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Earthquakes distribution and their focal mechanism in correlation with the active tectonic zones of Romania
Journal of Geodynamics 36 (2003) 129–145 www.elsevier.com/locate/jog
Earthquakes distribution and their focal mechanism in correlation with the active tectonic zones of Romania Andrei Bala*, Mircea Radulian, Emilia Popescu National Institute for Earth Physics, PO Box MG-2, Bucharest-Magurele, 76900, Romania
Abstract On the basis of an earthquake catalogue covering the time interval from 1929 to 1997 and comprising fault plane solutions, we analyze the distribution of the seismic activity on the Romanian territory in connection with the seismogenic zones, previously defined, and available geological and tectonic information. At the same time, the stress field characteristics, deduced from the available fault plane solutions, are investigated for different depth intervals. Predominant clusterings of the principal deformation axes and rupture plane orientation are observed in the Vrancea subcrustal domain in contrast with the earthquakes in the crust, which show no clear trending in the stress field. A number of 526 events occurred in the period 1929–1997 and having the magnitude 1.5 < MS < 7.4 are analyzed according to their fault plane solutions. The relatively large number of events provides important and reliable information to redefine from seismological point of view the limits of the seismogenic and active zones. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction The tectonic plate evolution of the whole Carpathian Arc and Pannonian back-arc Basin indicates that at least three tectonic units are in contact: the East European plate, the Moesian plate and the Intra-Alpine plate are responsible for the particular evolution in this region. The Carpathian Orogen is of Alpine age, composed of many Mesozoic and Cenozoic terranes. The Neogene subduction was accompanied by back-arc volcanism, and back-arc extension in the Pannonian area. A bent paleosubduction zone was recognized in the Eastern Carpathians, along which the original oceanic basement of flysch and the Subcarpathians nappes were consumed.
* Corresponding author. E-mail address: [email protected] (A. Bala). 0264-3707/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-3707(03)00044-9
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A small portion of this zone is still seismically active in the Vrancea area where a few strong earthquakes (1940, MW=7.7; 1977, MW=7.5; 1986, MW=7.2; 1990, MW=6.9) are located between 60 and 200 km depth. This is the only place in the entire Carpathian Orogen where folding and thrusting occurred during Pleistocene in the outermost zones between two deep faults in the Moesian Platform. These recent crustal movements are accompanied by moderate crustal seismic activity (between 10 and 50 km depth and Mmax=5.5), which together with the intermediate-depth seismicity produce high seismic risk in a densely populated area. In Vrancea area, the original oceanic lithosphere descended to more than 200 km depth, as indicated by the intense intermediate depth seismic activity. Recent work in seismic tomography indicates that regional scale anomalies of the seismic velocities can be identified down to 350 km depth (Martin et al., 2002). In a previous paper, Radulian et al. (2002) summarized all the available information to build up a catalogue of focal mechanisms for the earthquakes that occurred in the time interval 1929–1997 on the Romanian territory. This catalogue contains 526 events with magnitude MS between 1.5 and 7.4. In most of the cases the fault plane solution is determined using the first P polarities. In some cases the solutions obtained from the inversion of local or teleseismic waveforms are considered. All the fault plane solutions existing before 1980 have been recomputed tacking into account the convention of Aki and Richards (1980). Only the parameters of the fault plane solutions for the earthquakes occurred after 1994 were computed in the paper of Radulian et al. (2002). Based on this catalogue we analyze the stress field and redefine the seismogenic zones in Romania.
2. Seismic active zones in Romania The seismic activity in Romania is concentrated at the contact between the principal tectonic units. Most of the seismic activity in Romania is represented by the intermediate depth earthquakes concentrated in a narrow area of some 3000 km2 situated at the Eastern Carpathian Arc Bend (Vrancea zone)—Fig. 1. The existence of two kinds of major tectonic units—the orogenic units (the Carpathian Orogen and the North Dobrudjan Orogen) and platform units (the Moldavian Platform, as the south-western margin of the East-European Platform, and the Moesian Platform) leads to the hypothesis of a mobile and seismic active contact between these units. To ensure the comparison with previous results we adopt the following notation proposed by Radulian et al. (2000) in the Fig. 2. The VR zone contains the intermediate depth earthquakes of Vrancea zone, located between 60 and 200 km depth in a very narrow volume. It is the most active seismic zone in Romania in which four destructive earthquakes with magnitudes MW 57 occurred in the last century (Radulian et al., 1999). The crustal seismicity associated with the East Vrancea zone (EV zone) is partially overlapping the VR zone, but with an extension to east and south- east, to the Moesian Platform, between the Intramoesian Fault and the Trotus Fault (see Fig. 2). Most of the hypocenters are located in the lower crust (h520 km) and their energy is much lower than that of the intermediate-depth earthquakes. Only a small number of seismic events occurred in the upper mantle (404h460 km). At this depth range a zone with reduced seimic velocities was recently put into evidence by Hauser et al. (2001) on the refraction profile VRANCEA’ 99.
