Notes for IUGG 2007 talk
(Seismic imaging of the lithosphere beneath southeast Australia using data from
multiple array deployments)
Session: JSS015 - Crustal structure and Tectonophysics - Large-scale multidisciplinary programs for continental imaging.
Time: Thursday 12th July from 2.00 pm to 5.30 pm.
Location: Room 12, Law School.
Slide 1: Title
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In this talk, I will give you a brief overview of an intensive program run by
ANU to roll-out dense passive seismic arrays in southeast Australia.
Slide 2: Outline
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A brief outline of the talk
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Unravelling the deep geology and tectonics beneath the region provides the
underlying motivation for the deployments, but also provides an ideal
laboratory for technique development and verification.
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The passive seismic deployments span the last decade.
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The dominant use of short period recorders means that we cannot detect the
complete passive seismic wavefield, which has implications for the type of
studies we can perform.
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Given the flavour of this session, I thought a pertinent example to show was
one that involved the combined inversion of passive array data with a separate
active source wide-angle dataset from Tasmania.
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Future experiments will be directed towards trying to span as much of eastern
Australia as possible.
Slide 3: Geological background
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The western two-thirds of Australia comprises an assemblage of Archean and
Proterozoic terranes.
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The eastern third of Australia, sometimes referred to as the Tasman Orogen or
Tasmanides, records the break-up of Rodinia, followed by the growth of
orogenic belts along the eastern margin of Gondwana via a process of
subduction-related accretion.
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In southeast Australia, the Delamarian Orogen began in the Middle Cambrian,
with convergence along the proto-Pacific margin.
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The evolution of the Lachlan Orogen began in the Late Cambrian and was
largely complete by the Middle to Late Devonian. It is highly complex and
may have involved multiple subduction complexes.
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In the Early to Middle Phanerozoic, Tasmania comprised what is now referred
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to as the West Tasmania Terrane. The major event which shaped western
Tasmania was the Middle to Late Cambrian Tyennan Orogeny.
The East Tasmania Terrane contains no evidence of the Tyennan Orogen or
Proterozoic outcrop, and it is widely thought that the two terranes were
juxtaposed during th Middle Devonian Tabberabberan Orogeny.
The presence of exposed Precambrian blocks in western Tasmania appears to
exclude a tectonic affinity with the Lachlan Orogen; however, the similarity of
Devonian granites in southern Victoria and northern Tasmania suggests that
Tasmania was in its current position by the Middle Paleozoic.
Recent work, including tomography results from our passive array
experiments, suggests that the Tyennan Orogeny in Tasmania represents the
southern extension of the mainland Delamarian Orogeny.
Slides 4-12: Deployment history
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Over the last decade, nearly ten passive seismic arrays have been deployed
throughout southeast Australia.
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Originally, these arrays used vertical component short period seismometers
(L4Cs with a natural frequency of 1 Hz), but recently, they have been upgraded
to 3-component short period seismometers (LE-3Dlites with a natural
frequency of 1 Hz).
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The main objective of these array deployments has been 3-D P-wave
teleseismic tomography. However, other types of analyses are also possible.
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LF98, MB99 and AF00 are subsets of the MALT experiment, undertaken by
Monash and Adelaide Universities between 1998-2000. Each of these arrays
were deployed for a period of approximately 4 months.
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The TIGGER and SETA arrays in Tasmania have a station spacing of 15-20
km, compared to 40-60 km for the mainland stations.
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The SETA and SEAL2 arrays use 3-component short period seismometers.
Slide 13: Data return – MB99
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The use of short period recorders and a lack of local earthquakes means that
body wave teleseisms are the dominant class of data that is recorded.
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Thus, much of the global body wavefield is represented to some extent – P,
PcP, ScP, PP, PKiKP etc.
Slide 14: Data return – SEAL
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Core phases are particularly evident on the mainland arrays, some distance
from the coast.
Slide 15: Data return - SETA
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S-waves can be observed (mostly by the 3-component stations), although they
are generally less evident than P.
Slide 16: Data return – SETA
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Surface waves from large earthquakes are also evident, although the shape of
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the amplitude transfer function (exponential drop-off below 1 Hz) for the short
period sensors means that waves with periods above about 10s are hard to
detect.
In this example, we can clearly observe the arrival of a Love wave.
Slide 17: Data return – SEAL – ambient noise cross-correlation
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Long term cross-correlations of the ambient noise field between station pairs
reveals the high frequency Rayleigh wave empirical Green's function.
