This thesis is directed at exploiting information in the coda of
seismic phases and the ambient noise field to provide new constraints
on the structure of the Australian Continent.
¶
The exploitation of the immediate coda following the onset of P waves from
a distant earthquake using radial receiver functions is now a well
established method. The 40 sec interval following P contains
reverberations and conversions, by deconvolving the radial component
trace with the vertical components, the conversions are emphasized
by canceling the part of the response that are common to both
components. A member of different styles of such deconvolution, are
investigated and a variant of the multitaper method is adopted for
subsequent applications. The TASMAL experiment 2003-2005
spans the expected location of the transition between Precambrian and
Phanerozoic Australia. The 20 portable broadband stations were
exploited in receiver function studies to extract S wave crustal
structure through the inversion of stacked receiver functions using the
Neighbourhood Algorithm. There is no clear crustal transition
associated with the presence of Tasman Line. The Precambrian Cratons
tend to exhibit crustal thicknesses close to 40 km but such values
are also found in some Phanerozoic sites.
¶
The second part of the thesis is directed at the exploitation of
ambient noise or seismic coda to gain information on the Green's
function between seismic stations. The TASMAL experiment
covered a significant fraction of the Australian continent with a
simultaneous deployment of portable broadband stations. From these
continuous records, it has proved possible to extract very clear
Rayleigh wave signals for station separations up to 2000 km, and to
demonstrate the frequency dependent variations in group velocity
behaviour. The combination of the paths between the 20 stations localize such
behaviour, but detailed images needed more data. The entire archive of
portable broadband data recorded by RSES was mined, and
combined with data from permanent stations to provide more than 1100
estimates of interstation Green's functions within Australia. Group
velocity analysis as function of frequency was followed by nonlinear
tomography with the Fast Marching Method. The resulting images of
group velocity patterns as a function frequency show pronounced regions
of lowered group velocities, most of which match regions of thick
sediment. The frequency dependence is not consistent with just
sedimentary structure and low midcrustal velocities, most likely due
to elevated temperatures, are also needed.
¶
The surface wave portion of the interstation Green's function is the
most energetic, and is normally all that seen in ambient noise
studies. However, in the coda of events record at the broadband
Warramunga seismic array in the Northern Territory, the P and S
body wave components also emerge. The characteristics of these
arrivals match those observed from
nearby small earthquakes. The stacked cross-correlation is the normal
approach to enhance Green's function information from ambient noise,
but a broader spectral band width with the same phase response can be
found by spectral division. It appears advantageous to compare both
approaches and select the best result, since very little modifications
to procedures are needed.
¶
The properties of the ambient noise at a single station have been
investigated in the logarithmic spectral domain and a station
dependent signal can be extracted by stacking. The signal appears to
be related to the local structure beneath the station, and when fully
characterized may provide a new means of investigating structure.
| Identifer | oai:union.ndltd.org:ADTP/246508 |
| Date | January 2007 |
| Creators | Saygin, Erdinc, erdinc.saygin@anu.edu.au |
| Publisher | The Australian National University. Research School of Earth Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.anu.edu.au/legal/copyrit.html), Copyright Erdinc Saygin |
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