Earthquakes in Australia

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Earthquakes in Australia

• Earthquake of the week
  2009 Fall

http//www.seismicity.see.uwa.edu.au/welcome/seism
icity_in_australia

• Kate Huihsuan Chen 2009/11/23

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Main precambrian tectonic units

• Western Craton
• Pilbara Yilgarn cratons
• Northern Craton
• Small fragments
• Southern Craton
• Proto-Gawler craton

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Surface Geology and crustal units
T line the eastern limit of surface exposure of
Precambrian material
Indo-Australia plate
Pacific Ocean plate
Older
Younger
Antarctic plate
Kennett, 2003
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Two earthquakes Mgt5 per year

• 75 earthquakes of Mgt5.5
• NWA Northwest Australia
• SWA Southwest Australia
• SA South Australia
• SEA Southwest Australia
• NA North Australia
• All Australia excluding NWASWASASEA

NWA
SA
SWA
SEA
Continental crust
Leonard, 2008
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Development of seismic stations in Australia and
Mgt3.5 earthquakes
1880-1955
1956-1970
station
earthquake
Distribution of seismic stations is sparse, the
location accuracy is generally poor 0.4 lt 1 km
4 lt 2 km 14 lt 3 km 32 lt 5 km 62 lt10 km 85
lt 20 km
1971-1985
1986-2000
Leonard, 2008
7
Regions of smaller earthquake location uncertainty

• All events that are located to lt 3 km are in the
  SWA, SA, and SEA regions.
• Outside these regions, most events have location
  uncertainties of
• gt 5 km and typically gt 10 km.

NWA
SA
SWA
SEA
Continental crust
Leonard, 2008
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Historical earthquakes (very shallow, therefore,
more damage)
Mgt6 aftershock in 1 day
occurred in a seismic gap region
Leonard, 2008
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Depth distribution of seismicity

• Volc area isolated seismicity and includes the
  M6.5 event in 1987
• This region is of interest, since as recently as
  4500 yr B.P. there was volcanic activity
  immediately to the northeast of this area and the
  age of the volcanic activity increases with
  distance to the northeast (Johnson, 1989).
• 95 of earthquakes are relatively deep at 917 km.

Leonard, 2008
10
Depth distribution of seismicity

• Earthquakes in SWA are typically very shallow.
  95 of earthquakes are shallower than 5 km.
• This region has produced three surface faulting
  earthquakes in the last four decades (Gordon and
  Lewis, 1980).

Leonard, 2008
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Depth distribution of seismicity
Without aftershock removed

• In SEAS and SEAN earthquakes range between very
  shallow (lt4 km) and 17-km deep.
• Aftershocks in SEAN tend to be very shallow and
  numerous. Using a catalog containing aftershocks,
  Gibson et al. (1981) obtained a similar
  distribution for Victoria, except they had a
  greater proportion of very shallow earthquakes,
  reflecting the shallower depth of aftershocks.
• Earthquakes in SA tend to be relatively deep with
  75 of earthquakes between 8 and 14 km deep.

Leonard, 2008
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Depth distribution of seismicity

• Earthquakes in SA tend to be relatively deep with
  75 of earthquakes between 8 and 14 km deep.

Leonard, 2008
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Deployments of portable seismic recorders in
Australia
Kennett, 2003
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Coverage from Rayleigh waves for surface wave
tomography
The large-amplitude surface waves in the later
part of the seismogram travel nearly horizontally
and can be used in a tomographic inversion to
determine the 3-D variations in shear-wave speed.
Kennett, 2003
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3-D Vs model
80 km
200 km
Kennett, 2003
The areas of exposed Precambrian rocks largely
correspond to high seismic-wave speeds but there
are zones of enhanced wave speed extending to 150
km or more lying to the east of the conventional
Tasman line
Precambrian regions show the presence of
significant internal structure with an indication
of the separation of the major cratonic blocks,
especially at shallower depths.
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• The major change in mantle structure occurs on an
  approximately northsouth trend close to 140E.

The Phanerozoic belt in eastern Australia has a
thin zone of high wave speeds in the lithosphere
extending to 100 km, and beneath this there is a
zone of lowered wave speed that extends along
most of the east coast of Australia.
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3-D velocity structure for P and S waves
Kaiho and Kennett, 2000

• From body wave analysis

The short period results require a P velocity
contrast near 210 km depth to explain multiple
arrivals with a few seconds separation around
1400 km from the source. However, for S waves
the corresponding record sections only show a
clear arrival associated with the lithosphere
which cannot easily be traced beyond 2000 km, and
no branches associated with greater depth.
Considerable variation in seismic-wave speeds are
found through the asthenosphere and mantle
transition zone
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Cratonic lithosphere

• fast wavespeeds
• Very low differential attenuation (low t, high
  Q)
• No frequency dependence

Kennett, 2003
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3-D velocity structure for P and S waves
Reduced amplitude heterogeneity At 245 km the
amplitude of heterogeneity is much reduced
beneath the continent indicating that the base of
the lithosphere has been reached.
Oceanic Continental dichotomy there is a fast S
wave-speed structure in the oceanic region in
front of the Flores arc in Indonesia which does
not have an equivalent in the P wave-speed map at
245 km. This suggest some oceaniccontinental
dichotomy in the nature of the lithosphere.
Kennett et al., 2004
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Summary

• The cratonic region in the centre and west of
  Australia is underlain by a thick mantle
  lithosphere extending to around 210 km depth with
  fast wave speeds (especially for S waves).
• Beneath the eastern zone with Phanerozoic outcrop
  the lithosphere is generally thinner (less than
  140 km) and the asthenosphere has a pronounced
  low-velocity zone for S, with high attenuation.
• The variations in seismic-wave speeds extend
  through the upper mantle with noticeable
  differences in the transition zone.

• There is also evidence for pervasive small-scale
  heterogeneity (scale lengths of 100200 km)
  superimposed on the broader scale variations that
  can be imaged using tomographic methods.

Thicker lithosphere (210km) with high Vs
Thinner lithosphere (lt 140km) with low Vs for the
asthenosphere

• 140 km