The seismogenic zones are areas of grouped seismicity in which the seismic activity and stress field are assumed to be relatively uniform. The identification of long-term characteristics of the earthquake generation process in each seismogenic zone is of great significance for the seismic hazard assessment.
The most dangerous seismogenic zone in Romania is located in the subcrustal lithosphere at the bending of the Eastern Carpathians – Vrancea region.
Beside this intermediate-depth source, several shallow-depth seismic areas of local importance for the seismic hazard are pointed out: the East-Vrancea, Făgăraş – Câmpulung, Danubian, Banat and Crişana – Maramureş zones, the Bârlad Depression, the Predobrogean Depression, the Intramoesian Fault, and the Transylvanian Depression (Radulian et al., 2000).
The background seismicity – crustal events with magnitude Mw < 5 – is sporadically observed, mainly in northern Oltenia, Haţeg Depression, eastern part of the Romanian Plain, Moldavian Platform, Eastern Carpathians orogen.
Intermediate – depth seismic zone
Vrancea subcrustal Zone (VR)
The Vrancea region is a complex seismic region of continental convergence characterized by (at least) three tectonic units in contact: the East European plate, Intra-Alpine and Moesian subplates (Constantinescu et al., 1976).
The strongest seismic activity of Romania concentrates at intermediate depths (60–200 km) in an almost vertical downgoing high-velocity lithospheric body. Enhanced activity is observed within two depth ranges – 80 to 100 km, and 120 to 160 km, respectively. The strongest earthquakes of the XXth century occurred in both segments: on 1977 (Mw 7.4) and 1990 (Mw 6.9) in the upper one, and on 1940 (Mw 7.7) and 1986 (Mw 7.1) in the deeper one.
The occurrence of one to six events with Mw>7 per century in an extremely confined focal volume implies a high level of strain rate (~3.5×10-7 yr-1) at depth. Apparently, the tectonic process at depth is largely decoupled from the tectonics in the overlying crust.
Referring to the focal mechanism of the Vrancea intermediate-depth earthquakes, the reverse faulting – with nearly vertical tension (T) axis, and nearly horizontal pressure (P) axis – characterizes all the major events (Mw>6) and more than 90% of the studied events, regardless of their magnitude (Enescu, 1980; Oncescu şi Trifu, 1987; Enescu şi Zugrăvescu, 1990; Radulian et al., 2000). For the fault plane orientation, two typical solutions have been emphasized: (I) the fault plane oriented mainly in a NE–SW direction and dipping towards NW, and P-axis perpendicular to the mountain arc; and (II) the fault plane oriented mainly in a NW–SE direction and the P axis parallel to the mountain arc. The available macroseismic and instrumental data indicate the type (I) solution for all events with Mw>7.
Normal or strike-slip faulting have been rarely observed, for earthquakes localized mostly at the upper and lower edges of the seismogenic volume (Radulian et al., 2000).
The stress regime in the Vrancea subcrustal zone is predominantly compressive.
Normal – depth seismic zones
East Vrancea Zone (EV)
The shallow seismicity in the Vrancea region spreads diffusely eastward relative to the Carpathians arc bend, in the strip delimited by the Peceneaga-Camena fault to the north and Intramoesian fault to the south (so-called Black Sea subplate). It consists of only moderate-size earthquakes, not exceeding magnitude 6. Bursts of seismic activity – earthquake sequences or swarms – are relatively common in this zone (e.g., in Râmnicu Sărat – Focşani region, in Vrâncioaia area).
The seismic moment rate in the Vrancea overlying crust (~5.3×1015 Nm/year) is much smaller than the rate in the subcrustal domain (~1.2×1019 Nm/year) (Radulian et al., 2000).
Various focal mechanisms – reverse, strike-slip and normal faultings – are equally observed, pointing out a complex stress field, characterizing the transition from the predominant compressive regime at depth to the extensional regime in the crust.
Făgăraş – Câmpulung Zone (FC)
The Făgăraş – Câmpulung zone is sited in the eastern part of the Southern Carpathians. It is characterized by shocks as large as Mw~6.5, which are the largest shallow earthquakes recorded on the Romanian territory. The last major event occurred on January 26, 1916 (Mw=6.4), and was followed by significant aftershock activity.
