Natural Hazards 21: 331 345, 2000. 2000 Kluwer Academic Publishers. Printed in the Netherlands. 331 Earthquake Hazard for the Czech Republic, Poland and Slovakia Contribution to the ILC/IASPEI Global Seismic Hazard Assessment Program V. SCHENK 1,, Z. SCHENKOVÁ 1, P. KOTTNAUER 1, B. GUTERCH 2 and P. LABÁK 3 1 Institute of Rock Structure and Mechanics, Acad. Sci., CZ-182 09 Prague 8, The Czech Republic, e-mail: schenk@irsm.cas.cz; 2 Institute of Geophysics, Polish Acad. Sci., PL-01 452 Warsaw, Poland, e-mail: bguterch@igf.edu.pl; 3 Geophysical Institute, Slovak Acad. Sci., SK-842 28 Bratislava, Slovakia, e-mail: geofpela@savba.sk (Received: 13 March 1999; in final form: 29 December 1999) Abstract. The cross-bordering earthquake hazard map for three Central European countries, the Czech Republic, Poland and Slovakia (CZ-PL-SK) in the sense of the Global Seismic Hazard Assessment Program (GSHAP) was elaborated both in terms of macroseismic intensities and in terms of peak ground accelerations (PGA). A new earthquake parametric catalogue for CZ-PL-SK (Schenková et al., 1999) allows the source regions to be delineated with respect to tectonic structures. Regions for Austria and Germany were taken from the D-A-CH area with some modifications in the border zone with the Czech Republic and Poland. Regions of other surrounding countries were defined with respect to national earthquake catalogues and geologico-geophysical data of Central European countries. For each source region earthquake data were normalised to obtain a reliable annual recurrence graph and the maximum expected earthquakes. Attenuation laws were defined to allow more advanced earthquake hazard maps to be calculated by the standard probabilistic McGuire s (1976) approach. The obtained GSHAP hazard maps for the CZ-PL-SK area were calculated for the return period of 475 years. Besides a comparison with the hazard values for the D-A-CH area (Grünthal et al., 1995, 1996; Grünthal, 1997), the map was also compared with the effective ground acceleration map for Austria (Lenhardt, 1996) and in both cases a very good coincidence was found. Key words: earthquake hazard, macroseismic intensity, peak ground acceleration, Czech Republic Poland Slovakia, GSHAP Program. 1. Introduction The Global Seismic Hazard Assessment Program (GSHAP) was launched in 1992 by the International Lithosphere Program (ILP) with the support of the International Council of Scientific Unions (ICSU) and endorsed as a demonstration program in the framework of the United Nations International Decade for Natural Paper presented at the 7th International Conference on Natural and Man-Made Hazards HAZARDS-98, 17 22 May 1998, Chania, Crete (Greece). Author for correspondence.
332 V. SCHENK ET AL. Disaster Reduction (UN/IDNDR; Giardini and Basham, 1993). The primary goal of GSHAP is to ensure that national agencies should be able to assess seismic hazard in a regionally steered fashion and with the most advanced methods. The ultimate benefits will be national assessments of seismic hazards to be used by national decision-makers and engineers for land use planning and improved building design and construction. Three strategic elements were implemented to the program (Giardini and Basham, 1993): the establishment of Regional centres in all continents, the definition of test areas for seismic hazard assessments in regions and the compilation of a global seismic hazard map. The Czech Republic, Poland and Slovakia belong to the GSHAP-Region 3 (Europe N of 46 NandWof32 E). Earthquake hazard maps expressed in terms of macroseismic intensities for the return periods of 100 and 1000 years were first calculated for the Central European territory by Schenk et al. (1989). The first cross-border earthquake hazard maps expressed in the macroseismic intensities for the Czech Republic (CZ), Poland (PL) and Slovakia (SK) were presented at the 25th ESC General Assembly in Reykjavík (Schenk et al., 1996) and later at the 29th IASPEI General Assembly in Thessaloniki (Schenková et al., 1997). In 1997 the GSHAP Steering Committee decided to express the earthquake hazard in the peak ground accelerations. The main problem of the reliable hazard assessment was due to a finding of peak ground acceleration attenuation for the CZ-PL-SK area, because of missing acceleration records in this area. The hazard calculations and fitting of