58 



Symposium on Microseisms 



T, = period, corresponding to frequency 

 maximum, 



T„ = mean period, 



T a = period, corresponding to amplitude 

 maximum, 



n = number of observations, 



N = N-S component, 



E = E-W component, 



B = Bergen, 



C = Copenhagen, 



H = Helsinki, 



U = Uppsala, 



I = October 7, 1947, at 07 h M.E.T., 



II = October 28, 1947, 07 h M.E.T., 



III = January 14, 1949, 07» M.E.T., 



IV = March 23, 1949, 07 h M.E.T. 



The period spectra are given in Figs 2-5. 

 Table 2 contains values of Tf, T m , and T a with 

 standard errors for T m and T a and the number 

 of observations in each case. T f , T m , and T a 

 have also been indicated in Figs. 2-5 by vertical 

 lines, the shortest for Tf, the next longer for 

 T m , and the longest for T a . The whole in- 

 vestigation is based upon 7083 individual pe- 

 riod measurements on the records. The weath- 

 er situations at 07 h M.E.T. are obvious from 

 Fig. 6, copied from the official Swedish weath- 

 er maps. The agreement with the official Brit- 

 ish weather maps is very good. 



The aim in the following study has mainly 

 been to establish general rules for the periods. 

 The statistical significance of these rules has 

 been investigated in every case, usually by ap- 

 plication of Bernoulli's theorem. The devia- 

 tions from the general rules which may occur 

 in individual cases, require more detailed 

 studies of the particular cases in order to be 

 explained. 



1. The shape of the frequency curves. 



The frequency curves have in general only 

 one pronounced maximum around which the 

 curves are symmetrical. The microseisms may 

 therefore be characterized as regular. Notable 

 exceptions occur at Bergen, especially for N. 

 This component has usually two maxima at 

 Bergen. As the main source of the micro- 

 seisms in Scandinavia lies along the Norwegian 

 coast, we understand that the shape of a fre- 

 quency curve depends on the position of the 

 station in relation to this coast. If the posi- 

 tion is such that the coast length takes up a 

 large distance interval, from the station, as 

 the case is for Bergen, a wider and less regu- 

 lar spectrum is obtained. In case IV the mi- 

 croseisms are less regular at all stations than 

 in cases I-III. 



2 4 6 8 sec 



Bergen N-S 



25 



% 



2 4 6 8 sec 

 Bergen .E-W 



024b8 sec 02462 Sue 

 Copenhagen N-S 20r Copenhagen E-W 



2 4 6 8 $s . 

 Helsinki N-S 20r 



°/o 



15 



2 4 S 8 sec 

 Helsinki E-W 



2 4 6 8 sec 2 4 6 8 sec 

 Uppsala 'N-S Uppsala E-W 



Figure 2. Period spectra on October 7, 

 1947, at07 h M.E.T. The shortest vertical 

 line indicates T£ , the next longer T B , 

 and the longest T . 



2. Comparison of stations. 



The mean periods (n = 8 ; calculated from 

 Table 2) for both components and all situations 

 are as follows: 



For Tf it seems to be an increase from B to H : 

 B < C < U < H. The various differences, 

 B < C, C < U, U < H, B < U, and C < H are, 

 however, not statistically significant. But 



