Satellite Geodesy Uncovers 15 m of Slip on a Detachment Fault Prior to the 2018 Collapse at Anak Krakatau, Indonesia

On 22 December 2018, parts of the Anak Krakatau edifice collapsed, triggering a deadly tsunami. To investigate pre-collapse surface displacements, we analyzed Interferometric Synthetic Aperture Radar satellite geodetic data from 2006 to 2018, acquired from ALOS-1 (2006–2011), COSMO-SkyMED (2012–2018), and Sentinel-1 (2014–2018). We identified line-of-sight displacements on the southwestern flank throughout the study period. Inversion of COSMO-SkyMED data revealed a rectangular dislocation with a cumulative slip of 12 m from April 2012 to December 2018. Fixing the fault geometry, we found the optimal slip for time periods corresponding to slip rate changes, ranging from 1.2 to 3.1 m/yr. The slip estimates for ALOS-1 and Sentinel-1 data were 0.88 m/yr and 1.1 m/yr, respectively, over their individual time periods. Overall, the detachment fault experienced approximately 15 m of slip from 2006 to 2018 with acceleration and deceleration periods, and a notable acceleration prior to the 2018 collapse.