Strain Accumulation in the Southern Andes (25°-35°S) Observed by ERS and Envisat SAR Interferometry
Doin, Marie-Pierre1; Ducret, Gabriel2; Grandin, Raphael3; Métois, Marianne2; Socquet, Anne4; Vigny, Christophe2
1Université Joseph Fourier, Grenoble, FRANCE; 2ENS, Laboratoire de Géologie, Paris, FRANCE; 3IPGP, Paris, FRANCE; 4ISTerre, Univ. Joseph Fourier, Grenoble, FRANCE
Surface measurements of ground motion help constrain the along-strike and along-dip variations of interseismic coupling along subduction interfaces, defining their segmentation, and the rate of present-day strain accumulation. This is of primary interest as earthquake location and magnitude may be controlled by the propagation across one or several consecutive coupled patches.
We here focus on the 25-35°S portion of Chilean subduction zone, located between Taltal (~25°S) and Constitution (~35°S), where no large earthquake occurred since the mid-20th century. The Nazca plate there subducts under the South American plate with a velocity around 7 cm/year. Three main earthquakes in the last decade surround this area : the 1995 Antofagasta (M=8.1) and the 2007 Tocopilla (M=7.7) earthquakes to the North and the 2010 Maule (M=8.8) earthquake to the South. A few seismic swarms occurred in recent years : the Punitaqui (~31°S) crisis in 1997 and the swarms in the Caldera (~27°S) region in 1973 and 2006. The background seismicity rate is varying from South to North with a maximum located around La Serena area (~30°S).
The coupling distribution along the Chilean subduction interface is constrained by an heterogeneous GPS data set (Métois et al., 2012). Low coupling rates are inferred for La Serena area, framed by locked portions of the interface further North and South. GPS data also suggest that the Copiapo area corresponds to a local minimum of coupling. Further North, between 28°S and 25°S, the GPS network is too sparse to constrain coupling variations.
In this study, we use InSAR (Interferometric Synthetic Aperture Radar) to provide additional insights on interseismic strain accumulation. Its sensitivity to the vertical makes it prone to constrain the depth extent of the coupled interface. The available ERS and ENVISAT archive (1992-2010) on 4 tracks (53, 96, 325, 282) between 25°S and 35°S is particularly poor and with an heterogeneous latitudinal coverage. In order to benefit from most available SAR images, we processed interferograms with perpendicular baselines reaching up to 600 m and temporal baselines exceeding 7 years, thus with a poor coherence. Wrapped interferograms are corrected from stratified atmospheric delay, using empirical phase versus elevation relations varying with elevation and latitude. They are also compensated from DEM errors effects, before multilooking and filtering. The unwrapping starts in areas with the highest coherence and proceeds with a decreasing coherence threshold. Unwrapped interferograms are then flattened in range and azimuth, before being inverted to yield a time series of delay maps in the satellite Line Of Sight (LOS) direction. The last step is to separate atmospheric patterns from the deformation signal, and to isolate the interseismic component from the earthquake-related deformation.
The time series analysis allows to isolate a post- seismic deformation associated with the swarm of Caldera in 2006, of the same amplitude as the co-seismic deformation. They are inverted together with the constrain given by one continuous GPS station located further inland. Post-seismic motion on the interface appears deeper and slightly offset to the South with respect to the co-seismic rupture. Similarly, an InSAR time series analysis based on ERS data allows to isolate the effect of the Punitaqui earthquake sequence in 1996, that occurred at the southern end of the La Serena decoupled segment. Again, InSAR data suggest the occurence of aseismic slip on the subduction interface, associated with the main slab-push event.
We then construct average interseismic LOS velocity maps for the four parallel tracks. A qualitative comparison shows that the subduction segmentation observed in InSAR maps closely matches the coupling model derived from GPS velocities. This good agreement makes us feel confident in the methodology used here to extract the interseismic LOS velocity, despite unfavourable conditions.