GNSS Monitoring of Geodynamics in the Region Around Sofia and

South-Western Bulgaria

Nikolay Dimitrov

and Anton Ivanov

National Institute of Geophysics Geodesy and Geography, Bulgarian Academy of Sciences, Sofia, Bulgaria

Keywords: GNSS, Crustal Movements, Horizontal Velocity, Crustal Strain.

Abstract: For more than 25 years, the monitoring of geodynamic processes with modern GNSS technology in the region

of Sofia and Southwestern Bulgaria continues. To investigate modern crustal motions in the area, Global

Positioning System (GPS) data obtained between 1996 and 2022 are analyzed to obtain the velocity field for

southwestern Bulgaria. For this period, monitoring covered 28 stations. The active strain in the region is also

estimated from analysis of the results for velocity solutions. Some correlation between modern earth crust

movements, seismic events and tectonic structures is established. The obtained results in a general way

confirm previously data, but with much better accuracy and details at local level. The results can be used for

a detailed geodynamic and geological study of the active fault structures in the area.

1 INTRODUCTION

The area around Sofia and Southwestern Bulgaria is

characterized by a large number of active geological

fault structures, and the presence of tectonic and

seismic activity predetermines the development of

dangerous geodynamic processes. These processes

have the greatest impact on the changes

(deformations) of the geodetic networks built

specifically for their study. Using GNSS technology

for estimating natural destructive processes provides

specific quantitative values of recent crustal

movements. The area of interest of this study is

limited to the north by the southern slopes of Stara

Planina, to the south to the border of Bulgaria with

Greece, to the west - the western border of Bulgaria

and to the east - the beginning of the Upper Thracian

Plain (Fig. 1). The historical review of earthquake

activity shows that a significant number of strong

earthquakes have been recorded in the Sofia area. The

above shows the social importance of the studied

territory and the need to obtain geodetic data on its

current geodynamic activity in order to assess the

geological hazard. The main goal of the geodynamic

monitoring is to clarify the geodynamic setting of the

region and behavior of suggested active faults.

https://orcid.org/0000-0002-5369-5921

2 GPS NETWORK AND DATA

For more than 25 years, the monitoring of

geodynamic processes with modern GNSS

technology in the region of Sofia and Southwestern

Bulgaria continues. In order to study the modern

crustal movements in 1996, a geodynamic network

was built in the area around Sofia, covering SW

Bulgaria. The network is designed for high-precision

GNSS measurements, determination of coordinates

and velocities of points, calculation of active stress in

the area and long-term monitoring geodynamic

processes. The points are stabilized so that the grid

covers the main tectonic structures in the area. The

first GPS measurements of the Sofia Geodynamic

Network were made in 1996. GPS measurement of all

points of the network with processing and analysis of

the results first been performed only in two epochs

1997 and 2000 (Kotzev et al., 2001, Kotzev et al.,

2006). A new comprehensive measurement of the

geodynamic network was accomplished in the

summer of 2020 (Dimitrov and Nakov, 2020,

Dimitrov and Nakov, 2022). A new campaign was

performed in the summer of 2021. Three additional

points were measured – BELM, SATO and LOZ2

(Fig. 1). Point BELM was measured previously in

1997 and 2020, point SATO was measured in 1996

and 2003. Because the original point LOZE was

144

Dimitrov, N. and Ivanov, A.

GNSS Monitoring of Geodynamics in the Region Around Soﬁa and South-Western Bulgaria.

DOI: 10.5220/0011840400003473

In Proceedings of the 9th International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2023), pages 144-149

ISBN: 978-989-758-649-1; ISSN: 2184-500X

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

destroyed after the measurements in 2000, in 2021 we

measured the duplicating point LOZ2, which was

measured in 1997 but with shorter period of

observation. In this reason the obtained result for

velocity of this point has greater error, but still is

reliable (Dimitrov and Nakov, 2021). In the summer

of 2022, we measured the next epoch at 9 network

points. These points now have at least four

measurement epochs over a period of 25 years. The

epochs of all GPS measurements included in this

study are shown in Table 1.

3 VELOCITIES AND STRAIN

RATE ESTIMATION

The measurements were processed/reprocessed in a

two-step procedure using the GAMIT/GLOBK

software v10.71 (Herring et al., 2015, Herring et al.,

2018) to ensure the quality and homogeneity of the

solutions. In the first step, loosely constrained

estimates of station coordinates, Earth orientation,

orbital parameters, and atmospheric zenith delays

were determined using GAMIT. Major models and

parameters used in GAMIT GPS data processing are

given in Table 2.

Table 1: Epochs of GPS measurements.

