Екологично инженерство и опазване на околната среда, No 2, 2014, с. 13-21

SPATIAL ANALYSIS OF EARTHQUAKES IN AND NEIGHBORING AREAS Alexander N. Sadovski

Abstract: Statistical methods including Quadrat analysis, Cluster analysis, Descriptive statistics and Geographic Information Systems are used to study spatial distribution and to summarize data about earthquakes in Bulgaria and neighboring areas. Detection of areas with similar pattern of earthquakes occurrence is of interest for the Government and for general public. Seismic risk for NPP "" is discussed. Key words: Earthquakes, Spatial analysis, Statistical methods, Geographic Information Systems, NPP "Belene".

1. INTRODUCTION • On June 14, 1913 - Gorna Oriahovitsa - magnitude 7 on the Richter scale; The purpose of this article is study of the spatial On October 18, 1917 - - magnitude 5.3 distribution of earthquakes in Bulgaria and • on the Richter scale; neighboring areas and to present statistical On April 14, 1928 - - magnitude 6.9 characteristics related to their magnitude. It will help • on the Richter scale; for better understanding of the risk of such natural On 18 and 25 April, 1928 - Popovitsa and disaster (seismic hazard). Some perspectives on • / area/, earthquakes with spatial differentiation could be considered: (a) magnitude 7.0 and 5.6 on the Richter scale; interest in areal distribution; (b) interest in spatial On March 17, 1942 - - magnitude structure; and (c) concern with the explanation of • 7.0 on the Richter scale; areal variation. Since spatial data are neither the On June 30, 1956 - - magnitude 5.5 outcome of controlled experiments, nor do they • on the Richter scale; result from random samples, it is clear that beyond On March 4, 1977 - Vrancea /Romania/ - mapping and informal inference from patterns, • magnitude 7.2 on the Richter scale; specific spatial statistical methods are required. On November 3, 1977 - - Major earthquakes in our latitudes were in I, IX, • magnitude 5.3 on the Richter scale; XVI, XVII and XVIII century, information about On February 28, 1986 - Strajitsa - which is available only in old historical writings. • magnitude 5.1 on the Richter scale; According to some sources, the most destructive On December 7, 1986 - Popovo - magnitude earthquake in the country was with a magnitude • 5.7 degrees on the Richter scale; about sixth to ninth degree on the Richter scale and On October 27, 2004 - about 430 km. its epicenter was near Sofia, the earthquake • northeast of Sofia, outside the country - magnitude happened on 18 September 1858. Systematic 5.5 on the Richter scale; observations of earthquakes began in Bulgaria in On May 22, 2012 - earthquake with a 1892, when was founded Seismological Service at • series of seismic tremors that begin with a the Central Meteorological station in Sofia [27]. magnitude of 5.6. Here is a list [27, 28] of strong earthquakes with Territory of Bulgaria unconditionally should be magnitude greater than 5 from the early 20th century assigned to dangerous earthquake zones on Earth to present: [26]. Furthermore, the seismicity of the territory of On March 31, 1901 - the area between the • neighboring countries, Greece, Turkey, the former of Shabla and cape Kaliakra with magnitude Yugoslavia and Romania has a significant impact on 7.2 on the Richter scale; the assessment of seismic hazard for the territory of On April 4, 1904 - the river Struma valley - • Bulgaria (particularly a seismic impact on magnitude 7.8 on the Richter scale; intermediate-focus earthquakes in the Vrancea On October 8, 1905 - the river Struma valley • region, Romania). The literature describes the - magnitude 6.4 on the Richter scale; following major zones on epicenters of earthquakes On January 10, 1908 - Gorna Oriahovitsa - • with impacts on the territory of Bulgaria [18, 29]: magnitude 7 on the Richter scale; 1. Vrancea seismic zone (Romania); On April 14, 1909 - Gorna Oriahovitsa - • 2. Gorna Oriahovitsa seismic zone; magnitude 7 on the Richter scale; 3. Shabla seismic zone;

