S0225

ANNALS OF GEOPHYSICS, 60, 2, 2017, S0225; doi: 10.4401/ag-7285

Source parameters of the earthquake sequence that occurred close
to the BURAR array (Romania) between 24 June and 1 July 2011
Emilia Popescu1, Anica Otilia Placinta1,*, Mircea Radulian1,2, Felix Borleanu1,
Mihail Diaconescu1, Mihaela Popa1

1 National Institute for Earth Physics, Magurele, Romania
2 Academy of Romanian Scientists, Bucharest, Romania

Article history
Received October 13, 2016; accepted February 3, 2017.
Subject classification:
Earthquake sequence, Spectral ratio, Empirical Green’s function, Source parameters, Scaling relationships.

ABSTRACT
The seismic activity in the Eastern Carpathians area is poorly recorded (a few
hundreds of small-to-moderate earthquakes in the Romanian catalogue over
the last century). The installation in 2002 of the high-performance Bucovina
(BURAR) array in the Eastern Carpathians area contributed to a signifi-
cant growth of the capacity to monitor local seismicity. As a consequence,
the earthquake sequence occurred between 24 June and 1 July 2011 close to
the BURAR array is the best seismic data set ever recorded for this area. The
location of the events using all the available data provided by the real-time
seismic network of the National Institute for Earth Physics suggests a NE-
SW alignment along the western edge of the Avramesti - Suceava fault. This
fault is crossing the Carpathian Foredeep underthrusting the foreland units
to the orogeny area. The distribution of the first P-wave polarities is fitting
the geometry of this fault, indicating predominant strike-slip faulting, with
right-lateral movement. The compression axis oriented E-W is in agreement
with the stress field characterizing the region. We applied spectral ratios and
empirical Green’s function methods to estimate the source parameters (corner
frequency, seismic moment, source duration, rise time) for the events with mo-
ment magnitudes higher than 2.5 belonging to this sequence. The results show
a simple fracture model for the main shock of 24 June 2011 and an apparent
constant stress drop scaling. Source parameter scaling relationships fit well
the results obtained for other regions along the South-Eastern Carpathians
and those which are typical for intra-continental areas.

1. Introduction
The Carpathians area is generally characterized by

low seismic activity, except a strongly clustered activity
concentrated at the Carpathians arc bend in the Vran-
cea region, in Romania [Ismail-Zadeh et al. 2012] and
references therein). Whereas numerous investigations
focused on the Vrancea seismicity, other areas of the
Carpathians are still largely unexplored. The Eastern
Carpathians segment in Romania is a good example in
this respect. One explanation is the lack of high-quali-

ty observations as a consequence of rare events in the
region and poor monitoring infrastructure.

With the recent enlargement of the national sei-
smic network [Neagoe et al. 2009, 2011; Popa et al.
2015] in the Eastern Carpathians area and installation
of a high-performance array in the Bucovina region
(Grigore et al., 2004; Borleanu et al., 2011; Ghica,
2011) the occurrence of an earthquake sequence in
June - July 2011 in the northern part of the Eastern
Carpathians (Figure 1), even if it was of moderate size,
provided the best data set ever recorded for the region
of the northern Moldavia and the possibility to apply
advanced techniques to constrain source parameters.

The goal of the present paper is to study the se-
quence recorded between 24 June and 1 July 2011, ta-
king advantage of the station coverage improvements
in the region after 2005 and primarily of the running
of the Bucovina (BURAR) array, located at about 50
km distance from the sequence epicentral area. This
array of small aperture (~ 5 km radius) was installed in
2002 in cooperation with the Air Force Technical Ap-
plication Center (AFTAC) of the U.S.A. The array con-
sists of 9 short-period elements with vertical sensors
(BUR01, …, BUR09) and one 3-component broadband
element (BUR31).

The sequence was generated at the western edge
of an alignment extended from the Carpathian Fore-
deep and underthrust foreland units to the orogeny
area (Avramesti-Suceava Fault). In order to estimate
source parameters, we apply relative methods of de-
convolution, such as empirical Green’s functions and
spectral ratios methods.

Similar investigations were performed for earth-
quake sequences generated in the southern segment

POPESCU ET AL.

of the Carpathians [Enescu et al. 1996; Popescu 2000; Po-
pescu and Radulian 2001; Popescu et al. 2003, 2011, 2012;
Radulian et al. 2014, Placinta et al. 2016]. They basically
consist of applying relative techniques of investigation,
such as the spectral ratios and empirical Green’s fun-
ctions techniques that proved to be efficient in retrieving
source parameters for seismic sequences.

