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In this work, we investigate the past and present land deformation in Al Wagan area in the United Arab Emirates. The area is primarily an agricultural region where dependence on groundwater is documented. Such a reliance on ground water resources in a region which is characterized by very low precipitation can lead to significant land subsidence as was observed in this study which identified fast and localized deformation trends. The quantification of ground deformations of large magnitude and small amplitude in this area with SAR Interferometry is a challenging task using moderate resolution data due to the incoherent surface background. Even though SAR acquisitions were sparse over this region, the available ENVISAT, ALOS and Sentinel-1A imagery was analysed with differential interferometry and the Small Baseline Subset technique in order to provide estimates about the evolution of the deformation pattern in a limited area. A clear evidence of subsidence phenomena has been identified in the study area. During the period 2003–2010 the subsidence was estimated to reach 18 cm/year as observed in the DInSAR processing results of data from ENVISAT and ALOS Satellites. However it appears to be slightly more stable during the recent past (Dec/2016–March/2018) as observed in the results with recent Sentinel-1 data where a maximum localized subsidence in the order of 10 cm was estimated. The depletion of the aquifer resources which is confirmed from groundwater level data is speculated to be the most probable cause. [ABSTRACT FROM AUTHOR]

The seasonal freeze/thaw variations of the active layer cause seasonal ground deformation, such as thaw subsidence in summer and frost heave in winter due to the cyclic phase changes of soil water. The Synthetic Aperture Radar (SAR) remote sensing, especially interferometric SAR (InSAR) techniques, has been proven to be a very useful tool for observing surface displacements of permafrost environments. Most previous studies on seasonal InSAR displacement have been concentrated in several study sites in the Arctic tundra regions. The objective of this study is to extend the InSAR analysis of seasonal ground deformations to boreal forest regions, where monitoring ground deformation is particularly important in terms of better understanding active layer dynamics under various ecological and geological conditions. In this study, we presented a strategy to analyze the time series SAR data in the freezing season to estimate the differential phase for seasonal ground deformation while overcoming the problem of coherence losses due to the rapid growth of vegetation during the short growing season. We applied the SBAS-based InSAR processing method to Sentinel-1 SAR in the central Yakutian lowlands, where boreal forests and thermokarst landforms are widely distributed, and retrieved frost-related displacement signals successfully. In addition to the magnitude of ground deformation in the freezing season, we further examined the temporal deformation pattern related to the progression of the freezing front in the soil during active freezing. Based on the Stefan solution, we proposed a piecewise linear model for the time series deformation. The proposed model divides the time series displacement according to the progression of freezing into three linear segments, and the slope parameter of each linear model can be interpreted as indicating the hydrological characteristics of the upper, middle, and lower parts of the soil. The results for the Yakutian lowlands illustrated the potential of SAR remote sensing to observe and quantify spatial details of the thermal and hydrological properties inside the active layer soil even in highly vegetated subarctic boreal forest areas. • Observation of seasonal deformation in boreal permafrost using Sentinel-1 data. • Timeseries InSAR analysis in the freezing season allows retrieval of deformation signals • It is possible to observe spatial details on frost-related ground deformation patterns. • A new model for the temporal InSAR displacement pattern is presented. • Physical properties inside the soil layer can be inferred from the temporal InSAR pattern. [ABSTRACT FROM AUTHOR]

