Intraplate earthquakes in Australia - Intraplate Earthquakes - page 18

2003 ) , which allows dispersion of Lg-wave energy into the upper mantle (e.g., Bowman
and Kennett, 1991 ; Atkinson andMereu, 1992 ) . However, there appears to be no discernible
difference between the attenuation of Australian and ENA ground motions in the distance
range of engineering significance (i.e., < 100 km; Allen and Atkinson, 2007 ; Allen et al .,
2012b ) . Furthermore, attenuation rates at these near-source distances are also comparable
to those in active tectonic regions (e.g., Hanks and Johnston, 1992 ; Atkinson and Morrison,
2009 ; Campbell, 2011 ) .
Rupture dimensions have been modelled for only two of Australia's large earthquake
sequences - the 1968 Meckering earthquake and the 1988 Tennant Creek sequence
(Somerville et al ., 2010 ) - both of which occurred in the cratonic western and central
regions of Australia respectively (cf. Figure 2.1 ; Table 2.1 ) . Significant simplifications
were required to model the subsurface rupture geometries of both events, which involved
complex surface rupture on intersecting structural elements of varying orientation (e.g.,
Crone et al ., 1992 ; Dentith et al ., 2009 ) . While this might limit the confidence that can
be placed in the rupture models, earthquake scaling relations developed from the models
compare favourably with empirically derived relations for intraplate dip-slip earthquakes
(Johnston, 1994 ) , and self-similar scaling relations (Leonard, 2010 ) . At present there are
no estimates of the rupture dimensions of any earthquakes in the non-cratonic or extended
crust regions of eastern Australia.
2.3 A long-term landscape record of large (morphogenic) earthquakes
Australia is one of the lowest, flattest, most arid and slowly eroding continents on Earth.
Accordingly, large parts of Australia are favourable for the preservation of tectono-
geomorphic features, such as fault scarps, for tens of thousands to millions of years (e.g.,
Quigley et al ., 2010 ) . In regions with extremely low erosion rates, such as the Nullarbor
Plain ( Figure 2.4 ) , it has been claimed that a morphogenic earthquake record spanning the
last 15 Ma has been preserved essentially intact (Hillis et al ., 2008 ) . On this basis, Australia
boasts arguably the richest Late Neogene to Quaternary faulting record in all of the world's
SCR crust (Sandiford, 2003b ; Quigley et al ., 2010 ; Clark et al ., 2011a , 2012). Over 300
features (mainly fault scarps and folds) suspected or known to have been displaced under
the current crustal stress regime have been identified and recorded (Clark et al ., 2011a ,
2012) ( Figure 2.4 ) . The majority of these features, by virtue of their length and/or verti-
cal displacement (see Section 3.1), are likely to reflect multiple surface-rupturing events
(cf. Leonard, 2010 ) . This remarkable archive, while undoubtedly incomplete, has the poten-
tial to extend the historic record of seismicity to a timescale commensurate with the recur-
rence time of large earthquakes in this intraplate setting.
Variation in fault scarp length, vertical displacement, proximity to other faults, and
relationship to topography permits further division of this neotectonic record according
to fault character. Six onshore “neotectonic domains” are recognised, with an additional
offshore domain proposed by analogy with the Eastern United States (Clark et al ., 2011a ,