Intraplate earthquakes in Australia - Intraplate Earthquakes - page 14

In addition to the few documented surface-rupturing events, isoseismal maps have been
compiled for almost 400 historic Australian earthquakes (Everingham, 1982 ; Rynn et al .,
1987 ; McCue, 1996 ) . McCue ( 1980 ) derived a relationship between Richter magnitude ( M )
and the radius of a circle of equivalent area to the Modified Mercalli Intensity III isoseismal
for Australian earthquakes with recorded magnitudes ranging from 3.6 to 7.0 (reproduced in
McCue, 2004 ) . This permitted magnitudes to be assigned for pre-instrumental earthquakes
or recent earthquakes that were widely felt but either too close to or too far from the nearest
seismograph to measure the magnitude (McCue, 2004 ) .

2.2.1 Mechanism, geographic distribution, and strain rate

The Australian crustal stress regime is generally considered to be compressive (Denham
et al ., 1981 ; Hillis and Reynolds, 2000 , 2003). This assessment is supported by the majority
of earthquake focal mechanisms, which range from thrust to oblique strike-slip (e.g.,
Leonard et al ., 2002 ; Keep et al ., 2012 ) . Notable exceptions, with a dominant normal
component, are the 1985 Norseman and 2001 Ravensthorpe earthquakes ( Figure 2.1 - 16,
17), both located along the Albany-Fraser Orogen margin of the Yilgarn Craton, with the
latter being anomalously deep at 18 km (McCue, 1989 ; Clark, 2004 ) . The 1966 Mount
Hotham earthquake in the SESZ (Denham et al ., 1982 ) ( Figure 2.1 - 18), several of
the Burakin Swarm events in the SWSZ (Leonard et al ., 2002 ) , and two Tennant Creek
aftershocks (Clark and Leonard, 2003 ) also have a dominant normal component to their
focal mechanisms. Note that Leonard et al .( 2002 ) incorrectly transposes the P and T axes,
and dilatant and compressional quadrants, for the Norseman 1985 event (cf. McCue, 1989 ) .
Assuming a maximum possible magnitude ( M max )of 7.0 (refer also to Section 2.5),

the maximum seismogenic strain rate estimates for the continent (averaged over the last

10 − 17 to 10 − 16 s − 1 (Leonard, 2008 ; Braun et al ., 2009 ;
Sandiford and Quigley, 2009 ) . Comparable results (i.e., 10 − 17 s − 1 ) are obtained from
thin-plate finite element modeling using plate boundary conditions, heat flow, stress, and
geodetic data as inputs (Burbidge, 2004 ) . This has been equated to an east-west shortening
rate across southern Australia of approximately 0.3-0.4 mm/yr (Leonard, 2008 ) , which
compares to estimates based upon laser ranging and geodetic GPS of 0.65-3.0

50 years of complete data) are

± 2.0 mm/yr

(Smith and Kolenkiewicz, 1990 , ; Tregonning, 2003 ; Leonard, 2008 ) .
However, earthquake epicentres are not randomly distributed across the Australian con-
tinent in time or space (Denham, 1988 ; Leonard, 2008 ; Sinadinovski and McCue, 2010 ) .
At the continental scale, concentrations of epicenters occur in four major “seismogenic
zones” ( Figure 2.1 ) (Hillis et al ., 2008 ; Leonard, 2008 ; Sandiford and Egholm, 2008 ) , with
the continental margins also demonstrating a comparatively high number of epicentres
relative to the interior (Sandiford and Egholm, 2008 ) . Strain rates calculated for the higher
seismicity zones are no more than 10 − 16 to 10 − 15 s − 1 (Leonard, 2008 ; Braun et al ., 2009 ) .
Sandiford and Quigley ( 2009 ) provide an example from the Flinders Ranges Seismic Zone
(FRSZ, Figure 2.1 ) , where the bulk strain rate of 10 − 16 s − 1

implies a total shortening of