International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania

PROBABILISTIC SEISMIC HAZARD MAPS IN TERMS OF INTENSITIES

FOR ROMANIA AND BULGARIA

L. Ardeleanu 1, G. Leydecker 2, T. Schmitt 2, K.-P. Bonjer 3, H. Busche 2, D. Kaiser 2 , S.D. Simeonova4, D.E. Solakov 4

INTRODUCTION

Romania and Bulgaria, situated in the Balkan Region as a part of the Alpine-Himalayan seismic belt are characterized by high seismicity, and are exposed to a high seismic risk. Over the centuries, the two countries experienced strong earthquakes. Because both coun-tries are now members of the European Union, they are committed to introduce the EUROCODE 8 (EC8). For this building code seismic hazard maps have to be calculated and based on the results hazard zones have to be determined later. In this paper we present a special way to compute such hazard maps. We prefer to use the macroseismic intensity to calculate hazard maps, because this parameter is directly related to the degree of damage. A great advantage of using intensities here is that the irregular spatial attenuation field of the Vrancea intermediate depth earthquakes can be estimated by detailed macroseismic maps. The seismic hazard is computed with EQRISK (McGuire, 1976), based on the probabilistic approach developed by Cornell (1968).

SEISMIC SOURCE ZONES

The main base of our probabilistic analysis is the earthquake catalogue for SE-Europe (Shebalin et al, 1998) in combination with some local cata-logues. Epicenters and seismic source zones in an area of about 200 km around the territory of Romania and Bulgaria are shown in Fig. 1. The boundaries of these zones were chosen to reflect the seismicity ade-quately regarding tectonic units and lithospheric structure. For each seismic source the intensity-frequency relation was calculated and a maximum possible earthquake as well as a characteristic seis-mogenic depth has been estimated. For the parameters of the seismic zones see table 1 in Simeonova et al (2006).

ATTENUATION RELATIONSHIP

For crustal earthquakes we imple-mented the intensity attenuation function of Sponheuer (1960,) which is based on Kövesligethy (1907), in the program EQRISK.

1 National Institute for Earth Physics, Bucharest, Romania, ardel@infp.ro

2 Federal Institute for Geosciences and Natural Resources, Hannover, Germany, guenter.leydecker@web.de

3 Geophysical Institute University of Karlsruhe, Germany, kp-bonjer@online.de

4 Bulgarian Academy of Sciences, Geophys. Instit., Seismolog. Depart., Sofia, Bulgaria, stelas@geophys.bas.bg

Figure 1: Epicenter map (I0 = epicentral intensity) with the seismic source zones. Codes of source zones and their parameters are listed in Table 1 in Simeonova et al (2006).

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Isite = Iepic−3 log(r/h) −1.3 α(r−h) (1) Iepic is the epicentral intensity, Isite is the intensity at a site at hypocentral distance r (km); h is the focal depth (km); and α is the absorption coefficient, here 0.002 km

−1.

The macroseismic field of intermediate depth earthquakes of the Vrancea zone Vi (Roma-nia) is very irregular. As the attenuation law (Eq. 1) does not consider spatial differences in damping, a new empirical approach is chosen to take the directionality of attenuation and local site effects into account. Therefore a factor Ω (Ardeleanu et al, 2005) is introduced into Eq. 1: Isite = Iepic. -3log(r/h) -1.3 α * Ω (r-h) (2)

Using detailed macro-seismic maps of three Vrancea intermediate depth earthquakes, Ω has been calculated for each observation point assum-ing α = 0.001 km

-1. Strong

local variations of Ω were avoided by a gridding of 0.5 degree in longitude and 0.25 degree in lati-tude. The contribution of the Vrancea intermediate depth zone to each grid point was computed with the corresponding repre-sentative Ω of this point. Fig. 2 shows this Ω field. A seismogenic depth of 120 km was assumed. RESULTS

Each final seismic hazard map is a combination of two maps, the one for normal depth source zones (Fig. 3) and the one for the Vrancea interme-diate depth zone (Fig. 4).

The influence of the Vrancea intermediate depth zone on the seismic hazard is predominant for Romania, for Bulgaria it is important only for its Northern part. As recommended in EC8, the maps are calculated for a 475 years return period (probability of exceedance of 10% in 50 years) (Fig. 5) for the design earthquake and for a 95 years return period (probability of exceedance of 10% in 10 years) (Fig. 6) in order to limit the damage of buildings and the financial loss in case of weaker earthquakes with higher frequency of occurrence. Special structures like large dams are beyond the scope of EC8, as they require higher safety standards, including longer recurrence periods, e.g. 2475 years (2% in 50 years) (Fig. 6) or 5,000 years (Fig. 7).

Figure 2: The distribution of Ω. The Ω pattern mimics the trends of the general strong decay of the seismic intensity pattern of the big intermediate depth Vrancea events in NW-SE direction.

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INTENSITY

INTENSITY

Figure 3: Seismic hazard from source zones of normal depth for a recurrence period of 475 years; colours represent the intensities in MSK-1964 scale.

Figure 4: Seismic hazard from Vrancea events of intermediate depth for a recur-rence period of 475 years; colours represent intensities in MSK-1964 scale.

INTENSITY

INTENSITY

Figure 5: Seismic hazard from all source zones (Fig. 3 and Fig. 4) for a recurrence period of 475 years; colours represent in-tensities in MSK-1964 scale.

Figure 6: Seismic hazard from all source zones for a recurrence period of 95 years; colours represent intensities in MSK-1964 scale.

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INTENSITY

INTENSITY

Figure 7: Seismic hazard from all source zones for a recurrence period of 2,475 years; colours represent intensities in MSK-1964 scale.

Figure 8: Seismic hazard from all source zones for a recurrence period of 5,000 years; colours represent intensities in MSK-1964 scale.

REFERENCES

Ardeleanu, L., G. Leydecker, K.-P. Bonjer, H. Busche, D. Kaiser, T. Schmitt, 2005: Probabil-

istic seismic hazard map for Romania as a basis for a new building code. -- Natural Hazards and Earth Sciences, 5, 679-684. http://www.nat-hazards-earth-syst-sci.net/5/679/2005/

Cornell, C., 1968: Engineering Seismic Risk Analysis. -- BSSA, 5, 1583 - 1606. Kövesligethy von, R., 1907: Seismischer Stärkegrad und Intensität der Beben. -- Gerlands

Beiträge zur Geophysik, Band VIII, Leipzig. McGuire, R, 1976: FORTRAN Computer Program for Seismic Risk Calculations. -- U.S.

Geol. Surv. Open-File Rep. 76-67, p 90. Simeonova, S., D. Solakov, G. Leydecker, H. Busche, T. Schmitt, D. Kaiser, 2006: Probabil-

istic seismic hazard map for Bulgaria as a basis for a new building code. -- Natural Hazards and Earth System Sciences, 6, 881 - 887. http://www.nat-hazards-earth-sys-sci.net/6/881/2006/

Shebalin, N. V., G. Leydecker, N. Mokrushina, R. Tatevossian, O. Erteleva, V. Vassilev, 1998: Earthquake Catalogue for Central and Southeastern Europe. -- European Com-mission, Report No. ETNU CT93-0087, Brussels. (For catalogue download: http://www.bgr.de/quakecat.

Sponheuer, W., 1960: Methoden zur Herdtiefenbestimmung in der Makroseismik. (Methods for depths estimation in macroseismic). -- Freiberger Forschungs-Hefte C 88, 117 pp., Akademie Verlag Berlin.