Toss a stone into a pond, and ripples move in all directions away from the point of impact. As they collide with the shore and with one another, they bounce back and interfere, creating turbulence across the water. These ripples work on the small level of the pond, but also on the larger scale of entire oceans. The maps of tsunamis created by underwater or coastal earthquakes look surprisingly similar to ripples moving across a pond.
The map above was created by the National Oceanic and Atmospheric Administration (NOAA) Alaska Tsunami Warning Center. It shows how a tsunami created by an earthquake on the southern Alaskan coast would travel across the Pacific Ocean and beyond. On this map, each band of color represents an hour of travel time. A tsunami starting in Alaska could travel the 6,500 miles to Australia in only 15 hours. Click here or on the picture above to see the large version of the map.
As with the ripples in the pond, any interference with the tsunami wave will cause it to bounce back and interfere with itself. See, for instance, the Bering Sea in this map. Although relatively close to the epicenter of the earthquake, the Aleutian Islands and the enclosed nature of the sea itself force resistance upon the waves, slowing it down. On the contrary, however, the Pacific island nations present few obstacles to the tsunami wave due to their small land size and the large distances of open water between them.
The presence of the Great Barrier Reef off northeastern Australia adds more than two hours to the tsunami’s travel time in some places. And the wave slows down considerably across Indonesia’s Arafura and Java Seas. This map therefor demonstrates the importance of barrier islands in ensuring a safe coastline–not just from tsunamis, but also from high waves caused by hurricanes or other powerful storms.