Sea floor earthquake zones strengthen tsunamis more than expected

A multinational research team has discovered that sea floor earthquake zones can act like a ''magnifying lens,'' focusing and strengthening tsunamis beyond what was thought possible

The earthquake zones off of certain coasts - like those of Japan and Java - make them especially vulnerable to tsunamis, according to a new study. They can produce a focusing point that creates massive and devastating tsunamis that break the rules for how scientists used to think tsunamis work.

Until now, it was largely believed that the maximum tsunami height onshore could not exceed the depth of the seafloor. But new research shows that when focusing occurs, that scaling relationship breaks down and flooding can be up to 50 per cent deeper with waves that do not 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 and senior author of the study.

''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,'' Kanoglu adds.

The team observed this effect both in Northern Japan, which was struck by the Tohoku tsunami of 2011, and in Central Java, which was struck by a tsunami in 2006.