1 . Submarine Earthquakes . 



As shown by Iida (1970) , tsunamis are generated hy shallow-focus 

 earthquakes of a dip-slip fault type; i.e., vertical motion upward on 

 one side of the fault and downward on the other side (Fig. 5). Shepard, 

 MacDonald, and Cox (1950) indicate that tsunamis which travel long dis- 

 tances across the ocean are probably caused by unipolar disturbances. 

 (An example of a unipolar disturbance would be the uplift of a large 

 area of the sea floor where there is a net change in volume.) Waves 

 generated from a unipolar source decay much less rapidly with distance 

 than waves generated by a bipolar disturbance; i.e., a combination up- 

 lifting and subsidence, or other apparent transfer of material on the 

 sea floor, without a net change in volume. Hammack and Segur (1974) 

 studied the propagation of waves both experimentally and numerically. 

 They indicate that where there is a positive net change in volume (e.g., 

 a unipolar uplifting of the sea floor), waves of stable form (solitons) 

 evolve, followed by a dispersive train of oscillatory waves. The number 

 and amplitude of the solitons depends on the initial generating mechanism. 

 The wave record for the 1964 tsunami at Wake Island (see Fig. 6) illus- 

 trates this type of wave generation. Van Dorn (1965) discusses the 

 generating mechanism of the 1964 tsunami which originated in Alaska. 

 The ground motion was dipolar, having a positive pole (uplifting) under 

 the sea and a negative pole predominantly under the land. As the positive 

 pole was the main tsunami -generating mechanism, this was equivalent to a 

 unipolar source. 



Dip- Slip Fault 



Strike- Slip Fault 



Figure 5. Movement along faultlines. 



Heck (1936) indicates that horizontal motion of the sea floor does 

 not appear to generate large tsunamis. However, large "local" tsunamis 

 may be generated by horizontal motion. Iida (1970) shows that major 

 tsunamis (those that cause high water levels at many different coastal 

 locations) do not appear to occur as the result of deep-focus earthquakes 

 or the strike-slip fault type, i.e., horizontal motion along the fault- 

 line (Fig. 5). A general expression for the lower limit of the earth- 

 quake magnitude, M, of tsunamigenic earthquakes is given by Iida (1970) 

 as 



M = 6.3 + 0.005 D 



f 



(ID 



28 



