PROBABILISTIC SEISMIC HAZARD ASSESSMENT IN ...3 Probabilistic seismic hazard assessment in Romania...

PROBABILISTIC SEISMIC HAZARD ASSESSMENT IN ROMANIA:APPLICATION FOR CRUSTAL SEISMIC ACTIVE ZONES

I. A. MOLDOVAN, E. POPESCU, A. CONSTANTIN

National Institute for Earth Physics, C.P. MG-2, Calugareni 12, 077125, Magurele, Ilfov, RomaniaE-mail: [email protected]

Received August 8, 2007

The paper is adverted to a complex activity of research concerning thesettlement of the crustal seismic hazard in Romania for risk studies. The seismic riskassessments are very important to specialized organizations, as these studies reveal(physically meaning) the neuralgic points of certain sites that contain constructions,representing real centers of potential disasters when stroked by natural catastrophesand having large socio-economical impact.

To forestall such catastrophes, two categories of measures are necessary:(i) achievement of studies regarding the seismicity and evolution of the seismicitywithin the area, in order to detect abnormalities related to temporal evolution of thesystems, abnormalities that can be considered as forerunners of major earthquakes;(ii) hazard and seismic risk assessment for strategic sites and the dissemination of theinformation in the decision media.

The analysis that we propose implies: (1) geometrical definition of all seismicsources affecting Romania, (2) estimation of the maximum possible magnitude,(3) estimation of the frequency magnitude relationship, (4) estimation of theattenuation law and, finally, (5) computing PSH.

Key words: crustal seismicity, seismic hazard, probabilistic approach.

1. INTRODUCTION

The seismic hazard assessment in dense-populated geographical regionsand subsequently the design of the strategic objectives (dams, nuclear powerplants, etc.) are based on the knowledge of the seismicity parameters of theseismogenic sources which can generate ground motion amplitudes above theminimum level considered risky at the specific site and the way the seismicwaves propagate between the focus and the site.

Extremely vulnerable objectives, like large cities, hidroenergetic dams ornuclear power plants, are present all arround Romania, and not only in theVrancea intermediate earthquakes action zone. The best example is the westernpart of Romania, that is not affected by Vrancea intemediate earthquakes andwhere the crustal seismicity is high. In this part of the country are cities likeTimisoara, Arad and Oradea and the “Portile de Fier I and II” hidroenergetic

Rom. Journ. Phys., Vol. 53, Nos. 3–4 , P. 575–591, Bucharest, 2008

576 I. A. Moldovan, E. Popescu, A. Constantin 2

dams. Therefore, this region was primarily considered in all the studies ofseismic hazard in Balkan and Circum-Pannonian regions (see for example thetopical volumes “Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation”in Kluwer Academic Press, 1999 and “Seismic Hazard of the Circum-PannonianRegion” in Pure and Applied Geophysics, vol. 157, 2000).

The purpose of this paper is to provide a complete set of informationrequired for a probabilistic assessment of the seismic hazard in Romania relativeto the crustal sources. The analysis that we propose implies: (1) geometricaldefinition of all seismic sources affecting Romania, (2) estimation of themaximum possible magnitude, (3) estimation of the frequency magnituderelationship, (4) estimation of the attenuation law and, finally, (5) computingPSH with the algorithm of [1].

2. SEISMIC SOURCES CHARACTERISTICS

The first step in the determination of probabilistic crustal hazard consists indefining the seisogenic sources. It is necessary to point out and to delimit theseismic areas from the Romanian territory. The seismogenic crustal sources(Fig. 1) that affect the Romanian territory are: Vrancea crustal source (VRN),Barlad Depression zone (BD), Predobrogean Depression source (PD),Intramoesian fault-Shabla-Dulovo source (IMF-DUL-SH), Fagaras-Campulung-Sinaia crustal sources (CMP = FG + CP + SI), Transilvanian Depression (TD),Banat crustal source (BAN), Danubian crustal earthquakes (DAN) and IBARzone, Crisana-Maramures sources (CM = CMS1 + CMS2).

