IC/96/255
United Nations Educational Scientific and Cultural Organization
and
International Atomic Energy Agency
INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS
SEISMOGENIC ZONES OF ROMANIA
M. Radulian, N. Mandrescu
National Institute for Eearth Physics, Bucharest, Romania
and
G.F. Panza
International Centre for Theoretical Physics, SAND Group, Trieste, Italy
and
Dipartimento di Scienze della Terra, Universita degli Studi di Trieste,
Trieste, Italy.
MIRAMARE - TRIESTE
December 1996
Abstract
The seismogenic zone configuration for Romania is proposed, assuming that the
earthquake generation process is intimately connected with the geostructural
characteristics of the territory. Therefore, the boundaries of the seismogenic zones
follow closely the contact between the two major geotectonic units of the Romanian
territory: platform and orogen regions. The crustal seismicity is generally moderate and
follows the uplifting regions of the orogenic belt. The subcrustal seismic activity is
concentrated at the bend of the Carpathian arc (Vrancea region) affecting a confined
epicentral area. The area situated in front of the Vrancea region to the east-southeast,
delimited by two deep faults trending on a southeast-northwest direction seems to be
more mobile relative to the stable Moldavian platform to the north and Moesian platform
to the west.
Introduction
The information on the seismogenic zones, together with the information on
seismicity and focal mechanism provides the basic database required by seismic hazard
studies. Since the process of earthquake generation is closely connected with the
geotectonics, it is natural to define the seismoactive zones of a territory starting from its
geostructural characterization. Until now, two schemes concerning the division of the
Romanian territory into seismic areas were proposed (Radu et al., 1980; Constantinescu
and MSrza, 1980). Generally they agree with the geographical distribution of the
seismic activity, but the delimiting borders are in. a large measure conventional,
following especially the configuration of administrative provinces.
Our proposal for the identification of the seismogenic zones of Romania is
based on the revision and analysis of the relevant existing geological and structural data.
We present first a characterization of the Romanian territory from the geological point of
view and second the identified seismogenic zones are briefly described. In a companion
paper (Radulian et al., 1996) the proposed seismogenic zone configuration is analyzed
in connection with seismicity and focal mechanism data.
Geotectonic setting
Geological formations with different lithological compositions and ages are
disposed in a large variety of structures on the Romanian territory. This is a result of the
geological processes which have been active in this area since Precambrian up to the
present time. At the end of Paleozoic, the orogenic processes during the Hercynian
cycle contributed to the extension to the west and south of the rigid Eurasian platform.
As geographical position, the Romanian territory corresponds to the southern margin of
this platform.
The Alpine history of the Carpathians area can be divided into two stages
(Radulescu and Sandulescu, 1973; Sandulescu, 1984): an extensional period, which
began in the Middle Triassic and led to the opening and spreading of the Tethys ocean,
and a compressional or convergent period lasting from Neocomian until Miocene time.
The oceanic floor, generated during the extension period ,is now preserved as ophiolites
in the main Tethys suture, which can be detected in several areas of the inner part of the
Carpathians.
The Carpathians underwent several episodes of compressional deformations
during Cretaceous and Cenozoic times, and two main periods of compressional
deformation can be recognized in the Carpathians evolution: (1) the Dacidian period,
which includes Cretaceous events, and (2) the Moldavian period, which includes
Miocene events. The Cretaceous Dacidian events were mainly responsible for the
deformation and the present aspect of the inner Carpathians (or Dacides), while the
Miocene Moldavian events were restricted to the outer Carpathians (or Moldavides).
Two main specific geotectonic units are outlined on the Romanian territory: (1)
folding or orogen regions and (2) platform regions.
