Geologic expressions of faulting and earthquake strong ground motions in intraplate bedrock terrains

Abstract

Australian earthquakes offer unique opportunities to investigate environmental and landscape effects of reverse rupturing faults. All historic surface-rupturing earthquakes have occurred in arid, low-relief, bedrock dominated areas with little to no anthropogenic influence. Environmental earthquake effects identified following the 2016, reverse-mechanism, MW 6.1 Petermann earthquake in remote central Australia are categorised with the Environmental Seismic Intensity scale, the first application of this scale for an Australian earthquake. The intensity and distribution of environmental damage demonstrates strong asymmetry due to fault geometry, with damage increasing towards the surface rupture rather than epicentral region. The direction and distances of 1,437 co-seismically displaced rock fragments (chips) in the near-field of the Petermann earthquake provide a dense proxy-record of strong ground motions, both along- and across-rupture. Chips record preferred azimuths of displacement that are attributed to rupture fling effects. This unprecedented geological proxy-record of the distribution, directivity and intensity of strong ground motions has important implications for hazard analysis in the near-field of reverse earthquakes. Fine-scale mapping of the 2016 Peterman surface rupture and secondary fractures using field, drone-derived and remote-sensing datasets indicates surface rupture characteristics vary with changes in surface geology. Deformation zones are wider and less recognizable in granular materials (e.g. dunes, alluvium) compared with those in proximal bedrock. Kinematic analysis of bedrock fractures indicates sinistral-reverse faulting, consistent with published focal mechanisms, and a maximum compressive stress orientation generally consistent with the inferred regional SHMax orientation. Trenching and 10Be cosmogenic nuclide erosion rates provide preliminary evidence of absence for prior rupture on the Petermann faults within the last 200 to 400 kyrs. The 2016 earthquake is therefore hypothesized to be the first to rupture this fault in the near surface. Analyses of geological and geophysical data from ten moderate magnitude (MW 4.7 – 6.6) historical surface-rupturing earthquakes in cratonic Australia indicate that rupture likely propagated along pre-existing Precambrian bedrock structures. Six of seven events show evidence of multi-fault rupture across 2 to 6 discrete faults of greater than 1 km length, placing these events as some of the most structurally complex earthquake ruptures identified globally for this magnitude. No unambiguous geological evidence for preceding surface-rupturing earthquakes is present. This raises important questions regarding the recurrence behaviour of intraplate stable continental region faults, with implications for seismic hazard analysis. In summary, this thesis explores observational, seismic, and remote-sensing data of surface rupturing earthquakes in Australia to provide new (i) data regarding the recurrence patterns of Australian earthquakes (ii) insights into basement controls on these earthquakes (iii) and methods to quantify seismic directionality behaviour common to reverse earthquakes globally. These contribute to better understanding the why, what, when, where of intraplate earthquakes, and how seismic hazard varies across diverse tectonic and crustal environments.