Deceleration captured by InSAR after local stabilization works in a slow‑moving landslide: the case of Arcos de la Frontera (SW Spain)

Landslides Landslides DOI 10.1007/s10346-024-02292-y Technical Note Received: 22 May 2023 Accepted: 20 May 2024The Author(s) 2024 Guadalupe Bru · Pablo Ezquerro · Jose M. Azañón · Rosa M. Mateos · Meaza Tsige · Marta Béjar‑Pizarro · Carolina Guardiola‑Albert Deceleration captured by InSAR after local stabilization works in a slow‑moving landslide: the case of Arcos de la Frontera (SW Spain) Abstract Interferometric synthetic aperture radar (InSAR) is a remote sensing tool used for monitoring urban areas affected by geological hazards. Here we analysed the effectiveness of stabilization works on a slow-moving landslide in Arcos de La Frontera (Cádiz, Spain) using a persistent scatterer interferometric approach. The works consisted on jet grouting of cement-based injections and were applied locally to stabilize the most damaged neighbourhood. We processed a large stack of Sentinel-1 SAR satellite acquisitions covering the period January, 2016, to March, 2023, and obtained surface velocity and displacement trends measured along the line of sight (LOS) of the satellite on both ascending and descending orbits. The results show a clear deceleration of the landslide head after mid-2018, suggesting the local stabilization works were effective after that time. Prior to mid-2018, the maximum LOS velocity of the landslide head was 2.2 cm/year in ascending orbit and 1.3 cm/year in the descending orbit, decreasing to 0.43 cm/year and 0.23 cm/year, respectively. The InSAR results were compared to insitu monitoring data and revealed that the extent of the stabilization has influenced a much larger area beyond the zone of the local interventions. Overall, InSAR has proved a powerful and versatile tool to be implemented in operational geotechnical monitoring. Keywords InSAR · Geotechnical monitoring · Urban landslide · Stabilization works Introduction Landslides are a globally widespread natural hazard that occurs under various environments and conditions, with the potential to lead to fatalities and economic losses. They can develop not only in mountainous regions but also in moderate relief areas with unfavourable geotechnical conditions (weak rock and soil). A landslide, broadly defined, describes the movement of a mass of geological material, which can originate from diverse sources and possess various geomechanical properties, sliding down a slope (Hungr et al. 2014). This includes a large variety of phenomena, with their own peculiar features and mechanical behaviour. Within the wide spectrum of landslide types, slow moving slides are characterized by motion rates that span several centimetres per year and can persist for years to decades. Numerous natural, human-induced, or a combination of triggering factors can initiate or accelerate the motion, such as high rainfall, removal of the supporting forces of the toe by river erosion, topographic reshaping of the slope, or increased loading by buildings. While slow-moving landslides rarely claim lives, they can cause significant damage to housing and infrastructure; moreover, they may serve as precursors to faster and catastrophic mass movements. Satellite synthetic aperture radar interferometry (InSAR) is an Earth observation (EO) technique that measures the temporal evolution of ground surface deformation along the radar lineof-sight (LOS) direction, being particularly suitable to identify and/or monitor ground deformation associated with slowmoving landslides (Colesanti and Wasowski 2006; Herrera et al. 2013; Hilley et al. 2004; Solari et al. 2020; Wasowski and Bovenga 2014). For the particular case of landslides, the geometry of the slopes is a determinant factor to take into consideration, as the radar visibility for ascending or descending SAR images might be hampered. There are several InSAR methods depending on the criteria to select the punctual measuring targets (persistent or distributed scatters) and on the interferogram network formation (single of multi-reference) (Minh et al. 2020; Osmanoğlu et al. 2016). Among them, the Stanford method of persistent scatterer (StaMPS) is suited for measuring surface deformat0ion in urban and non-urban environments when the man-made and natural targets remain coherent over time, even with low-amplitude phase stability (Hooper et al. 2004). Sentinel-1 (S1) SAR is a two-satellite constellation operated by the European Space Agency (ESA) at C-band, providing a medium spatial resolution of 20 × 5 m. It was launched in 2014 and initially provided freely accessible global data with a revisit time of 6 days until December, 2021. Subsequently, the revisit time extended to 12 days due to the complete failure of satellite S1B. This data provides an exceptional opportunity to monitor long-term motion of slow-moving landslides and analyse areal extent or speed changes of different periods (Cigna and Tapete 2021; Cook et al. 2023; Kalia 2023). In the current scenario of Big Data SAR