What does stick in volcanoes mean

Tsunamis after volcanic eruptions: researchers want to improve early warning

"Everyone believed that the last days of the earth had come," noted the first officer of the US sailing ship W.H. Besse in his diary on August 27, 1883. On deck, the crew followed a huge eruption that took place 100 kilometers away. The volcanic eruption on the island of Krakatau was an event of the century, the echo of thunder could be heard on a third of the earth.

18 to 20 million cubic meters of rock and ash were hurled far into the atmosphere. The island collapsed completely in the explosion. Pressure waves raced around the globe several times; hot ash rain fell across the region. The worst consequence, however, was a tsunami that destroyed many settlements on the coasts of Sumatra and Java.

But the subsoil under the island did not come to rest: a good 40 years after this catastrophe, another volcano emerged from the sea at the same place - Anak Krakatau, child of Krakatau, was called the new vent. This volcano erupted every two to three years and grew rapidly in height. Nevertheless, despite constant monitoring, the catastrophe occurred again.

In a comparatively small eruption in December 2018, the unstable southern flanks of Anak Krakatau collapsed and slid into the sea. Another tsunami hit the coasts of Sumatra and Java, killing hundreds. There was no evacuation: the state-of-the-art tsunami early warning system installed in the region did not sound the alarm.

More than 50 volcanoes erupt on average each year. This has consequences for numerous cities and settlements - around 500 million people worldwide live near active volcanoes. The reinsurer Munich Re puts the damage caused by volcanic eruptions between 1980 and 2019 at 12 billion US dollars.

What are the reasons? "The early warning system is not yet perfect, it cannot identify all tsunami factors," says volcanologist Prof. Dr. Thomas Walter. Above all, landslides on volcanoes take place very slowly, often only a few millimeters per month, until a point is reached at some point where a volcanic flank finally gives way and falls into the sea.

The researcher at the German Research Center for Geosciences (GFZ) investigated the collapse of Anak Krakatau and evaluated numerous data from earth observation satellites, drones and control rooms. His conclusion: an increased activity of the volcano has been apparent for a long time, among other things due to the increased thermal radiation. But the Indonesian geological authority was not aware of the instability of the flank.

Walter now wants to improve the monitoring of so-called volcanoes with an international team and develop solutions for reliable early detection of landslides. To this end, the German-Indonesian project Tsunami_Risk started in March 2021, which is funded by the BMBF as part of the CLIENT II research program. In addition to the GFZ, the German Aerospace Center, the Disaster Research Center of the Free University of Berlin, the technical universities in Braunschweig and Berlin, the Federal Agency for Technical Relief and the Gempa company are involved in the project.

The first aim of the project partners is to identify those volcanoes and slopes in Indonesia that could trigger tsunamis at some point. This task alone is huge: there are almost 130 active volcanoes in the island state of Indonesia, which has a troubled geological underground. One of the world's longest subduction zones extends there, the Sunda Trench, where three tectonically active plates of the earth collide and repeatedly trigger quakes. There is often not much time for warnings.

Tsunami_Risk ties in with the major German-Indonesian project GITEWS in many respects, which produced the tsunami early warning system and was also funded by the BMBF. It has been in operation since 2008. Walter was also involved in the development. “The system has already saved many lives,” he emphasizes. Now it is a matter of improving warning sensitivity.

There are also quiet tsunamis that are not triggered by a thump, explosion, or strong earthquake, but by a landslide for which there are hardly any signs

Thomas Walter, German Research Center for Geosciences

“There are also quiet tsunamis that are not triggered by any oomph, explosion, or strong earthquake, but rather by a landslide for which there are hardly any signs,” explains Walter. The resulting low-frequency signals are difficult to locate and are superimposed by other frequencies. "That sounds like a scratching motion, but with an extremely low amplitude."

The research team wants to use existing seismic networks on site. However, where there are signs of danger, the monitoring should be modified. Satellite remote sensing is also integrated into the monitoring. “In the end, we want to better understand the dynamics of volcanic flanks, present solution concepts for improved early warning and inform decision-makers,” says Walter, who has been dealing with the topic for 20 years.

The researchers are therefore not only concerned with identifying and modeling the dangers posed by landslides and the resulting tsunamis, but also with communicating these dangers to the endangered coastal regions. The aim is to establish the risk chain and the necessary responses such as evacuation plans in guidelines Disaster management and Disaster Risk Reduction to be involved at national and local level and to implement the appropriate training and support on site. The Gempa company, which was spun off from the GITEWS project and was involved in the development and installation of the operational early warning system, plays an important role. Gempa will integrate modules for monitoring landslides and early warning in the existing system.

Dangerous landslides do not just happen thousands of kilometers away. The southeast flank of Mount Etna in Sicily is also slowly sliding towards the Mediterranean. Another example is Vesuvius. Other documented landslides occurred in 1956 at Bezymianny in Kamchatka, in 1980 with the eruption of Mount St. Helens in the USA, or in 1888 on Ritter Island in Papua New Guinea. The breaking off of flanks triggered huge eruptions.

If the corresponding data are available, it would be possible for the modelers involved in Tsunami_Risk to calculate how much rock mass could slide into the sea at certain volcanoes. In addition, tsunamis should be simulated better. “We are pursuing completely new approaches, but these will hopefully lead to better predictions of landslides,” says Walter.

There is great local interest in the project - especially in politics. In addition, the now large network with research institutions in Indonesia can be used. The long-term goal: As a module, the landslide simulations are to be integrated into existing tsunami early warning systems worldwide. It would be life insurance for many coastal residents.