## 1 Introduction  
Land subsidence is a common geological hazard that affects many regions in the world. Depending on the nature of the region and the man-made activities around it, subsidence phenomena may be triggered by anthropogenic activities such as excessive use of groundwater, mining activities and underground construction or by natural processes such as tectonic movements [1]. Land subsidence may result in infrastructure damage, ground surface ruptures, increasing flood risk and adverse socio-economic impact in the community [2; 3]. Therefore, detecting spatial extent of the deformation pattern and monitoring its temporal evolution is crucial to determine the primary cause of subsidence and conduct preventive activities accordingly to mitigate the negative effects [4]. Over the last two decades, Bolvadin in the province of Afyon (central west of Turkey) has been severely affected by ground deformation such as subsidence, fracturing and fissuring causing severe damage to the settlements and infrastructure. A field study to the region was organized on 12 and 13 February 2019 to observe such deformations in place. Previously reported deformation regions [5] and other potentially damaged areas were visited. Some of the observed surface deformations and their reflections on the buildings are shown in Figure 1a-f. The visited regions and the location of each photograph taken are shown in Figure 2b. Fractures and fissures are clearly seen on the ground in the regions \(\mathbf{a}\) and \(\mathbf{c}\) Although repaving has removed the effects of deformation on the roads in the regions \(\mathbf{b}\) and \(\mathbf{d}\), stone walls and roadside curbs along them reveals the evidence of deformation. No clear surface deformation was observed in the regions \(\mathbf{e}\) and \(\mathbf{f}\). However, the presence of quite a few cracks and openings on the wall of the buildings were observed. Figure 1: Ground deformations observed in Bolvadin and their effects on land, roads and structures. Figure 2: (**a**) Study area (Bolyadin, western Turkey) and surrounding regions with shaded topography, active fault lines and Sentinel-1 footprints. (**b**) Visited regions and location of photographs on Sentinel-2 true color view. (**c**) Geological map of Bolvadin taken from MTA. This kind of aseismic surface deformation is thought to result from a drastic decrease in ground water table due to excessive pumping or/and drought [5] or creep on nearby active faults [6]. In order to investigate the cause of the surface deformation in the study area, excavations were carried out by [5] in two different areas across fractures that display vertical offsets. Observations on the excavated trench walls reveal that the depths of vertical offsets of 20-30 cm are too shallow (down to 5 m) and offset rate is too high (1.5 to 3 cm/year) to be of tectonic origin, that is, surface creep on active faults buried by the Quaternary alluvium. Consequently, [5] conclude that surface deformations are caused by hydrological changes, not by fault creep or other tectonic motions. However, considering the presence of a complex active fault network [7] and their geodetic observations, [6] suggest that all the surface deformation cannot be explained by ground water table drops only and tectonic creep should also be partly responsible. In order to examine the seasonal behavior of these deformations, a continuous Global Positioning System (GPS) station was established by [6] in January 2015, and collected data for 2 years. For this specific point, the vertical subsidence rate was found to be 7.1 cm/year. In addition to GPS, a leveling network was established by [8] in August 2016. The network was reoccupied in May 2017. Between 18-24 mm of subsidence was observed during 8 months along 9 profiles [8]. Although ground-based tracking techniques, such as GPS, and leveling measurements provide high-precision measurements, they are obtained at predetermined scores points only. They cannot provide detailed and comprehensive information in the spatial domain.