Seismic hazard in Fennoscandia: A regionalized study

Tectcmophysics, 227 (1993) 205-213

Elsevier Science Publishers B.V., Amsterdam

205

Seismic hazard in Fennoscandia: a regionalized study

P. M~n~iemi **a, R. ~ahlstr~m b, C. Lind~olm ’ and A. Kijko ***d

a Institute of Seismology, P.O. Box 19, Et. Hesperiankatu 4, SF-o0014 University of Helsinki, Finland b Seismological Department, University of Uppsala, Box 2101, S-750 02 Uppsala, Sweden

’ NORSAR, P.O. Box 51, N-2007 Keller, Norway d Western Deep Levels Laboratory, Sekmic Research and Systems, P.0, Box 8044, Western Levels 2501, South Africa

(Received February 13,1992; revised version accepted June 9,1993)

ABSTRACT

A seismic hazard evaluation has been conducted for Fenno~andia, including Norway, Sweden, Finland, Denmark, areas of Russia, and adjacent sea areas. Several subregions were considered. Data were taken from a Fenn~andian earthquake catalog comprising both macroseismic and inst~mental observations. The analysis was based on a m~mum likelih~ approach using incomptete historical and complete instrumental data, different detection thresholds, and assessment of earthquake magnitude uncertainty. The output for the selected subregions consists of the probabilities that a given magnitude will not be exceeded in a given interval of time and mean return periods for earthquakes of magnitudes exceeding a given value ML. The evaluated parameters are the maximum regional magnitude, Gutenberg-Richter b-value, and annual activity rate, all with standard errors. The obtained parameters correspond well to the characteristic seismic@ features of inspected regions. However, discrepancies between the data used and the data model behind the applied method were observed: both the patterns of earthquake occurrence and the reporting of historical events tend to deviate from the model. Differences in output can be expected according to the decisions to deal with these discrepancies.

In global terms Fennoscandia belongs to a plate interior devoid of major seismic activity. However, earthquakes with felt effects occur there and areas with concentrated seismic activity can be discerned on a more regional scale. Several inspections of Fennoscandian seismicity on the basis of historical and/or instrumental data have been performed, e.g., by B&h (19531, Miyamura (19621, Husebye et al. (19781, Bungum and Fyen (19791, Bungum et al. (1986a) and Gregersen et al. (1991). Studies of the seismic potentia1 in the region can be classified in different categories. One type is concerned with the problem of earthquake recurrence, discussed in terms of b-values in the context of general seismicity (e.g., B&h,

* Corresponding author. * * On leave from Institute of Geophysics, Polish Academy of Sciences, Ksiecia Janusza 64, M-452 Warsaw, Poland.

1953; Miy~ura, 1962; Bungum et al., 1986a). More specific hazard/ risk evaluations for Fennoscandian countries (Ahjos et al., 1984; Ki- jko et al., 1993) and other subregions (B&h, 1979; Schenk et al., 1984) have also been carried out. One further type of study is for more local regions and often motivated by safety codes of critical structures (e.g., Varpasuo and Puttonen, 1985; Bungum et al., 1986b; Skordas and Wahlstriim, 1988).

The present paper is a probabilistic analysis of seismic hazard in Fennoscandia. It involves the utilization of a11 available earthquake data, as statistical models are mainly based on the space-time distribution of seismicity. Although the shortcomings of macroseismic data are well recognized, it has been noted that the seismic&y patterns derived from Fennoscandian data of different times are consistent in several ways (e.g., Bungum, 1989; Wahlstr(im, 1990). This study is based on a maximum likelihood procedure (Kijko and Sellevolf, 1992) that allows the use of both

~-1951/93/$~.~ 0 1993 - Elsevier Science Publishers B.V. Ail rights reserved

the incomplete historical and the recent instrumental catalogs. The uncertainty of magnitude determination is also taken into consideration. The probabihties that a given magnitude will not be exceeded in a given interva1 of time and the mean return periods for earthquakes of magnitudes exceeding a given value M are computed, and the parameters maximum regional magnitude, Gutenberg-Richter b-value, and annual activity rate, with standard errors, are estimated for eight Fennoscandian subregions.

