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Romanian Reports in Physics 70, 704 (2018)
EARTHQUAKE MECHANISM IN THE VRANCEA
SUBCRUSTAL SOURCE AND IN THE ADJACENT CRUSTAL
SEISMOGENIC ZONES OF THE SOUTH-EASTERN ROMANIA
E. POPESCU1, M. RADULIAN1,2, A. BĂLĂ1, D. TOMA-DĂNILA1
1 National Institute for Earth Physics, P.O.Box MG-2, RO-077125 Bucharest-Magurele, Romania
E-mail: mircea@infp.ro
2 Academy of Romanian Scientists, Bucharest, Romania
Received December 4, 2017
Abstract. Earthquake mechanism information is fundamental to determine the
stress field and to define seismogenic zones. At the same time, it is a basic input to
compute seismic hazard by deterministic approach. The present paper extends the
catalogue of the fault plane solutions for the earthquakes in Romania, previously
completed until 1997, for 1998–2012 time interval. The catalogue is limited
geographically to the Carpathians Orogeny and extra-Carpathians area located in the
south-eastern part of Romania because similar investigations cover the rest of the
country. The catalogue comprises 259 intermediate-depth seismic events and 90
crustal seismic events, recorded in the considered time interval with acceptably
constrained fault plane solutions. We use specific graphical tools in order to
emphasize statistically representative features of the stress field as coming out from
our results. The fault plane solutions of the Vrancea earthquakes generated in a
confined sinking plate in the mantle reflect the dominant geodynamic process in the
study region. The typical features revealed by all the previous studies on the
subcrustal seismic activity (predominant dip-slip, reverse faulting, characterizing both
the weak and strong earthquakes) are reproduced as well by our investigation. As
concerns the earthquake activity in the crust, a few new refined aspects are
highlighted in the present work: (1) a deficit of the strike-slip component over the
entire Carpathians foredeep area, (2) different stress field pattern in the Făgăraş-
Câmpulung zone as compared with the Moesian Platform and Pre-Dobrogean and
Bârlad Depressions, (3) a larger range for the dip angle of the nodal planes in the
Vrancea subcrustal source, ~ 400 –700 against ~ 700, as commonly considered.
Key words: earthquake mechanism, seismogenic zones, fault plane solution catalogue.
1. INTRODUCTION
The present paper is an extension of previous studies dealing with the focal
mechanism and related stress characteristics for the earthquakes recorded in
Romania (e.g., [1, 2, 3]). Thus, the catalogue of the fault plane solutions built up
until 1997 is updated and expanded for the time interval 1998–2012 and
Article no. 704 E. Popescu et al. 2
subsequently analyzed in correlation with specific cluster of events and active
faults.
We limit geographically our data set to the Carpathians Orogeny and extra-
Carpathians area located in the south-eastern part of Romania. Similar
investigations were carried out by [4] and [5], focused on the seismogenic zones
located in the western part of Romania: Danubian zone and Banat area. Their
results (140 earthquakes mechanisms) can be considered as a complement to our
work in order to characterize the earthquake mechanism data for the entire
Romania. Seismogenic areas that remains still uncovered includes the
Transylvanian Basin and Crisana – Maramures region (north-western Romania)
where we could not get enough data to compute reliable fault plane solutions.
The earthquake mechanisms are computed in all cases using the SEISAN
algorithm [6] and the polarities of the first P-wave arrivals. The catalogue of fault
plane solutions, presented in the Appendix A and accessible on line at
www.infp.ro, comprises 259 intermediate-depth seismic events and 90 crustal
seismic events, recorded in the time interval 1998–2012 (see also Table 1). We
selected only the solutions with minimum 10 reliable polarities and acceptable
stations coverage. The location of the events is presented in Fig. 1 together with the
seismogenic zones, as they were defined by [7] slightly changed here.
Table 1
Number of earthquakes recorded between 1998 and 2012, with fault plane solutions available,
associated with the study seismogenic zones
Seismogenic zone No. events Mw Depth [km] No. stations
Moesian Platform – MO 42 2.3–3.3 2–38 10–51
Barlad Depression &
Pre-Dobrogean depression
PD-BD
28 2.3–4.9 0–46 10–25
Fagaras – Campulung area – FC 20 2.3–3.3 0–25 10–34
Vrancea intermediate depth – VNI 259 2.7–6.0 64–167 10–60
Most of the study crustal earthquakes belong to the background seismicity,
with magnitudes Mw below 4, except the event of 3.10.2004 in the Northern
Dobrogea with Mw = 4.9. Almost three thirds of earthquakes occurred in the
Vrancea region (VRI) in the mantle range (64 to 167 km depth, Table 1). Most of
them have magnitudes Mw below 5, with only 5 earthquakes with magnitudes
Mw ≥ 5. The largest earthquake was recorded on 27.10.2006 with Mw = 6.
