Comparing geological and Persistent Scatterer Interferometry data of the Sele River coastal plain, southern Italy: Implications for recent subsidence trends

Geomorphology 351 (2020) 106953

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Comparing geological and Persistent Scatterer Interferometry data of the Sele River coastal plain, southern Italy: Implications for recent subsidence trends Vincenzo Amato a, Pietro P.C. Aucelli b, Giuseppe Corrado b,⁎, Gianluigi Di Paola a, Fabio Matano c, Gerardo Pappone b, Marcello Schiattarella d a

Dipartimento di Bioscienze e Territorio, Molise University, Pesche, Isernia, Italy Dipartimento di Scienze e Tecnologie, Parthenope University, Naples, Italy c Istituto Scienze Marine (ISMAR), Consiglio Nazionale delle Ricerche, Naples, Italy d Dipartimento delle Culture Europee e del Mediterraneo (DiCEM), Basilicata University, Matera, Italy b

a r t i c l e

i n f o

Article history: Received 25 July 2018 Received in revised form 1 October 2019 Accepted 6 November 2019 Available online 07 November 2019 Keywords: Coastal geomorphology Subsidence rates DInSAR data Southern Italy

a b s t r a c t Many coastlines around the world are subsiding, often due to tectonic causes. In tectonically active areas, the exploitation of natural resources has caused acceleration of subsidence rates. In these cases, it is difficult to separate the many different components of the subsidence process. We try to fill this gap, using both remote sensing data and geological evidence, in an area located in southern Italy (Sele River plain) and characterized by the presence of both human activities and natural processes. The coastal sector of the Sele plain shows a complex subsidence pattern, as testified by Persistent Scatterer Interferometry (PS InSAR) data related to a period of about two decades (1992–2010), that we put in correlation with stratigraphical data from both outcrops and boreholes (ca. 250). The Quaternary infill of the coastal sectors of the plain is not laterally isotropic and homogeneous because of the presence of layers of clastic sediments with different degrees of compaction. Dunal and beach sands, back-ridge (lagoonal and palustrine) silty clays, palaeosols and thick peaty layers are the main facies of this sedimentary prism. The back-ridge depressions hosted palustrine and marshy environments and were artificially drained in the last centuries. The main output of the PS-InSAR analysis is a map of vertical ground deformation (VGD), which has been a useful tool to understand the causes of the subsidence process if combined with geological data from field and boreholes. In fact, the overlay of the SAR interferometry VGD map with the geomorphological and stratigraphical features shows that the subsidence is higher in the northern sector of the plain, where the Quaternary alluvial-coastal deposits are thicker because of the structural asymmetry of the graben. The maximum subsidence rate, however, can be observed in the central part of the plain, in correspondence of the Sele, Tusciano and Picentino river mouths and in the coastal belt. This could be related to the thickness of loose sediments (N20 m) and to the presence of still compacting clay-rich sediments. In the southernmost sector the rates are the lowest recorded in the plain due to the minor thickness of Holocene deposits and to the presence of very thick sedimentary bodies of travertine. The understanding of subsidence patterns may represent a basic tool for future studies about the assessment of the coastal inundation hazard and related risk mitigation in the Sele River plain. Such a tool may reinforce the methodological approach to risk assessment in similar morphological settings. © 2019 Published by Elsevier B.V.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . Site description . . . . . . . . . . . . . . . . . . 2.1. Regional framework . . . . . . . . . . . . 2.2. Geological setting of the study area . . . . . 2.3. Geomorphological and land-use characteristics

⁎ Corresponding author. E-mail address: [email protected] (G. Corrado).

https://doi.org/10.1016/j.geomorph.2019.106953 0169-555X/© 2019 Published by Elsevier B.V.

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Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Borehole inventory and stratigraphic analysis . . . . . . . . 3.2. Vertical movements from DInSAR data . . . . . . . . . . . 3.3. GPS measurements . . . . . . . . . . . . . . . . . . . . 4. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Quaternary stratigraphy . . . . . . . . . . . . . . . . . . 4.1.1. Eboli supersynthem (CE) . . . . . . . . . . . . . 4.1.2. Battipaglia-Persano supersynthem (BP). . . . . . . 4.1.3. Aversana supersynthem (AS) . . . . . . . . . . . 4.1.4. Salerno supersynthem (SA) . . . . . . . . . . . . 4.2. Subsidence data . . . . . . . . . . . . . . . . . . . . . 4.2.1. LOS velocity fields . . . . . . . . . . . . . . . . 4.2.2. Vertical components of ground deformation velocity. 4.2.3. Subsidence map . . . . . . . . . . . . . . . . . 4.3. Stratigraphy and subsidence relations . . . . . . . . . . . 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Coastal belts of active orogenic chains are sensitive areas in terms of climate-tectonics interplay. Much interdisciplinary work currently focuses on the possible effects of global warming on coastal areas worldwide, and many recent papers were devoted to the estimation of flood and erosion risks in low-lying coastal areas as a consequence of increasing ice melting and thermal expansion of oceans (Meehl et al., 2012; Schaeffer et al., 2012; IPCC, 2014). In relation to flood risk and coastal vulnerability, the evaluation of rates and causes of the vertical movements must be taken in great consideration because these ground motions can be added or detracted to the effects of sea-level rise associated with global warming, so foreshadowing the possible increase or decrease of marine flooding and erosion (Wӧppelmann et al., 2009; Wöppelmann and Marcos, 2016; Di Paola et al., 2017). Many coastlines around the world are tectonically subsiding. The exploitation of natural resources (oil and fluid extraction, mining) in the past half-century has caused a dramatic acceleration of subsidence rates in several areas (Bird, 1993; Milliman and Haq, 1996). In Italy, many coastal and alluvial plains are currently subsiding, especially in the Po River plain and the Arno River plain (Cenni et al., 2013). Some areas of the Po plain, such as the drained land around the cities of Ravenna and Ferrara, are even below sea level and continue to subside (Teatini et al., 2012). We have adopted a multidisciplinary approach to define the stratigraphic architecture and the morphological setting responsible for the current vertical land movements of coastal plains of the Tyrrhenian flank of southern Italy. Our main aim is to identify subsiding zones featured by homogeneous geological and morphological conditions, taking into account the occurrence and thickness of compactable layers and the presence of more or less erosion-resistant landscape units. We also took into account land-use data to better define the role of anthropic impact, as recently performed in other subsiding areas (e.g. Chaussard et al., 2014). In this work, we present the detailed mapping of subsidence patterns from the flat coast of the Sele River coastal plain (Fig. 1), combining satellite-based Synthetic Aperture Radar (SAR) interferometry (InSAR) data, referred to a large time span (1992–2010), and geological evidence from both outcrops and boreholes (ca. 250). We wish to stress that local vertical ground motions of the Sele plain and, overall, the causes that provoked them are poorly known, despite they can contribute to define future risk scenarios. Based on satellite Permanent Scatterers radar interferometry (PSInSAR) data over the period from 1992 to 2000, Vilardo et al. (2009) showed that the Campania coastal plains are affected by strong subsidence. Yet, some remarks about the role exerted by tectonics and local

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faulting in the Sele River plain have been discussed here (cf. Section 5. Discussion) on the basis of PS-InSAR data from a longer time window and of more in-depth geological and geomorphological analyses. Satellite data allows accurate high-resolution mapping of the trend of the ground motions (Vallone et al., 2008; Kim et al., 2009; Martino et al., 2009; Perski et al., 2009; Hsieh et al., 2011; Amato et al., 2017). In detail, the largest subsiding area are located in the northern Campania sector with the coastal and alluvial plain of the Volturno River (Aucelli et al., 2016; Matano et al., 2018) and in the southern Campania sector with the coastal plain of the Sele River (Pappone et al., 2012; Di Paola et al., 2017). Based on stratigraphic and geochronological data, Pappone et al. (2012) recognized different rates of vertical land movement for the northern and southern sectors of the Sele coastal plain. These authors evidenced differential coastal subsidence behaviour: the northern sector shows a mean rate of 0.4 mm/year from 5.0 ky BP to 1.0 ky BP, while the southern sector appears almost stable. More recently, Di Paola et al. (2017) confirmed a south-eastward decrease of vertical subsidence rates along this coastal sector on the basis of PS-InSAR data, referred to 1992–2000 time span. These papers provide some hypotheses about the relationships between geological setting and ground motions, but are focused on the consequences of the subsidence phenomena with regard to coastal flooding hazard. Further, it is worth noting that we elaborated data from a longer chronological interval (1992–2010) and constrained some satellite data by means of DGPS measurements in the field, focusing on the genetic causes of the subsidence.

