A POSSIBILE BREACHED RELAY RAMP CAUSING THE 2013 LUNIGIANA EARTHQUAKE (NORTHERN ITALY) Marco Polcari 1 , Salvatore Stramondo 1 , Paola Vannoli 1 , Valentina Cannelli 1 , Daniele Melini 1 , Sergey Samsonov 2 , Marco Moro 1 , Christian Bignami 1 , Michele Saroli 3 1.Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy. 2. Canada Centre for Mapping and Earth Observation,Natural Resources Canada, Ottawa, ON, Canada. 3. DICeM - Dept. of Civil and Mechanical Eng., University of Cassino and Southern Lazio, Cassino, Italy ABSTRACT We applied the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique to investigate and model the Mw 5.3 (Ml 5.2), June 21st, 2013 earthquake, occurred in the discontinuity zone between the Lunigiana and Garfagnana areas, NW Italy. We analyzed the coseismic displacement from two differential interferogram generated using COSMO- SkyMed X-band and RADARSAT2 C-band data. Both interferograms show a pattern of subsidence of a few cm located between the Lunigiana and Garfagnana basins. We then modeled the observed SAR deformation fields using the Okada analytical formulation and found them to be consistent with an extensional fault plane dipping towards NW at about 50°. Our results suggest that the fault responsible for the June 2013 earthquake may correspond to a breached relay ramp connecting the Lunigiana and Garfagnana seismogenic sources Index Terms— DInSAR, earthquakes, relay ramp, surface displacement, normal fault 1. INTRODUCTION The Lunigiana and Garfagnana areas are located in the northwestern termination of the Etrurian Fault System (EFS) which is a NW-SE trending normal fault system with a length of about 350 km from Umbria to Tuscany [1]. Historical observations and studies of seismicity showed it is an active and seismogenic zone with most of the largest (M>5) historical earthquakes occurred in proximity of the transfer zone [2] of the EFS. The present work shows the outcomes concerning the Mw 5.3 (Ml 5.2) event occurred on June 21 , 2013 (10:33 UTC) in Tuscany region, Northern Italy, between the Lunigiana area to NW and the Garfagnana to SE. The mainshock was felt over a broad area if compared with its magnitude [3], but with minor damage in the epicentral area. The focal mechanism of the main event and of its aftershocks suggests that slip occurred along a ∼45° dipping fault plane, with a prevailing extensional sense of motion [4][5][6][7]. 2. SAR DATA PROCESSING AND RESULTS We applied the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique to a multiband SAR dataset to investigate and evaluate the surface displacement. We employed two differential interferograms: one was obtained from COSMO-SkyMed X-band data and the another from RADARSAT-2 C-band. Both SAR satellites imaged the epicentral area. The Cosmo-SkyMed X-Band interferogram was obtained from two SAR images along ascending orbit (incidence angle of 40°) characterized by a time interval of one day (June 21 - June 22) and 139 m spatial baseline, covering an area of about 40x40 km2 around the epicenter. The resulting signal measures a SW-NE trending displacement field with a maximum Line-Of-Sight (LOS) distance increase of about 3 cm. The C-band interferogram was computed from two RADARSAT-2 (RDR) Standard-3 mode images (incidence angle of 34°) acquired on June 18 and July 12, 2013 (24 days temporal baseline) with 69 m spatial baseline along ascending orbit over 100x100 km2 spatial extent. The larger temporal baseline of 24 days results in a more pronounced subsidence of about 5 cm in the satellite line-of- sight, probably caused by the cumulated effect of aftershocks following the June 21st event and/or by postseismic deformation. We used SRTM DEM and ASTER 30m DEM to remove topographic phase from CSK and RDR interferograms, respectively. The processing was performed with GAMMA software [8], and the interferograms were filtered by the adaptive