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Fig. 1. Tectonic sketch with the principal tectonic units and major faults in Romania. Black rectangle is the position of intermediate depth earthquake Vrancea zone (VR). SGF=Sfantu Gheorghe Fault.
Two seismic zones lie in the Scythian Platform: BD zone, roughly corresponding to the Birlad Depression and PD zone in the Predobrogean Depression. Several earthquakes from these areas have magnitudes between MS=4 and 5, only one having MS=5.2, in PD zone (along the Sf. Gheorghe fault). The Moesian Platform is relatively stable with the exception of zones EV and IM ( the last one is situated along the Intramoesian Fault, which is extended down to the base of the crust). The Intramoesian Fault has an extension to the south - east in the Shabla zone (Bulgaria), where a significant increase in the seismicity is observed. An earthquake with magnitude MW=7.2 occurred in 1901 (Radulian et al., 2000). On the Romanian territory there are some discontinuous zones along this fault where earthquakes of magnitudes MS=4–5 have been recorded. At least 3 of them are documented having hypocenter depths greater than 40 km. Two seismogenic zones are defined in Southern Carpathians: zone FC (Fagaras–Campulung) and zone DA (Danubian zone). In FC zone the majority of seismic events has magnitudes between 3 and 4 with only a few over 5. The DA zone is not so well put into evidence by this set of earthquakes. In the northern part of the zone DA lies the Banat zone—BA. This last one appears as an active seismic zone with many earthquakes with magnitudes greater than five located in the upper part of the crust (h=5–10 km) and having numerous aftershocks. The Transylvanian Depression (zone TD), is relatively stable for the considered interval of time.
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Fig. 2. Map of the epicentres for the earthquakes in the catalogue (magnitude in MS). The following seismic zones are represented: VR (Vrancea—intermediate depth earthquakes, h > 40 km); EV (Vrancea—crustal earthquakes, h < 40 km); BD (Barlad Depression zone); PD (Predobrogean Depression zone); IM (Intramoesian Fault zone); FC (Fagaras– Campulung zone); DA (Danubian zone); BA (Banat zone). TF=Trotus Fault. IMF=Intramoesian Fault.
In order to have a unique magnitude measure for all earthquakes of the catalogue we compute an empirical relation for converting the local magnitude (ML—based on duration measurement) to surface-wave magnitude (MS): MS ¼ 1:3156 ML 1:92131
ð1Þ
The relation was determined using 36 events having both magnitudes computed and it is represented in Fig. 3. These events covers the depth range between 10 and 166 km and the correlation coefficient is good enough (R=0.956), to be considered a reliable formula for our purpose. Except VR zone and partially EV zone, the seismic activity is restrained in the upper crustal layer. Nine events occurred along the Intramoesian fault on the Romanian territory, three of them having hypocenters below 40 km depth. Other crustal earthquakes (h 35 km) appear at the southern end of the Intramoesian fault according to Radulian et al. (1999). This could be enough evidence to consider the Intramoesian Fault a seismic active fault going down to the lithosphere base. The data from the catalogue of fault plane solutions as well as historical records (Oncescu et al., 1999) certify that almost all the seismic activity in Northern Dobrogea is concentrated along the Sf. Gheorghe fault (PD zone) and going to the north-west into EV zone (see Fig. 4).
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Fig. 3. Empirical equation to transform magnitude Ml in MS for 36 seismic events from the catalogue (10–166 km depth).
As for Peceneaga–Camena Fault, which separates Northern Dobrogea from Central Dobrogea, considered by some authors as a seismic active fault, we can not certify a significant seismic activity using our catalogue data. Only one earthquake (MS=3.2) is documented in Dobrogea in the neighborhood of this fault. Peceneaga–Camena Fault is continuing to the north-west (west of Danube), but at the same time it is affected by a number of secondary transverse faults, which interrupts its continuity in several places (Visarion et al., 1990). In this conditions its role of strike-slip fault is limited at Dobrogea. Central and Southern Dobrogea appear as stable regions, almost aseismic, even on the catalogue comprising all the historical events—ROMPLUS (Oncescu et al., 1999).
3. The seismicity catalogue The histogram of the number of earthquakes as a function of magnitude is shown in Fig. 5a. The overestimated magnitude value of 3.4 compared with 3.2 and 3.6 can be an artifact of the MS/ML correlation effect.
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However, the earthquakes number below magnitude MS=3.4 is underestimated due to the incompleteness of the catalogue as a result of the scarcity of the seismic stations, especially for the period before 1980. The distribution on the depth (Fig. 5b) outlines three enhancing zones of seismic activity: in the crustal domain (0–40 km ), in the upper part (60–100 km) and in the lower part (100–180 km) of the Vrancea subducting slab. In the crustal domain a great number of events is present in the depth range 5–15 km, the seismic activity is decreasing until 40 km depth, with a pronounced minimum at 50–60 km (Oncescu et al., 1999; Bala et al., 2001). For the period 1982–2002 the velocity model of Oncescu et al. (1984) was used to locate the events (for the period 1982–1984 the earthquakes were relocated ). Events that occurred earlier than 1982 were located by different authors through graphical means, so it is hard to appreciate the errors of the process.
