Seismicity in western part of Romania is the result of tectonic evolution, which created a fragmented structure at the crystalline basement level, with blocks that have suffered differential movements due to general tectonic stress in the area, and due to secondary factors such as erosion or lateral variations in density. Some of the faults formed during development of the units under survey were reactivated later in recent periods of stress and became seismogenic faults. The present paper is an analysis of tectonics and seismicity in western part of Romania (Pannonian depression and Transylvanian Basin, and the Apuseni Mountains Orogen). Several maps interpreted by different Romanian authors on local tectonics are presented and a final map with active faults in the region of study is constructed. The first part is a summary of the stress field in the Earth crust and tectonic evolution of the Carpathian area with particular reference to the units analyzed. Each unit is analyzed based on published sources, finally emphasizing the peculiarities of each area and tectonic fault lines known in particular. In a later chapter is a summary of information on the behavior of the stress field of study areas and seismicity zone, the range of magnitude and hypocenter depths registered with mention of the most significant events occurring over time and relative to areas where they were concentrated. In the last chapter fault systems in the study region are presented, their peculiarities as they appear in the available studies projected on the local tectonic structure for each of the areas under examination. Results are reported using the tectonic map of Romania, on which epicentres of earthquakes in the catalog ROMPLUS (NIEP Catalog) by the end of 2010 are projected.

A set of crustal data collected in the last decades across the Moesian platform are processed in order to derive a crustal model. New crustal model comprises maps with depths to the top of lower crust and Moho and thicknesses of the crystalline upper and lower crust. Some ranges of mean P-wave velocity in the upper and lower crust are assigned for the eastern and western sectors of platform.

The goal of the study is to analyze the distributions of peak ground acceleration, velocity and displacement from 18 moderate size undercrustal earthquakes (M-w 4.0-5.0), which occurred in the Vrancea region during the period 2008-2010. The patterns of the investigated ground motion parameters keep the general features observed for the strong Vrancea earthquakes - a rapid decrease of the ground shaking in SE-NW direction, and a prominent asymmetry of the isolines. The variability noticed among the individual distributions appears as a result of the combined effects of the hypocenter location within the seismogenic zone (focal depth and epicenter position) and diversity of the focal mechanisms.

A new crustal model for the central and north-eastern part of Romania is relying on the largest database available at this moment. In comparison with previous crustal models the new model takes into account all known data: old and new seismic refraction data, deep seismic reflection data and seismology data recorded by the broadband stations belonging to the Romanian seismic network for monitoring of earthquakes. The new crustal model reveals the topography of the Moho and top of the lower crust discontinuities as well as the areal distribution of the mean P-wave seismic velocities across the Moldavian and Scythians platform and partially for the Eastern Carpathians. The new configuration of the Moho topography map brings some details in the central study area.

The goal of this work is to test the capability of high frequency local waveform inversion to retrieve the source of low magnitude intermediate depth earthquakes of Vrancea region. Following a similar procedure to that previously developed to analyze the weak shallow events from the Eastern Carpathians bending zone, we obtain a fairly robust estimate of the orientation of the double couple component of the seismic moment tensor. The resolution of the resolved fault plane solution allows correlations with the stress field in the area; this opens perspectives to using the small subcrustal earthquakes, frequently recorded, to obtain detailed information on the process taking place in the Vrancea focal region.

In seismic microzonation we want to display the variation in seismic response of the subsurface and subsequently determine where the soil is being amplified to a level that may damage existing buildings or other structures. Frequently peak ground acceleration (PGA) is used to determine the maximum horizontal forces that can be expected. The method is not always adequate, because PGA often corresponds to high frequencies, which are out of range of the natural frequencies of most structures. The largest amplification of the soil will occur at the lowest natural frequency or its fundamental frequency, which corresponds to the characteristic site period. In situ measurements of shear wave velocity in the soil and the soil thickness, provide a direct measure of the characteristic site period. Extensively seismic noise measurements provides a more accessible method and computed H/V spectral ratios can also provide a good indication on the fundamental frequency of the site. Average shear wave velocity in the first 30 m depth (V(S-30)) as defined in EUROCODE 8 and Romanian Code P100-1 is a useful indicator in seismic microzonation, showing zones with low values of average seismic velocities in Bucharest.

