Keywords

1 Introduction

The evaluated seismic hazard for Bulgaria in terms of Peak Ground Acceleration (PGA) calculated for a 475 years return period [1] shows that the highest ground motion levels are predicted for Blagoevgrad (Krupnik-Kresna) region, where the calculated PGA values exceeds 0.2 g and in some places 0.3 g (Fig. 1).

Fig. 1
figure 1

Seismic hazard map in PGA for Bulgaria for a 475 years return period [1]

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The Krupnik-Kresna area which occupies the north-western part of the investigated region is considered to be one of the most dangerous earthquake zones in Bulgaria and the Mediterranean as well. The high seismic activity in this area is associated with the Struma fault system. Two of the strongest earthquakes in Europe in the twentieth century occurred in this area—the earthquakes of April 4, 1904 (MS = 7.1 and MS = 7.8). The second earthquake was felt even in Budapest. The villages of Krupnik and Simitli were completely destroyed. Serious damage has been reported in the towns of Dzhumaya (Blagoevgrad nowadays), Bansko and Razlog. At Dzhumaya (Blagoevgrad) damage was widespread and in places serious, particularly in the lower part of the town, where about 100 houses damaged by the first shock were ruined, including one mosque, three minarets, the barracks and the military hospital [2].The earthquake activated a fault line with a sub-parallel direction. There were also huge cracks in the neighbouring Brezhani graben. The subsidence at the Struma River was about 1.5–2 m, where a big step was formed. From the left bank of the Struma River the rupture was about 2 m high. In this region, where the thickness of the earth's crust is significant (below the western part of the Rhodope massif it reaches a depth of 40–50 km), originate the deepest earthquakes in Bulgaria.

In this research, the term risk follows the definition by the United Nations (UNDRO) and refers to the expected losses from a particular hazard to a specified element at risk in a particular future time period. Losses may be estimated in terms of human lives, buildings destroyed or in financial terms” [3]. In our case, as far as we analyse the population exposure to earthquakes, risk is connected with potential human losses (casualties) resulting from seismic hazard. One of the main parameters needed for calculating the casualties is the population exposure. It refers to the human occupancy of hazard zones, or the population present within the hazard area that would be potentially directly affected by an event [4].

2 Methodology

Mathematically, the risk is a function of hazard occurrence probability, element at risk (population) and vulnerability. We accept the hypothesis that the three factors explaining the risk are multiplying each other [5] which means that if the hazard is 0 (null), then the risk is 0 (null). Theoretically, the risk is also null if nobody lives in an area exposed to hazard and if the population is invulnerable. In the present research we use the census-based resident population (i.e. night-time) and do not account for spatio-temporal variation of population distribution in urban areas.

Population exposure is modelled by crossing the seismic hazard and population living in the potentially affected area. We determine a parameter called “population exposure index” (PEI) which has five classes: Minor, Low, Moderate, High and Major.

The input data are: (1) seismic hazard for the territory of Bulgaria [1] re-calculated according to the USGS global Vs30 model (the time-averaged shear-wave velocity to 30 m depth) (https://earthquake.usgs.gov/data/vs30/) and (2) one sq. km. population grid of the Republic of Bulgaria resulted from the census performed in 2011 (https://www.nsi.bg/en/content/12309/population-grid-1-sqkm-census-2011). Using the population grid rather than total number of population allows direct comparison between big cities and small villages (less populated towns have the same weight as more populated ones).

Similar analysis is performed by [6] but they use as input data the population density in Bulgaria which is calculated for each settlement using the total number of population living there and the area of the village/town/city. In this way, the population is artificially distributed over the entire area on the one hand and on the other hand the settlements that occupy a large area artificially reduce the value of the calculated density.

First, both variables are classified in five levels using equal interval classification schemes (Tables 1 and 2):

Table 1 Population density levels
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Table 2 Seismic hazard levels
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The number of levels and method for classifying the resultant map were chosen according to several criteria such as the optimum number of levels for visual representation or the number and level of errors between them.

The combination of both seismic hazard and population density provides the population exposure as an element of risk. A hypothesis is made that similar to the risk it follows the multiplicative formula that could be simplified as Eq. (1):

$$PopExp^{E} = \, H^{E} . \, Pop^{E} .$$
(1)

where:

PopExpE is the population exposure for the single spatial element E, one square kilometer in size.

