EARTHQUAKES OF MAY 19 60 IN CHILE — SAINT-AMAND 357 



the lakes, permitting it to rift and crack, and forming openings for 

 the escape of lava. 



ORIGIN OF THE MAREMOTO 



In general, the sea withdrew for a period of 10 to 20 minutes and 

 then returned. There were several successive waves, of which the 

 third or fourth seemed largest. The period of the waves was about 20 

 minutes. In almost all places, the first movement was a withdrawal of 

 the water. In Bahia Mansa, however, the first movement was re- 

 ported as a rise. A small rise may have preceded the drop in 

 Maullin (fig. 4). 



Watanabe and Karzulovic [20] have tried to indicate the epicenter 

 of the earthquake by assuming a velocity for the maremoto and calcu- 

 lating the distance run from some points on the shore, using a velocity 

 altogether too high. Following their lead and using calculated veloc- 

 ities, the following argument is adduced. 



The velocity depends on the depth of the water as shown by Airy 

 [21] and elaborated by Lamb [22] : 



V={gh)y^ 



where V is the velocity of the wave with respect to the shore and h 

 is the depth of the water. Although the velocity may reach speeds of 

 600 km./hr. in the deep sea, the velocity nearer to the shore will be 

 much less. Figure 6 shows the approximate position of a maremoto 

 wave as a function of time from the coast of Chile, using depths given 

 by the charts of the Department of Navegacion e Hidrografia of the 

 Chilean Navy. Since the time of arrival of the big wave was between 

 10 and 20 minutes after the main earthquake, the approximate origin 

 of the sea wave must be nearer to shore than the position indicated by 

 the dashed line. 



The times involved in the advance and retreat of the waves are 

 short, being possibly nothing more than that the period of the wave 

 and the distances from the coast are near that of the length of the 

 waves. Hence, such calculations may be meaningless. The shore 

 may have been in the zone of production of the wave. Also, the 

 time rate of change of velocity is high, and the equation does not 

 account for the drag of the bottom nor the change in shape of the wave 

 in shallow waters. 



It appears from the foregoing discussion that the wave may have 

 been created in two ways : 



1. The dropping of a large block of land offshore could explain the 

 wave, except that the primary movement in Bahia Mansa seems to 

 have been a rise. Perhaps vertical displacement of gigantic blocks 



