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
]. 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 1
a–f. The visited regions and the location of each photograph taken are shown in Figure 2
b. Fractures and fissures are clearly seen on the ground in the regions a
Although repaving has removed the effects of deformation on the roads in the regions b
, stone walls and roadside curbs along them reveals the evidence of deformation. No clear surface deformation was observed in the regions e
. However, the presence of quite a few cracks and openings on the wall of the buildings were observed.
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 scares points only. They cannot provide detailed and comprehensive information in the spatial domain. Furthermore, operating these techniques in the field is costly in terms of time and labor [9
Interferometric synthetic aperture radar (InSAR) is a powerful technique that measures ground surface deformation over a wide area with high precision [10
]. Satellite-based SAR systems provide very high spatial and high temporal resolution (4–6 days nowadays) over a large area at all weather conditions, day and night. Cost of the InSAR technique, in terms of measurement points per square kilometer, is much lower than other deformation measurement techniques such as GPS and leveling [12
]. Advanced InSAR techniques enable time series analysis of multi-temporal SAR data and provide a good set of tools for ground deformation measurements with millimeter precision [13
]. Previous studies used multi-temporal InSAR technique for analysis of land subsidence due to over exploitation of groundwater [16
], urban area deformation [21
], mining region stability [25
], landslide phenomena [28
], sinkhole detection [31
] and seismic events [33
The aim of this study is to understand the mechanism of ground deformation in the study area. To do this, we use multi-temporal InSAR technique with Sentinel-1 TOPSAR data. Sentinel-1 products that were acquired in both ascending and descending orbits between October 2014 and October 2018 were processed using Persistent Scatterer Interferometry (PSI) technique. The velocity fields obtained with PSI are compared with geology, groundwater level and water surface area of Eber Lake nearby.
2. Geographic and Geologic Setting of the Study Area
The study area, namely Bolvadin, is located on the eastern part of the Aegean sub-region of Turkey. Bolvadin is the largest town in the province of Afyon with a population of over 30 thousand. It is built on a wide fertile plain between Sultandağ and Emirdağ mountains on the Aegean coast. The plain is a rich agricultural and farming area watered by the river Akarçay and Lake Eber. Eber Lake is an important water source for the region, positively affecting economic and agricultural activities. However, the water surface area of Eber Lake varies from year to year due to the intense use of water in irrigated agricultural areas, climatic changes and newly built dams in the region [35
]. Historically, groundwater has been the primary source of water for irrigation and industrial use in the Bolvadin Region. Due to the soft alluvial sediments surrounding the Akarçay River and Lake Eber, many subsidence phenomena affect the area, making it potentially dangerous for civil infrastructures for the study area [5
Bolvadin is located in the middle part of the Afyon-Akşehir Graben system (Figure 2
), where the tectonic formation is extremely complex with frequent seismic events [6
]. In this work, the active fault database for Afyon-Bolvadin (Figure 2
) was taken from the renewed active fault map of Turkey prepared by [7
] under the establishment of the Geological Research Department of General Directorate of Mineral Research and Exploration (MTA) of Turkey.
Several studies have been carried out to investigate the causes of surface deformations in the Bolvadin region, [5
]. There is no debate about the primary cause of surface deformation which is thought to be hydrological changes. However, there are different approaches about the effects of the tectonics. While [5
] do not accept the effect of tectonic movements on ground surface deformations, [6
] suggest that even though there was no destructive earthquake, intensive micro-seismic activity, and related reasons may have affected the formation of surface deformations. Ground level measurements with GPS and leveling show the rate of deformation on specific points around faults and fissures. Although these kinds of techniques provide precise measurements, they are limited to discrete locations.
In this study, the ground deformation analysis of the entire Bolvadin region is performed by using the multi-temporal InSAR technique with Sentinel-1 TOPSAR data. Initially, LOS deformation velocity maps are produced for ascending and descending tracks as shown in Figure 4
. Later on, vertical and horizontal (E-W) displacements are generated by decomposition of ascending and descending LOS velocities, Figure 5
. It is quite clear that the southern and western parts of the city have been exposed to vertical deformation as a whole. Towards the south, the subsidence rate increases and reaches up to 35 mm/year, most probably due to increasing groundwater extraction in the agricultural fields and thickening of the alluvial deposits. Deformation rate highly correlates with the lithological units of the region as most of the vertical deformations is observed in soft alluvial sediments and approaches to zero as we move away from alluvial sediments.