It should be noted that the term Fennoscandia is here taken in a broad, geographical sense, and comprises Norway, Sweden, Finland, Denmark, Russian Karelia, western Kola Peninsula and all adjacent sea areas.

Earthquake data

The earthquake data were mainly taken from the FENCAT catalog (Ahjos and Uski, 1992). FENCAT provides more than 5000 entries in the

.x-3.9

.,-1.3 Imo-19% *s-s.9 (d

.___~__ _._- .-

SEISMIC HAZARD IN FENNOSCANDIA: A REGIONALIZED STUDY

years 1375-1989 and has been compiled from numerous sources, covering northern Europe (55”N to 80”N and 1O”W to 45”E). The data used in the present study are from FENCAT and in addition use is made of the information given by Muir Wood and Woo (1987) for this time period.

Figure 1 displays the epicentres given in FEN- CAT for different time periods. The available macroseismic data are very sparse during the earliest centuries of the study period, and the reporting of events remains occasional until the 1700s (cf. Figs. la, lb). During the 17OOs, earthquakes are beginning to be reported along the shores of the Gulf of Bothnia and a few also in Lapland in addition to the more densely populated areas in the south. A more abundant earthquake reporting commences in the 175Os, especially in Sweden and Finland in the first half of this decade. From this time on, observations become more regular and comprise a larger propor- tion of weaker earthquakes than earlier. The Iist of events in the interval from 1800 to 1879 (Fig. lc) is dominated by epicentres in the Telemark- Vginern region and around the Gulf of Bothnia, and by concentrated seismic&y near Kuusamo, Finland. An increase of events along the Norwe- gian coast, especially in the southwest, is also

207

noted in the 1800s. The interval from the year 1880 onwards shows an increase of epicentres in areas such as Lapland, the Gulf of Bothnia region, the Norwegian coast and the Lake Ladoga region (Fig. Id). The introduction of systematic macroseismic questionnaires took place in the 1880s.

The spatial coverage of the historical data agrees well with the distribution of the population. Owing to the low density of the population in Fenno~andia, the macroseismic parameters are often based on information that is unevenly distributed and inadequate to allow the construc- tion of detailed isoseismal maps. This results in uncertainties in location and magnitude. Besides a varying degree of incompleteness, the Fenno- scandian macroseismic catalogs most probably contain many events of doubtful seismic origin. One indication of this is that the number of entries is higher in the winter than in the sum- mer, which implies that superficial, frost-related phenomena have been mixed with earthquakes (see Renqvist, 1930; Wahlstrom, 1990).

The installation of highly sensitive seismo- graph stations beginning in the 1950s implied a significant increase in the number of observations (Figs. le, If). Th is applies most clearly to parts of

Fig. 1. FENCAT epicentres in: (a) 1375-1699; (b) 1700-1799, (c) 1800-1879; (d) 1880-1956; (e) 1957-1979; and (f) 1980-1989. Note that the time intervals are of different lengths. Magnitudes are expressed in the local magnitude scale of MJUPP).

I’ MANl’YNII:MI I:I’ 41,

,mognitude uncertainty

/’ /’

only extremes I I complete ports of cotologue -

t- SPAN OF THE CATALOOUE I

Fig 2. A qualitative sketch of an earthquake catalog that can be accepted by the present method (after Kijko and Setlevoll, 1992). The extreme part contains historical observations and the complete part earthquake data that are assumed to be complete above a

specified threshold magnitude Cm,, m2, . . . ). The largest known earthquake with magnitude X,,,, can have occurred outside the span of the catalog. Periods with no observations, seismic gaps TB, are permitted. The magnitude uncertainty is considered with the

help of an interval that is assumed to comprise the true magnitude.