2.1. VRANCEA CRUSTAL SOURCE

The seismic activity in Vrancea in the crustal domain (VN) is located infront of the Southeastern Carpathians arc, spread over a stripe area delimited tothe north by the Peceneaga-Camena fault and to the south by the Intramoesianfault (Fig. 1). The seismicity is more diffuse than for the subcrustal source andconsists only in moderate-magnitude earthquakes (Mw 5.5) generated in clusterslocalized in the eastern part (seismic sequences of Râmnicu Sarat area) andnorthern part (swarms in Vrâncioaia area). The catalog contains a singleearthquake of Mw = 5.9 occurred on March 1, 1894, with magnitude estimatedfrom historical information (possibly overestimated).

The rate of the seismic moment release, Mo = 5.3 1015 Nm/year, is fourorder of magnitude less than the moment rate characteristic for the Vranceaintermediate-depth domain. The analysis of the fault plane solutions shows acomplex stress field in the Vrancea crust, like a transition zone from thecompressional regime at subcrustal depths to extensional regime characteristic

3 Probabilistic seismic hazard assessment in Romania 577

Fig. 1 – Seismic crustal active zones in Romania and adjacent areas and their characteristics.

for the entire Moesian platform [2]. The largest earthquakes, for which the faultplane solutions could be relatively well constrained, are the main shocks of thesequences of February 1983 (Mw = 3.5), April 1986 (Mw = 3.7), August–September 1991 (Mw = 3.5) and December 1997 (Mw = 3.2) generated in theRâmnicu Sarat region [3].

2.2. PREDOBROGEAN AND BARLAD DEPRESSION SOURCES

Predobrogean Depression (PD) zone belongs to the southern margin ofPredobrogean Depression marked by Sfantul Gheorghe fault (Fig. 1). Onlymoderate-size events are observed (Mw 5.3) clustered especially along SfantulGheorghe fault. The fault plane solutions reflect the existence of the extensionalregime of the deformation field. In our opinion this consistently reflects theaffiliation of the Predobrogean Depression to the Scythian platform tectonic unit.The rate of the seismic moment release is Mo = 1.8 1015 Nm/year. Themaximum observed magnitude for the Predobrogean Depression crustal zone isMw = 5.3, assigned to the event occurred on February 11, 1871.

Barlad Deppression (BD), situated NE of the Vrancea zone, is characterizedonly by moderate size events (only four shocks with Mw > 5.0, but not exceedindMw = 5.6). Considering that from seismotectonic point of view the Predobrogean


Depression belongs to the Scythian platform as well as Barlad Depression weconsidered the observed maximum magnitude for both zones, Mw = 5.6.

2.3. INTRAMOESIAN FAULT

The Intramoesian fault (IMF) crosses the Moesian platform in a SE-NWdirection, separating two distinct sectors with different constitution and structureof the basement. Although it is a well-defined deep fault, reaching the base of thelithosphere [4], and extending southeast to the Anatolian fault region [5], theassociated seismic activity is scarce and weak. Geological and geotectonic dataindicate only a relatively small active sector in the Romanian Plain, situated tothe NE from Bucharest.

The geometry of the Intramoesian fault source and the distribution of theearthquakes with Mw 3.0 occurred between 1892 and 2001 (30 events) arepresented in Fig. 1. The magnitude domain of earthquakes is Mw [3.0, 5.4].The maximum magnitude was recorded in January 4, 1960 (Mw = 5.4) in thecentral part of the Romanian Plain.

2.4. DULOVO AND SHABLA SOURCES

A significant increase of seismicity is observed in the Dulovo and Shablaregion (DU and SH), NE Bulgaria, where an earthquake with an estimatedmagnitude of Mw = 7.2 occurred in 1901. The focal depth, whenever it can beconstrained, has relatively large values (h ~ 35 km), suggesting active processesin the lower crust or in the upper lithosphere.

Fig. 1 presents the geometry of the Intramoesian fault source and thedistribution of the earthquakes with Mw 3.0 occurred between 1892 and 2001(20 events). The magnitude domain of earthquakes is Mw [3.0, 7.2]. Wepointed out that the greatest magnitudes are attributed to the two historicalearthquakes: October, 14, 1892 (Mw = 6.5) and March, 31, 1901 (Mw = 7.2) andno event with magnitude greater than 5 was reported after 1950 since then theinstrumental earthquake monitoring has become more used.

