Séminaire ISTerre

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InSAR Monitoring of Climatic Forcing and Tectonic Deformation in North-Eastern Tibet.

jeudi 8 avril 2021 - 14h00

Simon Daout - ISTerre
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Until today, measuring slow deformations in mountainous areas with Synthetic Aperture Radar Interferometry (InSAR) has been challenging, in large part due to the sporadic nature of the available acquisitions. The recent launch of the Sentinel-1 satellites from the European Space Agency (ESA) in 2014 brings a new paradigm in Earth Science using space-based geodesy to measure at a greater scale (250 km swath), and with a better temporal resolution (6/12 days revisit time), subtle ground movements in remote, vast and largely inaccessible regions. In this presentation, I illustrate the benefits of such data sets by performing a multi-temporal InSAR analysis in the North-Eastern part of the Tibetan plateau to (1) characterise the spatial and temporal dynamics of permafrost’s active layer, and (2) study the tectonic deformation. I process three Sentinel-1 tracks acquired from 2014 to 2019, as well as, the complete Envisat data archive along four tracks between 2003 and 2011, extending from the Qilian Shan, in the north, to the Qaidam Basin, in the south.(1) Regional deformation maps show that seasonal thaw and frost effects are controlled by the sediment type and local topography, with concentrations of the deformation on shallow slopes and poor-drainage areas in unconsolidated, frost-susceptible, and fine-grained sediments. Fast subsidence due to thaw settlement takes place during June/July while frost heave is intense during December/January when two-sided freezing of pore water under pressure causes prolonged ice segregation near the permafrost table. The analysis also reveals pervasive subsidence of the ground, affected by freeze and thaw cycles, of up to 2 cm/yr, and increasing by a factor of 2 to 5 from 2003 to today in some areas, as well as high-rates of widespread gravitational mass movements of non-consolidated sediments. (2) The tectonic analysis illuminates the steady-state inter-seismic creep along thrusts within the Qaidam foreland basins, as well as spatio-temporal changes of the long-term post-seismic surface displacement rates and patterns that follow the three 2003, 2008, and 2009 Mw 6.3 Qaidam earthquakes. Long-term transient uplift coincides spatially with basin deformed sequences and bedding planes and continues more than ten years after the seismic events, providing evidence of co-seismic slip translated into distributed fold buckling during the post-seismic phases. The findings show that is critical to delineate the areas affected by surface processes to avoid erroneous interpretations of the tectonic loading along faults and demonstrate that periglacial processes currently participate in relief smoothing by moving quantities of loose sediments from the high ranges to the lowland regions in this internal part of the Tibetan plateau.

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Equipe organisatrice : Cycle sismique et déformations transitoires

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères