Surface deformation analysis in the Ischia Island (Italy) based on spaceborne radar interferometry
Introduction
The island of Ischia is located on the northwestern corner of the Gulf of Napoli (Italy) and represents the westernmost volcanic complex of the Phlegrean Volcanic District including the Procida Island and the Campi Flegrei. Ischia, whose morphology is dominated by the structural block of Mt. Epomeo (786 m above the sea level), extends over a surface of about 42 km2 and is composed of minor landslide deposits, marine sediments and volcanic rocks represented by alkali-trachytes, trachybasalts, latites and phonolites (Civetta et al., 1991, Orsi et al., 1991), see Fig. 1. Regarding these volcanic products, the oldest ones (130–150 ka) crop out along the coastline (Vezzoli, 1988). About 55 ka a major subaerial eruption, the ‘Green Tuff’ trachytic ignimbrite, occurred forming the calderic depression of Mt. Epomeo (Gillot et al., 1982). Subsequently, the caldera collapse allowed the deposition of the marine ‘Tuffite’ and ‘Colle Jetto’ formations in the central part of the island (Barra et al., 1992). Moreover, between 44 and 33 ka, the Citara Tuff eruption took place off the island western coast. Between 55 ka and 19 ka, probably around 33 ka, an 800–1100 m uplift phenomenon began affecting the structural block of Mt. Epomeo (Gillot et al., 1982).
Moderate volcanism in the last 33 ka mainly occurred outside the area of the resurgent dome and was localized in the eastern part of the island (Orsi et al., 1991). The more recent volcanism concentrated in the last 2.9 ka with effusive, extrusive and explosive activities (Civetta et al., 1991); this recent volcanic activity has been accompanied by subsidence, as testified by the presence of Greek and Roman ruins at about 2 m below the sea level (Friedlander, 1938, Buchner, 1986). The most recent eruption, the Arso lava flow, dates to 1302 AD; emplacement of still active landslides and mudflows, which concentrate in the northwestern and western sector of the island, preceded and followed the volcanic activity.
The structural setting of the island is characterized by four main fault systems which are highlighted in Fig. 1: (1) the E–W striking faults affecting the northern sector of Mt. Epomeo, (2) a NE–SW fault system cutting the deposits of the eastern sector, (3) a NW–SE striking fault affecting the southwestern corner of the island, and (4) the NNW–SSE to N–S faults bordering the eastern and western side of Mt. Epomeo. At the present, the southern flank of the Epomeo structural block and the northwestern corner of the island are affected by landslides that move in response to gravity instability phenomena on steep slopes, or to earthquake shaking (Vezzoli, 1988, Alessio et al., 1996). In particular, the slide of the northwestern corner was activated during the 1881 and 1883 earthquakes; rock-falls characterize the steep eastern and northern slopes of Mt. Epomeo.
We investigate in this work the surface deformations occurring in the island by applying the Differential Synthetic Aperture Radar Interferometry (DInSAR) technique and in particular, the approach referred to as Small BAseline Subset (SBAS) algorithm (Berardino et al., 2002) that allows us to analyze the spatial and temporal characteristics of the detected displacements in the imaged area. Our study is focused on the 1992–2003 time interval and SAR data acquired by the first and second European Remote Sensing (ERS-1 and ERS-2) satellites from both ascending and descending tracks have been used. The availability of these ascending/descending data set allows us to discriminate the vertical and east–west displacement components; this is a key issue because it allows us to provide a validation of the SAR products by comparing the vertical deformation estimated from the DInSAR measurements with those available from the spirit leveling network present in the area; in this case, the comparison shows a very good agreement between the two data sets. Moreover, in the final discussion we fully benefit from the overall DInSAR information, retrieved from the multi-track SAR data, in order to interpret the measured displacements.
Section snippets
Recent dynamics and geodetic monitoring
The most significant phenomena that have characterized the historical and recent activities of Ischia are seismicity, hydrothermal manifestations and ground deformations. The seismic catalog of the island is probably incomplete because catalog and historical records are not available for a large part of historical times. Anyway, the available information indicate that earthquakes occurred in 1762, 1796, 1828, 1881 and 1883 (Postpischl, 1985, Boschi et al., 1997). Shallow seismic activity
Rationale of the SBAS algorithm
In order to increase our knowledge of the characteristics and distribution of the deformations affecting the island, we have applied the DInSAR technique, which is based on exploiting the phase difference (interferogram) between SAR image pairs acquired at different times; this allows us to extract information on the radar line of sight (LOS) projection of the displacements that occurred between acquisitions (Gabriel et al., 1989, Massonnet et al., 1993). In particular, the algorithm we have
DInSAR/leveling comparison: quantitative analysis
Following the previous discussion it is evident that the availability of SAR data acquired from ascending and descending orbits allows us to gain insight into the detected deformation phenomena; in particular, we may benefit from these information to discriminate vertical and east–west displacement components.
To achieve this result we have combined the mean surface velocity maps computed from the ascending and descending orbits shown in Fig. 4a and b, respectively, on pixels common to both maps
Acknowledgments
We want to thank I. Aquino, S. Guarino, P. Lundgren, G. Macedonio, A. Pepe and V. Siniscalchi for their help. We also want to thank P. Euillades and P. Grosse that contributed to process the ascending SAR data set and the European Space Agency for providing SAR data within the Cat-1 1065 and 1318. This work has been partially sponsored by the European Community on Provision 3.16, under the project of the Regional Center of Competence “Analysis and Monitoring of the Environmental Risk”
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2022, GeomorphologyCitation Excerpt :The estimated deformation rates are close to the typical upper limit of detection, which is around ±0.4 m per year for C-Band and six-day temporal baselines according to Strozzi et al. (2020), who analyzed rock glacier kinematics using Sentinel-1 time series in Switzerland, Greenland and Argentina. Further, the absolute values of the deformation rates should be treated as first estimates as some inaccuracies might be present, listed as follows: (i) the movement rates were just estimated from short period and it is unknown if and how surface displacement takes place over the entire year/season (i.e. regarding potential reversible deformations and freeze-back), (ii) the conversion to vertical and horizontal displacement was calculated with the simplified approach proposed by Manzo et al. (2006), (iii) the location of the reference point was carefully selected and based on a field visit, even though no external validation on the stability of the reference point was available, (iv) the horizontal movement in north-south direction was not detectable, which is a general constrain arising from the polar orbiting, and (v) atmospheric corrections have not been applied but could have consequences in such high relief areas (cf. Dini et al. (2019)). Despite these limiting factors, movement patterns were very distinct and clearly visible thanks to the high quality interferograms and the high coherence allowing for a reliable phase unwrapping.
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