HR: 0800h
AN: T51D-0757    [Abstracts]
TI: GPS-geodetic monitoring of the South West Seismic Zone (SWSZ) of western Australia: progress after two observation epochs in 2002 and 2006
AU: Darby, D
EM: desmond.darby@gns.cri.nz
AF: GNS Science, PO Box 30368, Lower Hutt, 5040, New Zealand
AU: * Leonard, M
EM: Mark.Leonard@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
AU: Clarke, D
EM: Dan.Clark@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
AU: Dawson, J
EM: John.Dawson@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
AU: Hu, G
EM: Guorong.Hu@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
AU: Featherstone, W
EM: W.Featherstone@curtin.edu.au
AF: Western Australian Centre for Geodesy, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia
AU: Johnston, G
EM: Gary.Johnston@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
AU: Dentith, M
EM: mdentith@cyllene.uwa.edu.au
AF: School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
AU: McCarthy, B
EM: bob.mccarthy@landgate.wa.gov.au
AF: Landgate, PO Box 2222, Midland, WA 6936, Australia
AB: The Australian south-west seismic zone (SWSZ) is a northwest-southeast-trending belt of intra-plate earthquake activity that occurs in the southwest of Western Australia, bounded by 30.5S to 32.5S and 115.5E to 118E. This is one of the most seismically active areas in Australia, with nine earthquakes over magnitude 5.0 that have occurred in the SWSZ between 1968 and 2002. The largest of these was the M6.8 Meckering earthquake in 1968. Since the SWSZ lies as close as ~150 km from the ~1.4 million population of the Perth region, it poses a distinct seismic hazard. However, little is currently known about the magnitude and orientation of this deformation, and whether there is any associated ongoing surface expression. It is also not known how this intra-plate activity compares with that observed elsewhere in Australia or elsewhere on Earth. Earthquake activity recorded by Geoscience Australia over the past four decades suggests that the SWSZ could be deforming with strain rates between 10^-9/yr and 10^-8/yr, or with displacements between 0.1 mm/yr and 1 mm/yr across the 200km width of the currently active SWSZ. This estimate is derived by applying the Kostrov formula for a moment release of 3.3x10¹⁹ Nm in the 34 years from 1968 to 2002. Early geodetic studies of the SWSZ that used both terrestrial and Global Positioning System (GPS) techniques were inconclusive, due mainly to the imprecision of the technologies used in relation to the likely small amount of any surface deformation. Therefore, in 2002 a new 48-point campaign-reoccupation GPS network was established across the SWSZ to attempt to detect surface deformation, using ground-level forced-centred monuments. The first two observational epochs were in May 2002 and May 2006. In both surveys, the dual- frequency carrier-phase GPS data were collected continuously at each monitoring point over a 5-7 day observation period. For the both campaigns, after excluding outlying sites (due to equipment malfunctioning), the estimated internal horizontal precision was 1.0 mm; the vertical precision was generally better than 4 mm. Comparison of these two repeat surveys shows that strain rates in the order of 10^-9/yr can, in principle, ultimately be resolved if the deformation across the SWSZ occurs uniformly, but that the four-year time interval between the existing two surveys is at present not adequate to detect deformation with confidence given the noise in the estimated coordinates at each epoch. Further repeat surveys after, say, 8 and 12 years should begin to reveal any observable deformation significantly in excess of this observational error. These preliminary results suggest that the higher range of estimated of strain rates (10^-8/yr) is unlikely.
DE: 1209 Tectonic deformation (6924)
DE: 5475 Tectonics (8149)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: 2007 Fall Meeting