Microseismic Period Spectra and Related Problems Scandinavian Area 



63 



rests on the observation that the observed pe- 

 riod T a apparently increases with distance. 

 The method may briefly be described as follows. 

 For a mean apparent increase of T a with dis- 

 tance A from the source f=— r ~ 



dA 



we get the following equations, combining three 

 different stations two and two : 



T." _ T.' = f (A" — A' ) 

 T.'" — T.' = f (A'" — A' ) 

 TV" — T«" = f (A'" — A") 



f is assumed to be the same in all three equa- 

 tions as a first approximation. This set of 

 equations means that (A" — A') : (A"' — A") 

 : (A'" — A") is given, geometrically defining 

 the source. As f is not exactly known, the 

 source can be located by trial and error, until 

 all three hyperbolas intersect in a point, as- 

 suming a point source. When the source has 

 been located, it is possible to calculate f . The 

 method can only be used with success if the 

 source of the microseisms is exactly the same 

 for all stations for which the periods are used. 

 For a common point source the method may be 

 expected to lead to results. In our case, how- 

 ever, we have a line source, the length of which 

 | is comparable with and often larger than the 

 mutual distance between the stations. Differ- 

 ent parts of the Norwegian coast are of differ- 

 ent importance to the different stations as al- 

 ready indicated above, i.e. the source is not 

 exactly the same for all our stations. There- 

 fore the hyperbola method may not be expected 

 to lead to any useful results in these conditions. 



Among further desirable investigations 

 the following may be mentioned : 



a. Extension of the investigation of periods 

 by means of period spectra to greater distances. 



b. Application of the hyperbola method to 

 cases with a point source. 



c. Correlation of microseismic periods with 

 other phenomena, notably the periods of sea 

 waves and swell. Investigations of the last- 

 mentioned kind have been done by British in- 

 vestigators, but an extension to other localities 

 is desirable. 



Summary — Microseismic period spectra have 

 been constructed for four different situations 

 (I = Oct. 7, 1947, 11 = Oct. 28, 1947, 111 = 

 Jan. 14, 1949, IV = March 23, 1949) for both 

 components (N, E) at Bergen (B), Copenha- 

 gen (C), Helsinki (H), and Uppsala (U). The 

 microseisms studied are usually regular and 

 continuous in the general period range 3-8 sec. 

 The following results have been obtained for 

 the period Tf , corresponding to frequency maxi- 

 mum, for the mean period T m , and for the 

 period T a , corresponding to amplitude maxi- 

 mum. 



1. Especially T m increases clearly from B to 

 H : B < C < U < H. This is explained as a 

 distance effect. 



2. For all periods E < N; this is especially 

 clear for T a . It is also explained as a distance 

 effect. 



3. T m < T a is valid practically without excep- 

 tion. Furthermore T { ~ T„, corresponding to 

 the generally symmetrical nature of the fre- 

 quency curves. 



4. For all periods a comparison of the situa- 

 tions shows that II < I < III < IV. This is 

 explained as mainly due to different intensity 

 of the microseismic storms. 



5. A comparison of the upper and lower lim- 

 its of the period spectra at the different sta- 

 tions clearly indicates that there is a greater 

 extinction of the shorter waves, whereas there 

 is no indication of an actual period increase. 



6. The hyperbola method for locating the 

 source can be expected to lead to useful re- 

 sults only when the source is exactly the same 

 for all stations compared and preferably a 

 point source. 



10 sec 



2 A 6 8 sec 

 Uppsala N-S 



2 4 S 8 sec 

 Uppsala E-W 



Figure 5. Period spectra on March 23, 

 1949, at 07 h M.E.T. 