archives, the European Ground Motion Service (EGMS) from the Copernicus Land Monitoring Service (CLMS) provides free access to high-resolution monitoring of ground deformations over most of Europe, based on full-resolution processing of all S1 satellite images available (Costantini et al. 2022). EGMS employs advanced persistent scatterer (PS) and distributed scatterer (DS) InSAR processing techniques. At the time of writing, the EGMS products cover the period February, 2015, to December, 2022, and updates are planned annually thereafter (CLMS 2023). Landslide monitoring means the comparison of landslide conditions (like areal extent, speed of movement, topography, or soil humidity) from different periods in order to assess landslide activity (Mantovani et al. 1996) and to interpret its mechanical behaviour. Designing optimal engineering solutions to minimize the intensity of landslide hazard phenomena or reduce the Landslides Technical Note vulnerability of exposed urban elements at risk should follow a thorough understanding of the process and the characterization and quantification of geo-mechanical properties. The scope of geotechnical instrumentation during the construction or operation phase of a stabilization or remedial project is to monitor field performance and to evaluate and update the design judgements if necessary (Dunnicliff 1993). The implementation of InSAR techniques in the operational geotechnical monitoring of landslides provides valuable and complementary information to in situ instrumentation data (Bru et al. 2018; Ciampalini et al. 2021; Cigna et al. 2017; Guilhot et al. 2021) and allows the identification of urban landslides based on LOS velocity distribution (e.g. Guerriero et al. 2019). InSAR has the advantage of being cost-effective, as there is no need to involve staff in field campaigns, direct data collection, and maintenance of continuous recording systems, plus there are numerous open-source InSAR processing packages (Hooper 2008; Sandwell et al. 2011; Yunjun et al. 2019) and the option to access free satellite data. Moreover, it offers spatially extensive information compared to in situ methods and provides long observation periods with fixed-time acquisitions, unlike other remote techniques. The effectiveness of engineering stabilization works on unstable slopes has been assessed a posteriori using InSAR displacement rates from different satellites (Czikhardt et al. 2017; Del Soldato et al. 2018; Di Maio et al. 2018; Liu et al. 2020) and during and after the interventions (Confuorto et al. 2019). Arcos de la Frontera is a National historic-artistic monumental town located in the province of Cádiz (Andalusia, Spain) that went through a vast urban expansion in the first decade of this century. New housing blocks were constructed on a gentle slope area underlined by weathered clayey soil of the Guadalquivir Blue Marls (GBM) formation, which is extensively present in the region and typically undergoes serious geotechnical problems. GBM are high-plasticity clays that behave as a stiff soil with very low strength parameters when they are weathered (Escolano Sánchez et al. 2019; Tsige and Corral 2013). Numerous geotechnical failures such as landslide subsidence, collapse, soil creep, and expansiveness in the Guadalquivir basin are related to this formation (Alonso and Gens 2006; Oteo 2000; Tsige et al. 1995; Uriel and Fornes 1994; Uriel and Oteo 1976). Arcos de la Frontera landslide activity has caused damages of various degrees to infrastructures, urban assets, and buildings in the New Town area since the 1970s. The most affected site is La Verbena neighbourhood, comprised of five buildings that exhibited slope motion shortly after their construction in 2007. In October, 2009, 22 families were evacuated due to severe structural damage of one of the buildings, which was definitely declared derelict in March, 2010, after an intense precipitation period. Remediation measures to locally stabilize the slope motion in La Verbena were implemented intermittently between 2011 and 2021 with a cost of 4.1 million €. Previous research studies on this landslide have focused on the geological and mechanical characterization of the process, based on InSAR measurements along with a detailed geological interpretation and urban damage distribution (Bru et al. 2017). In another study, a methodology was developed to map vulnerable buildings in urban areas affected by active landslides using this site as a test scenario (Béjar-Pizarro et al. 2017). The landslide motion rates measured in these works using InSAR techniques reached up to 3 cm/year in the satellite line of sight (LOS) during the periods April, 2011, to February, 2012, and February, 2015, to July, 2016. In the present study, we comprehensively review the technical documentation related to urban damages caused by slope instability in Arcos de la Frontera New Town area since 1970, including the field investigations and the local stabilization works commissioned by the local authorities. We carried out on-site field assessments to document