Seismicity of Romania

The seismicity of Romania is clustered in several epicentral zones: Vrancea, Făgăraş-Câmpulung, Banat, Crişana, Maramureş and Dobrogea de sud. Other epicentral zones of local importance can be found in Transylvania, in the area of Jibou and Tarnava river, in northern and western part of Oltenia, in northern Moldova and in the Romanian Plain.

The Vrancea seismogenic zone is the most important among these seismic zones, having in mind the energy, the extent of the macroseismic effects and the persistent and confined character of the earthquakes that occur in this area. Two belts of moderate and shallower seismicity are emphasized in the other regions of the country, one along the Southern Carpathians and the eastern edge of the Pannonian Depression, the other along the Eastern Carpathians that extends towards SE on the Peceneaga-Camena line. Intracrustal earthquakes (depth of the foci between 5 and 30 km) of low energy and intensity, sometimes polikinetic (followed by a large number of aftershocks)  occur within these regions, on faults or at the crossing of the fractures such as: the faults that separate the Fagaras Mountains from the Transylvanian Basin and Lovistea Basin (Fagaras earthquakes), the fractures between the Southern Carpathians and the Pannonian Depression that are active in Timisoara area (Banat earthquakes - Banloc (M_w = 5.6) and Voiteg (M_w = 5.5)), the system of faults within Oradea region and the Sf. Gheorghe fault that bounds to the North the Dobrogea region.

Figure 1 Epicenters of the earthquakes occurred on the Romanian territory between 984 and 2010. (after ROMPLUS catalogue- http://www.infp.ro/catalog-seismic).

 

1. Definition of the Romanian seismogenic zones

Seismogenic zones represent areas with grouped seismicity where the seismic activity and the orientation of the stress field are considered relatively uniform. Identifying the long term characteristics of the earthquake generation process within each seismogenic zone is very important for seismic hazard assessment. This implies the existence of a dataset that needs to cover the time scale of the tectonic process.

The distribution scheme of the Romanian territory in seismogenic zones (Radu et al. 1980; Constantinescu şi Mârza, 1980) follows the geographic distribution of the seismic activity (Figure 2 a). Within these geographic regions, Radulian et al. (2000) proposed the definition of the seismogenic zones into narrower areas that take into account the geologic and seismotectonic characteristics of the Romanian tectonic units (Figure 2b). Ardeleanu et al. (2005), in their study for seismic hazard assessment of Romania, adapted the zones defined by Radulian et al. (2000) (Figure 2c). The seismogenic zones defined in the two publications mentioned above differ only by the way of their definition and not by their characteristics (for more details see the references). As a result, the characteristics of the seismic zones that are to be presented in the following section are the same for both interpretations.

 

 

Figura 2a

Figura 2b

Figura 2c

Figure 1. shows the distribution of epicenters of earthquakes in Romania. The Shabla seismogenic zone locatedin North-Eastern Bulgaria, close to the boarder with Romania is also included. In the East, the earthquakes are related to the subduction process at the Carpathian Arc Bend (Vrancea region); in the West, they follow the contact zone between the Pannonian Depression and the Carpathian orogen. The Eastern segment of the Romanian Carpathians is practically aseismic, except the Southern outermost part (Vrancea region). The Western segment is also aseismic. TheSouthern Carpathians have a more intense seismic activity, especially in the East (Fagaras-Campulung zone) and West (Danubian zone). The platform regions are stable, except one belt that crosses the Carpathian foreland on a NW-SE direction, in front of the Vrancea region. Several active faults that follow the same orientation are identified within this region, namely NW-SE (Intramoesian Fault, Peceneaga-Camena Fault, Sfântul Gheorghe Fault and Trotuş Fault). They represent the contact between different tectonic units where a relative increase of the seismicity is reported. At present, the Transylvanian Depression is almost aseismic. However, a small seismogenic zone was defined only on the basis of historical earthquakes.

