## 4 Data and Methodology
### SAR Data  
Sentinel-1 is the first mission of a Copernicus initiative, the European initiative for online dissemination of global satellite data free of charge. Copernicus is the European Union's Earth observation program fully operated since 2014 [66]. It is a joint program coordinated in partnership between the European Commission, European Space Agency (ESA), the EU Member States, and EU agencies providing an open information service with satellite and non-satellite Earth observation data. Sentinel-1 has two polar-orbiting satellites: Sentinel-1A and Sentinel-2B launched in 2014 and 2016, respectively. The Sentinel mission has four modes of distribution of its data. We chose the Interferometric Wide Swath (IWS) mode. Overall coverage of one Sentinel image in IWS mode is 250 km swath, 5 \(\times\) 20 spatial resolution [67]. Each Sentinel-1 IWS image is referred to as swath and is divided into three horizontal areas called sub-swaths. A sub-swath consists of nine vertical areas called bursts. Each satellite has 12 days revisit time cycle with 175 orbits/cycle. They are circulating around the globe with the path on which they travel from the South to the North belonging to the ascending orbit, while the descending orbit is directed vice versa. Therefore, the Sentinel images can be acquired in an ascending or descending orbit. The Sentinel-1 data products are distributed in three levels shown in Table 1. Since the energy of this earthquake \(M_{w}6.2\) caused effects on the building stock, such as the collapse of houses and the destruction of towns and villages, and on the natural environment, such as the activation of landslides, it was necessary to evaluate, quickly, the ground displacements of the land, in order to get a ground displacements assessment. For that purpose, we used a Level-1 images of Sentinel-1, Single Look Complex (SLC), partially processed data with preserved phase (see Table 1). Furthermore, since the most advantageous feature of SAR technology is that it enables acquiring images regardless of the clouds or any weather condition, we used interferometric wide (IW) sub-swaths. Even though SAR satellites record the distance along the radar path, it is not possible to say whether the Earth's surface has moved vertically or horizontally only by using one of the satellite orbits (ascending or descending). The displacement that is identified by SARs corresponds to the spatial displacement vector in the direction of the radar signal. To estimate ground movement, it is necessary to combine both orbits. Figure 4: Sentinel-1 SAR processing workflow. Therefore, we chose both Sentinel-1A and Sentinel-1B images. Sentinel-1A images were in ascending orbit, in relative orbit number T146, while Sentinel-1B images were in descending orbit, number T124. The two satellites are complementary, each has revisiting time of twelve days mutually spaced apart by six days, which makes it possible to get the images of the area of interest in a time span of six days. For the first stack of _before_, we have selected Sentinel-1A on 18 December 2020 and Sentinel-1B on 23 December 2020, and for _after_ earthquake state-of-the-art, we used Sentinel-1A on 30 December 2020. and Sentinel-1B on 4 January 2021 respectively (see Table 2). These images were the only ones available for the area of interest at the moment of research, which was in the first week after the earthquake. The images were selected and downloaded via Copernicus Sci Hub data site [68]. Later on, during January 2021, we downloaded two more pairs of corresponding images (Table 2) in order to analyze land displacement after the series of earthquakes. We conducted a well established procedure for interferometry processing [69] in a Sentinel Application Platform (SNAP), European Space Agency's (ESA) open toolbox for Sentinel analysis [70].