Hello butterfly..it have been long since i last posted but i was looking deeper for my research but i have come across a very interesting topic about tsunami and earthquakes.
Information about earthquakes and tsunamis.
Tsunamis for harbor wave is series of huge ocean waves caused by a rapid, large-scale disturbance of the sea water. Tsunamis can be caused by submarine volcanic eruptions, submarine landslides, meteor impact, and major earthquakes occurring beneath the seabed causing large vertical movements. In deep water, tsunami waves are less than a meter high, but they can travel at speeds exceeding 800 kilometers per hour and can easily cross an entire ocean basin. When they reach shallow water or narrow inlets the waves slow down and the height can build into a wall of water which causes devastation on the shore.
On January 26, 1700 one of the world's largest earthquakes occurred along the west coast of North America and created a tsunami which completely destroyed the winter village of the Panache Bay people with no survivors. These events are recorded in the oral traditions of the First Nations people on Vancouver Island. There is evidence of repeated tsunamis inundating our west coast following giant mega thrust in the Cascade seismic zone.
Characteristics of man made or triggered tsunamis.
Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris with it, even with waves that do not appear to be large.
While everyday wind waves have a wavelength from crest to crest of about 100 meters and a height of roughly 2 meters, a tsunami in the deep ocean has a much larger wavelength of up to 200 kilometers. Such a wave travels at well over 800 kilometers per hour, but owing 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 meter. This makes tsunamis difficult to detect over deep water, where ships are unable to feel their passage.
The velocity of a tsunami can be calculated by obtaining the square root of the depth of the water in meters multiplied by the acceleration due to gravity. For example, if the Pacific Ocean is considered to have a depth of 5000 meters, the velocity of a tsunami would be the square root of (5000 × 100) = 50000 =224 meters per second which equates to a speed of 806 kilometers per hour or about 500 miles per hour. This is the formula used for calculating the velocity of shallow waters waves. Even the deep ocean is shallow in this sense because a tsunami wave is so long by comparison.The reason for the Japanese name harbor wave is that sometimes a village's fisherman would sail out, and encounter no unusual waves while out at sea fishing, and come back to land to find their village devastated by a huge wave.
As the tsunami approaches the coast and the waters become shallow,wave shoaling compresses the wave and its speed decreases below 80 kilometers per hour. Its wavelength diminishes to less than 20 kilometers and its amplitude grows enormously in accord with greens law. Since the wave still has the same very long period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.
When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed as run up. Run up is measured in meters above a reference sea level. A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run-up.About 80% of tsunamis occur in the Pacific Ocean, but they are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, glacier calving, and collides.
All waves have a positive and negative peak; that is, a ridge and a trough. In the case of a propagating wave like a tsunami, either may be the first to arrive. If the first part to arrive at the shore is the ridge, a massive breaking wave or sudden flooding will be the first effect noticed on land. However, if the first part to arrive is a trough, a draw back will occur as the shoreline recedes dramatically, exposing normally submerged areas. The drawback can exceed hundreds of meters, and people unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed.
A typical wave period for a damaging tsunami is about twelve minutes. Thus, the sea recedes in the drawback phase, with areas well below sea level exposed after three minutes. For the next six minutes, the wave trough builds into a ridge which may flood the coast, and destruction ensues. During the next six minutes, the wave changes from a ridge to a trough, and the flood waters recede in a second drawback. Victims and debris may be swept into the ocean. The process repeats with succeeding waves.
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