2.5. FAGARAS-CAMPULUNG-SINAIAAND TRANSILVANIAN DEPPRESSION CRUSTAL SOURCES

The sources are located in the Southern Carpathians, Romania, adjacentlyto the West of Vrancea seismic region and are part of the major dome uplift ofthe Getic Domain basement. They are bordered at Northern and Southern edgesby first order crustal fractures and consist of three seismogenic zones: Fagaras


zone containing Lovistea Depression and North Oltenia (FG), Campulung (CP)and Sinaia (SI) zones. The earthquake activity is related to intracrustal fracturesextending from 5 to 30 km depth.

The earthquakes in this zone are generated at South, on deep fracturesextending on inherited hercynian lines along NW and NE alpine origin directionsand at North, throughout Transylvania, along a stepped fault system separatingthe Carpathian orogen from its intermountain depression. In the western part ofFagaras Mountains, the earthquakes have a typical polikinetic character, withmany delayed aftershocks, especially for large events, as the one produced in1916. Preferential centers and lines of seismicity were identified after theoccurrence of the large earthquakes and the subsequent aftershock activity.

Most of the earthquakes are of low energy, but once per century a largedestructive event with epicentral intensity larger than VIII is expected in Fagarasarea. The last major shock occurred in January 26, 1916, Mw = 6.5, Io = VIII–IX.Fagaras seismogenic region is the second seismic source in Romania as concernsthe largest observed magnitude (Mw = 6.5), after the Vrancea intermediate-depthsource (with maximum magnitude Mw ~ 7.7–7.8).

The Transilvanian Deppression (TD) seismogenic zone is defined onlybased on historical information, with the maximum reported earthquakeMw = 6.5. The seismic activity at present is mostly absent.

2.7. THE CRUSTAL SOURCES FROM THE WESTERN PART OF ROMANIA

The western and southwestern territory of Romania is the most importantregion of the country as concerns the seismic hazard determined by crustalearthquakes sources. The seismic risk in the region is also very high due to localrisk factors and vulnerabilities: weak dwellings, old and unprotected buildings inthe large cities, dams and chemical factories, high density of localities, greattowns, and so on.

The seismogenic Danubian zone (DA) represents the western extremity,adjacent to the Danube river, of the orogenic unit of the Southern Carpathians. Therate of seismic activity is relatively high, especially at the border and beyond theborder with Serbia, across the Danube river. The magnitude does not exceed 5.6.

The fault plane solutions are available for three earthquakes (the largestearthquake Mw = 5.6 occurred in July 18, 1991) and indicate normal faultingwith the T axis striking roughly N-S, in agreement with the general extensionalstress regime in the Southern Carpathians.

The contact between the Panonnian Depression and the Carpathian orogenlies entirely along the western part of the Romanian border. Even if nosignificant tectonic or geostructural differences are noticed, two enhancements in


the seismicity distribution can be identified in two relatively distinct active areas:Banat zone (BA) to the south, and Crisana-Maramures zone (CM) to the north.

The seismicity of the Banat zone is characterized by many earthquakeswith magnitude Mw > 5, but not exceeding 5.6. The largest earthquake occurredafter 1900 is the one from July 12, 1991 (Mw = 5.6). Historical informationsuggests potential earthquakes greater than 6 in Crisana-Maramures, but onlyone event approaching magnitude 5 was reported in this century. The largestreported earthquake was Mw = 6.5 on October 15, 1834.

The Serbian seismogenic source named IBAR [6] is characterised by theoccurrence of numerous crustal earthquakes with Mw > 5.0. The largest earthquakeoccurred in the zone on April 08, 1893, has the magnitude Ms = 6.6 [7].

3. THE FREQUENCY-MAGNITUDE DISTRIBUTIONFOR THE DEFINED SOURCES

The frequency-magnitude distribution for Vrancea crustal earthquakes isdetermined on the magnitude interval [3.0, 5.2]:

lgNcum= –(0.91 0.08)Mw + (4.98 0.32) (1)

with the correlation coefficient R = 0.98 and the standard deviation = 0.18. Thedistributions are plotted in Figs. 2 and 3.

For Predobrogean Depression zone the frequency-magnitude distribution,estimated for the magnitude interval [3.0, 5.5], is presented in relation (2):

lgNcum= –(0.65 0.06)Mw + (3.55 0.26) (2)

with the correlation coefficient R = 0.97 and the standard deviation = 0.15. Thedistributions are plotted in Fig. 4.

