[ckissick]

I'm a geologist. I'll take this one.

As you know, the gravitational pull between the earth and moon is what causes the tides. Over the billions of years that the earth and moon have been around, a stable "tidal wave" has developed. On the side of earth that faces the moon, you have your highest of the two daily high tides.

It seems odd, I know, but there is another, lower, high tide on the side of earth that is facing away from the moon. This is because the "tidal wave" is a normal ocean wave with a wavelength equal to one-half the earth's circumference. It has two peaks and two troughs, and it follows the moon as the moon does its orbit.

I suppose you could ask why the wavelength isn't equal to the entire circumference of the earth, with one peak and one trough. I have some ideas, but I'd have to look that one up.

In any case, it is the manifestation a true tidal wave. That's why we prefer to cause seismic waves tsunamis instead of tidal waves. Ironically, tsunami is Japanese for tidal wave, or so I hear.