Two orogen regions are defined by Sandulescu (1984): Alpine, including the
Carpathian orogen and North Dobrogea orogen, and Cadomian, which corresponds to
the green schist zone of the central Dobrogea. The Carpathian orogen is one of the most
complex, segments of the Europe Alpine system, consisting of three main sectors on the
Romanian territory: Eastern Carpathians, Southern Carpathians and Apuseni
mountains. It has a prominent bend in the Vrancea region, where the strongest
earthquakes of Romania are reported. After a first geotectonic division of the tectonic
units of Romania (Popescu Voitesti, 1929), the authors of the tectonic map of Romania
(Dumitrescu et al., 1962; Dumitrescu and Sandulescu, 1970; 1976) separated the
Dacides and Moldavides. The first units represent the Carpathian units with Cretaceous
tectogenesis and are located in the inner Carpathians zones (Internides zones); the
second units represent the Carpathians units with Neogene tectogenesis and are located
in the outer Carpathians zones (Outernides zones). From the inner to the outer parts of
the Carpathians, the following units are distinguished:
- Inner Dacides, consisting of units of continental origin (from the point of view
of the origin of the primary nature of the crust);
-Transylvanides, representing the major ophiolitic suture of the Carpathian
orogen;
-Pienides, probably characterized both by Cretaceous and Neogene
tectogenesis;
-Middle Dacides, which are units of continental origin;
-External Dacides, corresponding to a intra-continental suture;
-Marginal Dacides, in the south-western part of the Southern Carpathians
(Danubianunk);
-Moldavides, with Neogene tectogenesis.
The schematic seismotectonic map of Romania presented in Fig. 1 includes all these
units only into two larger units (Dacides and Moldavides), not to overcrowd the figure.
The North Dobrogea orogen is composed of three tectonic units, overthrusting
successively towards the Scythian platform. It is delimited from Scythian platform by
the Sfintul Gheorghe fault (SGF). It has a intra-cratonic position relative to the East
European platform and continues over the continental shelf of the Black Sea connecting
the Alpine orogen of the South Crimea through a series of left lateral strike slip faults.
To the north-west, the North Dobrogea orogen underthrusts the Carpathian foredeep
and flysch nappes deposits.
The platform regions are situated to the outer part of the Carpathians arc, in
contact with the Eastern and Southern Carpathians. A Precambrian platform (Moldavian
platform) and a Paleozoic platform composed of two distinct sectors, Scythian platform
and Moesian platform are distinguished. The Moldavian platform is the oldest geologic
unit of the Romanian territory and represents the extreme southwestern corner of the
East European platform. It has a kata- and meso-zonal metamorphic basement under an
undisturbed sedimentary cover, inside which Lower Paleozoic, Middle Cretaceous and
Neogene deposits are separated by disconformities reflecting long intervals of
geological times. To the west, the basement descends in step-faults under a border
depression framed by a marginal ridge, beyond which the crystalline rocks are sinking
to an undetectable depth.
The Scythian platform is in direct contact with the East European platform,
surrounding it to the west and to the south. It corresponds with the "Pre-Dobrogea
Depression", which includes the depression of Barlad too. It extends towards northwest, being completely covered by the outer thrust nappes of the Carpathians. The
basement is of Hercynian or Caledonian-Hercynian age. To the south, the Scythian
platform is in contact with the North Dobrogea orogen and the Moesian platform.
The Moesian platform has a basement of Precambrian age. It is covered by
Alpine nappes both in the southern part of the Eastern Carpathians, and in the southern
margin of the Southern Carpathians. It continues also over the continental shelf of the
Black Sea. Two important deep faults, Peceneaga Camena fault (PCF) and
Intramoesian fault (IMF), are crossing the Moesian platform from SE to NW. Their
extension to the south-east reaches probably the Anatolian fault region (Sandulescu,
1984). IMF is an important structural boundary which separates the Moesian platform
into two distinct sectors with different constitution and structure of the basement: the
Dobrogean sector situated to the north and east from this fault, and the Valahian sector,
located to the south and west.
