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The 1998 Sarno (Italy) landslide from SAR Interferometry

Salvatore Stramondo(1) and Andrea Arturi(2)

(1) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, 00143 - Rome, Italy
(2) Universita Tor Vergata, Via del Politecnico 1, 1-00133 Rome, Italy

Abstract

On May 4th - 5th , 1998 about 150 landslide movements succeeding in 10 hours, from 2 to 12 p.m. hit an area of 75 km2 near Naples (Italy), in the cities of Sarno, Siano and Bracigliano (in the district of Salerno) and in the city of Quindici (Avellino). This event caused several victims and shocked the entire nation for its alarming proportions. Actually a considerable part of Italian territory is affected by hydrogeological hazard and the use of remote sensed data like SAR offers an important tool for monitoring and studying these kind of events. The mountainous belt of Campania Region surrounding the Vesuvio are interested by characteristic landslide phenomena due to the pyroclastic deposits produced by the explosive and effusive eruptive activity and left on the limestone relief. May 4th -5th 1998 the landslide was favoured by a strong atmospheric perturbation that reached the highest values in the area comprised in Sarno, Quindici, Siano, Bracigliano and Lauro territories. Pluviometers, 40 km far from the landslide area, did not recorded particularly intense rains to confirm the very spatially limited extension of the perturbation. Meteoric waters (rain) saturated the pyroclastic low-coherent soil, and reached the carbonatic layer where they generated subsurface water streams. Besides the abandoned anthropic terracing could not develop its role of collecting meteoric waters. These produced the separation of the pyroclastic terrain from the underneath carbonatic layer, and generated the pyroclastic flow. Following meteoric events the full pyroclastic thickness has been rapidly moved on the slope, involving also the vegetation cover. In November 1997 the same area had still been struck by less dramatic landslide events. We present results of SAR Interferometry application to the study of the Sarno landslide. Two ERS-1/2 tandem pairs acquired over the Sarno area after the 1998 landslide have been processed to generate a SAR DEM. Due to the strong topographic relief of the study area, to avoid or limit layover and shadowing, we used either an ascending or a descending tandem pair. The two tandem pairs are characterized by a perpendicular baseline of 339 m such to have an ambiguity height of about 26 m, and 245 m, with an ambiguity height of 36 m. We used as reference a topographic map derived DEM dated 1997. The external DEM has been used because no ERS tandem pairs acquired before the disaster were available to produce a synthetic one. The resulting SAR DEM has been obtained by merging the two single pair SAR DEMs, also considering a weighting function based on the coherence of each interferometric pair. After coregistration, the map of the differences between the two DEMís has been generated. Except for the portions of the SAR interferogram characterized by low coherence and layover, where processing do not produce reliable results, the areas affected by the landslide have been identified as those showing the highest differences.

 

Full paper

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry