Доклади на Българската академия на науките
Comptes rendus de l’Académie bulgare des Sciences
Tome 66, No 5, 2013
GEOPHYSIQUE
Sismologie
THE MOHO DEPTH AND CRUSTAL STRUCTURE
BENEATH BULGARIA OBTAINED FROM RECEIVER
FUNCTION ANALYSIS
Gergana Georgieva, Svetlana Nikolova
(Submitted by Corresponding Member D. Solakov on January 17, 2013)
Abstract
Receiver function technique is applied to evaluate the Moho depth and
VP /VS ration on the territory of Bulgaria. The method was applied to eleven
stations of the Bulgarian National Seismological Network (BNSN) equipped
with broadband seismometers and the results are presented in this paper.
The results obtained show that the Moho depth varies between 30 and
50 km. The crust is shallower in northeast of Bulgaria and goes deeper in
southwest direction. The crust is thicker in the Rhodopean massif, where it
reaches more than 50 km. The VP /VS ratio in the study area varies between
1.60 and 1.90.
Key words: receiver function analysis, Moho structure, VP /VS
Introduction. In the last decade, receiver function analysis [1–4 ] became a
reliable method for mapping of the crustal and upper mantle structure close to
the seismic stations. After the upgrade of the Bulgarian National Seismological
Network (BNSN) to digital data acquisition and storage systems, the receiver
function technique can be applied to study the discontinuities in the crust on
the territory of Bulgaria and especially the structure and the depth of the Moho
boundary.
Data. The upgrade of the National Operative Telemetric System for Seismological Information (NOTSSI) in Bulgaria was completed in the last quarter of
2005 [5 ]. Currently BNSN consists of 14 stations and the recorded data are transmitted in real time to the Bulgarian National Data Centre for Automatic and
Interactive Data Processing [6 ]. Data from 11 stations, equipped with broadband
(BB) seismometers are used in this study.
Most of the events used for receiver function analysis are recorded in the
period from January 2006 to July 2010. The analyzed seismic events are of
magnitudes from 5.5 to 7.5, and with epicentral distances from 30◦ to 95◦ . After
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the removal of noisy traces from stacked seismograms, a set of about 200 traces
for each station was generated. A good azimuthal coverage was achieved for each
station.
Method. Receiver functions reflect the reaction of the Earth structures
beneath seismic station to the coming seismic wave. S wave is generated when the
P wave crosses the border between two layers with different velocities. Converted
S wave has a velocity smaller than the velocity of the P wave and it is coming
short after the P wave in the seismic station. The depth of the discontinuity
which generated the S waves could be restored if the time difference between
both phases is estimated.
The P wave coda from a teleseismic event contains P -to-S wave conversion.
The converted S wave is usually weak and it is necessary to remove P wave
to enable detection of the converted phase. This can be done by choosing a
local ray coordinate system LQT (P , SV and SH), where: the L component is
in the direction of the P wave; the Q component is perpendicular to L and is
selected so that the most of the P s energy is in this direction; the T component
is perpendicular to the LQ plane. In the optimal conditions for the horizontal
layered Earth, with the homogenous layers, no P s energy should be observed in
T direction. However, in the real data very often some energy can be observed
on the T component. The reason is usually lateral anisotropy in the Earth crust
beneath the seismic station.
Deconvolution of the seismic traces is performed after their rotation into
LQT coordinate system. Deconvolution is applied to the SV component by the
incident wave of P component. The resulting receiver functions are sorted by
azimuth and staked together. A summed receiver function is computed for each
station and it is used further for inversion and reconstruction of the velocity
model.
The depth of the Moho discontinuity is estimated using the method developed by Zhu and Kanamori [7 ]. The crustal thickness H and VP /VS ratio are
computed for each event and the results are plotted together in H-κ domain
defined as
s(H, κ) = w1 r(t1 ) + w2 r(t2 ) + w3 r(t3 ),
where t1 , t2 , t3 are arrival times of the P –S converted phase and following multiplies (P pP s and P pSs + P sPP
s), and w1 , w2 and w3 are the weighing factors for
each phase selected, so that
wi = 1. Function s(H, κ) reaches its maximum
when all phases are stacked coherently.
This method gives reliable results when the Moho discontinuity is flat and
there are no complex structures or strong anisotropy in the crust. The estimated
error of depth in this case is about ±0.5 km.
