Five to 10 minutes before it strikes, a tsunami usually gives a powerful warning that's hard to miss from the shore. It's not a roiling wave coming in, but the reverse - all the water in view going out to sea in the most massive and powerful undertow imaginable.
"If you're standing on the beach, the water can recede all the way out to the horizon," said Brian Yanagi, Hawaii's program specialist for earthquakes and tsunamis.
"Our biggest worry is for surfers, swimmers and Boogie Boarders because that giant undertow starts quickly and moves out at about 30 mph, pulling everything down beneath the surface.
"If you're standing in waist-high water or even less, it will pull you out and down and kill you," he said.
Those on the shore have 10 minutes to reach high ground before the tsunami waves - actually walls of tumbling water - strike.
On April 1, 1946, before a tsunami struck the town of Hilo on the eastern shore of Hawaii island, the town's entire mile-wide harbor drained into the ocean.
"People came rushing down to see what happened, there were fish flopping around on the ground - and then the wave hit," said Ray Novell, spokesman for Hawaii's Civil Defense Department. More than 150 were killed in that incident.
That first killer wave is just the beginning of the tsunami. The big damage is caused by the third, fourth or fifth wave, according to Yanagi.
An undersea earthquake causes seismic shocks that produce numerous waves, each more powerful than the previous one in the cycle of massive undertows and debris gathered up and into the incoming waves, Yanagi said.
A tsunami can last from 30 minutes to 10 hours depending on the power and configuration of the original quake.
Originally published on December 28, 2004
I'm not buying the preceding undertow idea, either, whereas the general rapid rise of water levels as described by the WaPost reporter --without large waves--makes more sense. I can't imagine a tsunami creating a vacuum out in front of its path, which is what it would take to create an undertow, unless a huge wave pulled water from below, stacked it up high, creating room for an undertow. Man oh man. Nature is mind blowing. Think of the force of a little 3 foot wave you've played in that's tossed you around and magnify it times a a million billion!Having went to college for Oceanography we studied these. The waves move at 500 to 600 mph usally just below the surface. When you see a wave break, it rises up when it's energy is transfered upwards due to shallowing. Thus the water goes up with the energy. Since it is building at such a fast rate it needs to draw more water, which it gets from in front of it. As it grows it draws water and keeps moving forward. We also need to remember the huge amount of water that was transfered from the top of the techtonic plate to the lower plate. Not all the water moves, just some of it, while it's energy is the driving force.
I have spoken to people who actually rode a small tsunami into a restaurant window on Long Island. It was the result of severe sharp turns by large oil tankers steaming to the docks for unloading.
Describing the apartment building coming at the surfer like a giant flyswatter was perfect The best thing about that book was it went into detail about the aftereffects of the impact unlike most other disaster books.
Another good set of books you might like is James Hogan "Cradle of Saturn"
Uses a lot of Velikovsky's whacked theories but good reading for seeing how absolutely fragile our civilization is.