Fig. 4. Seismic activity between 1000 and 1994 in the Romanian territory. Only shallow depth earthquakes (h < 60 km) are plotted. Geostructural units are given by pointed contours. PCF=Peceneaga–Camena Fault; IMF=Intramoesian Fault; SGF=Sfantu Gheorghe Fault.
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Fig. 5. Distribution of the earthquakes from the catalogue: (a). as a function of magnitude Ms; (b). as a function of depth.
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4. Distribution of the parameters of the focal mechanism versus depth In order to study the characteristics of the stress field using the information of the fault plane solution catalog, we analyze first the distribution of the parameters of the focal mechanism as a function of depth. To this aim, we consider four depth intervals: 1. 2. 3. 4.
0–40 km (the crust); 40–100 km (first domain of the intermediate-depth earthquakes); 100–140 km (first half of the second domain of the intermediate-depth earthquakes); 140–180 km (second half of the second domain of the intermediate-depth earthquakes).
The map of the earthquakes in these depth intervals is given in Fig. 6. The crustal earthquakes (0 60 km depth. The distribution of the parameters (strike, dip, slip) of the nodal planes are plotted in Figs. 7–10 for the four depth intervals. The depth limits for these intervals were chosen according to previous
Fig. 6. Map with the depth distribution of the earthquakes in the catalogue.
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Fig. 7. Diagrams of the parameters of the fault plane solutions for the crustal events (0–40 km, 127 events).
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Fig. 8. Diagrams of the parameters of the fault plane solutions for events in the 40–100 km depth domain (87 events).
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Fig. 9. Diagrams of the parameters of the fault plane solutions for events in the 100–140 km depth domain (203 events).
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Fig. 10. Diagrams of the parameters of the fault plane solutions for events in the 140–180 km depth domain (109 events).
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observations regarding the variation of the number of earthquakes and/or the energy released by the earthquakes with depth (Trifu and Radulian, 1991; Bala et al., 2001). The parameters of the nodal planes are plotted as angular histograms of the number of events on 5 intervals. The first and most striking outcome that comes from our diagrams is the quasi-random distribution of the geometry in the crustal domain in contrast with the subcrustal domain, where significant preferences are pointed out. For the crust, the only clustering which is visible is for the dip angle with two pronounced maxima around 50 and 90 . The lack of near horizontal fault planes ( <30 ) is an indicator of a strike-slip component predominance. The diagrams of strike and slip angles show roughly an uniform distribution with azimuth (see Fig. 7). A more refined analysis on seismogenic areas could reveal some preferences if some dominant tectonic features are present in the region (like major active faults). Passing to the first domain of intermediate depth earthquakes (40–100 km), the image is clearly changed. Enhancements of the strike angle distribution are outlined for two conjugate azimuths (around 30 and 300 , respectively), in agreement with the geometry of the Vrancea seismogenic zone (VR), oriented on a NE–SW distribution (see Fig. 8). The slip is strongly clustered around 90 direction, indicating a clear reverse faulting mechanism on steeply dipping faults. If we go deeper (100–140 km depth), it seems that the nodal planes are slightly rotated towards a N–S direction (see Fig. 9). It is interesting to note that this change is also emphasized by tomography studies, which show a rotation of the lower part of the high-velocity body beneath the Carpathians Arc Bend from a NE–SW direction to a N–S direction (Martin et al., 2002). Concerning the slip values, there is a tendency to be shifted from 90 direction to 90 45 directions, suggesting this tendency to a strike-slip component in the faulting process. In the deepest part (h >140 km) of the subcrustal body the strike angles are more randomly distributed, with some enhancements around 60, 160 and 300 . Again the slip angle indicates the pronounced dip-slip faulting, but with an increase of a secondary strike-slip component (see
Fig. 11. Equal area lower hemisphere projection of the P and T axes for the crustal earthquakes, (127 events with the depth range 0–40 km).
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Fig. 12. Equal area lower hemisphere projection of the P and T axes for the intermediate depth earthquakes : (a). 87 events in the depth range 40–100 km; (b). 203 events in the depth range 100–140 km; (c). 109 events in the depth range 140–180 km.
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Fig. 13. Hypocenters of the Vrancea intermediate depth earthquakes (in the time interval 1982–1996) projected on 2 vertical, orthogonal planes of direction: (a). SW–NE; (b). SE–NW. The origin is at 45 N/26 E.
