Abstract for the Asia Oceania Geosciences Society (AOGS) conference on 24-28 June 2013.

The first edition ACE - Australian Continental Elements dataset is a GIS representation of the lithosphere fabrics of the Australian plate, interpreted from linear features and associated discontinuities in the gravity anomaly map of continental Australia (Bacchin et al., 2008; Nakamura et al., 2011) and the global marine gravity dataset compiled from satellite altimetry (Sandwell & Smith, 2009). It should be used in context with these input data sources, at scales no more detailed than the nominal scale of 1:5 000 000.

The Cadell Fault, found in stable continental region (SCR) crust in southeastern Australia, provides a record of temporally clustered morphogenic earthquakes spanning much of the Cenozoic. The slip rate, averaged over perhaps as many as five complete seismic cycles in the period 70–20 ka, is c. 0.4–0.5 mm/a, compared with an average rate of c. 0.005–0.01 mm/a over the period spanning the late Miocene to Recent. If full length rupture of the 80 km long feature is assumed, the average recurrence for Mw 7.3–7.5 earthquake events on the Cadell Fault in the period 70–20 ka is c. 8 kyr. About 20 kyr, representing more than two average seismic cycles, have lapsed since the most recent morphogenic seismic event on the fault. It might therefore be speculated that this fault has relapsed into a quiescent period. Episodic rupture behaviour on the Cadell Fault, and nearby faults in Phanerozoic SCR crust in eastern Australia, might be controlled by their linkage into major crustal fault systems at depth, in apparent contrast with the style of deformation in non-extended Precambrian SCR crust. Periods of strain localization on these major crustal fault systems, effectively turning deforming regions ‘on’ and ‘off’, might be influenced by changes in distant plate boundary forces. If proved, this would have profound consequences for how the occurrence of large earthquakes is assessed in Australia, as the fundamental assumption of morphogenic earthquakes occurring as a result of the progressive build-up of strain, and thus being in some way predictable in their periodicity, is not satisfied. Documenting such fault behaviour in SCR crust assists in conceptualizing the points critical to understanding the hazards posed by SCR faults worldwide.

We present a seismic reflection section acquired across the western margin of the Lake George Basin near Geary's Gap which images the stratigraphy of the basin sediments and the interaction between faults and these sediments. When coupled with high resolution topographic data, key aspects of the evolution of the Lake George Basin may be deduced. The Lake George Basin formed as the result of west-dipping reverse faulting and associated fault propagation folding at the eastern margin of the Lake George Range in the interval between ca. 3.93 Ma and the present. Assuming that elevated gravels in Geary's Gap and to the west along Brooks Creek are correlative with similar lithology at the base of the basin (as suggested by previous workers), vertical displacement in the order of 250 m has occurred in this time interval. This is one of the larger rates of displacement recorded for an Australian intraplate fault, averaged over a timescale of several million years. Three prominent angular unconformities, separating packages of approximately parallel strata, indicate that deformation was episodic, with up to 1 million years separating active periods on the fault. The ~75 km active length of the Lake George Fault is consistent with a MW7.4 characteristic earthquake. An event of this magnitude has the potential to cause significant damage to the Australian Capital Territory, given that the surface trace of the fault approaches to within 25 km of Parliament House. Assuming periodic recurrence, a characteristic event might be expected every ~3040 kyr. However, the evidence for temporal clustering suggests that such events might be much more tightly spaced in time (perhaps by an order of magnitude) in an active period on the fault. This neotectonic activity is allied to the Late Pliocene to Pleistocene `Kosciuszko Uplift, which may be responsible for adding several hundred metres of relief to the Eastern Highlands of Australia. Few crustal fault systems which might have accommodated such large-scale uplift have yet been characterised. Consequently, the seismic hazard of the Eastern Highlands, which is based largely upon the short historic record of seismicity, is likely to be underestimated. Nearby candidate faults for similar activity include the Queanbeyan, Murrumbidgee, Shoalhaven, Crookwell, Mulwaree, Binda, Tawonga, Khancoban-Yellow Bog and Jindabyne faults.

