Special Report: What hit us on Nov. 15?

December 06, 2006 12:00 am

A tsunami describes the waves that ripple outward in an ocean from the location where a vertical earthquake or underwater landslide has reformed the sea floor.The radiating circle of waves happens because the sea floor that's jolted by an earthquake or moved by a landslide pushes other water out of its way. The movement continues until the water meets land.

As the sea floor becomes shallow, the wave is forced to slow its speed and rises in mass. Its huge mass and reverse action are what cause damage to coastal cities, along with the debris succeeding waves force against and through structures. The 25-ton concrete tetrapods placed near Crescent City Harbor are an example of the size of debris flung by tsunami waves.

Once on land, the waves continue moving forward until their momentum is slowed. Then they pull back hard.

The National Weather Service defines storm and tidal surges as:

•Storm surge is the onshore rush of sea or lake water caused by the high winds associated with a landfalling cyclone and secondarily by the low pressure of the storm.

•Tidal surge is often misused to describe a storm surge, but storm surge is independent of the usual tidal ebb and flow. In some inlets, such as the Bay of Fundy, rapid changes in sea level due to the tides will cause a tidal bore or surge to move into or out of the inlet.

So what struck us on Nov. 15? A tsunami.