The crustal earthquakes follow closely the areal distribution of the
seismogenic zones, which were set on the basis of seismicity trends as covered up
by the entire catalogue of earthquakes in Romania and on the basis of geotectonic
grounds.
The computed fault plane solutions are plotted in the inset of Fig. 1. For a
better graphical presentation/visualization, we kept for the Vrancea subcrustal
earthquakes the mechanism solutions only for the larger events (Mw > 4.5), which
are 22 subcrustal events out of 259 (Table 1).
Fig. 1 – Tectonic map of the south-eastern part of Romania (after [18]) and the epicentre location for the earthquakes considered in this paper.
In the inset the focal mechanisms for 90 crustal earthquakes (red circles) and 22 intermediate depth earthquakes with Mw > 4.5 (blue circles)
from Vrancea intermediate-depth zone (VNI) are represented.
Article no. 704 E. Popescu et al. 4
We can draw some conclusions from a simple visual examination of the
figure:
– Prevalence of reverse faulting in the Vrancea subcrustal source;
– A tendency of the nodal planes for the Vrancea subcrustal source to be
oriented either parallel or perpendicular to the Carpathians Arc bend;
– A large variety of fault plane solutions for the crustal events both as
faulting type and nodal plane orientation.
2. SEISMICITY DISTRIBUTION
Seismicity in Romania is concentrated at the Carpathians Arc bend in the
Vrancea region. Here, an isolated lithospheric slab downgoing in the mantle is
permanently releasing seismic energy in an extreme narrow volume. In average,
three earthquakes with magnitude above 7 were reported each century for a time
span of six centuries.
The origin of intermediate-depth seismicity in the Vrancea area is still an
ongoing debate. The lithospheric volume which is seismically active can be
approximated by a prism vertically oriented between 60 and 170 km depth, with a
horizontal cross section of 30 × 70 km2. Above 60 km and below 170 km the
seismicity is suddenly cut off, although the high-velocity body, as determined by
seismic tomography, extends notably beyond these limits ([8, 9]). The most
important intermediate depth earthquakes have been analyzed by [10].
The seismic activity in the crust is dispersed over the Carpathians orogeny
and foreland with significant enhancements in several seismogenic zones, as
defined first by [7].
The crustal earthquakes are commonly small to moderate (Mw < 6). Only in
the FC zone a few shocks of magnitude above 6 (Mmax = 6.5) were reported in the
Romanian catalogue, in about one millennium time interval [11]. The crustal
seismicity is generally associated with the basement fracture systems [12] and [13].
In the Fig. 1 we focus our attention on the seismogenic zones located in the
south-eastern part of Romania. The positioning of the other seismic areas in
Romania is represented in [7]. Given that tectonically and geologically the
seismogenic areas situated in front of the Carpathians Arc, south of the Peceneaga-
Camena fault in the Moesian Platform, do not differ notably, we prefer to consider
in our subsequent analysis a single area (MO) in this region. In the same way, we
combined the Pre-Dobrogean depression and Bârlad depression zones into a single
area (PD-BD). The Vrancea intermediate zone (VNI) and Făgăraş-Câmpulung zone
(FC) are the same as defined in [7]. We slightly adjusted the Pre-Dobrogean zone,
which was extended to the north-west in order to cover also the North Dobrogean
Orogeny and to have a common margin with Barlad Depression zone and to be
extended over the southern flank of Sf. Gheorghe fault (Fig. 1). The adjustments
5 Earthquake mechanism in the Vrancea subcrustal source Article no. 704
relative to the previously defined seismogenic zones are made to include all the
earthquakes in the catalogue and to keep at the same time their adherence to a
specific tectonic province (Fig. 1).