HE is the seismic hazard class for the single spatial element E determined accord. to Table 1.

PopE is the population density class for the single spatial element E determined according to Table 2.

As was expected, the seismic hazard levels of Blagoevgrad region are in the upper part of the classification scale (Fig. 2). The Krupnik-Kresna area is fully covered with the highest 5th level and the remaining part falls within level 4 interval limits. Only several small region in the south-east direction can be classified as having moderate level 3 values.

Fig. 2
figure 2

Levels of the seismic hazard obtained for Blagoevgrad region according to classification scheme in Table 2

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3 People Exposure Index in Blagoevgrad Region

Calculation of the population which is exposed to seismic hazard is performed in a gridded spatial network with a single element E with dimensions 1 × 1 km.

To determine the population exposure index (PEI) results obtained using Eq. 1 are also classified in five classes from 1 to 5: Minor, Low, Moderate, High and Major. Results, obtained for Bulgaria are given in the map of Fig. 3. According to those results, Sofia, Plovdiv and Blagoevgrad are the Top 3 municipalities with the major PEI level in Bulgaria. In the three districts live around 1.8 million people. A detailed breakdown for Blagoevgrad region (NUTS II classification) is given in Table 3.

Fig. 3
figure 3

Spatial distribution of the population exposure index obtained for the territory of Bulgaria

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Table 3 Settlements with major PEI level in Blagoevgrad region
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More than 130,000 people are exposed to the highest level of seismic hazard in the region. In the list of Table 3 are four of the major towns in this area. Spatial distribution of the results is displayed in the Fig. 4. Results obtained by [5] assessing global exposure and vulnerability towards natural hazards reveal that human vulnerability is mostly linked with country development level and environmental quality. Country development combines different indicators and parameters, but in the case of an earthquake the most important is the state of the norms for design and control over construction. The first building code in Bulgaria defining regulations and associated standards intended to control aspects of the design, construction, materials, etc. that are necessary to ensure human safety and welfare (including resistance to collapse and damage) is from 1956. It was updated several times and now Bulgaria has adopted modern standards according to the requirements of Eurocode 8.

Fig. 4
figure 4

Spatial distribution of the population exposure index (PEI) obtained for Blagoevgrad region. Each settlement is colored according to the maximum value of PEI calculated for its inner elements according to Eq. 1. Displayed are the names of municipalities

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4 Seismic Scenario for the City of Blagoevgrad

From the results obtained in the region (Fig. 4) it is seen that the city of Blagoevgrad has major PEI index which means that people living there are highly exposed to the seismic hazard. Due to this, a detailed study of the city is performed to outline which are the most exposed places and how many people are living at highest risk.

For obtaining the seismic hazard of the city of Blagoevgrad a deterministic earthquake scenario is used [7]. The scenario is defined on the base of the seismogenic potential of the causative faults defined in the SHARE project [8]. The fault system around the city of Blagoevgrad is complicated, comprising many inherited faults from the late orogenic stages. The active fault’s model of the region based on SHARE is presented in Fig. 5.

Fig. 5
figure 5

Spatial pattern of seismicity (historical and instrumental earthquakes with MW > 3.0) and active faults map for the Blagoevgrad region (modified from [7])

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The scenario considers earthquake with magnitude MW 6.5 on the closest to the city active fault (south of it, Fig. 5). The ground motion (in PGA), is calculated using Eq. 2:

$$GM\left( {x,y} \right) = \mathop \sum \limits_{i = 1}^{6} w_{i} g_{i} \left( {x,y} \right)$$
(2)

where GM(x,y) is the evaluated ground motion at point with coordinates (x,y). gi(x,y) is the median ground motion predicted by i−th GMPE (from 6 GMPE’s for active shallow crustal tectonic regime with different weights) and wi is the corresponding weight.

Results for the obtained PGA in the urban area very between 0.29 and 0.45 with the grater area being in the upper scale levels. They are presented in Fig. 6. The scenario maps account for soil amplification effects using the geotechnical zonation of the considered urban area. Representation of the soil properties is defined by the engineering parameter Vs30—average shear-wave velocity in the upper 30 m of the soil/rock profile. The Vs30 values for the city of Blagoevgrad are based on the results published in [9].