2. Site description 2.1. Regional framework The meandering lower reach of the Sele River extends over about 40 km of the Campanian coast, limited landward by the inner belt (i.e. western Tyrrhenian margin) of the southern Apennines orogenic chain (Bartole et al., 1984; Sacchi et al., 1994; Barra et al., 1998; Di Nocera et al., 2011; Aucelli et al., 2012; Amato et al., 2013). It is located on a stretched continental lithosphere representing the transition toward the oceanic area (i.e. the bathyal plain of the central Tyrrhenian Sea). This is a Neogene to Quaternary extensional basin generated by the anticlockwise rotation of the Italian peninsula (Malinverno and Ryan, 1986; Kastens and Mascle, 1988; Patacca et al., 1990; Nicolosi et al., 2006). The Campania margin was affected by normal blockfaulting in Quaternary times, causing a chequered pattern with NW-SE trending horst-and-graben morphostructures (Ortolani et al., 1992) and NE-SW trending half-grabens (Milia and Torrente, 1999). The Sele

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Fig. 1. Simplified geological map of the Salerno Bay - Sele Plain area (modified after Aucelli et al., 2012). 1) Laura-Sterpina sandy barriers (Holocene); 2) fluvial-marshy deposits (Late Pleistocene - Holocene); 3) Gromola-S. Cecilia-Arenosola-Aversana sandy barriers (Late Pleistocene, MIS 5); 4) marine, continental and transitional deposits of the Battipaglia-Persano supersynthem (Middle-Late Pleistocene); 5) travertine deposits (Middle Pleistocene - Holocene); 6) Eboli Conglomerates (Early-Middle Pleistocene); 7) Pre-Quaternary bedrock; 8) Main faults; 9) location of offshore and inshore (M1, ML1, MG1, and S1) cores; 10) traces of the section shown in Fig. 2.

plain is one of such graben-like structures, showing strong subsidence rates during the Late Pliocene – Early Pleistocene time interval (Neotectonic Map of Italy, 1983; Brancaccio et al., 1987). 2.2. Geological setting of the study area Marine, transitional, and continental deposits, and in a minor amount volcanic products, Quaternary in age and about 1 km in thickness, filled the Sele plain (Ippolito et al., 1973). The Sele alluvial-coastal plain formed, in fact, as the result of the sedimentary aggradation in a larger Pliocene-Quaternary tectonic depression (Salerno Bay - Sele Plain, Fig. 1). This graben is bounded by three major fault systems, oriented N50°, N110° and N150°, and bordered by the Lattari-Picentini Mts. positive morpho-structure to the north and by the Cilento horst to the south

(Fig. 1). Strips of faulted Lower Pleistocene conglomerate (Conglomerati di Eboli, cf. Cinque et al., 2009, and references therein) form a hilly landscape interposed between those highs and the alluvial-coastal plain. Such a plain was mainly filled during mid-Pleistocene times. During the Late Pleistocene the aggradation-progradation of the plain continued and formed also its more recent sector, coinciding at present with the 2to 6-km wide belt of dunal ridges. The Holocene beach-dune system marks the modern sea-land interface. The northern border slopes of the plain present a greater steepness and altitude than the southern ones. The same morphostructural asymmetry is present in the offshore sector, because of the NW tilting affecting the continental shelf (Argnani et al., 1989). Further, the seismic and deep borehole data collected both inshore (Sele 1 core, Ippolito et al., 1973) and offshore (Mina 1, Milena 1, and Margherita Mare 1 cores,

Fig. 2. Geological cross-section and stratigraphic logs showing the geometry of the Salerno Bay - Sele Plain structure (geological interpretation of the off-shore seismic cross section E-117, after Sacchi et al., 1994). Legend: 1) Limestone and dolomite (Upper Cretaceous); 2) Clay, marls, and marly limestone (Cretaceous); 3) Clays and sandstones (Miocene); 4) Clays and marls with interbedded sandstones and conglomerates (Upper Miocene); 5) Conglomerates, marly clays, and marls with interbedded sandstones (a: Lower Pleistocene – Upper Pliocene; b: Lower Pliocene); 6) Clays and marly clays with interbedded fine sands (Pleistocene); the black lines are the main faults.

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Sacchi et al., 1994), show a greater thickness of the Pliocene to Quaternary succession toward the NW sector of the structural depression (Fig. 2), with about 1000 m of Quaternary deposits and about 2000 m of Pliocene sediments. 2.3. Geomorphological and land-use characteristics The coastal plain is characterized by an elongated beach-dune ridge known as Gromola-S. Cecilia paleoridge and formed during the Last Interglacial period (MIS 5, Brancaccio et al., 1987, 1988). The same morphological element continues toward the north in the ArenosolaAversana paleoridge (Russo and Belluomini, 1992). Close to the present coastline, a composite sandy ridge testifies the evolution of the Holocene barrier-lagoon system (Amato et al., 2012b). It disappears inland under a flat, muddy, back-dune depression (Fig. 3). In the southern sector of the plain, moving from inland to the sea, the dune system shows at least three orders of paleoridges (Fig. 3): Laura ridge dates from 5.3 to 3.6 ky BP; Sterpina I and Sterpina II ridges, are, respectively, older than 2.6 ky BP and about 2 ky BP (Amato et al., 2012b, 2013; Barra et al., 1998, 1999; Brancaccio et al., 1986, 1988). These dune systems were emplaced against the Paestum travertine plateau (Amato et al., 2012a, 2013). The dune system cropping out in the northern sector of the Sele plain (Fig. 3) is characterized by an almost continuous beach-dune ridge between Salerno town and the Sele River mouth, known as Campolongo ridge (Amato et al., 2012b). These sandy ridges constitute the same dune system with a mean height of about 3 m a.s.l. interrupted

by rivers and artificial drainage channels. The back-ridge depressions, spread over a large area of the plain with a mean height of about 0.50–1.5 m above sea level (a.s.l.), hosted palustrine and marshy environments (Alberico et al., 2012a), were then artificially drained and are, today, prone to marine inundation. The infill of the plain is not homogeneous (see cross sections in Section 4.1. Quaternary stratigraphy), because of the presence of unconsolidated deposits with different degrees of compaction. Dunal and coastal sands, back-ridge (lagoonal and palustrine) silty clays, palaeosols, and thick peaty layers are the main facies of this sedimentary prism (Amato et al., 2013). Such features could promote local subsidence, mainly in sectors where the silty clays and peaty layers show higher thickness, as in the back-barrier sectors of the plain (Amato et al., 2013). Shoreline variations in the last 150 years show that the Sele coastal segments have been mainly affected by erosion. From 1870 to 1984 the shoreline progressively retreated, with the highest erosion rates around the main river mouths (Alberico et al., 2012a, 2012b; Cocco and De Magistris, 1988; Di Paola et al., 2014). The Sele plain is characterized by a high concentration of infrastructures (highway, roads, railways, etc.), industrial and urban areas. The urbanized environment represents a significant percentage of the area (15.6%), and extends both in coastal sectors (Capaccio and Paestum towns) and in inland areas (Pontecagnano, Bellizzi, Battipaglia and Eboli towns along highway and national roads). Several farming and agricultural activities are located throughout the plain. The land-use map (Fig. 4) shows a strong agricultural activity in the Sele plain, which indicates that fields are almost completely farmed (78.3%). Arable lands with fruit crops

Fig. 3. Geomorphological map.