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Fig. 10). Generally, our diagrams show an increase of dispersion in the intermediate-depth domain, as we go from the shallower part (II) to the deeper part (IV). This could indicate a more complicated process of tectonic stress distribution and consequently, of earthquake generation in the lower part of the subducted high-velocity body. In Figs. 11 and 12 the P and T principal axes are represented in equal area lower hemisphere projection for the same depth intervals. Prominent directions of P and T axes can be used to appreciate the stress field in the seismic zones: 1. If the T plunge is lower than 45 and the P plunge is greater than 45 then one has an extension regime; 2. If T plunge is greater then 45 and the P plunge is less than 45 then we have a compression regime; 3. If both axes are around 45 then the focal mechanism is of strike-slip type. For the crustal domain, the P and T axes are distributed randomly over the lower hemisphere, showing no preferred stress regime (Fig. 11). For the subcrustal domain, a clear tendency for the P axis to become horizontal and for the T axis to become vertical is outlined. This tendency is stronger as depth is greater (see Fig. 12a–c). It reflects a predominant compressive stress regime in the Vrancea subcrustal range (Oncescu, 1987).
5. Conclusions On the basis of an earthquake catalog presented by Radulian et al. (2002), covering the time interval from 1929 to 1997, we analyze the distribution of the seismic activity on the Romanian territory in connection with the seismogenic zones, previously defined, and geology and tectonics information. At the same time, the stress field characteristics, deduced from the available fault plane solutions, are investigated for depth intervals previously demonstrated to have different seismological characteristics (Oncescu and Trifu, 1987). The shallow earthquake activity is generally moderate and follows the regions of the orogenic belt. The spatial distribution of epicenters outlines the contact between the major tectonic units, which actually correspond to the lineaments of the paleo-subduction planes. Clusters of increased seismic activity are observed in Vrancea area, eastern and western parts of Southern Carpathians and Banat region. The largest part of the platform regions can be considered stable. Predominant clusterings of the principal deformation axes and rupture plane orientation are observed in the Vrancea subcrustal domain in contrast with the earthquakes in the crust, which indicate no clear trend in the stress field. At least two zones are present in the domain of the intermediate-depth earthquakes: the first from 60 to 100 km and the second from 100 to 180 km depth. These two zones have clearly different characteristics both in the number of earthquakes occurring here and also as the total released energy of the earthquakes occurring in each zone. They are also different with respect to the orientation angle of the blocks, which is changing at the same depth (see Fig. 13). The ‘‘transition zone’’ (100–110 km depth) appears as a low activity seismic zone (Trifu and Radulian, 1991; Bala et al., 2001).
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Acknowledgements This research has been made in the framework of the National Research and Development Programme (CERES) of the Minister for Education and Research, Romania, under Contracts no. 33/15.10.2001 and 39/15.10.2001. References Aki, K., Richards, P.G., 1980. Quantitative seismology. Theory and Methods. W.H. Freeman, San Francisco. Bala, A., Diaconescu, M., Biter, M., 2001. Spatial distribution of the earthquakes in the Vrancea zone and tectonic correlations. Romanian Journal of Physics 46 (7–8), 459–474. Hauser, F., Raileanu, V., Fielitz, F., Bala, A., Prodehl, C., Polonic, G., 2001. VRANCEA99—the Crustal structure beneath the southeastern Carpathians and the Moesian Platform from a refraction seismic profile in Romania. Tectonophysics 340 (3–4), 233–256. Martin, M., Wenzel, F., Achauer, U., Kissling, E., Mocanu, V., Musacchio, G., Radulian, M. and the CALIXTO Working Group, 2002. High-resolution images of a slab detachment process. Seismological Res. Letter (in press). Oncescu, M.C., Burlacu, V., Anghel, M., Smalbergher, V., 1984. Three-dimensional P wave velocity image under the Carpathian Arc. Tectonophysics 106, 305–331. Oncescu, M.C., 1987. On the stress tensor in Vrancea region. J. Geophys. Res. 62, 62–65. Oncescu, M.C., Trifu, C.I., 1987. Depth variation of the moment tensor principal axes in Vrancea (Romania) seismic region. Ann. Geophysicae 5B, 149–154. Oncescu, M.C., Marza, V., Rizescu, M., Popa, M., 1999. The Romanian earthquake catalogue between 1984–1997. In: Wenzel, F., Lungu, D., Novak, O. (Eds.), Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation. Kluwer Academic Publishers, pp. 43–49. Radulian, M., Mandrescu, M.N., Popescu, E., Utale, A., Panza, G.F., 1999. Seismic activity and stress field in Romania. Romanian Journal of Physics 44 (9-10), 1051–1069. Radulian, M., Mandrescu, M.N., Panza, G.F., Popescu, E., Utale, A., 2000. Characterization of Seismogenic zones of Romania. Pure Appl. Geophys 157, 57–77. Radulian, M., Popescu, E., Bala, A., Utale, A., 2002. Catalog of fault plane solutions for the earthquakes occurred on the Romanian territory, Romanian Journal of Physics (in press). Trifu, C.I., Radulian, M., 1991. Frequency- magnitude distribution of earthquakes in Vrancea: relevance for a discrete model. J. Geophys. Res. 96, 4301–4311. Visarion, M., Sandulescu, M., Rosca, V., Stanica, D., Atanasiu, L., 1990. La Dobrogea dans le cadre de l’Avant-pays Carpatique. Rev. Roum. Ge´ophysique 34, 55–65.