The eastern sector of the Moesian Platform, located between the
Intramoesian fault and Peceneaga-Camena fault is more seismically active as
compared with the sector located west of the Intramoesian fault, which is almost
aseismic. The seismicity concentrates close to the Carpathians Arc bend,
overlapping to some extend the epicentral area of the Vrancea subcrustal
earthquakes (VRI). Quite frequently, the earthquakes are generated in moderate
size sequences. Typical sequences were recorded in the Focşani – Râmnicu Sărat
and Vrancioaia areas ([14, 15]). Many times, the earthquake sequences in front of
the Carpathians Arc bend display alignments parallel to the orogeny, probably in
connection to a system of buried faults beneath the Focşani sedimentary basin
([16, 17]).
The Intramoesian fault crosses the Moesian Platform on the SE-NW
direction separating two distinct major sectors with different constitution and
structure of the basement. Although the Intramoesian fault is considered a major
fault, which extends from the continental platform of the Black Sea to the NW
under the Getic Nappe [18] and which is supposed to reach in depth the lithosphere
base [19], the associated seismicity is weak and scarce. The prolongation of this
fault under the Pericarpathian line and the relations with the Fagaras-Campulung
zone is still under debate (Fig. 1).
A scarce seismicity is noticed to the west of the Intramoesian fault, which
extends over the entire western part of the Moesian Platform, until it comes into
contact with the Southern Carpathians in the Danubian seismic region (not shown
in the Fig. 1).
For the seismogenic zones located in the western part of Romania, located
by [7], some earth sequences are analysed by [20] and mechanism data are
examined by [3] as well as in [8] and [9].
The Peceneaga-Camena fault separates the Moesian Platform unit from the
North Dobrogean Orogeny. The seismic activity to the northern side of the
Peceneaga-Camena fault cover roughly three tectonic units: North Dobrogean
Orogeny; Pre-Dobrogean Depression and Bârlad Depression. The Pre-Dobrogean
depression lies to the north North Dobrogean Orogen, being separated from it by
Sf. Gheorghe fault. The Pre-Dobrogean Depression is continuing to the north-west
into Bârlad depression, which is north of Trotus fault (Fig. 1, after [18]).
The Bârlad Depression is a subsiding depression on the Scythian Platform in
contact to the north with the southern edging of the East European Platform
(Moldavian Platform). As was also noted by other studies [2], the seismicity and
focal mechanism features in the PD and BD zones are similar, and therefore, we
can merge them in a single seismic active area (PD-BD), which includes also the
North Dobrogean Orogeny.
Article no. 704 E. Popescu et al. 6
The Făgăraş-Câmpulung area is the eastern seismic active segment of the
Southern Carpathians. It is characterized by strong shocks that reached up to
Mw ~ 6.5 (the maximum magnitude recorded in Romania for crustal earthquakes).
The last major seismic event was recorded on 26 January 1916 (Mw = 6.4) and it
was followed by a significant aftershock activity which lasted several weeks [21].
The geotectonics of the area is quite complex with relative distinct activities in the
central and western side (Făgăraş Mountains and Loviştea Depression) and eastern-
south-eastern side (Sinaia area). The earthquakes generated in the south-eastern
side could be eventually associated with the north-western edge of the
Intramoesian fault. A large sequence started on 4 May 1993 in the Sinaia area and
lasted through the entire year (maximum magnitude Mw = 4.7), estimated using the
seismic moment computed by [22].
2.1. FAULT PLANE SOLUTIONS
We follow the convention of Aki and Richards (1980) [23] to define the
nodal plane parameters (strike, dip and rake). Fault strike is the direction of a line
created by the intersection of a fault plane and a horizontal surface, measured
relative to North (0° to 360°). Strike is defined such as the fault dips always to the
right side of the trace when moving along the trace in the strike direction. The
hanging-wall block of a fault is therefore always to the right, and the footwall
block on the left. This is important because rake (which gives the slip direction) is
defined as the movement of the hanging wall relative to the footwall block.
Fault dip is the angle between the fault and the horizontal plane (Earth’s surface)
measured from the horizontal plane (0° to 90°). Rake is the direction in which the
hanging wall block moves relative to the foot wall during rupture, as measured on
the plane of the fault. It is measured relative to fault strike, ± 180°.
3. STATISTICAL ANALYSIS
One of our goals is to determine the main features of the stress field as
coming up from the fault plane solutions computed in this paper and to compare
the results with previous investigations. For this purpose, we used graphical tools
able to emphasize statistically representative features in our data set for each
seismogenic area considered in the study. In this way, simple polar diagrams were
employed to describe strike and dip behavior of the events in each active area.