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Fig. 4. Land use map.

and horticulture prevail, together with grassland, meadows and grazing land, mostly in association to zootechnical activities. 3. Methods 3.1. Borehole inventory and stratigraphic analysis Many stratigraphic data have been acquired by the analysis of a collection of borehole logs from previous works (Barra et al., 1998; Aucelli et al., 2012; Amato et al., 2013) in which numerous (e.g. paleoecological, radiometric) analyses have been applied to discriminate depositional environments. The same data have been used here to calibrate other log data coming from public institutions such as technical offices of municipalities and ISPRA (Istituto Superiore per la Protezione e la Ricerca

Ambientale, the Italian institute for the environmental research). Finally, where Quaternary sediments crop out, stratigraphic observations have been performed directly in the field. During this activity, the elevation above sea level of ten sites showing morphological features or outcrops of Quaternary formations have been measured by GPS-based methods, with the aim of comparing such a set of measurements with the ones previously recorded. This allowed validating subsidence data acquired by the PS-InSAR technique. Boreholes data and field survey have constituted the basis for the reconstruction of a Quaternary stratigraphic succession and for the draft of the geological map (cf. Section 4.1. Quaternary stratigraphy). The stratigraphic analysis has been performed through the recognition of discontinuity surfaces, mainly detected by the correlation of stratigraphic logs. The already known geometric relationships between the

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Table 1 Summary of Permanent Scatterer (PSI) datasets used in this study. Satellite - orbit

PS technique

Count

PS velocity - mean (mm/yr)

PS stand. dev. - mean

Mean PS coherence

PS density (num./km2)

ENVISAT - ascending ENVISAT - descending RADARSAT - ascending RADARSAT - descending ERS-1/2 - ascending ERS-1/2 - descending

PSP PSP PS-InSAR PS-InSAR PSP PSP

54049 50061 19544 17571 19656 23930

−0.28 −0.82 −1.30 −1.52 −0.06 −0.73

0.40 0.39 1.53 0.83 0.65 1.00

0.73 0.73 0.84 0.86 0.78 0.76

180.13 166.87 65.15 58.57 65.62 79.77

major geological bodies, and both facies changes and lithological contacts deduced from a new analysis of a large number of borehole data have been used to recognize such discontinuities. The stratigraphic subdivision of the units confined by discontinuities used here is based on the criteria of the Unconformity-Bounded Stratigraphic Units (UBSU, Chang, 1975; Salvador, 1987), as well as the procedure of construction of the geological map. Such a task has needed an effort to homogenize data from different sources by using log charts and their precise positioning in a GIS. Information about outcropping deposits and subsurface data have been also used to complete the geological and geomorphological maps and to draw geological cross-sections (cf. Section 4.1. Quaternary stratigraphy).

3.2. Vertical movements from DInSAR data The satellite-based radar differential Interferometry (DInSAR) techniques are used to assess ground-surface deformation related to subsidence processes (Burgmann et al., 2000; Strozzi et al., 2001; Herrera et al., 2009). The Persistent Scatterer Interferometry (PSI) approach is based on the use of a series of co-registered, multi-temporal, satellite SAR (Synthetic Aperture Radar) images. This approach allows for the survey of the displacement through the time of man-made or natural reflectors (Permanent Scatterers, PS) along the radar Line of Sight (LOS) with reference to a locally stable reference radar benchmark. The LOS is the straight line between the target on the Earth surface and the satellite radar sensor, and it is characterized by different incidence angle after orbit type (ascending or descending orbit) and satellite system. Each PS is identified by coordinates (North, East) and a set of attributes, including an identifier code, the average velocity (VLOS) expressed in mm year−1, the standard deviation of the average velocity, and the coherence. The accuracy of the average LOS deformation rate of every PS usually ranges between 0.1 and 1 mm/year, depending on the number of available interferograms and on the phase stability of each single PS (Colesanti et al., 2003b). The precision of the measured average velocity also strongly depends on the distance between the reference radar benchmark and the generic PS. After Colesanti et al. (2003a), when N40 images for each dataset were used for PS interferometric processing, the VLOS precision reduces to 0.5 mm/yr after filtering of linear components (atmospheric and orbital phases) for areas b2500 km2 wide.

The PS coherence parameter is a normalized index of the local signal-to-noise ratio of the interferometric phase and reflects the accuracy of PS measurements. By selecting only PS data, characterized by a coherence value N0.65, it is possible to obtain LOS velocity (VLOS) values with an accuracy within 1.0 mm/year (Vilardo et al., 2010). In this way, it was been possible to produce maps representing the yearly average VLOS field for each PSI dataset. With reference to the study area, a number of Italian national and regional remote sensing projects have recently reported several PSIprocessed datasets that are available for research activity and territorial planning and monitoring (Regione Campania – Settore Difesa Suolo, 2009a, 2009b; Terranova et al., 2009; EPRS-E, 2015a, 2015b). These datasets are related to images acquired by C-band sensors onboard ERS-1/2, ENVISAT and RADARSAT satellites, and have been implemented with different processing techniques, namely Permanent Scatterers (PS-InSAR) (Ferretti et al., 2000, 2001) and Persistent Scatterers Pairs (PSP) (Costantini et al., 2009, 2014), that are both referred to a linear deformation model. The PS-InSAR approach is the first PSI technique used for the detection of the displacements through the time of man-made or natural reflectors. This technique requires a master scene and a stable reference point, assumed motionless, to which the zero in the time series and the relative measurements of deformation are respectively referred. The PSP technique (Costantini et al., 2009, 2014) proposed a simplified method that only exploits the relative properties of neighbouring pairs of PSs. In this way, it guarantees very dense and accurate displacement measurements both for anthropic structures and natural terrains. The joint use of datasets with different radar sources and processing techniques is currently an active research topic (Colesanti et al., 2003a; Crosetto et al., 2016). In detail, we have collected and post-processed six PSI datasets (Table 1), including 184,811 PSIs (93,249 in ascending and 91,562 in descending datasets, respectively), characterized by a coherence higher than 0.65. Considering the PS density, we observed that ENVISAT datasets are characterized by values of 166–180 PS/km2, whereas ERS1/2 and RADARSAT datasets are characterized by lower values of PS density (58–79 PS/km2). The ERS-1/2 dataset includes data from July 1992 to December 2001, whereas ENVISAT acquisition period range from November 2002 to July 2010, having an overlap with RADARSAT data between March 2003 and August 2007 (Table 2). All PSI datasets have been post-processed using GIS software and georeferenced to

Table 2 Orbital parameters of interferometrically processed satellite images. Satellite - orbit

Track/frame

LoS incidence angle

Scenes

Time range

ENVISAT – descending ENVISAT – ascending

265/2781 129/801 358/801 204/S3 111/S3 129/801 358/801–819 265/2783 494/2799

22° 25°

49 51 65 52 51 60 45–47 67 79

08 Mar. 2003–19 Jun. 2010 13 Nov. 2002–14 Jul 2010 29 Nov. 2002–30 Jul 2010 28 Apr. 2003–29 Aug. 2007 5 Mar. 2003–23 Aug. 2007 10 Jan. 1993–13 Dec. 2000 08 Sep. 1992–24 Nov. 2000 11 Sep. 1992–23 Dec. 2001 10 Jul. 1992–08 Jan. 2001

RADARSAT - ascending RADARSAT - descending ERS-1/2 - ascending ERS-1/2 - descending

