*4.3. Detection of Morphological Changes (SAR)*

The intensity of SAR images is strongly dependent on the terrain slope, and is therefore useful to monitor morphological changes affecting the volcano. Because radar wavelengths penetrate through clouds, SAR intensity images provide crucial insights into the volcanic activity when optical imagery is obstructed by atmospheric and/or volcanic gas clouds. We here give two examples taken from very different volcanological settings: the summit crater collapse of K¯ılauea (Hawai'i) during the 2018 effusive eruption (Figure 8a), and the Anak Krakatau (Indonesia) island growth and destruction during the 2018 explosive eruption (Figure 8b).

#### **Figure 8.** *Cont*.

**Figure 8.** Two examples of morphological changes detected from SAR intensity (images on top rows, black markers on timeline), and closest S2 SWIR acquisition (images in bottom rows, orange markers on timeline). The temporal evolution of the most prominent morphological changes visible in SAR images are sketched on the right. (**a**) Caldera collapse of Kilauea (Hawai'i) during the first months of the 2018 flank eruption. (**b**) Anak Krakatau (Indonesia) island growth during the months preceding the 2018 tsunamigenic landslide, and horseshoe-shaped caldera after the landslide. In this figure speckle is removed from SAR images using a non-local means filter [78] (NDSAR, https://github.com/odhondt/ndsar), and tone mapping of SWIR images is fixed so that colors and contrasts are the same in each image. See Supplementary Material S3 and S4 for video animations spanning several months.

Kilauea is well known for its persistent active lava lake. In 2018, it experienced its largest flank eruption and caldera collapse in the last 200 years [79,80]. During spring 2018, the lava lake activity was high, which was clearly detected as a hotspot in the S2 SWIR images (Figure 8a, 13 April 2018). On 30 April 2018, seismicity indicated the intrusion of a dyke along the East Rift Zone, which generated a ~38 km long deformation zone, and multiple eruptive fissures with lava flows rapidly reaching the sea (see Supplementary Material S5 for analysis of S1 and S2 over the entire rift zone). During this time the summit underwent significant changes: lava lake withdrawal (i.e., hotspot disappears in SWIR images, Figure 8a), accompanied by summit subsidence (i.e., deflation detected in interferogram), progressively evolving in a ~3 km wide caldera collapse (see LIDAR digital elevation model in [80]), as the shallow magma reservoir was being drained. The progression of the caldera collapse is clearly imaged with SAR intensity images, which reveal the progressive formation of fractures and the profound summit morphological changes accompanying the flank eruption (Figure 8a and video in Supplementary Material S3). Ash deposits following the eruption onset is also captured, identified by a decrease in the SAR backscattered intensity on the SE flank of the volcano, also visible in the SWIR images. This decrease can be explained by the fact that fresh ash is less reflective than bare rock owing to its loose structure and high porosity, and that ash deposits smooth the surface, resulting in a more specular reflector which backscatters less energy towards when the slope is facing away from the sensor [23].

Krakatau is well-known for its volcano-induced tsunamis. Just over 135 years after the famous 1883 event, the volcano triggered on 22 December 2018 another deadly wave. Analysis of the SAR intensity images clearly shows the progressive island growth in the months preceding the tsunami, due to multiple lava flows reaching the sea and extending the island's coast line (Figure 8b and Supplementary Material video S4). This likely increased the instability of the volcano's flank, which on 22 December 2018 collapsed, generating a tsunami wave [81]. Post flank sector collapse images first reveal an amphitheater-shaped scar opened to the sea (Figure 8b, 31 December 2018 image), which was closed shortly after by an explosion tuff ring, resulting in a ~400 m wide water filled crater (Figure 8b, 12 January and 15 February 2019 images).