In order to classify the type of faulting, we represent the distribution of the
principal axes P, T and B using Kaverina’ projection [24], which improves the
Frohlich and Apperson (1992)’ ternary diagram [25].
A dedicated program is employed for this purpose, which was made
available by the author and which is described in [26] and [27].
7 Earthquake mechanism in the Vrancea subcrustal source Article no. 704
3.1. VRANCEA SUBCRUSTAL SOURCE – VNI
The polar diagrams for the azimuthal distribution of the two nodal planes
(A and B) and for the plunge angle are plotted in the Fig. 2. Note the predominance
of nodal planes oriented NE-SW, parallel to the Carpathians Arc bend and the
plunge angles greater than 40o.
It is worth mentioning that this geometry of the faulting system correspond
with the orientation of the rupture faults for the largest Vrancea events ([28, 29,
30]). Taking into account the difference in scale between a moderate earthquake
(Mw = 4–5) and a major earthquake (Mw > 7), it is not trivial such a feature. We
may assume that the process of generating moderate earthquakes is controlled
largely by the same tectonic forces as for generating the largest shocks.
Fig. 2 – Angular diagrams for azimuth and dip angles of the nodal planes –
Vrancea subcrustal source (VNI).
Article no. 704 E. Popescu et al. 8
The analysis of the Fig. 2 reveals also a secondary tendency for the
azimuthal orientation of the nodal planes, roughly perpendicular to the Carpathians
Arc bend. This kind of focal mechanism is observed from time to time at moderate-
to-large magnitudes (Mw 5 to 7). It looks like the rupture orientation and length
scale with the seismicity geometry (elongated along NE-SW, see Fig. 1): the
rupture process propagates always along NE-SW plane for the large events, while
for the smaller events propagate occasionally along a perpendicular direction
(NW-SE).
Fig. 3 – Diagram for P, T and B principal axes distribution for Vrancea zone (VNI). Top: all the
events; bottom: only events with at least 30 reliable polarities. The dots are proportional to Mw
magnitude; to the right, the color scale represents the depths of the events in km.
The distribution of the principal axes P, T and B, is represented in the Fig. 3
for the entire data set (259 Vrancea intermediate-depth earthquakes) and for the
9 Earthquake mechanism in the Vrancea subcrustal source Article no. 704
fault plane solutions obtained with minimum 30 polarities (31 events). In order to
classify the type of faulting we used the visualization procedure proposed by [24].
Clearly, the reverse faulting is dominating in the Vrancea subcrustal source,
independently of depth or magnitude ranges. This is even more evident when
plotting only the events with best constraint fault plane solutions (minimum 30
polarities). Practically, none of the events has well-defined normal or strike-slip
faulting; two events, located at the bottom edge of the descending seismically
active body, are characterized by strike-slip with normal components, and four
events by strike-slip with reverse components. Whereas, most of the events have
pure reverse faulting.
3.2. EASTERN MOESIAN PLATFORM SEISMOGENIC ZONE
The earthquakes with computed focal mechanism that we consider to fit in
the Moesian seismogenic zone are spread over a wide area, both to the south-east
and south of the Vrancea region and covering all the eastern Moesian Platform,
from Peceneaga Camena fault to Intramoesian fault (Fig. 1). Most of them (36) are
in connection with the Vrancea seismogenic area, while a few of them (6) can be
rather associated to the Intramoesian fault area. The events are of small-to-
moderate magnitude (Mw ≤ 3.3) belonging to the background seismicity. Four of
them are main shocks of local earthquake sequences: 30 April 2004 (15 events),
29 November–3 December 2007 (41 events), 6–30 September 2008 (42 events) and
6 December 2009 (23 events), while the other 38 are single events.
Fig. 4 – Ternary diagram for P, T and B principal axes distribution
for Moesian Platform seismogenic area.
Article no. 704 E. Popescu et al. 10
The diagram for P, T and B axes (Fig. 4) shows an equal distribution among
normal and reverse faultings and absence of pure strike-slip faulting. According to
our results, mechanisms of subsidence and folding are prevailing in the region
against transcurrent mechanisms.
3.3. PRE-DOBROGEAN DEPRESSION AND BÂRLAD DEPRESSIONS
The focal mechanisms were computed for 20 events located in the
Predobrogean Depression and some 8 events located in the Bârlad Depression. The
diagram for P, T and B axes (Fig. 5) shows broadly the same features as for the
eastern Moesian Platform (Fig. 4): an equal probability for normal and reverse
faulting and almost total lack of pure strike-slip faulting.