34° 32,5° 22° 23°

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the projection WGS-84 UTM Zone 33N. Their analysis allowed us to obtain a retrospective view on the spatial and temporal distribution of ground deformation for the study area with reference to a relatively long time period (July 1992 to July 2010). Six 50-m regularly spaced grids of the LOS velocity maps for ascending (VLOSasc) and descending (VLOSdes) orbits of ERS-1/2, RADARSAT and ENVISAT datasets were obtained by interpolating the scattered PSI points of each dataset with the Inverse Distance Interpolation Weighted (IDW) method (Vilardo et al., 2010). The availability of two VLOS raster maps related to coeval datasets with measurements from different acquisition geometries (ascending and descending orbits) allows for the evaluation of the vertical components of the deformation, based on the trigonometric calculations described in Vilardo et al. (2009). The obtained 50 m-spaced grid maps show the distribution of the vertical components of ground deformation velocity in the Sele plain for the used datasets related to 1992–2001 (ERS-1/2), 2003–2007 (RADARSAT) and 2002–2010 (ENVISAT). The described post-processing approach was thus able to recognize both magnitude and component directions of the deformation trends in other sectors of Campania region territory (such as Campi Flegrei, Vesuvius, Ischia island, and some coastline sectors) as confirmed by Vilardo et al. (2009, 2010) by a comparison with GPS and levelling data. In order to obtain a quantitative assessment of the subsidence process referred to the whole analysed period, and also to express the statistical consistence of rates, the cumulative amount of vertical ground deformation was calculated, after the procedure described in Matano et al. (2018). The rates obtained by the different datasets were averaged by taking into account the number of years of observation (i.e. 9 years for ERS-1/2, 2.5 years for RADARSAT and 5.5 years for ENVISAT, as the overlapping years of RADARSAT and ENVISAT datasets have been counted 0.5 year each). 3.3. GPS measurements To evaluate the uncertainties about vertical rates by satellite data and confirm their trends, a topographic survey was realized in 2018 on ten ground control points (cf. Section 4.2.2. Vertical components of ground deformation velocity), characterized by high and significant vertical movements, by using the DGPS (Differential Global Positioning Systems – Trimble© R6), with very high accuracy (vertical point error = ± 0.006 m; horizontal point error = ±0.009 m). The elevation values (ellipsoidal elevation) are compared with topographic data performed in 1994 by IGMI (the Italian Geographical Military Institute) and in 2002 by Regione Campania (the regional government agency) which have a comparable error level with regard to the DGPS survey, to better constrain the calculation of displacements related to the last twenty years. The altitude differences confirm the minimum and maximum values with an average error of 4 cm and standard deviation of 7 cm. 4. Results 4.1. Quaternary stratigraphy On the basis of a large review of the previous geological maps, integrated by targeted field survey and stratigraphic analysis of boreholes data, a new UBSU-based scheme is here proposed in order to better constrain the comparison between lithological data and subsidence pattern. Further, several papers (Brancaccio et al., 1986, 1987, 1988; Cinque et al., 1988a, 2009; Barra et al., 1998, 1999; Aucelli et al., 2012; Amato et al., 2012a, 2012b, 2013) constituted the grounds for both stratigraphic correlations of logs and attributions of depositional environments to different sediments recognized in the boreholes analysis. All the Quaternary sedimentary units forming the Sele plain have therefore been grouped into four supersynthems as follows (from the bottom): the Eboli supersynthem (CE), the Battipaglia-Persano supersynthem (BP), the Aversana supersynthem (AS), and the Salerno

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supersynthem (SA). CE is mainly constituted of Lower Pleistocene conglomerate and borders the plain toward east in the middle portion and to the northern boundary. It represents the Quaternary substratum on which the infill deposits of the plain lie. The erosion surfaces separating the Aversana supersynthem from the underlying Battipaglia-Persano supersynthem and the Salerno supersynthem from the underlying Aversana supersynthem could be set respectively to MIS 6 and MIS 4, as suggested by age constraints of the clastic successions (i.e. radiometric ages from the Eboli supersynthem, cf. Cinque et al., 1988a, 1988b, and isoleucine epimerization dating of the Tyrrhenian deposits, cf. Brancaccio et al., 1986) related to the frame of marine oxygen-isotope stages. Some synthems are represented in the sketch map (Fig. 5) by distinct coeval (i.e. heteropic) lithofacies referred to different depositional and morphological features. Such a map, due to the weak incision of the large alluvial-coastal lowland, has been generated also by using boreholes data from 269 drilling logs, and geomorphological interpretation and extrapolations. 4.1.1. Eboli supersynthem (CE) The sediments of this indistinct supersynthem are ascribed to the Early Pleistocene and related to a relict alluvial plain. This supersynthem lies directly on a heterogeneous substrate made of deformed preQuaternary units (both siliciclastic and calcareous rocks). The stratigraphic succession is composed almost exclusively of weakly cemented clast-supported conglomerates, roughly layered or locally massive, with interbedded reddish palaeosols. The thickness of the whole succession is difficult to evaluate because of erosional and tectonic processes; however it is not b150 m. 4.1.2. Battipaglia-Persano supersynthem (BP) This supersynthem includes fluvial and transitional sediments, grouped in an undifferentiated part (BPi), and travertine. This latter forms the Faiano synthem (BPt), constituted by lithofacies associations of alluvial, fluvial-marshy and low to middle gradient slopes environments with waterfalls toward the ancient coastal plain, which show a total thickness of about 50–60 m (for the classification of the facies associations see D'Argenio and Ferreri, 1988, and Capezzuoli and Gandin, 2004). Boreholes data do not cross its base, but it is probable that BP directly lies on CE for their field relationships in the internal portion of the plain. The upper discontinuity of BP often coincides with topography (mainly terraced surfaces, weakly incised valleys, and retreated fault scarp). Such an unconformity disappears below the Aversana supersynthem. The undifferentiated portion of the supersynthem (BPi), Middle Pleistocene in age, was partly deposited in an alluvial fan environment (Cinque et al., 2009; cf. also boreholes S85, S247, S248, S255, in Fig. 9) and consists of alternations of beds and lenses made of alluvial sandy gravel, sand and – less frequently – sandy silt. The upper part of the succession is dominated by fine grained alluvial beds rich in weathered pyroclastic material and interbedded soils. A thick clay-rich, decalcified, dark brown palaeosol is recognized in the field, for it is sometimes preserved at the top of the unit. The alluvial-transitional portion is mostly made of sandy and pelitic sediments, formed in different subenvironments of a coastal plain subject to sea-level fluctuations (Cinque et al., 2009; cf. also boreholes S85, S248, in Fig. 9). The whole thickness is unknown, but surely N250 m (Cinque et al., 2009). 4.1.3. Aversana supersynthem (AS) The AS includes the Gromola, Fasanara, and Cafasso-Gaudo synthems. This supersynthem is formed of a large variety of facies (beach-dune, marsh-lagoon, alluvial, colluvial, and travertine units, after Brancaccio et al., 1986, 1987; Cinque et al., 2009; Amato et al., 2012a). Its lower boundary is an erosional surface cutting the BP supersynthem. The age is Middle Pleistocene p.p. - Late Pleistocene p. p., and its thickness may vary from few meters to 40 m.

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Fig. 5. Geological map (see Section 5. Quaternary stratigraphy for the acronyms).

The Gromola synthem (ASd, ASm) comprises all the Late Pleistocene sediments that accumulated during the highstand of the Last Interglacial (OIS 5, after Brancaccio et al., 1987, 1988), including the remnants of a palaeo-coastal ridge (Cordone di Gromola Auctt.) and terraces corresponding to coeval back-barrier aggradation, as deduced by geological and geomorphological analyses (Figs. 3, 5). Near Paestum the synthem unconformably covers also the travertine units outcropping in the southernmost part of the plain. Toward the south-west it disappears under the Campolongo synthem (included in the Salerno supersynthem), whose fluvial and transitional facies penetrate also some valleys that had previously dissected the synthem. The synthem has been divided in two distinct lithofacies (Cinque et al., 2009; cf. also boreholes S95, S175, S221, S222, S238, in Figs. 9, 10), coinciding with the beach-dune system (ASd, reaching a thickness of a dozen of meters) and the lagoon and marsh environments (ASm, with a subsurface thickness up to about twenty meters in boreholes). Irregular beds of gravel and sandy gravel, with minor intercalations of sand and silt with interbedded palaeosols form the Fasanara synthem

(ASf) have been interpreted here as fluvial facies on the basis of sedimentological features observed in the field. The synthem constitutes an ancient, dissected alluvial fan of the Tusciano River that is partly inserted in the wide polycyclic terrace of the Battipaglia-Persano supersynthem. The thickness of this synthem extents up to about 20 m. Finally, the Cafasso-Gaudo synthem (ASt) includes travertine units of the Paestum area. They form thick sedimentary bodies constituting several flat platforms terraced on the alluvial plain (Figs. 3, 5), prograding toward the sea and migrating both laterally and frontally (Amato et al., 2013). The medium thickness of the synthem is about 40 m. 4.1.4. Salerno supersynthem (SA) The Salerno supersynthem is constituted of the Campolongo, Masseria Acqua Santa, Campanian Ignimbrite, and PontecagnanoPaestum synthems. Many different facies (beach-dune, marsh-lagoon, alluvial, colluvial, pyroclastic, and travertine units, after Barra et al., 1998; Amato et al., 2012a, 2013) concur to constitute such a complex

