"Detecting tsunamis is a very difficult thing to do.

As the tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, and it may only have a height of about twelve to twenty-three inches and look like nothing more than the gentle rise and fall of the sea surface."

Citation source: http://www.owlnet.rice.edu/~esci108/108_EQ_Lec_4.ppt

Source of picture: http://www.ngdc.noaa.gov/seg/hazard/tsu.shtml