7.2 Earthquake Off Japan's Coast

[Spidey2721]

http://www.weather.com/outlook/weather-news/news/articles/japan-earthquake_2011-03-08

 

 

 

"It warned that a tsunami of about 20 inches (50 centimeters) would hit the coast around noon Wednesday, but that time passed without any reports of a wave reaching shore. "

 

so they were out there watching for a 20'' wave?

[Trouble Maker]

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but due to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft).[15] This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.

 

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously, producing a distinctly visible wave. Since the wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like a fast moving tidal bore.[16] Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.

 

http://en.wikipedia.org/wiki/Tsunami#Characteristics

There is a good animation in that link.

 

A tsunami is a series of ocean waves generated by sudden displacements in the sea floor, landslides, or volcanic activity. In the deep ocean, the tsunami wave may only be a few inches high. The tsunami wave may come gently ashore or may increase in height to become a fast moving wall of turbulent water several meters high.

 

http://www.tsunami.noaa.gov/

 

It also seems like Japan had very bad Earthquake in 1995 (Kobe) which prompted a massive change to their natural disaster warning and response system. The Japaneses people really like to plan for every worst case scenario, so this instance might be a little over-reacting too.

 

http://en.wikipedia.org/wiki/Great_Hanshin_earthquake#Effect_on_disaster_prevention_planning