Flank instability and sector collapses, which pose major threats, are common on volcanic islands. On 22 Dec 2018, a sector collapse event occurred at Anak Krakatau volcano in the Sunda Strait, triggering a deadly tsunami. Here we use multiparametric ground-based and space-borne data to show that prior to its collapse, the volcano exhibited an elevated state of activity, including precursory thermal anomalies, an increase in the island’s surface area, and a gradual seaward motion of its southwestern flank on a dipping décollement. Two minutes after a small earthquake, seismic signals characterize the collapse of the volcano’s flank at 13:55 UTC. This sector collapse decapitated the cone-shaped edifice and triggered a tsunami that caused 430 fatalities. We discuss the nature of the precursor processes underpinning the collapse that culminated in a complex hazard cascade with important implications for the early detection of potential flank instability at other volcanoes.

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Abstract:The detection and monitoring of mass movement of susceptible slopes plays a key role in mitigating hazards and potential damage associated with creeping slopes and landslides. In this paper, we use observations from both Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) to assess the slope stability of the Sarcheshmeh ancient landslide in the North Khorasan province of northeast Iran. InSAR observations were obtained by the time-series analysis of Envisat SAR images covering 2004-2006, whereas repeated GPS observations were conducted by campaign measurements during 2010-2012. Surface displacement maps of the Sarcheshmeh landslide obtained from InSAR and GPS are both indicative of slope stability. Hydrogeological analysis suggests that the multi-year drought and lower than average precipitation levels over the last decade might have contributed to the current dormancy of the Sarcheshmeh landslide.

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Geodetic monitoring of infrastructures is one of the key tasks in surveying and engineering geology. Systematic monitoring and assessment of the exterior deformation of embankment dams for safety analysis are often difficult when using classical surveying techniques due to time-consuming surveying procedures and high labour costs. Modern remote sensing techniques play an important role in efficiently assessing deformation: changes in the geometry, position and orientation of dams. In this study, we present the feasibility of effective post-construction deformation monitoring of the Masjed-Soleyman dam in Iran using high-resolution (∼1 m) synthetic aperture radar (SAR) imagery in SpotLight (SL) mode from the X-band TerraSAR-X (TSX) satellite. This dam has been monitored over the last 15 years using classical surveying techniques, which provide horizontal and vertical deformation measurements of the structure. We show that high-resolution X-band SAR data provide a much more detailed identification of dam deformation in the crest and downstream that is not possible to infer from classical surveying techniques with few sparse geodetic monuments. High-resolution TSX data reveal that the dam is currently subject to two different deformation regimes: one is related to the crest and its adjacent area downstream, with a maximum rate of deformation of approximately 13 cm/yr in the radar line-of-sight (LOS). The other is related to the lower part of the downstream, with a maximum LOS velocity of 7 cm/yr. The effect of this centimetre displacement has been shown through several damage features on the dam body, including minor to large dislocation cracks on the crest and a significant deformation zone on the downstream slope.

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