The post-tectonic depressions represent another category of major geotectonic
units of the Carpathians. The Carpathian foredeep is a late and post-orogenic sinking
zone of Neogene age, filled with molasse sediments and forming a continuous border
with the Carpathian front. A thrust line of great regional significance, the Pericarpathian
line (PL), separates an internal limb (the epi-orogenic flank) from an external one (the
epi-platformic flank). Inside the external limb of the foredeep, a zone of special
seismotectonic interest is the Focsani-Odobesti depression where deep seismic
sounding measurements reveal a sedimentary sequence, about 18 km thick. The
depression has an asymmetrical structure with its steeper border located close to the
epicentral area of the Vrancea seismic zone.
A complex system of back-arc basins was formed during the Neogene. Two
extensional basins encircled by the Carpathians chain are emphasized: Transylvanian
and Pannonian basins. The Transylvanian depression is filled with a thick Neogene
molasse. The deposits of younger age are better developed in the crescent-shaped
zones migrating towards the Carpathians arc bending zone (Lazarescu, 1980). The
crystalline and Cretaceous basement has an Alpine structure, with deep thrust faults
along which some of the Dacidic nappes from Eastern and Southern Carpathians were
generated. The Pannonian depression behaved as a subsiding area since the Late
Tortonian time. It has an heterogeneous basement unconformably overlain by Upper
Miocene, Pliocene and Quaternary deposits.
The subsequent Alpine magmatism crosses or covers the folded units of the
Carpathian orogen, as it is the case of the Neogene magmatism of the inner parts of
Carpathian Dacides and partly Pienides. This magmatism has a post-tectonic character
and is the result of a subduction process with a Benioff plane dipping from east to west
(Bleahu et al., 1973; Stanica and Stanica, 1981), in a relic stage at present.
Seismogenic zones
The criterion considered in this study to delimit the seismogenic areas is related
to the characteristics of the main geostructural units of the Romanian territory. The
zones with different seismic potential, represented in Fig. 2, follow closely the
geotectonic units described briefly in the previous section. The hatched areas mark the
seismogenic zones we have identified, i.e. the zones where earthquakes with Ms> 5
occur. A short description of the seismic peculiarities of the geostructural units is given
in the following.
Zone I {Moldavian platform), located in the northeastern part of Romania,
overlies the western extremity of the East European platform. The seismicity is low due
to the relic character of the convergence between the East European platform and
Eastern Carpathians. The maximum magnitude earthquake is observed on July 10,
1970 (M s = 4.7).
Zone II {Scythian platform) could be subdivided into two subzones: the Barlad
depression (Ila) and the Danube Delta subzone (lib). The Barlad depression is a
subsiding area sited between the Moldavian platform and the Moesian platform. It is
seismically more active than Zone I. The strongest earthquake reported here occurred on
January 31, 1900 {M$ = 5.6). The Danube Delta subzone is situated at the northeastern
extremity of the Dobrogea region. The maximum magnitude reported in this area is for
the event of November 13,1981 (Ms = 5.2).
Zone III {Moesian platform) is separated into two distinct sectors, with different
constitution and structure of basement, by the Intramoesian fault (IMF): the Dobrogean
sector (Ilia) and the Valahian sector (Illb). The Dobrogea subzone is developed mainly
3
between IMF and PCF, but includes the North Dobrogea orogen, so that it is limited to
the north by the SGF. This subzone underwent the most recent displacements towards
the Carpathian orogen. The mobility of the area is reflected in the recorded seismic
activity which is more intense than in the western part of the Moesian platform
(Valahian subzone). It can be related to the Black Sea subplate movement from SE
towards NW (Airinei, 1977). The strongest known earthquake occurred on January 4,
1960 (M s = 5.4). The Valahian subzone is developed to the south and west from the
IMF. Generally, it is characterized by more stability, with the exception of the eastern
part, close to the IMF, and the contact with the Southern Carpathians to the north and
north-west. The strongest earthquake occurred on June 10, 1966 (Ms = 5.1) near to the
Carpathian mountains.
