‘Focusing points’ off certain coasts can create tsunamis much higher than previously believed possible.
Until now, it was generally believed that height of tsunamisonshore couldn’t exceed the depth of the seafloor. But new research shows that in some areas, such as Japan and Java, flooding can be up to 50 percent deeper, with waves that don’t lose height as they get closer to shore.
“It is as if one used a giant magnifying lens to focus tsunami energy,” says Utku Kanoglu, professor at the Middle East Technical University. “Our results show that some shorelines with huge earthquake zones just offshore face a double whammy: not only they are exposed to the tsunamis, but under certain conditions, focusing amplifies these tsunamis far more than shoaling and produces devastating effects.”
The team observed this effect both in Northern Japan, which was struck by the Tohoku tsunami of 2011, and in Central Java, hit in 2006.
During an earthquake, some sections of the sea floor lift up, while others sink. This creates tsunamis that propagate trough-first in one direction and crest-first in the other. But the researchers have discovered that on the side of the earthquake zone where the wave propagates trough-first, there’s a sweet spot where focusing occurs – strengthening it before it hits the coastline with an unusual amount of energy.
Based on the shape, location, and size of the earthquake zone, that focal point can concentrate the tsunami’s power right on to the coastline.
Another surprise of the study was the discovery that tsunamis don’t, as thought,decrease in height continuously as they move closer to shore, just as wind waves do. The study’s authors instead suggest that. Instead, it seems, the crest of the tsunami remains fairly intact close to the source.
“We are still trying to understand the implications,” says Costas Synolakis, director of the Tsunami Research Center at the USC Viterbi School of Engineering. “But it is clear that our findings will make it easier to identify locales that are tsunami magnets, and thus help save lives in future events.”