attained hazard values to the hazard values for the surrounding countries were elaborated. The final versions of the GSHAP hazard maps for CZ-PL-SK area were already presented in 1998 (Schenk et al., 1998; Schenková et al., 1998). The present paper describes these final cross-bordering earthquake hazard maps elaborated in terms of macroseismic intensities and of peak ground accelerations for the CZ-PL-SK area. 2. Input Data 2.1. SEISMICITY DATABASE FOR CZ-PL-SK AREA The new earthquake parametric catalogue for CZ-PL-SK (Schenková et al., 1999; Figure 1), elaborated according to general proposals of the CEC Project A Basic European Earthquake Catalogue and Database for the evaluation of long-term seismicity and seismic hazard (BEECD) (Stucchi, 1994) represents an up-dated revised and comprehensive seismicity database without boundary problems and allows more advanced earthquake hazard maps to be calculated. The working earthquake data file based on the recent versions of national catalogues of the Central European countries (Croatia, Hungary, Moldavia, northern Italy, Romania, Slovenia, Switzerland and Ukraine) is homogeneous for the last three five centuries. The data file was compiled for a broad area of the Central Europe (9-24 E, 38-55 N) and data were mutually compared mainly with respect to the individual epicentral intensity, because of an evaluation of historical earthquakes that are important for
EARTHQUAKE HAZARD FOR THE CZECH REPUBLIC, POLAND AND SLOVAKIA 333 Figure 1. The seismicity data for the Czech Republic, Poland and Slovakia and adjacent areas (Schenková et al., 1999). the hazard assessment. The applied method (Schenk, 1983) allows the representative period of the earthquake occurrence to be found with respect to the various seismic energetic levels of every source region. For computing of hazard maps in terms of strong motion parameters the seismicity data file was thoroughly analysed to introduce reliable magnitude M S. First of all, conversion relations of local magnitude M L to M S were introduced (Schenková et al., 1999). Then, for the seismicity data file two least-square-method approximations between the epicentre intensity I 0 and magnitude M S were calculated, i.e., the relations M S (I 0 )andi 0 (M S ). These two approximations allow the conversion formula between epicentre intensity I 0 and magnitude M S (Figure 2) to be determined as a mean relation of those approximations in the form M S = (0.6725 ± 0.0818)I 0 + (0.3354 ± 0.2704). (1) 2.2. SEISMIC SOURCE REGIONS FOR THE CENTRAL EUROPEAN AREA Eighty-eight source regions were delineated within and around the CZ-PL-SK area in accordance with seismotectonic criteria (Figure 3). Source regions for Austria and Germany were taken over from the D-A-CH scheme (Grünthal, 1997) with
334 V. SCHENK ET AL. Figure 2. Relation between magnitude M S and macroseismic intensity I 0. some slight modifications in the border zones with the Czech Republic and Poland. The other Central European source regions were defined using recent national earthquake catalogues and available geo-data. 2.3. RECURRENCE GRAPHS AND THE MAXIMUM EARTHQUAKES For every source region the annual density and cumulative recurrence graphs, their coefficients and standard deviations, were determined by applying the time normalisation method (Schenk, 1983). An example of such graphs for the Sudeten and Silesia region is drawn in Figure 4. If limited number of earthquake data for a region, the region had been extended for near surrounding area to find a reliable b-value that later allowed a-value with respect to the original limited data to be defined. For the most of recurrence graphs a following fact was detected: if their a-values are enlarged for one and half or two standard deviations, then they practically include all earthquake occurrences that can be expected in the regions. The maximum earthquake estimates based on the extreme value statistics, the 3rd Gumbel distribution, and the seismotectonic and expert assessments were applied for every source region. An earthquake sub-catalogue compiled for the
EARTHQUAKE HAZARD FOR THE CZECH REPUBLIC, POLAND AND SLOVAKIA 335 Figure 3. Seismic source regions for the Central European area. Figure 4. Annual recurrence graphs for the Sudeten and Silesia region (thin line density graph, bold line cumulative graph, dashed lines standard deviations σ ).
