Intraplate earthquakes in Australia - Intraplate Earthquakes - page 9

Reinecker et al ., 2003 ; Schulte andMooney, 2005 ) indicates that compressive stress regimes
dominate within continental interiors, with maximum compressive stresses oriented pre-
dominantly in accordance with absolute plate motion (see also Richardson, 1992 ) . Australia
is anomalous in this respect. While earthquake focal mechanisms suggest that the crustal
stress field at seismogenic depths in Australia is everywhere compressive, with signifi-
cant strike-slip components along the northwest margin (NWSSZ, Figure 2.1 ) and in the
Flinders/Mount Lofty Ranges (FRSZ, Figure 2.1 ) (Leonard et al ., 2002 ; Clark et al ., 2003 ) ,
stress orientations in southern Australia are typically not parallel to the north-northeast-
directed plate motion vector (Coblentz et al ., 1995 ; Hillis and Reynolds, 2000 , 2003;
Sandiford et al ., 2004 , 2005; Hillis et al ., 2008 ) .
In the southern half of the continent, the maximum horizontal stress orientation ( S Hmax )is
essentially east-west in western and central Australia and rotates to northwest-southeast in
eastern Australia ( Figure 2.1 ) . In the northern half of the continent, the stress field transitions
from the generally east-west trend in the south, to a broadly northeast-southwest trend. To
a first order these regional stress orientations are not influenced by tectonic terrane, crustal
thickness, heat flow, regional structural trends, geological age, or by the depth at which
orientations are sampled (e.g., Hillis et al ., 2008 ; Sandiford and Quigley, 2009 ; Holford
et al ., 2011 ) . The trends have been satisfactorily modelled in terms of a balance between
plate driving and resisting torques generated at the margins of the Indo-Australian Plate
(Cloetingh and Wortel, 1986 ; Coblentz et al ., 1995 , 1998; Reynolds et al ., 2002 ; Burbidge,
2004 ; Sandiford et al ., 2004 ; Dyksterhuis and Muller, 2008 ) ( Figure 2.2 ) .
Stratigraphic relationships establish that fault-related and presumably seismogenic
deformation consistent with the present stress field commenced in the late Miocene, in the
interval 10-6 Ma (Dickinson et al ., 2002 ; Sandiford et al ., 2004 ; Keep and Haig, 2010 ) , as
the result of complex evolving plate boundary conditions (Sandiford and Quigley, 2009 ) . It
has been proposed that the onset of deformation at specific locations may reflect rising stress
levels related to the combination of all plate boundary forcings (Sandiford and Quigley,
2009 ) , with variations in the thermal structure of the Australian lithosphere influencing the
localisation of deformation at the regional or terrane scale (e.g., Celerier et al ., 2005 ; Sandi-
ford and Egholm, 2008 ; Holford et al ., 2011 ) . Analysis of the spatial and temporal patterns
of contemporary seismicity might therefore be improved by studying the extraordinarily
rich Neogene to Quaternary record of seismicity in the Australian landscape (e.g., Clark
et al ., 2012 ) . In the sections that follow, earthquake occurrence in Australia is therefore
examined from both the historic and prehistoric (Neogene and Quaternary) records.
2.2 Two centuries of earthquake observations in Australia
Between 1788, when European colonists reported the first earthquake felt in Australia
(Historical Records of Australia, 1914), and the early 1900s, newspaper articles were
the main source of information about earthquakes (e.g., Malpas, 1991 ) , and continued
to be important until the 1960s (McCue, 2004 ) . The first seismographs in Australia for