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supersynthem. The age is Late Pleistocene p.p. – Holocene. The thickness of SA varies from few meters to 20 m. The Campolongo synthem represents the prism of transgressive and prograding sediments that formed on the lower Sele plain during the Holocene (SAd: beach and dune deposits; SAa: fluvial deposits; SAc: colluvial deposits; SAm: marsh, pond, and lagoon deposits), whose facies have been identified through field work and boreholes analysis (cf. S1, S41, S171, S173, S241, S243, S260, in Figs. 9, 10). Near the present coastline, that prism reaches its maximum thickness (15 to 20 m). Its oldest part (Early Holocene) is related to a barrier-lagoon system, whereas its Late Holocene part shows progradation of 3–4 m-thick coastal sand ridges (Laura and Sterpina ridges, cf. Section 2.3. Geomorphological and Land Use features) and the passage from lagoonal to marshy conditions in the back-barrier zone (see the heteropic contact in the borehole S243, Fig. 9) (thickness up to some metres). Near the Sele and Tusciano river mouths, the synthem received also fluvio-deltaic inputs, while calcareous tufa accumulated in the area near Paestum. The last survived wet areas were reclaimed (filled and drained) during the XIX and XX centuries. In the coastal area, the synthem lower boundary is an erosional surface moulded in the Gromola and Cafasso-Gaudo synthems. Inland, its fluvial terms are hosted into incisions cutting also the Middle Pleistocene deposits, with thickness of few metres up to 10 m. Other terms of the supersynthem are represented by the Late Pleistocene p.p. – Holocene alluvial fans of the lowermost branch of the Tusciano River and other minor streams (Masseria Acqua Santa synthem, SAf), constituted of irregular beds of sandy gravel and sand with silty loam up to 15 m in thickness, whose apexes are morphologically inserted in the Fasanara synthem and whose base descends under the present sea level. Although locally dissected, they substantially preserve their original (fan shaped) upper boundary surfaces that disappear downslope under the Campolongo synthem. The Salerno supersynthem contains also a regional-scale volcanic marker, the well-known Campanian ignimbrite (SAi), mapped in the northern sector of the Sele plain (Pappone et al., 2009) and largely diffused also in other Tyrrhenian coastal plains (Corrado et al., 2018). It consists of sanidine-rich scoriae with different degrees of flattening, embedded in a grey ashy matrix with subordinate lithic inclusions. This unit is separated by an erosional discontinuity from the underlying units. Its radiometric age is 39.28 ± 0.11 ka (De Vivo et al., 2001). Also SA includes travertine units (Pontecagnano-Paestum synthem, SAt) outcropping in two distinct areas of the Sele River alluvial coastal plain (Pontecagnano area to the NE and Paestum area to the SE). They form thick sedimentary bodies, constituted by Upper PleistoceneHolocene lithofacies associations from alluvial and fluvial-marshy environments, showing stromatolithic and phytohermal textures, morphologically expressed by flat platforms terraced on the alluvial plain. The medium thickness of this synthem is about 20 m. 4.2. Subsidence data 4.2.1. LOS velocity fields The LOS velocity values are mainly characterized by negative values in both ascending and descending orbits for all satellite datasets (Table 3). They show similar values for all datasets with wider ranges only for Radarsat datasets. The mean and median of the LOS velocity

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values are negative (respectively ranging from −1.52 to −0.06 mm/yr and from −1.34 to +0.16 mm/yr) for almost all datasets except than median values for both ENVISAT and ERS ascending orbits. The datasets show an asymmetrical distribution toward negative values (skewness ranging from −3.05 to −2.14) due to the general subsidence process affecting the plain. The Quartile (Q1, Q3) values and the interquartile range (IQR) are similar for all datasets (Table 3). The “normal” values, that are referred to a narrow range, approximately between Q1-IQR and Q3 + IQR, account for approximately 90% of all data for each dataset (Table 3). The extremes of velocity values over the last decades, which represent approximately 10% of data, show significant differences, with minima ranging from −38.13 to −14.1 and maxima from +4.6 to +24.61 (mm/time-span). They are usually due to local causes (e.g., water extraction from the subsurface, instabilities of anthropic structures or buildings, water table oscillations, mass movements). The almost homogenous distribution of most of the data in each dataset describes a constant and persistent process of ground deformation at the regional scale in the examined time period (1992–2010). Different maps (Fig. 6) show the spatial distribution of PS LOS velocity values, averaged over the time period of production of each type or radar images and expressed in mm/year, for both ascending and descending orbits of ERS-1/2, RADARSAT and ENVISAT datasets. ENVISAT descending dataset is not available for the southernmost sector of the study area (Fig. 6f). In all maps most of the PS are concentrated over urban areas and along linear infrastructures (railway, highway, roads, etc.), where the concentration of buildings is very high. The overall PS distribution made possible a regional analysis of ground deformation because the local consistency of the data is satisfying almost everywhere in the study area. The mapped velocity classes show common general trends, even though some differences can be recognized in the different maps. The largest negative LOS velocity values (lower than −5 mm/yr) are recorded along the Sele River and in the central part of the coastal strip. In more detail, RADARSAT datasets display usually higher negative values in the northern and central sectors and less negative values in the southern sector of the study area when compared to ERS-1/2 and ENVISAT datasets. The difference between the LOS angles for the orbital systems (32–34° for RADARSAT compared to 22–25° for ERS-ENVISAT; Table 1) can justify a difference of a few mm/yr in the values of LOS velocity related to the same ground deformation process. Moreover, some datasets show differences in the displacement rates obtained from ascending and descending geometries. RADARSAT datasets show some differences along the coastal area near Capaccio and in the inner sector, which display higher subsidence in descending datasets. ERS datasets are characterized by lower negative values in descending datasets in the southern sector. These differences can be interpreted as the effect of non-vertical movements affecting those sectors, such as local horizontal component of ground deformation. After Vilardo et al. (2009), during 1992–2000, the horizontal velocity pattern shows a general westward movement of the coastal area with local spots characterized by eastward deformation; an eastward movement is present in the central sector of the inner area. 4.2.2. Vertical components of ground deformation velocity The post-processing of ascending and descending LOS velocities maps has allowed us to obtain maps of the vertical component of the

Table 3 Data distribution statistics of Line of Sight (LoS) velocity data in the used datasets. Satellite - orbit

Min

Max

Mean

Std. dev.

Skewness

Q1

Median

Q3

IQR

Q1-IQR

Q3+ IQR

Norm. val. %

ENVISAT - ascending ENVISAT - descending RADARSAT - ascending RADARSA-descending ERS-1/2 - ascending ERS-1/2 - descending

−15.0 −14.1 −38.13 −31.95 −17.41 −23.74

+5.40 +4.6 +24.61 +7.8 +6.22 +9.71

−0.28 −0.82 −1.30 −1.52 −0.06 −0.73

1.42 1.41 2.11 2.45 1.51 1.75

−2.51 −2.43 −2.14 −2.23 −3.05 −2.82

−0.5 −1.1 −2.21 −2.14 −0.39 −1.07

+0.1 −0.5 −1.06 −1.34 +0.16 −0.4

+0.5 −0.1 −0.05 −0.51 +0.71 +0.2

1.00 1.00 2.16 1.63 1.10 1.27

−1.50 −2.10 −4.37 −3.86 −1.49 −2.34

+1.50 +0.90 +2.11 +1.12 +1.81 +1.47

86.7 86.6 92.2 88.3 89.4 88.2

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Fig. 6. Map view of range-change rate measurements (LOS velocity) of PS with coherence ≥0.65 related to the six used datasets: (a) ERS-1/2 ascending orbit; (b) ERS-1/2 descending orbit; (c) RADARSAT ascending orbit; (d) RADARSAT descending orbit; (e) ENVISAT ascending orbit; and (f) ENVISAT descending orbit.

