Measurement of interseismic crustal deformation in San Juan, Argentina, by Satellite Radar Interferometry

ROSELL, PATRICIA A.; EUILLADES, PABLO; EUILLADES, LEONARDO; BLANCO, MAURO; ACOSTA, GEMMA

San Juan

Workshop; First international Andes-Geo workshop; 2018

Seismic activity in Argentina is focused on the central west and north of the country. Several destructive earthquakes took place in this region in the past, which unveiled the need of seismic safety regulations and further research to assess the related risk. A variety of geophysical prospectingmethods and geodetic techniques are being used for modelling  interseismic displacement.The purpose of this project is to develop methodologies for processing Synthetic Aperture Radar images (SAR) which grant characterization of slow crustal deformation in time and space able to characterize the slow inter-seismic deformation signal.Synthetic Aperture Radar (SAR) is a technique that allows high resolution images to be formed from data acquired by side looking radar instruments and is characterized by amplitude and phase. The difference in phase between two images results in an interferogram with topographic contribution. The process of correcting for topography is referred as Differential Interferometric SAR (DinSAR). Thismethodology permits to measure displacements produced by several phenomena (earthquakes, landslides, land subsidence, volcanic eruptions, among others) with sub centimeter resolution. One of the approaches for studying deformation dynamics and their behaviour is called Small BAseline Subsets (SBAS) that allows obtaining deformation time series.A dataset of 84 Sentinel-1 images (descending pass and VV polarization) over San Juan province were used, covering the period between October 2014 and April 2018. 256 interferograms with SBAS technique could be generated with a temporal baseline of 1500 days and spatial baseline of 200m. Topography phase was removed using 30 m C-band STRM DEM. In order to display ground motion, a mean deformation velocity map and temporal deformation series were computed. Additionally, positional signal from permanent GNSS stations belonging to the study area (UNSJ,OAFA and CASLO) were processed for comparison purposes with the DinSAR results. For modelling crustal structures between San Juan and Mendoza, a geophysical model is being calculated from the data of three gravity-magnetic campaigns carried out in April 2017 and August 2018.DinSAR results show a downward/westward movement in the order of 3 cm/year, however it appears to be inconsistent with the eastward movement of the South American plate. However, it is possible that the post-seismic relaxation after the Illapel Earthquake (Chile, September 2015, Mw 8.4) is still ongoing. Further analysis of the relative baselines between GNSS stations within the AOI is being addressed for clarifying the point. Furthermore, a relatively high percentage of interferograms are affected by atmospheric stratification (correlation between height and atmosphere) introducing noise in the results.Due to atmospheric stratification it was not possible to assert that the movement detected contains a inter-/post-seismic deformation correlation. Phase propagation delay in the atmosphere is the mainsystematic error of interferometric SAR measurements. Hence, for rectification of each image used atmospheric corrections methods are being developed with the purpose of reduce atmospheric noise.Is expected that the geophysical model could indicate deformation rate that allow correlating its results with DinSAR.