Tsunamis

Stupid Question ™
Jan. 24, 2005
By John Ruch
© 2005

Q: Was the Dec. 26, 2004 tsunami the biggest and deadliest ever?
—William R., from the Internet


A: Tsunamis, you mean. Like virtually all of its brethren, the Indian Ocean tsunamis of Dec. 26 came as a series in which the first is not usually the worst.

The Dec. 26 tsunamis, which killed an estimated 230,000 people, were indeed the deadliest ever, at least in terms of clear record-keeping. But they were in the average height range of 10-30 feet—nothing compared to the mind-blowing, 1,720-foot freak wave that some claim as the tsunami king.

In terms of death toll, tsunamis killed about 70,000 people worldwide in the century between 1880 and 1980. The Dec. 26 tsunamis killed more than three times that amount in one day.

Runners-up are muddled by being more closely associated with direct earthquake damage. More than 200,000 people died in India’s Bengal area in 1876 from an earthquake and tsunami, but it’s unclear how many died from each. No other tsunami-related disaster comes close to the 200,000 mark.

Tsunami height is a tricky business that varies widely with locality. Tsunamis begin as long-period, high-velocity submarine waves that are barely perceptible in deep water. It is only on sloping shorelines and sharp inlets that they slow down enough to get squashed together and built up to devastating heights.

A 1960 tsunami that smashed Hawaii harbor towns hit as only a six-inch rise in the tide on an island with very steep sides. (And on Hawaii itself, a devastating 1946 tsunami ranged from 2 to 55 feet high, depending on the locale.)

So there’s no such thing as an absolute height for a tsunami. It all depends on what gets hit. The average range for past destructive tsunamis is 10 to 30 feet—which is plenty scary, as you know if you’ve seen any of the Dec. 26 amateur video footage.

Also, many tsunamis don’t hit as the big, breaking wave familiar from disaster movies. They’re more of a flood surge, a big swell that pushes in and just keeps on coming. (The undertow as it pulls back out to sea can be even more deadly than the incoming wave.)

Even when they do come as surf (as the Dec. 26 ones frequently did), it’s hard to get a height measurement because people either run away or die if they stick around to get a look.

So what many tsunami reports discuss is not wave height, but wave “run-up.” The terms are often confused and are related but not identical. Run-up is the elevation on land above mean sea level the tsunami floodwaters attain once they reach shore.

Run-up can be higher than the wave if the wave breaks violently and sprays upwards. And a wave can be higher than its run-up; for example, a wave could break at 30 feet high, but then result in a 10-foot-deep push of water into the shore.

Furthermore, run-up is generally calculated after all the waves have come and gone, so it’s not a good measure of any one particular tsunami in the series.

Run-up confusion clouds reports of the most astonishing tsunamis—ones so big they’re known as “mega-tsunamis.”

The potential highest tsunami of all time was an almost unimaginable 1,720-foot-high splash in an Alaskan bay, apparently caused by a huge earthquake-driven rockslide. It can be measured with precision because it denuded a forested bluff to that height; eyewitnesses also saw the water spraying up from behind a mountaintop.

However, this freak phenomenon, which happened on July 9, 1958 in Lituya Bay, was not necessarily a tsunami, and not necessarily that high if it was.

It might be better classed as simply the biggest splash ever witnessed, rather than a wave. Even if it was a wave, 1,720 feet was its run-up, not necessarily its height. A wave of 500 feet high or even less could still splash up that high once it struck the bluff. Granted, that would still make it the biggest tsunami ever known.

And in any case, the splash was certainly followed by a tsunami, possibly the largest ever witnessed, ranging anywhere from 100 to 500 feet high at its peak according to witnesses—four of whom, incredibly, rode it out in boats. This aftershock wave swept the length of the bay and out to sea, stripping four square miles of shoreline as far as 3,600 feet inland.

Based on scanty historical records and geological evidence, it appears Lituya Bay has a past of similar freak tsunamis that could vie for the height record. Records indicate run-ups of 150 feet in 1936; 180 feet in 1899; and 350 feet in 1854, among other, lesser events. All were big splashes related to rock or ice falls.

There are also speculative claims of even bigger mega-tsunamis produced by the toppling of entire mountains or asteroid impacts. The idea of the entire Eastern seaboard of the U.S. being inundated all the way to the Appalachian Mountains is credulity-stretching. But the claim has been put forth based on rather meager geological evidence.

In the realm of classical tsunamis caused by undersea earthquakes and rockslides, the biggest claim is a wave about 250 feet high that supposedly washed over Ishigaki in Japan’s Okinawa island chain on April 24, 1771. The claim relies on geological evidence and an eyewitness account, but again, seems more about run-up than wave height. It’s still widely debated.

The Aug. 27, 1883 volcanic explosion of Krakatoa produced tsunamis reportedly in excess of 100 feet high, possibly up to 130 feet, in some parts of Sumatra and Java in Indonesia, killing more than 36,000 people. Again, these estimates are largely based on run-up.

An April 1, 1946 Alaskan tsunami destroyed a lighthouse in the Aleutian Islands built 50 feet above sea level, and had run-up onto a promontory 118 feet above sea level. By some estimates, the wave was at least 100 feet high; no one survived in the lighthouse to say.

Japan’s major island of Honshu has a lengthy history of tsunamis which credibly supports repeated strikes by waves of 80 feet in some inlets.