Seismic Hazard, Vulnerability and Risk for Vrancea Events
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International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania SEISMIC HAZARD, VULNERABILITY AND RISK FOR VRANCEA EVENTS Dan Lungu 1, Cristian Arion 1, Alexandru Aldea 1, Radu Vacareanu 1 ABSTRACT The paper presents the probabilistic seismic hazard analysis used for constructing the actual seismic hazard map of Romania. The new edition of the code for design of earthquake-resistant buildings and structures in Romania P100 was just issued in 2006 and follows the format and contents of Eurocode 8. Based on the available data obtained from more than 400 boreholes and using the GIS techniques, significant soil parameters were mapped for the territory of city of Bucharest and will allow seismic microzonation of Bucharest to be used as a tool for urban planning and earthquake risk reduction. The paper also explains the synergy between national programs and international projects as: JICA Project “Seismic Risk Reduction for Building and Structures in Romania” (2002-2008), World Bank Hazard Risk Mitigation and Emergency Preparedness Project in Romania (2004-2009) - Component B: Earthquake Risk Reduction, RISK-UE Project “An advanced approach to earthquake risk scenarios with application to 7 European towns” (2001-2004) and European Project PROHITECH “Earthquake Protection of Historical Buildings by Reversible Mixed Technologies” (2004 -2008). INTRODUCTION With about 2 millions inhabitants and 110,000 buildings Bucharest can be ranked as the megacity having the highest seismic risk in Europe due to (i) soft soil condition in Bucharest characterized by long predominant period (1.4 ÷ 1.6s) of ground vibration during strong Vrancea earthquakes and (ii) high fragility of tall reinforced concrete buildings built in Bucharest before 1940 and even before the 1977 big Vrancea earthquake. The city is located in the alluvial Romanian Plain, between the Danube and the Carpathian Mountains. Bucharest city is built in the meadow area of two rivers, Colentina and Dambovita, that cross the region from NW to SE., Fig. 1. VRANCEA EARTHQUAKES CATALOGUES AND BUCHAREST EARTHQUAKE RECORDS Seismic hazard in Romania is due to the Vrancea subcustral source located at depths between 60 and 180km where the Carpathians Mountains Arch bends, Fig.2. Vrancea subcrustral source affects more than 2/3 of the territory of Romania and an important part of the territories of Republic of Moldova, Bulgaria and Ukraine. According to the 20th century seismicity, the epicentral Vrancea area is confined to a rectangle of 40 x 80 km 2, having the long axis oriented NE-SW and being centred at about 45.6 o Lat N and 26.6 o Long E (i.e. about 130 km NE from Bucharest) Fig. 2. 1 Technical University of Civil Engineering, 124 Lacul Tei Blvd., Bucharest 020396, Romania Email: [email protected], [email protected], [email protected], [email protected] 292 D. Lungu et al. Altitude, m 90 85 Colentina River 80 75 City Centre 70 Dambovita River 65 60 Figure 1) Topography of Bucharest Figure 2) Vrancea seismic zone in Carpathians Mountains of Romania Vrancea earthquakes prove a significant mobility of their epicenters: 1940 and the 1990 events epicenters were located towards NE, while the 1977 event epicenter was located towards SW (i.e. Bucharest), and in 15 seconds during the 1977 event, that epicenter moved about 65 km from NW to SE. It is emphasized that the damage of Vrancea subcrustal earthquakes is the combined result of both magnitude and depth: evidence shows that in Bucharest, the 1977 earthquake (moment magnitude M W ≅ 7.5 at depth h=109 km) produced much greater damage and losses than the 1940 earthquake (M W ≅ 7.7 at h = 150 km). Two catalogues of the earthquakes that occurred on the territory of Romania were compiled, more or less independently, by Radu (1974, 1980, 1995) and by Constantinescu & Marza (1980, 1995), Table 1. The Radu’s catalogue is more complete, while the majority of significant events are also included in the Constantinescu & Marza catalogue. The magnitude in Radu catalogue is the Gutenberg-Richter magnitude, M GR . The magnitude in Constantinescu & Marza catalogue is the surface magnitude, M S, later tacitly assimilated as M GR (Marza, 1995). The Constantinescu & Marza catalogue has been converted in terms of moment magnitude, M W, into infp.ro catalogue. Even the 1802 (M GR =7.5) event is generally considered the largest Vrancea earthquake ever occurred, the largest seismic losses