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The seismicity of Fennoscandia
7’ectnnophysics, 204 (1992) 193- 195 Elsevier Science Publishers B.V.. Amsterdam
The seismicity of Fennoscandia Rune Olsson and Efthimios Skordas Seismological Department, Uppsafa University, Box 12019, S-75012 @pala,
Sweden
(Received November 4, 1991; revised version accepted January 10, 1992)
ABSTRACT Olsson, R. and Skordas, E., 1992. The seismicity of Fennoscandia.
Tecronophysics, 204: 193-195.
The randomness of the seismic activity in different parts of Fennoscandia has been tested. The results have been compared to an earlier corresponding investigation for the Mid-Atlantic Ridge and we have noticed a behaviour of the time-frequency relation of earthquake occurrence similar to that of the Ridge.
Fencat catalogue (see Skordas et al., 1991). In order to examine the Fennoscandian earthquakes with respect to randomness we have made chisquare tests of the time lapses between consecutive shocks. The test variable has been computed from the differences between the observed and the expected frequencies of intervals. The chisquare tests strongly reject an exponential distribution of intervals and, consequently, a Poisson distribution of the shocks themselves. This result holds for different time periods, for different subdivisions of the Fennoscandian area and for different magnitude thresholds. In case some earthquakes should be foreshocks or aftershocks, we have also, as an alternative, excluded earthquakes which took place within one day and within an area of 0.25 degrees square from the central shock. In this case the test variable becomes smaller but not small enough to make the randomness hypothesis acceptable (Table 11. This
introduction
Several authors have found through statistical investigations of the seismicity in different regions of the world that earthquakes take place randomly (see e.g., Schlanger, 1960). However, Francis and Porter (1971) established that for Mid-Atlantic Ridge earthquakes the number of small intervals between shocks is too great to establish a Poisson distribution of earthquake occurrence. They interpret this result by suggesting that a fraction of shocks is clustered. In the present study we examine the time distribution of Fennoscandian shocks. Methods and observational material Our data set comprises the earthquakes of M 3 2.5 for whole Fennoscandia for the years 1900-1987. We have made use of the Finnish
NOTATION
f(t)
no n(t) n,(t) I I
The The The The The The
frequency of shocks total number of shocks in a region during the time period accumulated expected frequency of shocks modified accumulated expected frequency of shocks time in unit interval length (one unit interval length = l/75 years) parameter of the exponential distribution fr is the inverted value of the mean interval length between consecutive shocks)
~4~-1951/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
R. OLSSON
194
and E. SKORDAS
TABLE 1 Earthquake characteristics Area
Period
Num.of shocks
Magn M
Degof freedom
Parameter r
Test.var. chi’
Limit of chi’ l%, 5% level
f
Fennoscandia Fennoscandia Fennoscandia SW Norway and SW Sweden lat. d - 11/25*long.+ 65” North. Fennoscandia lat. > - 1l/25 * long. + 65” Norway and NW Sweden lat. > 17/25 * long. + 49.9” South and Central Sweden and Finland lat. d 17/25 * long. + 49.9 South. Fennoscandia lat G 63”N North.Fennoscandia lat. > 63”N West.Fennoscandia long d 1O”E EastFennoscandia long. > 10”E The V&tern Region 11”E Q long. < 14”E 58”N d lat. < 60.5”N
1900-1987 1900-1987 1950-1984
573 257 599
> 3.0 > 3.5 > 2.5
21 18 16
0.1590 0.1406 0.2409
99.9, 75.3 30.6, 29.0 77.3, 60.0
38.9 34.4, 28.9 32.0
0.77 0.92 0.82
1900-1987
254
> 3.0
17
0.1309
52.5, 44.8
33.4
0.77
1900-1987
319
> 3.0
17
0.1809
57.4, 40.9
33.4
0.77
1900-1987
294
> 3.0
17
0.1493
53.6, 37.9
33.4
0.79
1900-1987
279
> 3.0
17
0.1766
48.8,40.0
33.4
0.76
1900-1987
338
> 3.0
17
0.1360
52.0, 36.5
33.4
0.75
1900-1987
235
> 3.0
13
0.2136
27.7, 24.9
27.7, 24.7
0.83
1900-1987
256
> 3.0
14
0.1617
47.8, 42.2
29.1
0.76
1900-1987
317
> 3.0
17
0.1494
67.0, 57.4
33.4
0.77
1900-1987
210
> 2.5
11
0.1518
71.6, 60.6
24.7
0.77
X freq.
40
-
0
Fig. 1. Frequency diagram (SW Norway and SW Sweden, 1900-1987). o is observed frequencies; f(t) = n, exp(-rf) expected frequencies ft is measured in interval units, one unit = l/75 years); X = reduced observed frequencies. Shocks, which occur within 1 day and within an area of 0.25 degrees square from the largest shock in the group, are excluded as considered as foreshocks or aftershocks.