 

2. Characterization of the Romanian seismogenic zones

Vrancea subcrustal zone (VR)

The Vrancea region is a complex continental convergent seismic region characterized by at least three tectonic units in contact (insertion in Figure 2): East-European plate, Intra-Alpine and Moesic sub-plates (Constantinescu et al., 1976). The strongest seismic activity concentrates at intermediate depths (60 – 200 km), within an almost vertical old subducted slab. The occurrence of one to six events with magnitude Mw > 7.0 per century within a narrow focal volume implies a high level of active deformation (~3.5 x 10^-7year^-1) that is not clearly observed in the deformation of the crust.

Local shallow earthquakes occur at the outer part of the Vrancea intermediate-depth seismic region within a few regions, some of them associated with deep fractures ( >15 km) (e.g. Focsani-Odobesti zone), some with superficial focus in Mărăşeşti-Nămoloasa, T. Vladimirescu, Cudalbi and Tecuci in Moldova, or on the Galaţi-Isaccea-Tulcea line in Northern Dobrogea. The intensities associated with the earthquakes within these regions are III-IV, even V for Cudalbi region, while the isoseists are oriented NW-SE (Polonic, 1986). The orientation of the isoseists suggests a link with the reactivated hercinic fractures of the basement that prolonged the geologic structures of Dobrogea. The fact that some of these regions (e.g., Focsani-Odobesti and Tecuci) show increased seismic sensitivity to the Vrancea intermediate-depth earthquakes, suggests that the faults of the basement of the outer Carpathians bend are not welded yet. The increased seismicity shortly after the 4th of March, 1977 earthquake on the Intra-Moesian Tinosu-Călăraşi fault is supporting the same model.

Important earthquakes occurred in the region:

• 29 Aug. 1471 h=110 km Mw=7.5 I=9
• 24 Noe. 1516 h=150 km Mw=7.5 I=9
• 30 Apr. 1590 h=100 km Mw=7.3 I=8.5
• 09 Aug. 1679 h=110 km Mw=7.5 I=9
• 11 Iun. 1738 h=130 km Mw=7.7 I=9.5
• 05 Apr. 1740 h=150 km Mw=7.3 I=8.5
• 06 Apr.1790 h=150 km Mw=7.1 I=8
• 26 Oct.1802 h=150 km Mw=7.9 I=10
• 26 Noi.1829 h=150 km Mw=7.3 I=8.5
• 23 Ian. 1838 h=150 km Mw=7.5 I=9
• 17 Aug.1893 h=100 km Mw=7.1 I=8
• 31 Aug.1894 h=130 km Mw=7.1 I=8
• 06 Oct. 1908 h=125 km Mw=7.1 I=8
• 10 Noi. 1940 h=150 km Mw=7.7 I=9.5
• 07 Sept.1945 h=80 km Mw=6.8 I=7.5
• 04 Mar.1977 h= 94 km Mw=7.4 I=9
• 30 Aug.1986 h=131 km Mw=7.1 I=8.5
• 31 Mai 1990 h=86.9 km Mw=6.4
• 27 Oct. 2004 h=98.6 km Mw=6.0

East Vrancea Zone (EV)

The superficial seismicity associated with the subduction process in Vrancea propagates diffusely towards East relative to the Carpathian Arc, within a stripe marked by Peceneaga-Camena fault to the North and Intramoesian fault to the South (so called sub-plate of Blak Sea). Seismicity consists of moderate earthquakes with magnitudes not larger than 5.6, and seems to be decoupled of the seismic activity in the subducted lithosphere. This seismicity shows time (main shocks of the sequences are associated with aftershocks and often pre-shocks or earthquake swarms) and space clustering. Seismic sequences are common for the eastern part of the zone (Râmnicu Sărat region), and swarms predominate in the northern part of the zone (Vrâncioaia region)

Theseismic activity in the Râmnicu Sărat subzone is characterized by crustal moderate earthquakes with magnitudes less than 5.2 (Radu, 1979), that often occur grouped in space and time, as sequences. The hypocenters are generally located at depths between 15 and 30 km within the foredeep of the Carpathians Arc Bend. In this region the sedimentary layers reach the largest thickness (> 10 km) of the whole Carpathian Arc system.  