First, source parameters are determined, and then,
source scaling properties are subsequently investigated.
Finally, the contribution of the new results in improving
our understanding of the seismotectonics along the Car-
pathians orogeny is discussed.

2. Regional seismotectonics
The northern and central parts of the Moldavian

Platform and Carpathians Orogen show a low-to-mode-
rate crustal seismic activity (Figure 1), in contrast with
the sharp concentration of earthquakes in the Vrancea
region, located south of the Trotus Fault, at the Eastern
Carpathians bending zone. According to the Romplus
catalog [Oncescu et al. 1999], during January 1900 - April
2014, 874 small-to-moderate events (1.1 ≤ Mw ≤ 5.5) oc-
curred in this area, that we consider to belong mainly to
a low background seismicity and to man-made activity.
One single event has the magnitude Mw greater than 5,

but it is an event with no instrumental recordings, whi-
ch occurred on 31 January 1900, 09:00, lon. 27.300E, lat.
46.500N. The earthquake is located close to the Bistriţa
Fault. Most probably the magnitude (Mw = 5.5) is ove-
restimated.

The locations of a few significant earthquakes in the
region are represented in Figure 1. They can be associa-
ted with the principal faults crossing the platform region
to the Carpathians orogen. Thus, we mention: events re-
corded along the Trotuş Fault that occurred on 18 April
1956 (Mw = 4.5), 12 October 1959 (Mw= 4.1) and 16 Sep-
tember 1965 (Mw = 4.5), along the Bistriţa Fault on 17 Oc-
tober 1906 (Mw = 4.9) and 6 November 1997 (Mw = 3.1),
along the Vaslui Fault on 8 November 1905 (Mw = 4.2)
and 5 May 1981 (Mw = 3.2) and along the Avrămeşti-Suce-
ava Fault on 20 January 1903 (Mw = 4.1), 20 October 1979
(Mw = 3.7) and 24 June 2011 (Mw = 3.8). Note that all
the events with magnitude larger than 4 took place until
1970, when the Romanian seismic network performance
was modest and the accuracy in magnitude and location
parameters was poor.

The relative enhancement of seismicity in the area
between the Trotuş and Vaslui Faults reflects perhaps
the transition from a stable segment (Moldavian Pla-
tform) to the active segment related to the Vrancea
seismic activity, located south of the Trotuş Fault. The
Trotuş Fault is an old Jurassic fracture, separating the
Scythian Platform from the Moesian Platform, which
looks like to be still active [Enciu et al. 2009, Van der
Hoeven et al. 2005, Săndulescu 2009]. Another concen-
tration of events is visible along the Neogene volcanic
chain Călimani - Gurghiu - Harghita (but only events
below Mw 4.0 magnitude), located in the inner side of
the Carpathians. However, the hourly distribution of
the number of events indicates that a significant per-
centage of the activity in the Trotuş Fault region and
especially in the Călimani - Gurghiu - Harghita region
is due to quarry activities.

In the northern part of Moldavia the reported sei-
smicity is sparse and is probably related to marginal
fractures of the Moldavian Platform (part of the Ea-
stern European Platform). In fact, the marginal fractu-
res of the Platform are situated to the east of the epi-
centers [Polonic 1986]. In general, the epicenters are not
following the main faults alignments excepting some
clustering around central segment of the Trotus Fault.
Non-coincidence between the epicenters and identified
faults and the observation that the region exhibits an
uplift neotectonic movement, lead to the idea that these
earthquakes are caused by flexure that break and give
small normal faults, with the eastern side having a ten-
dency to upraise, while the western side of the fault is

Figure 1. Sketch of seismotectonic map of the contact between
the Moldavian and Scythian Platforms (east side) and Carpathians
Orogen (west side). Setting in Europe is given in the upper right
corner inset. The study region is marked by the blue square. The
epicenters of the events recorded in the Romplus catalog (Oncescu
et al., 1999) are plotted with red symbols for crustal earthquakes
and yellow symbols for subcrustal earthquakes (Vrancea seismic
region). The seismic stations of the National Institute for Earth
Physics – Măgurele (Romania), located inside the map, in opera-
tion at that time, are plotted with solid triangles. The epicenters of
the sequence of 24 June – 1 July 2011 are plotted as black dots (the
main shock – black star). Significant events recorded in the region
since 1900 are represented by green stars.