velocity of ground deformation over the 1992–2000 (Fig. 7a), 2003– 2007 (Fig. 7b) and 2003–2010 (Fig. 7c) time intervals, showing similar vertical velocity patterns. All the vertical velocity maps (Fig. 7) indicate significant negative ground deformation in the northern part of the coastal sector with subsidence rates are in the order of −15 to −5 mm/year. The process is mostly concentrated, at least since the early ‘90s, between Pontecagnano town and Aversana village, and along the lower course of the Sele River. Wider areas affected by moderate subsidence processes with rates of −5 to −1 mm/year occur along adjacent inner sectors. The remaining wide sectors of the plain are substantially stable during the two decades of observation. A small sector with positive

rates of +1 to +2.5 mm/year is detected around Battipaglia in the eastern sector of the study area only during 1992–2000 time period. A comparison between the PSI-derived data presented above and GPS data from a dedicated topographic survey performed in 2018 on ten significant sites allowed validation of the satellite-based dataset. In fact, InSAR data and DGPS measurements are well fitting (rate difference of 0–2 mm/yr) in four localities. Rates from other five sites vary from 2.3 to 5.3 mm/yr. Only a single point is not fitting, since a rate difference of about 20 mm/yr (Table 4) is recorded. In Table 4, GPS data from all surveyed sites are reported with values rounded to the first decimal place and compared to different time intervals of SAR take-over.

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Fig. 7. Vertical component of ground deformation velocity obtained by post-processing of (a) ERS-1/2, (b) RADARSAT and (c) ENVISAT interferometric datasets.

4.2.3. Subsidence map In this study we have obtained a quantitative assessment (expressed in mm) of the subsidence processes affecting the Sele plain during 1992–2010. The map of the cumulative amount of the ground surface vertical deformation (Fig. 8) has been derived by the merging of the three different maps of the average vertical velocity maps related to the processed datasets (Fig. 7) by accounting for the different duration of the time intervals they are referred to. This map shows that the coastal sector of the Sele plain was characterized by a complex vertical velocity pattern during the entire time interval (i.e. 1992–2010) considered here. The whole coastal belt from Pontecagnano to Capaccio was characterized by a subsidence process of variable intensity ranging from −10 to −210 mm during about twenty years. The process was more intense (N30 mm of subsidence) along the mouth and the lower reach of the Sele River and along the Aversana to Pontecagnano coastline. In a 2 km-wide coastal strip of the northern sector (around Pontecagnano) of the plain the subsidence process had its peak and generated a lowering of the ground surface of up to N20 cm during

1992–2010 time-span. In the inner sectors of the plain and south of the mouth area of the Sele River along the coast, a general condition of stability with small subsiding areas has been recognized. Small spots of cumulative uplift, characterized by apparent vertical motion of 10 to 30 mm during 19 years, are present in the inner sectors of the plain around the towns of Battipaglia and Eboli, but they should be considered as due to local (anthropic or natural) causes, and therefore not relevant for our study. 4.3. Stratigraphy and subsidence relations In order to explore the relationships between stratigraphic setting and ground deformation spatial variations, several log-based geological cross-sections have been constructed (Figs. 9, 10); cumulative subsidence profiles, obtained by the local values of the areas crossed by the sections (Fig. 8), have been associated to each geological section. From the subsidence rates viewpoint, sections A-A′ and F-F′ are more or less similar; as a matter of fact, both present subsidence

Table 4 Comparison between GPS and vertical SAR data. Id point (# IGMI; ^ Regione Campania)

B1 = 197902# B2 = 467386^ B3 = 486387^ B4 = 486423^ B5 = 486438^ B6 = 486447^ B7 = 486461^ B8 = 487407^ B9 = 487465^ B10 = 487466^

Site (major toponyms of reference)

Eboli Pontecagnano Pontecagnano Eboli Eboli Capaccio Capaccio Eboli Roccadaspide Capaccio

1992–2010 SAR vertical deformation (mm)

SAR vertical annual rate (mm/year)

2002 (1994*)-2018 GPS deformation (mm)

GPS rate (mm/year)

V vert

V vert/19 years

GPS d

GPS d/16 years (*24 years)

−51,0 −400,0 −111,0 8,0 9,0 −103,0 −34,0 6,5 0,7 4,4

−2,7 −21,1 −5,8 0,4 0,5 −5,4 −1,8 0,3 0,0 0,2

−100,0 −10,0* −140,0 −40,0 −20,0 −50,0 −40,0 −80,0 0,0 40,0

−4,2 −0,6* −8,8 −2,5 −1,3 −3,1 −2,5 −5,0 0,0 2,5

GPS-SAR rate difference

−1,5 20,4 −2,9 −2,9 −1,7 2,3 −0,7 −5,3 0,0 2,3

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5. Discussion

Fig. 8. Subsidence map of Sele Plain and location of GPS measurements and cross-sections.

values that reach about 30 mm/19 yr not far from the coastline, along a strip in which 10 m-thick Holocene back-barrier deposits, mainly consisting of clays, silts and peat (SAm), lie on alluvial sand and gravels (SAf). Greater values are found in the profile related to the E-E′ section, showing a peak of 60 mm/19 yr across the heteropic passage between dunal sands (SAd) and back-dune silty clays (SAm). C-C′ and D-D′ sections are characterized by subsidence values up to 120 mm/19 yr in coincidence of both clayey deposits (SAm) of back-dune depressions and sandy deposits (SAd) of dunal systems. The B-B′ subsidence profile displays a totally different shape, articulated in two main segments whose boundary is located at the passage between the Tyrrhenian dunal/back-dune system and the Upper Pleistocene p.p. – Holocene homologue deposits, represented by few metres thick loam and peat lying on coarse-grained alluvial deposits. Indeed, the thickness of back-barrier sediments shown in the area crossed by the B-B′ section reaches here its minimum with regard to the other log-based cross-sections. In the related subsidence profile the highest subsidence rate (about 200 mm/19 yr), recorded close the coastline, also marks the difference. In conclusion, two main sets of subsidence curves can be observed in the study area. The first, represented by almost all profiles, presents curves featured by a flat, sub-horizontal line in which more or less pronounced sag is inserted; in this group, the D-D′ profile displays a more articulated shape with a pair of negative peaks. The second one is represented by the anomalous B-B′ profile, featured by a sharp change in angular coefficient of two straight segments.

