Tsunami Termination
Tsunami Termination : Upon reaching shoaler water, the speed of the advancing tsunami wave diminishes, its wave length decreases, and its height may increase gready, owing to the piling up of water. Configuration of the coastline, shape of the ocean floor, and character of the advancing waves play an important role in the destruction brought by tsunamis along any coast, whether near the generating area or thousands of kilometers from it. At present, detection of tsunamis is possible only near shore where the shoaling effect can be observed. The first visible indication of an approaching tsunami is often a recession of water caused by the trough preceding an advancing wave. Any withdrawal of the sea, therefore, should be considered a warning of an approaching wave. A rise in water level also may be the first event. Under certain conditions, the crest of an advancing wave can overtake the preceding trough while some distance offshore. This causes the wave to proceed shoreward as a bore - a wave with a churning front. The force and destructive effects of tsunamis should not be underestimated. At some places, the advancing turbulent front is the most destructive part of the wave. Where the rise is quiet, the outflow of water to the sea between crests may be rapid and destructive, sweeping all before it and undermining roads, buddings, and other works of man with its swift currents. Ships, unless moved away from shore, can be thrown against breakwaters, wharfs, and other craft, or washed ashore and left grounded during withdrawals of the sea. In the shallow waters of bays and harbors, a tsunami frequently will initiate seiching. If the tsunami period is related closely to that of the bay, the seiche is amplified by the succeeding waves. Under these circumstances, maximum wave activity often is observed much later than the arrival of the first wave. A tsunami is not one wave, but a series of waves. The time that elapses between passage of successive wave crests at a given point usually is from 10 to 45 minutes. Oscillations of destructive proportions may continue for several hours, and several days may pass before the sea returns to its normal state
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