Analysis of historical seismic events reveals that the region near the trench affects the severity of the tsunami


The 2004 Sumatra earthquake generated one of the most destructive tsunamis on record, with 100-foot waves killing nearly 230,000 people and causing damage estimated at $10 billion. It also ushered in a new understanding that powerful tsunamis are triggered by shallow earthquake ruptures of submarine fault lines. Future tsunamis will likely be just as severe, if not worse, potentially killing even more people and wiping out entire communities. Although current research indicates that rupture depth is a key factor in predicting tsunami severity, these models fail to explain why large tsunamis still occur after relatively small earthquakes.

Now, USC researchers have found a correlation between tsunami severity and the width of the outer wedge – the area between the continental shelf and the deep trenches where large tsunamis emerge – which helps explain how seismic events under- sailors generate large tsunamis. Drawing on a survey of previous tsunamis, the authors analyzed geophysical, seismic, and bathymetric data from global subduction zones to identify and discuss potential tsunami hazards.

Their latest study found that current predictive models underestimate tsunami severity by up to 100%. The book appears in the journal Earth Science Reviews.

“Almost half of the human population is coastal, leaving our population and infrastructure vulnerable to seismic hazards and tsunamis,” said USC’s Sylvain Barbot, associate professor of Earth Sciences at USC Dornsife College of Letters, Arts and Science and study co-author. “To maintain our livelihoods and our economy, we need to protect ourselves from these very violent dangers which are relatively infrequent but still occur. We cannot stop this danger, so we must mitigate its effects.

“That means having evacuation plans for tsunamis and making an urban development plan to avoid having schools and hospitals in flood prone areas. There are preventative measures we can take to protect ourselves from tsunamis and long-term floods, and our study describes how to delineate the area affected by these hazards.

Tsunami threat: width of the excitation zone strongly correlated with gravity

To develop their new model, Barbot and co-author Qiang Qiu, now at the Institute of South China Sea Oceanology under the Chinese Academy of Sciences, analyzed the structural and tectonic parameters of nearly a dozen global tsunamis generated by earthquakes. Varying in location and intensity, the analysis revealed that particularly large tsunamis emerge after horizontal motion transfers to uplift in the outer corner of the sediments between the continental shelf and the deep ocean trench. The many faults and folds in the outer corner of the accretionary prisms effectively redirect the sub-oceanic horizontal motion generated by large, giant trench-breaking earthquakes into potentially devastating tsunamis.

“We can very quickly determine where and how earthquakes occur in subduction zones,” Barbot said. “If they are shallow enough, our results can quickly determine the height of the tsunami they may generate. This can help improve already existing short-term mitigation strategies for early warning systems.”

The investigation of tsunamis generated by earthquakes has shown a correlative relationship between the width of the outer wedge and the maximum tsunami strength resulting from earthquakes with a moment magnitude (Mw) of 7.1 to 8.2. By doing so, the researchers were able to generate estimates of the future severity of tsunamis generated by a series of seismic events.

Middle East, Alaska and Pacific Northwest among regions at risk from tsunami

The authors investigated 30 other active subduction zones. Using the correlation between the width of the outer wedge and tsunami momentum, they shed light on the threat posed by potential tsunamis. The authors identified the West Makran (Iran), West Aleutian, Lesser Antilles, Hikurangi (New Zealand) and Cascadia subduction zones as having the potential to produce the strongest tsunami surges. For example, the Cascadia Subduction Zone – located off the west coast of the United States near Oregon and Washington – could experience tsunamis 160 feet high from a major earthquake, either the double what current models provide.

“The region that should be most vigilant about this is Iran and Pakistan,” Barbot said. “Much of their industry and population is located on their south coast, which exposes them to the greatest potential tsunami danger – possibly up to 90 meters. [nearly 300 feet] in the event of a 9.0 Mw earthquake. However, the threat is almost as severe in other subduction zones. In the Pacific Northwest, they already have tsunami mitigation measures in place, but they may be preparing for a lower acceleration than what will happen.”

While these findings better explain how severe tsunamis result from shallow seismic events, future efforts should incorporate three-dimensional imagery of the outer corner, the authors say. Understanding the pathway from earthquake to tsunami depends on identifying the structural and rheological controls that transform a rupture into a trench rupture earthquake.

“With this study, we were able to find this correlation simply because we now have a lot of data,” Barbot said. “It was the benefit of hindsight that allowed us to discover this really, really simple correlation. There’s a lot of that that we don’t know yet, so it requires more detailed research, but the relationship between wedge width exterior and the rise of the tsunami is clear enough to be extrapolated.”


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