The purpose of our study is to investigate the ground motion characteristics in 15 sites located in the Eastern part of Romania by applying the H/V spectral ratio method to the data (S and coda waves) recorded during the CALIXTO'99 tomography experiment. The results show no significant differences as regarding the resonant frequencies of the spectral ratios computed for the two types of waves, while the level of the amplification for S-wave is slightly higher than for coda waves. Only for two sites, located on thick Quaternary deposits, the amplification obtained from S-wave is larger by a factor of 2 than the amplification obtained from coda waves at low frequencies. In the studied locations the ground motion amplification varies by a factor of 2 to nearly 6 over the frequency range of 0.5 to 10-12 Hz.

Molinari I. , Kaeser M. , Morelli A. , Effects of the representation of the crustal structure on seismic wave propagation modeling on the continental scale, AGU abstract, 2009 Behm et al., 2006. A new seismic model of the E Alpine Crust, in: First European Conference on Earthquake Engineering and Seismology (13thECEE & General Assembly of the ESC), 55Geneva Switzerland, 3-8 Sept.2006. Sumanovac, F., Orescovic, J., Grad, M., and ALP02 working group, 2009. Crustal structure atthe contact of the Dinarides and Pannonian basin based on 2-D seismic and gravity interpretation of the ALP07 profile in the ALP02 experiment. Geophys.J.Int. 179, 615-633. Sroda, P et al., 2006, Crustal upper mantle structure of the W Carpathians from CELEBRATION 2000 profiles CEL01 and CEL 04: seismic models and geological implications. Geophys.J.Intern. 167, 737-760. Guterch A, Grad, M., 2006. Lithospheric structure of the TESZ in Poland based on modern seismic experiments. Geological Quaterly, 2006, 50(1), 23-32. Guterh et al., (2003). CELEBRATION 2000 seismic experimen, Stud.Geoph.geod., 47(3), 659-669. Ruzek, B., Hrubcova, P., Novotny, M., Spicak, A., Karousova, O. 2007. Inversion travel times obtained during active seismice refraction experiments CELEBRATION 2000, ALP02 and SUDETS 20003. Studia Geophysica et Geodaetica, 51 (2007), 141 – 166. Hrubcova et al., 2005. Crustal and uppermost mantle structure of the Bohemian Massif based on CELEBRATION 2000 data. Journ.Geoph.Res, 110, B11305. Grad, M., et al., 2009. Crustal structure of the Eastern Alps and their foreland: seismic model beneath the CEL10/Alp04 profile and tectonic implications. Geophys.J.Intern. 177, 279-295 Hrubcova, P. and Sroda, P. 2008. Crustal structure at the easternmost termination of the Variscan belt based on CELEBRATION 2000 and ALP 2002 data. Tectonoph. 460, 55–75. The DOBREfraction’99 Working Group, 2003. ‘‘DOBREfraction’99’’—velocity model of the crust and upper mantle beneath the Donbas Foldbelt (East Ukraine), Tectonoph., 2003, 371, 84-110. Tybo et al., 2003. Upper lithospheric seismic velocity structure across the Pripyat Trough and the Ukrainian Shield along the EUROBRIDGE’97 profile. Tectonoph.371, 41-97. Kostyucenko, S.L. et al., 2004. The evolution of the southern margin of the East European Craton based on seismic and potential field data. Tectonoph. 381, 101-118. Grad, M. et al., 2006. Lithospheric structure of the western part of the East European Craton investigated by deep seismic profiles. GeologQuaterly, 2006, 50(1), 9-22. Grad, M. and Tripolsky, A.A., 1995. Crustal structure from P and S seismic waves and petrological models of the Ukrainian shield. Tectonoph., 250, 89-112. Hauser, F., Raileanu, V., Fielitz,W., Bala, A., Prodehl, C., Polonic, G., Schulze, A., 2001. VRANCEA'99-the crustal structure beneath SE Carpathians and the Moesian Platform from a seismic refraction profile in Romania. Tectonophysics 340, 233–256. Hauser, F., Raileanu, V., Fielity, W., Dinu, C., Landes, M., Bala, A., Prodehl, C., 2007. Seismic crustal structure between Transylvanian Basin and the Black Sea, Romania. Tectonophysics, 430, 1-25. References: Becen, A., et al, 2009, Moho, crustal architecture and deep deformation under the N Marmara Trough, from SEISMARMARA Leg 1 offshore-onshore reflection-refraction survey, Tectonophysics, 467, 1-21. Karahan. A., Berckhemer, H, Baier, B. 2001. Crustal structure at de western end of the N Anatolian Fault Zone from deeps seismic sound. Annali di geofisica, 44(1), 49-68. Angus, D.A., Wilson, D.C., Sandvol, E., Ni, J.F., 2006, Lithopheric structure of the Arabian and Eurasian collision in easter Turkey from S-wave receiver functions, GJI, 2006, 166, 1335-1346. Makris, J and Yegorova, T. 2006. A 3-D density-velocity model between the Cretran sea and Libia, Tectonophyscis, 417, 201-220. Makris, J., Papoulia, J., Papanikolaou, D., Stavrakakis, G., 2001. Thinned continental crust below northern Evoikos Gulf, central Greece, detected from deep seismic soundings, Tectonophysics, 341, 225-236. Zelt, B., Taylor B., Sachpazi, M., Hirn, A., 2005. Crustal velocity structure beneath the Gulf of Corint, Greece. Geoph.Journ.Intern., 162, 257-268. Papazacos, 1998. Crustal Pand S-velocity structure of the serbomacedonian Massif (Northern Greece) obtained by a non-linear inversion traveltimes. Geophys.J.Int.134, 25-39. 0 10 20 30 40 50 D ep th 0 167 334 501 668 835

