New scientific analysis has revealed that a colossal megatsunami in Alaska, which occurred in 2015, was the second largest ever recorded globally. Researchers from institutions including the University of Washington and the Alaska Earthquake Centre have pinpointed that surprisingly small, previously unrecognised earthquakes were the catalysts for the massive landslide that generated the enormous wave. The study, which has been peer-reviewed and published in the journal Science Advances, sheds new light on the complex interplay between seismic activity and landscape stability in glacial environments.
The event, which took place in Taan Fjord, Icy Bay, saw an estimated 180 million tonnes of rock detach from a mountainside and plunge into the narrow fjord. This immense displacement of material generated a wave that reached an astonishing height of over 190 metres – taller than London's Gherkin building – as it surged up the opposite slope. While the scale of the tsunami was immediately evident, the precise trigger for the landslide had remained a subject of ongoing investigation.
The new research utilised a combination of seismic data re-analysis, satellite imagery, and field observations to reconstruct the sequence of events. Scientists discovered that a series of small, shallow earthquakes, with magnitudes ranging from 2.5 to 3.0, occurred in the hours leading up to the landslide. These tremors, individually too small to be considered a major threat, are now understood to have progressively destabilised the already fractured rock mass, which was likely under stress from glacial retreat and permafrost thawing.
This finding challenges previous assumptions that only large-magnitude earthquakes could initiate such catastrophic landslides and subsequent tsunamis. The researchers suggest that the cumulative effect of these minor seismic events, coupled with the unique geological and environmental conditions of the glaciated fjord, created a 'perfect storm' for the disaster. The study's lead authors, including Dr. Bretwood Higman, emphasised the importance of considering the role of subtle seismic activity in hazard assessments for similar regions worldwide.
The implications of this research extend beyond Alaska, offering crucial insights for other seismically active, mountainous regions with fjords or deep valleys, including parts of Scandinavia, British Columbia, and New Zealand. Understanding that small earthquakes can trigger such devastating events is vital for refining early warning systems and land-use planning in areas susceptible to landslides and tsunamis. It underscores the need for continuous, high-resolution seismic monitoring to detect even minor tremors that could precede major geological hazards.
This research builds upon existing knowledge of tsunamigenic landslides, such as the 1958 Lituya Bay megatsunami, also in Alaska, which remains the largest recorded. The Taan Fjord event provides a contemporary example of these powerful natural phenomena and highlights the evolving understanding of their triggers in a changing climate, where glacial retreat can further destabilise mountain slopes.
Source: University of Washington, Alaska Earthquake Centre