Intraplate earthquakes in Australia - Intraplate Earthquakes - page 12

M 3.5 by the 1980s, the Australian National Seismic Network (ANSN - http://www.

ga.gov.au/earthquakes/seismicSearch.do ) remains relatively sparse, with no more than 70
seismic stations suitable for general earthquake monitoring in a continent of a similar size
to the conterminous United States or Western Europe (Leonard, 2008 ) . This results in
generally poor constraints being placed on the location of the more than 36,000 onshore
earthquakes recorded in the current Australian earthquake catalogue (Allen et al ., 2012a ) .
Approximately 20 earthquakes of M > 6.0 have been recorded in Australian continental
crust, at the rate of one every

+

6-8 years (McCue, 1990 ; Leonard, 2008 ) . On average, less
than one M > 5.0 event has occurred per year (Leonard, 2008 ) . The largest instrumentally
recorded earthquake occurred in 1941 near Meeberrie, Western Australia (Everingham,
1982 )( Figure 2.1 - 1). The event is associated with a surface wave magnitude of M S 6.8
(Allen et al ., 2012a ) . In January 1988 three large “mainshock” earthquakes of magnitude
M S 6.3-6.7 (Choy and Bowman, 1990 ) occurred in a 12-hour period near Tennant Creek
in the Northern Territory (Jones et al ., 1991 ) ( Figure 2.1 - 2). The events were preceded by
a year-long foreshock sequence, and followed by a continuing aftershock sequence (e.g.,
Bowman et al ., 1990 ) . The most notable pre-instrumental earthquake sequence occurred in
Bass Strait, off the northeast coast of Tasmania ( Figure 2.1 - 3), in the 1880s-1890s. Over
2,400 events were felt during that period (Ripper, 1963 ) , four of which caused damage in
Tasmania, and two of which are estimated from felt effects to rival the Meeberrie earthquake
in magnitude (Michael-Leiba, 1989 ) . The Dalton-Gunning region of southeast Australia
( Figure 2.1 - 4) continues to experience a less intense earthquake sequence (comprising
more than 600 recorded events) that began with an M L 5.6 event in the 1930s (Cleary, 1967 ;
Denham et al ., 1981 ; Michael-Leiba et al ., 1994 ) .
Earthquake clusters (e.g., Dalton-Gunning) are considered to be distinct from swarms,
which are defined as occurring within a limited volume, lasting over a period from hours to
months, with the largest event occurring well after the swarm commences, and not having a
magnitude significantly greater than the second largest event (

0.5 magnitude unit; Gibson
et al ., 1994 ; Dent, 2008 ) . Geomechanical testing suggests that earthquake swarms occur
preferentially in regions of extremely heterogeneous geological structure (Mogi, 1963 ) .
Increasing regional stress manifests as high stress concentration around numerous cracks
and faults within the structured volume, resulting in failure on many local fractures at low
stress. This has the effect of reducing the probability of failure on a single large fracture.
Earthquake swarms are an important component of Australian seismicity, and can represent
a large percentage of events in earthquake catalogues, particularly in regions of granitic
geology (Dent, 2008 ) . A non-exhaustive list of 42 earthquake swarms was compiled by
Dent ( 2008 , 2009), which includes perhaps the most significant of recent swarms, the
2001-2 Burakin Swarm (Leonard, 2002 , 2003) in the Southwest Seismic Zone (SWSZ;
Doyle, 1971 ) of Western Australia ( Figure 2.1 ) . This swarm involved six events in the
magnitude range M L 4.5-5.2, with over 18,000 smaller events, all purportedly occurring
within a volume of 5 km diameter (Leonard, 2003 ) . The centre of activity for the swarm

occurs approximately 20 km north of the surface rupture relating to the 1979 Cadoux M S 6.0
earthquake ( Figure 2.1 - 11). Several other swarms in the area (Kalannie, Beacon) suggest