The Dec 22nd 2018 flank collapse and tsunami at Anak Krakatau in Indonesia is a key event in geosciences as little is known about the lead-up processes and deformation changes prior to flank failure. We processed Sentinel-1 satellite radar data in both ascending (orbit 171) and descending (orbit 47) acquisition using multi-temporal InSAR with the Small BAseline (SB) method during the 4 years prior to the collapse. The data shows that the flank was already moving for years prior to collapse, demonstrating that developing instability in volcano can be monitored long before a collapse. The southwest flank movement rates averaged approx. 27 cm/yr, but underwent intermittent accelerations coinciding with distinct intrusion events in Jan/Feb 2017 and in Jun 2018. The data archived here supplements the material detailed in Zorn et al. (202X, https://doi.org/XXXXX).

We processed the displacement flank evolution at Anak Krakatau using multi-temporal InSAR technique and exploited the Sentinel-1 data in both ascending (orbit 171) and descending (orbit 47) acquisition orbit in the period between 08th Oct 2014 and 19th Dec 2018. We adopted the Small BAseline (SB) method as implemented in the ENVI SARscape® software, using standard processing methods (Berardino et al., 2002). SB allows for a maximisation of the spatial and temporal coherence, and therefore maximises the displacement measurements over the flank, owing to the combination of interferograms with small normal and temporal baselines. We generate interferograms connecting each image with two previous and two following acquisitions. The original data has been multi-looked resulting in a pixel size of 15 m. We used the 30m resolution Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) via the Earth Resources Observation And Science (EROS) Center 2017 to remove the topographic component from the interferograms and we filter the results using Goldstein filter (Goldstein and Werner, 1998) with tile sizes of 18×18px. Finally we unwrap the interferograms masking out coherence lower than 0.2 and applying two-dimensional phase unwrapping algorithm Snaphu and we refer to the displacement measurement using a reference point located on the most northern part of the island considered stable (Fig. 2A). The final results are 1D displacement maps in ascending and descending line-of-sight (LOS). Due to a significant increase in eruptive activity of Anak Krakatau beginning towards the end of May 2018, coherence was significantly reduced on the SW-flank and many unwrapped points became unreliable and had to be filtered out. To capitalise on the previously better coverage, we split our analyses into two separate datasets, one (short) for interpreting data before the 29th May 2018, containing more reliable points on the SW-flank, and one (full) dataset with fewer points used to interpret activity after.