What is the difference between tidal waves and tsunamis?
Most people who haven’t spent the last fifteen years in a cave, wearing a blindfold and earmuffs, with their head buried in three feet of sand know what a tsunami is, or at least what it looks like. While I enjoy horsing around as much as anyone—and more than most people—there was nothing remotely funny about the Christmas Day tsunami that struck the islands of Thailand. And that was a small one, as tsunamis go. Most of us understand the mechanisms behind tsunamis, mainly because of that horrific event. However, there remains considerable confusion about tidal waves and their source. The one thing most of us unobstructed non-cave-dwellers who hold our heads high can agree on is that tidal waves and tsunamis are not the same thing.
Tsunamiscan arise from a number of sources. The most common of these is an earthquake near a coastline or near a submarine drop-off, which causes an underwater landslide. The severity of the slide usually determines the size of the tsunami more than the scale of the earthquake that caused it. This was what led to the devastation in Southeast Asia. Of course, landslides don’t always need an earthquake to trigger them. I saw a particularly frightening science documentary showing a giant cleft in one of Hawaii’s giant volcanoes (Mauna Loa, I believe, but don’t quote me). The tear in the mountain is gradually cutting it in half. Now, keeping in mind that, including what’s hidden under the Pacific, Mauna Loa is taller than Mount Everest, the scientists predicted that roughly half of the mountain would sheer away one day, creating a mega tsunami that would annihilate much of America’s West Coast… which happens to be where I reside, which is what made the show so scary.
Another source of tsunamis—one that we will hopefully never experience—is one generated by the impact of an asteroid or comet. Did you see the movie Deep Impact? Probably not. Most people were preoccupied with the more popular but juvenile movie Armageddon, which came out about the same time and was about essentially the same thing. Such an ocean impact need not be what they call an “extinction event” in order to generate a destructive tsunami.
Travelling at hundreds of miles per hour in the deep ocean, tsunamis remain quite small until they enter shallower water, where the up-sloping sea floor forces the wave’s energy upward, slowing the wave and making it considerably taller.
Tidal waves are a completely different phenomenon. They do not travel at great speeds in the open ocean; earthquakes, sea-slides or meteoric impacts do not spawn them. The name says it all: tidal forces create tidal waves. Such events are rare, often occurring annually in specific areas when the gravitational forces of the moon and the sun combine in just the right way that it draws water into a channel, river, harbor or some other inlet in much greater abundance than a normal tide, and much more quickly. Tidal waves are generally rather predictable, therefore.
One thing holds true for both tidal waves and tsunamis: the beach is not a great place to be when either occurs. You can avoid tidal waves easily enough, but if you happen to be near the shore when an earthquake hits, or if a tsunami alert sounds (so far, this only happens in Hawaii in regards to the United States), take it seriously and get away from the beach. Seek high ground immediately; if none is available, get into a building two or preferably more stories in height and get upstairs in a hurry. A tsunami will gut out the bottom floor of any building, taking people right along with it.
