# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
## 5 Geodetical Investigations and Geological Interpretation  
For the geodetic aspect of this study, satellite-based Synthetic Aperture Radar (SAR) images were used. Recently, SAR has become a well-established technology that is commonly used in studies of land displacements [18; 19] and earthquakes [20; 21; 22; 23]. SAR images can be acquired almost in real time and regardless of clouds or any weather condition. Therefore, we used multiple spatiotemporal images that were acquired by Sentinel-1, which is a constellation of satellites of the European Union's Earth observation program [24] Copernicus. It is jointly coordinated by European umbrella institutions and provides global satellite data in real time and free of charge. The Sentinel-1 satellite constellation consists of two polar-orbiting satellites that are each equipped with a C-SAR (C-band SAR) sensor. The satellites are continuously circulating around the globe in two directions, namely ascending and descending, each with a revisiting time of 12 days, and they provide multiple images over the same area in different time periods. We used Level-1 images of the Sentinel-1 Single Look Complex (SLC) that were acquired in Interferometric Wide Swath (IWS) mode with 5x20 spatial resolution. We analysed pairs of Sentinel-1A and Sentinel-1B images, wherein Sentinel-1A images were captured in ascending orbit number T146 and Sentinel-1B images were captured in descending orbit number T124. For the before images, we used Sentinel-1A images that were captured on 18 December 2020 and Sentinel-1B images that were captured on 23 December 2020, and for the after-earthquake state-of-the-art images we used Sentinel-1A images that were captured on 30 December 2020 and Sentinel-1B images that were captured on 4 January 2020, respectively. The images were selected and downloaded via the Copernicus Sci Hub data site [25]. Interferometry processing was conducted using a well-established procedure from InSAR processing (additional details are provided in [26; 27]), and the analysis was performed in the Sentinel Application Platform (SNAP), which is the European Space Agency's (ESA) open toolbox for Sentinel analysis [28]. The most suitable feature of SAR images for earthquake detection is the phase that enables the determination of land displacement. It is expressed as a wavelength and can be mapped on an interferogram. The interferometric phase represents the difference in distance between recorded signals that were acquired from two SAR images captured with the same orbit direction. The interferograms in Figure 7 show a satisfactory correlation with the signal dominated by deformations with negligible coherence. Subsequently, such interferograms are a reliable foundation for the detection of the range and behaviour of land deformations after hazardous earthquakes. The interferograms mutually differ due to the difference in the incident angles of the orbits, which range from 36.24 to 45.78 and 35.03 to 41.73 degrees for Sentinel-1A and Sentinel-1B, respectively. Unwrapping the interferometric phase, we obtained the displacement in line of sight (LOS) of the SAR radar. It represents a spatial displacement vector in the direction of the radar signal [29]. LOS displacement is not a value that expresses the real ground movement of the Earth's surface, but it demonstrates the occurrence and implies the behaviour of land motion. The results of the SAR analysis (Figure 8) clearly show a fault zone that stretches from NW to SE, which is identified as the Pokupsko Fault, and it corresponds entirely to the geological data. The SE extension of the fault, which is not marked by clear surface ruptures, is also visible. In addition, the strike of the Petrinja Fault (NE to SW) is also recognized although it cannot be clearly identified by SAR analysis, which is likely due to the resultant vector of the dextral and sinistral movements along the fault system.