Tsunamis are impulsively generated sea waves by a disturbance to or near the ocean.
21. {{U}}Earthquakes, submarine volcanic explosions, landslides and the detonation of nuclear devices near the sea can give rise to such destructive sea waves. By far the most destructive tsunamis are generated from large shallow-focus earthquakes with an epicenter or fault line near or in the ocean. Vertical displacements of the earth's crust along the rupture resulting from the ocean. Vertical displacements of the earth's crust along the rupture resulting from such earthquakes can generate destructive tsunami waves which can travel across an ocean spreading destruction across their path.{{/U}} Similar displacements of the ocean floor can also be produced by volcanic eruptions and submarine avalanches or landslides. However, these sources are considered as point sources and, although the tsunami waves generated can be very destructive locally, the energy of the waves is rapidly dissipated as they travel across the ocean.
To forecast tsunamis and determine terminal run-up and destructiveness, one must be able to evaluate the parameters of the tsunami source mechanism in real time, often, from inadequate date. 22. {{U}}Tsunami source mechanism analysis is difficult given the time constraints of a warning situation. It will suffice to say that forecasting the run-up and potential destructiveness of a tsunami at a distant shore will depend greatly on determining the seismic parameters of the source location such as magnitude of the earthquake, its depth, its orientation, the length of the fault line, the size of the crustal displacements, and depth of the water.{{/U}} 23. {{U}}Refraction(折射) and diffraction(衍射) processes will affect the energy and height of the tsunami waves as they travel across the ocean. These effects must also be determined. Finally, terminal height, run-up, and inundation of the tsunami at a point of impact will depend upon the energy forcusing effect, the travel path of the waves, the coastal configuration, and the offshore bathymetry, only to name a few.{{/U}}
Tsunami run-up is the vertical distance between the maximum height reached by the water on shore and the mean-sea-level surface. 24. {{U}}Contrary to meteorological predictions, tsunami run-up, the final product of earthquake and tsunami investigations is not possible to forecast with a great degree of accuracy. The reason for this inadequacy is that the Tsunami Warning System works in a real time frame of short duration, often with inadequate date and information. Problems of communication and lack of sufficient station density, often complicate the process.{{/U}} Forecasting tsunamis requires adequate understanding of the phenomenon, good and expeditious collection of earthquake and sea level date, and accruate and expeditious assessment and interpretation of this data.