A search begun in 2011 identified a deformation of the topographic surface (subsidence) of about 15 mm, within two basins near the epicentral area of the 2009 L'Aquila earthquake, probably linked to the preparatory phase of the earthquake. The research, conducted by INGV, in collaboration with the University of Cassino (DICeM) and L'Aquila (DICEAA) was published in Scientific Reports of the NATURE group
Earthquake prediction is a goal that is still far from being achieved, however an important contribution could come from satellite interferometric techniques, capable of measuring the deformations of the earth's surface and providing useful information on the probability of occurrence of a seismic event in a given area . This conclusion was reached by research, which began in 2011 and lasted about 6 years, conducted by the National Institute of Geophysics and Volcanology (INGV) in collaboration with the Department of Civil and Mechanical Engineering (DICeM) of the University of Cassino and of Southern Lazio and the Department of Civil, Building-Architecture and Environmental Engineering (DICEAA) of the University of L'Aquila. The New insights into earthquake precursors from InSAR study, published in Scientific Reports of the NATURE group (www.nature.com/articles/s41598-017-12058-3), identified and measured a topographic surface deformation (subsidence) of about 15 mm, within two basins near the epicentral area of the 2009 L'Aquila earthquake, which began about three years before the seismic event and probably linked to the preparatory phase of the earthquake.
"The deformation observed before the earthquake", explains Marco Moro, INGV researcher and first author of the work, "was induced by the subsidence of some stratigraphic levels, caused by the progressive lowering of the superficial aquifers, determined, in turn, by the migration of deep fluids".
In fact, it is known in the literature that, before a seismic event, the rocks present in the volume of the hypocentral area (focal volume) are subjected to shear stress, with consequent formation of fractures.
“The voids of the fractures are consequently filled by the surrounding fluids which, under favorable geological and hydrogeological conditions, can lead to a migration of the more superficial fluids. In order to attribute the measured signal to the preparatory phase of the earthquake, it was therefore necessary to exclude the additional causes that could have influenced the displacement of the topographic surface”, continues Moro.
The research required a multidisciplinary approach and the extensive use of satellite interferometric techniques, applied to InSAR (Interferometric Synthetic Aperture Radar) radar images, suitable for measuring the deformations of the earth's surface (processed in collaboration with TRE ALTAMIRA srl e-GEOS and GAMMA Remote Sensing Research and Consulting).
"The detected signal was interpreted thanks to the geological, hydrogeological, geotechnical and seismological knowledge acquired for the area following the L'Aquila earthquake", adds the researcher. “Hence the idea of applying and verifying this research to strong earthquakes that have already occurred in different tectonic and geological contexts, to see if the phenomenon can be observed and measured in a similar way. Only in this way could the observation of the trend of deformations over time, in seismically active areas, in the near future be able to represent a useful tool for predicting seismic events with the subsequent activation of interventions to mitigate seismic risk”.
ABSTRACT
New insights into earthquake precursors from InSAR
Authors: Marco Moro, Michele Saroli, Salvatore Stramondo, Christian Bignami, Matteo Albano, Emanuela Falcucci, Stefano Gori, Carlo Doglioni, Marco Polcari, Marco Tallini, Luca Macerola, Fabrizio Novali, Mario Costantini, Fabio Malvarosa, Urs Wegmüller.
We measured ground displacements before and after the 2009 L'Aquila earthquake using multi-temporal InSAR techniques to identify seismic precursor signals. We estimated the ground deformation and its temporal evolution by exploiting a large dataset of SAR imagery that spans seventy-two months before and sixteen months after the mainshock. These satellite data show that up to 15 mm of subsidence occurred beginning three years before the mainshock. This deformation occurred within two Quaternary basins that are located close to the epicentral area and are filled with sediments hosting multi-layer aquifers. After the earthquake, the same basins experienced up to 12 mm of uplift over approximately nine months. Before the earthquake, the rocks at depth dilated, and fractures opened. Consequently, fluids migrated into the dilated volume, thereby lowering the groundwater table in the carbonate hydrostructures and in the hydrologically connected multi-layer aquifers within the basins. This process caused the elastic consolidation of the fine-grained sediments within the basins, resulting in the detected subsidence. After the earthquake, the fractures closed, and the deep fluids were squeezed out. The pre-seismic ground displacements were then recovered because the groundwater table rose and natural recharge of the shallow multi-layer aquifers occurred, which caused the observed uplift.
A. Ground acceleration map obtained by processing RADARSAT-2 data. The map shows the two sectors affected by negative acceleration values (in red), located within two basins filled by Quaternary deposits.
B. Post-seismic velocity map derived from COSMO-SkyMed data showing for the same basins an opposite behavior (uplift, in blue) caused by the elastic recovery of the settlement.
C. Time series of the deformation inside the basins, in red before the earthquake (subsidence) and in blue after the earthquake (uplift and recovery of the elastic portion of the settlement).
Simplified scheme describing the mechanism of variation of the groundwater level observed on the basis of the theory of dilatancy
Rome, 21 September 2017