James MacWhyte has posted a video on Facebook that clarifies the issue for all of us who live hundreds of meters above sea level.
I never really understood what a "tidal wave" was until watching this video. You may have thought, as I did, that a tsunami was just a great big breaking wave on the beach that smashed everything in its path. Clearly this is what the visual-effects guys on Deep Impact imagined.
Only, it's not, and MacWhyte's on-scene video makes the terror of a tsunami clear.
The United States Geological Survey has logged hundreds—hundreds—of earthquakes of 5+ magnitude off Japan since last Wednesday, but only one (or possibly two) disrupted the sea floor sufficiently to displace a humanly-incomprehensible volume of seawater. Water, unlike air, can't expand. You can detonate a massive nuclear bomb and it's likely no one upwind of the blast will feel a puff of air. But when a few hundred hectares of seabed changes location, the entire world feels it.
I thought I understood the physics of tsunamis, how a massive displacement of water causes surges all along the nearby coastline, but seeing what MacWhyte experienced really brought it home.
Watch that video: the ocean just keeps coming. Even a dam break, or a seiche on Lake Michigan, or a molasses tank rupture, has a single discrete glob of fluid that causes all the destruction. But just watch the ocean here: not only does it keep coming for five minutes, but there's another tsunami right behind it.
When you think about what Japan has had to deal with in the last three days, just try to imagine which was more terrifying: the ground liquefying, or the ocean arriving, unstoppably, on your front door.