Shallow intraplate earthquakes in Western Australia observed by Interferometric Synthetic Aperture Radar

Abstract

We investigate two intraplate earthquakes in a stable continental region of southwest Western Australia. Both small‐magnitude events occur in the top ∼1 km of crust and their epicenters are located with an accuracy of ∼100 m (1σ) using satellite Interferometric Synthetic Aperture Radar (InSAR). For theMw4.7 Katanning earthquake (10 October 2007) the average slip magnitude is 42 cm, over a rupture area of ∼1 km2. This implies a high static stress drop of 14–27 MPa, even for this very shallow earthquake, which may have important implications for regional seismic hazard assessment. The earthquake rupture extends from a depth of around 640 m to the surface, making it a rarely observed intraplate, surface‐rupturing event. Using InSAR observations, we estimate the coseismic slip distribution of the shallow earthquake, such estimates being rarely available for small magnitude events. For theMw4.4 composite Kalannie earthquake sequence (21–22 September 2005), we use a long‐term time series analysis technique to improve the measurement of the co‐seismic signal, which is a maximum of 27 mm in the line‐of‐sight direction. Double difference seismic analysis shows some relocated cluster seismicity which corresponds in timing, location, and source parameters to the InSAR‐observed deformation. This earthquake is the smallest magnitude seismic event to have been investigated using InSAR and demonstrates the capability of the technique to provide important constraints on small‐magnitude coseismic events. The shallow depth of both these events adds weight to the suggestion that earthquakes associated with tectonic processes in this area of Western Australia often initiate in the upper 1 km of crust.