62 



2 4 6 8 sec 



Bergen E-W 



2 4 6 6 sec 



Copenhagen N-S 



15 

 % 



10 

 5 



2 4 6 8 sec 

 Copenhagen^ E-W 



2 4 6 



Helsinki N-S 



8 sec 



2 4 6 8 sec 



Helsinki E-W 



2 4 6 8 sec 



Uppsala N-S 



2 4 6 



Uppsala E-W 



Figure 4. Period spectra on January 14, 

 1949, at 07 h M.E.T. 



5. Comparison of situations. 



The mean periods (n = 8) for all stations 

 and both components are as follows : 



These values indicate the sequence II < I < 

 III < IV for all three periods. This sequence 

 is in general well established from the individ- 

 ual cases. All differences II < I, I < III, 

 III < IV are significant for T a (BE is the only 

 partial exception). The differences are also 

 significant for T„. possibly with exception for 

 the difference II < I (exceptions occur for T m 

 for CN and BE). On the other hand, the dif- 

 ferences for T f are not significant, not even 

 II < IV ; nevertheless, the general trend is the 

 same for T f as for T m and T a . 



Symposium on Microseisms 



The differences between the different situ- 

 ations can hardly be explained as only distance 

 effects, e.g. in II both active coast and cyclone 

 center are at the greatest distance, neverthe- 

 less the periods are shortest. On the other hand, 

 the different intensities of the storms seem to 

 afford an explanation. This is another in- 

 stance of the parallel behaviour of amplitudes 

 and periods. The maximum ground ampli- 

 tudes, expressed in n, are given in the follow- 

 ing table. 



If the anomalous amplitude BNIII is excluded, 

 we get the amplitude sequence II < I < III < 

 IV, i.e. the same as for the periods. Plotting 

 the periods against the mean amplitudes we 

 find that I-II-III forms a reasonable sequence, 

 whereas the small amplitude difference between 

 III and IV is not in good accord with the rela- 

 tively large period differences. This could pos- 

 sibly be due to a distance effect, as in IV the 

 more active part of the Norwegian coast is in 

 the northern part. A numerical calculation 

 of the rate of change of T a with distance to cen- 

 ter of the active coast gives approx. 2.10 " 3 sec/ 

 km, a value which lends further support to this 

 idea (1949). 



6. Upper and lower limits of the period 

 spectra. 



From an inspection of the period spectra 

 (Figures 2-5) it is clear that the upper limit 

 is remarkably constant from station to station 

 in a given situation with no general variation, 

 whereas the lower limit shows a very pro- 

 nounced increase in the direction B-C-U-H. 

 This increase occurs for both components in 

 every case without exception. The increases 

 of the lower limit are in the mean from B to C 

 about 0.25 sec, from C to U about 0.7 sec, from 

 U to H about 0.6 sec. The total increase from 

 B to H is in the mean about 1.5 sec. 



The results concerning the upper and low- 

 er limits of the period spectra strongly support 

 the conclusion that the change of the spectrum 

 with distance is due to a more rapid extinction 

 of the shorter waves rather than to an actual 

 increase of periods. It would naturally be 

 valuable to extend such investigations to great- 

 er distances, wherever possible, provided the 

 source of the microseisms is the same for the 

 whole area investigated. 



7. The hyperbola method. 



A hyperbola method for locating the 

 source of microseisms from the periods ob- 

 served at a number of stations has been given 

 in my paper (1951 a, p. 374). This method 