ever experienced were during the 1977 event (M GR =7.2), Figs.3, 4 and 5. Table 1. Catalogue of 20th century subcrustal Vrancea earthquakes ( Mw ≥ 6.3 ) Date Lat. N 0 Long. E ° Radu Marza www.infp.ro Catalogue, 1994 Catalogue, 1980 Catalogue, 1998 1) h, km I0 MGR Mw I0 Ms Mw 1903 Sept 13 45.7 26.6 >60 7 6.3 - 6.5 5.7 6.3 1904 Feb 6 45.7 26.6 75 6 5.7 - 6 6.3 6.6 1908 Oct 6 45.7 26.5 150 8 6.8 - 8 6.8 7.1 1912 May 25 45.7 27.2 80 7 6.0 - 7 6.4 6.7 1934 March 29 45.8 26.5 90 7 6.3 - 8 6.3 6.6 1940 Oct 22 45.8 26.4 122 7 / 8 6.5 - 7 6.2 6.5 1940 Nov 10 45.8 26.7 150 9 7.4 - 9 7.4 7.7 1945 Sept 7 45.9 26.5 75 7 / 8 6.5 - 7.5 6.5 6.8 1945 Dec 9 45.7 26.8 80 7 6.0 - 7 6.2 6.5 1948 May 29 45.8 26.5 130 6 / 7 5.8 - 6.5 6.0 6.3 1977 March 4 2) 45.34 26.30 109 8 / 9 7.2 7.5 9 7.2 7.4 1986 Aug 30 45.53 26.47 133 8 7.0 7.2 - - 7.1 1990 May 30 45.82 26.90 91 8 6.7 7.0 - - 6.9 1990 May 31 45.83 26.89 79 7 6.1 6.4 - - 6.4 1) Maximum seismic intensity 2) Main shock International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 293 $ 29 $32 $33 $1 $ Collapsed building $31 8 2 Land use 9 $ 17 $ 3 16 18 Street $ $ $ $ $ 4 19 Urban built zone $ 27 $ 24 $ $ Rural built zone 5 Lake, river, canal 20 21 10 $ $$ 28 Park $ 22 $ $ 6 7 Forest 25 $ 12 11 $$ 26 $ $ 23 13 $ Garden $$ Cemetery Economic zone Agricultural zone 15 $ $14 N W E 1 0 1 2 Kilometers S ArcView GIS 3.2 - ESRI California Figure 3) Collapsed buildings during the 1977 Vrancea earthquake in central Bucharest Figure 4) 1977 earthquake: Dunarea Figure 5) Partial collapse of Faculty of building collapse (pre-war RC structure) Chemistry on Dambovita river The first strong ground motion recorded in Romania was the 1977 record in Eastern Bucharest, at seismic station of INCERC, National Institute for Building Research, on a Japanese SMAC - B instrument. The ground motion was digitized and analysed by Building Research Institute, Ministry of Construction , Japan, 1978. "Digitized data of strong-motion earthquake accelerograms in Romania (March 4, 1977)" by Observational Committee of Strong Motion Earthquake, Kenchiku Kenkyu Shiro No.20, January.1.4. The unusual 1977 record, characterized by a long predominant period of ground vibration , T p ≅ 1.6s, has been used for calibrating design response spectra in Romanian seismic code for the period 1977- 1992 when almost 40% in Bucharest buildings stock has been built. After the 1977 earthquake a significant ground motion database of about 40 records was collected in Bucharest during the 1986 & 1990 Vrancea earthquakes. Based on the very important conclusions from 1977, 1986 and 1990 earthquakes, the Seismic instrumentation of Bucharest has been recently extended and improved by various national and international efforts. Presently Romania has more than 100 digital K2 & ETNA, Kinemetrics instruments and only in the last 2 years Romania installed 50 digital K2 and ETNA , Kinemetrics instruments, more than half in Bucharest. 294 D. Lungu et al. ISC, State Inspectorate in Constructions, JICA, Japan International Cooperation Agency, Project Reduction of Seismic Risk for Buildings and Structures in Romania, SFB, German Research Foundation, Project 461 on Vrancea earthquakes. Figure 6) Seismic networks of Romania Figure 7) Seismic networks of Bucharest BUCHAREST SEISMIC HAZARD Magnitude recurrence From regression of recent events data, the moment magnitude and the Gutenberg-Richter magnitude can be related as: Mw ≅ M GR + 0.3 6.0 < M GR < 7.7 (1) th From 20 century catalogue of events having Mw > 6.3, the average number per year of Vrancea subcrustal earthquakes with magnitude equal to and greater than Mw is: log n( ≥Mw) = 3.76 - 0.73 M w (2) Since the source magnitude is limited by an upper bound magnitude Mw,max , the recurrence relationship can be modified (Elnashai and Lungu, 1995) as: − .1 687 1.8( −M ) − 1− e w n()≥ M = e .8 654 .1 687 Mw (3) w − − 1− e .1 687 1.8( )3.6 were the threshold lower magnitude is Mw0 =6.3 and the maximum credible magnitude of the source has been considered: Mw,max = 8.1. Attenuation relationships for Vrancea subcrustal source The envelope of the peak ground acceleration recorded in Romania during the last 3 strongest Vrancea events are interpolated in Fig.8 and shows the NE directivity of the subcrustal Vrancea source. The database of Vrancea strong ground motions contains records from 47 free-field stations in Romania distributed on networks and events. The free field accelerograms were obtained at the ground level or the basement of 1 - 2 storey buildings.