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100 km wide zone, which could be accommodated by 10 faults
with slip rates consistent with the current topography and neotectonic faulting record (e.g.,
Sandiford, 2003b ; Quigley et al ., 2006 ; Clark et al ., 2011a ) . Braun et al .( 2009 ) estimate
that half of the current relief of the Flinders Ranges, and a non-negligible proportion of
the relief in the Eastern Highlands (SESZ, Figure 2.1 ) , might plausibly have been built
since the inception of the current stress regime. In contrast, similar seismogenic strain rates
calculated for the SWSZ (Leonard, 2008 ; Braun et al ., 2009 ) ( Figure 2.1 ) are not consistent
with the present low-relief landscape (Clark, 2010 ) , the distribution of paleo-earthquake
fault scarps (Leonard, 2008 ; Leonard and Clark, 2011 ) , or GPS strain rates (Leonard et al .,
2007 ) . This implies migration of the locus of seismicity with time (Leonard, 2008 ; Leonard
and Clark, 2011 ) , an assertion supported by evidence suggesting that the rate of seismic
activity in the SWSZ has increased dramatically in only the last 50 years (Michael-Leiba,
1987 ; Leonard and Clark, 2011 ) .
250 m/Ma across the
2.2.2 Seismogenic depth
Depth constraint for Australian earthquakes is generally poor. Leonard ( 2008 ) selected a
subset of the Australian earthquake catalogue for detailed hypocentral depth analysis using
only depth values whose uncertainty was either less than the depth itself, or less than 5 km.
More than 75% of epicentres from the current earthquake catalogue are rejected using these
criteria. The selected data correlate with the regions of highest station density in the ANSN
(i.e., southern Australia).
The concentration of epicentres in the SWSZ ( Figure 2.1 ) occurs in non-extended
cratonic crust. The Leonard ( 2008 ) dataset shows that earthquakes in this region are typically
very shallow (see Figure 2.3 ) , with 95% of events found to have occurred within the upper
5 km of crust, consistent with previous analyses (Doyle, 1971 ; Everingham and Smith,
1979 ; Denham, 1988 ; McCue, 1990 ) . This region has produced four surface-rupturing
earthquakes in the last five decades ( Figure 2.1 ; Table 2.1 ) . The largest of these earthquakes
(1968 Meckering; Figure 2.1 - 9) is calculated to have initiated at 1.5 km depth and to
have ruptured both upwards to the surface and down to
6 km depth (Langston, 1987 ;
Fredrich et al ., 1988 ) , while the smallest (2008 Katanning; Figure 2.1 - 15) ruptured from
0.6 kmdepth to the surface (Dawson et al ., 2008 ) . Earthquake swarms are also particularly
prevalent in the SWSZ, with well-located depths being in the upper 2-3 km (Leonard, 2002 ,
2003). Analysis of the characteristics of modern and prehistoric fault scarps (Clark, 2010 ;
Leonard and Clark, 2011 ; Clark et al ., 2012 ) suggests that large earthquake characteristics
in the SWSZ (e.g., shallow depth) are typical of what might be expected throughout non-
extended cratonic crust in Australia. However, Australia's deepest known earthquakes are
also from this crustal setting: the 1989 Uluru earthquake ( Figure 2.3 - 1), with a calculated
depth of 31 km (Michael-Leiba et al ., 1994 ) , and a 1992 earthquake beneath the Arafura
Sea, north of the Northern Territory coastline ( Figure 2.3 - 2), which had a depth of 39 km
(McCue and Michael-Leiba, 1993 ) .
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