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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS005 Poster presentation 1922
Reconciling neotectonic and seismic recurrence rates in SW Australia
Dr. Mark Leonard
Geoscience Australia Geohazards IASPEI
Dan Clark
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 the 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 (Clark 2006 & Clark and Leonard 2007). Half of these scarps 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 (Estrada et al. 2006). Using recently developed fault scaling relations
(Leonard 2007) 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 & 7.2 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 overrepresented by a factor of perhaps 3-6. This could be explained by the
longer preservation age of these large scarps. 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. 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.
Keywords: paleoseismicity, recurrence, earthquake