EARTHQUAKE SEQUENCE CLOSE TO BURAR ARRAY

immobilized under molasses deposits. Such an interpre-
tation is consistent with the generally accepted concept
of termination of under pushing east-west movement
of the foreland from the Moldavian Platform under
Carpathians. The cluster of events recorded in the inner
side of Carpathians (Harghita region) is rather associa-
ted with man-made activity due to quarry blast exploi-
tation. The deficit of seismicity in comparison with the
southern and south-western parts of Moldavia can be
partly explained by the poor coverage of the Romanian
seismic network in the northern part of Moldavia.

3. Description of the earthquake sequence
The crustal seismic sequence produced in the vici-

nity of Bucovina seismic array in June 2011 is a singular
seismic phenomenon for this region, as far as we have
available information. As Figure 1 shows, the back-
ground seismicity in this region is diffuse and poor,
and therefore no preferential alignments of seismic
sensibility and seismo-genetic contoured areas can be
identified. However, the epicenters distribution of the
studied seismic sequence apparently follows the align-
ment of the Avrămeşti - Suceava Fault (more precisely,
the western edge of this fault) which crosses perpen-
dicularly the entire fault system oriented NNW-SSE
(Figure 1). Nevertheless, we should consider carefully
this assumption taking into account not necessarily the
location errors (the maximum axis or errors ellipsis is
below 6 km), but rather the configuration of stations
with a large gap towards N-NE azimuth.

More than 40 events were identified as belonging
to the sequence, but only 9 of them are well located
(Table 1) using all the available data recorded by the re-
al-time seismic network of National Institute for Earth
Physics (short-period and broadband seismometers).
We retained in our data set only the events located with
minimum 6 stations and 8 phases. The locations were
performed using LOCSAT routine which runs under
ANTELOPE (BRTT) software, routinely operated by
NIEP (National Institute for Earth Physics). To locate
the events, we used P - and S - wave travel times ma-
nually read on the seismograms. The aftershocks spre-
ading around the main shock epicenter (Figure 1) seem
to indicate a unilateral rupture for this event along the
Avrămeşti-Suceava Fault, dipping toward SE. Certainly,
a rupture length close to 10 km, as suggested by after-
shocks distribution, extends far beyond the rupture di-
mension typically observed for an earthquake of magni-
tude 3.8 (no more than 1 km). Therefore, the apparent
length is most likely excessively large due to the uncer-
tainties in the locations.

The number of stations used in location process
varies from 6 (for the events of 24 June 2011, at 13:31
and 16:18) to 30 stations (for the main shock). Three sta-
tions belonging to the seismic network of the Republic
of Moldova (LEOM, MILM, SORM) were included as
well. The largest azimuthal gap between azimuthally
adjacent stations (GAP) is around 120°, while the RMS
value spans the interval 0.34 to 0.75 s. The quality of re-
ading the P- and S-wave phases is good for BURAR and
5 stations (SORM, TESR, MILM, JOSR, ARCR), while is
generally poor for the more distant stations.

It is not possible to constrain the fault-plane so-
lution of the main shock by inverting the first P-wave
polarities (10 polarities picked with high confidence).
However, to test if the Avrămeşti - Suceava alignment
coincides with a possible nodal plane, we projected
this fault on a lower hemisphere (Figure 2). The fault
azimuth (N48°E) and dip (56°) are estimated simply
from the geometry of the fault mapping in connection
with the orientation of earthquake locations (Figure 1)
and focal depth (Table 1). The conjugate nodal plane
that approximates to some extent the distribution of
reliable P-wave polarities is drawn in the same figure
(plane 2). However, some stations are slightly outside
the nodal plane 1 (fitting the assumed Avrămeşti - Su-
ceava Fault). As a conclusion of our investigation, we
can assume either that the Avrămeşti - Suceava Fault
is shifted a bit to the NW relative to the epicenters (or

Figure 2. A possible fault plane solution of the main shock of the
BURAR sequence, 24 June 2011, 13:08, Mw = 3.8. The nodal planes
with solid lines correspond with the Avrămeşti - Suceava Fault vs.
epicentral distribution geometry. As shown in the text, a correction
like that represented by dashed lines is matching better the P-wave
polarities. Empty and solid circles are for compression and dilata-
tion, respectively.

POPESCU ET AL.

vice versa), or the focal depth of the main shock (h = 10
km) is smaller (h ~ 5 km).