336 V. SCHENK ET AL. given source region passed the Gumbel distribution calculations which involved all combinations of time intervals and observation periods (Figure 5; Schenk and Kottnauer, 1991). In principle, an observation period chosen for the Gumbel analysis is divided with respect to duration of time intervals into different sets of input data. For example, for the period of one hundred years (e.g., 1890 1989) and for 1-year interval, the input data set will contain one hundred of inputs for the Gumbel analysis. But, if an interval is equal to 2-years, two sets of data can be created: one with fifty inputs for 1890 1989 and another with forty-nine inputs for 1891 1988. Consequently, when 3-years interval is applied, then three input sets are obtained: the first one with thirty-three inputs for 1890 1988, the second one again with thirty-three inputs for 1891 1989 and the third one with thirty-two inputs for 1892 1987. The upper threshold of time interval duration is given by the Gumbel distribution definition. To attain a statistically reasonable set of the most probable earthquakes for the given region obtained by applications of different time intervals, it was adopted to introduce to this set always only the mean value of resulting values got for the same time intervals. If different observation periods were applied, other sets of the most probable earthquakes were obtained. Such an approach allows the most probable earthquake for every return period T [years] to be assessed from the individual Gumbel approximations calculated for this return period. It was found that these distributions fit well the Gauss distribution. It means that for every return period T the most probable earthquake can be determined as the mean value of that Gauss distribution. Then, its standard deviations characterise an uncertainty of the most probable earthquake determination. If T is equal for example to ten thousands years (the value is frequently applied as a return period of the maximum earthquake of nuclear devices) then the most probable maximum earthquake can be assessed. 2.4. ATTENUATION RELATIONS As it is commonly known, attenuation relations greatly influence the earthquake hazard values (e.g., Schenk et al., 1997). Therefore, for the earthquake hazard assessments presented in this paper the existing macroseismic intensity and the PGA attenuation relations had to be reassessed. The known intensity-attenuation laws were revised and, because of the lack of strong-motion records for the CZ- PL-SK area, the PGA attenuation relations published for areas of similar geological structures were analysed and tested for the whole area under study. 2.4.1. Macroseismic Intensity The problem of the macroseismic intensity attenuation relation was thoroughly analysed for the Czech Republic in Schenk et al. (1997) together with related hazard output values. The intensity attenuation law based on data of more than
EARTHQUAKE HAZARD FOR THE CZECH REPUBLIC, POLAND AND SLOVAKIA 337 Figure 5. Set of the Gumbel distributions found for the Sudeten and Silesia region for all combinations of time intervals and observation periods. 250 macroseismic fields known for the Central European area was given in the form and for the epicentre distance R<8.3 km: I = 0 (2a) for the R 8.3 km: I = α β ln(r + r), (2b) where parameter r = exp(α/β) R E. The regression coefficients α = 4.044 ± 1.326 and β = 1.914 ± 0.035 were determined by the least square method, and R E = 8.275 km is the radius of the mean pleistoseismal area for seismogenic zones of the Central Europe, especially those situated in units of the West European platform. It was decided that the intensity attenuation relations (2) could be applied without changes to the GSHAP earthquake hazard calculations. 2.4.2. Peak Ground Acceleration (PGA) Because of the lack of a sufficient number of strong motion records for the CZ-PL-SK area within the range of magnitudes and distances required for the earthquake hazard assessment, the authors expected to apply the same PGA attenuation that had been used for the GSHAP-Region 3. In the GSHAP-Region 3 Report (Grünthal, 1997), where the last version of the earthquake hazard PGA map is described, there is a reference that the map is based on a 1/3 weight of the used attenuation relations after Ambraseys et al. (1996), Sabetta and Pugliese
338 V. SCHENK ET AL. (1996) and Spudich et al. (1997).... Unfortunately, such a kind of the attenuation relation can be hardly applied for the two following fundamental reasons: a. Three relations mentioned above were originally determined from strong ground motions recorded in areas of orogenic geological structures (e.g., Turkey, Iran, Greece, Italy) while the CZ-PL-SK area is built mainly by old crystalline rocks. b. The reliability of any relation based on a 1/3 weight of three other relations is doubtful because each of three creative relations had to be statistically determined from a different number of observations, i.e., each of them has another statistical credibility with respect to the other two relations. It is evident that under these conditions statistical deviations of the new sophistically created relation cannot be adequately determined at all. The territory of the Czech Republic, Poland and Slovakia belongs geologically to three main European structural units