The History Channel recently had a “Nature Tech” story that included info on some of Japan’s tsunami defenses. The northern town of Taro (sp.?) had 2 tsunamis, in 1896 and 1933. They killed 20,072 and 3,000 respectively, with the latter wave being 95 feet high. In 1934 they started building a series of seawalls appearing to be about 30 feet high and with the seaward side sloping down to the beach at about a 45 degree angle. It has rectangular indentations to help dissipate wave energy. There are 6 openings about 12 feet tall with massive 2-ton steel doors that take a crew of 3 volunteer firemen to close and secure them. One man stays on the seaward side to lock them. He then has to climb a stairway to get over the wall. It was said that the walls were built to the hight of previous tsunamis but that was no guarantee they would suffice for higher waves in the future. They also have tide gauges and tv surveillance of the waterfront to give additional info after a warning is received that a 'quake has occurred that could cause a tsunami.
The program also mentioned evidence found in 1953 of a “magatsunami” that had hit Lituya Bay, Alaska, that was wiped out all vegetation up to the 500 foot level. . An oil prospector, George Plafker, now a Ph.D. with the USGS, saw that the old growth tree line was up so high and wondered about it for 5 years. In 1958, Howard Ulrich and his 8-year-old son were on a boat in the bay and saw a massive landslide on its other side. The wave from it carried the boat into the treetops and back to the bay and wiped out all vegetation on the shore up to an elevation of 520 metres or about 1,640 feet above sea level. He contacted Mr. Plafker and his investigation showed the immense power of what are called megatsunamis.
The program then reported on the volcano, La Coimbra Vieja (sp.?) on the island of La Palma in the Canary Islands. A 1949 'quake there resulted in a 12-metre drop in the west side of the south end of the island. The prediction is that it will slide into the sea, causing an up to 1200-foot high wave to reach the US east coast in 8 hours. No estimate on the timing. The wave size depends on the size of velocity of the rockfall when it hits the water.
This week the US reported that they will be installing a tsunami warning system in the Atlantic over the next 2 years at an estimated cost of $20 million IIRC.
The Pacific warning system, Deep Ocean Assessment and Reporting of Tsunamis (DOART), was installed by the USA in the Pacific after the 1949 tsunami that inundated Hilo, Hawaii, with a 30-foot wave. 26 other Pac countries joined in that effort after the 1960 tsunami that hit Chile, killing more than 2,000.
The Pacific warning system consists of 100 surface buoys sensing wave action and 6 ocean-bottom sensors that are tethered to surface buoys. The deep ones measure the weight of water above them. All data are sent hourly to satellites and then down to ground station(s?) whence warnings are issued. Presently it is one-way communication from the sensors up, but they are working on two-way so they can query the sensors at any time. It takes only two minutes to read the data, but longer to decide on, and then to issue, warnings.
/cite/ notes taken during a rerun of the program. There may be some e-links.