In order to analyse the possible correlation between subsurface geological features (i.e. lithology of stratigraphic units) and subsidence rates we made an overlay of the subsidence map derived by PSI data processing with a lithological map of the study area. The unconformity-bounded stratigraphic units (Fig. 5) have been grouped and/or converted into different lithological units (Fig. 11a) based on the prevailing characters of the outcropping deposits (lithology, compactness, grain size, etc.). On this basis, descriptive statistics of the subsidence behaviour of the different lithological units have been elaborated (Table 5). Such an approach can be applied to similar subsiding coastal plains all around the world to define the effective causes of ground motion and to obtain suitable risk scenarios. The average values of subsidence clearly show that the lithological unit loam clay (corresponding synthems: ASm, SAm in the geological sketch map of Fig. 5) presents the greatest values of subsidence in the study area, consistent with the lagoon-marsh nature of its deposits. Even the slight subsidence values that characterize the categories sandy gravel (dt, BPi, SAf, ASf, SAa, Fig. 5) and sandy silt (SAc, Fig. 5) are consistent with their lithological nature. The categories sand, corresponding to sandy beach and dune deposits (b, ASd, SAd, Fig. 5), and fluvial deposit (i.e. the current river bed units, unit a in Fig. 5) present high subsidence values. The outcrop areas of bedrock units (PQU), Campania Ignimbrite (UBSU: SAi), travertines (UBSUs: BPt, ASt, SAt), and Lower Pleistocene conglomerates (CE supersynthem) result stable. The above reported remarks only allowed us to assess how the areal arrangement of the lithological units may influence the subsidence trends of the study area. In order to consider also the contribution of subsurface stratigraphy (Figs. 9, 10) and geomorphological setting (Fig. 3) to the subsidence processes, a zoning map has been made (Fig. 11b). In this way, we identified subsiding zones characterized by homogeneous geomorphological and stratigraphic conditions. As you can see in Figs. 9 and 10, the Quaternary infill of the coastal sectors of the plain is not laterally homogeneous because of the presence of unconsolidated layers with different degrees of compaction. Dune-beach sands, back-ridge silty clays, palaeosols, and thick peaty layers are the main facies of unconsolidated deposits present in the sedimentary prism, variably represented in the cross sections reported here. These deposits are systematically associated with the highest cumulative values of subsidence. Yet, such a lateral variation does not reflect the shape of the present coastline, which shows a regular, slightly concave, planimetric trend without significant changes along the whole coastal segment. This can be easily explained by the fact that the Sele plain beach is a young landform (Cinque et al., 1988b) whose sedimentation/erosion processes due to SE-trending long-shore currents have obliterated eventual past coastal articulation due to differentiated subsidence. Furthermore the present-day straight coast line and the less pronounced fluvial mouths are related to human-induced erosive coastal processes (Alberico et al., 2012a, 2012b). The obtained subsidence zoning encompasses several zones from inland to the coast. In detail, we have recognized the inner border zone, the alluvial belt zone, the travertine zone, the paleodune zone, the river mouth zones, and the coastal zone (Fig. 11b). The different average values of vertical ground deformation for each zone are reported in Table 6. The most subsiding areas are those labelled “river mouth”, in coincidence of the youngest landforms characterizing those areas (i.e. PRM, TRM, SMA, and partly SMB), characterized by very high and persistent negative average values over time (Table 6). Such behaviour is likely due to the presence of lagoon deposits in the subsoil, as shown in the Section 4.1. Quaternary stratigraphy, with a significant thickness of clayey deposits as a result of the fluvial paleovalleys filling (cf. Figs. 8, 9, 10, 11). The back-barrier lagoon, current dunes and beach strips (Figs. 3, 5), denoted as “coastal zone”, show two subsectors (Fig. 11b) with very different behaviours (Table 6). A central sector (CZA) is in fact strongly subsiding, whereas the northernmost and southernmost

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Fig. 9. Geological cross sections (A, B, C) with subsidence profile during 1992–2010 time-span (expressed in mm). The uncertainty of subsidence data falls within ±1 mm.

coastal sectors (CZB) are slightly subsiding or almost stable. Also the “paleodune” sectors (Figs. 3, 5) are divided into a slightly subsiding central zone (PA) and a stable southern zone (PB). The large area occupied by the BP supersynthem (Fig. 5), indicated with the label “alluvial belt”, is divided into three zones (Fig. 11b; Table 6) corresponding to stable or very slightly uplifting internal portions of the plain (ABB), to the stable central sector of the plain (ABA) and to the slightly subsiding area (ABC) located immediately to the south of the Sele River lower reach, maybe in correspondence of its ancient palaeomouth. The weak subsidence of the elongated sector (ABC) behind the Arenosola-Aversana Tyrrhenian dune ridge may be due to the presence of more compactable back-barrier sediments, detected on the basis of field investigations and boreholes log interpretation. Yet, it is not possible to exclude the contribution of the fault motion of the NW-SEstriking buried structure located just east of that strip. Nevertheless,

the sum of these two causes should provoke a more intense subsidence phenomenon, whereas it appears more or less equal to the one from other zones with the same lithological features. The subsidence curve related to the profile B-B′ displays a different shape with regard to the other sections, passing from about 2.1 mm/yr up to 10 mm/yr (almost two times faster than elsewhere) in a more continuous way. This could mean – together with the observation that the major subsidence spot is located south of Pontecagnano not far from the B-B′ section, at the intersection of N70°- and N110°-striking faults – that such a subsidence is due to the effects of tectonics. This aside, it appears clear that the same area is characterized by the presence of 4–5 m-thick soft sediments. The loss of about 4% of total thickness as a result of 20 year-long subsidence may be suitably compatible with dewatering and compaction of such deposits. On the other hand, tectonic processes – such as hangingwall subsidence by fault linkage

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Fig. 10. Geological cross sections (D, E, F) with subsidence profile during 1992–2010 time-span (expressed in mm). The uncertainty of subsidence data falls within ±1 mm.

or basin-scale subsidence induced by main border faults – could be adequately represented by similar rates (i.e. about 1.6 mm/yr) but it doesn't seem supported by other evidence. The general pattern displayed by the subsidence profiles coupled with geological cross-sections has shown significant anomalies (from 1.3 mm/yr up to 10 mm/yr) linked to the presence of marsh and lagoonal deposits (Figs. 3, 5), easily compactable, which appear as “clustered” areas in a planimetric view (Fig. 8), superposed to a probable “background noise” due to tectonic subsidence (values from 0.26 mm/yr to 2.1 mm/yr), affecting a larger portion of the plain and represented by the plateau-shaped segments of the curves (Figs. 9, 10). The compaction signal is detectable in all profiles and more subdued in the E-E′ section, where the compaction-prone sediments are also largely present, maybe due to the presence of a greater amount of

interbedded gravel deposited by a likely lateral (i.e. northward) migration of the Sele River lower reach. Only very discontinuous and slight “uplift” values have been revealed by the cumulate SAR signal in the study area (Fig. 8). They have been referred by Vilardo et al. (2009) to prevailing dip-slip movements (based only on the 1992–2000 ERS dataset, Fig. 7a) along a NNWSSE-striking blind fault located in the inner sector of the plain. As in the same areas null to slight negative values have been observed during the following 2002–2010 periods (Fig. 7b–c), we considered these “uplifts” not referable to the inferred tectonic input but due to local transient causes. Indeed, significant and long lasting (almost two decades) subsidence processes have been clearly documented in the central sectors of the coastal zones and related paleodunes and river mouths (i.e. PA, CZA, SMA, TRM and PRM, Fig. 11b).

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Fig. 11. a) Map of lithological units; b) Map of subsidence zones; yellow to red areas are characterized by increasing subsidence, whereas green areas are stable. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

On a regional scale, the overlay of the SAR interferometry data with the geomorphological and stratigraphical features confirms that the subsidence is high in the northern-central sector of the plain, where the Quaternary alluvial-coastal deposits are thicker because of the structural asymmetry of the graben in which they are hosted. In relation to a very short-term evolution, the maximum subsidence rate can be observed in the central part of the plain, in correspondence of the river mouths and of the coastal belt. This could be related to the larger local thickness of unconsolidated sediments (N20 m) and to the presence of still compacting clay-rich sediments. In the southern sector of the Sele Plain, the general subsidence rates are lower due to the minor thickness

of Holocene deposits and to the presence of very thick sedimentary bodies of travertine. The land use of the analysed territory (Fig. 4) does not seem to have an important influence on the recognized regional subsidence trends (Fig. 8). The different average values of vertical ground deformation for each mapped land-use class are shown in Table 7. As expected from the lithology and stratigraphy data, the highest subsidence values (−30 to −60 mm) are found for “dune, beach, river and humid zones” classes, while intermediate subsidence values (−11 to −25 mm) are showed by areas with agricultural activities, forest, grazing lands, and urban areas. Lower values (−8 to −10 mm) characterize fruit grove,

Table 5 Mean and extreme values of vertical ground deformation referred to the defined lithological units. For geological units labels see units in Fig. 4. Lithology

Geological units (litho- and UBS-units)

Mean areal subsidence

Standard deviation

Minimum value

Maximum value

FLYSCH (Ub) IGNIMBRITE (Ig) TRAVERTINE (Tr) CONGLOMERATE (Co) SANDY GRAVEL (Sg) SANDY SILT (Ss) FLUVIAL DEPOSIT (Fd) SAND (Sa) LOAM CLAY (Lc) All units

PQU SAi BPt, SAt, ASt CE dt, BPi, SAf, ASf, SAa SAc a b, ASd, SAd ASm, SAm

−8, 2 −6,9 −9,6 −6,1 −12,0 −17,6 −26,7 −33,5 −41,3 −16,9

5,6 3,2 7,7 5,8 25,2 29,5 30,0 38,6 38,4 28,7

−47,8 −12,9 −64,5 −37,3 −209,8 −146,6 −136,2 −213,9 −185,7 −213,9

9,1 2,1 7, 8 12,2 20,4 25,9 11,0 12,5 18,6 25,9

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Table 6 Mean and extreme values of ground deformation for the different subsidence zones. Zone