The focal mechanisms of the events occurred in the Râmnicu Sărat zone are of two types. In the first case, the mechanisms indicate a predominant component of strike-slip faulting type, with the pressure axis orientated NW-SE. These mechanisms fit well with the general tendency of the stress field in Vrancea region: compression caused by the movement of the Black Sea sub-plate from SE towards NW (Constantinescu şi Enescu, 1984). The second type of mechanism is reverse faulting, suggesting the presence of a relative complex stress field in the region.

 

Bârlad Depression (DB)

Depresiunea Bârlad este o depresiune de subsidenţă situată la nord est de regiunea Vrancea în platforma Scitică, şi reprezintă prelungirea către NV a Depresiunii Predobrogene (Figura 4). In zonă au fost observate numai evenimente de mărime moderată (patru şocuri cu MW>5.0 dar care nu depăşesc MW=5.6). Toate soluţiile de plan de falie disponibile indică un regim extensional orizontal predominant, cu o importantă componentă normală. Deşi mecanismele disponibile aparţin cutremurelor mici (3.0MW3.6), consistenţa lor pare a reflecta existenţa unui câmp de tensiuni regional caracteristic. Falierea normală este probabil legată de falierea de tip treaptă evidenţiată în Depresiunea Bârlad (Mutihac and Ionesi, 1974).

Important earthquakes occurred in the region:

• - ______1800 Mw = 4.5 h = 10km I=6
• - 02 Noi. 1871 Mw = 5.3 h = 10km I=6
• - 31 Ian 1900 Mw = 5.5, h = 10km I=6.5

 

Predobrogean Depression (DP)

This seismogenic zone belongs to the southern edge of the Predobrogean Depression marked by the Sfântul Gheorghe fault. The seismicity and focal mechanisms are similar with those mentioned for Bârlad Depression: the moderate seismic activity (M_W £5.3), grouped mainly along the Sfântul Gheorghe fault, and the extensional regime of the deformation field. This reflects the fact that the two seismogenic zones belong to the same tectonic unit (Scitian platform) and, from this point of view, they could form a single seismogenic zone.

Important earthquakes occurred in the region:

• 11 Sept.1980 Mw = 4.2 h = 20km I=5 (zona Galati-Braila)
• 13 Noi. 1981 Mw = 5.1 h = 4 km (vecinatatea orasului Tulcea)
• 03 Oct. 2004 Mw = 5.1 IM = 4 (NE orasului Tulcea)
• 07 Mai 2008 Mw=5-5.2 h=5km (NV Marii Negre)

 

Intramoesian Fault (FI)

The Intramoesian Fault crosses the Moesian platform on SE-NW direction delimiting two sectors with different composition and basement. Although it is a deep and well defined fault, reaching the base of the lithosphere (Enescu, 1992) and extending southeastwards to the Anatolian fault region (Săndulescu, 1984), the seismic activity is scattered and week, with only two events with magnitude greater than 5. A significant increase is observed in Shabla region (SH), Bulgaria, where in 1901 an earthquake with magnitude Mw = 7.1 occurred. The focal depth has a relative high value (h ~ 35 km), suggesting an active process in lower or upper crust.

It is difficult to draw any conclusion regarding the characteristic model of the stress field in this region only on the basis of the four available fault plane solutions, even if they are compatible with the extensional regime outlined in the whole Carpathian foredeep (with the exception of the Vrancea crustal region). One single event (M_W=3.2) has a large slip component on left lateral direction with a nodal plan oriented N30^oW, parallel with the Intramoesian fault.

Important earthquakes occurred in the region:

• 04 Jan 1960  Mw = 5.4 h = 40km I_max = 5.5
• 27 Feb 1967 Mw = 5.0 h = 42km I=5

 

Făgăraş-Câmpulung Zone (FC)

The Făgăraş-Câmpulung zone is located at the contact between Moesian Platform and the Southern Carpathian orogen. It is characterized by strong shocks with magnitudes M_w up to 6.5. These are the strongest crustal earthquakes that occur on the Romanian territory. The last major event occurred on 26th of January 1916 (M_W=6.4) and was followed by a significant activity of aftershocks.

