the 2015 Eidsvold earthquakes (yellow stars) are also shown. The grey dotted contours for the posterior mainshock location distribution were determined using the Griffin et al. (2019) Bayesian inversion method.
Revised Identification of the P wave Arrival at Riverview.
The seismograms recorded on the Weichert and Mainka seismometers at Riverview are shown in Figure 3. The original compressional wave (P-wave) pick listed in the RIV station bulletin is marked on the original Weichert seismogram (Fig. 3a, inset). The blue arrow in Figure 3a shows that the observatory personnel interpreted the P-wave arrival to be a pulse with a period of about 6 seconds marked with the blue P symbol. However, Martin et al. (2024) interpreted this arrival to be ocean wave generated microseisms, which typically have a period of 6 seconds. Martin et al. (2024) suggest that the actual P arrival (red arrow) is instead the sharper pulse appearing about 31 s after the original P pick, marked with a red arrow and P symbol (Fig. 3). The time between the P and S (shear wave) arrivals indicates the distance between the seismograph station and the earthquake epicentre. The revised S–P time of 95.1 s reduces the estimated distance between the epicentre and RIV from 10.5° (about 1166 km) to 8.4° (about 932 km).
The relocation of the epicentre of the Bundaberg earthquake shown in Figures 1 and 2 is based on the distance from Riverside from this revised S-P time at Riverview, and estimated distances from Sydney, Melbourne, Adelaide and Jakarta. The grey dotted contours for the posterior mainshock location distribution in Figure 2 were determined using the Griffin et al. (2019) Bayesian inversion method.
Implications for Seismic Hazard in Southeastern Queensland
The 2023 revision of the National Seismic Hazard Assessment (NSHA23; Allen et al., 2023) was based on the RIV location of the Bundaberg earthquake shown in Figure 1. The Bundaberg earthquake moment magnitude Mw in NSHA23 is 5.87, slightly less than the value of 6.0 obtained by Martin et al. (2024), and similar to the Mw of 5.9 for the 22 February 2022 Woods Point earthquake in Victoria. If NSHA23 were to be revised using the relocated epicentre and magnitude of the Bundaberg earthquake, the seismic hazard level would increase slightly in the coastal towns of Southeastern Queensland and increase more in the inland regions within about 200 km of the coast.
References
Allen, T. I., J. D. Griffin, D. J. Clark, P. R. Cummins, H. Ghasemi, and R. Ebrahimi (2023). The 2023 National Seismic Hazard Assessment for Australia: Model overview, Geoscience Australia Record 2023/53, doi: 10.26186/148969.
Everingham, I. B., D. Denham, and S. A. Greenhalgh (1987). Surface wave magnitudes of some early Australian earthquakes, BMG J. Aust. Geol. Geophys. 10, 253–259.
Griffin, J., N. Nguyen, P. Cummins, and A. Cipta (2019). Historical earthquakes of the Eastern Sunda Arc: Source mechanisms and intensity-based testing of Indonesia’s national seismic hazard assessment, Bull. Seismol. Soc. Am. 109, no. 1, 43–65, doi: 10.1785/0120180085.
Hedley, C. (1925). The Queensland earthquake of 1918, Trans. R. Geograph. Soc. Aust. 1, 151–156.
Martin, Stacey S, Phil R. Cummins, Jonathan D. Griffin, Dan Clark, Trevor I. Allen (2024). Resolving the Location and Magnitude of the 1918 Queensland (Bundaberg), Australia, Earthquake. Bulletin of the Seismological Society of America 2024; doi: https://doi.org/10.1785/0120240029
