On the geodynamics of the Eastern Balkans based on some ... · On the geodynamics of the Eastern...
2
150 БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2010“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2010” On the geodynamics of the Eastern Balkans based on some seismic and GPS data Върху геодинамиката на Източните Балкани по сеизмични и GPS данни Emil Botev, Ivan Georgiev, Dimitar Dimitrov Емил Ботев, Иван Георгиев, Димитър Димитров National Institute of Geophysics, Geodesy and Geography, BAS; E-mail: [email protected] Abstract. Some aspects of geodynamics of the Eastern Balkan region are discussed mainly on the basis of stress and strain fields modelling by seismological and geodetic data. Key words: seismicity, fault plane solutions, GPS velocities, stress and strain fields modelling, geodynamics. Fig. 1. Distribution of the events with fault plain solutions In this report an attempt to elucidate the geodynamics of Bulgarian territory and the very adjacent lands from seismological and geodetic view points is proposed. For this purpose a short analysis of the historical and nowadays seismicity is made. The fault plane solutions of about two hundred earthquakes (Fig. 1) and corre- and corre- and corre- sponding stresses are also analysed. For the purposes of the quantitative stress and strain modelling some seismic zoning is discussed and the main seismologi- zoning is discussed and the main seismologi- zoning is discussed and the main seismologi- cal and focal mechanisms characteristics for each zone are presented. The mean stress tensors for most zones are calculated by means of an inversion of the focal mechanisms data using the technique of Gephart. The released strain is computed from the moment tensors of the focal mechanisms according to the relation of Kostrov. In general, a predominant strike-slip stress field is observed in Struma zone and Rhodopean zone. The significant number of vertical movements from the focal mechanisms data is supported by geologi- cal and geodetic data in Gorna Oryahovitsa zone and Shabla zone, where the stress inversion shows a nor- mal stress regime. The maximum compressive stress oriented approximately E–W for the north–eastern zone could be associated with the remnant effects of the pushing of the west Black Sea basin, resulting of the long time lasting riftogenesis in the central part of the north Black Sea. The normal stress regime act- ing in Srednogorie zone results also from geological and tectonic evidence of rifting processes and agrees with the interpretation of the present day extension- al tectonics. This result is in a good agreement with the geological concepts of the Late Alpine evolution of the depressions with the general trend of continu- ous subsidence of the grabens itself (Alexiev, 1999). According to some authors (Tzankov et al., 1998; Alexiev, 1999), all used geomorphological criteria for recognising the Quaternary active faults in many parts of the investigated territory evidence a predominant listric faulting. This listric faulting, together with the basins formation, allows us to accept the presence of a subhorizontal extension of the upper part of the crust. The misfit between the principal axes of the obtained stress and strain tensors for almost all zones could be explained with the dominated Quaternary listric fault- ing in the zones with many inherited faults from previ- ous tectonic activities. The GPS-measurements of the present state of crustal deformation in the Eastern Mediterranean indi- cate that the biggest movements (around 30 mm/y) are observed towards SSW in the central and southern parts of the Aegean plate (Fig. 2). At the same time, a few measurements in northern Greece reveal much smaller movements in the south direction (around 5–6 mm/y). According to our investigations, the SW part of Bulgaria is characterised by predominant south movements with velocity around 3–4 mm/y, and some places in Northern Bulgaria are characterised by simi-
Transcript of On the geodynamics of the Eastern Balkans based on some ... · On the geodynamics of the Eastern...
150
БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2010“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2010”
On the geodynamics of the Eastern Balkans based on some seismic and GPS dataВърху геодинамиката на Източните Балкани по сеизмични и GPS данниEmil Botev, Ivan Georgiev, Dimitar Dimitrov Емил Ботев, Иван Георгиев, Димитър Димитров
National Institute of Geophysics, Geodesy and Geography, BAS; E-mail: [email protected]
Abstract. Some aspects of geodynamics of the Eastern Balkan region are discussed mainly on the basis of stress and strain fields modelling by seismological and geodetic data.
Key words: seismicity, fault plane solutions, GPS velocities, stress and strain fields modelling, geodynamics.
