Microseismic Period Spectra and Related Problems Scandinavian Area 



57 



usually all larger than the microseismic peri- 

 ods and there seems to be no influence from the 

 different constants. This is obvious for the 

 following reasons. 



a. The periods corresponding to frequency 

 maxima and amplitude maxima are not con- 

 sistently larger at Helsinki, nor are they con- 

 sistently lower at Copenhagen. 



b. The upper limit of the period spectra is 

 not larger at Helsinki than at the other stations. 



c. The lower limit of the period spectra in- 

 creases from Bergen to Uppsala in spite of 

 equal free periods ; it also increases from Ber- 

 gen to Copenhagen in spite of generally lower 

 free periods at Copenhagen than at Bergen. 



d. Also the mean periods increase from 

 Bergen to Copenhagen and from Bergen to 

 Uppsala. 



Therefore the conclusion seems to be justi- 

 fied that the different seismographs have the 

 same response to the microsisms under con- 

 sideration, and the spectra at the different sta- 

 tions are directly comparable. 



"1 2 3 4 ~5 6 7 8 9 T se( !C 



Figure 1. Dynamic magnification curves. 

 Explanation: 



Curve T V E Valid for 



a 9.3 180 2.4 Bergen N 



b 9.2 129 2.7 Bergen E 



c 7.7 220 1.4 Bergen E 



d 8.2 200 4.3 Copenhagen N,E 



e 12 140 3.5 Helsinki N,E 



f 9.2 189 3.9 Uppsala N,E 



All curves have been displaced so as to 

 pass through the point T - 5 sec, V = 200. 



2. The ground (from the source to the 

 station) also acts as a filter. Bergen (gneiss), 

 Uppsala (granite), and Helsinki (gneiss) are 

 all within Fennoscandia, and no general dif- 

 ferences exist. Copenhagen (chalk) is outside 

 Fennoscandia, but owing to the relatively small 

 part of the path, consisting of sediments, as 

 well as owing to the uncertainties of any cor- 

 rection for their influence, the Copenhagen pe- 

 riod spectra have also been used without modi- 

 fication. 



3. An essential requirement is that the 

 microseisms at the different stations compared 

 have the same source. This has earlier been 

 shown to be the case for Bergen and Uppsala 

 by the author (1951 a) as well as for Copen- 

 hagen (1952). In the light of our present ex- 

 perience the statement is justified that the 

 microseisms at Helsinki also have the same 

 source as at the other three stations (for fur- 

 ther discussion see below). 



4. Only measurable periods have natural- 

 ly been included. The smaller number of ob- 

 servations in a few cases depends on weak mi- 

 croseisms, i.e. fewer measurable periods. This 

 procedure necessarily entails a certain selec- 

 tion, depending on the sensitivity of the seismo- 

 graph. With regard to what has been said in 

 1. above as well as to the results (Table 2) this 

 circumstance does not seem to be of any impor- 

 tance. 



5. The drum speeds are 12mm/min at 

 Copenhagen, 15 mm/min at Bergen and Upp- 

 sala, and 20 mm/min at Helsinki. The meas- 

 urements were made to 0.1 mm and then con- 

 verted into seconds and tenths of seconds. 

 There was a clear tendency in all cases of ob- 

 taining frequency maxima at 1.0, 1.5, 2.0 mm 

 (the measurements were made with a glass 

 scale, divided into half millimeters). As they 

 were certainly not real, smoothing has been 

 made according to the formula V& (a + 2 b + 

 c). The frequency curves thus obtained are 

 certainly nearer to the truth than curves based 

 directly on the original observations. 



6. In a procedure like this where succes- 

 sive periods are measured, false periods may 

 arise at points where one wave train gives 

 place to another wave train. A too long false 

 period may arise if a small quiet interval sep- 

 arates the two wave trains. Likewise too short 

 false periods may occur where one wave train 

 is replaced by another wave train without sep- 

 aration. However, this source of error is of 

 no influence if due account is taken of it in the 

 measurements; moreover, the microseisms 

 measured are generally regular and continuous. 



III. Discussion of the Results — For conveni- 

 ence in writing we introduce the following no- 

 tations : 



