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

PATRICIA ROSELL; ING. PABLO EUILLADES; LEONARDO D. EUILLADES; GEMMA ACOSTA

San Juan

Workshop; First International Andes Geo Workshop; 2018

Varias

Seismic activity in Argentina is focused on thecentral west and north of the country. Several destructive earthquakes tookplace in this region in the past, which unveiled the need of seismic safetyregulations and furtherresearch to assess the related risk. A variety of geophysical prospectingmethods and geodetic techniques are being used for modelling interseismicdisplacement.The purpose of this project is to developmethodologies for processing Synthetic Aperture Radar images (SAR) which grantcharacterization of slow crustal deformation in time and space able tocharacterize the slow inter-seismic deformation signal.Synthetic Aperture Radar (SAR) is a techniquethat allows high resolution images to be formed from data acquired by sidelooking radar instruments and is characterized by amplitude and phase. Thedifference in phase between two images results in an interferogram withtopographic contribution. The process of correcting for topography is referredas Differential Interferometric SAR (DinSAR). This methodology permits tomeasure displacements produced by several phenomena (earthquakes, landslides,land subsidence, volcanic eruptions, among others) with sub centimeterresolution. One of the approaches for studying deformation dynamics and theirbehaviour is called Small BAseline Subsets (SBAS) that allows obtainingdeformation time series.A dataset of 84 Sentinel-1 images (descendingpass and VV polarization) over San Juan province were used, covering the periodbetween October 2014 and April 2018. 256 interferograms with SBAS techniquecould be generated with a temporal baseline of 1500 days and spatial baselineof 200m. Topography phase was removed using 30 m C-band STRM DEM. In order todisplay ground motion, a mean deformation velocity map and temporal deformationseries were computed. Additionally, positional signal from permanentGNSS stations belonging to the study area (UNSJ, OAFA and CASLO) were processedfor comparison purposes with the DinSAR results. For modelling crustalstructures between San Juan and Mendoza, a geophysical model is beingcalculated from the data of three gravity-magnetic campaigns carried out inApril 2017 and August 2018.DinSAR results show a downward/westward movementin the order of  3 cm/year, however itappears to be inconsistent with the eastward movement of the South Americanplate. However, it is possible that the post-seismic relaxation after theIllapel Earthquake (Chile, September 2015, Mw 8.4) is still ongoing. Furtheranalysis of the relative baselines between GNSS stations within the AOI isbeing addressed for clarifying the point. Furthermore, a relatively highpercentage of interferograms are affected by atmospheric stratification(correlation between height and atmosphere) introducing noise in the results.Due to atmospheric stratification it was notpossible to assert that the movement detected contains a inter-/post-seismicdeformation correlation. Phase propagation delay in the atmosphere is the mainsystematic error of interferometric SAR measurements. Hence, for rectificationof each image used atmospheric corrections methods are being developed with thepurpose of reduce atmospheric noise. Is expected that the geophysical modelcould indicate deformation rate that allow correlating its results with DinSAR.