Results. We computed the receiver functions for 11 stations from the BNSN
(Fig. 1) equipped with BB seismometers. Because the receiver function calculations require initial velocity model, we combined two velocity models for the
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G. Georgieva, S. Nikolova
Fig. 1. Bulgarian National Seismological Network: the triangles show the stations used in
the study; the stars show stations not used in the study; the circles show stations from Local
Seismological Network of Provadiya
purposes of the present study: the velocity model for the Earth crust down to
100 km depth, as obtained by Raykova [8 ], and the IASPEI velocity model for
depth greater than 100 km. The velocity model by Raykova [8 ] has granularity
of 1◦ × 1◦ and therefore for each station a specific initial model was applied for
better representation of the local structure. We assume as representative the
estimates of the thickness of the crust and VP /VS ratio if both values have probability higher than 95%. These H and k correspond to coherently stacked phases
(Figs 3c and 4c).
The results from 11 seismic stations can be divided into three groups based on
their location: Northern Bulgaria (3 stations), Upper Thracian Plain (3 stations)
and the region of Rila and the Rhodopes (5 stations).
We could reliably identify two discontinuities in stacked seismograms for the
stations in Northern Bulgaria. The first one corresponds to the boundary between sedimentary layer and consolidated crust. A thick sediment layer reaching
about 10 km was delineated in the receiver functions of stations MPE, PVL, SZH
(Fig. 2a). The depth of the Moho discontinuity is estimated to about 29–31 km,
and VP /VS is in the range of 1.80–1.87 in north and north-west direction from
MPE station. Figure 2 shows receiver functions sorted by azimuth from 0◦ to
360◦ and stacked together from each event. In some azimuthal ranges, the phase
Compt. rend. Acad. bulg. Sci., 66, No 5, 2013
727
MPE_Q
–10
0
–10
0
Ps
10
PpPs
PpSs
20
30
40
20
30
40
sum
a)
10
Delay time (s)
MPE_T
–10
0
10
20
30
40
0
10
20
30
40
sum
b)
–10
Delay time (s)
Fig. 2. Receiver functions for Q (a) and T (b) components of
MPE station
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G. Georgieva, S. Nikolova
JMB_Q
0
–10
10
Ps
PpPs
PpSs 20
30
40
20
30
40
sum
a)
0
–10
10
Delay time (s)
JMB_T
–10
0
10
20
30
40
0
10
20
30
40
sum
b)
c)
–10
Delay time (s)
Fig. 3. Receiver functions for Q (a) and T (b) components of
JMB station; (c) H-κ values for each event are represented as
isolines with different probability
from Moho is missing or comes at different time. For these azimuthal ranges,
there is energy on the T component (Fig. 2b), which is indication for existence of
anisotropy and/or complex structures in the crust beneath the station. Therefore, the depth of Moho for these directions is not estimated (Fig. 2b). The Moho
depth beneath PVL station is about 31-32 km (VP /VS is in the interval 1.86–1.90)
and the depth beneath SZH station is in the range of 32–34 km with velocity ratio
in the range 1.79–1.87.
The results for the Upper Thracian Plain show flat and well defined crustal
structure. From all the stations used in the present study JMB is the only one,
where no complex structures or anisotropy indications are observed (Fig. 3a).
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Compt. rend. Acad. bulg. Sci., 66, No 5, 2013
729
MMB_Q
0
–10
Ps
10
PpPs PpSs
20
30
40
20
30
40
sum
a)
0
–10
10
Delay time (s)
MMB_T
–10
0
10
20
30
40
0
10
20
30
40
sum
b)
c)
–10
Delay time (s)
Fig. 4. Receiver functions for Q (a) and T (b) components of
MMB station; (c) H-κ values for each event are represented as
isolines with different probability
The Moho discontinuity is flat and well defined for all back azimuths around the
station. In this case, distinct results can be obtained using Zhu and Kanamori
method [7 ] (Fig. 3b). The depth beneath JMB is in the range of 30–32 km (VP /VS
is in the range of 1.77–1.85). The Moho depth beneath PLD station is estimated
between 32 and 34 km and the VP /VS ratio is in the range of 1.76–1.83. PGB
station is situated in the highest part of Sredna Gora Mountain. The calculated
depth for Moho discontinuity is 37–39 km and VP /VS is in the range of 1.69–1.77.
The Rhodopean massif occupies the southwestern part of the study area. The
analysis of the receiver function indicates the Moho depth of 28–33 km beneath the
Eastern Rhodopes (KDZ station) and the VP /VS ratio in the interval of 1.74–1.87.