Entering the SAR's golden era began with the launch of Sentinel-1A/B satellites in 2014 and 2016 with 6–12 day revisit time, much larger stacks of high-resolution SAR images are available over a given area to perform time series analysis. Algorithms that deal with large stack sizes face several challenges, including interferometric phase quality degradation due to signal decorrelations, phase closure error caused by applied multilooking, and tropospheric phase delay. Here, we present an improved SBAS-type algorithm suitable for processing a large stack of SAR images at an arbitrary resolution. We develop a new pair selection strategy that applies dyadic downsampling combined with widely used Delaunay Triangulation to identify an optimal set of interferometric pairs that minimize systematic errors due to short-lived signals and closure errors. We develop and apply a novel statistical framework that selects elite pixels accounting for distributed and permanent scatterers. Also, we implement a new tropospheric error correction that takes advantage of smooth 2D splines to identify and remove error components with fractal-like structures. We demonstrate the effectiveness of the algorithms by applying them to 3 large datasets of Sentinel-1 SAR images measuring non-linear surface deformation over various terrains. Compared with independent GNSS observations, we find that over the rural/natural terrains adjacent to San Andreas fault in southern California, our approach yields a standard deviation of 0.48 cm for time series differences in both ascending and descending tracks. While in urban areas, such as Los Angeles, standard deviation difference with GNSS time series is 0.30 cm. • Present an improved SBAS-type algorithm for processing a large stack of SAR images. • Develop a new pair selection strategy to identify interferometric pairs. • Apply a novel statistical framework that selects elite pixels. • Implement a new tropospheric error correction that uses smooth 2D splines. [ABSTRACT FROM AUTHOR]

The dynamics of rock glaciers in marginal periglacial environments are poorly understood, especially in Eastern Europe where the enhanced continentality produces a distinct pattern of periglacial phenomena. Multi-temporal image analysis of high-resolution optical and radar satellite imagery of the Southern Carpathian Mountains, Romania revealed the small dynamic nature and a slow geomorphologic evolution of rock glaciers over a 46-yr period of record (1968–2014). Nine rock glaciers located in glacial cirques and troughs within the central area of Retezat Mountain were included in this study. Overall, the estimated displacement rates are extremely low (i.e., a few cm/year) compared with other active rock glaciers from all over the world. Despite their relative attenuated activity, it appears that Judele, Valea Rea, and Pietrele are still active rock glaciers, but in an evident disequilibrium/imbalance with the actual climate. These findings document the lowest altitude and easternmost active rock glaciers at this latitude from Europe. Quantitative investigations were concentrated on Pietrele rock glacier, subject of recent field campaigns. Optical data analysis indicated a slight acceleration of the horizontal velocities at the surface of Pietrele rock glacier in recent years. This acceleration appears to be caused by an increase in the temperature of permafrost, resulting from an evident warming of external air temperature. Radar data analysis suggested seasonal variability of surface motion, with higher deformation in autumn, whereas in early summer or spring the deformation is negligible. Overall, the results obtained with cross-correlation analysis, interferometric synthetic aperture radar (InSAR) coherence analysis, and Small BAseline Subset (SBAS) multitemporal interferometry, are consistent, displaying similar deformation patterns with the highest creep rates located in the southern and western portions of the glacier. These findings are supported by thermal and geophysical measurements, which suggest the probable presence of permafrost within these areas. The observed displacements were interpreted as permafrost creep, and their very low velocity rates suggest the deforming frozen layers are very thin. These results provide (i) baseline information and decadal-scale trends and a (ii) strategy for future monitoring of the health and integrity of the rock glacier environment in the Carpathian Mountains. [ABSTRACT FROM AUTHOR]

The foreland fold-and-thrust belt of northern Pakistan is located at the western end of the Himalayan arc, where salt tectonics is responsible for structural styles, hydrocarbon source areas and seismicity. Three sub-regions of fold-and-thrust belt are the Potwar Plateau–Salt Range, Kohat Plateau–Surghar Range and the Bannu Basin–Khisor Range. The difference in deformation intensity between these regions is mainly attributed to the presence, or absence, of an Infra-Cambrian salt layer. This study investigates the active tectonics of the Potwar Plateau and Salt Range region, emphasizing the role of salt, using the Small Baseline Subset Interferometry (SBAS) technique and 2-D seismic interpretations. Ten PALSAR images and four seismic profiles from western Potwar Plateau–Salt Range region were used. SBAS results, derived from PALSAR images spanning from 2007 to 2010, suggest that the Potwar Plateau–Salt Range is active with the western portion of the region experiencing an uplift at an average rate of around 10 mm/year. Two uplift anomalies were observed, which were interpreted as zones of transpression and salt diapirism. Time-migrated seismic profiles show that the salt layer acts as a detachment. Our results show that salt has different roles in different regions of the fold-and-thrust belt; in the Potwar Plateau region the salt layer acts as a detachment, while in Salt Range salt flow-induced structures are prominent. The integration of SBAS and 2D seismic interpretation reveals that the deformation of western Potwar Plateau–Salt Range is influenced by two main faults, the dextral Kalabagh fault and the Main Frontal Thrust. Finally, this work was compared to the Kuqa fold-and-thrust belt in western China due to their structural similarities. [ABSTRACT FROM AUTHOR]