The earthquakes from Făgăraş Mountains and Loviştea Depression occur in South, along a NW alignment (that corresponds to deep fractures oriented on inherited hercinic directions, argument for a weak Alpine regeneration of the Făgăraş Mountains), or NE (directions of Alpine origin) and towards Transylvania, along several step faults that divide the Carpathian orogen and the depression zone. These earthquakes are characterized by intensities up to VII and have typical polikinetic character, over long time intervals, but with moderate energy. Thus, the major event of 1916 that exceeded intensity VII-VIII on a very limited area had produced repeated aftershocks (named by Atanasiu (1961) late aftershocks) during four months of which epicenters migrated from NW to SE. This behavior could be explained through a successive rearrange of several small intracrustal blocks. Note also that the Carpathian Arc Bend operated like a screen for the seismic waves generated by the 1916 Făgăraş earthquake.

Important earthquakes occurred in the region:

• 26 Oct 1550 Mw = 6.5 h = 10km I = 9
• 10 Aug 1590 Mw = 6.5 h = 10km I = 8.5
• 07 Dec 1746 Mw = 5.9 h = 10km I = 8
• 08 Dec 1793 Mw = 6.2 h = 10km I = 7
• 19 Feb 1832 Mw = 5.6 h = 10km I = 8
• 26 Ian 1916 Mw =5.2 h = 10km I = 8
• 18 Apr 1919 Mw =4.1 h = 10km

 

Danubian Zone (DA)

The seismogenic zone named after Atanasiu (1961) „Danubian Zone” represents the western extremity, adjacent to river Danube, of the orogenic unit of the Southern Carpathians. The rate of seismic activity is relatively high, especially close and across the boarder with Serbia. The magnitude of the earthquakes is smaller than 5.6 (Oncescu et al., 1988; Radulian et al., 1996). 

Fault plane solutions are available for three earthquakes (including the largest known one MW 5.6, of 18 July 1991) indicating a process of normal faulting with T axis oriented approximately N-S, in agreement with the extensional stress regime in the Southern Carpathians (Oncescu et al., 1988; Radulian et al., 1996). However, for a firm conclusion, there are necessary more fault plane solutions for Danubian zone.

Important earthquakes occurred in the region:

• 18 Ian 1991 Mw = 5.6 h = 12km I = 8

 

Banat (BA) and Crişana-Maramureş (CM) zones

The contact between the Pannonian Depression and the Carpathian Orogen extends along the western part of the Romanian boarder. Although no tectonic and major geostructural differences are observed, two relatively distinct seismic areas can be defined on the basis of the seismicity distribution: Banat zone at South, and Crişana-Maramureş at North. Seismicity of Banat zone is characterized by several earthquakes with magnitudes greater than 5, but less than 5.6. Historical information suggests for Crişana-Maramureş area possible earthquakes with magnitudes greater than 6, but in the last century only one earthquake with magnitude closer to 5 was reported.

Banat earthquakes have polikinetic character, with a large number of aftershocks in case of larger events. Thus, we can mention: the earthquakes occurred between October 1879 and April 1880 in Moldova Noua area; the earthquake occurred at a depth of 5 km near Timisoara city on 27th of May 1959, Mw=5.6, followed by two shocks occurred in 1960; the earthquakes in Banloc, 12 July 1991, Mw=5.6, depth 11 km, and Voiteg, 2 December 1991, Mw=5.6, depth 9 km, followed by a large number of aftershocks.

In Banat region the reverse and strike-slip faulting are predominant. Even the orientation of the P axis is not well constrained, a regional compressional field is outlined on N-E direction. As shown by Radulian et al. (1996), the available fault plane solutions along the Eastern edge of the Pannonian Basin and within the area of Eastern Carpathians (only for rare and weak events) suggest the compressional character of the stress field. This result is in agreement with the work of Grunthal and Stromeyer (1992) which outlines that the radial model of the extensional regime of the Pannonian Basin implies a compressional field oriented E-W at the East of the basin.

