Research has shown that the Anak Krakatau volcano in Indonesia displayed clear warning signals before it collapsed into the Sunda strait between the Sumatra and Java islands on December 22, 2018. The event triggered a tsunami which resulted in 430 fatalities. The study, led by Thomas Walter of the German Research Center for Geosciences GFZ in Potsdam, was published on October 1, 2019, in the journal, Nature Communications. The authors 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 research team came up with the results after analyzing a large amount of data from various sources gathered during ground-based measurements, drones, and satellites.

Satellite data showed a rise in temperatures and ground movement on the southwestern flank several months before the event. Seismic information and low-frequency sound waves from a smaller quake prior to a partial collapse of the volcano had also signaled the incident.

The team aims to utilize the analysis of the complex event cascade to further develop the early detection and monitoring of other volcanoes. Some volcanic islands like Krakatau usually have unstable material. Thus, a collapse of volcanic flanks happens every now and then on the islands.

However, it had not been accurately measured until lately. Walter explained, "At Krakatau, we were able to observe for the first time how the erosion of such a volcanic flank took place and which signals announced it."

The researchers were able to present that the movement of the southeast flank over months formed an accelerated slide into the sea. The collapse occurred in two minutes and was measured by seismographs and infrasound networks before the initial impacts of the tsunami had reached the coast.

Image credit: GFZ

"We used an exceptionally broad range of methods: From satellite observation to ground-based seismic data, from infrasound to drone data, from temperature measurements to chemical analysis of eruption products," Walter said.

"Today's almost unrestricted access to worldwide data was critical in this. In the days following the tsunami, it allowed us to analyze this event at different locations in different countries at the same time."

Walter also stated that "tsunami early warning systems must also take into account events caused by landslides," and those volcanoes that show slipping threats should be combined into the monitoring systems.

The unusual seismic pattern of the flank collapse was a specific challenge when studying the data, said seismologist Frederik Tilmann from GFZ and the Free University of Berlin, who was part of the research team.

The Anak Krakatau volcano, before and after the explosion which caused the deadly tsunami on December 22, 2018 [source: https://t.co/HQDyu39UOn] pic.twitter.com/P0YgKHjraV — Massimo (@Rainmaker1973) September 27, 2019

Prior to the collapse of a section of the volcano Anak Krakatau in December 2018, which triggered a deadly tsunami, it exhibited several signs of activity for months, according to a @NatureComms study. https://t.co/zZuAB43wRP pic.twitter.com/nAsW55jxIo — Nature Research (@nresearchnews) October 3, 2019

According to Tilmann, in contrast to tectonic earthquakes, only a small portion of this pattern is composed of high frequencies around 1 Hertz (1 oscillation per second). The earthquake waves had stronger components in the range of low frequencies about 0.03 Hertz (1 oscillation per 35 seconds).

"This property was the reason why the event was not detected in any routine evaluation."

Furthermore, Walter said the efforts of monitoring systems will pay back since a lot of victims of volcano-related incidents in the past 200 years have not been killed by the explosions, but by landslides and tsunamis.

The new results show that the risks of collapsing volcanoes have been underestimated, and the first step in taking action is by identifying the volcanoes at specific risk. It is also vital to supplement existing measurement methods with more sensors and new algorithms for evaluation. "We are confident that our findings will lead to the development of improved monitoring systems," Walter stated.

Complex hazard cascade culminating in Anak Krakatau sector collapse: “prior to its collapse, volcano exhibited elevated state of activity, including precursory thermal anomalies, increase in surface area, & gradual seaward motion of its southwestern flank” https://t.co/ck9mfmSKZE pic.twitter.com/sDgIOw7SSr — Jascha Polet (@CPPGeophysics) October 3, 2019

Satellite monitoring & ground observations reveal Anak Krakatau was in an elevated stage of activity throughout 2018

Infrared data of radiometer indicates that a new intense eruptive phase initiated at Anak Krakatau on 30 June 2018 https://t.co/Bv8qAw77bt pic.twitter.com/eq5MCepQEC — PeterRMare (@PeterRMare) October 2, 2019

Reference

"Complex hazard cascade culminating in the Anak Krakatau sector collapse" - Walter, T.R. et al. - DOI: 10.1038 / s41467-019-12284-5

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.

Featured image credit: GFZ