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4. Provadiya seismic zone; n k+−x1 kx 5. Negotinska Kraina and Timoc seismic (3) P3 (x)=−∑( x )pp(1) zone (Serbia); x=1 6. Sofia seismic zone; corresponds to clustered spatial dispersion. 7. Struma () seismic zone; It is known that the following inequalities exist [9]: 8. Maritsa seismic zone; Binomial distribution Variance < Mean 9. Xanthi seismic zone (Greece); Poisson distribution Variance = Mean 10. Marmara Sea seismic zone (Turkey). Negative binomial distribution Variance > The nature of modern geodynamic processes in Mean Bulgaria and neighboring areas can be outlined in a They could give indication about the type of general way by the grouping of epicenters of statistical distribution and corresponding point process. earthquakes with magnitude M ≥ 4. Data are from If the hypotheses of randomness and regularity several sources [2, 4, 20] and publications of Bulgarian are rejected, that it is convenient to apply some authors [1]. Stands out is the high seismicity, some method of cluster analysis. For our purposes we earthquakes with a magnitude of more than 7, although select the minimal dispersion method [21] and not with high frequency of repetition. Canberra Metric, calculated from original data. Statistical analysis of earthquakes aims to 2. CONCEPT AND METHODS describe and summarize data about their magnitude, The concept, which is used in this study, is that of and to reveal natural groups (clusters) of theirs “randomness”. If N points are located randomly in a places. Data from 137 sources - places of planar region, then the probability distribution of this earthquakes with magnitude M ≥ 4 are object of the random spatial point pattern follows the Poisson law. analysis. Such set of data is sufficient for reliable An observed frequency distribution, which does not inferences and conclusions. conform to the expected from a random point process, leads to rejection of the hypothesis of randomness in 3. SPATIAL DISTRIBUTION OF favor of the alternative - more regular or more EARTHQUAKES clustered, than the random model. A digitized map of Balkan area with scale 1:3 The method applied for study spatial distribution 000 000 is used as a base map [6]. The places of of the earthquakes in Bulgaria and neighboring areas earthquakes with their longitude/latitude coordinates is called “Quadrat analysis” [3, 13, 14, 19]. In the have been overlaid on this base map. The whole area quadrat method a planar study region is divided into is divided into a grid with cells of equal size a grid with cells of equal size, called “quadrats”, and (quadrats 1.0o longitude by 1.0o latitude) and the the number of points in each cell is counted. A pattern is shown on Fig. 1. regular point process generates a large number of The number of points in each cell on the quadrats containing only a single point, some empty territory of interest is counted and sample statistics quadrats and a very few quadrats with more than one are calculated. To make conclusion about the point in them. Conversely, a clustered point process character of observed spatial point pattern, three is expected to produce a very large number of empty statistical distributions are fitted to the data: quadrats, a few quadrats with one or two points, and Binomial, Poisson and Negative binomial several quadrats with many points in them. distribution. Results are presented in Table 1. Analyses have shown that the Poisson Two statistical tests are applied: Chi-square distribution goodness of fit test x ()λa k 2 (1) P1 (xa)=−exp()λ , (x=0,1,2,...,n) 2 ()OEii− x! χν = ∑ i=1 Ei corresponds to random spatial dispersion. (4) The Binomial distribution and variance-mean ratio test n nxnx− m2 (2) P2(x)=−()r pp(1) −1 ∑ m x=0 t* = 1 is connected with regular spatial pattern, and the 2 Negative binomial distribution (5) N −1

14 Екологично инженерство и опазване на околната среда, No 2, 2014, с. 13-21

Fig. 1

Table 1. Observed and expected distributions of the earthquakes No. of Observed Binomial Poisson Neg. Binom. points frequ- per ency n = 137 λa = w = 0.31786 cell p = 0.00232 0.31786 k = 0.07748

0 375 313.53 313.64 379.87 1 30 99.88 99.69 23.66 2 11 15.79 15.84 10.25 3 5 1.65 1.68 5.71 4 1 0.13 0.13 3.53 5 6 0.01 0.01 2.31 6 0 0.00 0.00 1.57 7 0 0.00 0.00 1.10 8 0 0.00 0.00 0.78 9 0 0.00 0.00 0.56 10 2 0.00 0.00 0.41 11 0 0.00 0.00 0.30 12 0 0.00 0.00 0.23 13 0 0.00 0.00 0.17 14 0 0.00 0.00 0.13 15 0 0.00 0.00 0.10 16 1 0.00 0.00 0.00