In order to evaluate the possible effects of the Bolvadin fault entering the city from the east, deformation rates are examined with profiles of vertical velocity field across the fault. Figure 5
shows four profiles that perpendicularly cross the fault strike and its probable southwest continuation beneath the alluvium. Vertical and horizontal (E-W) mean deformation velocities over profiles are shown in Figure 10
. There is no obvious or consistent vertical or horizontal deformation velocity change across the Bolvadin fault that might be indicative of tectonics origins. However, especially AA’ and BB’ profiles passing through soft alluvium sediments show significant variation across the Bolvadin fault. These asymmetric deformation patterns must be mainly due to the different amount of materials types within alluvial sediments because aseismic slip on the Bolvadin fault is expected to produce subsidence not on the northern side, but on the southern side (i.e., hangingwall) of the fault. The deformation rates of the CC’ and DD’ profiles are more stable in short time periods as they are situated over more compacted lithological units. When the profiles are examined, vertical velocities in the far field away from the Bolvadin fault are in general roughly the same, that is, there is no differential motion (i.e., downthrown of hangingwall southern block) across the fault that might have been caused by creep on a normal fault that dips to the south, implying that the fault has no apparent contribution to the observed subsidence.
We compare water surface area of Eber Lake and LOS displacement time series for ascending and descending orbits. As shown in Figure 8
b,c, a clear correlation between the seasonal variation of water surface area of Eber Lake and displacement time series is revealed when the long-term linear subsidence trend is removed from the LOS displacements. There is a lag time between the peak level of the lake water surface area and the displacement time series. It can be observed from the time series of individual points that correlation of LOS displacements with seasonal variation of water surface area increases towards the south of the region. The effect of seasonal change on different regions can also be examined by looking at the local standard deviation of the mean deformation velocity. Statistics provided in Figure 7
include the whole area for each geological unit. In order to observe how seasonal variation affects different regions, additionally, the standard deviations for local regions alone are also calculated. The average standard deviation for the deformation rate increases for both dataset from 0.3 mm/year to 1.7 mm/year as from north to the south. The increase of standard deviation is a direct effect of seasonal variation. Although the water source of Eber Lake has no direct relation with groundwater, both Eber Lake water surface and ground water level are affected by climate and fed by precipitation. Moreover, LOS displacement time series for both datasets are compared with groundwater level changes. There is quite a good correlation between groundwater level changes and LOS deformation trends both in long term and short term as shown in Figure 9
a,b. As a result, the relation of groundwater level and lake water surface area with displacement time series show effects of hydrological changes to deformation.
Correlation of the high deformation rate with soft lithological units of the Bolvadin region, the similarity between deformation rate and groundwater level changes and the high correlation of deformation rate change with seasonal variations in water surface of Eber Lake suggest that subsidence phenomenon might be a direct result of hydrological effects in the region. Besides, mean deformation velocities over the profile lines AA’-DD’ suggest that there is no clear spatial evidence about the effects of the Bolvadin fault on deformation. Furthermore, the surface deformations in the a, c regions such as fracturing and fissuring are almost in the south-west north-east direction which is not parallel to the Bolvadin fault. As a result, it is very likely that those surface deformations observed in the region are the consequences of generic deformation trend affecting the whole graben system which is mainly caused by overexploitation of ground water and hydrological changes in the region.
We have mapped the ground surface deformation in Bolvadin using Sentinel-1 SAR images on both ascending and descending tracks within the time period of October 2014 and October 2018. As a result of multi-temporal InSAR analysis, two main products are produced: (i) Mean deformation velocity maps, (ii) Time-series of displacements for measurement points. The mean velocity maps show that the town of Bolvadin almost entirely subsides at different rates reaching 35 mm/year. The subsidence is taking place not only in Bolvadin, but also almost in the entire Afyon-Akşehir Graben.
We observe a strong spatial correlation between subsidence and lithology, a high similarity between deformation rate and groundwater level changes, and a temporal correlation between subsidence rate and Eber Lake water surface changes. There is no specific deformation pattern observed around the Bolvadin fault that might continue westward into the city center. As a result, we conclude that the primary cause of surface deformations is the overexploitation of ground waters and hydrological changes that affect the entire basin.
This study is the first step of the project related to the continuous monitoring of land deformation in the country scale using SAR data. The main purpose of this study is to investigate deformation phenomenon in a region-scale and extend our understanding of it to all subsidence regions across the country. Continuous monitoring of deformations throughout the country is crucial for sustainable planning and risk management. Sentinel-1 data show a high potential in the field of near real-time land deformation monitoring with wide coverage and regular data acquisition. This study suggests that the temporal and spatial behaviors of subsidence basins in Turkey can be investigated using Sentinel-1 data and PSI technique. To achieve this, all the processing steps used in this study will be updated to be capable of working on multiple processor stations and computer clusters to handle the enormous workload.