Mean Standard subsidence deviation

Minimum value

Maximum value

Inner Border Zone – conglomerate (IBZ) Inner Border Zone – flysch (IBZ) Inner Border Zone – ignimbrite (IBZ) Inner Border Zone (IBZ) Travertine Zone (TZ) Alluvial Belt A (ABA) Alluvial Belt B (ABB) Alluvial Belt C (ABC) Alluvial Belt Zone (ABZ) Coastal Zone A (CZA) Coastal Zone B (CZB) Coastal Zone (CZ) Paleodune Zone A (PA) Paleodune Zone B (PB) Paleodune Zone (PZ) Picentino river mouth (PRM) Tusciano river mouth (TRM) Sele river mouth A (SMA) Sele river mouth B (SMB) Sele river mouth (SMZ) River Mouth Zone (RMZ)

−6,4

6,2

−37,3

12,2

−7,5

5,8

−47,8

13,3

−6,9

3,2

−12,8

2,1

−7,0 −10,3 −0,9 7,3 −13,9 −4,9 −82,1 −15,2 −46,4 −30,7 2,8 −18,3 −90,3 −85,5 −81,9 −30,1 −40,2 −55,9

5,9 7,3 5,0 3,7 11,5 11,1 30,1 10,9 40,3 22,4 4,5 24,2 39,6 27,9 15,1 19,1 27,62 37,3

−47,8 −64,5 −38,4 −26,2 −124,0 −124,0 −213,9 −67,6 −213,9 −123,4 −25,1 −123,4 −209,8 −142,9 −125,6 −111,5 −125,6 −209,8

13,3 7,8 16,3 20,3 20,4 20,4 −10,6 7,1 7,1 25,9 12,5 25,9 −19,3 −6,4 −32,8 8,7 8,7 8,7

vineyards, shrub and bush areas. Overall, small differences (in the range of 15 mm) characterize the different land use related to the different agricultural activities and urban areas. The different land use may influence subsidence only at local scale. For example, immediately south of the lower reach and mouth of Sele River, the irrigation needs of a wide area devoted to fruit crop and horticulture (Fig. 4) may be identified as the cause of an excessive groundwater withdrawal that is mirrored by a stronger subsidence recorded along the lower fluvial segment. The overlay (Fig. 12) of vertical ground deformation map with urban areas, derived from the land-use map of Fig. 4, and industrial sites, derived by CTR (Carta Tecnica Regionale) topographic map at 1:5000 scale, shows a low correlation between subsidence and anthropogenic activities. On the contrary, several stable or fairly uplifting areas are located within urban and industrial areas near Salerno, Bellizzi, Battipaglia, and Eboli towns.

6. Conclusions The Sele River plain is the long-term product of the infill of a grabenlike structure originated since Pliocene times, evolving in Quaternary

Table 7 Mean and extreme values of ground deformation for the different land use classes. Land use class

Mean Standard subsidence deviation

Minimum value

Maximum value

Shrub, bush Fruit and olive grove, vineyards Complex cultural systems Urban and industrial areas Arable land Cereal, cattle fodder Forest Protected fruit crop, horticulture Grassland, meadow, grazing land River and humid zone Dune, beach

−8.8 −9.2 −11.8 −13.3 −17.9 −20.4 −21.6 −22.2

15.4 18.8 20.4 26.7 29.3 31.8 24.7 33.0

−110.0 −172.0 −142.6 −213.9 −209.8 −130.7 −120.2 −181.9

+7.1 +20.3 +13.1 +20.4 +25.9 +13.7 +7.6 +18.4

−23.7

36.0

−182.4

+8.8

−32.0 −60.7

33.9 54.2

−135.9 −194.2

+11.0 −3.0

times in the frame of a continuous interplay between eustatic changes and tectonics. According to some authors (Neotectonic Map of Italy, 1983; Brancaccio et al., 1987; Carta Geologica d'Italia, Foglio 486 - Foce del Sele, Cinque et al., 2009), the plain was strongly subsiding during the Late Pliocene – Early Pleistocene, slightly rising in the Late Pleistocene, and substantially stable in the last millennium. A 20-year period of PSI data acquisition now allows the estimation of vertical movements of such a coastal lowlands during the period 1992–2010. These data revealed that the whole coastal belt from Pontecagnano to Capaccio is characterized by a subsidence process of variable intensity ranging from −10 to −210 mm. Subsidence peaks in a 2 km-wide coastal strip of the northern sector of the plain (around Pontecagnano), generating a subsidence of about 200 mm and evidencing a significant differential spatial variation in vertical displacement within a few kilometres. In the inner sectors of the plain and to the south of the Sele River mouth there is a general condition of stability with minor subsiding areas. This could be partially put in relation with the structural asymmetry provoked by Pliocene to Pleistocene tilting of the Salerno graben, which favoured a larger accumulation of easily compactable Quaternary alluvial-coastal deposits in the northern sector of the plain as compared to the southern one. The overlay of PSI data with land-use classes, geomorphological features, and unpublished stratigraphic logs from boreholes shows that the highest subsidence – recorded in the northern coastal area and around the Sele River lower reach and mouth – has a clear geological underpinning. In fact, the subsidence is emphasized in the back-dune areas due to sediment compaction following the rapid aggradation and progradation of the coastal environments during Holocene sea-level rise. These areas are characterized by sedimentary successions with organic clays and peat layers and alluvial brackish deposits up to 20 m thick. At the river mouths, such a condition has been particularly promoted by the presence of thick soft sediments, highly subject to compaction, related to the Holocene infill of pre-existing deep fluvial incisions produced by river down-cutting during the Last Glacial Maximum (22–18 ky). In the area of Sele mouth a significant thickness of alluvial and brackish deposits is present, as compared to the other sectors of the alluvial coastal plain. In addition, the subsidence also affects other sectors of the plain as the back-ridge strips that hosted lagoonal and marshy environments during the Holocene, characterized by stratigraphic successions with organic clays and peats layers (10 m in thickness on average). In the southern sector of the plain, near to the Paestum archaeological site, the vertical land movements rates are the lowest recorded, probably due to the minor thickness of the Holocene deposits and to the presence of thick sedimentary bodies of travertine (Amato et al., 2012a, 2013) that are, not conducive to dewatering and compaction processes. The subsidence trend recorded by PSI data is in agreement with the data available on the Holocene paleo-sea level markers analysed by Pappone et al. (2012) and Amato et al. (2013). In these papers the authors highlighted that in the sector of the plain located to the north of Sele River the sea-level indicators fall below the Holocene sea-level curve estimated in Lambeck et al. (2011). This indicates the existence of subsidence phenomena – due to compaction of sediments or other any causes – since the beginning of the Holocene. The methodology applied in this study is based on the integration of different analyses (geological, stratigraphic, structural, geomorphological, land use, and interferometric datasets) that are able to provide different inputs for the understanding of spatial and temporal trends and causes of the complex process of subsidence in coastal plain in tectonically active and urbanized regions. This procedure can be a reference for further investigations in several coastal areas along the Mediterranean Sea and in other regions of the Earth that have become more vulnerable under the effects of undergoing global changes. The Volturno and Garigliano plains along the Tyrrhenian coastal segment of central-southern Italy, coastal areas located around the Arno River plain and the Po River

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Fig. 12. Overlay of urban and industrial areas on the subsidence map of Sele Plain.

plain in central and northern Italy, south-eastern coast of England, the Netherlands, northern Germany, and south-western Denmark in northern Europe, are subsiding for multiple causes and appear to be suitable candidates for the multidisciplinary approach proposed in this paper. Acknowledgements We wish to thank three anonymous referees and the editor of the journal, Professor Andy Plater, for helpful comments and accurate revision. This study was financially supported by University of Basilicata RIL 2015 granted to M. Schiattarella and by research found University of Naples Parthenope granted to P.P.C. Aucelli and G. Pappone.

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