Tripartite Stations and Direction of Approach of Microseisms 



19 



Observatory. The comparisons were made for 

 selected, simple, meteorological situations when 

 there was little doubt of the actual source of 

 the microseisms. The experiments were most 

 disappointing and the main conclusion was that 

 the ground movements at Kew were so com- 

 plicated, presumably by the interference of 

 waves which have suffered multiple refractions 

 and reflections, that comparison of the two 

 horizontal components gave little information 

 about the direction of the source. During the 

 experiments Fourier analyses were made of 

 simultaneous recordings of waves and the three 

 microseism components. The most striking 

 features of these analyses was the striking 

 similarity of the period spectra of the north- 

 south, east-west, and vertical components. 

 Though it was fairly certain that the direction 

 of the storm changed over a time of 30 hours 

 from west through northwest to north, the 

 relative amplitudes of the east-west and north- 

 south records and spectra remained the same. 



The horizontal components always had the 

 same range of periods as the verticals ; this 

 might indicate that there was no appreciable 

 movement due to Love waves which would be 

 likely to widen the period range of the hori- 

 zontal components. 



The amplitudes of the vertical movements 

 were roughly twice those of the horizontal com- 

 ponents. 



The use of tripartite stations is based on 

 the assumption that the microseisms approxi- 

 mate to a regular, simple wave system, with 

 the wave crests travelling as straight lines. 

 The fact that tripartite stations in some places, 

 and at some times, give excellent results shows 

 that the microseisms sometimes do travel as 

 simple waves, but experience in Great Britain 

 indicates that such behavior is exceptional 

 there. 