Fig. 5 – Diagram for P, T and B principal axes distribution for Pre-Dobrogean
and Bârlad Depressions.
This result suggests that despite the lateral differences among the tectonic
units acting in the foredeep area of the Carpathians the faulting processes are quite
similar, with prevalent subsidence and folding mechanisms.
Due to the reduce number of earthquakes in this zone and to the fact that the
orientation of the P, B, T axes is very much alike the distribution presented in
Fig. 4 for eastern Moesian Platform zone, the polar diagrams are represented for all
the crustal earthquakes considered in the two zones, a total of 70 events (Fig. 6).
The distribution of dip angles in Fig. 6 is close to the distribution find for
104 events in the same crustal region after the old catalog of earthquake
mechanisms [3], while the strike angles show the same almost equal distribution to
all directions.
11 Earthquake mechanism in the Vrancea subcrustal source Article no. 704
Fig. 6 – Angular diagrams for azimuth and dip angles of the nodal planes –
Eastern Moesian Platform area and PD-BD area.
3.4. FĂGĂRAŞ – CÂMPULUNG SEISMOGENIC ZONE
The catalogue of earthquakes with computed focal mechanism belonging to
the Făgăraş-Câmpulung seismogenic area includes 20 events, most of them located
in the southern side, towards the contact between Făgăraş Mountains and Moesian
Platform. All the events occurred in the 1998–2012 time interval belong to the
background seismicity (magnitudes no greater than 3.3).
Fig. 7 – Angular diagrams for azimuth and dip angles of the nodal planes for the events of Făgăraş –
Câmpulung area (FC).
The polar diagrams for the azimuthal distribution of the two nodal planes
(A and B) and for the plunge angle are plotted in the Fig. 7. The statistics is too low
to provide reliable trends in nodal plane orientation. Apparently, the fault planes
are equally distributed on azimuth, while the dip angles have some three principal
directions to 30, 60 and 85o.
Article no. 704 E. Popescu et al. 12
Fig. 8 – Diagram for P, T and B principal axes distribution for the Făgăraş –
Câmpulung (FC) seismogenic zone.
The picture shown by the diagram for P, T and B axes (Fig. 8) for the
Făgăraş – Câmpulung area, is somewhat different from that pointed out for the
other seismogenic areas in the crust. It is closer to the typical distribution of the
Vrancea subcrustal earthquakes, with a particular emphasis on reverse faulting
component. This result suggests the prevalence of folding processes as a
consequence of the convergent contact between Moesian Platform and Carpathians.
4. CONCLUSIONS
The catalogue of the focal mechanism for the earthquakes recorded in
Romania, available until 1997 ([2, 3]) is updated for the period 1998–2012. Thus,
the fault plane solutions for 259 intermediate-depth earthquakes (Vrancea source)
and 90 crustal earthquakes (Moesian Platform, Predobrogean Depression, Bârlad
Depression and Făgăraş–Câmpulung) are added to the existing catalogue (appendix
A). The fault plane solutions are computed in all cases by inverting the polarities of
the P-wave first arrivals manually picked up at the seismic stations of Romania,
Republic of Moldova, Bulgaria and Ukraine. To this aim, the SEISAN algorithm
[6] was applied. Only the solutions with minimum 10 polarities and acceptable
coverage on the lower hemisphere are selected.
The seismic activity that took place during 1998–2012 time frame was
limited to moderate-size events. The largest event was generated on 27 October
2006 (Mw = 6.0) in the Vrancea subcrustal source. The largest event generated in
the crust occurred on 3 Oct. 2004 (Mw = 4.9) in the North Dobrogean Orogeny.
13 Earthquake mechanism in the Vrancea subcrustal source Article no. 704
For this magnitude range, the fault plane solutions of the shallow earthquakes is
more likely related to local secondary faults than to the major faults located in the
Carpathians foredeep region. We can simply explain on this line the variety of
solutions in the Moesian Platform, Pre-Dobrogean Depression and Bârlad
Depression zones. The existence of a mosaic of blocks named “buffer plates” or
“sub-plates” have been postulated in the area located in front of Eastern
Carpathians Bend, between the zone of the major crustal faults like Intramoesian
fault, Peceneaga–Camena fault, Capidava fault and Trotus fault, which are
intersected by a secondary fault system which is roughly parallel with Carpathians.
