## 1 Introduction  
The role of geodesy in assessing the extent and consequences of earthquakes on surface deformation has proven to be a crucial and valuable asset, enriching our understanding of seismic events in conjunction with other scientific disciplines like seismology, geophysics, and geology [1, 2, 3, 4]. Historically, geodetic measurements have been integral to surface measurement, yielding positional or combined positional and height determinations for observed points, respectively. However, until a few decades ago, geodetic processes were labor-intensive and time-consuming. The advent of technology and the evolution of space-borne geodesy in recent decades have transformed these measurements into rapid, reliable, and precise real-time solutions, introducing time as a fourth dimension in the resultant measurements. This temporal component has become pivotal in utilizing remote sensing technologies for monitoring crustal deformation, allowing for the observation and identification of Earth's surface movements during specific periods and the detection of ground surface displacements. In the realm of satellite geodesy, Global Navigation Satellite Systems (GNSS) have revolutionized geodetic measurements throughout history. GNSS technology now facilitates the real-time determination of horizontal and vertical locations worldwide within seconds, achieving unprecedented accuracy at the scale of a few centimeters or less [5; 6; 7; 8]. The current high level of GNSS measurement precision forms the cornerstone of geodetic and geophysical research, applied in monitoring short-term crustal movements, regular inter-seismic deformations associated with plate movements [9; 10], and crustal deformation monitoring [11]. Recent studies have extensively relied on GNSS observations to investigate the impact of earthquakes and subsequent ground deformations [12; 13; 14; 15; 16; 17; 18]. Conversely, the Satellite Aperture Radar (SAR) interferometric technique has facilitated near-real-time data acquisition over earthquake-affected regions with a short time span of just six days between repeated footage. Leveraging radar remote sensing, this technique operates independently of weather conditions or cloud cover. Its heightened sensitivity to the C-band phase enables the identification of seismic waves captured within the radar's line of sight. Recent studies have increasingly concentrated on earthquake analysis using SAR input data, yielding multi-temporal surface deformation assessments over an extended time scale [19; 20; 21]. On 29 December 2020, a formidable earthquake with a magnitude of Mw = 6.2 struck NW Croatia, causing substantial damage to buildings, posing a threat to human lives, and impacting the Croatian Positioning Reference System (CROPOS). The reliability of geodetic measurements hinges on the stability of the reference framework. In the realm of GNSS measurements, the reference and accuracy are defined by a national reference frame anchored by corresponding stations ensuring geodynamic stability. CROPOS, established through a network of GNSS Continuously Operating Reference Stations (CORS), achieves high accuracy and long-term stability on a subcentimeter level [22; 23]. To furnish such precision to end-users, the reference frame must exhibit exceptional stability. This stability is not only pivotal for practical applications but also serves as a crucial reference point for geodynamic and geophysical studies, where even minute-scale values play a significant role. The aftermath of the earthquake in Sisak and Glina prompted the displacement of the Cadastral Departments, while a national GNSS CORS station in the Sisak region was rendered inoperative due to substantial displacement. Motivated by these circumstances, our study delves into the analysis of surrounding national reference GNSS stations of CROPOS in a broader area around the earthquake's epicenter.