The modeling of the high frequency waveforms from low magnitude shallow earthquakes located in the Vrancea region and surroundings is used to evaluate depth-dependent models for the quality factor in the crust, optimal for ray paths crossing the area from the bending of the Eastern Carpathians. The study points out lateral variations of the attenuation of the seismic waves with frequencies up to 5 Hz, indicating high values of the Q factor in the extra-Carpathian region, in the East European, Scythian, and eastern Moesian Platforms, and higher attenuation in the orogen from the major bend of the Carpathians, and in the Carpathian foredeep area.

Processing techniques were applied to seismic data acquired by reflection methods. The seismic methods are efficient research methods for civil engineering and environmental geology, which invite to develop specific methodologies. Therefore, soft-programs for processing data collected with refraction seismic techniques (based on head and transmitted waves) and by transmission tomography for velocity were made. The visual programming medium Borland Delphi was utilized to create the program MEDCONT, whose abilities, by menus and dialog windows, are both commanded and controlled. The accuracy and the adaptability of the program to field cases are validated by data resulted from forward models and also collected by applications on field objectives.

Besides the intermediate depth earthquakes (Mw>7.0), in the SE Carpathians and its foreland a crustal seismicity (Mw<=5.6) is as well present. A correlation of the crustal hypocenter distribution with the deep structure displayed by the deep seismic lines is done. Some deep crustal faults extended down to the upper mantle are found out both in the SE Carpathians and its foreland. They are correlated with the seismic cross-sections of the Vrancea 99 and Vrancea 2001 lines. The faults are overlapped on the inflection zones of the interfaces between the crustal layers.

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 Pand 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 10-15 km under the Carpathians. Moho depth is variable from. 38 km near Bacau to 46 km under the Focsani Basin, 41-43 km under the Carpathians, and 34 km under Transylvania.

In seismic microzonation we want to display the variation in seismic response of the subsurface and subsequently determine where the soil is being amplified to a level that may damage existing buildings or other structures. Frequently peak ground acceleration (PGA) is used to determine the maximum horizontal forces that can be expected. The method is not always adequate, because PGA often correspond to high frequencies, which are out of range of the natural frequencies of most structures. The largest amplification of the soil will occur at the lowest natural frequency or its fundamental frequency, which corresponds to the characteristic site period. In situ measurements of shear wave velocity in the soil and the soil thickness, provide a direct measure of the characteristic site period. Extensively seismic noise measurements is a much accessible method and computed H/V spectral ratio can also provide a good indication on the fundamental frequency of the site. Average shear wave velocity in the first 30 m depth (Vs_30) as defined in EUROCODE 8 and Romanian Code P100-1 is a useful indicator in seismic microzonation, showing zones with low values of average seismic velocities in Bucharest. PEAK GROUND ACCELERATION DETERMINATION IN BUCHAREST Bucharest is one of the most affected cities by earthquakes in Europe. Situated at 140 – 170 km distance from Vrancea epicentral zone, Bucharest had suffered many damages due to high energy Vrancea intermediate-depth earthquakes. For example, the 4 March 1977 event produced the collapse of 32 buildings with 8-12 levels, while more than 150 old buildings with 6-9 levels were seriously damaged. Since then the occurrence of 3 other earthquakes (1986 /M=7.1; 1990 /M=6.9; 2004 /M= 6.0) demonstrated that the Vrancea seismic activity is continuing, permanently threatening the Bucharest City area. The studies done after 1977 earthquake had shown the importance of the surface geological structure upon ground motion parameters and emphasized the need for new methods of quantifying the site effects. The earthquake from 27.10.2004 was one of the most studied as there were many good recordings in the Bucharest City area. The accelerometer network of National Institute for Earth Physics have recorded this earthquake and the PGA map for Bucharest was computed for the 3 components. Considering only the EW horizontal component, they show variation in the PGA with amplitudes with ratio from 1 to 4 (16 to 65 cm/s) in the city area (Fig. 1). Most of this variation is due first to the package of the Quaternary sedimentary layers which amplify the original strong motion arrived from the earthquake to the bedrock. 1 National Institute for Earth Physics, Bucharest Magurele, Romania, bala@infp.ro 2 University of Karlsruhe, Dept. of Applied Geology, Karlsruhe, Germany International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 249 415000.00 420000.00 425000.00 430000.00 435000.00 440000.00 4915000.00 4920000.00 4925000.00 4930000.00 4935000.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00 65.00