the East European Platform, the Hercynides and the West Carpathians. The first two units consist of hard crystalline rocks. The outer part of the West Carpathians is built by nappes over-thrusting the crystalline rocks of the Bohemian Massif. According to geophysical evidence these hard rocks continue below the West Carpathians for more than 100 kilometres in the south-east direction. This fact is supported by gravity field interpretations that do not indicate the existence of deep orogenic roots below the Carpathians that are known from the Alpino-Himalayan belt. The inner part of the West Carpathians is assembled of individual granite blocks coated with Mesozoic hard rocks. Therefore, in the hazard calculations the applicability of other peak acceleration attenuation laws was tested and the output hazard values were compared with the maximum macroseismic effects observed in the CZ-PL-SK area. The relation (Schenk, 1988) log PGA [cm. sec 2 ]=0.346I [ MSK] 0.332 (3) allows the hazard values and real observations to be mutually compared and for the macroseismic effects, observed at sites located on the West European units, the regression coefficients of the attenuation relation ln PGA [cm. sec 2 ]=a + bm c ln(r [km]+d) (4) approved for the application to the program EQRISK (McGuire, 1976) were found: a = 5.42 ± 0.32, b = 0.65 ± 0.02, c = 1.54 ± 0.16 and d = 17. Then, the GSHAP earthquake hazard calculations in terms of the PGA were performed for the safety-margin probability 90%, i.e., for the 1 1 multiple of the 2 standard deviations σ of all regression coefficients (Schenk et al., 1997) and for the radius of the epicentre zone R E = 8.275 km. The use of relation (4) supported the fact that Lenhardt (1996) applied the original McGuire s relation to the calculation of the earthquake hazard map for Austria in terms of effective ground accelerations with 10% probability of an exceedance in 50 years. It allowed our
EARTHQUAKE HAZARD FOR THE CZECH REPUBLIC, POLAND AND SLOVAKIA 339 Figure 6. Comparison of the PGA relation (4) with three relations from which the 1/3 weight attenuation relation was created for the GSHAP-Region 3. output hazard values for a border zone close to Austria to be compared with these effective accelerations. Figure 6 shows the PGA estimated by relation (4) as a function of distance R for the magnitude M S = 5.5 and their comparison with acceleration values of three relations, which created the 1/3 weight attenuation relation used for the GSHAP- Regional Centre 3 (Grünthal, 1997). Every relation is coupled with its plus/minus one standard deviation belt to assess uncertainties in its statistical determination. Figure 7 illustrates relationship between the relation (4), applied for the GSHAP hazard calculations of the CZ-PL-SK area, and the relations given by Ambraseys et al. (1996), Sabetta and Pugliese (1996) and Spudich et al. (1997). All coefficients of the relation (4) were shifted for 1 1 standard deviation σ towards the safetymargin side (Schenk et al., 1997) that represents 90% probability of earthquake 2 occurrences. A lower PGA decrease in the relation (4) is caused by the fact that it has been determined mostly from data observed on crystalline igneous and/or metamorphic rocks of the West European Platform. Generally, one can see that in the pleistoseismal area the applied relation (4) fits well to the relation given by Ambraseys et al. (1996). For the distances greater than 15 km, the relation (4) approaches slowly to the 1 σ standard deviation belt of the relation published by Ambraseys et al. (1996) and approximately to 1 1 up to 2 σ standard deviation 2 belts of the other two relations.
340 V. SCHENK ET AL. Figure 7. Variations of the PGA relation (4) of the safety-margin probability 90% to other three relations used to the 1/3 weight attenuation relation created for the GSHAP-Region 3; examples for the magnitudes 4.5, 5.5 and 6.5. 3. Hazard Maps in Terms of Macroseismic Intensities The earthquake hazard calculations for the CZ-PL-SK area were made by the program EQRISK (McGuire, 1976). This procedure had been applied formerly for the Central European area (Schenk et al., 1989) and recently suggested for the GSHAP hazard calculations too. When the newly calculated hazard map was compared with the results previously obtained (Schenk et al., 1989), several local changes of the hazard values were identified. They were caused by an introduction of newer input data joined to the source region delineation, to a definition of their seismogenic regimes (because of a revised seismic database) and to an application of a more reliable attenuation law. The earthquake hazard map for the CZ-PL-SK area in terms of macroseismic intensities with the 90% probability of nonexceedance within 50 years (i.e., with the return period of 475 years; Schenková et al., 1997; Schenk et al., 1998) involved the possibility to compare the results with published GSHAP-Region 3 hazard maps for the D-A-CH test region (Grünthal et al., 1996). A relatively good coincidence of calculated values for the border areas Poland Germany, the Czech Republic Austria and Slovakia Austria was found. Only few relatively small differences appeared in two border areas of the Czech Republic with Germany:
EARTHQUAKE HAZARD FOR THE CZECH REPUBLIC, POLAND AND SLOVAKIA 341 Figure 7. Continued.