A possible focal mechanism that we selected as
the one best fitting all the polarities has the nodal pla-
ne 1 with the same azimuth (N48°E), while the dip is
lower (43°) - see nodal planes represented by dashed
lines in the Figure 2. The focal mechanism is predomi-
nantly of strike-slip type, with right-lateral movement
of the south-east compartment. The compression axis
oriented E-W is in agreement with the stress field cha-
racterizing the region. A slight underthrust of the nor-
thern compartment under the southern compartment
is also noticed (southern compartment is lifted and shi-
fted towards south-west).

4. Source parameters
The relative deconvolution methods (spectral ra-

tios and empirical Green’s function deconvolution) are
efficiently retrieving source parameters (seismic mo-
ment, source radius, rupture duration, rise time and
stress drop) when waveforms from pairs of co-located
events are available at common broadband stations
[Frankel et al. 1986, Hough et al. 1989, Lindley 1994,
Mueller 1985, Mori and Frankel 1990, Popescu et al.
2016]. Typically for this class of methods, the path, site
and instrument effects are removed by deconvolving
the waveform of a lower magnitude event from the
main event waveform. The same approach was ap-
plied to other earthquake sequences occurred in the
South-Eastern Carpathians area [Popescu 2000, Pope-
scu and Radulian 2001, Popescu et al. 2003, 2011, 2012,
Radulian et al. 2014, Placinta et al. 2016].

We applied the spectral ratios technique in paral-
lel with the empirical Green’s function deconvolution
for the earthquakes given in Table 1. The quality of re-
cordings for the last event (1 July) is poor and therefore,
the application of relative methods in retrieving seismic
source parameters cannot be properly done for this

event. We limit in this case to the estimation of magnitu-
de and seismic moment.

Spectral ratios depend essentially only on the source
when the selection of earthquakes pairs is properly done
and, in this case, it is not necessary to apply path, local
and instrument response corrections. Another advantage
of the method is the possibility to simultaneously deter-
mine the corner frequencies for both earthquakes of a
selected pair, as long as the instrument is broadband and
signal-to-noise ratio (SNR) is high enough in the frequen-
cy of interest. As concerns the source size, we obtain only
the ratio of seismic moments. To estimate the absolute
values, we need an independent determination for one
event (reference value). We selected the largest earth-
quake as reference event and applied relation (3) below
to compute its seismic moment. Then we compute the
absolute values for all the other events using the spectral
ratios values.

For a source model with uniform rupture and ω-2
spectral fall-off at high frequencies, the spectral ratios
(R(f )) can be approximated by the theoretical function:

(1)

where Ω0
P, Ω0

G are the low-frequency asymptotes of am-
plitude spectra of principal and Green’s earthquakes, and
fc

P, fc
G are the corresponding corner frequencies and γ is

the coefficient of the spectral fall-off at high frequency.
Selecting as free parameters the ratio of seismic moments
a = lg(Ω0

P/Ω0
G) and the corner frequencies, we apply a

nonlinear regression procedure in order to find the fun-
ction (1) that best approximates observed spectral ratios.

We used all the components (Z, E, N) of the wave-
forms and all the common stations available to estimate
the parameter values a, fc

P, fc
G resulting from spectral ra-

tio method application. The final estimates are the ave-
rage values over all the specific values for different event

R( f )
Ω0

P 1+( f / fc
G)2γ⎡⎣

⎤
⎦
1/2

Ω0
G 1+( f / fc

P)2γ⎡⎣
⎤
⎦
1/2

No. Date
(yyyy/m/day)

Origin time
(hh:mm:ss)

Lat.
(°N)

Lon.
(°E)

Depth
(km)

MD/MW No.
stations

RMS GAP

1. 2011/6/24 13:06:36.8 47.399 25.768 11 2.9/3.1 10 0.73 136

2. 2011/6/24 13:08:40.5 47.372 25.777 10 4.6/3.8 30 0.69 107

3. 2011/6/24 13:31:29.1 47.359 25.732 3 2.8/3.1 6 0.34 110

4. 2011/6/24 16:18:57.9 47.355 25.756 5 2.6/2.9 6 0.47 118

5. 2011/6/25 00:13:47.3 47.370 25.776 8 3.0/2.9 7 0.57 119

6. 2011/6/25 01:43:21.6 47.344 25.765 10 3.2/3.1 10 0.73 120

7. 2011/6/30 21:21:18.5 47.349 25.764 5 3.0/2.8 22 0.65 116

8. 2011/6/30 21:22:01.5 47.345 25.730 4 2.9/2.8 18 0.58 120

9. 2011/7/01 22:20:25.4 47.333 25.692 3 2.3/2.5 15 0.75 116

Table 1. Earthquake parameters for the study sequence. The main shock is marked with bold.

pairs, stations and components. They are given in Annex.
Examples of the spectral ratios for two earthquake pairs
and two stations are plotted in the Figure 3.