Lapland, the Norwegian west coast, the Kola Peninsula and Russian Kareha. The most recent decade contributed with numerous weak shocks owing to the further improved station coverage and sensitivity. Although the spatial homogeneity remains a problem also for the instrumental data at the microearthquake level, the data are complete above some threshold magnitude.

Method for seismic hazard as~ssment

The maximum likelihood procedure described in Kijko and Sellevoll (1992) was used. This method allows the utilization of all available seismicity information, as it makes use of both incomplete historical data (extreme part of the catalog) and more coherent data (complete part). It is possible to divide the complete part into subcatalogs, each with a specified threshold magnitude. Periods with no entries in the catalog can be taken into account by introducing so-called gaps (see Fig. 2). In the procedure used here, the uncertainty of the magnitude determination is modelled with the help of a magnitude interval, SO that each magnitude is associated with an upper and a lower bound. The magnitude interval determined by these bounds is assumed to comprise the true, unknown magnitude (cf. Kijko and Sellevoll, 1992). The procedure can handle magnitude intervals associated with every ind~idual event or, alternatively, the span of the catalog can

be divided into several parts, each with its estimated magnitude error, according to the quality of the data.

The computations are based on the assump- tions of the Poisson distribution of earthquakes with an annual activity rate A and the doubly truncated Gutenberg-Richter distribution of earthqu~e mag~tude x. The probab~it~es that a given magnitude will not be exceeded in a given interval of time and mean return periods for earthquakes of magnitudes exceeding a given value M, are computed. The parameters sought are the b-value, A, and maximum regionai magni-

tude, mmax, with their standard deviations. The maximum likelihood procedure, used to estimate b and h, does not provide an evaluation of mmax. This parameter is instead evaluated by setting the largest observed magnitude, X,,, equal to the largest expected ma~itude (see Kijko and Dessokey, 1987; Kijko and Sellevoll, 19921. Xm,, can have occurred before the beginning of the actual catalog (cf. Fig. 2).

Data treatment

Magnitude homogenization was necessary, since the values associated with the entries in FENCAT derive from various sources. The macroseismic magnitudes were computed according to ~~~UPP~, the scale derived by Wa~str~m and Ahjos (19841, and the inst~ent~ magni-

SEISMIC HAZARD IN FENNOSCANDIA: A REGIONALIZED STUDY 209

Fig. 3. The eight selected subregions with the number of events given in FENCAT.

tudes were converted into the local magnitude scale of M,(UPP). Since MM (UPP) is calibrated to M,(UPP), all magnitudes are compatible.

The analysis was conducted for the eight subregions shown in Figure 3. In selecting them, the spatial distribution of epicentres and the princi- pal tectonic features were considered. The amount of data does not allow for a more detailed division of source zones. One advantage of larger zones is that the effect of location errors is diminished.

Each data set was inspected to find the magnitude range, the maximum magnitude of the region, and the number of events vs. magnitude. Generally speaking, two populations of subregions can be discerned: the non-Norwegian (l-6) and the Norwegian (7 and 8) regions. In the former ones, the magnitudes range from about 0 to above 4, but the great majority of events falls in the interval 2-3.5. There are only scattered occurrences of the highest magnitudes, which has been suggested to imply nonlinearity of the earthquake recurrence relationship (Gregersen et al., 1991). The Norwegian data comprise fewer mi- croearthquakes, have higher maximum magnitudes and are more abundant in the upper magnitude range than the other data.

