How do volcanoes trigger tsunamis?

Volcanic eruptions often have far-reaching consequences, both in their impact and geographical extent. A deeper understanding of the complex processes that cause such events can enable more accurate hazard forecasting while improving preparedness and mitigation strategies.

Complex and dangerous

Volcanic activity can also trigger multiple hazards.

One type of hazard is tsunamigenic flows. These are mass flows that generate tsunamis hitting a body of water like the sea or lake. They can be landslides on the volcano flank or pyroclastic density currents (PDCs) - fast-moving, hot mixtures of gas, ash, and volcanic rock fragments that flow down the slopes of a volcano during an eruption.

Tsunamigenic flows are particularly complex because they involve interactions between geological, hydrological, and atmospheric processes.

The sudden nature and rapid propagation of the tsunamis caused by these flows make them particularly hazardous to coastal communities, both near and far from their point of origin.

The recent 2018 Krakatoa eruption and subsequent landslide-induced tsunami showed how these hazards can devastate communities and ecosystems.

What can we do to better understand what's going on and be ready for the consequences?

This was the starting question for a recent study in which researchers modelled past events at the volcanic island of Stromboli (Italy) to gain more insight into the possible consequences of tsunamigenic flows and the impacts they can cause.

The Stromboli study

A common problem with studying these phenomena is that the data is often limited.

Stromboli, however, is a highly monitored volcano and data for recent events are available thanks to seismic stations, thermal cameras, gas sensors, ground deformation instruments, and buoy wave gauge networks.

The study focuses on the small tsunami generated during the eruption that occurred at Stromboli on the 3rd of July 2019. This eruption, which sadly caused one fatality, was followed by PDCs and landslides. A 1.5m tsunami wave was registered in front of the Sciara del Fuoco - the slope of the volcano where most of the activity, PDCs and landslides usually happen.

The main aims of the researchers were to understand the characteristics of the primary source of the tsunami, assess the volumes and characteristics of the mass flow involved, determine how the morphology of the area influenced the wave height and propagation and assess the possible impacts of such events (through the interdisciplinary collaborations of the authors). 

The study combined field observations, remote sensing, numerical modelling, seafloor surveys and impact chain study.

Simulation

To achieve their goals, the researchers conducted a back analysis of the 3rd of July event using a numerical model. Input parameters included digital elevation models (DEMs) of the subaerial area and bathymetries of the submarine area before and after the event.

Once they determined the type of flow and rheology, the researchers were able to run a simulation in order to assess how variations in mass flow volumes and rheological properties affect tsunami generation.

Results

Study results suggest that the tsunami at Stromboli was mainly caused by a PDC with a volume of about 1 million cubic meters.

In further simulations, researchers tested different PDC volumes, eruption times, and cohesion values. It appeared that larger PDC volumes led to higher tsunami waves, whereas more prolonged eruptions with the same volume (in other words, lower discharge rates) resulted in reduced wave height.

This shows that being able to accurately estimate flow volumes and discharge rates is crucial for the hazard assessment of this type of phenomenon.

Impact chain analysis

Tsunamis caused by volcanic hazards can have cascading effects across various sectors and lead to long-lasting socio-economic and environmental impacts.

To visualise this, researchers from different and multi-disciplinary backgrounds applied the principles of impact chain analysis.

Impact chains help to break down risk scenarios into understandable components.

In volcanic environments, this approach is particularly useful due to the multi-hazard nature of volcanic activity. The impact chain methodology enables the visualisation of hazard events, as well as their direct physical impacts, including fatalities, damage to infrastructure, and environmental damage.

In addition, researchers can identify secondary and tertiary impacts, such as the need for temporary shelters and long-term socio-economic consequences.

Impact chain analysis also looks at risk factors and preparedness levels. This structured approach can serve to enable mitigation and adaptation measures.

Snapshots of the simulation video showing the propagation of PDCs into the sea and the generation of tsunami waves
Manzella et al. (2024)

Example of impact chain in case of an eruption at Stromboli volcano with focus on tsunamigenic flows hazard
Manzella et al. (2024)

Conclusion

In conclusion, the researchers emphasise the complexity of volcanic hazards.

Understanding tsunamigenic flows and assessing and managing the related hazards requires a multifaceted approach that incorporates numerical modelling, continuous monitoring, and impact chain analysis

Although modelling has improved our understanding of how volcanic mass flows entering the sea can generate tsunamis, significant challenges remain, especially when it comes to accurately capturing underwater dynamics.

On the other hand, remote sensing by satellites and drones has provided useful insights into volcanic landforms above sea level, making it easier to identify changes that may indicate increased instability.

Regarding future work, the researchers advocate more interdisciplinary collaborations, where geologists, geophysicists, engineers, social scientists, and local community representatives join forces to develop early warning systems, outline mitigation measures to enhance resilience to complex volcanic hazards.

This story is an adaptation of a journal article: Manzella, I., Makris, S., Casalbore, D., Cole, P., Kelfoun, K., Georgiopoulou, A., ... & van Westen, C. (2024). Cascading hazards in volcanic environments: monitoring, modelling and impact analysis of tsunamigenic flows for risk reduction. Annals of Geophysics, 67(4). It has been adapted with permission by the authors and in accordance with the copyright license CC BY 4.0 

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