Intraplate earthquakes in Australia - Intraplate Earthquakes - page 17

of hypocentres in the 10-20 km depth range in the central Flinders Ranges, and less than
10 km depth in the southern Flinders Ranges (Cummins et al ., 2009 ) , consistent with
previous work (Greenhalgh et al ., 1994 ) . The high proportion of hypocentres with depths
greater than 10 km suggests that the depth to brittle-ductile transition in the Flinders Ranges
is deeper than might be inferred from heat flow data (cf. Celerier et al ., 2005 ; Holford et al .,
2011 ) .
Within the Southeast Seismic Zone (SESZ - Figure 2.3 ) , and non-cratonic eastern
Australia in general, hypocentral depth estimates are bimodal (Close and Seeber, 2007 ) ,
ranging between very shallow ( < 5 km; 30% of data) and mid-crustal depths (10-20 km;
40% of data) (Allen et al ., 2012a ) . Aftershocks tend to be very shallow and numerous,
perhaps accounting for the shallower mode (Gibson et al ., 1981 ; Leonard, 2008 ) . Within
a sub-zone that encompasses the Pliocene to Holocene Newer Volcanic Province (e.g.,
Sutton et al ., 1977 ; Sheard, 1995 ) , mid-crustal earthquakes are generally deeper, with 95%
of recorded events occurring in the range 9-17 km (Leonard, 2008 ) .
The Otway and Gippsland basins of southeast Australia ( Figure 2.3 ) form part of an
aulacogen developed in non-cratonic Paleozoic crust that was extended by rifting in the
late Mesozoic-early Cenozoic. Earthquake hypocentres are deep in this region compared
to those in the non-extended parts of the same Paleozoic basement province that is more
typical of the SESZ. More than 70% of hypocentres in the extended basins are in the
10-25 km depth range (Allen et al ., 2012a ) , exemplified by the well-located 19 June 2012
M L 5.4 Moe earthquake at a depth of 17 km (Sandiford and Gibson, 2012 ) . It is probable
that the contribution from these basins accounts for the greater depth element present in the
southern southeast Australia (SEA-S) zone of Leonard ( 2008 ) , as he does not distinguish
between these two geological settings.

2.2.3 Attenuation and scaling relations

Several studies have shown that earthquakes in SCRs, such as Australia, are generally felt at
larger distances than earthquakes in active tectonic regions (McCue, 1990 ; Frankel, 1994 ;
Bakun and McGarr, 2002 ; Atkinson and Wald, 2007 ; Wald et al ., 2011 ) . This is because the
seismic energy propagates more efficiently through cold, relatively homogeneous continen-
tal crust, which is less susceptible to anelastic and scattering effects. It is recognised from
analysis of isoseismal radii (Gaull et al ., 1990 ) that attenuation of seismic wave energy
varies transversely across the Australian continent, with relatively low attenuation in the
Archaean and Proterozoic terranes of western and central Australia and higher attenuation
in the younger Phanerozoic terranes of eastern Australia (cf. Figure 2.1 ) . Moreover, it is also
observed that ground-motion amplitudes at large distances ( > 100 km) for an earthquake
of given magnitude are lower in southeast Australia (SEA) than in eastern North America
(ENA) based on macroseismic intensities (Bakun and McGarr, 2002 ) and instrumental data
(Allen and Atkinson, 2007 ) . The higher attenuation observed in SEA at distances greater
than

100 km is likely to be due to the broad crustal velocity gradient (Collins et al .,