*4.2. Tephra fall*

During the February 2018 paroxysm the eruption column reached over 6000 m height (Figure 2) and was mostly dispersed to the west of the volcano. Figure 4 shows the tephra deposit extent obtained from the NDVI difference method. One other image source (Sentinel 2) was also processed in a similar way for comparison. For the PS images, the NDVI difference is calculated using an image collected in January 2018 and another right after the eruption. In order to have a meaningful comparison between PL and Sentinel 2, the post-eruption images are both from 4 February (Figure 5), with pre-eruption images from 9 January (PS) and 10 January (Sentinel 2). Slightly different thresholds were used to delineate the tephra fall covered areas for each image source; for the PS imagery a threshold of −0.125 was used for the NDVI difference values (i.e., pixels with an NDVI difference <−0.125 were considered to be covered with ash) which gives an area of 41.5 km<sup>2</sup> for the ash covered vegetation. For Sentinel 2 data, a NDVI difference threshold of −0.1 was used, which results in an ash covered area of 35.6 km2. Deposits derived from the two data sources are similar to each other in shape and orientation (Figure 4), and both show limitations in detecting the very proximal tephra fall deposits (i.e., in non-vegetated areas).

**Figure 4.** *Cont*.

**Figure 4.** Tephra fall deposit mapped using NDVI difference analysis of (**A**) PlanetScope and (**B**) Sentinel 2 images from 9th 10th January and 4th February 2018. Threshold values of –0.1 for Sentinel 2, and–0.125 for PlanetScope were used to identify the ash covered areas shown in red in (**A**) and (**B**). (**C**) Shows the areas that can be identified as ash covered by the NDVI differences in each set of satellite images after applying a low-pass filter and converting them to polygons (only taking the connecting polygons along the expected dispersal axis). BOA = Bottom of Atmosphere. Background image is a PlanetScope image from February 4, 2018 at 15:58 UTC.


**Figure 5.** Schematic three-month timeline for satellite data coverage of Landsat 8 (L8), Sentinel 2 (S2) and Planet Labs (PL). Only usable data, with no cloud coverage over the study area are considered. Images selected for NDVI analysis are marked with darker colours. The red box shows the February 2018 event period.