urban damages and geomorphological features. We conducted an analysis of landslide activity in the Arcos de la Frontera New Town area spanning a 7-year period, from January, 2016, to March, 2023. The InSAR analysis is based on Sentinel-1 SAR data acquired in both ascending and descending orbits. The displacement time series (TS) reveal a change in the trend after mid-2018, suggesting a deceleration in sliding. We further analyse the changes in landslide activity and establish correlations with the local stabilization works undertaken in the La Verbena area. Additionally, we compare our InSAR results with those provided by the European Ground Motion Service (EGMS). Study area Geology and geomorphology Arcos de la Frontera (Andalusia, Spain) is located in a small postorogenic intramountain basin that is part of the geological domain of the Guadalquivir Basin (Sanz de Galdeano and Vera 1992). The site’s geomorphology is characterized by the erosion of different Miocene materials by the Guadalete River, which flows at the base of the town (Fig. 1). The old town is perched on a nearly vertical 100-m cliff, where calcarenites are exposed. In the gentler slopes to the west, where a portion of the new town is settled, high-plasticity silty clays from the GBM formation are prevalent (VORSEVI 2009). Moving towards the lower part of the slope, the GBM exhibits a higher clay content, contributing to a smoother landscape with slopes measuring less than 15°. Alluvial plain materials are deposited in the inner bank of the Guadalete River. Soil samples collected at the head of the landslide and at the Tablellina water channel during various field investigations, as documented in technical reports (García 1970; VORSEVI 2009, 2010), reveal a distinct stratification in the GBM. The upper layer consists of weathered high-plasticity GBM, identified as brown silty clays with variable thickness, transitioning to an increased clay content. Immediately below, at depths of around 15–20 m, unweathered high-plasticity GBM are found, characterized by a green-grey colour. Dynamic probing super heavy (DPSH) and standard penetration tests (SPT) conducted in these investigations classified the consistency of weathered GBM as soft to medium in the shallower part and stiff to hard below. Meanwhile, the unweathered GBM were classified as hard. The investigated landslide develops on the western slope where the GBM formation is present (Fig. 1). According to the classification by Hungr et al. (2014), it falls into the category of a planar slide earthflow. The displaced material initially undergoes planar slide movements before transitioning into a flowing state. The lower sections of the slope are the first to fail, primarily influenced by river erosion. Consequently, new slides form through a series of progressive retrogressive failures upslope (Béjar-Pizarro et al. 2017). The location of the main scarp is determined by the contact between the calcarenites and the GBM formation. The cross section of the slope in Fig. 1b illustrates the planar surface Landslides and shallower slips, as interpreted from field characterization along with inclinometric data (which will be further described in the “Geotechnical investigations and local stabilization works” section). The head of the landslide covers a relatively flat area of 0.17 km2 and is entirely urbanized by the New Town (Figs. 1 and 2). In the medium and lower parts of the slope, soil piping and creeping phenomena are observable, along with mudflows exhibiting lobular morphologies (Fig. 3). The principal direction of motion of the landslide in La Verbena area is identified as 330° NE, following the local maximum slope path, as observed in numerous pavement cracks (inset Fig. 2) and supported by geotechnical instrumentation data (Cobo 2021; MCH 2009). These cracks indicate the alignment of movement along this direction. Additionally, cracks observed in buildings parallel to the main scarp line suggest a N-S component of movement in the middle and eastern header area. Urban development and damages Arcos de la Frontera has a population of more than 30,000 inhabitants. Approximately the 80% of its buildings were constructed after the 1960s, being the most productive construction period the 1995–2009 (Figs. 2 and S1 in the Supplementary Material) Fig. 1 a Location and geological map of the studied area. Buildings belonging to the Arcos de la Frontera Old Town are depicted in a different colour than those constructed after 1960 (New Town). La Verbena neighbourhood is situated at the head of the landslide, and the Guadalete River flows at the foot of the landslide. b Geological cross section illustrating the complex landslide. Please note that the cross section is in proximity to inclinometer S-1 but does not directly intersect with it Landslides Technical Note coinciding with the Spanish housing bubble. The historical old town flourished between the fifteenth to eighteenth centuries and settles on the top of a calcarenite ridge; meanwhile, the newer areas are located over gentler slopes