Fig. 1. Distribution of the events with fault plain solutions
In this report an attempt to elucidate the geodynamics of Bulgarian territory and the very adjacent lands from seismological and geodetic view points is proposed. For this purpose a short analysis of the historical and nowadays seismicity is made. The fault plane solutions of about two hundred earthquakes (Fig. 1) and corre- and corre-and corre-sponding stresses are also analysed. For the purposes of the quantitative stress and strain modelling some seismic zoning is discussed and the main seismologi- zoning is discussed and the main seismologi-zoning is discussed and the main seismologi-cal and focal mechanisms characteristics for each zone are presented. The mean stress tensors for most zones are calculated by means of an inversion of the focal mechanisms data using the technique of Gephart. The released strain is computed from the moment tensors of the focal mechanisms according to the relation of Kostrov. In general, a predominant strike-slip stress field is observed in Struma zone and Rhodopean zone. The significant number of vertical movements from the focal mechanisms data is supported by geologi-cal and geodetic data in Gorna Oryahovitsa zone and Shabla zone, where the stress inversion shows a nor-mal stress regime. The maximum compressive stress oriented approximately E–W for the north–eastern zone could be associated with the remnant effects of the pushing of the west Black Sea basin, resulting of the long time lasting riftogenesis in the central part of the north Black Sea. The normal stress regime act-ing in Srednogorie zone results also from geological and tectonic evidence of rifting processes and agrees with the interpretation of the present day extension-al tectonics. This result is in a good agreement with the geological concepts of the Late Alpine evolution of the depressions with the general trend of continu-ous subsidence of the grabens itself (Alexiev, 1999). According to some authors (Tzankov et al., 1998; Alexiev, 1999), all used geomorphological criteria for recognising the Quaternary active faults in many parts
of the investigated territory evidence a predominant listric faulting. This listric faulting, together with the basins formation, allows us to accept the presence of a subhorizontal extension of the upper part of the crust. The misfit between the principal axes of the obtained stress and strain tensors for almost all zones could be explained with the dominated Quaternary listric fault-ing in the zones with many inherited faults from previ-ous tectonic activities.
The GPS-measurements of the present state of crustal deformation in the Eastern Mediterranean indi-cate that the biggest movements (around 30 mm/y) are observed towards SSW in the central and southern parts of the Aegean plate (Fig. 2). At the same time, a few measurements in northern Greece reveal much smaller movements in the south direction (around 5–6 mm/y). According to our investigations, the SW part of Bulgaria is characterised by predominant south movements with velocity around 3–4 mm/y, and some places in Northern Bulgaria are characterised by simi-
151
Fig. 2. GPS velocities obtained for the Aegean region
lar oriented movements with almost the same velocity (1–3 mm/y). The orientations and magnitudes of the present crustal movements in southern and northern Greece, south-western and northern Bulgaria can ex-plain the predominance of extensional subhorizon-tal stresses in Central–Northern Greece and Eastern Balkans. The extensional regime in the north Aegean area and the influence of the Anatolian plate have caused one additional opening of the south-eastern parts of the Srednogorie zone (Tzankov et al., 1998).
In Bulgaria and the very adjacent lands, the focal mechanisms and stress tensor inversion show that the present acting state of stress is strictly connected to the complex geodynamics of the Eastern Balkan area. The prevailing normal extensional stresses obtained
from the seismological data could be explained by the above discussed two main regional processes. The first one is the post-collisional extensional collapse of the Central Balkan orogen under influence of the paleosubduction in the Vardar paleobasin (Zagorchev, 1996) and the present days pushing of the Adriatic plate towards ENE. The second process is the com-plex influence of the existing SW horizontal move-ments along the North Anatolian fault while passing through the Aegean Sea; it causes extension of the south-eastern parts of Srednogorie zone and forma-tion of an extensional province northern of the North Aegean Trough. In general, the obtained mean strain tensors of deformation show some agreement with the calculated mean stresses.
ReferencesAlexiev, G. 1999. The role of intense destructive taphrogenesis
in shaping the morphostructural patterns of Bulgaria terri-tory. – Problems of Geography, 4, 46–57.
Burchfiel, B. C., R. W. King, A. Todosov, V. Kotzev, N. Dur-murdzanov, T. Serafimovski, B. Nurce. 2005. GPS results for Macedonia and its importance for the tectonics of the Southern Balkan extensional regime. – Tectonophysics, 413, 239–248.
Hollenstein, Ch., M. Müller, A. Geiger, H.-G. Kahle. 2008. Crustal motion and deformation in Greece from a decade of GPS measurements, 1993–2003. – Tectonophysics, 449, 17–40.
Kotzev, V., R. Nakov, Tz. Georgiev, B. C. Burchfiel, R. W. King. 2006. Crustal motion and strain accumulation in western Bulgaria. – Tectonophysics, 413, 127–145.
McClusky, S., S. Balassanian, A. Barka, C. Demir, S. Ergintav,
I. Georgiev, O. Gurcan, O. Hamburger, K. Hurst, H. Kahle, K. Kastens, G. Kekelidze, R. King, V. Kotzev, O. Lenk, S. Mahmoud, A. Mishin, M. Nadariya, A. Ouzounis, D. Paradissis, Y. Peter, M. Prilepin, R. Reilnger, I. Sanli, H. Seeger, A. Tealeb, M. Toksoz, G. Veis. 2000. Global Positioning System Constraints on Plate Kinematics and Dynamics in the Eastern Mediterranean and Caucasus. – J. Geophys. Res., 105 (B3), 5695–5719.
Tzankov, Tz., K. Kurtev, S. Shanov, G. Nikolov. 1998. On the genesis of the Upper Trachian depression (SE part of the Balkans). – In: Proc. of the Symp.“Geodynamic investiga-tions on the 1928 Plovdiv earthquake”, Sofia, 113–118.
Zagorchev, I. 1996. Late Alpine (Paleogene-earliest Neogene) and neotectonic development of the central parts of the Balkan Peninsula. – Zeitschrift der Deutscher Geologischen Gesellschaft, 24, 1–2, 91–112.
