Microseismic Period Spectra and Related Problems Scandinavian Area 



65 



A few similar studies have been published 

 in the past for other regions. But one won- 

 ders what is the overall significance of a peri- 

 od spectrum which seems to take no account of 

 superposition of wave trains simultaneously ar- 

 riving from different directions. There would 

 be less likelihood of inhomogeneity if only the 

 periods accompanying the maximum ampli- 

 tudes of well-formed groups are considered ; but 

 care is required even in this case to select only 

 groups of regular waves. Neither from one 

 microseismic storm to another nor within a 

 group in the same storm has the writer found 

 the period to increase consistently with ampli- 

 tude. 



Recently Gotch under the writer's direc- 

 tion made a study of the relation between am- 

 plitude and period in the most regular portions 

 of wave groups in nineteen microseismic storms 

 recorded by Galitzin-Wilip seismographs at 

 Florissant in 1949, 1950 and 1951. The aver- 

 age period of the maxima for all nineteen 

 storms was 6.66 seconds. The shortest period 

 was 5 seconds and the longest was 8.8 seconds. 

 In the four storms in which the average peak 

 period was over eight seconds the amplitudes 

 were only moderate; while in those storms in 

 which the mean peak amplitudes were largest 

 the corresponding periods were less than the 

 overall average. The writer has not succeeded 

 in finding a tenable storm to storm relationship 

 between amplitudes and periods in the mid- 



continent area of North America. Within a 

 given microseismic storm after it is fully de- 

 veloped the period tends to remain approxi- 

 mately constant. 



The periods generally recorded at Floris- 

 sant and Saint Louis are sensibly longer than 

 those shown by Donn and by Gilmore in rec- 

 ords of east coast stations. This might be and 

 has been interpreted as a distance effect. How- 

 ever, the Florissant periods are approximately 

 the same as those listed by Thompson for the 

 Palmer Land station in Antarctica and yet the 

 probable source fronts were often very near 

 to that station and sometimes over relatively 

 shallow waters. 



Many seismologists will fail to see the 

 cogency of the argument for a linear source at 

 a given distance drawn from the variation of 

 the plane of vibration of microseismic waves at 

 single stations. Even the actual instantaneous 

 directions of travel of well-formed groups of 

 waves across a tripartite station vary through 

 many degrees of arc so that an average of many 

 observations is necessary to determine a bear- 

 ing that is representative of the energy flow 

 across the network. How then can it be shown 

 from observations at single stations that the 

 source is linear and not a real? And in view 

 of the inconclusive relationship of period to 

 distance how can the source be considered as 

 certainly lying along a given coast line ? 



Measurements Made by Mr. Gotch on 

 Florissant Seismograms 



