122 



Symposium on Microseisms 



V 



/W 



1.4 



I.O 



0.6 



0.2 



I.O 



1.2 



1.4 



1.6 



1.8 



2.0 



sin (5 



Figure 4 



The maximum of the first mode appears 

 at vh/b zz 0.85; supposing b = 2.8 km /sec and 

 h = 3 km we have v = 0.79 (period of about 

 8 sec). At a distance of 3000 km the ampli- 

 tude of the vertical component = Q X 1-5 X 

 10~ ,!l cm. In order to obtain a microseismic 

 amplitude of 5 u the total force Q has to be 

 about 3.10"'"' dynes; assuming the radius of the 

 storm area zz 18 km it appears that a mean 

 pressure variation of 1/3 mb is necessary to 

 produce the observed microseisms. 



In this calculation it has been assumed that 

 the pressure variations at widely separated 

 points are correlated ; as this will not be the case 

 in actual circumstances, the obtained value of 

 1/3 mb has to be interpreted as the effective 

 pressure variation. 



Supposing the phases of these pressure os- 

 cillations at points separated by a distance 

 greater than a to be incorrelated the effective 

 pressure of the storm area b- is about a/b 

 times the mean pressure. 



An explanation of such a pressure varia- 

 tion has to be found either in the atmospheric 

 or in the hydrodynamical circumstances during 

 a storm. With regard to the first and most 

 obvious explanation we refer to the observa- 

 tions at Wei-ka-wei by Gherzi (1921) ; if the 

 atmospheric pressure in the "eye" of the ty- 

 phoon changes periodically with an amplitude 

 of 0.5 mb this would be sufficient to generate 

 microseisms at large distances of the track of 

 the storm (Scholte 1943). It is perhaps pos- 

 sible to obtain more data about this phenome- 

 non by placing a network of microbarographs 

 in the regions where typhoons often occur. 



Recent observations by Donn (1951) may 

 also elucidate the connection between atmos- 

 pheric disturbances and microseisms in the 

 western hemisphere. 



REFERENCES 



Algue, J., 1904, The cyclones of the Far East, Manila. 