Abstract: We extensively exploit advanced Differential SAR Interferometry (DInSAR) techniques for enhanced landslide investigations. We focus on the Ivancich area, Assisi, Central Italy, which is affected by a deep-seated landslide investigated through in-situ surveys. For this area, large data sets of SAR acquisitions were collected by the C-band ERS-1/2 and ENVISAT sensors (from April 1992 to November 2010), and by the X-band radars of the COSMO-SkyMed (CSK) constellation (from December 2009 to February 2012). We concentrate on the advanced DInSAR technique referred to as Small BAseline Subset (SBAS) approach, benefiting of its capability to generate deformation time series at full spatial resolution and from multi-sensor SAR data. This allows us to present one of the first examples for a landslide area of ERS-1/2 — ENVISAT deformation time series exceeding 18 years. The results allowed characterizing the long-term behaviour of the landslide, and identifying sectors of the unstable slope affected by different deformation dynamics. Analysis of the CSK data set, characterized by a reduced revisit time and improved spatial resolution, resulted in a 15-time larger point density with respect to the ERS-ENVISAT measurements, allowing to investigate nearly all the buildings (and, in many cases, portions of buildings) in the landslide area. Lastly, we present an innovative modelling approach based on the effective integration of the DInSAR measurements with traditional geological and geotechnical information, providing deeper insights on the kinematical evolution of the landslide. We consider our analysis a prototype example that can be extended to different geological and geotechnical conditions, providing significant advances in the understanding of ground deformations induced by active landslides. [Copyright &y& Elsevier]

Abstract: Rockslides have a high socioeconomic and environmental importance in many countries. Norway is particularly susceptible to large rockslides due to its many fjords and steep mountains. One of the most dangerous hazards related with rock slope failures are tsunamis that can lead to large loss of life. It is therefore very important to systematically identify potential unstable rock slopes. Traditional landslide monitoring techniques are expensive and time consuming. Differential satellite interferometric synthetic aperture radar (InSAR) is an invaluable tool for land displacement monitoring. Improved access to time series of satellite data has led to the development of several innovative multitemporal algorithms. Small baseline (SB) methods are based on combining and inverting a set of unwrapped interferograms that are computed with a small perpendicular baseline in order to reduce spatial phase decorrelation. Another well proven technique is the persistent scatterer interferometric method (PSI) that is based on analysis of persistent point targets. In this paper, we apply both approaches to study several rockslide sites in Troms County in the far north of Norway. Moreover, we take the opportunity to address the difference and similarities between the SB and the PSI multitemporal InSAR methods for displacement studies in rural terrain. [Copyright &y& Elsevier]

