20. MICROSEISMS 



J. Darbyshire 



1. Relation between Sea Waves and Microseisms 



Microseisms 1 are small vibrations on the Earth's surface of period 4-10 sec 

 and amplitudes up to 20 jj.. It had long been suspected that they were in some 

 way connected with meteorological factors. Some investigators considered that 

 they were due to barometric fluctuations but as the incidence of high micro- 

 seism activity in Europe often coincided with high surf on the Atlantic coasts, 



0^ 20 



Mean wave period 



Mean microseism period x 2 



Peak wove amplitude 

 Mean microseism amplitude 



Ith 



12th 13th 



14th 



15th 16th 17th 

 March 1945 



18th 



19th 20th 



Fig. 1 . Comparison of the amplitudes and periods of the pressure variations below waves 

 at Perranporth with the amplitudes and periods of microseisms recorded at Kew. 

 (After Deacon, 1947, Fig. 1.) 



Professor E. Wiechert in 1905 suggested that they were caused by the breaking 

 of waves on rocky shores. This view gained the support of B. Gutenburg (1912, 

 1915, 1921) and others. It was confirmed by the work of E. Gherzi in India, 

 (1923, 1927), but S. K. Banerji (1930) noticed, however, that, though micro- 

 seisms in India were often associated with monsoon activity in the Indian seas, 



1 This is the definition adopted in this article; oscillations outside this range are 

 sometimes included. 



[MS received December, 1959] 



700 