at its sides. The geological materials underlying the western New Town area are the GBM. Evidence of mass movements in this area was documented as early as the 1970s (García 1970), with severe impacts on linear infrastructures such as the artificial water channel known as the Tablellina and the old railway station (Figs. 3 and S2 in the Supplementary Material). The latter was dismantled at the beginning of the 1980s. In 1995 a new Urban Development Plan was approved by the municipality (Table 1) and new, large areas around the town were declared as urban (PGOU 1995), including the head of the landslide. La Verbena neighbourhood is composed of five buildings constructed between 2001 and 2004, starting from those located further West at the top of the slope (inset Fig. 2). Each building is divided by two or three expanding joints and the type of foundation is reinforced concrete continuous slab. First damages appeared in building R-6 (inset Fig. 2) in 2007 in the form of cracks in partition walls, pillars and pavement, the opening of the dilatation joint, and the breakage of sewer and water systems (Dictum 2014). The rest of the buildings were gradually affected by the same structural issues (Fig. S3 in the Supplementary Material), decreasing the degree of damage from R-5 to R-2 (EDARTEC 2009; EXPERTA 2009). By October, 2009, severe structural damage of building R-6 led to the evacuation of 22 families. The reinforced concrete slab forming the foundation of the building had failed in the area near the expansion joint. This caused the settlement and tilting of the southern part of the building with respect to the other (Fig. 4), as well as the appearance of horizontal cracks in the pillars near the expansion joint and damage to the structural elements of the floor slabs. The opened joint resulted in a preferential entry of rainwater. Between December, 2009, to February, 2010, more than 800 mm of accumulated rainfall aggravated the damages and building R-6 was declared derelict by the Local Government Board. The opening of this joint in the NS direction measured in the western facade was 10 cm in 2009 (MCH 2009). In December, 2022, we measured in the same place 60 cm of opening in the NS direction, 32 cm in the WE direction, and a difference of 40 cm between buildings height vertically (Fig. 4). Geotechnical investigations and local stabilization works Geotechnical investigations in the La Verbena area have been ongoing since 2009, encompassing a comprehensive characterization Table 1 Timeline sequence of reported damages associated with landslide motion, building construction, and local stabilization works Year Events 1970 A technical geological report by García (1970) identifies severe damages in the Tablellina water channel and around the old railway station caused by shallow slip processes occurring in the weathered GBM formation. 1995 The land use designation of the area where La Verbena is now settled was changed to urban land (PGOU 1995). 2000 The geotechnical investigations carried out before the construction of La Verbena buildings (TEDECO 2000) did not consider landslide hazard. 2001–2004 Construction of La Verbena buildings. 2007 First damages reported in building R-6: cracks inside the flats and opening of the dilatation joint (Dictum 2014). 2009 Preliminary survey reports and analysis of pathologies of La Verbena buildings exposing the building degradation (EDARTEC 2009; EXPERTA 2009). An expert report at the end of the year recommended declaring building R-6 derelict (MCH 2009), leading to the evacuation of 22 families. Subsequently, geotechnical works were initiated (VORSEVI 2009) to investigate possible stabilization measures. 2010 Between December, 2009, to February, 2010, more than 800 mm of accumulated rainfall aggravated the damages in building R-6, which was finally declared derelict by the Local Government Board. The geotechnical works continued (VORSEVI 2010). 2011–2014 Phases I–II (BOE 2011a, b) of local stabilization works in La Verbena, involving water drainage and jet grouting, with a total cost of €2.6 million. 2014 The families evacuated from building R-6 lose their trial against the construction company. The court determined that the damages were a result of the landslide and not due to building defects (Dictum 2014). The company argued that the slope failure was unpredictable and cited the Urban Development Plan of the city (PGOU 1995), which did not mandate a landslide hazard analysis due to the characteristics of the La Verbena site—specifically, the slope inclination being less than 15%, and the affected area or volume being less than 2500 m2 and 5000 m3, respectively. 2018–2021 Phase III of local stabilization works in La Verbena, involving jet grouting with a total cost exceeding €1.5 million (Moncloa 2017). 