DEEP SEISMIC SOUNDING ACROSS THE VRANCEA REGION

Transcription

1 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania DEEP SEISMIC SOUNDING ACROSS THE VRANCEA REGION V. Raileanu 1, F. Hauser 2, 4, A. Bala 1, W. Fielitz 2,5, C. Prodehl 2, C. Dinu 3, and M. Landes 2 ABSTRACT Two major seismic refraction lines have been recorded across the Vrancea region: VRANCEA'99, in the N-S direction from Bacau to south of Bucharest and VRANCEA'2001, in the E-W direction from the Black Sea near Tulcea to Aiud in Transylvania. Based on P- and S-wave data, two crustal models were derived which display a multi-layered structure for the sediments and the crystalline crust with variable thicknesses along the lines. The thickness of the sediments increases from 1-2 km in N Dobrogea to about 20 km in the Focsani Basin and reaches km under the Carpathians. Moho depth is variable from. 38 km near Bacau to 46 km under the Focsani Basin, km under the Carpathians, and 34 km under Transylvania. INTRODUCTION The Vrancea region is known as a seismic active area where 2-3 strong earhquakes at intermediate depths occur per century. They evidently result from the geodynamic activities connected to the Eastern Carpathians orogen. The geodynamic setting of the Vrancea region is however highly complex and not yet fully understood. This is why the Eastern Carpathians of Romania became the target of a joint German-Romanian research program, aiming to study the structure that cause the strong and deep-seated intermediate-uppermantle earthquakes. Seismic-refraction studies and a large-scale tomography study of southeastern Romania were part of an interdisciplinary research program, carried out by the Collaborative Research Center 461 (CRC 461) Strong Earthquakes - A Challenge for Geosciences and Civil Engineering at the University of Karlsruhe (Germany) and the Romanian Group for Vrancea Strong Earthquakes (RGVE) at the Romanian Academy in Bucharest (Wenzel, 1997, Wenzel, 1998a, b). Two major active-source seismic refraction experiments were carried out in the eastern Carpathians of Romania in 1999 and 2001 as a contribution to this research program (Hauser, 2001, 2002, 2007). They were designed to study the crustal and uppermost mantle structure to a depth of about 70 km underneath the Vrancea epicentral region and were jointly performed by the Geophysical and Geological Institutes of the University of Karlsruhe (Germany), the National Institute for Earth Physics in Bucharest (Romania) and the University of Bucharest (Romania). The two seismic refraction lines traverse several major tectonic units: the Moesian and Scythian platforms, the North Dobrogea Orogen, the Eastern Carpathian Orogen and the Transylvania Basin, (Fig.1). The Carpathian Orogen and its contact with adjacent units are the most complex structure both near surface level and at the greater depth as will be seen from the seismic sections of two seismic refraction lines. 1 National Institute for Earth Physics, Bucharest, Romania, 2 University of Karlsruhe, Germany, 3 University of Bucharest, Romania, 4 Dublin Institute for Advanced Studies, Ireland, 5 University of Heidelberg, Germany.

2 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 81 Figure 1. Location map of the seismic-refraction VRANCEA profiles 1999 (N-S line) and 2001 (W-E line) across the seismogenic Vrancea zone in the eastern Carpathians of Romania (Hauser, 2002). V = Vrancea seismogenic zone with strong intermediate-depth earthquakes between 70 and 160 km depth. VRANCEA 99 AND VRANCEA 2001 EXPERIMENTS The VRANCEA 99 seismic refraction experiment (Hauser, 2001) was a 300 km long refraction profile and was recorded between the cities of Bacau and Bucharest with 12 shot points A to N, traversing the Vrancea epicentral region in NNE SSW direction (Fig.1). It was complemented by a short perpendicular line, which ran transverse to the geological structures along the Putna Valley, north of the seismogenic Vrancea zone intersecting with the main refraction line at shot point D and with two additional shot points R and S. Along these two segments a total of 140 recording sites were occupied with an average station spacing of 2 km. Seismic recording equipment for the experiment was provided by the GeoForschungsZentrum Potsdam (Germany), by Leicester University (UK) and by the NERC geophysical equipment pool (UK). The spacing of shot points ranged from km, with an average of 22 km. The charge sizes varied from 300 kg to 900 kg with the larger shots at the end points of the main line. All shots were recorded simultaneously along the main line and the transverse line. Hauser (2000) and Landes (2004b) have compiled data and other technical details in data reports. The second experiment across the Vrancea zone, VRANCEA 2001, was a 700 km long WNW ESE trending (Fig.1) seismic refraction line and was carried out in August/September 2001 in Romania with a short extension into Hungary (Hauser, 2002, 2007). The main part ran from the Transylvanian Basin across the East Carpathian Orogen and the Vrancea seismic region to the foreland areas with the very deep Neogene Focsani Basin and the North Dobrogea Orogen on the Black Sea. Ten large drill hole shots with kg charges (O to Z in Fig.1) were fired in Romania between Aiud at the western margin of the Transylvanian Basin and the Black Sea which resulted in an average shot point spacing of 40 km. To the west an additional shot (500 kg) was fired in Hungary. They generated 11 seismogram sections recorded by almost 800 geophones. The spacing of the geophones was variable. It was around 1 km from the eastern end to Aiud (ca. 450 km length), 6 km

