Offshore Islands Don't Break Tsunamis, They Amplify Them

Tsunamis can be one of the most destructive forces of nature to hit a location. These massive waves can destroy coastal towns and cause damages that last for years to come. Now, scientists have taken a closer look at tsunamis and have found that contrary to popular belief, offshore islands don't protect the mainland from tsunamis; instead, it appears that the exact opposite is true.

"This is where many fishing villages are located, behind offshore islands, in the believe that they will be protected from wind waves," said Costas Synolakis, one of the researchers, in a news release. "Even Southern California residents believe that the Channel Islands and Catalina will protect them."

The new study was conducted after a field survey of the impact of the 2010 tsunami on the Mentawai Islands off of Sumatra. The survey data revealed that villages located in the shadow of small offshore islets suffered some of the strongest tsunami impacts. In fact, these impacts were worse than on villages located along open coasts.

That's why the researchers designed a computer model that took into consideration various island slopes, beach slopes, water depths, distance between the island and the beach and wavelength of the incoming tsunami. This allowed them to see how a tsunami might impact the coast when islands were in front of the coastline or when the coastline was open.

"This work is applicable to some of our tsunami study sites in New Zealand," said Jose Borrero, one of the researchers. "The northeast coast of New Zealand has many small islands offshore, similar to those in Indonesia, and our modeling suggests that this results in areas of enhanced tsunami heights."

The findings reveal that offshore islands don't protect coastlines from tsunamis, but instead enhance them. This has important implications for advance warning systems and planning throughout regions where tsunamis are a possibility.

The findings are published in the journal Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.