342 V. SCHENK ET AL. Figure 8. Joint earthquake hazard map for the Czech Republic, Poland and Slovakia in terms of macroseismic intensities with the 90% probability of nonexceedance of the intensity within 50 years, i.e., the return period of 475 years. In Vogtland the hazard values for the Czech Republic are about 0.5 MSK-64 higher than hazard values obtained for Germany, On the contrary, in Zittau border area these values for the Czech Republic and Poland are about 0.5 MSK-64 lower than those for Germany. The latest version of the hazard map in terms of macroseismic intensity was calculated under the same assumptions as for the PGA hazard maps (Schenk et al., 1998; Schenková et al., 1998). Great attention was given to the attenuation law of the macroseismic intensities (Paragraph 2.4.1 and Schenk et al., 1997). The earthquake hazard map was computed for the attenuation function (4), in which the regression coefficients altered for 1 1 σ of their standard deviations. A nonexceedance probability level of 90% of macroseismic intensity occurrence in the 2 sense of safety margins was used for a period of 50 years, i.e., with the return period of 475 years (Schenk et al., 1997). The final hazard map for the Czech Republic, Poland and Slovakia is in Figure 8.
EARTHQUAKE HAZARD FOR THE CZECH REPUBLIC, POLAND AND SLOVAKIA 343 4. Hazard Maps in Terms of Peak Ground Accelerations The peak ground acceleration hazard map for the GSHAP-Region 3 (Grünthal, 1997) presented at the 29th IASPEI General Assembly, Thessaloniki 1997, gave for the CZ-PL-SK area low and rather smoothed hazard values. They were unacceptable not only from the viewpoint of observed data but also from existing national building codes of all three countries. This smoothing is probably caused by the application of the attenuation relation for the GSHAP-Region 3 based on a 1/3 weight of three different attenuation relations (Paragraph 2.4.2). It was decided that the earthquake hazard map in terms of the PGA for the CZ-PL-SK area has to be calculated separately with respect to local geological conditions. The hazard maps computed by Grünthal (1997), Schenk et al. (1998) and Schenková et al. (1998) were compared to remove the discrepancies appeared at the border of the Czech Republic and Germany. When regional seismotectonic characteristics and statistically assessed seismogenetic potentials (e.g., Přimda 1902 earthquake) were emphasised, then the final version of the joint PGA hazard map for the CZ-PL-SK area with the 90% probability of non-exceedance within 50 years could be compiled (Figure 9). The PGA earthquake hazard map (Figure 9) was also compared with the effective acceleration map compiled for Austria (Lenhardt, 1996). The PGA values obtained for the CZ-PL-SK area are about 40 50% higher than the effective acceleration values for Austria. It fits Lenhardt s assumption that the effective value represents 70% of the maximum ground acceleration. When the values were scaled, their differences at the border area attain in their absolute values round 20 cm.s 2 only, i.e., a good agreement from the viewpoint of the earthquake engineering practice. 5. Conclusion The final versions of the cross-bordering earthquake hazard maps for the Czech Republic, Poland and Slovakia (CZ-PL-SK) in the sense of the Global Seismic Hazard Assessment Program (GSHAP) were elaborated both in terms of macroseismic intensities and in terms of peak ground accelerations (PGA). The GSHAP earthquake hazard maps were computed for the return period of 475 years, i.e., for the 90% probability of nonexceedance of the PGA and the macroseismic intensities within 50 years. A good coincidence of the obtained maps with the last hazard maps for the D-A-CH area (Grünthal, 1997) and with the effective ground acceleration map for Austria (Lenhardt, 1996) allows these maps to be implemented into the GSHAP European version and also to its global version without any changes.
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