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°.