Fig. 2 – (a) necumulative; (b) cumulative for Vrancea crustal earthquakes.


Fig. 3 – Reccurence relation and magnitude distribution function for Vrancea earthquakes.

Fig. 4 – (a) necumulative; (b) cumulative relation for earthquakes occurred in PredobrogeanDepression zone.

The frequency-magnitude distribution for Barlad Depression zone isdetermined on the magnitude interval [2.5, 5.5]:

logNcum = –(0.75 0.05)Mw + (4.15 0.20) (3)

with the correlation coefficient R = 0.99 and the standard deviation = 0.1, andthe annual number of earthquakes: 1.53479 eq/year. The distributions are plottedin Figs. 5 and 6.

The frequency-magnitude distribution for Intramoesian fault-Shabla-Dulovo crustal sources, determined for the magnitude interval [4.5, 7.2], ispresented in relation (4) and plotted in Fig. 7:

logNcum = –(0.51 0.03)Mw + (3.54 0.17) (4)

with R = 0.99 and = 0.08.


Fig. 5 – (a) necumulative; (b) cumulative relation for earthquakes occurred in Barlad Depressionzone.

Fig. 6 – Reccurence relation and magnitude distribution function for earthquakes occurred inBarlad Depression zone.

The frequency-magnitude distribution for Fagaras-Campulung-Sinaiacrustal source regions for a magnitude interval [4.0, 6.5], leads to equation (5):

lgNcum = –(0.67 0.21)Mw + (4.62 0.26) (5)

with the correlation coefficient R = 0.91 and the standard deviation = 0.24, forM > 5.0, and

lgNcum = –(0.47 0.1)Mw + (3.38 0.56) (6)

with the correlation coefficient R = 0.91 and the standard deviation = 0.21, forM > 4.0. The distributions are both plotted in Fig. 8.

The frequency-magnitude distribution for crustal earthquakes ocurred inCrisana-Maramures zone, was computed for two distinct regions CSM1(equation 7, Table 1) and CSM2 (equation 8, Table 1) for a magnitude intervalof [4.0, 5.6]:


lgNcum = –( 0,56 0.15)I + (4,6673 0.23) (7)

with R = 0.91 and = 0.24 and

lgNcum = –( 0,46 0.18)I + (3,670 0.25) (8)

with R = 0.90 and = 0.21.

Fig. 7 – (a) necumulative; (b) cumulative(c) reccurence relation for earthquakes occurred in IMF-Shabla-Dulovo zone.


Fig. 8 – The frequency-magnitudedistribution for Fagaras-Campulung-Sinaia crustal sources, for 2 sets of data.

Banat region on the magnitude interval [4.0, 5.6], for the entire timeinterval of the catalogue (equation 9) and after 1900 (equation 10):

lgNcum = –(0.82 0.08)Mw + (4.73 0.38) (9)

with the correlation coefficient R = 0.98 and the = 0.12.

lgNcum = –(0.74 0.06)Mw + (4.31 0.27) (10)

with the correlation coefficient R = 0.99 and the = 0.09.Danubian crustal earthquakes was determined for magnitudes between

[4.0, 5.6] for two time intervals. One for the whole catalogue of earthquakes(equation 11) and the other for the earthquakes occurred after 1900:

lgNcum = –(0.82 0.09)Mw + (4.94 0.46) (11)

with R = 0.97 and = 0.15.

lgNcum = –(0.62 0.06)Mw + (3.74 0.31) (12)

with R = 0.98 and = 0.10.The necumulative and cumulative distributions are plotted in Fig. 9, for

both time intervals and regions.The frequency-magnitude distribution for the Serbian seismogenic source

named IBAR is estimated for the magnitude interval [3.7, 6.6] in equation (13)and for the intensity interval [4.5, 9.0] in equation (14)

lgNcum = –(0.87 0.03)Ms + (5.68 0.18) (13)

with R = 0.99 and = 0.11,

lgNcum = –(0.50 0.03)Io + (4.82 0.19) (14)

with R = 0.98 and = 0.18, and are both plotted in Fig. 10.


Fig. 9 – necumulative; cumulative relation for Banat and Danuabian crustal sources ( M = 0.3).