The size of the rupture area is directly related to the
corner frequency [Madariaga 1976]:

(2)

r representing the equivalent radius of the source while
k is a constant value of 0.32 for P waves and 0.21 for S
waves, fc is the corner frequency and VS is the S-wave
velocity in the focus. With relationship (2) we determine
the source radius from corner frequencies (rGrs - radius
of Green function obtained from spectral ratios, rPrs ra-
dius of the main event obtained from spectral ratios).

The source radius estimated from corner fre-
quency is an average between the estimations using
P-wave and S-wave corner frequencies. Since the rela-
tion (2) assumes a ratio about 1.5 of fc

P/fc
S and for our

data fc
P≈ fc

S, the radius computed from fc
P is systema-

tically greater than the radius computed from fc
S rou-

ghly by a factor of 1.5 (for example, for main event,
r = 274 m from fc

P and r = 177 m from fc
S).

The seismic moment for the earthquakes analy-
zed in this study is estimated using:

(3)

ρ is the density at the source depth, VP is the velocity

of P-waves at source depth, Ω0 is the long period displa-
cement spectral level, R is the hypocentral distance and
Rθφ is the source radiation pattern (average values of
0.52 for P waves and 0.63 for S waves, according to [Aki
and Richards 1980]. We adopted for VP and ρ parame-
ters the values as resulted from the velocity structure
model estimated by Raileanu et al. [2012].

After seismic moment and source radius are cal-
culated, the Brune stress drop [Brune 1970] is compu-
ted using:

(4)

For the same pairs of events considered in the
spectral ratios method, we applied in parallel the
method of deconvolution with empirical Green’s fun-
ctions. The source rise time τ1/2, and the source du-
ration τ, for the main events, are estimated from the
source time function each time it had a pulse - like sha-
pe. In this case the source radius was computed using
Boatwright’s formula [Boatwright 1980]:

(5)

where τ1/2 is the source rise time, v is the rupture veloci-
ty in the source, considered as v = 0.9 β (with β- S-wave
velocity at the seismic source depth), α - P-wave velocity

r = kVs / fc

M0 =(4 πρ VP
3Ω0R)/Rθϕ

r = (τ1/2v) /1−v /αsinθ)

EARTHQUAKE SEQUENCE CLOSE TO BURAR ARRAY

Figure 3. Spectral ratios obtained for the main event of 24 June 2011, 13:08 and empirical Green’s functions of 30 June 2011, 21:21
(MW=2.8) – left column and 21:22 (MW=2.8) – right column: a) at BURAR station and b) at TESR station. The dashed line represents the
best approximation with a theoretical function given by relation (1).

ΔσB =
7Mo
16r3

POPESCU ET AL.

at the source depth, θ - the angle between the normal
to the fault and the output direction of P waves from
hypocenter. In case the main event has more Green’s
functions associated, rise time is the average of all
obtained values of τ1/2 (different Green functions and
different stations).

An example of main event - empirical Green’s
function pair is given in Figure 4. We plotted the main
shock of 24 June 2011, 13:08 with two associated empiri-
cal Green’s functions of 24 June, 13:06 and 25 June 2011,
01:43 as recorded at two stations, BURAR and TESR.

The average source time function for the main
event, calculated as the arithmetic mean of the sour-
ce time functions obtained for each pair of co-loca-
ted events at each station, whenever these functions
show well-defined patterns, is represented in Figure 5.
In this case eight STFs were accepted for the average,
including different station components. Source time
function is a simple unipolar pulse, which supports the
hypothesis of a homogeneous rupture pattern over
0.18 s duration.

We applied for the same selected pairs of events
the method of deconvolution with empirical Green’s
functions to obtain the source duration τ and rise time

τ1/2, for the main event, using the available stations for
all co-located pairs.

