Most tsunamis are caused by earthquakes that extend at least partly under 

 the sea, although not all submarine earthquakes produce tsunamis. Severe 

 tsunamis are rare events. 



Tsunamis may be compared to the wave generated by dropping a rock in a 

 pond. Waves (ripples) move outward from the source region in every direc- 

 tion. In general, the tsunami wave amplitudes decrease but the number of 

 individual waves increases with distance from the source region. Tsunami 

 waves may be reflected, refracted, or diffracted by islands, sea mounts, 

 submarine ridges, or shores. The longest waves travel across the deepest part 

 of the sea as shallow-water waves and can obtain speeds of several hundred 

 kilometers per hour. The traveltime required for the first tsunami dis- 

 turbance to arrive at any location can be determined within a few percent of 

 the actual traveltime by the use of suitable tsunami traveltime charts. 



Tsunamis cross the sea as very long waves of low amplitude. A wavelength 

 of 200 kilometers (124 miles) and an amplitude of 1 meter (3 feet) is not 

 unreasonable. The wave may be greatly amplified by shoaling, diffraction, 

 convergence, and resonance when it reaches land. Seawater has been carried 

 higher than 11 meters (36 feet) above sea level in Hilo, Hawaii, by 

 tsunamis. Tide gage records of the tsunami of 23-26 May 1960 at these 

 locations are shown in Figure 3-46. The tsunami appears as a quasi-periodic 

 oscillation, superimposed on the normal tide. The characteristic period of 

 the disturbance, as well as the amplitude, is different at each of the three 

 locations. It is generally assumed that the recorded disturbance results from 

 forced oscillations of hydraulic basin systems and that the periods of 

 greatest response are determined by basin geometry. 



Theoretical and applied research dealing with tsunami problems has been 

 greatly intensified since 1960. Preisendorfer (1971) lists more than 60 

 significant theoretical papers published since 1960. Recent research on 

 tsunamis is discussed by Camfield (1980) and Murty (1977). 



3. Lake Levels . 



Lakes have insignificant tidal variations, but are subject to seasonal 

 and annual hydrologic changes in water level and to water level changes caused 

 by wind setup, barometric pressure variations, and seiches. Additionally, 

 some lakes are subject to occasional water level changes by regulatory control 

 works . 



Water surface elevations of the Great Lakes vary irregularly from year to 

 year. During each year, the water surfaces consistently fall to their lowest 

 stages during the winter and rise to their highest stages during the summer. 

 Nearly all precipitation in the watershed areas during the winter is snow or 

 rainfall transformed to ice. When the temperature begins to rise there is 

 substantial runoff — thus the higher stages in the summer. Typical seasonal 

 and yearly changes in water levels for the Great Lakes are shown 'in Figure 3- 

 47 The maximum and minimum monthly mean stages for the lakes are summarized 

 in Table 3-4. 



3-93 



