## 5 Discussion
### Geological Interpretation
#### 5.1.3 Ground Deformation in the City of Crotone  
Across the whole P-SBAS datasets, the other zone with negative LOS velocity values that can be clearly detected is located over the harbors of the city of Crotone (Figure 6). In particular, this pattern comes out very distinctively even with the naked eye in the ascending geometry dataset (Figure 6a), where LOS velocity is estimated at up to \(-1\) cm/year. The rest of the city instead appears overall stable in both geometries, as expected for an historic urban settlement (Figure 6a,b). However, this evidence links with the reference point that was set within the urban area (see green star in Figure 6), and so any deformation signal is relative to such point.  
By analyzing the P-SBAS data in more detail, and specifically the vertical displacement field (Figure 8a), coherent targets with \(V_{U}\) values reaching \(-0.75\) to \(-1.3\) cm/year are distributed along the 1.7 km-long breakwater of the new harbor, as well as along the smaller breakwater and over the commercial and residential buildings overlooking the old harbor. A reasonable explanation of such subsidence might be the typical settlement of the harbor infrastructure, also as a consequence of recent interventions and maintenance works. In this regard, it is worth recalling that LOS displacement rates between \(-0.2\) and \(-0.5\) cm/year had been already found with ERS-1/2 and ENVISAT data in the period 1995-2009 [38].  
The horizontal field (Figure 8b) provides a further interesting insight, given that both the breakwaters and most of the old town (including the massive Castle of Charles V) fall within a wide pattern of eastward deformation, with \(V_{E}\) ranging between +0.2 and +0.6 cm/year. Such pattern corroborates the evidence of +0.3 to +0.5 cm/year found with COSMO-SkyMed data for the period 2011-2014 [28]. Any slight difference between the two InSAR analyses could mainly be due to the different time periods surveyed, as well as the different techniques used to process the SAR data and the respective processing parameters that were set up, above all the reference point and its precise location within a known tectonically active region. Continuous geodetic monitoring data from two GPS stations installed in Crotone (CROT and KROT) indeed confirmed the presence of significant eastward horizontal motions, alongside northward ones [93]. The latter, however, are not captured in the P-SBAS results due to the known relatively poor visibility of north-south horizontal movements to the SAR sensor LOS, and the \(V_{N}\) = 0 assumption that was made to estimate the vertical and east-west deformation field (see Section 3.3). In this case, such an assumption prevents any north-south displacement to be estimated, and also introduces an error in the other two components (vertical and east-west), especially at locations where a significant north-south displacement occurs [81]. Detailed date-by-date comparison of the P-SBAS and GPS monitoring data at the two stations would require the setting of a local reference frame for the GPS stations and the calibration of the P-SBAS results to the same reference system, as done in [28]. This would enable the quantification of the P-SBAS datasets accuracy against external data, though such analysis is beyond the scientific goals of this study.