124 



Symposium on Microseisms 



be large ratios of horizontal to vertical am- 

 plitudes. Romneij pointed out that there are 

 some microseisms with periods as great as 20 

 seconds, but with much lower amplitudes than 

 those of shorter period.) 



Longuet-Higgins. The occurrence of micro- 

 seisms in "groups" appears to be an example 

 of a very general phenomenon which is ob- 

 served whenever a disturbance can be con- 

 sidered as the sum of a large number of dis- 

 turbances of about the same frequency. Such 

 a sum was first considered by Rayleigh in 1880 

 in connection with sound from many different 

 sources — this is sometimes called "the bee-hive 

 problem." He showed that the probability 

 P(a)da of the sound wave amplitude being be- 

 tween a and a-da was given by 



P(a): 



(a)* 



11 

 a2 



where a was the r.m.s. amplitude. The height 

 of sea waves (denned as the difference in ele- 

 vation between a crest and the preceding 

 trough) has been shown to obey the same sta- 

 tistical law. In this case the generating area 

 of the swell can be considered as divided up 

 into regions, each large compared with the 

 length of a sea wave, and each giving a sinus- 

 oidal contribution of independent phase. A 

 similar concept probably applies to micro- 

 seisms. An analysis of the statistical distribu- 

 tion of microseismic amplitude over a fairly 

 short interval of time would be of interest. 



(Donn commented that frontal micro- 

 seisms are weaker in the summer than in the 

 winter. Carder replied that recently there 

 have been some intense summer cold front mi- 

 croseisms in Washington.) 



Bath. Cold fronts cannot be located sufficient- 

 ly accurately by interpolation from weather 

 maps, especially not when they pass over 

 oceans. Hydrographic records of pressure and 

 temperature have to be used. 



There is no microseismic effect observed 

 when cold fronts pass the limit of the conti- 

 nental shelf outside the Norwegian coast, 

 whereas there is generally rapid increase of 

 the microseisms when the cold fronts pass the 

 coast itself. 



Jardetzky. It is yet difficult to understand in 

 all details the mechanism of transmission of a 

 disturbance in the air to be a recorder of mi- 

 croseisms. There are three media involved. 

 The wave propagation in the ocean bottom (or 

 coast) does not present any difficulty, but there 

 is no agreement about the kind of disturbance 

 at the sea surface and the behaviour of the sea. 

 The observations are interpreted in different 

 ways and the surf, the strong wind in the cold 

 front of a cyclone, the atmospheric pulsations, 



the ocean swell or the interference of gravity 

 waves are made responsible for microseisms in 

 different theories. Neither of them seems to 

 be convincing when all observations are taken 

 into account, but each one might be true for a 

 corresponding group of microseisms. It is dif- 

 ficult to see whether a sufficient amount of ener- 

 gy is communicated to the sea by the air masses 

 directly in the form of compressional waves or 

 it is transmitted or increased by the action of 

 gravitational waves. There is no doubt more 

 that the movement of air masses in a cyclone 

 has to be taken as a primary disturbance. The 

 atmospheric pulsations can be one of charac- 

 teristics of this movement. On assuming the 

 existence of these pulsations (or some other 

 cause producing compressional waves) at the 

 interface air-water, one can determine the part 

 played by the latter. The signed has com- 

 puted from the theory of propagation of a 

 plane wave in the vertical direction a curve 

 mentioned by Dr. Press. This curve repre- 

 sents the amplitude of the vertical displace- 

 ment at the sea bottom in terms of ^ H , where 

 H is the depth, a the velocity of sound in 

 water, w the circular frequency) . The shape 

 of this curve suggests that the ocean acts as a 

 filter. For example, if a = 1.430 m/sec, H 

 = 1000 or 3000 m. periods of waves, which will 

 reach the bottom with amplitudes not chang- 

 ing essentially, will vary from 1.5 to 4 sec. 

 Making clear such an interpretation of the 

 behaviour of the water layer, this result does 

 not explain the conditions at the sea surface. 

 It seems that far more systematized data should 

 be correlated with each of the factors involved 

 in order to clarify those conditions. 



{Longuet-Higgins brought up the subject 

 of gusts again. He made a plea for measure- 

 ment of their intensity at sea. Van Straten 

 pointed out that there is a strong land-sea 

 breeze on the East coast, and suggested it 

 might make a difference between the day and 

 night frontal microseisms.) Deacon agreed 

 with the remarks about the possibility of mi- 

 croseisms of all periods up to a certain maxi- 

 mum being caused by one source, although they 

 were ground oscillations of short and long pe- 

 riods produced by other causes such as traffic, 

 wind on mountains, buildings, etc. Sixteen 

 second ocean waves produced 8 second micro- 

 seisms at Kew, and it seemed very likely that 

 the 2 second microseisms observed by Father 

 Lynch might be caused by 4 second waves on 

 the Great Lakes. Dr. Longuet-Higgins had 

 used V2 second waves to reproduce 14 second 

 microseisms in the bottom of his tank. 



With regard to the common explanation 

 of microseisms associated with weather dis- 

 turbances it was likely that when the explana- 

 tion was found it would be universally satis- 

 factory. The position at present was very 

 difficult to understand. On the eastern side of 

 the ocean the microseism records looked like 



