Academic News
On New Year’s Day 2024, a magnitude 7.6 earthquake struck Japan’s Noto Peninsula and triggered a tsunami. Scientists were surprised when tsunami waves reached Toyama Bay in only two to three minutes, much earlier than the roughly ten minutes predicted by conventional tsunami theory. The observed wave heights were also significantly larger than expected.
Professor Tso-Ren Wu of National Central University (NCU), together with Yushan Scholar Professor Kenji Satake and their research team, published a study in *Engineering Geology* showing that the anomalous tsunami was not caused solely by earthquake fault displacement. Submarine landslides near Toyama Bay also played a major role.
The research team explained that traditional tsunami simulations often assume that seafloor deformation occurs instantaneously. In reality, submarine landslides behave more like underwater landslides or debris flows, involving acceleration, movement, deceleration, and eventual stopping over tens of seconds. Even though the process is short, it can strongly affect how tsunami waves are generated.
To reconstruct this hidden underwater process, the team used the three-dimensional computational fluid dynamics model SPLASH3D to simulate the movement of submarine sediments. The simulations showed that the landslide reached peak kinetic energy about ten seconds after initiation and continued strongly disturbing the surrounding seawater for roughly 20 to 30 seconds before gradually stopping. The researchers then incorporated this time-evolving seabed motion into the tsunami propagation model COMCOT to calculate how the waves traveled toward the coast.
The results closely matched actual tide-gauge observations, including tsunami arrival times, waveform patterns, and maximum wave heights. The study further showed that the submarine landslides occurred only a few kilometers from the coastline, allowing tsunami waves to reach nearby coastal areas within two to three minutes. This successfully explains why the tsunami arrived much earlier than expected. The interaction of earthquake-generated tsunami waves and landslide-generated tsunami waves amplified the overall tsunami height beyond what would have been expected from fault displacement alone.
According to Professor Wu, the study highlights the need for future tsunami warning systems to consider not only earthquake fault motion, but also the possibility of submarine landslides, especially in steep coastal bays such as Toyama Bay. Building precomputed simulation databases for different submarine landslide scenarios could help improve tsunami forecasting and coastal hazard mitigation in the future.