2 χ  - 694.43 694.43 20.645 df - 15 15 15 P0.001 - 37.70 37.70 24.32 - p < 0.001 p < 0.001 p < 0.148 D at 1%=0.0785 0.7402 0.7400 0.9997

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where: Oi are observed frequencies, Ei are expected (8) m frequencies and ν - degrees of freedom; m1 is Mean, Dc(a,b)=∑|xaj−−xbj|/||xxajbj m2 is Variance and N - number of observations. j=1 2 It is evident that values of χ for Binomial and where xaj is value of variable j for object a, m is total Poisson distributions exceed the critical value from number of variables. chi-square table (37.70), which lead to rejection of The application of Ward's method gives a the null hypothesis of regularness and randomness of dendrogram, presented on Fig. 2. Nine clusters are spatial pattern. Only χ2 value for Negative binomial found and their members are listed in Table 2. distribution is less than the critical value and shows It is interesting to notice that cluster 1 includes a clustered point process. earthquakes in Eastern Bulgaria. In cluster 2 are some The results from variance-mean ratio test are: places in middle part of Bulgaria, Eastern Greece and m1 = 0.31786, m2 = 1.62198, m2/m1 = 5.10273, European part of Turkey. Cluster 3 consists of t* = 65.5795, ν = 511, p < 0.001, earthquakes in Western Bulgaria and Macedonia. In which confirms that the observed spatial dispersion cluster 4 are all earthquakes in Romania, a few in is significantly different from the random Poisson Bulgaria and one in Moldavia. Cluster 5 covers places model. in Turkey and Ukraine of the Black Sea region. The standard Kolmogorov-Smirnov test for Cluster 6 consists of Central Greece only. In cluster 7 comparing two frequency distributions can also be are earthquakes from Albania, Macedonia and Serbia. applied. The test statistic “D” is simply given by the Cluster 8 covers places in Hungary, Slovakia and largest difference (irrespective of sign) between Western Romania. Cluster 9 presents Eastern Turkey observed cumulative frequency and expected and part of Caucasus region. cumulative frequency: Separate contour maps, which are two- dimensional representation of three-dimensional D = max [Cum Obser. Freq – Cum Expect. Freq] data from places of earthquakes, are produced with The critical value at the 1% level is 0.07851. help of procedure "Kriging" [10, 11]. It is one of the Observed frequencies differ significantly from the most useful methods for generating regularly spaced theoretical distributions, which confirms a clustered arrays of Z values from irregularly spaced XYZ point process. data. The derived contour maps are overlaid on the intermediate map and could be easily interpreted [5, 4. SPATIAL STRUCTURE OF 17] (see Fig 3). EARTHQUAKES 5. DESCRIPTIVE STATISTICS OF The study is accomplished with help of EARTHQUAKES statistical package STATISTICA, system for surfaces mapping SURFER and geographic The main index selected for the study is information system (GIS) MapInfo. magnitude of earthquakes on the Richter scale (M ≥ Relationships between 137 places of earthquake 4). Basic descriptive statistics for the characteristics are examined by the method of hierarchical of all 137 places of earthquakes are calculated, agglomerative cluster analysis [7, 12, 16]. It is namely: mean, median, minimal and maximal known that all combinatorial methods for such values, standard deviation, skewness and kurtosis analysis have common equation [15, 22]. They are given in Table 3.

(6) dhk = αidhi + αjdhj + βdij + γ|dhi - dhj|, 6. DISCUSSION ABOUT SEISMIC RISK FOR NPP "BELENE" where parameters αi, αj, β and γ determine method. Here Ward's method is used [21], for which The report of the Bulgarian Academy of (7) Sciences from 1990 - NPP "Belene" Studies and n + n n + n − n α = h i ; α = h j ; β = h ; γ = 0 . Position of BAS" [23, 24] makes the following i n + n j n + n n + n h k h k h k conclusion: "The possible maximum magnitude of This method minimizes within-group sum of earthquakes in the area of the site /fault line of the squares and it is called method of the minimal river/ 5,5; node of Nikopol /30 km west of it/ dispersion. and node /15 km east of it/ is 6.0, and about For similarity measure so-called Canberra 25 km southeast of it - 6.5. These data should be Metric is used considered in determination of the seismic risk."