An attempt was made to remove the events of doubtful seismic origin (see section Earthquake Data). This was based on the information pro-

vided by FENCAT. Regions 3 and 5 have the biggest numbers of questionable events, whereas the Norwegian regions have only a few. Fore- and aftershocks were also omitted from the data sets. The identification of the dependent events was based on a space-time window search. The windows were estimated with respect to the event size (see, e.g., B%th and Duda, 1964). The time windows ranged from a few days up to a few weeks, which means an aftershock characteriza- tion slightly different from that of Havskov et al. (1989). With the space windows, some attention was paid also to the possible location error in the respective time periods (see, e.g., Ahjos and Arhe, 1983; Bungum et al., 1991; Gregersen et al., 1991).

TABLE 1

Input data from the inspected Fennoscandian subregions for the selected extreme and complete parts of the catalog

Region Extreme part(s) Complete part(s)

span NE ME span NE TH ME

1 1375-1879 69 0.4 1904-1954 24 2.8 0.3 1880-1903 33 0.3 1956-1989 32 2.3 0.2

2 1657-1729 8 0.5 1880-1955 36 3.3 0.3 1752-1879 110 0.4 1956-1979 36 2.6 0.2

1980-1989 40 2.4 0.2

3 1706-1749 6 0.5 1882-1955 27 2.8 0.3 1750-1869 30 0.4 1956-1979 36 2.4 0.2

1980-1989 46 2.1 0.2 4 1659-1794 9 0.4

1829-1944 44 0.3 1951-1989 31 2.4 0.2 5 1610-1739 5 0.5 1882-1956 34 2.7 0.3

1751-1870 33 0.4 1957-1989 38 2.3 0.2 6 1731-1792 15 0.5

1800-1917 51 0.4 1924-1956 38 0.3 1957-1989 158 2.4 0.2

7 1799-1881 25 0.4 1887-1955 42 3.0 0.3 1956-1982 100 2.5 0.3 1983-1989 100 2.6 0.3

8 1612-1884 103 0.4 1885-1955 169 2.8 0.3

1956-1982 111 2.7 0.3 1983-1989 274 2.2 0.2

TH = threshold magnitude, NE = number of events above TH, ME = magnitude error. Given years start on 1 January and end on 31 December. Introduced seismic gaps, i.e. time periods with no observations available, can be seen as breaks between chosen intervals.

‘IO

The characteristics of the input data of each inspected zone are shown in Table 1. The division

into extreme and complete catalog parts is not

identical for each region, as it was adjusted according to the number of events available. How- ever, the milestones of improvement in event detection (188Os, 195Os, 1980s) were used as start- ing points. The standard procedure of cumulative frequency vs. magnitude plots was applied to estimate the threshold of completeness of each complete subcatalog. Earlier literature (e.g., Bhh, 1978; Havskov et al., 1989) also gives estimates of the level of completeness and the magnitude un-

certainty, which were used as guidelines.

Results and dj~ssion

The obtained b, h and mmax parameters for the selected eight regions are shown in Table 2.

The evaluated maximum magnitudes tend to correspond to the observed ones, but their relia- bility is also dependent on the observations. The magnitude error a(X,,> of the largest observed earthquake in each subregion used in the computation equalled 0.5 magnitude units and has a somewhat formal character. The ratio of the error of the evaluated mmax and a(X,,) can be regarded as a transmission coefficient that trans- fers the uncertainties of observation to the mmax estimate. For the subregions 1, 2, 3 and 5, mmax was obtained from historical events. As pointed

TABLE 2

Evaluated earthquake hazard parameters, with standard errors, for the inspected Fennoscandian subregions

Region b

1 0.70* 0.05 2 0.93 f 0.02 3 0.94 f 0.08 4 0.18 f 0.23 5 0.76 f 0.01 6 0.94 f 0.05 7 0.63 & 0.06 8 0.86*0.02

0.83+0.10 3.36*0.13 1.57kO.19 0.46*0.19 0.85 f 0.01 4.03 f 0.35 5.70* 0.81

11.58*0.51

4.92*0.60 5.51+ 1.06 4.34 of: 0.66 4.28 f 0.67 4.66 f 0.64 5.87* 1.65 6.25 f 0.70 5.28 *OS8

b = Gutenberg-Richter parameter; &,a = annual activity rate for magnitude 2.0 [M&JPPfl or greater; m,,, = maximum magnitude; ofA&.J= magnitude error of the largest observed earthquake in each subregion used in the ~putation.

