Christchurch earthquake, February 2011 – Preliminary InSAR Observations
John Elliott, Zhenhong Li, Barry Parsons
Following the recent earthquake to hit Christchurch, a satellite radar interferogram of the land deformation due to this large, shallow event has been produced. If you are interested in the ground displacement measurements of the interferogram for academic research, please contact John Elliott (email@example.com) for the InSAR data (courtesy of METI/JAXA).
The earthquake struck Christchurch city at lunchtime on the 22nd February 2011 resulting in multiple fatalities. The interferogram (Figure 1a) shows the location of the ground deformation, with the peak deformation centred just south-east of the city. The fringes in the figure indicate the contours of the ground motion towards and away from the satellite. The place where these fringes are tightest indicates where the fault rupture projects up towards the surface, which in this case is more towards the centre of Christchurch. The fault plane therefore dips to the south-east, and the rupture is not likely to have significantly broken the surface (unlike the September 2010 Darfield earthquake further west that caused the surface to be offset by up to 4 m). The peak ground motion is almost 50 cm of motion towards the satellite (up and to the west given the look orientation of the satellite). The Landsat satellite image (Figure 1b) shows that position of slip on the fault during the earthquake lies right beneath the south-east edge of the city of Christchurch (urban areas appear as purple in this false colour image, vegetated areas as green). The aftershocks (Figure 1c) are from the GNS Geonet and are located around where the fault broke.
The type of faulting is oblique – that is to say it is a mixture of strike-slip (where the two sides of the fault move sidewards passed each other) and thrust (where one side of the fault moves on top of the other). The proximity of the quake to the city, as well as it shallow depth, has resulted in greater shaking in the urban area when compared to the previous main-shock in the Canterbury plains in September 2010. This has resulted in much more damage, despite the size of the more recent earthquake in 2011 being almost 20 times smaller than the previous main-shock in 2010.
Figure 1: (a) Satellite radar observations of ground displacement towards and away from the satellite. The interferogram is a contour map of ground motion with each colour cycle of blue through green to red indicating 12 cm of ground motion away from the satellite. The SAR images used in this research were provided by the Japanese Aerospace Exploration Agency (JAXA) under a JAXA AO project. The ownership of PALSAR data belongs to METI (Ministry of Economy, Trade and Industry) and JAXA. (b) Landsat false colour image (RBG Bands 742) of the epicentral region. (c) Aftershock distribution in the 7 days following the main shock as recorded by the GNS Geonet (http://magma.geonet.org.nz/resources/quakesearch/) of New Zealand. The aftershocks’ magnitude is shown by the size of the circle and the circles are coloured by the number of days after the main shock. The main focal mechanism determination (black and white beachball) suggests an mixture of strike-slip and thrustal motion on a steeply south-easterly dipping fault. A clear ENE-WSW alignment of aftershocks is observed to occur over a distance of about 15 km.