When two tectonic plates move past each other along fault lines deep within the Earth, the rupture is often felt as an earthquake at the surface. If the quake happens below water, such a rupture can also cause a tsunami. The complexity of ruptures—variations in the direction of motion and length of displacement along the fault, known as heterogeneous slip—can, in turn, affect the size of an ensuing tsunami and the extent of damage that it can cause in coastal communities.
However, current methods that scientists and policy makers use to assess tsunami hazard do not account for rupture complexity. The problem is that the more complex a slip is, the higher the computational load is for a prediction model. Here Li et al. examine how heterogeneous slips can affect tsunami risk in the South China Sea.
To understand the contribution of complex slips to tsunami risk, the researchers modeled earthquakes with a wide range of magnitudes originating from a megathrust—a subduction zone where one tectonic plate is being forced underneath another. They focused on the megathrust along the Manila Trench in the South China Sea.
Using a Monte Carlo probabilistic tsunami hazard assessment (PTHA), the authors calculated the height of potential tsunamis generated from an earthquake source. In the assessment, the scientists subdivided the area of the fault zone into discrete regions to account for the effects of complex slip along the rupture. The team also compared the projected risks of the tsunami resulting from a complex slip to a more uniform, homogeneous rupture.
The complex and uniform models for earthquakes with shorter return periods tend to converge on the same result. But the study revealed that if the subduction zone in the Manila Trench generated a massive tsunami-producing earthquake, nearby regions like southern Taiwan and the Philippines’ western Luzon Island would likely face the greatest tsunami hazard in the South China Sea.
Of considerable note is the finding that more complex slips—tsunami-producing earthquakes big enough to happen only once every 500 years—can lead to bigger tsunami waves in heavily populated southern China. These waves are 20%–60% larger than waves produced by more uniform slip models.
In other words, the complex model showed an average wave height increase of roughly 0.5 meter across the region for tsunamis with a 500-year return period. Thus, areas like Hong Kong and Macau are at greater risk from tsunami inundation than previously thought.
The authors note that because current models do not account for the heterogeneity of slips, they may be significantly underestimating tsunami risk in vulnerable areas. They recommend that tsunami-prone coastlines close to large subduction zones should account for slip complexity in their tsunami prediction models. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013111, 2016)
—Wudan Yan, Freelance Writer