Unstable Slopes and Their Surprising Impact: A Hidden Tsunami Threat
When we think of massive waves, our minds often wander to the vast open ocean and powerful earthquakes. However, a recent global study has revealed a surprising truth: some of the tallest and deadliest tsunamis ever recorded were not born from fault lines, but from unstable slopes collapsing into water. These landslide-triggered tsunamis, or LTTs, account for a significant portion of recorded tsunami events and pose a unique and urgent threat.
But here's where it gets controversial... these LTTs are not just a rare occurrence. They are the second most frequent source of tsunamis worldwide, and their impact can be devastating. With waves reaching heights of over thirty meters and, in extreme cases, hundreds of meters, these events highlight a highly localized and fast-acting hazard that scientists are still striving to fully comprehend.
The new global catalog, led by Katrin Dohmen and colleagues, provides a comprehensive overview of 317 landslides and their resulting tsunamis. Each case is meticulously described, from the triggering event to the wave's magnitude and the subsequent damage and loss of life. The researchers categorized these events into six main causes, with earthquakes taking the lead at 44% of documented LTTs. Volcanic activity, paraglacial conditions, and human activity also contribute significantly to this phenomenon.
And this is the part most people miss... the catalog reveals that these waves are not just a coastal issue. About a third of LTTs occur along open coasts, but a staggering 41% take place in enclosed marine environments like bays and fjords, and a quarter in inland waters such as lakes and reservoirs. These confined settings trap wave energy, intensifying the impact and creating a unique and unpredictable hazard.
For instance, in 1958, a rockslide into Lituya Bay, Alaska, produced a wave that stripped vegetation up to an astonishing 524 meters above the water. Similarly, the Vajont reservoir disaster in Italy in 1963 sent water surging up surrounding slopes, killing over 2,000 people. These events serve as stark reminders of the power and unpredictability of landslide-triggered tsunamis.
Climate change and human activities further exacerbate this risk. Many worrying cases are linked to changing climate conditions in cold regions, where glaciers retreat and leave unstable landscapes. As ice thins and retreats, steep rock walls lose their frozen support, permafrost degrades, and sediment-built deltas become more unstable. Events like the 2017 Greenland rockslide, which sent a tsunami sweeping through nearby villages, and the 2023 Greenland rockslide, which created a wave estimated at two hundred meters high, highlight the potential consequences of a warming climate in high-latitude fjord coasts.
Human activities, such as the construction of large dams and reservoirs, also contribute to this risk. Fluctuating water levels and heavy rainfall can trigger landslides along the shores, especially in areas like the Three Gorges region in China, where thousands of landslides have occurred post-impoundment. Open-pit mines, quarries, and artificial embankments are also potential sources of LTTs, creating waves that threaten workers, communities, and infrastructure.
The challenge of warning systems is twofold. Firstly, most landslide waves arrive in minutes, leaving little time for official alerts to be effective. Secondly, uncertainty plagues key parameters such as landslide volume, material, and location, especially for submarine slides. Publicly available seafloor maps often lack the resolution to identify smaller features that could generate dangerous waves. This makes it incredibly difficult to predict the height and impact of an LTT, leaving coastal towns vulnerable.
Existing early warning systems are primarily designed for classic tectonic tsunamis, performing well in regions like Indonesia and Japan. However, when earthquakes trigger underwater landslides, the resulting waves can arrive earlier and be taller than predicted by models. This combined threat warrants special attention for coastal regions near large strike-slip faults, such as Palu Bay and parts of the Sea of Marmara.
Scientists argue that the first step towards mitigating this risk is to identify unstable slopes. This requires high-resolution bathymetry along coasts, especially in active tectonic margins and around major reservoirs. Additionally, landslide susceptibility mapping should be applied to offshore areas, not just hillsides above sea level. In places like Tafjord, Norway, and some Chinese reservoirs, authorities already monitor known unstable slopes with advanced sensors and instruments, providing alerts to residents when failure seems imminent.
For most coastlines, however, protection relies on a simpler approach: education. People living or vacationing near steep fjords, volcanic islands, or large reservoirs need to be aware that strong shaking, rockfalls, or loud roars near the shore could be signs to evacuate immediately, without waiting for official warnings.
Landslide tsunamis may not be the most common waves, but they highlight the intricate connection between mountains, dams, volcanoes, and the sea. When a slope fails at the wrong time and place, coastal life can be forever altered in a matter of seconds. This study, published in the journal "Natural Hazards," emphasizes the urgent need for further research and awareness to address this hidden tsunami threat.
What are your thoughts on this surprising phenomenon? Do you think enough is being done to address the risks posed by landslide-triggered tsunamis? Share your opinions and let's spark a discussion!