Fig. 10 – The frequency-magnitudedistribution for IBAR zone.

In Table 1 are presented the characteristics of each source that will be used inthe seismic hazard assessment. In the table the Fagaras-Campulung-Sinaia zonehave been devided in 3 subzones: FG, CP and SI. The i value has been computedfrom the intensity-frequency distribution, using the formula from equation (15):

ln10i b (15)


Table 1

Input parameters for probabilistic hazard assessment using crustal sources

Seismicsources

Coordinates Averagedepth

Mmin Mmax b Imin Imax bi iActivity

rate

VRN 25.70/45.20-26.60/44.8026.82/46.10-27.72/45.87

20 3.0 5.9 0.91 2.5 6.5 0.60 1.38293 0.514526

PD 27.45/45.85-29.00/44.9027.72/45.87-29.30/45.20

10 3.0 5.6 0.65 2.5 6.5 0.42 0.99011 0.360254

BD 25.70/45.20-26.60/44.8026.82/46.10-27.72/45.87

10 2.5 5.6 0.75 2.0 6.5 0.49 1.12826 1.534712

IMF 26.06/44.76-27.36/44.0026.39/45.14-27.33/44.80

15 4.5 5.4 0.51 5.0 6.5 0.32 0.74466 0.034600

DUL 26.70/43.50-27.50/43.5027.00/43.92-27.92/43.78

15 4.5 7.2 0.51 5.0 9.0 0.32 0.74466 0.028000

SH 28.03/43.36-28.73/43.2828.31/43.72-28.90/43.71

15 4.5 7.2 0.51 5.0 9.0 0.32 0.74466 0.092600

FG 24.10/45.40-24.60/45.1524.10/45.90-24.80/45.60

15 4.0 6.5 0.76 5.0 8.5 0.50 1.15325 0.247403

CP 24.95/45.00-25.30/45.1024.95/45.50-25.30/45.50

15 4.0 5.0 0.66 5.0 6.0 0.44 1.01820 0.0865384

TD 23.50/46.00-24.50/46.1023.50/46.90-24.50/46.90

10 4.0 6.5 0.89 5.0 7.0 0.59 1.36774 0.010254

SI 25.05/45.70-25.50/45.4025.20/45.80-25.50/45.60

15 4.0 4.9 0.65 4.5 6.0 0.43 0.98781 0.076923

(continues)


Table 1 (continued)

Seismicsources

Coordinates Averagedepth

Mmin Mmax b Imin Imax bi iActivity

rate

CMS1 47.34/21.48-48.31/23.0047.96/24.84-47.00/23.36

15 4.0 6.5 0.56 4.5 8.0 0.56 1.29815 0.3762

CMS2 48.26/19.60-48.72/22.7248.24/23.24-47.81/20.24

15 4.0 6.5 0.46 5.0 8.0 0.46 1.06699 0.1027

DAN 21.00/44.90-22.30/44.1521.80/45.70-23.00/44.80

15 4.0 5.6 0.71 5.5 9.0 0.43 0.98091 0.276700

BAN 21.00/44.90-21.90/45.8020.70/4610-21.30/46.40

10 4.0 5.6 0.82 5.5 9.0 0.50 1.16056 0.128800

IBAR 19.80/44.00-20.80/44.6021.00/43.10-21.80/43.80

14 3.7 6.6 0.87 4.5 9.0 0.54 1.24340 0.556203

4. ATTENUATION LAWS

It is essential, for a probabilistic estimation of the seismic hazard, toconstrain as much as possible how the energy of the seismic waves attenuateswhen propagating from the source to the site. The attenuation law for the crustalsources is given in the equation (16):

I = Io – c1 log(Dh/h) – c2 log(e) (Dh – h); (16)

where: c1, c2 and a are different for each region; log(e) = 0.006514; Dh is the hypo-central distance, and h is the depth presented, for each seismic source, in Table 1.

For Vrancea, Barlad and Predobrogean Deppression we have used:c1 = 3.16, c2 = 3.02 and a = 0.0015 (1/m).

For Fagaras-Campulung-Sinaia and Transilvanian Depression zones wehave used: c1 = 3.46, c2 = 3.12 and a = 0.0013 (1/m).