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Fig. 2

Still following words: "... if a 7.2 magnitude In the article by Investor.bg from 03/11/2013 /earthquake "Vrancea"- 1977/ of NPP "Belene" is "NPP and future NPP Belene could established intensity of seventh degree, then with a withstand an earthquake" [8] the chairman of the magnitude greater than 7.5 could be expected Nuclear Regulatory Agency Sergey Tzotchev says intensity exceeding eight degrees." that "NPP Belene is designed with 40% "reserve" for Follows citation of Soviet "Guidelines on an earthquake, that when set parameters for ground location of nuclear stations" - 1987 and "Regulation acceleration of 0,24 g, or 8th grade on the scale of about design of seismostable nuclear stations - NP- Medvedev the plant could sustain acceleration of AЭ D-006-87," according to which sites with 0,34 g, or ninth degree in the same scale." It should engineering-geological conditions as those of NPP be noted that the intensity of the earthquake I /by "Belene" are unfavorable to construction of nuclear Medvedev/ to a location on the surface of the Earth power plants, in accordance with paragraph 2.3.1.3 depends on the magnitude of the earthquake M /by of the same Guidelines in a zone of intensity for Richter/ at the centre and the distance to hypocenter MP3, higher than the eighth degree, the placement the earthquake. An earthquake of 10 grade scale of of nuclear power plant is not allowed. Medvedev corresponds to a magnitude 8.0 on the In his analysis of the future of nuclear energy Richter scale at a depth of 45 kilometers of the and energy security of Bulgaria F. Dudin [25] states: outbreak. "In this region studies have shown the danger of The peculiarity of the Vrancea earthquakes is earthquakes at 7 - 8 degrees on the Richter scale on the large focal depth. Pockets of the largest Bulgarian territory and more than 8 degrees in earthquakes are mainly located at a depth of 110 - Romania with an average frequency once every 50 150 km. In connection with this, area of the years (the last such was in 1977). Unfortunately 8 destructive and tangible shock extends over large degrees is the maximum for the chosen building site distances. With such a large magnitude and large can take as a design basis earthquake and in focal depth Vrancea earthquake is felt by the account of the enormous radiation risks must be population for a vast territory of Greece in the south envisaged reserve to 9 degrees for safe shutdown to Finland in the north and from the European part earthquake." of Russia in the east to the west of Germany [30].

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In connection with the foregoing, the assessment the East-European platform must consider the of seismic risk for the planned and under impact of the earthquake or seismic potential of construction objects in the south-western regions of local seismic source zones and Vrancea.

Table 2. Clusters and their members Cluster 1 Cluster 2 Cluster 3 Cluster 4 Cluster 5 Galabovo Chirpan Cherna gora Duzce Kaliakra G. Oriahovica KCM Razgrad Gediz Kamen bryag G. Oriahovica Kralev dol Tirnovo Golcuk Popovitsa Pernik Kishinev Crimea Kurdjali Sofia Rila M. Braila Kherson Mirovo Struma Smolian Buzau Sudak Ovchi kladenec Alexandropoli Sofia Covasna Popovo Kavala Sofia Covasna Popovo Kavala Sofia Kovasna Kavala Struma Kovasna Mitilini Troian Nehoiu Ruse Panorama Velingrad Nehoiu Shabla Canakkale Drama Nehoiu Canakkale Panorama Vrancea Triavna Gelibolu Basibos Vrancea Triavna Dink Vrancea Varna Bashibos Vrancea Varna Belotino Vrancea Varna Gevgelija Vrancea Negotino Nehoiu Yambol Nikolik Budeni Novo Selo Prsten Sveti Nikole Valandovo