2023 Planned works for phase IV involve the renovation and restoration of urban elements, with an estimated cost of €1.5 million. Landslides of materials through in-situ and laboratory testing. This data is documented in various technical reports commissioned by the local authorities (Table 1). It is important to highlight that these reports, along with the plans for implementing the stabilization works, did not approach the complex landslide as a holistic problem. Instead, the focus was specifically directed towards addressing the most damaged area, which is La Verbena. On the contrary, research conducted using InSAR, geological and geomorphological mapping, and urban damage inventory (Béjar-Pizarro et al. 2017; Bru et al. 2017) revealed that the extension of the active area goes beyond La Verbena, and they delineated the boundaries of the complex landslide. The field investigations described in the technical reports involved the installation of manual recording piezometers, inclinometers, and crack gauges, as well as tachymeter surveys to monitor building movements and cross-hole seismic surveys to analyse changes in soil strength parameters after the stabilization works (which aimed to enhance the geotechnical properties of the soils). All of these aspects will be further explained in this section. The initial geotechnical report, conducted before the construction of La Verbena, analysed soil performance to a depth of 10 m (TEDECO 2000). Despite evidence of previous damages near this site (Fig. 3) to the old railway and Tablellina channel (García 1970), the report failed to identify potential landslide hazard. Subsequently, after the complex landslide started causing damage to the buildings in 2007, more detailed investigations were conducted. In July, 2009, crack gauges were installed inside building R-6 in response to emerging damages. Monthly readings were conducted until November, 2009, revealing a progressive opening of cracks with a maximum displacement of 5 mm over the 4-month period. This observation exposed the gradual degradation of the structure (EXPERTA 2009). To investigate landslide activity, two inclinometers and three piezometers were installed in La Verbena (VORSEVI 2010). Only two manual inclinometric readings were conducted between September, 2008, and February, 2009. Inclinometer SS-1, with a length of 25 m, detected a basal slip surface at a depth of 24 m, exhibiting approximately 30 mm of cumulative landslide displacement towards the maximum slope direction. In contrast, in inclinometer SS-2, the basal slip surface was distinctly identified at a depth of 6 m, displaying cumulative displacements of less than 30 mm towards the maximum slope direction. Other inclinometers installed below the landslide head in 2003 identified a shallow slip surface at 4.5 m (S-1) and 7 m (S-2) (VORSEVI 2003). Phreatic level measurements on 11/09/08 indicated a level at 9 m depth in SS-1, while SS-2 was dry. The only available data from the piezometers, measured on 03/09/2010, showed phreatic levels Fig. 2 Stages of building construction in the New Town of Arcos de La Frontera (see Fig. S1 in the Supplementary Material). It highlights both, the landslide and urban features. Circled letters indicate sites corresponding to the photos shown in Figs. 3 and 4. The upper right inset showcases the La Verbena neighbourhood, constructed between 2001 and 2004, with discernible damages indicated by surface cracks and openings of building dilation joints. The five buildings comprising La Verbena are labelled R2 to R6 Landslides Technical Note at 2.5 m depth in SP-1, 4.5 m depth in SP-2, and 3.2 m depth in SP-3. These measurements were taken 2 years apart. Between December, 2009, and February, 2010, the region experienced a substantial accumulation of over 800 mm of rainfall, which could potentially explain the observed rise in the water table, reaching almost 5 m (Bru et al. 2017). High-precision tachymeter surveys were performed over La Verbena buildings between February, 2013, and June, 2021, to monitor their movement during the stabilization works. Cross-hole seismic pair surveys were also performed between 2017 and 2021 to analyse the soil properties before and after the last phase of stabilization works (Cobo 2021). The primary purpose of the stabilization measures was to enhance the geotechnical properties of the soil beneath La Verbena neighbourhood, located at the head of the landslide, to mitigate slope movements at this specific location. The works involved draining the western part of the slope head and implementing jet grouting via boreholes to improve both bearing capacity and relative density of the ground. Phases I and II of jet grouting works were carried out Fig. 3 Photographs of landslide impacts and features. a, b Soil piping at the old railway (site A in Fig. 2) in 1970 and 2022, respectively. c, d The Tablellina water channel in 2017 (site B in Fig. 2) in 2017 and 2022, respectively. e Creep processes and mudflows forming lobular morphologies (site C in Fig. 2) Landslides in La Verbena neighbourhood soils between 2011 and 2014, starting in the areas labelled as TA-1 and TA-2 in Fig. 5a. Small diameter holes (76 mm) were drilled with inclinations ranging between 0° (vertical) and 45°, reaching depths up to 40 m and a drill spacing between 