Zone IV (Eastern Carpathians) follows the trend of the eastern branch of the
Carpathians. In the northern part of this zone the magnetotelluric observations (Stanica
and Stanica, 1981) identified a relic subduction from the NE to the SW, as a
confirmation of the convergence of the East European platform and the Carpathian
orogen. The geological structure of the area is characterized by nappes belonging to
Dacides and Moldavides with uni-directional vergence to the subducting plate. The
folded ensembles overlie both the subducting and overriding plates. Thus, the
Moldavides folds and nappes belong to the subducting plate, while the Inner Dacides
nappes belong to the overriding plate. The seismic activity in the zones of the Neogene
volcanic chain (Harghita-Calimani) and of the Eastern Carpathians are similar.
Therefore, we consider both zones as a single zone, characterized by events with
magnitudes Ms < 5.
Zone V (Southern Carpathians) had a different evolution as compared with the
Eastern Carpathians (the process of geosynclinal migration did not take place and the
flysch zone is not so extended in the Southern Carpathians). The history of the
Southern Carpathians is typical Dacidian, the whole crystalline basement and
sedimentary deposit being thrusted during the Cretaceous events (Austrian and Laramic
events) into two main tectonic units: Getic nappe and Danubian Autohton. Taking into
consideration seismicity data, the area could be divided into two subzones: the FagarasCimpulung and Danubian subzones, representing approximately the eastern and
western halves of the entire zone. The Fagaras-Cdmpulung
subzone (Va) is
characterized by a relatively high seismicity level, with earthquakes with magnitudes Ms
> 5 (the maximum magnitude M$ = 6.4 was recorded on January 26, 1916). The main
shocks are frequently followed by many aftershocks. The Danubian subzone is also
characterized by a relatively high seismicity. The maximum observed magnitude is Mg
= 5.6 (October 10, 1879). Another large event was recorded on July 18, 1991 (M s =
5.5) followed by a significant aftershock activity.
Zone VI (Apuseni mountains) constitutes the western branch of the Romanian
Carpathians, closing to the west the Transylvanian depression. As the Eastern and
Southern Carpathians, the Apuseni Mountains were made up in a geosynclinal area, at
the beginning of the Alpine orogen. The seismicity is low and scarce (the largest
earthquake occurred on October 6,1880, Ms = 4.4).
Zone VII (Transylvanian depression) is the largest molassic Neogene
depression on the Romanian territory. The first outline of the depression occurred at the
end of the Lower Miocene, after the old Styrian tectogenesis (17 My). Two distinct
tectonic stages are emphasized in its composition: (1) the deformed elements of the
different Dacidian segments; (2) the post-tectogenetic cover. Geophysical data show
that the crystalline basement is located at a depth of 7-8 km, and even shallower in some
zones. The seismic activity is poor at present, but several strong shocks are reported in
historical documents (the strongest one occurred on November 8, 1620, Ms - 6.5).
Zone VIII (Eastern margin of the Pannonian depression). The basement is
partitioned in blocks which are dislocated following a system of faults which are
intersecting almost perpendicularly; the Pannonian system of faults oriented in parallel
to the trend of the mountains edifice, and the Carpathian fault system oriented
approximately E-W. According to seismicity we can define two subzones: Banat
subzone and Crisana subzone. The Banat subzone has a high seismic activity with
many earthquakes with magnitudes Ms> 5. They occur in the upper part of the crust (h
= 5-10 km) and are followed by numerous aftershocks lasting for relatively large time
intervals. Significant sequences occurred on April 17, 1974 (Mmax = 5.6), August 27,
1991 (M max = 5.3) and December 2, 1991 ( M ^ = 5.1). Two strong events are
reported from historical data for the Crisana subzone: January 7, 1829 (Ms = 6.2) and
October 15, 1834 (M s = 6.5). After 1900 only one earthquake was observed with M s >
5 (March 23, 1939, M s = 5.1).