THE
SEISMICITY
195
OF FENNOSCANDIA
I
I
10
30
20
(0.5 y )
40
t
Fig. 2. Accumulated frequency diagram (SW Norway and SW Sweden, 1900-19872 o observed accumulated frequencies: n = n,r[l - exp( -rtJl is expected accumulated frequencies; n, = n,[l -fexpt --@I] is modified expected accumulated frequencies.
outcome contradicts to some extent the conclusion by B&h (19781, who has noticed a random behaviour for Swedish earthquakes. His study comprises a smaller area and a relatively short time interval, 1967-1976. The diagrams (Figs. 1 and 2) concerning SW Norway and SW Sweden for the time period 1900-1967 may be chosen as representative for the earthquake occurrence in our study. In Figure 1 we have plotted the observed frequencies together with the expected exponential frequencies. The accumulated observed and expected frequencies are shown in Figure 2. As can be observed in the diagrams, the greatest deviation from the expected curve takes place for small intervals. Following Francis and Porter (1971) we have also plotted the modified accumulated exponential frequencies in the same diagram. The ratio f is given by a least-squares calculation. This ratio has been interpreted by Francis and Porter (1971) as the fraction of shocks which occur randomly. The seismic&y map of Fennoscandia shows a clustered activity in regions such as SW Norway, NW Norway, the V&rem region in Sweden and the Bothnian coast. There is a tendency that the shocks are ciustered in space as is the case in the Mid-Atlantic Ridge earthquakes. For instance,
NW Norway is relatively calm during periods of more frequent outburst in SW Norway. Conclusion In an earlier study of the seismicities of the Mid-Atlantic Ridge and of Fennoscandia (Skordas et al., 1991) we found great similarities between the time functions of energy release. Since the frequency diagrams of different parts of Fennoscandia resembles the corresponding frequency diagrams of the Ridge as shown in this study, we support the assumption that the seismicities of Fennoscandia and of the Mid”Atlantic Ridge are interrelated. References Bbth, M., 1978. Suspected periodicities of earthquakes in Sweden. Tectonophysics, 51: T55-T62. Francis, T.J.G. and Porter, LT., 1971. A statistical study of Mid-Atlantic Ridge earthquakes. Geophys. J.R. Astron. sot., 24: 31-50. Schlanger, A., 1960. Some consequences of earthquake statistics for the years 191881955. Gerlands Beitr. Geophys., 69: 68-72. Skordas, E., Meyer, K., Olsson, R. and Kulhanek, 0, 1991. Causality between interplate (North Atlantic) and intraplate (Fennoscandia) seismicities. Tectonophysics, 185: 295-307.
The seismicity of Fennoscandia Rune Olsson and Efthimios Skordas Seismological Department, Uppsafa University, Box 12019, S-75012 @pala,
Sweden
(Received November 4, 1991; revised version accepted January 10, 1992)
ABSTRACT Olsson, R. and Skordas, E., 1992. The seismicity of Fennoscandia.
Tecronophysics, 204: 193-195.
The randomness of the seismic activity in different parts of Fennoscandia has been tested. The results have been compared to an earlier corresponding investigation for the Mid-Atlantic Ridge and we have noticed a behaviour of the time-frequency relation of earthquake occurrence similar to that of the Ridge.
Fencat catalogue (see Skordas et al., 1991). In order to examine the Fennoscandian earthquakes with respect to randomness we have made chisquare tests of the time lapses between consecutive shocks. The test variable has been computed from the differences between the observed and the expected frequencies of intervals. The chisquare tests strongly reject an exponential distribution of intervals and, consequently, a Poisson distribution of the shocks themselves. This result holds for different time periods, for different subdivisions of the Fennoscandian area and for different magnitude thresholds. In case some earthquakes should be foreshocks or aftershocks, we have also, as an alternative, excluded earthquakes which took place within one day and within an area of 0.25 degrees square from the central shock. In this case the test variable becomes smaller but not small enough to make the randomness hypothesis acceptable (Table 11. This
introduction
Several authors have found through statistical investigations of the seismicity in different regions of the world that earthquakes take place randomly (see e.g., Schlanger, 1960). However, Francis and Porter (1971) established that for Mid-Atlantic Ridge earthquakes the number of small intervals between shocks is too great to establish a Poisson distribution of earthquake occurrence. They interpret this result by suggesting that a fraction of shocks is clustered. In the present study we examine the time distribution of Fennoscandian shocks. Methods and observational material Our data set comprises the earthquakes of M 3 2.5 for whole Fennoscandia for the years 1900-1987. We have made use of the Finnish
