The presence of slow fault movements during the sequences of low-magnitude earthquakes has been highlighted for the first time in Italy, in the Pollino area, which contributes to explaining why, compared to the rest of the Apennines, in this area the earthquakes of elevated are less frequent. The study was published in Nature's Scientific Reports
During the long seismic sequence that affected the Pollino from 2010 to 2014, there were not only earthquakes but also slow and continuous slips of faults without seismic activity. This was revealed for the first time by the study just published in the journal Scientific Reports of Nature by a team of researchers from the National Institute of Geophysics and Volcanology (INGV), the Institute for electromagnetic sensing of the environment of the National Research Council (IREA -CNR) in collaboration with the Civil Protection Department. The results of the study shed new light on seismicity in historical times in the Pollino area.
“In recent years”, explains Daniele Cheloni, INGV researcher and first author of the work, “it has been highlighted that the seismic sequences of low-magnitude earthquakes are often accompanied by aseismic slips, even if the lack of a sufficient number of deformation measurements of the soil during these sequences has so far prevented the verification of this hypothesis in the Italian area”.
Earthquakes, as is known, are caused by movements of faults, i.e. fractures in the earth's crust, which occur very quickly (in a few seconds). In other cases, the same faults can move slowly (over weeks or months) without generating earthquakes (aseismic slip). To demonstrate the simultaneous presence of seismic activity and aseismic movements, the researchers relied on data from the GPS stations (constellation of satellites of the Global Positioning System) of the INGV RING network (http://ring.gm.ingv.it), installed in 2011 as part of an INGV project to study tectonic deformation in the Pollino area, and to the radar images collected by the COSMO-SkyMed satellites of the Italian Space Agency (ASI), provided as part of the ASI initiative COSMO-SkyMed Open Call.
“The satellite data at our disposal”, explains Eugenio Sansosti Senior Researcher of IREA-CNR, “have guaranteed a high detail in space and time unimaginable with other sensors, allowing us to measure even very small and slow ground deformations, such as those related to aseismic slips”.
This was also possible thanks to the intensification of satellite acquisitions on the Pollino area implemented by ASI, on the advice of the Civil Protection, during the seismic sequence.
The huge amount of satellite data available required an accurate and delicate processing operation. "We have used innovative techniques, developed at our Institute over the years, to trace the variations over time of the strain signal", explains Gianfranco Fornaro, Senior Researcher of IREA-CNR, "and the subsequent comparison of the results with the GPS left no doubts about the reliability of our measurements”.
The results obtained are important for the understanding of seismicity in the Pollino area. The historical testimonies of the last centuries do not show evidence of significant seismic events which instead affect the adjacent areas of the Apennines and Calabria. “Aseismic motion contributes to the release of some tectonic strain that would otherwise be released by earthquakes. This may explain why, compared to the rest of the Apennines, earthquakes of higher magnitude are relatively less frequent in the Pollino area. Further progress in understanding seismogenic phenomena in the Italian area cannot ignore observation systems such as the GPS RING network, the COSMO-SkyMed mission and the National Seismic Network", concludes Nicola D'Agostino, Senior Researcher of INGV and research coordinator .
Figure 1 – GPS station of the RING network in Mormanno (Cosenza) (source: http://ring.gm.ingv.it)
Figure 2 – Map of the velocity of ground deformation in the Pollino area between 2012 and 2014. The colored dots represent the radar measurements made by the satellite. The areas in green are still; those in red move away from the satellite with an average speed of about 2.5 cm per year; the areas in blue approach the satellite with an average speed of about 1.5 cm per year