The epicenter distribution outlines two clusters: one located to the west, which includes the largest shocks (Mw~6), the other one located to the east (Sinaia region), with smaller events (Mw<5).
The focal mechanisms are of strike-slip and normal type, indicating an extensional stress pattern. The strike-slip faultings predominate, with the fault planes oriented NW–SE (Enescu et al., 1996).
Danubian Zone (DA)
The seismogenic Danubian Zone represents the western extremity, adjacent to the Danube river, of the orogenic unit of the Southern Carpathians.
The rate of seismic activity is relatively high, especially at the border and beyond the border with Serbia, across the Danube river. The magnitude does not exceed 5.6.
The few fault plane solutions available indicate normal faulting with the T-axis striking roughly N–S, in agreement with the general extensional stress regime in the Southern Carpathians (Oncescu et al., 1988; Radulian et al., 2000).
Banat Zone (BA)
The contact between the Panonnian Depression and the Carpathian orogen lies entirely along the western part of the Romanian border.
Two enhancements in the seismicity distribution were identified, in two relatively distinct active areas: Banat to the south, and Crişana-Maramureş to the north, although no significant tectonic or geostructural differences can be noticed between them.
The seismicity of the Banat zone is characterized by many earthquakes with magnitude Mw>5, but not exceeding 5.6. In general, the larger shocks, which are frequently followed by aftershock sequences, occur in clusters (within a few month intervals).
In contrast with the focal mechanisms observed in the Carpathians foredeep area (with the exception of Vrancea crustal zone) and in the Southern Carpathians, where reverse faultings have not been noticed, here reverse and strike-slip faultings predominate. A regional horizontal compression field with E–W direction has been outlined (Radulian et al., 2000), in agreement with an approximately radial pattern of the extensional regime in the Pannonian Basin (Grünthal and Stromeyer, 1992), which implies E–W compression east of the basin, in the intra-Carpathian region.
Crişana-Maramureş Zone (CM)
The historical earthquake catalogues report the occurrence of events greater than magnitude 6 in Crişana-Maramureş (ROMPLUS catalogue, Oncescu et al., 1999 – 1 event, Shebalin et al., 1998 – 2 events, in the first half of the XIXth century). Several damaging earthquakes with magnitude above 5 are also reported on the basis of historical information. During the instrumental period (starting with the XXth century), however, only one event approaching magnitude 5 has been localized in the zone.
Bârlad Depression (BD)
The Bârlad Depression is a subsiding depression situated to the NE of the Vrancea region on the Scythian platform, and it represents the prolongation towards the NW of the Predobrogean Depression. Only moderate-size events are observed, not exceeding Mw=5.6.
The available fault plane solutions indicate a predominant extensional regime. The normal faulting is probably related to the step-like faulting outlined in the Depression (Mutihac şi Ionesi, 1974).
Predobrogean Depression (PD)
This seismogenic zone belongs to the southern margin of the Predobrogean Depression, following the Sfântul Gheorghe fault alignment.
Roughly, the seismicity and focal mechanism characteristics are similar to those outlined for the Bârlad Depression: the moderate seismic activity (Mw≤5.3), clustered especially along the Sfantul Gheorghe fault, and the extensional regime of the deformation field. This consistently reflects the affiliation of the two zones to the same tectonic unit – the Scythian platform.
Intramoesian Fault (IM)
The Intramoesian fault crosses the Moesian platform in a SE–NW direction, separating two distinct sectors with different constitution and structure of the basement. Although it is a well-defined deep fault, reaching the base of the lithosphere (Enescu, 1992), and extending southeast to the Anatolian fault region (Săndulescu, 1984), the associated seismic activity is scarce and weak (only two events above magnitude 5, both reported in the instrumental period). The focal depth (whenever it can be constrained) has relatively large values (h~35 km), suggesting active processes in the lower crust or in the upper mantle.
The very few fault plane solutions available for this zone are all consistent with the extensional regime outlined throughout the Carpathians foredeep area (with the exception of Vrancea crustal region).
Transylvanian Depression (TD)
This seismogenic zone is defined only on the basis of historical information.
The seismic activity is mostly absent at present; nevertheless, several earthquakes with magnitude above 5 (a couple of events with Mw > 5.5) have been reported on the basis of historical documents, which notify important damaging effects in Transylvania (ROMPLUS catalogue, Oncescu et al., 1999).
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