What is a “tidal wave”?

A lot of people wonder how “tidal waves” compare to tsunamis, but in fact they are both alternative names for the same thing. More specifically, what scientists call a tsunami today was more commonly called a “tidal wave” up until a few decades ago. Among the public, the term “tidal wave” is still common. So to dig a bit deeper, you might wonder: Why were tsunamis called “tidal waves” in the past, and why don’t scientists like to call them that anymore?

The old name (“tidal wave”) came from the way the waves appear as they come ashore. When we think about shore waves, we usually picture waves that crest and break, like those that surfers ride on. But every day along coastlines, you can also see the daily tides roll in and out. No waves break with these tides; instead, we simply see a “rising tide” as the water comes in and then a falling tide as the water moves back out.

Like tides, the huge waves associated with tsunamis appear to simply roll in without cresting or breaking. In other words, if you watched a tsunami coming ashore, it would look (at least at first) more like an incoming tide than the kind of wave we normally see at a shore break. This similarity of appearance explains why tsunamis were called “tidal waves.”

However, a tsunami can make the water rise far higher and move far deeper inland than any tide, and the water can move in much faster as well. Moreover, tides are caused by gravitational effects of the Moon and Sun (as we’ll discuss in Chapter 6), while tsunamis are caused by events here on Earth, such as undersea earthquakes, volcanic eruptions, or landslides. That’s why scientists no longer like to use the name “tidal wave” — because it implies a connection to tides that doesn’t actually exist.