Equivalent displacement of water surface 



-Equivalent vertical motion 

 -Horizontal motion 



Figure 7. Horizontal motion normal to continental slope (scale exaggerated). 



variation of the uplift within the area of uplift, and the depth of 

 water and coastal characteristics in the generating area. While ordi- 

 nary sea waves are assumed to have a cnoidal shape as they approach a 

 shore (i.e., high crests and shallow troughs), the waves in a tsunami 

 may have various combinations of forms . 



Visual observations of tsunamis have led to reports that the initial 

 wave was often a negative wave causing an initial drawdown of the water 

 level at the shoreline. Shepard, MacDonald, and Cox (1950) show that 

 some tide gage records indicate a small positive wave followed by a very 

 deep trough, the amplitude of the trough being about three times the 

 amplitude of the initial wave crest. This may have been misinterpreted 

 by observers who reported initial negative waves. However, Striem and 

 Miloh (1975) indicate that an initial drawdown may occur when the tsunami 

 is generated by slumping of the continental slope. Tsunamis may sometimes 

 produce waves with narrow, deep troughs and low, wide crests at the shore- 

 line, the opposite of the cnoidal waveform. 



Wave records from Wake Island for the March 1964 tsunami (Van Dorn, 

 1964) show a positive surge with a period of 80 minutes (see Fig. 6). 

 There was a series of positive wave crests with the elevations of the 

 intervening troughs above the normal expected tide level . This was 

 followed by a series of crests and troughs with the elevations of the 

 troughs below the normal tide level. Using a shallow-water wave celerity 

 at the source and an average depth of approximately 100 meters (325 feet) 

 for the generating area, the period of the initial positive surge is 

 approximately equivalent to the time required for the trailing edge of 

 the initial uplifted water surface to travel completely across the area 



35 