3 82 V. Raileanu et al. from Aiud to Oradea (Romanian-Hungarian border) and about 2 km on the Hungarian territory. Between shot points T and U the geophones were deployed at a spacing of 100 m (Panea, 2005). In total, 790 recording instruments were available. The 640 one-component geophones (TEXAN type) were mostly deployed in the open field outside of localities, while, for safety reasons, the 150 three-components geophones (REFTEK and PDAS type) were deployed in guarded properties within towns and villages. The experiment was jointly performed by the research institutes and universities of Germany, Romania, the Netherlands and the United States. They included the University of Karlsruhe and The GeoForschungszentrum Potsdam, Germany, the Free University of Amsterdam, Netherlands, the National Institute for Earth Physics and the University of Bucharest, Romania, and the Universities of El Paso and South Carolina, USA. Field recording instruments were provided of University of El Paso and PASSCAL, USA (TEXAN 1-component stations), and the GeoForschungsZentrum, Germany (3-component REFTEK and PDAS stations). DATA INTERPRETATION AND VELOCITY MODELS The P-wave interpretation of the VR 99 seismic line (Fig.2) showed up to 10 km thick sedimentary sequences. Both the sedimentary and crystalline upper-crustal layers appeared to thicken underneath the seismogenic Vrancea zone and the adjacent southern margin of the Carpathians. As a result, the intra-crustal discontinuity varies strongly in depth, between 18 km at both ends of the line and 31 km at its centre. Strong wide-angle PMP reflections indicated the existence of a first-order Moho, whose depth also varies, from 30 km near the southern end of the line and 41 km near the centre. Within the uppermost mantle a low velocity zone was interpreted from a reflection, named PLP, which defined the bottom of this low velocity layer at a depth of 55 km (Hauser, 2001). Figure 2. Crustal P-wave cross section along the north-south profile VRANCEA 1999 (from Hauser, 2001, Fig. 9). P-wave velocities in km/s. S-wave velocities in between brackets.

4 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 83 The data also showed reasonable appearance of S-waves, which were incorporated into the P-wave model (corresponding velocities are shown in brackets in Fig. 2) and physical properties were calculated for the individual crustal layers (Raileanu, 2005). The data interpretation for the eastern 450 km long segment of the Vrancea 2001 seismic line (Hauser et al., 2007) indicated a multi-layered structure with variable thicknesses and velocities. The sedimentary stack comprised up to 6 layers (L1-L6 in Fig. 3) with seismic velocities of km/s and reached a maximum thickness of more than 15 km within the Focsani Basin area. The underlying crystalline crust (L7-L10 in Fig. 3) showed considerable thickness variations in total as well as in its individual subdivisions. The model (Fig. 3) shows lateral velocity structure of these blocks along the seismic line which remained almost constant with about 6.0 km/s along the basement top and with 7.0 km/s above the Moho. Under the Transylvanian basin the crust appeared to be 34 km thick with low velocity zones in its uppermost 15 km. Under the Carpathians the Moho deepens to below of 41 km, reaching 46 km under the Focsani basin, but the crystalline crust does not exceed 25 km in thickness and is covered by up to 15 km of sedimentary rocks. The North Dobrogea crust reaches a thickness of about 43 km and was interpreted as thick Eastern European crust overthrusted by a thin 1-2 km thick wedge of the North Dobrogea Orogen (Hauser, 2007). Figure 3. Crustal P-wave cross section along the west-east profile VRANCEA P-wave velocity in km/s (from Hauser, 2007, Fig. 11). The two seismic-refraction campaigns and the teleseismic tomography survey of 1999 (Wenzel et al.,1998b) produced a wealth of data, concentrating on the Vrancea zone proper and allowed a three-dimensional tomographic interpretation of the crustal data which were presented in contour maps at various depth levels (Landes, 2004a). The 3-D model reveals lateral velocity variations, which allows to distinguish between foreland platform areas, foreland basins and the Carpathian Orogen. Clear velocity differences between the foreland

5 84 V. Raileanu et al. basins and the Focsani Basin indicate different pre-miocene sedimentary compositions and geological evolutions of the foreland platforms. The involved Moesian and Scythian platforms are separated by the Trotus Fault system, which is observed as a velocity discontinuity. An upper crustal high-velocity zone is explained as a Middle Pliocene to Pleistocene E W oriented out-of-sequence thrust of the crystalline basement, below the decollement of the flysch nappes. (b) Figure 4. (a) Horizontal tomographic slices of the P-wave velocity model at 4.5, 8.5 and 12.5 km depth. The VRANCEA 1999 and 2001 seismic profiles are indicated with its shot points (red stars). Region marked by polygon represents Vrancea zone.vertical ( y) and horizontal (x) axes in km. (b) Geological interpretation of the horizontal tomographic P-wave velocity slices of Fig. 4.a. (Landes, 2004b, Figs.10.a and 11).

