Current Models Underestimate Tsunami Threats

deep ocean trench. The many faults and folds of the outer wedge of accretionary prisms 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.”

The survey of earthquake-generated tsunamis illuminated a correlative relationship between the width of the outer wedge and maximum tsunami strength resulting from earthquakes measuring 7.1 to 8.2 in moment magnitude (Mw). In doing so, the researchers were able to generate estimates of future tsunami severity generated by a range of seismic events.

Middle East, Alaska and Pacific Northwest Among Regions Facing Tsunami Threat
The authors investigated another 30 active subduction zones. Utilizing the correlation between the width of the outer wedge with tsunami run-ups, they shed light on the threat posed by potential tsunamis. The authors identified the Western Makran (Iran), Western Aleutian, Lesser Antilles, Hikurangi (New Zealand) and Cascadia subduction zones as having the potential to produce the highest tsunami run-ups. For instance, the Cascadia subduction zone — located off the U.S. West Coast near Oregon and Washington — could suffer tsunamis 160 feet high in the wake of a major quake, double what current models project.

“The region that should be the most alert to this is Iran and Pakistan,” Barbot said. “Much of their industry and population is located on their southern coast, exposing them to the largest potential tsunami run-up hazard — perhaps up to 90 meters [nearly 300 feet] in the event of a 9.0 Mw earthquake. However, the threat is nearly as bad in other subduction zones. In the Pacific Northwest, they already have tsunami mitigation measures in place, but they may be preparing for a lower run-up than will happen.”

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.”