The active seismic zone around Oradea and Carei is located at the crossing of some fractures with NE and E-W orientations (Cornea and Spânoche, 1978) and is characterized by normal foci, with a burst of activity between 1829 and 1834. The earthquakes in Maramures are known for the shocks occurred in the period 1876-1926 with maximum intensity V, sometimes with many aftershocks. They occur on an alignment from Sighetul Marmaţiei towards West, along Tisa river, or on NW-SE alignments between Sighet - Ocna Şugatag and Costiui – Strimtura. These last alignments are parallel with the structures of the Eastern Carpathians and reflects probably fractures of dipping steps of the crystalline basement and of raising of the base of the crust towards the contact with Neogene vulcanites. Four earthquakes had occurred in the area of Baia Mare city that were felt with intensities of V-VI, one in 30.06.1978 (MW=4) and three in 1979. This area is located on a deep fracture oriented W-E that extends towards East and corresponds to the root of Poiana Botizei Nappe, and at West it prolongs as the limit of the Szolnok geosynclinals.

Several alignments of earthquakes oriented NE occur in the western part of Oltenia and Mehedinţi: Drobeta Turnu Severin - Târgu Jiu line, with two events with magnitude 4.5 in 1962 and 1963, respectively.

Important earthquakes occurred in the region:

• 12 Iul 1991 M = 5.6 h = 11km I = 8 (zona Banloc-Timis)
• 18 Iul 1991 M = 5.6 h = 12km I = 8 (zona Herculane-Ofsenita)

 

Transylvanian Depression (DT)

A seismic zone is located between Târnava Mare and Târnava Mică, in the center part of the Transylvanian Depression with some historical earthquakes with magnitude between 5 and 6. The earthquake in 1880 had a diffuse epicentral zone, with two directions of increased intensity, towards N and NW. It is possible that the foci are located on fractures beneath Transylvanian Depression. Note that this seismogenic zone is defined based only on historical data. Presently, the seismic activity is practically absent.

Important earthquakes occurred in the region:

• 08 Ian 1223 Mw = 5.9 h = 10km I = 8
• 19 Oct 1523 Mw = 5.9 h = 10km I = 8 (in apropiere de Medias)
• 18 Mar 1223 Mw = 5.6 h = 10km I = 7
• 15 Feb 1786 Mw = 5.3 h = 10km I = 7
• 03 Oct 1880 Mw = 5.3 h = 10km I = 8

 

 Table 1. Seismogenic zones and their characteristics

 * From the past century; ** normalized for 1 year period and a epicentral area of 5000 km2; *** only earthquakes with MW>5

Table 1. Earthquakes with MW>5.0 occurred between 984 and 2003 and the associated fault plane solutions with minimum 15 polarities of the first arrivals for each seismogenic zone. The maximum value of the seismic moment (M0,max) refer to the all time interval considered, and the coefficients used in the magnitude formula (a and b) are computed only for earthquakes’ occurred after 1900.

 

3.  Fault plane solutions

 

Fig 3. Fault plane solutions for major Romanian crustal earthquake (after Radulian et al., 1999). For more information see www.infp.ro

 

4.  Remarks

After the installation of new seismic sensors, some of the „aseismic” zones proved to be characterized by a microseismicity (M<2) that was impossible to detect in the past. The significant increase of the number of stations in the last years and their better distribution over the Romania territory contribute to a more complete database, due to the increase of the detected events and to the quality of the records that, in the same time, improve the quality and the precision of the earthquake locations.

 

5. References:

Ardeleanu, L. et al. (2005) Probabilistic seismic hazard map for Romania as a basis for a new building code. Natural Hazards and Earth System Science 5, 679-684.

Atanasiu I., 1961. Earthquakes in Romania. Academia R.P.R., Bucarest, 194pp. (in Romanian)

Oncescu, M., Marza, V. I., Rizescu, M., and Popa, M. (1999). The Romanian earthquake catalogue between 984-1997, Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation (edited by F. Wenzel et al.), 43-47.

Radulian, M., Mândrescu, N., Panza, G.F., Popescu, E., Utale, A. (2000), Characterization of Seismogenic Zones of Romania, Pure appl. geophys. 157, 57-77.