Earthquakes distribution and their focal mechanism in correlation with the active tectonic zones of Romania Andrei Bala*, Mircea Radulian, Emilia Popescu National Institute for Earth Physics, PO Box MG-2, Bucharest-Magurele, 76900, Romania
Abstract On the basis of an earthquake catalogue covering the time interval from 1929 to 1997 and comprising fault plane solutions, we analyze the distribution of the seismic activity on the Romanian territory in connection with the seismogenic zones, previously defined, and available geological and tectonic information. At the same time, the stress field characteristics, deduced from the available fault plane solutions, are investigated for different depth intervals. Predominant clusterings of the principal deformation axes and rupture plane orientation are observed in the Vrancea subcrustal domain in contrast with the earthquakes in the crust, which show no clear trending in the stress field. A number of 526 events occurred in the period 1929–1997 and having the magnitude 1.5 < MS < 7.4 are analyzed according to their fault plane solutions. The relatively large number of events provides important and reliable information to redefine from seismological point of view the limits of the seismogenic and active zones. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction The tectonic plate evolution of the whole Carpathian Arc and Pannonian back-arc Basin indicates that at least three tectonic units are in contact: the East European plate, the Moesian plate and the Intra-Alpine plate are responsible for the particular evolution in this region. The Carpathian Orogen is of Alpine age, composed of many Mesozoic and Cenozoic terranes. The Neogene subduction was accompanied by back-arc volcanism, and back-arc extension in the Pannonian area. A bent paleosubduction zone was recognized in the Eastern Carpathians, along which the original oceanic basement of flysch and the Subcarpathians nappes were consumed.
* Corresponding author. E-mail address: [email protected] (A. Bala). 0264-3707/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-3707(03)00044-9
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A small portion of this zone is still seismically active in the Vrancea area where a few strong earthquakes (1940, MW=7.7; 1977, MW=7.5; 1986, MW=7.2; 1990, MW=6.9) are located between 60 and 200 km depth. This is the only place in the entire Carpathian Orogen where folding and thrusting occurred during Pleistocene in the outermost zones between two deep faults in the Moesian Platform. These recent crustal movements are accompanied by moderate crustal seismic activity (between 10 and 50 km depth and Mmax=5.5), which together with the intermediate-depth seismicity produce high seismic risk in a densely populated area. In Vrancea area, the original oceanic lithosphere descended to more than 200 km depth, as indicated by the intense intermediate depth seismic activity. Recent work in seismic tomography indicates that regional scale anomalies of the seismic velocities can be identified down to 350 km depth (Martin et al., 2002). In a previous paper, Radulian et al. (2002) summarized all the available information to build up a catalogue of focal mechanisms for the earthquakes that occurred in the time interval 1929–1997 on the Romanian territory. This catalogue contains 526 events with magnitude MS between 1.5 and 7.4. In most of the cases the fault plane solution is determined using the first P polarities. In some cases the solutions obtained from the inversion of local or teleseismic waveforms are considered. All the fault plane solutions existing before 1980 have been recomputed tacking into account the convention of Aki and Richards (1980). Only the parameters of the fault plane solutions for the earthquakes occurred after 1994 were computed in the paper of Radulian et al. (2002). Based on this catalogue we analyze the stress field and redefine the seismogenic zones in Romania.
2. Seismic active zones in Romania The seismic activity in Romania is concentrated at the contact between the principal tectonic units. Most of the seismic activity in Romania is represented by the intermediate depth earthquakes concentrated in a narrow area of some 3000 km2 situated at the Eastern Carpathian Arc Bend (Vrancea zone)—Fig. 1. The existence of two kinds of major tectonic units—the orogenic units (the Carpathian Orogen and the North Dobrudjan Orogen) and platform units (the Moldavian Platform, as the south-western margin of the East-European Platform, and the Moesian Platform) leads to the hypothesis of a mobile and seismic active contact between these units. To ensure the comparison with previous results we adopt the following notation proposed by Radulian et al. (2000) in the Fig. 2. The VR zone contains the intermediate depth earthquakes of Vrancea zone, located between 60 and 200 km depth in a very narrow volume. It is the most active seismic zone in Romania in which four destructive earthquakes with magnitudes MW 57 occurred in the last century (Radulian et al., 1999). The crustal seismicity associated with the East Vrancea zone (EV zone) is partially overlapping the VR zone, but with an extension to east and south- east, to the Moesian Platform, between the Intramoesian Fault and the Trotus Fault (see Fig. 2). Most of the hypocenters are located in the lower crust (h520 km) and their energy is much lower than that of the intermediate-depth earthquakes. Only a small number of seismic events occurred in the upper mantle (404h460 km). At this depth range a zone with reduced seimic velocities was recently put into evidence by Hauser et al. (2001) on the refraction profile VRANCEA’ 99.