Cluster 6 Cluster 7 Cluster 8 Cluster 9 Athens Tirana Karvina Badamdar Athens Ulcinj Beregdaroc Surami Chania Bitola Budapest Bingol Iraklion Debar Budapest Bingol Kalamata Dolna Banjica Gyor Erzincan Lefkada Ohrid Oroszlany Erzincan Loutrakion Skopie Pishkol Karakose Patras Zajas Hateg Oltu Sparti Orsava Timisoara Van Zakinthos Gnjilane Voiteg Zakinthos Kopaonik Vrsac Kraljevo Zilina Kursumilja Dolge Mionica Novi Pazar Trstenik

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Table 3. Descriptive statistics of earthquakes (N=137) Variable Mean Median Min Max Std.Dev. Skew. Kurt. Longitude 25.55 24.80 18.20 49.80 5.462 2.084 5.139 Latitude 42.62 42.59 34.20 49.91 2.741 -0.031 0.336 Magnitude 5.64 5.50 4.00 8.00 1.061 0.225 -1.085

Our analysis shows that the estimated magnitude underestimates seismic hazard in the Northern of an earthquake at the Belene site is approximate Bulgaria. It is clear that despite all the safety 7,5 on the Richter scale. Comparison of the Fig. 3 measures in the construction, the selected and Fig. 4 shows that the USGS - Bulgaria Seismic construction site of NPP "Belene" does not provide Hazard Map is not enough detailed and reserve up to 9 degree seismic certainty [23, p 324].

Fig. 3 Estimated magnitudes

Fig. 4 From: USGS - Bulgaria Seismic Hazard Map

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7. CONCLUSIONS 12. Peter A. Rogerson, G. Lee, I. Yamada, 2006. Technical Report: Statistical Methods for the Use of several statistical methods such as Detection and Monitoring of Spatial Clusters. Quadrat analysis, Cluster analysis, Descriptive 13. Rogers A., 1974. Statistical analysis of statistics and Geographic Information Systems has spatial dispersion. Pion Ltd London. allowed revealing spatial distribution and spatial 14. Rogers A., N. G. Gomar, 2010. Statistical structure of earthquakes in Bulgaria and neighboring Inference in Quadrat Analysis. UC Berkeley, areas. The study rejects the hypothesis of random http://onlinelibrary.wiley.com/doi/10.1111/j.1538- dispersion of earthquakes over the country's territory 4632.1969.tb00631.x/pdf. in favor of the alternative - more clustered, than the 15. Rogerson, P.A., 2001. Statistical Methods of random model. Cluster analysis allocates all 137 Geography. Sage. London. places into 9 clusters with distinct properties. Tables 16. Rogerson, P. 2001a. A statistical method for 2 - 4 give bases for more profound interpretation. the detection of geographic clustering. Geographical The result of present study indicates needs for Analysis, 33: 215-27. more detailed research of seismic risk for Bulgaria 17. Sadovski L. A., A. N. Sadovski, 1998. and neighboring areas. It confirms that the territory Geographic information systems - Indispensable tool of Bulgaria unconditionally should be assigned to for Ecology. J. Balkan Ecology, vol. 1, No. 3, 9 - 15. dangerous earthquake zones on Earth. 18. Shebalin N.V., G. Reisner, A. Drumea, J.