For the active zones from the southern and western part of Romania (IMF,BA, DA, CSM1 and CSM2), from Bulgaria and from Serbia (IBAR) we haveused the attenuation law (equation 17) obtained by [8], with c1 = 3.0, c2 = 3.0and = 0.0015 (1/m).

I = I0 – 3.0log(Dh/h) – 3.0 log(e) (Dh – h) (17)


5. CRUSTAL SEISMIC HAZARD ASSESSMENT

For the input data set obtained in the present work, we applied thealgorithm of [1] to compute the seismic hazard map of Romania in the case ofcrustal earthquakes. We present, in the following pictures, the hazard maps interms of macroseismic intensities for different return periods (50, 100 years –Fig. 11, 150, 200 and 475 years – Fig. 12 and 1000 and 2000 years – Fig. 13).

The hazard values are in good agreement with the deterministic approach.If we compare our results for this case for 50 years return period with thecomputed intensities (converted from the peak ground displacement, velocity oracceleration) obtained by [9] using the deterministic approach, the differencesare not exceeding 0.2 degrees in intensity, less than 0.5, the minimum measuringunit for intensities.

We see that all the input parameters are directly related to the hazardpattern, as expected, and each of them is a crucial parameter in the seismic

Fig. 11 – Hazard map for returnperiods of 50 and 100 years.

Fig. 12 – Seismic hazard curves forRomania, in terms of macroseismicintensities for return periods of 150,200 and 475 years – using only crustal seismic active zones.


Fig. 13 – Seismic hazard curvesfor Romania, in terms of macro-seismic intensities for returnperiods of 1000 and 2000 years –using only crustal seismic active zones.

hazard probabilistic approach. The Romanian seismic data used in the paperwere obtained from [10] and outside the borders from [7] and [11].

6. CONCLUSIONS

This work is a useful tool for the assessment of the seismic risk andimplementation of antiseismic protection measures in the case of specialconstructions and strategic objectives, such as, nuclear power plants (CernavodaNPP), large cities situated outside the Vrancea intermediate seismic sourceinfluence zone (Timisoara, Turnu Severin, Arad, Cluj) and hidroenergetic largeconstructions.


REFERENCES

1. R. K. McGuire, EQRISK-Evaluation of Earthquake Risk to Sites, United States Department ofthe Interior, USGS, Open File Report No. 76–67, 1976.

2. M. Radulian, N. Mândrescu, G. F. Panza, E. Popescu, A. Utale, Characterization ofseismogenic zones of Romania, Pure Appl. Geophys., 157, 57–77, 2000.

3. E. Popescu, M. Radulian, Source characteristics of the seismic sequences in the EasternCarpathians foredeep region (Romania), Tectonophysics, 338, 325–337, 2001.

4. D. Enescu, B. D. Enescu, Contributions to the knowledge of the genesis of VranceaEarthquakes, Romanian Reports in Phys. 45, 777–796, 1993.

5. M. Sandulescu, Geotectonics of Romania. Ed. Tehnica, Bucuresti, 1984, p. 334 (inRomanian).

6. R. M. W. Musson, Generalised seismic hazard maps for the Pannonian Basin usingprobabilistic methods, Pure Appl. Geophys., 157, 147–169, 2000.

7. N. V. Shebalin, G. Leydecker, N. G. Mokrushina, R. E. Tatevossian, O. O. Erteleva, V. Yu.Vassiliev, Earthquake Catalogue for Central and Southeastern Europe 342 BC - 1990 AD.European Commission, Report No. ETNU CT 93 - 0087, Brussels, 1998.

8. T. Zsiros, Macroseismic focal depth and intensity attenuation in the Carpathian Region, ActaGeod. Geophys. Hung., 31 (1–2), 115–125, 1996.

9. M. Radulian, F. Vaccari, N. Mandrescu, G. F. Panza, C. Moldoveanu, Seismic Hazard of theCircum-Pannonian Region, eds. G. F. Panza, M. Radulian, C.-I. Trifu, Pure Appl. Geophys.157, 221–247, 2000.

10. ***, Romplus catalog-updated until 2006 by the department of Data Acquisition of theNational Institute for Earth Physics-Bucharest.

11. ***, International Seismological Center, On-Line Bulletin, http://www.isc.ac.uk, Thatcham,U.K.

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