6 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 85 ACKNOWLEDGEMENTS The field works for the data acquisition was made possible by the effort of many students from the Universities of Amsterdam, Bucharest and Karlsruhe. The National Institute for Earth Physics and the University of Bucharest provided the logistics for the fieldwork. PROSPECTIUNI S.A. company, Bucharest conducted the drilling and shooting operations. Data were collected using the seismic equipment from the GeoForschungsZentrum Potsdam as well as from IRIS /PASSCAL at Socorro, New Mexico and the University of Texas at El Paso. The Deutsche Forschungsgemeinschaft funded the project through the Collaborative Research Centre 461 at the University of Karlsruhe, Germany. The Romanian Ministry for Education and Research funded the Romanian researchers in this project via the CERES program. The NATO Science Collaborative Research Linkage Grant assisted the project by additional travel funding. REFERENCES Hauser, F., V.Raileanu, C.Prodehl, A.Bala, A.Schulze and P.Denton, The Seismic- Refraction Project VRANCEA-99. Open-File Report, Geophysical Institute, University of Karlsruhe. Hauser, F., V.Raileanu, W.Fielitz, A.Bala, C.Prodehl and G.Polonic, The crustal structure between the southeastern Carpathians and the Moesian platform from a refraction seismic experiment in Romania. Tectonophysics, 340: Hauser, F., C.Prodehl, M.Landes and the VRANCEA Working Group, Seismic experiments target earthquake-prone region in Romania. EOS, Trans. Am. Geophys. Un., 83: 457, Hauser, F., V.Raileanu, W.Fielitz, C.Dinu, M.Landes, A.Bala and C.Prodehl, Seismic crustal structure between the Transylvanian Basin and the Black Sea, Romania. Tectonophysics, 430: Landes, M., W.Fielitz, F.Hauser, M.Popa and the CALIXTO Group, 2004a. 3-D upper-crustal tomographic structure across the Vrancea seismic zone, Romania. Tectonophysics, 382: Landes, M., F.Hauser, V.Raileanu, A.Bala, C.Prodehl, J.Bribach, S.Harder, E.Hegedues, R.G.Keller, R.A.Stephenson, V.Mocanu, C.Dinu and C.Diaconescu, 2004b. VRANCEA 2001 data processing and seismic sections. Open-file report, Geophys. Inst. Univ. Karlsruhe, 57pp. & seismic sections. Panea, I., R.Stephenson, C.Knapp, V.Mocanu, G.Drijkoningen, L.Matenco, J.Knapp and C.Prodehl, Near-vertical seismic reflection image using a novel acquisition technique across the Vrancea Zone and Foscani Basin, south-eastern Carpathians (Romania), In: Cloetingh, S., Matenco, L. Bada, G. Dinu, C., and Mocanu, V. (eds.), The Carpathians - Pannonian Basin System: Natural Laboratory for Coupled Lithospheric - Surface Processes.Tectonophysics, 410 (1-4): Raileanu, V., A.Bala, F.Hauser, C.Prodehl and W.Fielitz, Crustal properties from S- wave and gravity data along a seismic refraction profile in Romania. In: Cloetingh, S., Matenco, L. Bada, G. Dinu, C., and Mocanu, V. (eds.), The Carpathians - Pannonian Basin System: Natural Laboratory for Coupled Lithospheric - Surface Processes. Tectonophysics, 410: Wenzel, F., Strong Earthquakes: A challenge for geosciences and civil engineering - a new Collaborative Research Center in Germany, Seismol. Res. Lett., 68: Wenzel, F., D.Lungu and O.Novak (eds), 1998a. Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation. Selected papers of the First International Workshop on Vrancea Earthquakes, Bucharest, November 1-4, Kluwer Academic Publishers, Dordrecht, Netherlands, 374pp. Wenzel, F., U.Achauer, D.Enescu, E.Kissling, R.Russo, V.Mocanu and G.Mussachio, 1998b. The final stage of plate detachment; International tomographic experiment in Romania aims to a high-resolution snapshot of this process. EOS, 79: 589,