60 



Symposium on Microseisms 



B < H is significant. Therefore we may con- 

 clude that there is no pronounced difference of 

 T f between the different stations except for a 

 slight indication in the manner already men- 

 tioned. For T m there is a very clear increase 

 from BtoH:B<C <U <H, very well sub- 

 stantiated by the individual cases. Every dif- 

 ference is significant, possibly with the excep- 

 tion for U < H. For T a there is no very ob- 

 vious difference between the different stations, 

 except that H has generally larger values. The 

 only difference which is no doubt significant is 

 C < H. The relatively large values of T a at B 

 for our situations are remarkable. Earlier re- 

 sults (1951 a, p. 373) have indicated that in 

 general T a is less at B than at U. By means 

 of the data used in (1951 a, p. 373) this was 

 shown to be significant. By means of the cor- 

 responding data for Copenhagen (1952) it has 

 been shown that for T a B < C with a high de- 

 gree of significance, whereas for the same data 

 there was no difference of T a between C and U. 



The main reason for the sequence B < C < 

 U < H, especially clear for T m , is the stronger 

 extinction of shorter periods (see below) . The 

 fact that C comes between B and U is probably 

 explained by a relatively larger importance at 

 C of the southwestern part of the Norwegian 

 coast, from where the microseisms arrive at C 

 before they arrive at U. At U and H the 

 whole Norwegian coast is of about equal im- 

 portance. 



3. Comparison of components. 



The mean periods (n = 16) for all sta- 

 tions and all situations are as follows: 



For all three periods E < N. This result may 

 be considered significant only for T a , but the 

 tendency of E < N exists for all three periods. 

 The result that E < N for Ta confirms my earl- 

 ier results for Bergen and Uppsala (1951 a, pp. 

 372-373). It has a high degree of significance 

 for Uppsala, but not for Bergen. It has also 

 been proved for Copenhagen with a high de- 

 gree of significance, using the observations cor- 

 responding to those at Bergen and Uppsala in 

 (1951 a, p. 373). This is therefore a charac- 

 teristic feature of Scandinavian microseisms. 

 The most probable reason is a distance effect. 

 The N component is most sensitive to actions 

 at the more remote northern part of the Nor- 

 wegian coast, whereas the E component reacts 

 strongest to actions at the west coast (around 

 B). This has been established beyond doubt 

 from numerous cases at Uppsala. 



An increase of the period T a with increas- 

 ing distance has been observed in several in- 

 vestigations. But the nature of the phenome- 

 non is not clear: if it is mainly an actual in- 



crease of period with distance, or if it is only a 

 more rapid extinction of the shorter periods. 

 No decision seems to be possible only from a 

 knowledge of the increase of T a with distance 

 as both effects produce the same result (for 

 further discussion see 6, below) . 



4. Comparison of the different periods. 



The mean periods (n = 32) for all sta- 

 tions, both components, and all situations are 

 T f == 5.18 sec, T ra == 5.15 sec, T a = 5.69 sec. 

 The result T m < T a is valid in practically every 

 case (exceptions are UIIE and HUE) and has 

 a very high degree of significance. The result 

 Tf < T a is also highly significant, but there are 

 a few more exceptions to this rule than to the 

 rule T m < T a . This result is a reflection of the 

 fact that the amplitudes corresponding to the 

 shorter periods are relatively small. Further- 

 more, 2f ^.To, i.e. no significant difference, cor- 

 responding to the generally symmetrical nature 

 of the frequency curves. 



20 

 % 



15 • 



8 sec 



Berqen .E-W 



8 sec 



2 4 6 8 sec 

 Copenhagen N-S 



20r K 2U 



% l\ % 



15 



2 4 6 8 sec 



Copenhagen E-W 



2 4 6 



Helsinki N-S 



8 sec 



2 A 6 8 sec 



Helsinki E-W 



6 

 E-W 



8 sec 



02458 024 



Uppsala N-S sec Uppsala 



Figure 3. Period spectra on October 28, 



1947, at 07 h M.E.T. 