Fig. 6
figure 6

Deterministic earthquake scenario for the city of Blagoevgrad in Peak Ground Acceleration (PGA). Obtained PGA very between 0.29 and 0.45 g with the grater area being in the upper scale levels, colored in red

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5 Distribution of Buildings and Population in the City of Blagoevgrad

According to the last census performed in 2011, the total number of population in the Blagoevgrad city is 69,178 people. They are living in 5994 residential buildings. The distribution of buildings, by number of floors and total floor area—TFA (m2) is given in Table 4.

Table 4 Distribution of buildings (by pieces and TFA) in different levels of damage
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Most of the buildings are low-rise constructions with up to 3 floors but there are also 806 buildings which are above 6 floors. These are the homes of 73% of the people. According to the cadaster information most of the buildings were built in the period 1966–1977. The distribution of buildings by typologies shows that main part of the building stock are masonry structures, unreinforced masonry with wooden beams without reinforced concrete belts, not framed by columns. There is also a significant number of masonry structures, unreinforced masonry with reinforced concrete slabs, beams and belts, not framed or framed by columns.

6 People Exposure in the City of Blagoevgrad

Similar to the calculation of the people exposure in Blagoevgrad region we implemented the same methodology which crosses the seismic hazard and population living in the potentially affected area. According to the results obtained for the regional assessment which are described in the previous chapter seismic hazard and population density of Blagoevgrad follows within the major PEI category. That is why for the detailed assessment of the exposure inside the city we did not make a re-classification of the input variables. Population exposure is obtained for every single building using Eq. 3:

$$PopExp^{b} = \, H^{b} . \, Pop^{b} \ldots .$$
(3)

where:

PopExpb is the population exposure for a single building.

Hb is the seismic hazard for the site of the building.

Popb is the population for a single building (residents).

Thus, we obtain a list of the population exposure for all residential buildings in the city of Blagoevgrad. To perform a quality assessment of the results we classified the results in the three upper categories: Moderate, High and Major (Fig. 7).

Fig. 7
figure 7

Population exposure in the city of Blagoevgrad. In red are marked places where population is majorly exposed to the effects of a potential earthquake

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Results of the assessment are summarised in Table 4. Majorly exposed are 21,820 people living in 398 buildings. All of those building have more than 3 floors. High levels of exposure have 1466 buildings with 32,284 residents. Both categories encounter for 75% of the Blagoevgrad citizens (Table 5).

Table 5 Categories of the population exposure obtained for Blagoevgrad
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7 Conclusion

Determination of the population exposure index (PEI) in one of the most seismically active zone in Bulgaria–Blagoevgrad region shows that less than 30% of the settlements are characterised with Moderate (3) values of PEI. These are 85 small villages where the population is between 2 and 99 people. For all 14 towns, the obtained PEI values are High (4) and Major (5). Purpose of the calculated index is to give a qualitative information about the size of negative consequences which may occur in case of a strong earthquake. For quantitative analysis, measures of the exposure should be combined with the specific vulnerability of the exposed elements to seismic hazard, to estimate the quantitative risks and to determine the potential loses.

An usable and realistic seismic scenario for the city of Blagoevgrad is proposed for detailed assessment of the people exposed to seismic hazard. The obtained values of Peak Ground Acceleration (PGA) varying between 0.29 and 0.45 g are crossed with the population living there to determine the levels of population exposure.

The population of the Blagoevgrad city numbers about 70,000 people living in 5994 residential buildings. Most of them are low-rise constructions with up to 3 floors but there are also 806 buildings which are above 6 floors. We consider a deterministic earthquake scenario with magnitude MW 6.5 on the closest to the city active fault, located south of it. When combining the earthquake scenario with the distribution of residents in the city we obtain the population exposure as a parameter of the seismic risk. Our results show that people living in 398 buildings are majorly exposed to the seismic hazard in Blagoevgrad city. Another 1465 buildings are determined as highly exposed to this threat. In total, 75% of the Blagoevgrad citizens are largely exposed to a potential earthquake’s effect.

The obtained results are intended to serve as input for developing of detailed earthquake damage scenarios for the settlements and can be used for disaster preparedness and emergency reaction planning.