1 and 3.5 m (Fig. 5a and b). The jet grouting scheme is shown in Fig. 5c. A low-mobility cement mortar with high internal friction (Abrams cone between 3 and 8 cm) was pumped with a flow ranging between 10 and 90 L/min at high-pressure (between 5 and 30 bars). The pumped mortar induced lateral displacement of the soil in the vicinity of the application points, resulting in densification, hardness, and increased strength of the soil surrounding the treated area. Additionally, the reorganization of soil particles significantly reduced the percentage of voids. The vertical stress in the treated soil layer ensured that the low-mobility mortar displaced the soil horizontally without causing uplift at the surface. Tachymeter surveys in 2014 showed that the movement of the buildings in the areas TA-1 and TA-2 had not only stopped but was also partially reversed. However, the following survey in October, 2017, detected movements downslope in buildings R-2, R-3, and R-4, highlighting the urge of a third phase. Phase III started in February, 2018, and the first operation was the controlled drainage of La Verbena area with the objective of increasing shear strength and achieve primary soil consolidation. Hereafter the jet grouting works were performed in the zone TA-3. The mortar injections also facilitated soil water drainage, but disabled the draining pipes. Tachymeter surveys conducted in August, 2019, and February, 2020, indicated stabilization in area TA-3. However, adverse movements were detected in buildings within TA-1 and TA-2, leading to the spatial densification of injections along the same axes as the previous ones. The latest survey in June, 2021, revealed a reversal of movement in the zone corresponding to TA-2, showing upward movements with respect to the slope of the hillside. No movement was detected in TA-1 and TA-3 at that time. The cross-hole seismic surveys in the TA-2 zone, conducted in 2017 and September, 2021, aimed to analyse changes in strength soil parameters before and after the jet grouting works of phase III. The results revealed a notable increase in the bulk modulus (K) by 33 to 115% and an increase in the shear modulus (G) between 60 and 250% at depths of 10 to 20 m. This enhancement in both K and G signifies improved stiffness (resistance to compression) and strength (resistance to shear forces) in the treated soils. The significance of these parameter increases lies in reinforcing the stability of the slope. They directly contribute to the soil’s resistance against sliding and shear forces, thereby enhancing load-bearing capacity. Additionally, the filling of microfissures within the GBM formation reduces permeability, minimizing water infiltration and preventing further degradation of the structural integrity of the slope. The jet grouting process induced the horizontal propagation and impregnation of the cement mortar, extending several tens of metres (more than 100 m in some cases) from the injection points at La Verbena. This suggests an improvement in geotechnical soil properties in a much wider area. However, it is important to note that the Fig. 4 Photos of building R-6 (site D in Fig. 2) showing the gradual opening of the dilatation joint through the years a 2009 (Google StreetView), b 2015, and c 2022. d Graphical representation of the displacement of the R-6 building from the time of its construction to the present day (2023) Landslides Technical Note effectiveness of these improvements beyond the head, particularly in the middle part and foot of the slope, remains uncertain due to data limitations. Therefore, while positive effects on the stability of the landslide head are observed, the overall improvement in stability across all regions of the slope cannot be conclusively stated. At the time of writing, the fourth phase had not yet started. It will consist on the underpinning of the foundations of the buildings, improvement and reinforcement of soils, drainage, surface protection, and repair of wet networks, with an estimated cost of 1.5 million €. Fig. 5 a Map showing the location of the grouting boreholes in La Verbena neighbourhood. b Design of the inclined grouting boreholes that intersect the local shear surface (dashed line). c Jet grouting scheme Landslides InSAR monitoring Data and processing We used SAR images acquired by the Sentinel-1 satellite in the Interferometric Wide (IW) swath mode to measure ground movements over the studied area. Two stacks of 172 ascending (track 147) and 179 descending orbit (track 154) acquisitions were processed separately, covering the periods January, 2016–March, 2022, and May, 2016–March, 2022, respectively (Table 2). We limited our analysis to Sentinel-1A (S1A) data having a 12-day repeat cycle. Furthermore, we conducted a more recent InSAR analysis spanning from June, 2022 (coinciding with the completion of the stabilization works), to March, 2023, in both orbits. The list of SAR images is available in Tables TS1, TS2, TS3, and TS4 in the Supplementary Material. errors. For the phase unwrapping we used the 3-D