Based on the seismic moment, corner frequency
and source duration estimations, we determine the
source area and stress drop using relations (2) - (4).
To apply equation (4), we adopted an average value
of 30°for the take-off angle with respect to the nor-
mal to the fault. This angle takes into account source
directivity effects which are likely to be negligible for
such small earthquakes. For θ varying between 0 and
45°, the variation in r is slightly higher than 30%. The
difference between the source radius inferred from
the duration and that inferred from corner frequency
would suggest some inadvertencies in the parameters
of relations (2) and (4).

The results are presented in Table 2 for the main
event and for the empirical Green’s function events.

5. Scaling relationships
The scaling relationships for earthquake sequen-

ces are valuable indicators of geotectonic peculiarities
of the area under investigation. Such studies have been
done previously for earthquake sequences occurred in
the South-Eastern Carpathians foredeep region [Ene-

Figure 4. Examples of waveforms: a) main event of 24 June 2011, 13:08 and the associated empirical Green’s function of 24 June 2011, 13:06
at the broadband element of the BURAR array (right); b) main event of 24 June 2011, 13:08 and the associated empirical Green’s function
of 25 June 2011, 01:43 at TESR station (left).

EARTHQUAKE SEQUENCE CLOSE TO BURAR ARRAY

scu et al. 1996, Popescu 2000, Popescu and Radulian
2001, Popescu et al 2003, 2011, 2012, Radulian et al.
2014, Placinta et al. 2016]. Up to now, there is no sy-
stematic investigation of the seismic activity recorded
in the northern part of the Moldavian Platform. From
this point of view, the determination of source para-
meters and of the corresponding scaling relationships
for the sequence in the northern part of Moldavia pro-
vides new insights in the seismotectonics of this area.

The scaling of the seismic moment M0 with dura-
tion magnitude MD is shown in Figure 6. The data are
approximated by the linear regression:

(6)

However, the regression is based on only nine
points, eight of them covering a narrow magnitude

range with practically no correlation. For this reason,
the regression parameters depend critically on the sin-
gle earthquake with magnitude above 4. It is highly re-
commended to adopt such scaling only after including
more observation data. The corresponding moment
magnitude - duration magnitude scaling tends to ove-
restimate the earthquake size inferred from duration
for moderate earthquakes and to underestimate it for
small earthquakes.

The inconsistency between the two magnitude
scales could be explained by the significant difference
in S/N ratio as we go to smallest or biggest events.
Below a certain magnitude, it becomes difficult to

distinguish signal from noise and duration tends to
saturate. By contrary, for larger events the S/N is hi-
gher and there is a higher probability to limit the du-
ration measurement before some later phases.

Scaling of seismic moment with source radius (Fi-
gure 7) is well approximated by the linear regression:

(7)

The slope of the regression line comes close to
the theoretical value (3) which characterizes the sei-
smic source scaling in case of homogeneous rupture
process.

The scaling of stress drop with earthquake size
(Figure 8) indicates a constant stress drop over the en-
tire magnitude range. However, we should keep in
mind that the uncertainties in stress drop are ampli-
fied relative to the source radius (corner frequency)
uncertainties because of the power law dependence

logM0 = (0.88±0.13)MD +(10.89±0.43)

Figure 5. Source time function for the main shock of 24 June 2011,
13:08 (continuous line) calculated as the arithmetic mean of the va-
lues obtained for empirical Green’s function deconvolution at all sta-
tions and components. The dashed lines represent the standard error.

R = 0.94,σ = 0.22

Event Seismic moment
(Nm)

τ (s) fc (Hz) Source radius (m) Stress drop (MPa) Mw MD
from τ1/2 from fc

24.06.2011, 13:08 1.01x1015 0.180 3.72 282 227 37.8 3.8 4.6

Event Seismic moment
(Nm)

Source radius (m)
from fc

P from fc
S average

Stress drop (MPa) Mw MD

24.06.2011, 13:06 6.25x1013 89 72 81 51.5 3.1 2.9

24.06.2011, 13:31 6.03x1013 105 70 88 38.7 3.1 2.8

24.06.2011, 16:18 3.58x1013 - 71 71 43.8 2.9 2.6

25.06.2011, 00:13 4.20x1013 115 83 99 18.9 2.9 3.0

25.06.2011, 01:43 8.21x1013 133 84 109 27.7 3.1 3.2

30.06.2011, 21:21 2.25x1013 83 53 68 31.3 2.8 3.0

30.06.2011, 21:22 2.08x1013 75 53 64 34.7 2.8 2.9

01.07.2011, 22:20 1.35x1013 - - - - 2.5 2.5

Table 2. Final source parameters for the main shock and for the empirical Green’s functions used in this study.

logM0 = (2.92±0.31)logr+(8.04±0.60)

R = 0.97,σ = 0.14

Figure 6. Scaling of seismic moment with duration magnitude.

