Tsunami threat grossly underestimated in existing models

(ORDO NEWS) — The 2004 Sumatra earthquake produced one of the most destructive tsunamis on record, with 100-foot waves that killed nearly 230,000 people and cost an estimated $10 billion in damage.

It also led to a new understanding that powerful tsunamis are caused by underwater fault lines breaking in shallow earthquakes. Future tsunamis are likely to be just as strong, if not worse, potentially resulting in even more deaths and the destruction of entire communities.

Although current research points to rupture depth as a key factor in predicting tsunami strength, these models cannot explain why large tsunamis still occur after relatively small earthquakes.

Now UCLA researchers have found a correlation between tsunami strength and the width of the outer wedge – the area between the continental shelf and deep troughs where large tsunamis occur – that helps explain how undersea seismic events generate large tsunamis.

Building on previous tsunami research, the authors analyzed geophysical, seismic, and bathymetric data from global subduction zones to identify and discuss potential tsunami hazards.

Their latest study found that existing forecasting models underestimate tsunami strength by as much as 100%. The work was published in the journal Earth–Science Reviews.

“Nearly half of humanity lives in coastal areas, making our populations and infrastructure vulnerable to seismic and tsunami hazards,” said Sylvain Barbot, assistant professor of geosciences at the Dornsife College of Letters, Arts and Sciences at the University of Southern California and co-author of the study.

“In order to preserve our livelihoods and our economy, we must protect ourselves from these very strong dangers, which are relatively rare, but do happen. We cannot stop this danger, so we need to mitigate its effects.”

“That means having tsunami evacuation plans and developing an urban development plan to avoid locating schools and hospitals in flood zones. There are proactive measures we can take to protect ourselves from tsunamis and floods in the long term, and our study provides a description how to identify the area exposed to these hazards.

Tsunami Threat: Excitation Zone Width Highly Correlates with Severity

To develop the new model, Barbot and his co-author Qiang Qiu, now at the South China Sea Institute of Oceanology at the Chinese Academy of Sciences, analyzed the structural and tectonic features of nearly a dozen global tsunamis caused by earthquakes.

The analysis showed that especially strong tsunamis occur after the horizontal motion passes into the rise of the outer wedge of sedimentary rocks, located between the continental shelf and a deep ocean trench.

Numerous faults and folds in the outer wedge of accretionary prisms effectively redirect the suboceanic horizontal motion generated by strong and giant rupture earthquakes in the trenches into potentially destructive tsunamis.

“We can very quickly determine where and how strong earthquakes are in subduction zones,” Barbot said. “If they turn out to be quite small, our results will allow us to quickly determine what height of a tsunami they might generate. This could help improve existing short-term mitigation strategies for early warning systems.”

A study of tsunamis generated by earthquakes has found a correlation between the width of the outer wedge and the maximum strength of tsunamis generated by earthquakes with magnitudes between 7.1 and 8.2 (Mw).

At the same time, the researchers were able to obtain estimates of the future strength of the tsunami caused by various seismic events.

The Middle East, Alaska and the Pacific Northwest are among the regions threatened by tsunamis
. The authors examined 30 additional active subduction zones. Using the correlation between the width of the outer wedge and tsunami waves, they shed light on the threat posed by potential tsunamis.

The authors identified the Western Makran (Iran), Western Aleutian, Lesser Antilles, Khikurangi (New Zealand), and Cascadia subduction zones as the zones capable of generating the highest tsunami surges.

For example, the Cascadian subduction zone, off the US west coast near Oregon and Washington, could experience a 160-foot tsunami from a large earthquake, twice as high as existing models predict.

“The region that should be most vigilant in this regard is Iran and Pakistan,” Barbot said. “Most of their industry and population is located on the south coast, which exposes them to their greatest potential tsunami threat – perhaps up to 90 meters [nearly 300 feet] in the event of a 9.0 MW earthquake.

However, in other subduction zones, the threat is almost the same. On The Pacific Northwest has already taken tsunami mitigation measures, but they may be bracing for a smaller upsurge than will happen.”

While these results better explain how large tsunamis are generated by shallow seismic events, future work should include a 3D survey of the outer wedge, the authors say. Understanding the path from earthquake to tsunami depends on identifying the structural and rheological factors that turn a rupture into a trough-destroying earthquake.

“In this study, we were able to find this correlation simply because we now have a lot of data,” Barbot said. “It was the advantage of hindsight that allowed us to discover this really very simple correlation.

Much of this we don’t know yet, so it requires more detailed investigation, but the relationship between the width of the outer edge and the tsunami run is clear enough to be extrapolated.”

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