out (e.g., Husebye et al., 19781, the largest location uncertainties might accompany the largest historical earthquakes, some of which could even have been localized to an erroneous subregion. m ,,;values for the sparseiy populated subregions 4 and 6 were obtained from inst~mentally observed magnitudes. Possible larger historical earthquakes in these areas could have escaped documentation or could have been mislocated. For subregion 6, the maximum observed magnitude is for an event located close to the eastern border of the subregion, and large earthquakes seem to have occurred east of this subregion (cf. Figs. 1 and 3). An earthquake in 1976 off the coast of Estonia, located not far south of subregion 5, had a magnitude of 4.8 ~~~(UPP}), i.e., exceeding m,._ for this region (see Table 2).

As mentioned in the preceding section, two categories of subregions are discerned on the basis of the input data. In output, the Norwegian subregions (7 and 8) display the highest h-values. The non-Norwegian subregions can be divided into three regions of lower seismic activity (1, 4, and 5; low b and A) and two intermediate- seismicity regions (2, 3 and 6). Even though subregion 3 has a low A-value, it clearly belongs to this group because of the small area of this region. All the estimated b-values are in the lower or intermediate range of the previously reported results (Miyamura, 1962; B&h, 1979; Bungum et al., 1986a; Kijko et al., 1993).

Estimates of both the activity rate and b-value are dependent on the number of events available. The amount of data did generally not allow a too specific division into regions; Norway is an exception as abundant observations and a detailed correlation between the tectonic features and the seismic&y has recently become available (Bun~m, 1989; Bungum et al., 1991). The Norwegian terri- tory was also divided into nine smaller subregions, but the b-values turned out to be fairly similar to the ones presented for regions 7 and 8. b-estimates based on too sparse data could be biased (e.g., Duda and Rashidi, 1972). The valid- ity of the Gutenberg-Richter equation for source regions comprising individual faults has also been questioned (e.g., Wesnousky et al., 19831. Here, subregion 4 comprised a fairly large source area

SEISMIC HAZARD IN FENNOSCANDIA: A REGIONALIZED STUDY 211

with few reported events, but computation was feasible although quite sensitive to changes in input parameters.

An attempt was made to obtain a data set of independent events to be used under the assump- tion of the Poisson distribution of earthquakes. The success of removing fore- and aftershocks depends on the criteria to identify them. Havskov et al. (1989) discussed some of the different pat-

i Al// I I I ‘I

terns of the temporal extent of aftershock activity that have been noted in Fennoscandia: large Nor- wegian events can start a period of prolonged activity, whereas large Swedish earthquakes often are followed by only a relatively brief sequence of aftershocks. In addition, seismicity concentrations as both fore- and/or aftershocks (e.g., Ekstriim, 1939; Porkka and Vesanen, 1958; Kulhinek et al., 1981) and swarm-type activity (Gregersen, 1979;

7 I I I I I i! 11 I

I I 31’ 121’ 161

Fig. 4. Estimated mean return periods for earthquakes exceeding given magnitudes: (a) for the ‘non-Norwegian, low-seismicity’ regions 1, 4 and 5; (b) for the ‘non-Norwegian, intermediate-seismicity’ regions 2, 3 and 6; (c) for the ‘Norwegian’ regions 7 and 8.

The dashed line denotes the evaluated maximum magnitude.

