Surface liquefaction effects detected and measured by DInSAR: 2012 Emilia (Italy) earthquake

We have investigated the liquefaction phenomenon occurred on May 20, 2012 Emilia earthquake (ML 5.9), induced by the water pressure increase in the buried and confined sand layers, exploiting the Differential Interferometry Synthetic Aperture Radar (DInSAR) capability to detect and estimate their surface displacements. The level-ground liquefaction is the result of a chaotic ground oscillation caused by the earthquake shaking and the observed failures are due to the upward water flow caused by the excess of pore pressures. Here, using a SAR dataset composed of four COSMOSkyMed X-band SAR images, covering the period April 1 – June 6, 2012, amultidisciplinary (remote sensing,geological and geotechnical) approachhas been appliedin order to relate local changes detected by DInSAR measurements to the observed liquefaction phenomena. The detection of induced subsidence measured by the DInSAR phase, and of surface changes revealed by SAR derived features (i.e., interferometric coherence and intensity correlation), allowed the identification of zones affected by differential compaction occurred in urban areas, probably associated to a volumetric deformation of the ground induced by liquefaction of saturated sandy deposits. Complex coherence and intensity correlation revealed themselves capable to identify different sources of changes, respectively small random changes in urban settlements and abrupt changes due to urban damage or soil ruptures. Geological-geotechnical analysis has been also performed in order to assess if the detected SAR-based surface effects could be due to the compaction induced by liquefaction of deep sandy layers. With this regard, the results obtained from 13 electrical cone penetrometer tests show the presence of a fine to medium sandy layer at depths ranging between 9 and 13 m, which probably liquefied during the earthquake, thus inducing vertical displacements between 3 and 16 cm. The quantitative results from geological-geotechnical analysis and the surface punctual effects measured by DInSAR are in good agreement, even if some differences are still present, probably ascribable to the local thickness and depth variability of the sandy layer, or to lack of deformation detection due to DInSAR decorrelation. One of the most relevant results is the capability of this approach to detect settled areas where no surface effects, sandy boils or cracks, can be observed through ground surveys. Moreover, with the already operational SAR satellite constellations, such as COSMO-SkyMed and TerraSAR-X, and the next launch of the second Sentinel-1 satellite (two satellites working for 7 years with a revisit time of six days), more outcomes are expected. A faster processing of data might also provide a fundamental contribution to the management of seismic crisis, giving a more complete and detailed information on the “event scenario” in order to provide authorities with elements for a rational planning of the emergency.