Where we prepare for the worst, and hope for the best
Can You Outrun a Tsunami?
by Gordan Gumpertz
Try to imagine a solid block of ocean hundreds of miles long, 3 miles deep, and as wide as the coastline, coming toward you at 500 to 600 miles an hour. That describes a tsunami in deep water racing toward land. A tsunami's speed slows as it encounters the coastline but the total water mass is still moving at 20 to 25 mph just before surging ashore. Maybe a world class distance runner who runs a 4- minute mile can stay ahead of an oncoming tsunami, but most of us are not world class runners. If it comes down to a race, the tsunami will win and the runner will lose almost every time. A tsunami rolling onshore is massive, powerful, and destroys everything in its path.
A major tsunami can start when sudden slippage occurs in a seafloor fault, triggering a major earthquake in the magnitude 8.0 to 9.0 range, and causing the seafloor on one side of the fault to sink and on the other to uplift, all in a matter of minutes. The earthquake plus the radical seafloor deformation displaces vast amounts of water, upsets the ocean's equilibrium, and sets a tsunami in motion.
A tsunami produced by this kind of event will tend to have long wavelengths, deep water columns, and high rates of travel speed. A wavelength is the distance between the crest of one wave and the crest of its trailing wave. The speed is determined by the ratio of the depth of the ocean to the length of the wave. The deeper the water and the longer the wave, the faster the tsunami moves. For example, a major earthquake and seafloor deformation at a depth of 20,000 ft. can initiate a tsunami with a wavelength of 175 miles, a water column depth of 15,000 ft., and a speed of between 500 and 600 miles an hour.
To illustrate the gradual drop in tsunami speed, when the depth decreases to 13,000 feet, the 15,000 ft. water column begins to drag the bottom, the wave length shortens to about 130 miles, and the speed drops to around 440 mph. At 6500 ft. depth, the wavelength shortens to less than 100 miles and the speed drops to around 300 mph. At 150 ft. depth, the wavelength reduces to 30 miles and the speed to approximately 100 mph. In 30 feet of water just before surging onshore, the wavelength is down to 6 miles and the tsunami's speed is 20 to 25 mph.
The height of the wave on the ocean's surface in deep water will tend to be only 2 to 3 feet and hardly noticeable among the normal ocean swells. A tsunami will usually pass unnoticed under the keel of a boat or ship in mid ocean, even though the wave may be as high as 100 feet when it surges onshore.
When the water depth decreases and the leading edge of the wave slows, the following wave at the tail end of the long trough is still traveling at a higher speed and rapidly closes the gap. The height of the tsunami increases dramatically as it nears shore due to compression from shoaling and from the rapidly closing trailing wave, and may be squeezed up to 100 feet high when it hits the beach. The 100-ft. towering wall of water is most often associated with shallow bays and narrow inlets where the tsunami acts like a giant tidal bore. On a broad beach type of coastline, the tsunami tends to come ashore as a rapidly rising sea. Along the broad beaches of Sumatra, Sri Lanka, and Thailand, the 2004 Indonesian tsunami produced a sudden 30-ft. rise in sea level that surged onto land so quickly that few could get away. Over 225,000 people died in 8 countries bordering the Indian Ocean.
Following the magnitude 9.1 Indian Ocean earthquake on December 26, 2004, in
areas east of the epicenter Sumatra and Sri Lanka the trough of the first
tsunami wave reached land ahead of the crest. When the trough arrives first, a
phenomenon called drawdown occurs. If recognized, it can act as a warning for
people in the area to move to higher ground before the tsunami hits. When
drawdown happens, the sea level appears to sink and the tide recedes rapidly,
leaving hundreds of yards of empty ocean bottom exposed. The drawdown is
followed almost immediately by the oncoming crest and accompanying sudden rise
in sea level as the tsunami charges full blown onto land. In areas west of the
epicenter Thailand and India the crest and sudden rise in sea level struck
first, without warning.
The distance a tsunami can travel inland once it hits the coast depends on the size of the wave and the slope of the land. The size of the wave is expressed as runup, a term meaning the height of the wave over mean high tide. In flat, low-lying areas, a major tsunami with a runup of 30 feet or more can reach areas 2 miles or more from the shoreline with devastating power.
If you are on or near the coastline when a tsunami warning is issued by your local authorities, follow evacuation directions and clear the area immediately. If you hang around to see how big the wave is, and then try to outrun it, you are almost certain to lose the race and your life.
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