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Fig. 1. Tectonic sketch with the principal tectonic units and major faults in Romania. Black rectangle is the position of intermediate depth earthquake Vrancea zone (VR). SGF=Sfantu Gheorghe Fault.
Two seismic zones lie in the Scythian Platform: BD zone, roughly corresponding to the Birlad Depression and PD zone in the Predobrogean Depression. Several earthquakes from these areas have magnitudes between MS=4 and 5, only one having MS=5.2, in PD zone (along the Sf. Gheorghe fault). The Moesian Platform is relatively stable with the exception of zones EV and IM ( the last one is situated along the Intramoesian Fault, which is extended down to the base of the crust). The Intramoesian Fault has an extension to the south - east in the Shabla zone (Bulgaria), where a significant increase in the seismicity is observed. An earthquake with magnitude MW=7.2 occurred in 1901 (Radulian et al., 2000). On the Romanian territory there are some discontinuous zones along this fault where earthquakes of magnitudes MS=4–5 have been recorded. At least 3 of them are documented having hypocenter depths greater than 40 km. Two seismogenic zones are defined in Southern Carpathians: zone FC (Fagaras–Campulung) and zone DA (Danubian zone). In FC zone the majority of seismic events has magnitudes between 3 and 4 with only a few over 5. The DA zone is not so well put into evidence by this set of earthquakes. In the northern part of the zone DA lies the Banat zone—BA. This last one appears as an active seismic zone with many earthquakes with magnitudes greater than five located in the upper part of the crust (h=5–10 km) and having numerous aftershocks. The Transylvanian Depression (zone TD), is relatively stable for the considered interval of time.
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Fig. 2. Map of the epicentres for the earthquakes in the catalogue (magnitude in MS). The following seismic zones are represented: VR (Vrancea—intermediate depth earthquakes, h > 40 km); EV (Vrancea—crustal earthquakes, h < 40 km); BD (Barlad Depression zone); PD (Predobrogean Depression zone); IM (Intramoesian Fault zone); FC (Fagaras– Campulung zone); DA (Danubian zone); BA (Banat zone). TF=Trotus Fault. IMF=Intramoesian Fault.
In order to have a unique magnitude measure for all earthquakes of the catalogue we compute an empirical relation for converting the local magnitude (ML—based on duration measurement) to surface-wave magnitude (MS): MS ¼ 1:3156 ML 1:92131
ð1Þ
The relation was determined using 36 events having both magnitudes computed and it is represented in Fig. 3. These events covers the depth range between 10 and 166 km and the correlation coefficient is good enough (R=0.956), to be considered a reliable formula for our purpose. Except VR zone and partially EV zone, the seismic activity is restrained in the upper crustal layer. Nine events occurred along the Intramoesian fault on the Romanian territory, three of them having hypocenters below 40 km depth. Other crustal earthquakes (h 35 km) appear at the southern end of the Intramoesian fault according to Radulian et al. (1999). This could be enough evidence to consider the Intramoesian Fault a seismic active fault going down to the lithosphere base. The data from the catalogue of fault plane solutions as well as historical records (Oncescu et al., 1999) certify that almost all the seismic activity in Northern Dobrogea is concentrated along the Sf. Gheorghe fault (PD zone) and going to the north-west into EV zone (see Fig. 4).
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Fig. 3. Empirical equation to transform magnitude Ml in MS for 36 seismic events from the catalogue (10–166 km depth).
As for Peceneaga–Camena Fault, which separates Northern Dobrogea from Central Dobrogea, considered by some authors as a seismic active fault, we can not certify a significant seismic activity using our catalogue data. Only one earthquake (MS=3.2) is documented in Dobrogea in the neighborhood of this fault. Peceneaga–Camena Fault is continuing to the north-west (west of Danube), but at the same time it is affected by a number of secondary transverse faults, which interrupts its continuity in several places (Visarion et al., 1990). In this conditions its role of strike-slip fault is limited at Dobrogea. Central and Southern Dobrogea appear as stable regions, almost aseismic, even on the catalogue comprising all the historical events—ROMPLUS (Oncescu et al., 1999).
3. The seismicity catalogue The histogram of the number of earthquakes as a function of magnitude is shown in Fig. 5a. The overestimated magnitude value of 3.4 compared with 3.2 and 3.6 can be an artifact of the MS/ML correlation effect.
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However, the earthquakes number below magnitude MS=3.4 is underestimated due to the incompleteness of the catalogue as a result of the scarcity of the seismic stations, especially for the period before 1980. The distribution on the depth (Fig. 5b) outlines three enhancing zones of seismic activity: in the crustal domain (0–40 km ), in the upper part (60–100 km) and in the lower part (100–180 km) of the Vrancea subducting slab. In the crustal domain a great number of events is present in the depth range 5–15 km, the seismic activity is decreasing until 40 km depth, with a pronounced minimum at 50–60 km (Oncescu et al., 1999; Bala et al., 2001). For the period 1982–2002 the velocity model of Oncescu et al. (1984) was used to locate the events (for the period 1982–1984 the earthquakes were relocated ). Events that occurred earlier than 1982 were located by different authors through graphical means, so it is hard to appreciate the errors of the process.
