## 5 Discussion
### Geological Interpretation
#### 5.1.1 Coastal Land Subsidence at the Capo Colonna Promontory
Therefore, the present study is the first that, at least for the last six years, is able to fully constrain the geometry of the land subsidence process at the promontory not only along the LOS, but also along both the vertical and east-west directions. Furthermore, a closer look at the temporally dense InSAR time series highlights that the displacement trend of land subsidence has been almost constant over the study period (Figure 10a), with a magnitude of total displacement that makes a clear separation between the subsiding points and the more stable ones (Figure 10b). Pending an in-depth time series analysis (also through statistical methods, which is beyond the scope of this research), the nonlinearities visible throughout the time series do not seem to coincide with any significant deceleration or inversion of the land subsidence trend. Future research will also need to focus on the temporally confined trend variations occurred in July-October 2016 and September-October 2018, which will have to be further investigated through comparison with independent in situ monitoring data (not available for this research). In this regard, the present analysis allows a revisit of the assessment made based on an earlier Sentinel-1 dataset, in which a slight decrease in the annual LOS velocities of subsidence around the gas extraction sites was suggested [46]. That dataset consisted of 46 ascending mode images only, collected by the Sentinel-1A satellite alone, every 12 days, from October 2014 to May 2016, and was processed with an intermittent SBAS [89] processing method capable of extending the coverage of motion results across the full range of land-cover types by relaxing the approach to selecting image pixels to process [90]. Therefore, despite the high density of coherent pixels over the agricultural area provided in [46], the limited length of the image datasets somehow might have affected the estimation of the deformation velocity by tying it to a shorter time period. On the other hand, with nearly 300 displacement estimates composing the InSAR time series in each Sentinel-1 observation geometry (Figure 11), the current P-SBAS analysis provides a more robust evidence of the uninterrupted land subsidence long-term behavior and its rate of occurrence. \begin{table}
\begin{tabular}{c c c c} \hline
**Reference** & **Satellite Data** & **Period** & **Observed Deformation Rates** \\ \hline [PERSON] et al. 2010 [38] & ERS-1/2 asc. & 1995–2000 & More than \(-0.5\) cm/year LOS rate \\  & ENVISAT asc. & 2002–2009 & \\ \hline [PERSON] and [PERSON] 2012 [39] & ERS-1/2 asc./desc. & 1992–2000 & Up to \(-1.0\) (1992–2000) and \(-1.3\) \\  & ENVISAT asc. & 2003–2010 & (2003–2010) cm/year LOS rate \\ \hline [PERSON] et al. 2016 [45] & TerraSAR-X asc./desc. & 2008–2010 & Up to \(-1.5\) cm/year LOS rate, and \(-1.6\) cm/year vertical rate \\ \hline  & ERS-1/2 asc./desc. (from [39]) & 1992–2000 & Up to \(-2.0\) (2011–2014), \(-1.0\) \\  & ENVISAT asc. (from [39]) & 2003–2010 & (2014–2015) and \(-0.8\) (2014–2016) \\ [PERSON] et al. 2016 [46] & TerraSAR-X asc./desc. (from [45]) & 2008–2010 & cm/year LOS rate; up to \(-20\) cm \\  & COSMO-SkyMed asc./desc. & 2011–2015 & vertical displacement (1992–2016; \\  & Sentinel-1A asc. & 2014–2016 & stitched time series) \\ \hline [PERSON] et al. 2018 [28] & COSMO-SkyMed asc./desc. & 2011–2014 & Up to \(-0.5\) cm/year east–west rate \\ \hline [PERSON] and Tapete 2020 1  & Sentinel-1A/B asc./desc. & 2014–2020 & Up to \(-2.3\) cm/year vertical and \(-1.0\) cm/year east–west rate \\ \hline \end{tabular}
\end{table} 
Table 1: Existing InSAR literature covering the Capo Colonna promontory, with details on the input SAR data, monitoring period covered, and the observed deformation rates. Notation: asc. = ascending; desc. = descending.