## 4 Co-Seismic Ground Deformation
### Interferometric Data and Results  
The term SAR stands for Synthetic Aperture Radar [38]. Differential Interferometry (Differential InSAR technique-DInSAR) is an advanced technique [39] aimed at detecting surface movements due to geophysical phenomena or human interventions. Since the 1990s, the DInSAR technique has proven to be an interesting tool for measuring and observing ground deformation suitable for analyzing geodynamic processes, e.g., [40; 41; 42; 43; 44]. To map the co-seismic ground deformation due to the 27 September 2021 mainshock, we used two SAR image pairs, one on ascending and one on descending orbital geometries (Table 3), acquired from ESA's Sentinel-1A and Sentinel-1B satellites ([[https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)]([https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)), accessed on 31 May 2022). In both cases, the reference image was the one before the earthquake occurrence, while the repeat image was the one that refers to the date after the event. Each reference-repeat pair was processed using the ESA's SNAP software and two individual interferograms were generated. The topographic phase was subtracted using the SRTM 1 arc-second Digital Terrain Model, a 30-m resolution  
Figure 9: Cross-sections of vs. (%) perturbations. The cross-section traces are the same with the first four ones of Figure 5.  
Shuttle Radar Topography Mission Digital Elevation Model (USGS 1 ARC-second SRTM DEM, [[https://doi.org/10.5066/F7](https://doi.org/10.5066/F7) DF6 PQS]([https://doi.org/10.5066/F7](https://doi.org/10.5066/F7) DF6 PQS), accessed on 31 May 2022), while the signal to noise ratio was enhanced by applying the adaptive power spectrum filter of [45] with a coherence threshold of 0.4. The two-phase wrapped interferograms were then used to estimate co-seismic ground deformation. To calculate the terrain displacement, an unwrapping process was performed, and the phase unit was transformed into distance units in the satellite line of sight (LoS) for each interferometric pair. Finally, decomposition of the ascending and descending LoS displacement vectors was performed to extract the vertical (up-down) and horizontal (east-west) ground deformation components. The two wrapped interferograms are of good quality and contain the phase difference, between reference and repeat images, produced by the main seismic event and its aftershocks until the 29 th of September. Due to the low temporal geometric baselines (six days), there are no areas of low coherence in the interferograms. Six fringes, forming a lobe, are evident both in ascending and descending wrapped phase interferograms (Figure 10, upper part). This asymmetrical displacement pattern is characteristic of normal-faulting earthquakes indicating that the subsidence is larger than the uplift. Each interference fringe is a phase change that corresponds to a motion of 2.8 cm in the satellite line of sight. The LoS displacement map in the ascending geometry shows negative LoS displacement values up to 18 cm, after the Mw = 6.0 earthquake including the ground deformation caused by all the seismic events that occurred in the time interval from 23 September to 29 September, (Figure 10, lower part), while the LoS displacement map in the descending geometry shows negative LoS values up to 20 cm after the Mw = 6.0 earthquake, including all the aftershocks, occurred until the 1 st of October. On the descending orbital geometry, the maximum value of ground deformation has shifted east of the epicenter of the main earthquake (Figure 10) with respect to the ascending displacement map. After the decomposition of ascending and descending LoS displacement maps, the ground deformation in vertical (up-down) and east-west directions were extracted (Figure 11a). Subsidence up to 18 cm has been calculated from the displacement decomposition in the vertical (up-down) direction while no uplift was detected.