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.

The 2004 Sumatra-Andaman tsunami flooded coastal northern Sumatra to a depth of over 20 m, deposited a discontinuous sheet of sand up to 80 cm thick, and left mud up to 5 km inland. In most places the sand sheet is normally graded, and in some it contains complex internal stratigraphy. Structures within the sand sheet may record the passage of up to 3 individual waves. We studied the 2004 tsunami deposits in detail along a flowparallel transect about 400 m long, 16 km southwest of Banda Aceh. Near the shore along this transect, the deposit is thin or absent. Between 50 and 400 m inland it ranges in thickness from 5 to 20 cm. The main trend in thickness is a tendency to thicken by filling low spots, most dramatically at pre-existing stream channels. Deposition generally attended inundation-along the transect, the tsunami deposited sand to within about 40 m of the inundation limit. Although the tsunami deposit contains primarily material indistinguishable from material found on the beach one month after the event, it also contains grain sizes and compositions unavailable on the current beach. Along the transect we studied, these grains become increasingly dominant both landward and upward in the deposit; possibly some landward source of sediment was exposed and exploited by the passage of the waves. The deposit also contains the unabraded shells of subtidal marine organisms, suggesting that at least part of the deposit came from offshore. Grain sizes within the deposit tend to fine upward and landward, although individual units within the deposit appear massive, or show reverse grading. Sorting becomes better landward, although the most landward sites generally become poorly sorted from the inclusion of soil clasts. These sites commonly show interlayering of sandy units and soil clast units. Deposits from the 2004 tsunami in Sumatra demonstrate the complex nature of the deposits of large tsunamis. Unlike the deposits of smaller tsunamis, internal stratigraphy is complex, and will require some effort to understand. The Sumatra deposits also show the contribution of multiple sediment sources, each of which has its own composition and grain size. Such complexity may allow more accurate modeling of flow depth and flow velocity for paleotsunamis, if an understanding of how tsunami hydraulics affect sedimentation can be established.

A huge earthquake of magnitude M 9.0 occurred at 00:58 (UT), December 26, 2004, in the sea off the west coast of northern Sumatra, Indonesia, followed by a huge tsunami that hit almost all coasts facing the Indian Ocean. We conducted a field survey in the residential area of Banda Aceh, the town of the severest damage by the tsunami, on the west coast of the northernmost point Sumatra, Sigli City, about 80 kilometers east of Banda Aceh three-four weeks after the event. In Banda Aceh, almost all houses in the residential area about 2 km from the coast were swept away, while houses more than 3 km rarely were. Inundation continued about 5 to 6 km from the shoreline. In Lhoknga and several villages on the west coast of Sumatra Island near Banda Aceh, where tsunamis 15 to 30 meters high hit coastal villages, nobody survived. Along the valley about 1 km north of the cement plant, seawater rose to a height of 34.8 m above MSL, which is the highest recorded inundation measured in our survey.

We established an extensive database of tsunami field evidence in order to facilitate various numerical models of the 26 December 2004 tsunami in the Banda Aceh district of Sumatra, Indonesia. Inputs provided by 12 weeks of efforts during seven field trips include field measurements of runup, tsunami heights, flow depths, flow directions, and events’ chronology. We deliver detailed inundation maps of the western and northern coasts of Banda Aceh.About ten waves reached the coast. The maximum tsunami height reached 35 m during the third wave on flat areas and the maximum runup was 51 m measured on hill slope. This value is the highest runup ever measured in human history for a co-seismic tsunami event. The main hydrologic characteristic of the 26 December 2004 tsunami is the break of the main wave up to 3 km inland from the shoreline. The data collected, such as the numerous warning signs, could help further prevention actions.

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