On 23 March 2012 a MW 5.4 intraplate earthquake occurred in the eastern Musgrave Ranges of north-central South Australia, near the community of Ernabella (Pukatja). This was the largest earthquake recorded on mainland Australia in the past 15 years and resulted in the formation of a 1.6 km-long surface deformation zone that included reverse fault scarps with a maximum vertical displacement of ~0.5 m (average ~0.1 m), extensive ground cracking, and numerous rock falls. Fifteen months later, on 09 June 2013 a MW 5.6 earthquake (the Mulga Park earthquake) occurred ~15-20 km northwest of the 2012 rupture. The P-axes of the focal mechanisms constructed for both events indicate northeast-oriented horizontal compressive stress. However, the focal mechanism for the Mulga Park earthquake suggests strike-slip failure, with a sub-vertical northerly-trending nodal plane favoured as the failure plane, in contrast to the thrust mechanism for the 2012 event. Despite being felt more widely than the 2012 event, ground cracking and minor dune settlement were the only surface expressions relating to the Mulga Park earthquake. No vertical displacements were evident, nor were patterns indicative of a significant lateral displacement. An 18 km long north to north east trending arcuate band of moderate to high cracking density was mapped parallel to the surface trace of the Woodroffe Thrust, a major crust-penetrating fault system. A lobe of high-density cracking ~5km long, coincident with the calculated epicentral location, extended to the north from the centre of the main arc. We speculate that the rupture progressed to the south beneath the northern high-density lobe (consistent with the dimensions expected from new scaling relations), and that the larger arcuate band of cracking might relate to positive interference resulting from reflection of energy from the Woodroffe Thrust interface. Both events provide new insight into the rupture behaviour of faults in non-extended cratonic crust.

Long-term temporal and spatial patterns in large earthquake occurrence can be deduced from the Australian landscape record and used to inform contemporary earthquake hazard science. Seismicity source parameters such as fault slip-rates, large earthquake recurrence times and maximum magnitude vary across the continent, and can be interpreted within a framework of large-scale neotectonic domains defined on the basis of geology and crustal setting. While the suite of neotectonic fault behaviours may vary across Australia, as implied by the neotectonic domains model, one individual fault characteristic appears to be common to most Australian intraplate faults studied active periods comprising a finite number of events are separated by much longer periods of quiescence. Studies elsewhere in the world identify similar episodic behaviour on faults with low slip rates and suggest that the time between successive clusters of events (deformation phases) is highly variable but significantly longer than the times between successive earthquakes within an active phase. Furthermore, there is some indication that the temporal clustering behaviour emerging from single fault studies may be symptomatic of a larger picture of the more or less continuous tectonic activity from the late Miocene to Recent being punctuated by pulses of activity in specific, actively deforming regions. At present the underlying tectonic processes driving the observed variability in Australian seismicity are poorly understood. Questions remain as to whether stress accumulation and/or strain release is predictable, and at what scale. In this talk, we will outline some of the key challenges facing earthquake hazard scientists in Australia, and how these are being addressed.

In probabilistic seismic hazard modelling the choice of whether faults behave with Characteristic or Gutenberg-Richter recurrence statistics has a high impact on the hazard level. Compared to a model that does not include fault sources, the addition of a high slip rate (by intraplate standards) Characteristic fault results in a modest increase in hazard for a 500 years return period event, and a modest increase at longer return periods (i.e. ~2500 years). A Gutenberg-Richter fault with the same slip rate will result in a comparatively higher hazard at 500 years, similar hazard at 2500 years and a lower hazard a very long return periods (i.e. ~5000 years). Results from interplate and active intraplate paleoseismological investigations since the mid-1980s have been used to suggest that earthquakes recurrent on a given fault often have the same characteristic rupture length and amount of slip (i.e. a Characteristic Rupture Model). Stable asperities and barriers, which survive many earthquakes, are proposed to explain these results. The scarcity of data precludes definitive validation of the model in Australian Stable Continental Region crust. However, preliminary indications are that the Characteristic Rupture Model has some merit in cratonic regions of the country while faults in non-cratonic regions may behave in a more complex fashion.

