Go ahead and call them tidal waves

A tsunami made the news in December 2004: a monster wave, started by a magnitude-9 earthquake off the west coast of Sumatra. It overwhelmed long stretches of coastline along the Indian Ocean shore and drowned more than 100,000 people.

Here's what tsunamis are like. . .

You're sitting in your seaside house and you notice that the surf sounds different. You look out and see that the water has receded, as if the tide had pulled out in a hurry. The sea stays low for several minutes.

If you don't know about tsunamis, you stand and stare, or you ignore it and go back to your business. If you don't know better, you might even go down to the shore and start poking around the exposed seafloor with its stranded fish. Then the sea rises, as quickly and quietly as it left—then it keeps on rising, higher than you've ever seen it go.

The sea grows louder and outruns you, catches your ankles, knocks you down, and smashes you against trees and rocks and buildings as you drown in its muddy, turbulent flow.

If you're smart, you start running for high ground immediately, and with luck you may survive. You may watch this flood come and go several times over the course of a few hours. With some tsunamis the water doesn't recede first, and that funny-sounding surf is the sea rising around your house without warning.

The Web has a lot of sites with tsunami information. I've got a tsunami list with the best of them. But I think the Pacific Tsunami Museum in Hilo, Hawaii, is special because it has a human face and a human basis.

Hilo was heavily damaged by tsunamis in April 1946 and again in May 1960, so tsunamis are on Hiloans' minds more than for the average Hawaiian. Even so, as the years passed people who remembered began to die and new residents came who didn't take the threat as seriously, and in 1994 the museum was founded to help keep the population prepared and alert. It takes that extra effort to mobilize people against something they haven't seen.

On November 26, 1999, such efforts paid off for the South Pacific island nation of Vanuatu. A magnitude-7 quake struck late that night, and a tsunami completely wiped out the village of Baie Martelli. But only five lives were lost. A research team reported a month later in Eos:

"The small number of casualties was due to prior education and a party. Because of a wedding on the day of the earthquake, most everyone was still up celebrating when the earthquake occurred. A lookout was sent to note the condition of the sea. When he reported that the water was receding, villagers concluded that a tsunami was coming, and they ran to a nearby hillside to escape the wave. Villagers credited their response to a video of the 1998 Papua New Guinea tsunami, which they had seen a few months before. The only casualties were those too elderly to escape the wave, those who returned for possessions after the passage of the first wave, and a man so drunk on kava that he ignored people who were directing him to safety."

Tsunami researcher Andy Moore has photos taken after the Baie Martelli tsunami.

That slow rise and fall of a tsunami, too slow to be an ordinary wave but eerily fast for a tide, is what gives it the common name "tidal wave." Scientists don't use the term, and neither do mariners—geologists say a tidal wave means the tide itself, and sailors say it means any big surge, such as a storm surgebut I think the rest of us can call tsunamis tidal waves without confusion.

Besides, scientists have their own blinders. Many think of tsunamis as caused strictly by earthquakes, or underwater volcanic eruptions or seafloor landslides. They even call them "seismic sea waves." But in fact the most significant kind of tsunamis, for the geologist, are created not by quakes or any earthly process but by cosmic impacts. Earthquakes cannot be bigger than about magnitude 9.5, as I explained in another article, but comet and asteroid impacts have no such limit at all.

Impacts that could unleash large tsunamis onto the whole shoreline of the Pacific Ocean occur, on average, once in 1,000 years. Over geologic time, events of that frequency leave traces in sedimentary rocks. Geologists ought to be looking for those traces, but they too can find it hard to keep in mind something they haven't seen.

The last significant tsunami before 2004's was in June 1998, in New Guinea. The last time a major disaster like Sumatra's happened was on 23 May 1960, when tsunamis triggered by the great Chile earthquake struck Hawaii. I was a little kid at that time, and thanks to the news reports I've never forgotten the hazard that tidal waves present. But a few billion people born since then had not had their awareness raised, until 26 December 2004. That awareness could mean your life.

PS: The 1964 Alaska earthquake sent a tsunami down the Pacific coast of North America. It was news all the way down in California. Here's a first-person account, with photos, from a Canadian who lived through it.