NOTATION
f(t)
no n(t) n,(t) I I
The The The The The The
frequency of shocks total number of shocks in a region during the time period accumulated expected frequency of shocks modified accumulated expected frequency of shocks time in unit interval length (one unit interval length = l/75 years) parameter of the exponential distribution fr is the inverted value of the mean interval length between consecutive shocks)
~4~-1951/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
R. OLSSON
194
and E. SKORDAS
TABLE 1 Earthquake characteristics Area
Period
Num.of shocks
Magn M
Degof freedom
Parameter r
Test.var. chi’
Limit of chi’ l%, 5% level
f
Fennoscandia Fennoscandia Fennoscandia SW Norway and SW Sweden lat. d - 11/25*long.+ 65” North. Fennoscandia lat. > - 1l/25 * long. + 65” Norway and NW Sweden lat. > 17/25 * long. + 49.9” South and Central Sweden and Finland lat. d 17/25 * long. + 49.9 South. Fennoscandia lat G 63”N North.Fennoscandia lat. > 63”N West.Fennoscandia long d 1O”E EastFennoscandia long. > 10”E The V&tern Region 11”E Q long. < 14”E 58”N d lat. < 60.5”N
1900-1987 1900-1987 1950-1984
573 257 599
> 3.0 > 3.5 > 2.5
21 18 16
0.1590 0.1406 0.2409
99.9, 75.3 30.6, 29.0 77.3, 60.0
38.9 34.4, 28.9 32.0
0.77 0.92 0.82
1900-1987
254
> 3.0
17
0.1309
52.5, 44.8
33.4
0.77
1900-1987
319
> 3.0
17
0.1809
57.4, 40.9
33.4
0.77
1900-1987
294
> 3.0
17
0.1493
53.6, 37.9
33.4
0.79
1900-1987
279
> 3.0
17
0.1766
48.8,40.0
33.4
0.76
1900-1987
338
> 3.0
17
0.1360
52.0, 36.5
33.4
0.75
1900-1987
235
> 3.0
13
0.2136
27.7, 24.9
27.7, 24.7
0.83
1900-1987
256
> 3.0
14
0.1617
47.8, 42.2
29.1
0.76
1900-1987
317
> 3.0
17
0.1494
67.0, 57.4
33.4
0.77
1900-1987
210
> 2.5
11
0.1518
71.6, 60.6
24.7
0.77
X freq.
40
-
0
Fig. 1. Frequency diagram (SW Norway and SW Sweden, 1900-1987). o is observed frequencies; f(t) = n, exp(-rf) expected frequencies ft is measured in interval units, one unit = l/75 years); X = reduced observed frequencies. Shocks, which occur within 1 day and within an area of 0.25 degrees square from the largest shock in the group, are excluded as considered as foreshocks or aftershocks.
THE
SEISMICITY
195
OF FENNOSCANDIA
I
I
10
30
20
(0.5 y )
40
t
Fig. 2. Accumulated frequency diagram (SW Norway and SW Sweden, 1900-19872 o observed accumulated frequencies: n = n,r[l - exp( -rtJl is expected accumulated frequencies; n, = n,[l -fexpt --@I] is modified expected accumulated frequencies.
outcome contradicts to some extent the conclusion by B&h (19781, who has noticed a random behaviour for Swedish earthquakes. His study comprises a smaller area and a relatively short time interval, 1967-1976. The diagrams (Figs. 1 and 2) concerning SW Norway and SW Sweden for the time period 1900-1967 may be chosen as representative for the earthquake occurrence in our study. In Figure 1 we have plotted the observed frequencies together with the expected exponential frequencies. The accumulated observed and expected frequencies are shown in Figure 2. As can be observed in the diagrams, the greatest deviation from the expected curve takes place for small intervals. Following Francis and Porter (1971) we have also plotted the modified accumulated exponential frequencies in the same diagram. The ratio f is given by a least-squares calculation. This ratio has been interpreted by Francis and Porter (1971) as the fraction of shocks which occur randomly. The seismic&y map of Fennoscandia shows a clustered activity in regions such as SW Norway, NW Norway, the V&rem region in Sweden and the Bothnian coast. There is a tendency that the shocks are ciustered in space as is the case in the Mid-Atlantic Ridge earthquakes. For instance,
NW Norway is relatively calm during periods of more frequent outburst in SW Norway. Conclusion In an earlier study of the seismicities of the Mid-Atlantic Ridge and of Fennoscandia (Skordas et al., 1991) we found great similarities between the time functions of energy release. Since the frequency diagrams of different parts of Fennoscandia resembles the corresponding frequency diagrams of the Ridge as shown in this study, we support the assumption that the seismicities of Fennoscandia and of the Mid”Atlantic Ridge are interrelated. References Bbth, M., 1978. Suspected periodicities of earthquakes in Sweden. Tectonophysics, 51: T55-T62. Francis, T.J.G. and Porter, LT., 1971. A statistical study of Mid-Atlantic Ridge earthquakes. Geophys. J.R. Astron. sot., 24: 31-50. Schlanger, A., 1960. Some consequences of earthquake statistics for the years 191881955. Gerlands Beitr. Geophys., 69: 68-72. Skordas, E., Meyer, K., Olsson, R. and Kulhanek, 0, 1991. Causality between interplate (North Atlantic) and intraplate (Fennoscandia) seismicities. Tectonophysics, 185: 295-307.