## 3 Characteristic Deformation and Associated Physical Processes at Piton de la Fournaise volcano
### Short-Term Rapid Variations and Typical Deformation Patterns Associated with Eruptions
#### 3.2.2 Post-Eruptive Displacements  
At Piton de la Fournaise volcano, short revisit times (i.e., higher repeat cycles) of ESA's S1A and S1B satellites (one ascending and one descending IW acquisition every 12 days, and one ascending and one descending SM acquisition every 12 days, 6 days apart from the IW acquisitions) allow for the recognition of subtle displacement signals that might not be detectable with coarser temporal sampling due to superposition of pre-, co-, and post-eruptive surface deformation. For instance, high data sampling of S1 SM data has facilitated the detection of inverse displacements upon the first April 2018 eruption, when post-eruptive line-of-sight (LOS) subsidence of up to ~7 cm was observed approximately at the northeastern tip of the preceding larger scale co-eruptive uplift (Figures 6 and 7). This eruption started on 03 April 2018 at 7H00 UTC, and the timing of ascending S1 SM SAR acquisitions spanning this eruption is particularly interesting: co-eruptive uplift is observed from an interferogram spanning 22 March 2018 (14H52 UTC) to 03 April 2018 (14H52 UTC) (not shown in Figure 7), while post-eruptive subsidence is already detected from interferograms starting on 03 April 2018 (14H52 UTC) (Figure 7a), i.e., less than 8 hours after the onset of the eruption. While the process behind this subsidence remains to be investigated, we note that interferograms spanning longer time periods, or being less fortunate regarding their timing, might not allow for the distinction of such subtle or short-lived (elastic) deformation processes. For example, the TSX interferograms as provided in Figure 6a,b span both the co-eruptive uplift and the post-eruptive subsidence. Moreover, the X-Band sensitivity causes coherence loss in the close proximity of the eruption site (i.e., at the northeastern tip of the uplift pattern that is related to the magma propagation towards the eruption site, cf. Figure 6a-d). At the same time, the closest GNSS station FJAG was not located at the center of this post-eruptive subsidence pattern and, therefore, did not pick up on the signal (Figure 6e,f and Figure 7).  
Figure 6: Deformation associated with the 03–04 April 2018 eruption as imaged by TSX SM interferometry: (**a**) unwrapped interferogram spanning 24 January to 22 April 2018 (ascending); (**b**) unwrapped interferogram spanning 28 January to 26 April 2018 (descending); (**c**) near-vertical displacements as derived from the combination of both ascending and descending interferograms, black triangle marks the location of the closest GNSS station (FJAG); (**d**) near E–W displacements as derived from the combination of both ascending and descending interferograms, black triangle marks the location of the closest GNSS station (FJAG); (**e**) vertical displacement component and (**f**) eastward displacement component of GNSS station FJAG (mm; at a daily sampling), where red bars mark the two April 2018 eruptions and dashed lines mark the time periods spanned by interferograms: red—time spanned by Figure 6a, blue—time spanned by Figure 6b, green—time spanned by Figure 7a, black—time spanned by Figure 7b.