Landslides originating from remote steep slopes render people living downhill vulnerable, unaware of the impending danger. Identifications of slow-moving mountain slopes are possible now due to time series measurement from space using microwave satellite data and the InSAR technique, which potentially can detect displacement at millimetre level. Availability of open-source Sentinel-1 data has revolutionised the study involving landslide kinematics and predicting the time of failure. However, identification of accelerating trend, demarcation of release area and prediction of flow path after failure initiation are still challenging. In this paper, we present a novel method for time and path prediction of landslides using two large landslides (Kikruma and Kotropi) located in the Himalayas in India. Sentinel-1 data stack was processed using the Persistent Scatterer and Small Baseline Subset interferometric techniques to analyse the trend of ground deformation leading to slope failures. The displacement time series of the measurement points, analysed using inverse velocity and modified inverse velocity methods, show that the instability had commenced almost a year or more with the final onset of acceleration triggered by heavy rainfall, couple of weeks prior to the actual failure. The acceleration image created from displacement time series data was clustered using image segmentation techniques to demarcate the release area of landslides. The flow simulation was done using the Voellmy friction model with a high-resolution DEM to predict the flow path. The analysis done for Kikruma and Kotropi landslide case studies with the proposed method provided a safe prediction of the time of landslide with ~90% accuracy of the flow path prediction. Results show that the method demonstrated in this study may evolve as an effective tool for landslide early warning in hilly areas. • A novel method to map landslide release area from InSAR derived slope acceleration. • PSI and SBAS are used to detect the deformation trend leading to slope failures. • Velocity (Inverse and Modified Inverse) are used for failure time prediction. • Flow path simulated using the Voellmy friction model. [ABSTRACT FROM AUTHOR]

We present a quantitative assessment of the capability of the differential SAR interferometry (DInSAR) technique referred to as Small BAseline Subset (SBAS) approach to investigate fault creep phenomena. In particular we have computed, via the SBAS-DInSAR algorithm, time series of the surface displacements relevant to the Hayward fault zone, within the San Francisco Bay Area (California), from the European Space Agency''s ERS-1/2 satellite radar data for the 1992 to 2000 time period. Starting from the DInSAR time series we measured the relative displacements across the fault with no need for any atmospheric filtering step. These results have been systematically compared to the measurements available from the alignment arrays that are located along the fault. Our analysis shows that the standard deviation of the differences between the DInSAR and the in situ measurements is on the order of 2 mm. Moreover, the estimated mean deformation rates have an accuracy that is better than 1 mm/year. [Copyright &y& Elsevier]

To address increasing water demands in expanding cities, many aquifers in Mexico are overexploited and deplete. The resulting land subsidence often combines with ground faulting/fracturing and damage to infrastructure. This study provides the longest Synthetic Aperture Radar (SAR) survey ever undertaken for the Aguascalientes Valley, aimed to constrain its structurally-controlled subsidence process and the induced risk. 275 ERS-1/2 1996–2002, ENVISAT 2003–2010 and Sentinel-1 2014–2020 C-band SAR images are processed with change detection, differential Interferometric SAR (InSAR) and Small Baseline Subset (SBAS) methods. Aguascalientes notably expanded over the last four decades, as revealed by Seasat 1978 L-band SAR, Landsat 1985–2010 and Sentinel-2 2020 optical imagery. The observed subsidence pattern involves alluvial/fluvial deposits within the N-S trending graben. Maximum settlement rates are −14 cm/year in 1996, −10 cm/year in 2000–2010 and over −12 cm/year in 2015–2020. An acceleration (−0.70 cm/year2) is recorded in 2015–2020 close to recently developed industrial plants and housing districts. Satellite estimates agree with in-situ observations, static GPS surveying and continuous GPS monitoring data. Rough correlation is found with piezometric level drop rates, whereas aquifer thickness plays a stronger role in the subsidence process. While these outcomes align with the existing literature, this InSAR survey: (i) unveils previously unknown E-W deformation affecting two N-S oriented bands within the valley, with up to ~ ±3 cm/year in 2015–2020 towards its center; (ii) identifies zones of sagging and hogging with horizontal strain (ε) of up to 0.05–0.1%; (iii) retrieves differential rates reaching 6–8 cm/year and angular distortions (β) of 1/500 along the Oriente fault; and (iv) investigates the statistical distribution of β across field surveyed faults and fissures, and marks areas with potentially yet-unmapped ground discontinuities. A new surface faulting risk matrix embedding β and ε is therefore proposed to estimate subsidence impact on properties and population. Given its scale-dependency, the risk assessment provides a lower bound to the percentage of urban areas at risk within the Aguascalientes state: at least 2% of the urban areas were at high and very high risk in 2003–2010 (involving ~12,000 properties and ~39,000 inhabitants), but this increased to 6% in 2015–2020 (~25,600 properties, ~85,200 inhabitants). The evidence of a subsidence process evolving spatially and temporally highlights the need for continuous updating of hazard information. • Subsidence in Aguascalientes is unveiled with ERS-1/2, ENVISAT and Sentinel-1 InSAR. • Up to −12 cm/year vertical and ±3 cm/year E-W rates are observed in 2015–2020. • Subsidence acceleration is recorded near new industrial/housing developments. • Angular distortions reach 1/500 and horizontal strain 0.1% along surface faults. • 6% urban areas are at high to very-high risk in 2015–2020, i.e. ~85,200 inhabitants. [ABSTRACT FROM AUTHOR]

