GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 2.1 23 migration of the Po River and other smaller watercourses during the Late Quaternary. Recent horizontal GPS measurements (Serpelloni et al. , 2006; Devoti et al. , 2011) suggest ~1 mm yr -1 of shortening, confirming the occurrence of active thrusting in the area. The epicentral area of the 2012 Emilia seismic sequence is characterized by low historical seismicity. Since at least the year 1000 A.D. this area has never experienced earthquakes of magnitude larger than ML ~ 4.7 [1574 Finale Emilia earthquake: Rovida et al. (2011)], whereas larger magnitude earthquakes occurred west and east of the 2012 epicentral area. The 2012 sequence shows two mainshocks occurred on May 20 and 29, 2012, of magnitude ML 5.9 and 5.8, respectively. Pezzo et al. (2013) investigated the related coseismic surface deformation using COSMO-SkyMed and Radarsat-1 satellite SAR images and GPS data: SAR and GPS data inversions suggested that the main activated faults were the Ferrara thrust (for the May 20 shock) and the Mirandola one (for the May 29 shock). They also observed that the maximum deformation measured by InSAR data corresponds to the top of the Mirandola and Ferrara folds: in the long-term, the growth of these folds cause the wide northward bend of the Po river course and the deviation of the Secchia and Panaro rivers, as suggested by Burrato et al. (2003). An interesting finding of Pezzo et al. (2013) is an aseismic deformation, well modelled by slip on the eastern side of the frontal Mirandola thrust later activated during the May 29 mainshock, occurred in the time span between the two mainshocks. InSAR data. We processed InSAR data sets for the X-Band (COSMO-SkyMed) and C- Band (ERS and Envisat) sensors to retrieve the largest possible spatial and temporal coverage. We used both ascending and descending COSMO-SkyMed image data sets, spanning the June 2012- December 2013 time interval, to study the post-seismic deformation, as well as ASAR and ERS image data sets, spanning from 1992 to 2010, for the inter-seismic period (Tab. 1). Using the multi-temporal InSAR technique named SBAS (Small BAseline Subset, Berardino et al. , 2002), we obtained mean ground velocity maps and displacement time series for each coherent pixel in the temporal span of the processed images (Figs. 1 and 2). We use time series and mean ground velocity retrieved from CGPS to calibrate InSAR velocity maps and remove spurious signals during the SAR data processing. The CGPS displacement rates are used to validate the InSAR results. Tab. 1 - Available SAR data set for COSMO-SkyMed image beams (Stripmap mode), ERS and ASAR sensors for both ascending and descending geometries, covering almost the entire Emilia Region. Sensor Orbit Track-Frame Temporal Span N° of images N° of pairs COSMO-SkyMed Ascending 05 west 8/7/12-28/12/13 26 86 COSMO-SkyMed Ascending 05 east 9/7/12-17/11/13 22 70 COSMO-SkyMed Ascending 04 13/7/12-23/12/13 25 88 COSMO-SkyMed Ascending 03 30/5/12-14/8/13 17 48 COSMO-SkyMed Ascending 10 4/7/12-14/12/13 34 128 COSMO-SkyMed Ascending 11 28/7/12-26/12/13 39 251 COSMO-SkyMed Descending 03 20/6/12-20/12/13 25 75 COSMO-SkyMed Descending 05 6/7/12-16/12/13 25 76 ERS Descending 165-2705 30/9/92-5/6/99 36 134 ASAR Descending 166-2705 25/1/03-21/8/10 33 139 ASAR and ERS mean velocity maps (1992-2010) show deformation patterns (Fig. 1) ascribable to different natural and human phenomena. We observe some strong subsidence patterns (red colours) mainly due to the intensive water pumping carried out in the Po plain. Subsiding patterns are mainly located in correspondence with the maximum thickness of soft