ON THEORIES OF THE ORIGIN OF MICROSEISMS 



by J. G. Scholte 



In the last decennium the concept that 

 many microseisms are generated by a storm at 

 sea has been more and more generally accepted ; 

 detailed investigations as for instance by Ber- 

 nard (1941) as well as the successful detection 

 of hurricanes by means of tripartite stations 

 prove the validity of this view beyond any doubt 

 (Gutenberg 1952). 



It is however still uncertain by which proc- 

 ess these seismic movements come into exist- 

 ence; the observations often point in different 

 directions and it is therefore not possible to 

 formulate a theory covering all observed data. 



Perhaps the most useful way to treat this 

 matter theoretically is to ignore various mete- 

 orologic and oceanographic circumstances and 

 to start from the undisputed fact that a dis- 

 turbance at the surface of the ocean causes at a 

 distance of the order of 10 8 cm. microseisms 

 with an amplitude of say 5 |i and a mean period 

 of about 6 seconds. 



The movement of the ocean in the vicinity 

 of the storm area has an amplitude which is of 

 course several times greater than 5 [i and as 

 the vertical motion at the bottom has to be con- 

 tinuous the same is true for the movement of 

 the water. In view of the well known fact that 

 the amplitude of gravity waves decreases ex- 

 ponentially with the depth, it is evident that 

 the motion of the water which generates at the 

 bottom the seismic waves is not a gravitational 

 but a compressional wave and that we may 

 neglect the effect of gravity on this process. 



Consequently we have to consider waves of 

 compression in a purely elastic system consist- 

 ing of a fluid layer of finite depth h covering a 

 solid body. Consider a cartesian coordinate sys- 

 tem with the x axis in the free surface paral- 

 lel to the direction of propagation and the z 

 axis vertically downward. In order to avoid 

 complications which are irrelevant to this prob- 

 lem we suppose this body to be semi-infinite. 

 Denoting the horizontal component of the move- 

 ment by u and the vertical one by w, 



»- x 



Figure 1 



114 