method (Hooper 2010), which is performed both spatially and temporally. We opted for a short unwrapping time window of 10 or 30 days to prevent irregular trends in the time series caused by reaching extreme values of − 𝜋 ∕+𝜋 . Additionally, we chose a small unwrapping grid size of 50 m to prevent undersampling of the signal, considering the localized nature of the phenomenon under study and its reduced spatial size. Finally, spatially correlated DEM and orbit errors were estimated and subtracted. A comprehensive description of the StaMPS processing chain steps and its parameterization is provided in the StaMPS/ MTI Manual (Hooper et al. 2010). With this analysis we have retrieved the average line-of-sight (LOS) ground deformation rate maps and displacement time series of the four SAR stacks. Post‑processing We have applied a moving average window of three acquisitions to smooth the displacement time series. In addition, we calculated the 2-D deformation field (eastward and vertical motion) from the PS results combining the two different viewing geometries of the ascending and descending satellite orbit passes, and assuming that the north-south component is negligible. Firstly, we rasterized the PS measuring points of both orbits into a common grid with a pixel size of around 60 × 60 m. Then, we calculated for each pixel the vertical and east-west velocity with the raster calculator tool of the open-source software Q-GIS, using Eqs. (1) and (2) (Béjar-Pizarro et al. 2017), where • Hd,Ha = vertical directional cosine of descending and ascending LOS local unit vector; • Ed,Ea = east–west directional cosine of descending and ascending LOS local unit vector; • vLOSd,vLOSa = mean velocity in the line of sight (LOS) of the descending and ascending satellite orbit modes. (1) V east−west = ( vLOSd Hd − vLOSa Ha ) Ed H d − Ea H a (2) V vertical = ( vLOSd Ed − vLOSa Ea ) Hd E d − Ha E a Table 2 Characteristics of the two datasets processed Satellite Dataset Orbit N. of images Track First image Last image Years Ref. image S1A 1 ASC 172 147 13/05/2016 31/03/2022 5.9 16/04/2019 DES 179 154 15/01/2016 20/03/2022 6.2 26/10/2019 2ASC 51 147 02/06/2021 02/03/2023 1.7 06/01/2022 DES 51 154 05/06/2021 19/02/2023 1.7 12/02/2022 For the data processing we have employed the open source python scripts snap2stamps and the persistent scatterer (PS) method of the StaMPS software package (Hooper et al. 2007, 2004). The snap2stamps scripts operate within the open-source ESA SNAP software, facilitating the automated generation of single reference interferograms from Sentinel-1 data and exporting them to the StaMPS format (Foumelis et al. 2018; Delgado Blasco et al. 2019). The single reference approach involves using one reference SAR image from which all the interferometric pairs are generated. The reference images presented in Table 2 were selected to minimize the dispersion of the perpendicular baseline in each SAR stack as much as possible. To correct the topographic contribution to the radar phase in the interferograms, we used a digital terrain model of the project PNOA-LIDAR (from the National Center of Geographic Information; CNIG) with a resolution cell of 5 m. The StaMPS software package extracts ground deformation data throughout the entire SAR observation period. We have processed a small area of 300 km2 in both geometries. StaMPS carries out the initial selection of PS candidates based on the amplitude dispersion (DA), which is defined as the ratio between the standard deviation and mean deviation of the amplitude over time. We chose a DA threshold value of 0.4 to minimize random amplitude variability and exclude pixels exhibiting high temporal decorrelation. The final PS selection is based on a combination of amplitude and estimated phase stability, rejecting those that appear to be persistent only in certain interferograms. After the PS are selected, the PS weeding process drops those that are due to signal contribution from neighbouring ground resolution elements and those deemed too noisy. For the noise-based weeding, we have selected standard deviation values ranging from 0.8 to 1.2 that balances selecting the maximum amount of points with reducing the noise levels. Then, the wrapped phase is corrected for spatially uncorrelated DEM Landslides Technical Note Berti M, Corsini A, Franceschini S, Iannacone JP (2013) Automated classification of Persistent Scatterers Interferometry time series. 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Mateos · Marta Béjar‑Pizarro · Carolina Guardiola‑Albert Geohazards InSAR laboratory and modelling group (InSARlab), Geological Risk and Climate Change Department, Geological and Mining Institute of Spain (IGME-CSIC), Ríos Rosas 23, 28003 Madrid, Spain Email: [email protected] Jose M. Azañón Department of Geodynamics, Faculty of Science, University of Granada, Av. de la Fuente Nueva, 18071 Granada, Spain Meaza Tsige Department of Geodynamics, Stratigraphy and Paleontology, Faculty of Geological Sciences, Complutense University, José Antonio Novais, 12, 28040 Madrid, Spain