Among the fastest sinking cities globally, the metropolitan area of Mexico City is the target of an unprecedented satellite investigation based on over 300 Sentinel-1 Synthetic Aperture Radar (SAR) Interferometric Wide swath mode scenes acquired in 2014–2020. Two-pass differential Interferometric SAR (InSAR) and the parallelized Small BAseline Subset (SBAS) repeat-pass InSAR approach provide a complete account of spatial patterns, long-term trend and present-day settlement rates affecting the city. The 3D deformation field reveals that foremost is the role of the vertical velocity V U , with peaks of −38.7 cm/year in Nezahualcóyotl, −32.0 cm/year in Gustavo A. Madero and Venustiano Carranza, and −39.1 cm/year in Iztapalapa. Settlement at the metropolitan Cathedral in Cuauhtémoc is ongoing at up to −8.8 cm/year, consistently with the last six decades. Volcanic edifices mark ground stability "islets" inside the main subsidence bowls. East-west rates are limited, except for some horizontal strain (up to ±5 cm/year) within the subsidence bowls. Comparison with surface geology and geotechnical zoning confirms that aquitard compaction is the predominant process. The power relationship between V U [cm/year] and the thickness of lacustrine clay deposits H C [m] is: V U = 1 76 ∗ H C 1.8. The 2019–2020 deformation scenario shows that subsidence still involves most of Nezahualcóyotl, with −3.0 cm/month V U. A well-defined long-term deformation behaviour comes out from 2014–2019 InSAR time series and comparison with 2008–2020 GPS data. RMSE of 0.9 cm is found at MMX1 station deployed within the lacustrine unit, and 0.6 cm at TNGF station onto hard volcanic rocks. The sharpest subsidence gradients and angular distortions β in 2017–2019 (up to over 1/400, i.e. 0.14°) – thus the greatest vulnerability to surface faulting and cracking as a consequence of large tensile stress in the soil caused by differential settlement – are found at the foothills of Sierra de Santa Catarina, Peñón del Marqués, Cerro Chimalhuachi, Peñón de los Baños and Sierra de Chichinautzin, where the transition unit is narrower (or absent). Faults and cracks develop where β > 1/3000, i.e. 0.03%, in 2017–2019. The observed width of the influence zone (i.e. damage band), where β is still significant to induce damage, is 250 m. Differential settlement and surface faulting could compromise the serviceability of housing and utility infrastructure. It is estimated that over 457,000 properties and 1.5 million inhabitants of the Valley of Mexico Metropolitan Area (ZMVM) are in zones at high to very high surface faulting risk, mainly in Iztapalapa, Tláhuac, Chimalhuacán and Chalco. Increased flood exposure due to formation of topographic depressions involves over 751,000 properties and ~2.7 million inhabitants of the ZMVM, mainly in Nezahualcóyotl, Tláhuac, Venustiano Carranza, Iztapalapa, Gustavo A. Madero and Ecatepec de Morelos. These municipalities are often hit by catastrophic floods. • Over 300 Sentinel-1 SAR scenes are used to monitor land subsidence in Mexico City. • InSAR reveals vertical displacement rates as high as −39 cm/year in 2014–2020. • Urban infrastructure is vulnerable to cracking, as angular distortions reach 1/400. • 1.5 million inhabitants live in zones at high to very high risk of surface faulting. • Increased flood exposure due to subsidence involves over 751,000 properties. [ABSTRACT FROM AUTHOR]