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Comerci, Valerio; Vittori, Eutizio; Cipolloni, Carlo; Di Manna, Pio; Guerrieri, Luca; Nisio, Stefania; Succhiarelli, Claudio; Ciuffreda, Maria; Bertoletti, Erika
doi: 10.1007/s00024-015-1066-1pmid: N/A
Within the PanGeo project (financed by the European Commission under the 7th Framework Program), the Geological Survey of Italy (ISPRA) and the Urban Planning Department of the City of Roma developed a geodatabase and map of the geological hazards for the territory of Roma, integrating remotely sensed data (PSInSAR—Permanent Scatterer Interferometry Synthetic Aperture Radar) and in situ geological information. Numerous thematic layers, maps and inventories of hazards (e.g., landslides, sinkholes, cavities), geological and hydrogeological data added to historical and recent urbanization information were compared to the permanent scatterer (PS) data from the European Remote Sensing satellites (ERS-1/2, 1992–2000) and ENVISAT (2002–2005) descending scenes, in order to produce a ground stability layer (GSL). Based on the PS data, most of the territory appears stable (almost 70 % of PS velocities are within ±1 mm/year). About 14 % of the PSs show positive line-of-sight (LOS) velocities (measured along the LOS of the satellite) between 1 and 3 mm/year and more than 2 % exceed 3 mm/year; more than 11 % of PSs show negative LOS velocities between −1 and −3 mm/year, while about 3 % exceed −3 mm/year (with tens of the PSs showing velocities over −20 mm/year). The GSL is comprised of polygons or multi-polygons (multipart polygons grouping individual polygons under a single identifier geohazard) enclosing areas where geohazards have been pointed out by PS data and/or in situ surveys (observed instabilities), and by polygons enclosing areas potentially affected by geohazards, based on the available knowledge of the territory (potential instabilities). In Roma’s GSL, 18 multi-polygons (covering ca. 600 km2) related to observed instabilities have been outlined, where ground movements could be detected through InSAR data or where landslides and sinkholes are known to have occurred. Other 13 multi-polygons (covering nearly 900 km2) concern areas where the potential occurrence of geohazards was inferred by combining geological and/or geothematic data (potential instabilities). The geohazards mapped in Roma have been: landslides, collapsible grounds, compressible grounds, groundwater abstraction, mining, man-made ground, tectonic movements, and volcanic inflation/deflation. The lattermost is the likely cause of the significant uplift observed in the Alban Hills area. However, this paper focuses on two more currently impending hazards: subsidence and sinkholes. In general, sinkhole-prone areas (areas of dense underground cavities) are hard to discern from satellite data, but can be revealed by ruling out other potential causes of observed ground movement based on in situ data. Subsiding zones are effectively detected by the available PSInSAR dataset over a total extent of about 60 km2, mostly overlapping the recent alluvial areas of the Tiber and its tributaries. PSs show a very different behaviour inside and outside the historical centre. Inside, loading by anthropogenic construction and man-made ground since ancient times has led to an almost complete consolidation of the recent river deposits, marked by modest to absent subsidence. In contrast, outside, subsidence clearly stands out, with negative LOS velocities that, although generally within several mm/year, can locally exceed −25 mm/year. PS data have provided motion information at both a regional and local scale (up to the scale of a single building). Closer to the sea, in the Tiber delta area, velocities increase, especially above recently reclaimed marsh areas, rich in peat and organic clays. Velocities can change significantly over short distances, as in the international airport area, reflecting the local stratigraphic setting. The same occurs in the two subsiding areas located within the Alban Hills volcanic complex, which is generally affected by ground uplift. As a whole, PSInSAR ground motion velocities offer a significant contribution to susceptibility and hazard recognition studies. In particular, such a method provides a fast and effective tool available to local authorities to monitor ground and building behaviour, possibly allowing for timely prevention activities, especially when coupled with appropriate in situ knowledge.