POPESCU ET AL.

(equation (4)). The average stress drop value (~ 30 - 40
MPa = 300 - 400 bar) is characteristic for faulting pro-
cesses in intra-continental areas (large stress drops).
According to our results, the earthquake sequence was
generated in an area that has been less fractured before.

It is interesting to notice (as shown in Table 2) that
the highest value of the stress drop was recorded for
the foreshock on 24 June at 13:06, in agreement with
the hypothesis of a poorly fractured area prior the se-
quence triggering.

The scaling of the corner frequency and source
duration with duration magnitude is represented in
Figures 9-10. We combine in the graphical represen-
tation the estimates of the present study with estima-
tes previously obtained for earthquakes generated in
different other areas of the South-Eastern Carpathians

(see references above). In all cases, the same procedure
was applied to retrieve corner frequency and source
duration parameters.

The regression lines, approximating the scaling
relationships, are:

(8)

(9)

The slopes in the relations (8) and (9) are close
each other in absolute value (with opposite signs).
Therefore, we can assume that the corner frequency
scales as a simple inverse of duration:

(10)

Figure 7. Scaling of seismic moment with source radius.

Figure 8. Scaling of stress drop with seismic moment. The regres-
sion line is close to a constant stress drop scaling around 35 MPa.

Figure 9. Scaling of corner frequency with duration magnitude.
Data for South Carpathians from Radulian et al (2014), for Vrin-
cioaia region from Popescu et al. (2012) and for Vrancea foredeep
from Popescu et al. (2001) and Popescu et al. (2011). The regression
line is estimated for all the data points.

Figure 10. Scaling of source duration with duration magnitude.
Data for South Carpathians from Enescu et al. (1996) and Radulian
et al. (2014); for Vrincioaia region from Popescu et al. (2012) and
for Vrancea foredeep from Popescu et al. (2001) and Popescu et al.
(2011). The regression line is estimated for all the data points.

log fc = −(0.20±0.03)MD +(1.40±0.10)

r = 0.76,σ = 0.12

logτ = (0.19±0.02)MD −(1.50±0.09)

r = 0.87,σ = 0.06

fc ~τ
-1

in agreement with the relation (6) used by Boore
[1983].

6. Conclusions
The earthquake sequence produced in the area

adjacent to Bucovina Seismic Array (BURAR) betwe-
en 24 June and 1 July 2011 is the single such a seismic
phenomenon recorded until now in this region. Hypo-
center locations using all the available data recorded by
the real-time seismic network of the National Institu-
te for Earth Physics (NIEP) show a NE-SW alignment
along the Avrămeşti-Suceava Fault. It is likely that the
study seismic sequence was a consequence of a sudden
activation of the south-western edge of this fault. The
fault plane solution is not constrained by the available
data. The reliable P-wave polarities suggest a strike-
slip faulting with a nodal plane close to the Avrăm-
eşti-Suceava Fault.

The source parameters (corner frequency, seismic
moment, source duration, rise time) are estimated by
applying spectral ratios technique and empirical Gre-
en’s function deconvolution. The results show a sim-
ple fracture model for the main shock of 24 June 2011,
with a unipulse source time function and a constant
stress drop scaling. The stress drop level is compatible
with stress regime in intra-continental settings. Sour-
ce parameter scaling relationships fit well the results
obtained for other regions along the South-Eastern
Carpathians and those which are typical for intra-con-
tinental areas.

Acknowledgements. Data used in the present study were
provided by the National Institute for Earth Physics. The work
was partially supported by the project “Nucleu”(PN 16 35 01 08)
of the National Plan for Research, Development and Innovation
of the Romanian Ministry of National Education and by Project
69/2014, DARING, under The Executive Unit for Financing Hi-
gher Education, Research, Development and Innovation, Program
Partnership in Priority Areas, Collaborative Applied Research
Projects C2013.

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*Corresponding author: Anica Otilia Placinta
National Institute for Earth Physics, Magurele, Ilfov, Romania;
email: anca@infp.ro.

2017 by Istituto Nazionale di Geofisica e Vulcanologia.
All rights reserved

ANNEX
Parameter values “a”, fcP, fcG resulting from spectral ratios method analysis. Event P corresponds to main

event (event 2 in Table 1). The number assigned to each Empirical Green’s Function (EGF) is the same as in Table 1.

