Stress-field constraints from recent intraplate seismicity in southeastern Australia

Abstract

Well-constrained fault-plane solutions are determined for five recent moderate-magnitude intraplate earthquakes recorded in southeastern Australia: Thomson Reservoir, Tatong, Corryong and Boolarra South in Victoria, and Appin in eastern New South Wales. The solutions, determined from analysis of vertical-component first-motions, provide an indication of the maximum horizontal stress influencing contemporary regional deformation. The fault-plane solution for the 1996 M _L 5.0 Thomson Reservoir earthquake indicates reverse faulting with northwest – southeast-trending horizontal compression. The Yallourn Fault is the most likely source for this event given the orientation of the fault-plane solution and the hypocentral location of the earthquake. The 1997 M _L 4.2 Tatong earthquake also indicates reverse faulting with approximate north – south compression. The 1998 M _L 4.7 Corryong event indicated a strike-slip mechanism with a compressional axis trending northwest – southeast. Despite local complexities identified within the stress field of the Sydney Basin, the fault-plane solution determined for the 1999 M _L 4.6 Appin earthquake indicated strike-slip faulting and is consistent with the proposed weak northeast – southwest compressional regime in the region. The 2000 M _L 4.7 Boolarra South earthquake indicates a mechanism consistent with reverse faulting with horizontal compression oriented northwest – southeast. Further analysis of horizontal-component seismograms indicated that the earthquake was possibly a result of movement on the Yarragon Fault. A composite focal mechanism was constructed comprising data from the four Victorian earthquakes (which lie in a similar stress province) used in this study. The solution indicated horizontal compression, consistent with reverse faulting, with an overall maximum horizontal stress trending at approximately 325°.

Keywords:

• earthquakes
• focal mechanism
• intraplate stress
• southeastern Australia

ACKNOWLEDGEMENTS

We would like to thank the staff at the ES&S Seismology Research Centre for access to resources and their generous cooperation in the support of this research. We also thank Scott Reynolds for his feedback on the quality of our fault-plane solutions with respect to WSM protocols and review of the paper. Dan Clark, Mark Leonard, Peter Betts, Caroline Forbes, Amy Brown, and Christian Noll are thanked for their critical reviews and thoughtful suggestions. Mike Sandiford and an anonymous reviewer are also thanked. In addition, the authors acknowledge Matt Purss for ERMapper assistance. Gridded airborne geophysical data were kindly supplied courtesy of the Department of Primary Industries, Victoria and the New South Wales Department of Mineral Resources. This research was supported by a Monash University postgraduate scholarship.