Fig. 4. Seismic activity between 1000 and 1994 in the Romanian territory. Only shallow depth earthquakes (h < 60 km) are plotted. Geostructural units are given by pointed contours. PCF=Peceneaga–Camena Fault; IMF=Intramoesian Fault; SGF=Sfantu Gheorghe Fault.
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Fig. 5. Distribution of the earthquakes from the catalogue: (a). as a function of magnitude Ms; (b). as a function of depth.
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4. Distribution of the parameters of the focal mechanism versus depth In order to study the characteristics of the stress field using the information of the fault plane solution catalog, we analyze first the distribution of the parameters of the focal mechanism as a function of depth. To this aim, we consider four depth intervals: 1. 2. 3. 4.
0–40 km (the crust); 40–100 km (first domain of the intermediate-depth earthquakes); 100–140 km (first half of the second domain of the intermediate-depth earthquakes); 140–180 km (second half of the second domain of the intermediate-depth earthquakes).
The map of the earthquakes in these depth intervals is given in Fig. 6. The crustal earthquakes (0
Fig. 6. Map with the depth distribution of the earthquakes in the catalogue.
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Fig. 7. Diagrams of the parameters of the fault plane solutions for the crustal events (0–40 km, 127 events).
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Fig. 8. Diagrams of the parameters of the fault plane solutions for events in the 40–100 km depth domain (87 events).
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Fig. 9. Diagrams of the parameters of the fault plane solutions for events in the 100–140 km depth domain (203 events).
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Fig. 10. Diagrams of the parameters of the fault plane solutions for events in the 140–180 km depth domain (109 events).
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observations regarding the variation of the number of earthquakes and/or the energy released by the earthquakes with depth (Trifu and Radulian, 1991; Bala et al., 2001). The parameters of the nodal planes are plotted as angular histograms of the number of events on 5 intervals. The first and most striking outcome that comes from our diagrams is the quasi-random distribution of the geometry in the crustal domain in contrast with the subcrustal domain, where significant preferences are pointed out. For the crust, the only clustering which is visible is for the dip angle with two pronounced maxima around 50 and 90 . The lack of near horizontal fault planes ( <30 ) is an indicator of a strike-slip component predominance. The diagrams of strike and slip angles show roughly an uniform distribution with azimuth (see Fig. 7). A more refined analysis on seismogenic areas could reveal some preferences if some dominant tectonic features are present in the region (like major active faults). Passing to the first domain of intermediate depth earthquakes (40–100 km), the image is clearly changed. Enhancements of the strike angle distribution are outlined for two conjugate azimuths (around 30 and 300 , respectively), in agreement with the geometry of the Vrancea seismogenic zone (VR), oriented on a NE–SW distribution (see Fig. 8). The slip is strongly clustered around 90 direction, indicating a clear reverse faulting mechanism on steeply dipping faults. If we go deeper (100–140 km depth), it seems that the nodal planes are slightly rotated towards a N–S direction (see Fig. 9). It is interesting to note that this change is also emphasized by tomography studies, which show a rotation of the lower part of the high-velocity body beneath the Carpathians Arc Bend from a NE–SW direction to a N–S direction (Martin et al., 2002). Concerning the slip values, there is a tendency to be shifted from 90 direction to 90 45 directions, suggesting this tendency to a strike-slip component in the faulting process. In the deepest part (h >140 km) of the subcrustal body the strike angles are more randomly distributed, with some enhancements around 60, 160 and 300 . Again the slip angle indicates the pronounced dip-slip faulting, but with an increase of a secondary strike-slip component (see
Fig. 11. Equal area lower hemisphere projection of the P and T axes for the crustal earthquakes, (127 events with the depth range 0–40 km).
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Fig. 12. Equal area lower hemisphere projection of the P and T axes for the intermediate depth earthquakes : (a). 87 events in the depth range 40–100 km; (b). 203 events in the depth range 100–140 km; (c). 109 events in the depth range 140–180 km.
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Fig. 13. Hypocenters of the Vrancea intermediate depth earthquakes (in the time interval 1982–1996) projected on 2 vertical, orthogonal planes of direction: (a). SW–NE; (b). SE–NW. The origin is at 45 N/26 E.
