We’ve seen the damage a tsunami can do with videos of the 2011 Tōhoku earthquake and tsunami in Japan, but the phenomenon has been difficult to describe.
An analysis of tsunami science published on the National Oceanic and Atmospheric Administration website notes that there’s still no agreed-upon metric for measuring a tsunami.
The tsunami page of ready.gov still measures tsunamis vertically, as if they’re regular ocean waves, and states, “A tsunami is a series of enormous ocean waves caused by earthquakes, underwater landslides, volcanic eruptions, or asteroids.”
That often leaves vertical measurements that mimic the gauging of daily ocean waves, but the two waves are quite different. Last night, Gov. Josh Green tried to explain the difference as he noted that a 3-foot pulse from the ocean was likely in Hawaii.
“This is not like a wave that comes across the shore, a 3-foot wave,” he said at a news conference. “This is a longitudinal wave with great force.”
That language mimics the NOAA-hosted analysis, which states, “Tsunamis are long waves of small steepness.”
In other words, much of their weight isn’t coming down from top to bottom, but rather from back to front as they rush onto dry land. A top-to-bottom breaker of a tsunami has happened, but the typical tsunami seems to take the form of a stealth wave below the surface and rushes in rapidly.
Ocean scientists at the University of Western Australia note that a tsunami gains speed as its ocean path loses depth. It also gains vertical height.
“It is not just a 3-foot wave,” Green said. “It is a forceful wall of water.”