Detecting Australian Earthquakes with InSAR
John Dawson1,2 and Paul Tregoning,1
1 Research School of Earth Sciences, Australian
National University, Canberra, ACT 0200, Australia
2 Geoscience Australia, Canberra, Australia.
Figure 1. Slip distribution estimate of the Katanning earthquake, October 2007. A) Estimated slip. View is normal to the fault plane. b) 1σ uncertainties of the slip. c) Plan view of the rupture in an arbitrary local coordinate system. d) Viewing azumuth and elevation is 210° and 10°, respectively (Dawson et al., 2008).
Analysis of Interferometric Synthetic Aperture Radar (InSAR) images has detected two shallow, intraplate earthquakes over the past three years. Each of these small earthquakes occurred in the top ~1 km of the crust and caused sufficient surface deformation that the location and fault parameters (orientation, dip, slip direction) could be estimated. These are the smallest magnitude earthquakes ever imaged by the InSAR technique (Dawson et al., 2008).
The Mw 4.7 October 2007 Katanning earthquake ruptured ~1 km2 with an average slip of ~ 42 cm. This implies a static stress drop of 14-27 MPa which is much higher than previously expected for such a small event. The quality of the InSAR deformation estimates is sufficiently high that the depth of the event can be estimated with a precision of ~10 m. It was even possible to invert for the distribution of slip on the fault plane - the first time that this has been achieved for such a small event. These results have been published by Dawson et al. (2008).
The fine spatial resolution and accuracy that InSAR analysis can provide to the study of earthquakes in Australia opens up exciting new possibilities. Given the vastness of the Australian continent, it is likely that most earthquakes will not occur close to a seismic station. Therefore, the accuracy with which seismic data can be used to locate earthquakes is limited. Given that InSAR observations are made remotely from space platforms, the deformation from any shallow earthquake (ie < 5 km depth) can be captured by InSAR provided an image of the region exists prior to the earthquake. It is even conceivable that the highly accurate locations estimated by InSAR could be used as constraints in seismic inversions for crustal rheology modelling and/or as master event locations in a bootstrapping process to relocate other Australian earthquakes.
Dawson, J., P. Cummins, P. Tregoning, and M. Leonard, 2008. Shallow
intraplate earthquakes in Western Australia observed by InSAR, J. Geophys.
Res. 113, B11408, doi:10.1029/2008JB005807.