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These waveforms can be exploited using seismic tomography to map shallow
to mid-crustal structure.
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The example shown is not very symmetrical due to the one-sidedness of the
oceanic disturbances that provide the ambient noise source.
Slide 18: Research Potential
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Data recorded by the short period passive arrays provide a variety of research
opportunities.
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This flow diagram summarizes some of the possibilities.
Slide 19: Lithospheric structure of Tasmania – Active and passive source datasets
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This example shows how we can combine data from one of the passive arrays
(TIGGER) with data from an active source experiment which was carried out
independently. This is an example of value-adding.
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The 3-D wide-angle experiment (TASGO) was carried out in 1995, and
involved the deployment of 44 analogue and digital seismometers throughout
Tasmania to record approximately 36,000 airgun shots fired off-shore in a
complete circumnavigation.
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A total of 3,172 Pg, PmP and Pn traveltimes are extracted from this dataset for
stations in northern Tasmania.
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The teleseismic dataset comprises 6,520 arrival time residuals from 101
teleseismic events.
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The total number of traveltimes used to constrain structure is therefore 9,692.
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Note that the azimuthal distribution of teleseismic sources is not even, with
much fewer events from the south and west that the north an east.
Slide 20: Lithospheric structure of Tasmania – Tomographic method
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An iterative non-linear inversion scheme designed to integrate multiple classes
of body wave traveltime data is used to constrain structure.
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Structure is parameterised in 3-D spherical coordinates, and can include both
continuous velocity variation and undulating interfaces, both of which are
represented by cubic B-spine meshes.
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The central innovation of the scheme is its use of a grid based eikonal solver,
known as the Fast Marching Method, to solve the forward problem of
traveltime prediction. Sources may be teleseismic, local earthquake, and active
(e.g. for wide-angle and reflection experiments).
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A subspace inversion scheme is applied to solve the inverse problem of
adjusting model parameters to better satisfy data observations subject to
damping and smoothing regularization.
The iterative process of traveltime prediction and inversion ensures that the
non-linearity of the problem is accounted for.
Slide 21: Lithospheric structure of Tasmania – Imaging results
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Comparison of slides at 15 and 52 km depth shows that the velocity structure
of the crust and upper mantle is not very similar.
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The elevated crustal velocity region in the east, which is underlain by a
shallower mantle, is quite well resolved. This feature helps to confirm the
hypothesis that eastern Tasmania is underlain by rocks with an oceanic crustal
affinity, which contrasts with the continentally derived siliciclastic core of
western Tasmania. It is likely that this region of Tasmania began as a passive
margin in the Ordovician, with subsequent episodes of orogenisis and
sedimentation compressing and thickening the oceanic crust.
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Notably, the Tamar Fracture System, which has traditionally been used to mark
what was thought to be the juxtaposition of two crustal elements, does not
overlie the transition in crustal character.
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The thicker crust beneath the central northern part of Tasmania may well be a
consequence of crustal shortening associated with the mid-Devonian
Tabberabberan Orogeny.
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The easterly dipping zone of higher velocity material is well constrained, and
is unlikely to be an artifact related to smearing. This feature may represent
remnants of a Mid Cambrian easterly dipping subduction zone, associated with
the mainland Delamerian Orogeny. Recently, boninites (relatively siliceous
group of basaltic or andesitic rocks normally found in island-arc settings, and
may be a product of the early stages of subduction) of a similar age and
composition have been found in both locations.
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The thinner, higher velocity crust in northwest Tasmania is bounded to the
southeast by exposed Cambrian ophiolites (mafic to ultramafic igneous rocks
associated with pelagic sediments, which represent segments of oceanic crust
emplaced in the continent by plate collision) at the surface, which suggests that
this change in crustal character may be related to a prior collision zone in the
region (also supported by the dipping higher velocity structures).
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The shallow high velocity anomaly beneath central northern Tasmania
correlates quite well with the surface exposure of the Cambrian Mt. Read
Volcanics, a highly mineralized metal province.
Slide 22: Future experiments
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In the short term, one of the aims is to try and combine all the teleseismic data
in a single inversion for upper mantle structure. One could also combine the
ambient noise data to image the shallow to mid-crustal structure beneath the
entire region.
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Contingent on future funding, we will attempt to propagate the array
northwards as far as possible, where there is no shortage of juicy geological
targets.