An assessment of the spectral ratios H/V of seismic records in 18 sites from south Covasna to Aiud in the Transylvania province were achieved based on the data provided by the Vrancea 2001 seismic experiment. A global analysis of the spectral ratio curves shows a transition from 1-3 well outlined peaks with high amplifications (>1.5-2 units) to poorly differentiated peaks and low amplifications on the frequency scale.

In order to study the lithospheric structure in Romania a 450 km long WNW-ESE trending seismic refraction project was carried out in August/September 2001. It runs 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. A total of ten shots with charge sizes 300-1500 kg were recorded by over 700 geophones. The data quality of the experiment was variable, depending primarily on charge size but also on local geological conditions. The data interpretation indicates a multi-layered structure with variable thicknesses and velocities. The sedimentary stack comprises up to 7 layers with seismic velocities of 2.0-5.9 km/s. It reaches a maximum thickness of about 22 km within the Focsani Basin area. The sedimentary succession is composed of (1) the Carpathian nappe pile, (2) the post-collisional Neogene Transylvanian Basin, which covers the local Late Cretaceous to Paleogene Tarnava Basin, (3) the Neogene Focsani Basin in the foredeep area, which covers autochthonous Mesozoic and Palaeozoic sedimentary rocks as well as a probably Permo-Triassic graben structure of the Moesian Platform, and (4) the Palaeozoic and Mesozoic rocks of the North Dobrogea Orogen. The underlying crystalline crust shows considerable thickness variations in total as well as in its individual subdivisions, which correlate well with the Tisza-Dacia, Moesian and North Dobrogea crustal blocks. The lateral velocity structure of these blocks along the seismic line remains constant with about 6.0 km/s along the basement top and 7.0 km/s above the Moho. The Tisza-Dacia block is about 33 to 37 km thick and shows low velocity zones in its uppermost 15 km, which are presumably due to basement thrusts imbricated with sedimentary successions related to the Carpathian Orogen. The crystalline crust of Moesia does not exceed 25 km and is covered by up to 22 km of sedimentary rocks. The North Dobrogea crust reaches a thickness of about 44 km and is probably composed of thick Eastern European crust overthrusted. by a thin 1-2 km thick wedge of the North Dobrogea Orogen. (c) 2006 Elsevier B.V. All rights reserved.

O6 - 01 REGIONAL GEOPHYSICS & GEOTECTONICS RELATIONSHIPS BETWEEN TECTONIC ELEMENTS AND CRUSTAL REFLECTIVITY - A CASE FROM MOESIAN PLATFORM 1 2 3 Justin Matresu Victor Raileanu Corneliu Dinu 1 Petrom SA Romania 2 National Institute for Earth Physics Bucharest Romania 3 Bucharest University Faculty of Geology and Geophysics Romania During the Paleozoic time the western part of the Moesian platform underwent a series of major tectonic events. Relicts of their effect are preserved in the present structures displayed on the reflection seismic sections. The structural patterns of these tectonic events are visible not only over the sedimentary cover pile

SEISMOLOGY & EARTHQUAKES P11 - 05 LOCAL SEISMIC EFFECTS AS RESULTED FROM A SEISMIC EXPERIMENT IN ROMANIA Victor Raileanu Andrei Bala Bogdan Grecu National Institute for Earth Physics Bucharest Romania Introduction Observational and experimental data proved that the local geology strongly influence the size and distribution of earthquake damages. Anywhere one or more instrumental recordings of the seismic events are available a spectral analysis can identify the frequencies where the soil amplification occurred. For a more complete analysis – densely distributed seismic instruments over the studied region are required. An empirical alternative solution to the instrumental data for estimation of