Notti, Davide; Calò, Fabiana; Cigna, Francesca; Manunta, Michele; Herrera, Gerardo; Berti, Matteo; Meisina, Claudia; Tapete, Deodato; Zucca, Francesco
doi: 10.1007/s00024-015-1071-4pmid: N/A
Recent advances in multi-temporal Differential Synthetic Aperture Radar (SAR) Interferometry (DInSAR) have greatly improved our capability to monitor geological processes. Ground motion studies using DInSAR require both the availability of good quality input data and rigorous approaches to exploit the retrieved Time Series (TS) at their full potential. In this work we present a methodology for DInSAR TS analysis, with particular focus on landslides and subsidence phenomena. The proposed methodology consists of three main steps: (1) pre-processing, i.e., assessment of a SAR Dataset Quality Index (SDQI) (2) post-processing, i.e., application of empirical/stochastic methods to improve the TS quality, and (3) trend analysis, i.e., comparative implementation of methodologies for automatic TS analysis. Tests were carried out on TS datasets retrieved from processing of SAR imagery acquired by different radar sensors (i.e., ERS-1/2 SAR, RADARSAT-1, ENVISAT ASAR, ALOS PALSAR, TerraSAR-X, COSMO-SkyMed) using advanced DInSAR techniques (i.e., SqueeSAR™, PSInSAR™, SPN and SBAS). The obtained values of SDQI are discussed against the technical parameters of each data stack (e.g., radar band, number of SAR scenes, temporal coverage, revisiting time), the retrieved coverage of the DInSAR results, and the constraints related to the characterization of the investigated geological processes. Empirical and stochastic approaches were used to demonstrate how the quality of the TS can be improved after the SAR processing, and examples are discussed to mitigate phase unwrapping errors, and remove regional trends, noise and anomalies. Performance assessment of recently developed methods of trend analysis (i.e., PS-Time, Deviation Index and velocity TS) was conducted on two selected study areas in Northern Italy affected by land subsidence and landslides. Results show that the automatic detection of motion trends enhances the interpretation of DInSAR data, since it provides an objective picture of the deformation behaviour recorded through TS and therefore contributes to the understanding of the on-going geological processes.
Sarychikhina, Olga; Glowacka, Ewa; Robles, Braulio; Nava, F.; Guzmán, Miguel
doi: 10.1007/s00024-015-1067-0pmid: N/A
Ground deformation and seismicity in Mexicali Valley, Baja California, Mexico, the southern part of the Mexicali-Imperial valley, are influenced by active tectonics and human activity. In this study, data from two successive leveling surveys in 2006 and 2009/2010 are used to estimate the total deformation occurred in Mexicali Valley during 2006–2009. The leveling data span more than 3.5 years and include deformation from several natural and anthropogenic sources that acted at different temporal and spatial scales during the analyzed period. Because of its large magnitude, the aseismic anthropogenic deformation caused by fluid extraction in the Cerro Prieto geothermal field obscures the deformation caused by other mechanisms and sources. The method of differential interferograms stacking was used to estimate the aseismic (interseismic tectonic and anthropogenic) components of the observed displacement, using SAR images, taken in 2007 during a period when no significant seismicity occurred in the study area. After removing the estimated aseismic signal from the leveling data, residual vertical displacement remained, and to identify possible sources and mechanisms of this displacement, a detailed analysis of records from tiltmeters and creepmeters was performed. The results of this analysis suggest that the residual displacement is mainly caused by moderate-sized seismicity in the area of study. Modeling of the vertical ground deformation caused by the coseismic slip on source fault (primary mechanism) of the two most important earthquakes, May 24, 2006 (Mw = 5.4) and December 30, 2009 (Mw = 5.8), was performed. The modeling results, together with the analysis of geotechnical instruments data, suggests that this moderate-sized seismicity influences the deformation in the study area by coseismic slip on the source fault, triggered slip on secondary faults, and soft sediments deformation.
Carmona, Enrique; Almendros, Javier; Alguacil, Gerardo; Soto, Juan; Luzón, Francisco; Ibáñez, Jesús
doi: 10.1007/s00024-014-1018-1pmid: N/A
Analyses of seismograms from ~1,100 north-Moroccan earthquakes recorded at stations of the Red Sísmica de Andalucía (Southern Spain) reveal the systematic presence of late phases embedded in the earthquake codas. These phases have distinctive frequency contents, similar to the P and S spectra and quite different to the frequency contents of the earthquake codas. They are best detected at near-shore stations. Their amplitudes decay significantly with distance to the shoreline. The delays with respect to the P-wave onsets of the preceding earthquakes are consistently around 85 s. Late phases are only detected for earthquakes located in a small region of about 100 × 60 km centered at 35.4°N, 4.0°W near the northern coast of Morocco. Several hypotheses could, in principle, explain the presence of these late phases in the seismograms, for example, the occurrence of low-energy aftershocks, efficient wave reflections, or Rayleigh waves generated along the source-station paths. However, we conclude that the most-likely origin of these phases corresponds to the incidence of T-waves (generated by conversion from elastic to acoustic energy in the north-Moroccan coast) in the southern coast of the Iberian Peninsula. T-waves are thought to be generated by energy trapping in low-velocity channels along long oceanic paths; in this case, we demonstrate that they can be produced in much shorter paths as well. Although T-waves have been already documented in other areas of the Mediterranean Sea, this is the first time that they have been identified in the Alboran Sea.
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