High-Resolution (0.5m) Optical Imagery and InSAR for Constraining Earthquake Slip: The 2010-2011 Canterbury, New Zealand Earthquakes

We present a successful example of sub-pixel tracking applied to high-resolution (~0.5 meter) optical imagery to image earthquake surface deformation. We examine the 2010-Sept-4 Mw 7.1 Darfield and 2011-Feb-22 Mw 6.3 Christchurch, New Zealand earthquakes using space-based interferometric synthetic aperture radar (InSAR) and high-resolution optical imagery from the Worldview-1 and Geoeye-1 commercial satellites made available through the U.S. National Geospatial Agency. The Darfield event created nearly 30 kilometers of E-W surface rupture and generated mappable offsets of roads, hedgerows, tree stands, and train tracks, among other features. The low relief of the Canterbury Plains and available short InSAR repeat intervals resulted in excellent coherence of interferograms. In addition, rapid acquisition of high-resolution optical imagery following the Darfield and Christchurch earthquakes allowed us to select pre- and post-seismic scenes with minimal cloud cover and good spatial coverage of the earthquakes, although the pairs of scenes that span the full Darfield rupture have a long temporal baseline. We perform normalized cross-correlation on optical imagery pairs, using AMPCOR from the ROI_PAC software package, to derive surface offsets. We find that orthorectification and terrain correction are the most difficult tasks because of the lower resolution of publicly available DEMs (90 m SRTM, 15 m NZDEM_SoS_v1.0) compared to the high resolution of the imagery. Coregistration and orthorectification though are greatly enhanced by using information in the imagery .ntf wrapper. For the Darfield scenes, orthorectification is not as critical due to the flat topography and near-nadir look angle of the scenes. However, precise orthorectification and terrain correction is required over Christchurch because of the steep relief of the Banks Peninsula. The long temporal baseline between the Darfield pre- and post-seismic scenes prevents us from obtaining offsets within agricultural fields where surface rupture is clearly visible in the imagery; however, we are able to successfully obtain offsets on roads and hedgerows which are oriented approximately orthogonal to the rupture trace. We demonstrate with post-Darfield pairs, obtained within days of each other, how correlation over agricultural regions changes with temporal baseline. We include the offsets imaged along roads with down-sampled interferograms in our inversion for co-seismic slip during the Darfield earthquake. For the inversion, the InSAR provides information about the deeper nature of slip, particularly because of decorrelation near the surface trace, while the optical offsets provide information about shallow deformation close to the fault. We invert for slip onto a three-fault model discretized according to model resolution. We conduct inversions using InSAR only and InSAR-optical offsets data sets to demonstrate the ability of the surface offsets to better constrain inversions at shallow depths near the surface rupture.