HR: 0800h
AN: S11A-0268    [Abstracts]
TI: Reconciling Neotectonic and Seismic Recurrence Rates in SW Australia
AU: * Leonard, M
EM: mark.leonard@ga.gov.au
AF: Geoscience Australia, PO Box 378, Canberra, ACT 2601, Australia
AU: Clark, D C
EM: dan.clark@ga.gov.au
AF: Geoscience Australia, PO Box 378, Canberra, ACT 2601, Australia
AB: The southwest corner of Western Australia is an area of stable continental crust (SCC) with the geology being Protozoic and the area undergoing no tectonic activity in the last 40Ma. However in the last 50 years it has had the highest level of seismicity of any area of worldwide SCC. It not been glaciated in the last 20Ma and has had a cool dry climate for at least 200ka, thus providing an ideal environment to preserve fault scarps. The availability of high resolution digital elevation model data (DEM) in this area has lead to the identification of over 50 new features that are thought to be scarps of surface rupturing earthquakes. Half of these features have been the subject of field work and all are thought to be fault scarps. One feature has been trenched and the presence of a fault scarp verified. Using recently developed fault scaling relations the fault length and displacement are used to estimate magnitude and in some cases identifying multiple events, these scarps have been used to generate a neotectonic earthquake catalogue. Between M6.5 and M7.2 the Log N vs Mag plot of the data has a slope of 1, indicating that the neotectonic derived catalogue is complete above M6.4. Below M6.4 the number of earthquakes rapidly decreases, with 20% of the expected number of M6.2 earthquakes and 3% of M6.0 earthquakes being identified via neotectonic methods. This is likely a combination of the lower likelihood of the earthquakes causing a surface rupture and the smaller rupture more rapidly reducing in size to below the detection threshold. Scarps formed by earthquakes of ≥M7.3 appear over represented with M7.4-7.5 earthquakes being over represented by a factor of perhaps 3 to 6. This could be explained by the longer preservation age of these large scarps. Alternatively, as the majority of these large scarps are in a a small section of the study area which could be considered extended continental crust, they might be representing a separete tectonic regime. The three key results of this study are: 1. Under the right geological and climatological conditions fault scarps from ≥M6.5 earthquakes can be preserved for 100ka or more and scarps from ≥M7.3 earthquakes for perhaps >150ka. With approximately 50% of M6.3 and 100% ≥M6.4 earthquakes in SCC are expected to form scarps. 2. Mmax in stable continental crust is more likely to be M7.3-7.5 than the M7.0-7.2 generally used in hazard studies. If the extended continental crust hypotesis is applied then the Mmax will be more like M7.4-7.5 and M7.3- 7.4 for extended and non-extended SCC respectively. 3. The recurrence rate for this neotectonic catalogue and historical earthquakes in the whole SCC of Australia are similar. However the contemporary level of seismicty in this area is an order of magnitude higher than that needed to generate the scarps. This supports the hypothesis that seismicity in SCC is episodic and migrates on time frames of 1-10ka.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8103 Continental cratons
DE: 8175 Tectonics and landscape evolution
SC: Seismology [S]
MN: 2007 Fall Meeting