The Geoscience Australia Structural Measurements Database contains field measurements of geological structure features such as bedding, foliation, lineation, faults and folds from field sites, measured sections, and boreholes. The database is delivered as a layer in Geoscience Australia's "Geological Field Sites, Samples and Observations" web service.

Abstract: Compressional deformation is a common phase in the post-rift evolution of passive margins and rift systems. The central-west Western Australian margin, between Geraldton and Karratha, provides an excellent example of a strain gradient between inverting passive margin crust and adjacent continental crust. The distribution of contemporary seismicity in the region indicates a concentration of strain release within the Phanerozoic basins which diminishes eastward into the cratons. While few data exist to quantify uplift or slip rates, this gradient can be qualitatively demonstrated by tectonic landforms which indicate that the last century or so of seismicity is representative of patterns of Neogene and younger deformation. Pleistocene marine terraces on the western side of Cape Range indicate uplift rates of several tens of metres per million years, with similar deformation resulting in sub-aerial emergence of Miocene strata on Barrow Island and elsewhere. Northeast of Kalbarri near the eastern margin of the southern Carnarvon Basin, marine strandlines are displaced by a few tens of metres. A possible Pliocene age would indicate that uplift rates are an order of magnitude lower than further west. Relief production rates in the western Yilgarn Craton are lower still - numerous scarps (e.g. Mount Narryer) appear to relate individually to <10 m of displacement across Neogene strata. Quantitative analysis of time-averaged deformation preserved in the aforementioned landforms, including study of scarp length as a proxy for earthquake magnitude, has the potential to provide useful constraints on seismic hazard assessments in a region containing major population centres and nationally significant infrastructure.

Australia is one of the lowest, flattest, most arid, and most slowly eroding continents on Earth (Quigley et al. 2010). The average elevation of the continent is only c. 330 m above sea level (asl), maximum local topographic relief is everywhere <1500 m (defined by elevation ranges with 100 km radii) and two-thirds of the continent is semi-arid to arid. With the exception of localized upland areas in the Flinders and Mt Lofty Ranges (Quigley et al. 2007a, Quigley et al. 2007b) and the Eastern Highlands (Chappell 2006, Tomkins et al. 2007), bedrock erosion rates are typically 1-10 m/Ma (Wellman & McDougall 1974, Bishop 1985, Young & MacDougall 1993, Bierman & Caffee 2002, Belton et al. 2004, Chappell 2006, Heimsath et al. 2010) (Fig. 1A). Despite this apparent geomorphological longevity (e.g. Fig. 1B), Australia has had a dynamic Neogene to Recent tectonic history. In the last five decades seven locations in intraplate Australia are documented as having experienced earthquakes large enough to rupture the ground surface (Clark et al. 2013). These earthquakes produced scarps up to 2 m high and 37 km long. Several hundred features consistent in form to the historic ruptures have since been identified Australia-wide (Fig. 2), mainly through interrogation of digital elevation data (Clark et al. 2011, Clark et al. 2012). Palaeoseismic analysis of these features indicates that periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence. While morphogenic earthquake events in an active period on a given fault may be separated by a few thousand years (-0.4 mm/a uplift rates in an active period), active periods might be separated by a million years or more (long term uplift rates -0.001mm/a). A rupture sequence of this kind has the potential to have a dramatic effect on the landscape, especially in regions of low local topographic relief, such as the Murray Basin. For example, uplift across the Cadell Fault (see Fig. 2 for location) in the interval 70 - 20 ka resulted in the formation of a 15 m high and 80 km long scarp which temporarily dammed, and ultimately diverted the Murray and Goulburn Rivers (McPherson et al. 2012). Even in upland regions, the effects can be marked, as demonstrated by the formation of Lake George over the last ca. 4 Ma as the result of uplift on the Lake George Fault (Pillans 2012). Over timescales of millions of years, such activity, in combination with mantle-related dynamic topographic effects (Sandiford 2007, Sandiford et al. 2009, Quigley et al. 2010), might be expected to have a significant influence on the distribution and thickness of regolith over large areas.