Complex hazard cascade culminating in the Anak Krakatau sector collapse

Abstract

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.

Mapping Recent Shoreline Changes Spanning the Lateral Collapse of Anak Krakatau Volcano, Indonesia

“…Starting from May 2018, the shoreline changes show no increase in the island area, despite the elevated eruptive activity in Summer 2018 noted by Walter et al (2019) [32] but a slight decrease from 2.93 km 2 in May 2018 to 2.83 km 2 in November 2018. Within this period, there is a small increase in area from August to October 2018, due to the shoreline advancement towards the south given by the newly erupted material.…”

“…Rakata is a remnant of the island that was volcanically active in 1883, whereas Sertung and Panjang are remnants of a caldera wall that pre-dates this. Since its subaerial emergence, eruptive activity at AK has continued periodically [27] and the shoreline has undergone substantial changes as the planform of the island has evolved-from an aerial extent of 1.55 km 2 [32]. These fluctuations reflect the many eruptive episodes [24] during which the island has grown through emission of lava flows and deposition of tephra [27].…”

“…After 15 months of quiescence AK began a new eruption phase on 21 June 2018 (and currently ongoing), characterized by Strombolian activity that generated low-level ash plumes and ballistic ejecta, alongside lava flows [24]. This activity was typical in style of that which occurred in preceding decades, although there is evidence that the overall magmatic flux was notably high in this period [32]. On the 22 December 2018, several months after eruptive activity had begun, a lateral collapse event reduced the height of the island from 320 to 120 m [32] and approximately halved the island area [33].…”

“…On the 22 December 2018, several months after eruptive activity had begun, a lateral collapse event reduced the height of the island from 320 to 120 m [32] and approximately halved the island area [33]. The south-west flank is thought to have collapsed, partially as a result of its over steepening due both to long-term growth on this side of the island, which is built on a submarine scarp of the 1883 caldera and the additional volume erupted between June and December 2018 [32,33].…”

Volcanic Hot-Spot Detection Using SENTINEL-2: A Comparison with MODIS–MIROVA Thermal Data Series

“…For instances, fumaroles at Chaitén lava dome (Chile, [57]) are perfectly localized by our algorithm, which detect several small (< 9 pixels) clusters composed by α-triggered warm pixels only (Figure 4 a,b). Instead, glowing inside the main vent of Anak Krakatau Island (Indonesia, [58]) triggered a central hot pixel (β condition) and a surrounding perimeter of eight warm pixels (α condition), related to a new Strombolian activity producing incandescence and grey plumes at the beginning of the eruptive phase culminating with the December 2018 island flank collapse (Figure 4c,d; see [59]). These cases definitively reveal the meaning of the 9 pixels threshold, working as a minimal criterion to detect thermal signals produced by a very hot and localized source, with a hotter core extending heat in the nearby area of at least of eight pixels.…”