In the sense of [32] these are small blocks situated between major plates, which
accommodate the relative displacements, like in a puzzle game.
As a general trend which is statistically consistent, note the deficit of strike-
slip faulting. This result suggests the prevalence of subsidence and folding
processes as stress release mechanisms in the entire Carpathians foredeep region,
as well as in Moesian Platform and Barlad depression. The hypothesis raised by a
few authors in the last century on a trans-current movement along the major faults,
crossing the aria situated between the Black Sea and Vrancea [31–33] is not
supported by our results.
More than that it appears that the seismic activity in eastern Moesian
Platform zone is limited to the east by Danube. In Central and Southern Dobrogea,
although they are considered to be tectonic provinces belonging to Moesian
Platform, there exist just a few earthquakes recorded in ROMPLUS catalogue.
Central and Southern Dobrogea are more stable zones.
There is no evidence for strike-slip earthquakes along the major faults in the
area, like Peceneaga – Camena fault and Capidava – Ovidiu fault, and they show
no signs of mobility for the interval of time considered. More than that in his study
of paleostress resulted from a great number of fault observations in Dobrogea, in
[34] the author considers that after Paleogene the Peceneaga-Camena fault, as other
faults in Dobrogea, were reactivated at certain moments only as reverse faults
without any strike-slip component until present. The historic seismic activity along
this faults in Dobrogea is clearly reduced, if we compare it to the seismic activity
along Sf. Gheorghe fault in Northern Dobrogea (Fig. 1).
To the western part of the Moesian Platform (Wallachian sector), located
west of the Intramoesian fault, very few earthquakes were recorded. The seismic
events are not clustering along Intramoesian Fault, so there is no evidence that this
crustal fault has still an active character. In fact, the Intramoesian fault have been
introduced following mostly tectonic considerations (the presence of a large,
potentially active fault) and not seismic data [7]. Although the fault is prolonged to
the south of Calarasi, in Bulgaria, until Shabla area, there is no seismic activity
evidence in this sector, until Shabla area, near the Black Sea.
The crustal seismic activity in this region can rather be seen as a response of
the intense processes taking place beneath the Carpathians Arc Bend (Vrancea
Article no. 704 E. Popescu et al. 14
area) and materialized by a triple number of earthquakes occurred at intermediate
depth in the same time interval, almost the same like in the previous catalogue [2].
For the combined seismogenic areas MO and PD-BD (70 events) the distribution of
the dip angles is close to the distribution find for 104 events in the same crustal
region after the previous catalogue of earthquake mechanisms [3], while the strike
angles show the same almost equal distribution to all directions.
Crustal earthquakes appear not to reflect the significant trans-current
motions between the tectonic units in the region along the main faults, but they
rather express the moving and re-positioning of different blocks in the area,
delimited by either the main fault system, which runs parallel to the Carpathians
Arc Bend or by secondary faults, distributed to a general direction which is roughly
perpendicular to the direction of the main fault system.
The fault plane solutions of the Vrancea earthquakes generated in sinking
plate in the mantle are preserving the typical features revealed by all the previous
studies on the subcrustal seismic activity in the Vrancea region: predominant dip-
slip, reverse faulting, characterizing both the weak and strong earthquakes [28, 29].
T axis close to vertical and P axis close to horizontal tending to be oriented
perpendicularly to the Carpathians arc.
The considerable increase of fault plane solutions accuracy for the
earthquakes recently recorded, due to the significant improvement of the Romanian
seismic network (www.infp.ro) and of their statistical representativeness, allows us
to individualize a few refined aspects not obvious in the previous investigations,
such as: (1) deficit of the strike-slip component over the entire Carpathians
foredeep area, (2) different stress field pattern in the Făgăraş – Câmpulung zone as
compared with the Moesian Platform and Pre-Dobrogean and Bârlad Depressions,
(3) a larger range for the dip angle of the nodal planes in the Vrancea subcrustal
source, ~ 40o –70o against ~ 70o, as commonly considered.
Acknowledgments. The database for this work is organized in Appendix A (www.infp.ro) and
it is part of this paper. The ternary diagrams in this paper are realized with a dedicated program which
was kindly made available by the author and which is described properly in Álvarez-Gómez (2014).
The work in this paper was realized in the frame of NUCLEU Program, project no. PN 16.35.01.08
and PN 16.35.01.03 (2016–2017). Part of the work was completed in the frame of the project no.
90/2014.
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