The promising role of A-DInSAR time series analysis in investigating subsoil characters for seismic risk assessment and mitigation: the case study of L'Aquila historical downtown

Abstract

Statistical analyses of time series of A-DInSAR post-seismic data (April 6th 2009 L’Aquila earthquake), acquired in the time range 2010-2021 from the Cosmo-SkyMed (by ASI) and Sentinel-1 (by ESA) missions, have been carried out. These have allowed investigating the relationships between ground deformations and geological, hydrogeological, and geomorphological features of the study area, located in L’Aquila (Italy) historical centre (LAHC). The analysis of these data is still ongoing and offers promising research perspectives in the field of geomechanical/geotechnical subsoil characterization, based on satellite ground deformation data, also useful in seismic hazard characterization and mitigation. L’Aquila downtown is placed in the L'Aquila-Scoppito intermontane basin (Central Italy) which is a half-graben bordered by SW-dipping normal mostly active faults, filled with approximately maximum 600 meters of Plio-Quaternary continental slope, colluvial and alluvial deposits which overlie unconformably the carbonate bedrock. To assess the relationship between the geological-geomorphological and hydrogeological study area features and ground deformation in L’Aquila downtown, a correlation analysis has been carried out, between subsidence velocity and the following driving factors: water table depth, ground slope, shear wave velocity of outcropping lithologies and Red Soil thickness. Furthermore, cluster analysis and various filtering and time series treatment have been applied to these time series with the aim of analysing seasonal and deseasonalized trends. The correlation between subsidence velocity and the above-mentioned driving factors is statistically significant. It is presumable that the subsidence process is mainly controlled by the kind and thickness of lithologies involved. The above illustrated correlation analysis provides a first result, which may be improved by a multivariate approach. Let us consider a simple vertical strain model, made up of overlying layers (e.g., Red Soil, L’Aquila Breccia, etc.) in the consolidation phase. The integrated analysis of A-DInSAR and well data may allow determining the subsurface structure for the studied urban area, with particular attention to Red Soils, which, due to their high compressibility, have been recognized as lithology responsible for site-specific seismic amplifications. This may provide a promising powerful method of geotechnical characterization at urban scale and at a relatively low cost. The main achieved results can be summarized as follows: The A-DInSAR post-seismic data, recorded in the time range 2010-2021, revealed a post-seismic subsidence phenomenon, still ongoing. The correlation analysis allows us to conclude that subsidence velocities are mainly controlled by the properties and thicknesses of shallower rock layers. The subsidence velocity is positively correlated with the damage level of buildings. The cross-correlation analysis highlighted a significant correlation between seasonal fluctuations in subsidence rate and rainfall variations. The seasonally adjusted A-DInSAR time series highlighted an anomaly in the subsidence trend, observable during the Amatrice-Norcia seismic sequence of 2016. The study of this anomaly deserves attention and will be the subject of future research. Ground deformations detected by means of A-DInSAR technology may provide a promising inversion criterion, enabling us to perform a geotechnical characterization of shallow rock layers, over large areas, at relatively low costs.