Tsunami threats are greatly underestimated in current models, new research shows
The 2004 Sumatra earthquake generated one of the most destructive tsunamis ever recorded, with 100-foot waves that killed nearly 230,000 people and resulted in an estimated $10 billion in damage.
It also ushered in a new understanding that potent tsunamis are triggered by shallow earthquake ruptures of underwater fault lines.
Although current research pop[ints to rupture depth as a key factor in predicting tsunami severity, those models fail to explain why large tsunamis still occur following relatively small earthquakes.
Now, Earth scientists at the USC Dornsife College of Letters, Arts and Sciences have found a correlation between tsunami severity and the width of the outer wedge — the area between the continental shelf and deep trenches where large tsunamis emerge — that helps explain how underwater seismic events generate large tsunamis.
USC Dornsife researchers found that large earthquake-generated tsunamis emerge after horizontal oceanic water movement is transferred to uplift in the tsunami excitation zone, the outer wedge of sediment between the continental shelf and the deep ocean trench. (Image: Mesa Schumacher and Edward Soleto.)
Drawing insights from a survey of previous tsunamis, the authors analyzed data of global subduction zones to identify and discuss potential tsunami hazards.
Their latest study revealed that current predictive models underestimate tsunami severity by as much as 100%. The work appears in the journal Earth-Science Reviews.
To develop their new model, Associate Professor of Earth Sciences Sylvain Barbot and co-author Qiang Qiu, now at the South China Sea Institute of Oceanology under the Chinese Academy of Sciences, analyzed the structural and tectonic settings of nearly a dozen global earthquake-generated tsunamis.
The analysis found that particularly large tsunamis emerge after horizontal movement is transferred to uplift in the outer wedge of sediment located between the continental shelf and the deep ocean trench. The many faults and folds efficiently redirect the sub-oceanic horizontal motion generated by great and giant trench-breaking earthquakes into potentially devastating tsunamis.
“We can very quickly determine where and how big earthquakes are at subduction zones,” Barbot said. “If they happen to be fairly shallow, our results can quickly determine what tsunami height they can generate. This can help improve already existing short-term mitigation strategies for early warning systems.”
While these findings better explain how severe tsunamis result from shallow seismic events, future efforts should incorporate three-dimensional imaging of the outer wedge, according to the authors. Understanding the pathway from earthquake to tsunami depends on identifying the structural and rheological controls that turn a rupture into a trench-breaking earthquake.
“With this study, we were able to find this correlation simply because we have a lot of data now,” Barbot said. “It’s the benefit of hindsight that allowed us to discover this really very simple correlation. There is much of this we don’t know yet, so it needs more detailed research, but the relationship between outer-wedge width and tsunami run-up is clear enough that it can be extrapolated.”