## 7 Discussion
### InSAR-Detected Ground Motions in Ireland (AOI-1 and AOI-2)  
In general, the ground motions detected in AOI-1 can be related to compaction of unconsolidated sediments, particularly peatlands, and to coastal erosion processes. Most of the detected velocities are negative in both LOS geometries, indicating that the motions are dominantly vertical (subsidence), and they range between \(-1.5\) and \(-14\) mm/year. One exception to this general pattern occurs in the peatland north of Galway city, where positive velocities are detected in both LOS geometries indicating uplift of between 1.5 and 8.9 mm/year. Another exception occurs on parts of the Cliffs of Moher, where the opposite sign of the detected velocities indicates a large horizontal component of movement. A plausible, but unverified, explanation for the ground motions detected in urban areas (Galway city) and along infrastructure in rural areas is the compaction of unconsolidated sediments (peat or lacustrine deposits) due to the load produced by the recent construction of roads and buildings. [19; 20] proposed this cause also for some ground motions indicated by PS-InSAR in Cork and Dublin through analysis of ERS-1/2 and ENVISAT data and comparison to historical Ordnance Survey maps. Since some sections of the motorway and the railways in AOI-1 pass over highly karstified limestone regions (e.g., the Gort lowlands), however, a relationship of detected motions to karst or epi-karst processes cannot be excluded. Sub-surface geophysical studies or drilling may be required to further test such hypotheses. Our study also highlights the suitability of the PS-InSAR technique for monitoring both man-made coastal defense structures and natural coastal landforms in Ireland. The detected instabilities on the Cliffs of Moher are not currently registered in the national landslide database [68], but local warning signs indicate that the local community and authorities are aware of instability issues there. As in the other areas of detected ground motion, our study does not enable a definitive cause to be ascribed to the observed motions. Motions on the Hag's Head, for instance could relate either to superficial creep of talus or to a deeper-seated subsidence related to undercutting of the cliff and cave formation by wave action. Nonetheless, these results show that PS-ISAR can provide local and national authorities in Ireland with new sets of information useful to mitigate risks associated with landslides and coastal instability. The peatlands of AOI-1 provide the most striking example of the greater capacity of PS-InSAR based on Sentinel-1 data to detect ground motions compared to older C-Band satellites (ERS-1/2 and ENVISAT). Over these peatlands, we detected over 10,500 PSs with Sentinel-1 and only 1482 with ENVISAT. The simple explanation again lies in the greater acquisition frequency of Sentinel-1, which provides more highly coherent points. The radar back-scattering mechanism in peatland is complex, however, as it involves a combination of surface and volume scattering [69]. The areas of higher temporal coherence are found in the uncut peatland centers, where the Sphagnum moss-dominated vegetation cover, which generally remains stable over the year, provides a consistent volume backscattering while the humid soil acts as an effective surface backscatterer. The harvested margins of the peatland areas provide far fewer PSs, because backscattering there is limited by the growth of leafy or grassy vegetation and by drier soil conditions. The growth rates of Sphagnum moss in such climate conditions has been estimated to be in the order of 1 cm/year [70], rates that are compatible with the uplift up to 8.9 mm/year measured with Sentinel-1 over one of the presented raised peat example (Figure 15a). Although most of the displacements measured here show a linear trend, some of them present short-term variations that could be related to the water levels in the peat.