EARTHQUAKE SEQUENCE CLOSE TO BURAR ARRAY

Pair P-1
EGF:11/06/24, 13:06

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB 1.79 1.87 1.84 4.79 3.89 3.89 7.80 5.21 5.21

SORM - 1.65 1.84 - 3.38 3.30 - 7.00 7.92

TESR 1.84 1.70 1.79 3.22 3.35 3.24 10.40 9.40 9.50

Average/component 1.820
±

0.035

1.740
±

0.115

1.823
±

0.029

4.00
±

1.11

3.51±0.30 9.10
±

1.84

7.37±1.92

Average/EGF 1.793±0.079 3.633±0.541 8.235±1.223

Pair P-3
EGF:11/06/24, 13:31

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB 1.91 1.93 1.98 3.71 4.15 3.94 7.50 6.80 7.20

TESR 2.07 1.92 1.93 3.28 3.55 3.78 9.32 9.00 10.00

Average/component 1.990
±

0.113

1.925
±

0.007

1.955
±

0.035

3.495
±

0.304

3.855±0.254 8.410
±

1.287

8.250±1.509

Average/EGF 1.957±0.061 3.735±0.303 8.330±0.113

Pair P-4
EGF:11/06/24, 16:18

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB - 2.44 2.37 - 4.92 4.66 - 9.00 7.00

SORM - 2.09 2.14 - 3.18 3.96 - 6.65 10.00

Average/component - 2.265
±

0.247

2.255
±

0.163

- 4.180
±

0.780

- 8.163±1.604

Average/EGF 2.260±0.171 3.735±0.303 8.330±0.113

Pair P-5
EGF:11/06/25, 00:13

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB 2.13 2.20 219 3.89 3.59 3.80 6.56 5.39 5.61

SORM 1.92 2.16 2.18 4.62 3.20 3.22 9.52 8.85 10.00

TESR 2.13 2.02 2.18 2.97 4.61 3.29 10.00 8.30 9.20

Average/component 2.060
±

0.121

2.127
±

0.095

2.183
±

0.006

3.827
±

0.827

3.618±0.540 8.693
±

1.863

7.892±1.934

Average/EGF 2.123±0.094 3.712±0.571 8.293±0.556

POPESCU ET AL.

Pair P-6
EGF:11/06/25, 01:43

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB - 1.73 1.74 - 4.40 4.34 - 6.00 6.00

JOSR 1.78 1.80 1.73 3.49 3.12 3.21 7.20 8.30 7.80

MILM 1.67 1.60 1.81 3.04 3.78 3.60 5.39 5.79 7.16

SORM 1.60 1.62 1.65 3.36 3.70 3.48 6.25 6.00 6.15

TESR 1.82 1.94 1.96 3.84 4.07 4.54 7.63 7.13 8.34

Average/component 1.718
±

0.101

1.738
±

0.139

1.778
±

0.116

3.433
±

0.331

3.820±0.498 6.618
±

1.001

6.867±1.001

Average/EGF 1.746±0.115 3.712±0.480 6.743±0.167

Pair P-7
EGF:11/06/30, 21:21

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB 1.91 1.81 1.81 3.02 3.41 3.46 5.25 4.90 5.40

JOSR 1.95 - 2.25 3.5 - 2.61 8.81 - 9.31

TESR 1.96 2.12 2.00 3.50 3.51 4.40 8.85 9.88 10.03

Average/component 1.940
±

0.026

1.965
±

0.219

2.020
±

0.221

3.340
±

0.277

3.478±0.634 7.673
±

2.067

7.904±2.534

Average/EGF 1.976±0.150 3.426±0.507 7.789±0.163

Pair P-8
EGF:11/06/30, 21:22

a
(spectral ratio)

fc (Hz)
(main shock)

fc (Hz)
(Green function)

Z E N Z E N Z E N

BURB 1.74 1.81 1.79 4.20 3.60 3.14 6.93 9.00 7.00

JOSR 1.96 2.01 2.12 3.46 3.07 2.92 9.11

TESR 1.90 2.04 2.01 3.93 4.22 4.16 9.71 6.65 10.00

Average/component 1.867
±

0.114

1.953
±

0.125

1.973
±

0.163

3.863
±

0.374

3.515
±

0.563
-

8.583
±

1.463

7.975±2.480

Average/EGF 1.931±0.129 3.633±0.516 8.279±0.430