Monitoring terrain stability using radar interferometry in Lebanon

Cataloging and quantifying rates of mass wasting in mountainous regions provides key constraints on the interplay between surficial and tectonic processes, as well as critical information for hazard assessments. Interferometric Synthetic Aperture Radar (InSAR) is an effective tool for tracking coherent mass movements, such as earth flow and slope creep. This study demonstrates the utility of InSAR to contribute to hazard assessments. Specifically, InSAR is used to document mass movements in Lebanon, an area of active uplift within a 200-km-long restraining bend along the Dead Sea transform system. SAR data from several platforms were analyzed: 27 ERS 1 & 2 images spanning 1992 - 2001; 29 ENVISAT ASAR images spanning 2003 - 2009; and six 8 ALOS-PALSAR- 8 images covering 2007 - 2009. Challenges to the analysis included temporal decorrelation (particularly for C-band SARs) and DEM errors. The latter were identified and reduced using a multi-baseline baseline approach. For coherent regions, the multi-temporal analysis produced a time-series of displacements associated with down-slope movement. The detected mass movements are predominantly located on the west-sloping flank of Mount Lebanon, rather than the semi-arid Anti Lebanon range to the east. The majority of these mass movements occur on the slopes exposed to the south and west that are warmer with higher evaporation rates and lower moisture storage capacity. The outcrop geology is either dominated by the impermeable, friable and chalky Senonian (Upper Cretaceous) marls or by the Neocomian-Barremian (Lower Cretaceous) sandstone. Locally, rates exceeding ~8 cm/yr are observed. The identified mass movements also correlate with previously identified zones of high mass movement hazard in Lebanon, and collectively will contribute to an improved assessment of the risk zonation.