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Fig. 10). Generally, our diagrams show an increase of dispersion in the intermediate-depth domain, as we go from the shallower part (II) to the deeper part (IV). This could indicate a more complicated process of tectonic stress distribution and consequently, of earthquake generation in the lower part of the subducted high-velocity body. In Figs. 11 and 12 the P and T principal axes are represented in equal area lower hemisphere projection for the same depth intervals. Prominent directions of P and T axes can be used to appreciate the stress field in the seismic zones: 1. If the T plunge is lower than 45 and the P plunge is greater than 45 then one has an extension regime; 2. If T plunge is greater then 45 and the P plunge is less than 45 then we have a compression regime; 3. If both axes are around 45 then the focal mechanism is of strike-slip type. For the crustal domain, the P and T axes are distributed randomly over the lower hemisphere, showing no preferred stress regime (Fig. 11). For the subcrustal domain, a clear tendency for the P axis to become horizontal and for the T axis to become vertical is outlined. This tendency is stronger as depth is greater (see Fig. 12a–c). It reflects a predominant compressive stress regime in the Vrancea subcrustal range (Oncescu, 1987).
5. Conclusions On the basis of an earthquake catalog presented by Radulian et al. (2002), covering the time interval from 1929 to 1997, we analyze the distribution of the seismic activity on the Romanian territory in connection with the seismogenic zones, previously defined, and geology and tectonics information. At the same time, the stress field characteristics, deduced from the available fault plane solutions, are investigated for depth intervals previously demonstrated to have different seismological characteristics (Oncescu and Trifu, 1987). The shallow earthquake activity is generally moderate and follows the regions of the orogenic belt. The spatial distribution of epicenters outlines the contact between the major tectonic units, which actually correspond to the lineaments of the paleo-subduction planes. Clusters of increased seismic activity are observed in Vrancea area, eastern and western parts of Southern Carpathians and Banat region. The largest part of the platform regions can be considered stable. Predominant clusterings of the principal deformation axes and rupture plane orientation are observed in the Vrancea subcrustal domain in contrast with the earthquakes in the crust, which indicate no clear trend in the stress field. At least two zones are present in the domain of the intermediate-depth earthquakes: the first from 60 to 100 km and the second from 100 to 180 km depth. These two zones have clearly different characteristics both in the number of earthquakes occurring here and also as the total released energy of the earthquakes occurring in each zone. They are also different with respect to the orientation angle of the blocks, which is changing at the same depth (see Fig. 13). The ‘‘transition zone’’ (100–110 km depth) appears as a low activity seismic zone (Trifu and Radulian, 1991; Bala et al., 2001).
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Acknowledgements This research has been made in the framework of the National Research and Development Programme (CERES) of the Minister for Education and Research, Romania, under Contracts no. 33/15.10.2001 and 39/15.10.2001. References Aki, K., Richards, P.G., 1980. Quantitative seismology. Theory and Methods. W.H. Freeman, San Francisco. Bala, A., Diaconescu, M., Biter, M., 2001. Spatial distribution of the earthquakes in the Vrancea zone and tectonic correlations. Romanian Journal of Physics 46 (7–8), 459–474. Hauser, F., Raileanu, V., Fielitz, F., Bala, A., Prodehl, C., Polonic, G., 2001. VRANCEA99—the Crustal structure beneath the southeastern Carpathians and the Moesian Platform from a refraction seismic profile in Romania. Tectonophysics 340 (3–4), 233–256. Martin, M., Wenzel, F., Achauer, U., Kissling, E., Mocanu, V., Musacchio, G., Radulian, M. and the CALIXTO Working Group, 2002. High-resolution images of a slab detachment process. Seismological Res. Letter (in press). Oncescu, M.C., Burlacu, V., Anghel, M., Smalbergher, V., 1984. Three-dimensional P wave velocity image under the Carpathian Arc. Tectonophysics 106, 305–331. Oncescu, M.C., 1987. On the stress tensor in Vrancea region. J. Geophys. Res. 62, 62–65. Oncescu, M.C., Trifu, C.I., 1987. Depth variation of the moment tensor principal axes in Vrancea (Romania) seismic region. Ann. Geophysicae 5B, 149–154. Oncescu, M.C., Marza, V., Rizescu, M., Popa, M., 1999. The Romanian earthquake catalogue between 1984–1997. In: Wenzel, F., Lungu, D., Novak, O. (Eds.), Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation. Kluwer Academic Publishers, pp. 43–49. Radulian, M., Mandrescu, M.N., Popescu, E., Utale, A., Panza, G.F., 1999. Seismic activity and stress field in Romania. Romanian Journal of Physics 44 (9-10), 1051–1069. Radulian, M., Mandrescu, M.N., Panza, G.F., Popescu, E., Utale, A., 2000. Characterization of Seismogenic zones of Romania. Pure Appl. Geophys 157, 57–77. Radulian, M., Popescu, E., Bala, A., Utale, A., 2002. Catalog of fault plane solutions for the earthquakes occurred on the Romanian territory, Romanian Journal of Physics (in press). Trifu, C.I., Radulian, M., 1991. Frequency- magnitude distribution of earthquakes in Vrancea: relevance for a discrete model. J. Geophys. Res. 96, 4301–4311. Visarion, M., Sandulescu, M., Rosca, V., Stanica, D., Atanasiu, L., 1990. La Dobrogea dans le cadre de l’Avant-pays Carpatique. Rev. Roum. Ge´ophysique 34, 55–65.