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G. Georgieva, S. Nikolova
The discontinuity sinks down in west direction and reaches about 36 km (34.5–
37.5 km and VP /VS 1.73–1.82) in the central part of the Rhodopes (RZN station).
The thickest Earth’s crust observed on the territory of Bulgaria is beneath MMB
station. The results indicate very complex crustal structure and the observed
energy on the T component (Fig. 4b) indicates strong anisotropy. The estimated
Moho depth here is about 50 km (49–50 km) (Fig. 4a). Two areas with probability
higher than 95% could be identified in Fig. 4c. The results for the first area
correspond to a Moho depth within the range of 38–40 km and VP /VS between
1.78 and 1.83. The second area indicates the range of 49–52 km for the Moho
depth and 1.60–1.65 for the velocity ratio. Previous geophysical studies [9, 10 ]
of the region define the depth of Moho at 50 km, and our results confirm the
existence of a well-defined boundary at this depth. The low VP /VS ratio is typical
of regions with felsic rocks, which is the case in the Western Rhodopes, east of
MMB station [11 ]. Most of the events (170 out of 220 events) used for construction
of the receiver function for MMB are located in the azimuthal range of 0◦ –100◦ ,
coinciding with the formation of the felsic rocks. Therefore, for MMB station we
accept that the Moho is at 49–52 km depth with 1.60–1.65 VP /VS . The estimate
for Moho depth beneath KKB station is between 27 and 30 km and the VP /VS
ratio is in the range of 1.84–1.92. The discontinuity is not well defined in the back
azimuthal range between 300◦ –45◦ . The receiver function analysis is also sensitive
to vertical discontinuities in the medium; therefore a plaussible explanation for
this pattern could be the presence of Krupnik fault penetrating through the whole
crust [12 ]. The most complex patterns in the stacked receiver function plot are
observed for VTS station. Two peaks are identified as P s on the summed trace.
The Moho depth is shallower in the back azimuth range between 70◦ –180◦ and the
converted phase from this direction comes 3.9 s after the main phase. For all other
back azimuths, the P s phase comes 5 s after P . Furthermore, detailed analysis
of these data needs to be performed as the current approach does not provide
reliable results for the depth of Moho discontinuity if structures are anisotropic
and/or there are strong lateral inhomogeneities or faults.
Two more stations – Provadiya (PRD) and Preselentsi (PSN) are located
in Eastern Bulgaria, close to the Black Sea. The standard analysis of the data
from these stations could not provide reliable results, due to the high microseismic
noise from the Black Sea. The sea microseisms have frequency maximum at about
4–8 s [13 ], which is within the frequency range of the analyzed receiver functions,
and therefore standard suppressing/filtering was affecting the data and results.
Several filters were used to suppress the high microseismic noise from the Black
Sea but another technique needs to be applied in order to distinguish the phases
from Moho.
Conclusions. The structure of the crust in Bulgaria is evaluated using
the receiver function technique beneath 11 seismic stations on the territory of
Bulgaria. The crustal thickness on the territory of Bulgaria varies between 30
Compt. rend. Acad. bulg. Sci., 66, No 5, 2013
731
and 50 km. The depth of the Moho boundary is about 30 km beneath stations
PVL, MPE, SZH, PLD and JMB. The crust is thickening beneath the Rhodopean
massif and Pirin Mountain, where it reaches about 50 km. The Moho structure in
Southwestern Bulgaria is very complex and further more detailed analysis of data
should be performed. A sedimentary layer with thickness of 10 km was delineated
in Northern Bulgaria (PVL, MPE, SZH).
The VP /VS ratio in Northern Bulgaria varies between 1.80 and 1.90 and
decreases in south and southwest direction. It is between 1.76–1.85 in the Upper
Thracian Plain and for the Rhodopean massif the range is about 1.72–1.82. The
western part of the Rhodopean massif is characterized by low VP /VS (1.6–1.65)
and could be explained with the presence of felsic rock formation.
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∗
Faculty of Physics
St. Kliment Ohridski University of Sofia
5, J. Bourchier Blvd
1164 Sofia, Bulgaria
e-mail: [email protected]
732
National Institute of Geolody,
Geodesy and Geography
Bulgarian Academy of Sciences
Acad. G. Bonchev Str., Bl. 3
1113 Sofia, Bulgaria
e-mail: [email protected]
G. Georgieva, S. Nikolova