In the Zone IX (East Vrancea), the most recent deposits affected by
deformations are of Lower Pleistocene age (Valahian tectogenesis). These deposits are
located only in the outer part of the Carpathians arc bend, extending to the PCF to the
north-east and IMF to the south-west. The structure of the inner part of the foredeep
situated westward from the IMF was deformed during the Intra-Sarmatian or IntraPliocene. This fact demonstrates the independence of the foreland movement of one
side relative to the other of the IMF. A rather large number of earthquakes is reported
here (often occurred in clusters) but only with moderate magnitudes (the largest
earthquake occurred on July 14, 1914 with M$ = 5.3). Most of the focal depths are
greater than 20 km.
Zone X (Vrancea) delimits the epicentral area of the Vrancea intermediate depth
earthquakes (an area of approximately 3000 km2 oriented NE-SW). It is the most active
seismic area of Romania, including the epicentres of the strongest earthquakes reported
for this territory (Ms > 7). The crustal seismicity related to Vrancea region partly
overlaps Zone X, but it is spread mainly towards east and southeast (Zone IX). A
seimic gap is seen approximately in the depth interval from 40 km to 60 km between the
intermediate depth earthquakes and crustal earthquakes of the East Vrancea area. It is
associated probably with a low velocity zone which does not allow a brittle behaviour.
Conclusions
Our study shows the existence of a good correlation between the seismic activity
and the geostructural peculiarities of the major tectonic units. As it is well known, in the
Vrancea region the major seismic activity of the whole Romanian territory is
concentrated over a small epicentral area (Zone X) and it is characterized by intermediate
depth earthquakes with magnitudes as large as 7. The shallow earthquake activity is
generally moderate and follows the uplifting regions of the orogenic belt. The largest
part of the tectonic units belonging to the Carpathian foreland represents old stable
platforms which are not seismically active (Moldavian and Moesian platforms). To the
west, the Apuseni Mts. and the largest part of the Transylvanian depression are
characterized by very low seismic activity. The seismogenic zones (hatched areas in
Figure 2) have been defined to be used in seismic hazard computation. These zones are
located at the contact between the platform and orogen regions to the east-southeast and
at the contact between the eastern margin of the Pannonian depression and Carpathian
orogen to the west. This is in accordance with the modern concepts of seismogenesis
which are based on the idea that the lithosphere is made up of relatively homogeneous
and rigid block structures, separated by narrow active (mobile) zones, where
earthquakes are mostly generated. The genetic interdependence between the geotectonic
units and earthquake process, which is the fundamental hypothesis in defining the
seismogenic zones, will be tested in a companion paper (Radulian et al., 1996) using an
extended seismological database (seismicity and focal mechanism information).
Acknowledgments
This research has been carried out under the EEC-COPERNICUS project,
contract CT 94-0238.
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Figure captions
Figure 1. Tectonic sketch map of Romania (simplified after Sandulescu, 1984, with
some modifications). (1) Dacides; (2) Moldavides; (3) North Dobrogea
orogen; (4) Moldavian platform; (5) Moesian platform; (6) Scythian
platform; (7) Post-tectogenic depressions; (8) Volcanic rocks: CretaceousPaleogene (8a); Neogene (8b); (9) Isopach of the Pliocene-Quaternary
deposits; (10) Thrust fault; (11) Normal fault; (12) Flexure.
Figure 2. Geostructural units and seismogenic zones of the Romanian territory.
Borderlines of the geostructural units are plotted as pointed lines. Epicentres of
the strongest earthquakes are indicated by open circles (circle size is
proportional with the earthquake magnitude. The seismogenic zones are marked
by hatched areas.
Fig-1
27°
21
TRANSYLVANIAN
\
DEPRESSION
if
-U
50
Fig. 2
29°