Acehtsunami

Newly discovered tsunami deposits imply that the Japanese coastline was hammered by a series of massive waves thousands of years ago. The finding adds to growing proof that the region is regularly pounded by killer waves, and could help in planning for future inundations. The northern Japanese island of Hokkaido is huddled up against the Kuril-Kamchatka trench, a place where the Pacific tectonic plate dives beneath the Eurasian plate and home to terrible earthquakes in excess of magnitude 8.0.
Now Wesley Nutter and a team of researchers say nine waves, each at least 33 feet high, thrashed the coastline before the dawn of civilization on the island. “In recorded history, tsunamis have hit the Hokkaido coast over and over again,” Wesley Nutter of Earlham College in Indiana said. “But something of that size has never been recorded here.”
Nutter and a team of researchers dug down into the sediments of a saltwater marsh on the island looking for signs of past tsunamis. Team member Kazuomi Hirakawa of Hokkaido University had first detected a series of sand deposits several years ago there that had no business in a marsh mostly made of peat. Tracing the sand deposits away from the coast, the team found they extend up to more than a mile inland and get thinner further from the sea.

Tsunamis are different from wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths.The wind-generated swell one sees at a California beach, for example, seeded by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m.

A tsunami, on the other hand, can comprise a wavelength in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis usually behave as shallow-water waves.A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very less.Shallow-water waves travel at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth.

If the first part of a tsunami to reach land is a trough known as drawback, rather than a wave crest. The water along the shoreline recedes dramatically, exposing usually submerged areas.

A drawback generally occurs because the tectonic plate on one side of the fault sinks suddenly during the earthquake, causing the overlaying water to propagate outwards with the trough of the wave at its front. This is why there would not be any drawback when the tsunami traveling on the other side arrives ashore, as the tectonic plate is “raised” on that side of the fault line.

Drawback starts to begin before the wave arrives at an interval equal to half of the wave’s period. If the slope of the coastal seabed is small, drawback can exceed about hundreds of meters. People who are unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed. During the Indian Ocean tsunami, the sea withdrew and a number of people went onto the exposed sea bed to investigate. Photos show people walking on the normally submerged areas with the advancing wave in their background.

It has been planned and designed as a symbolic reminder of the disaster, as well as an educational centre. In addition it will also serve as an Emergency disaster shelter in case the area is ever hit by a tsunami again.

Aceh served as home for more than half the 240,000 people who died in the disaster. The outburst of aid which followed was the largest in history. Almost all that aid money has now been spent - gone to pay for more than 130,000 houses and thousands of kilometers of road, bridges, as well as schools, and other infrastructure.

Yet this new museum building, paid for by Ache’s Reconstruction Fund, breaks with the tradition of post-disaster construction.

Tsunamis can be generated when the sea floor hurriedly deforms and vertically displaces the overlying water. Tectonic earthquakes are a certain kind of earthquake that are associated with the earth’s crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is relocated from its equilibrium position.

Waves are formed as the displaced water mass, which acts under the pressure of gravity, attempts to regain its equilibrium. When large areas of the ocean floor elevate or subside, a tsunami can be created. Huge vertical movements of the earth’s crust can occur at plate boundaries.

Plates act together along these boundaries called faults. Around the margins of the Pacific Ocean, for instance, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are mainly effective in generating tsunamis.

In general, if you think tsunami is going to hit or the ground shakes under your feet or you hear there is a warning, you should move immediately to higher ground.

If you are at home and hear there is a tsunami warning, you should make sure people around you are also aware of the warning. You should quickly move to a safer place if you live in an evacuation zone. Follow the guidance of local emergency and law enforcement.

If you are at the beach or anywhere near the ocean and you feel the ground shake, move immediately to higher ground; DO NOT wait for a warning to be announced. Stay away from rivers and streams that lead to the ocean so that you would stay away from the beach and ocean. A regional tsunami from a local earthquake could smack some areas before a tsunami warning could be announced.

Tsunamis generated in distant locations will generally give a time lapse so that people can move to higher ground. For locally-generated tsunamis, where you might feel the ground shake, you may only have a few minutes so you should react sooner and move to higher ground.

On November 29, 1975, at 14:48 GMT, an earthquake occurring off the coastline of the Island of Hawaii. A close by felt tsunami was triggered by the earthquake, which had a surface-wave scale of 7.2, an epicenter of 19.3° N, 155.0° W, and a focal deepness of 8 km. The greatest missing was at Halape, a beach park at the base of a large precipice, on the land mass of Hawaii. At Halape, of the 32 campers 19 suffered injury and 2 died. It was the sounds of the lessening rocks from the cliff and the wobbly that cause the campers to awake and a few moved to a coconut grove that was closer to the ocean. The campers were awaken by a second quake that sent big boulders down the cliff and forced the rest of the campers to flee toward the sea. However, these campers were forced back to precipice when the campers at the coconut grove fleeing the rising ocean with cries of tsunami.

The first wave will be alarmed the campers was only 1.5 m. The second most wave, however was 7.9 m accepted campers into a trench near the base of cliff where they remained until the ordeal ended. The coconut grove that a few campers took shelter in received everlasting subsidence between 3.0 and 3.5 meters.

The largest record run-up was 14.3 m at Keauhou Landing, Hawaii Island. Also on the Island of Hawaii in the small cove of Punaluu the run-up reached 7.6 m. At Punaluu houses were swept off their basics and property were damaged. By the time local responsibilities should sound the coastal sirens the first wave had already arrived. As in the 1964 in Alaska the best caution to the possible danger of a local tsunami is the wavering from the earthquake that triggers it.