Aptekman, V. Sholpo, N. Stepanenko, A. REFERENCES Zacharova, 1976. Earthquake origin zones and 1. E. et al., 2003 - 2012. Data and distribution of maximum expected seismic intensity analysis of the events recorded by NOTSSI. for the Balkan region. Proc. Sem. on Seismic Zoning Bulgarian Geophysical Journal, BAS. Maps, Skopje 1975, II, pp. 68-171. 2. Compare Eastern Europe Earthquakes, 19. Thomas R. W., 1977. An introduction to 2013. National Geophysical Data Center (NGDC). quadrat analysis. Concepts and techniques in modern http://earthquakes.findthedata.org/d/b/Eastern- Geography No. 12. Europe. 20. USGS, 2012. Bulgaria - Seismic Hazard 3. Cressie N., Ch. Wikle, 2011. Statistics for Map. Spatio-Temporal Data, John Wiley and Sons. http://earthquake.usgs.gov/earthquakes/world/bulgar 4. Earthquake Track, 2013. Worldwide ia/gshap.php. Earthquake Stats. All Earthquake Data Courtesy 21. Ward J. H. Jr., 1963. Hierarchical Grouping USGS. http://earthquaketrack.com/. to optimize an Objective Function. Amer. Stat. 5. ESRI, 2001. ArcGIS Geostatistical Analyst. Assoc. Journal, vol. 58, 236-244. ESRI White paper. 22. Wikle Ch. K., 2012. Statistics for Spatio- 6. Google, 2013. Map data. www.balkan- Temporal Data. University of Missouri. hotel.com_map-of-balkans. 23. БАН, 1990. АЕЦ "Белене": Изследвания и 7. Gruijter J. J. de, 1977. Numerical становище на БАН, ред. П. Цветанов. Изд. БАН. classification of soils and its application in survey. 24. Доклад на БАН за проект "АЕЦ Белене", Soil Survey Papers, No. 12. Wageningen. 1990 г., ред. ст. н. с. Пламен Цветанов, 8. Investor.bg, 2013. NPP Kozloduy and future http://www.zelena-ikonomika.ekonetbg. NPP Belene could withstand an earthquake. org/01_upload/doc/02_library/ http://www.investor.bg/ikonomika-i- ban_belenereport1990_scanbczi_2011_ocr2. politika/332/a/aec-kozlodui-i-bydeshtata-aec-belene- pdf. biha-izdyrjali-pri-zemetresenie,128375/. 25. Дудин Ф., 2013. Анализ на АЕЦ 9. Kendall M., A Stuart. 1958. The Advanced „Белене”, бъдещето на ядрената енергетика и Theory of Statistics, Vol. I. Ch. Griffin, London. енергийната сигурност на България. http://e- 10. Li J., A. D. Heap, 2008. A Review of Spatial bookbg.com/index.php? Interpolation Methods for Environmental Scientists. option=com_docman&task=doc_view&gid= Geoscience Australia. 3945&Itemid=61. 11. Nelson Mandela Metropolitan University, 26. Солаков Д., 2009. Сеизмично райониране 2013. Lectures: Spatial Interpolation. на Република България, съобразено с http://tutor.nmmu.ac.za/uniGISRegisteredArea/Mate изискванията на Еврокод 8 и изработване на rial/Module3/AdobeFiles/Section05.pdf карти за сеизмичното райониране с отчитане на

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сеизмичния хазарт върху територията на полуостров. Курс по Сеизмотектоника. страната. БАН, Геофизичен Институт. http://www.geology.bas.bg/lecture/seismo/seismotec 27. Тотков Н., 2012. История на tonics-lecture.pdf. земетресенията в България през ХХ век. 30. Шаров Н., А. Маловичко, Ю. Щукина http://ntotkov.blogspot.com/2012/12/blog-post.html. (Ред.), 2007. Зелметрясения и микросеизмичность в 28. Уикипедия, 2013. Земетресения в задачах современной геодинамики Восточно- България. http://bg.wikipedia.org/wiki/. Европейской платформы – Книга 1, Землятресения. 29. Шанов Ст., 2006. Регионална сеизмичност Карельский Научный Центр, Российская Академия на Източната част на Централния Балкански Наук, Петрозаводск

ПРОСТРАНСТВЕН АНАЛИЗ НА ЗЕМЕТРЕСЕНИЯТА В БЪЛГАРИЯ И СЪСЕДНИТЕ ТЕРИТОРИИ Александър Н. Садовски

Резюме: Статистически методи, включително Quadrat анализ, Клъстърен анализ, Дескриптивна статистика и Географски Информационни Системи се използват за изследване пространственото разпределение и да се обобщят данни за земетресенията в България и съседните области. Откриването на райони с подобна картина за поява на земетресения е от интерес за Правителството и за широката общественост. Дискутира се сеизмичният риск за АЕЦ "Белене".

Ключови думи: Земетресения, пространствен анализ, статистически методи, Географски информационни системи, АЕЦ "Белене"

Проф. Д-р Александър Н. Садовски Prof. Dr. Alexander N. Sadovski Академик на МАНЕ Academician of the IEAS Тел. 02-981-5586 Phone: 02-981-5586 e-mail: [email protected] e-mail: [email protected]

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