Bungum et al., 1979, 1982; Ahjos and Arhe, 1983: M~n~niemi and Ahjos, 1990; Kijko et al., 1993) have been observed in Fennoscandia. The available obse~ations comprise a large variety of occurrences and thus the removal of dependent events could imply that some impo~ant information about earthquake occurrence in the region of study for which the model is insu~cient becomes eliminated. Earthquakes of low magnitude are insignificant for hazard assessment, but they af- fect the relative abundance of low- and high-magnitude earthquakes. Had the dependent events not been removed, higher b-values would have been obtained in some cases (e.g., subregion 7). Also, many dependent ones belong to the intermediate magnitude range.

The division of data into extreme and complete parts affects the number of events available for computation. The FENCAT catalog does not seem to adequately fit the format of input data as illustrated in Figure 2, since its historica data are certainly incomplete but not necessarily extremes only, the magnitude ranges for the macroseismic and the instrumental periods being similar, i.e., if the microearthquake reports of the 1980s are disregarded. Using complete subcatalogs means remo~ng the events below a given threshold magnitude, the lower part of the total magnitude range. Therefore, in the case of FENCAT, a longer extreme part-where all events are in-

cluded- means that a bigger porti0.t of the low-ma~itude events is utilized in the computation. If a brief extreme part and complete subcatalogs with high assigned completeness thresholds are used, the number of events available for imputation could become too small to charac- terize the data set. This may have in~uence on the b-value, and thus all estimated hazard parameters may be affected by the way the division into extreme and complete parts is made. One crite- rion behind the hazard parameter estimates of Table 2 was that the data sets that gave the greatest stabifity of parameters and of the itera- tive procedure, as judged by its robustness and the size of the different error estimates, were considered the most appropriate.

The estimated mean return periods at different magnitudes for the selected regions are shown

TABLE 3

Probabilities that the given magnitudes will not be exceeded

in I and SO years in the inspected subregions

M,_@JPP) Regions

1 2 3 4 S 6 7 8

Probability for 1 year:

3.0 1.000 0.710 l.ooO l.@lCI 1.000 0.654 0.279 0.222

3.5 l.ooo 0.913 1.000 1.000 1.000 0.879 0.548 0.585

4.0 1.000 0.994 1.000 l.ooo 1.000 0.970 0.757 0.831

4.5 I.000 1.000 - - 1.000 1.000 0.882 0.944

5.0 - l.OOO- - - 1.000 0.949 0.988

5.5 - 1.000 - - - 1.000 0.982 -

6.O - _. - - - - 0.999 -

Probabili~ for 50 years:

3.0 0.001 0.000 0.000 0.036 o.olX 0.000 0.000 0,000

3.5 U.042 0.002 0.089 0.327 0.076 0.001 0.000 0.500

4.0 0.301 0.127 0.606 0.790 0.433 0.090 OX@0 0.000

4.5 0.714 0.527 - - 0.884 0.462 0.004 0.057

5.0 - 0.850 - - - 0.795 0.062 0.570

5.5 - 0.998 - - - 0.951 0.348 -

6.0 - - - - - - 0.793 -

in Figure 4. Their relative comparison generally agrees with the h-values displayed in Table 2, so that smaller activity rates at lower magnitudes mean longer return periods in the total magnitude range available for different regions. The No~egian regions make an exception as region 8 shows a more enhanced activity at low magnitudes but longer return periods at high magnitudes than region 7. A further way to quantify the interoccurrence times of earthquakes in the selected regions are the probabilities that the given magnitudes will not be exceeded in I and 50 years (Table 3).

The present probabilistic analysis of seismic hazard in Fennos~~a is based on the available space-time distribution of seismici~, and relies on the m~mum effects recorded during the past six centuries. The definable geological and seis- mogenic properties of the region should be incor- porated to obtain a more complete picture of the seismic hazard, especially since Fennoscandia is situated away from plate boundaries. Therefore, the results reported here should be regarded as indicators, not as final values that can be used directiy in site-specific hazard evaluation.

SEISMIC l3AZARD IN FENNOSCANDIA: A REGIONALIZED STUDY 213

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Seismic hazard in Fennoscandia: A regionalized study