Point Year of measurements number

1996

1997

2000

2001

2002

2003

2004

2012

2017

2020

2021

2022

epochs

BANK * * * * 4

BELI * * * 3

BELM * * * 3

BERK * * * * 4

BOGS * * * * * 5

BOSN * * * * 5

BUHO * * * 3

CARV * * * * * 5

DELA * * * * * 5

DOB1 * * * * * * 6

DSEC * * * 3

FROL * * * 3

GURM * * 2

KRAL * * * * * * 6

LOZ2 * * 2

MALC * * * * 4

MECH * * 2

MUHO * * * 3

PADA * * * 3

PLA1 * * * * * * * * 9

SATO * * * 3

SLI1 * * * 3

SOFI * * * * * * * * * * * 11

VERI * * * * 4

VETR * * * * 4

VITI * * * 3

VLAD * * * * 4

ZEME * * * * 4

GNSS Monitoring of Geodynamics in the Region Around Soﬁa and South-Western Bulgaria

145

Table 2: Major models and parameters used in GAMIT

GPS data processing.

In the second step, a global Kalman filter was applied

using GLOBK, to the combined, loosely constrained

solutions and associated covariances in order to

estimate a consistent set of station coordinates and

velocities. Six parameter transformation was

estimated by minimizing the horizontal velocities of

10 globally distributed IGS stations with respect to

the IGS14 realization of the ITRF2014 reference

frame (Altamimi et al., 2016). In the region is located

the IGS station Class A – SOFI which is included in

the estimation and analysis. The obtained results for

the velocities of the point from geodynamic network

are shown in Fig. 1.

In the analysis we examined the time series for all

of the stations, removing obvious outliers and further

Figure 1: Horizontal GPS velocity with respect to Eurasian plate with 95% confidence. Slim lines show suggested active

faults in the area. The topography is shown from green to brown – lower to higher elevation. The elevation of the highest

peaks in the area is given in meters.

Cutoff angle and data

weighting

10º, depending on the elevation angle

Data sampling and

data weighting

30 s for data editing, and 120 s for parameter

estimation

Antenna phase center

IGS ANTEX files to correct absolute PCVs of

satellite and receiver

Ionospheric refraction Iono-free linear combination

VMF1 for dry delay and parameter estimation in

2-hour intervals for wet delays.

Troposphere horizontal gradients in 24-hour

interval are estimated.

Atmospheric tidal loading corrections VMF1.

Ocean tide

FES2004 model [12] with correction for the

center-of-mass motion

Solid Earth tide, pole

tide

Models recommended by IERS Conventions

2010

Troposphere refraction

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146

downweighting those for which the normalized root-

mean-square (nrms) was greater than 0,7. Four of

these time series are shown in Fig. 2. It should be

noted that unlike full velocity solution, the time series

do not account rigorously for all correlations.

For this study all measurements were reprocessed

with the GAMIT/GLOBK software, to obtain loosely

constrained daily solutions saved in SINEX (Solution

Independence Exchange format). For estimation of

strain rate, the QOCA software (https://qoca.

jpl.nasa.gov/) is used to model site displacements,

which involve all the campaign sites. We use

velocities adjustment in strain rate analysis.

Coseismic jumps and post-seismic deformations are

removed in the time series. Principal strain axes of the

horizontal strain rate tensors are estimated over the

Delaunay triangles (Fig. 3).

Figure 2: Long-term repeatabilitie of the horizontal station

positions.

4 DISCUSSIONS

The present-day geodynamics of the area as well as

the entire territory of South-West Bulgaria is defined

by the neotectonic extensional processes in the South

Bulgarian Extensional Region (Burchfiel et al.,

2000), part of the broad East-Mediterranean – Balkan

Extensional system. Presently, based on geological

data the extension is suggested to be in a general N-S

direction, which results in tectonic structures with

general trend NW-SE to E W (Dimitrov and Nakov,

2020). For a period of more than 25 years the

monitoring has covered 28 stations. They have been

measured in different years and in different number

of campaigns (Table 1). Despite the different number

of measurements, the obtained results are quite

GNSS Monitoring of Geodynamics in the Region Around Soﬁa and South-Western Bulgaria

147

Figure 3: Topography (brown elevated areas, green low lands) and suggested active faults with black lines (Barrier et al.,

2004) with principal axes of the horizontal strain rate tensor with blue arrows.

homogeneous in the different deposits of the studied

territory and show clear uniform trends. In this

regard, the velocities of stations PLA1 and SOF1 with

the largest number of measurements, which suggests

very high reliability, do not differ significantly from

the surrounding stations.

Recently obtained velocities are extremely

reliable because theirs 3σ errors are very small. All

velocities are in southern direction. All velocities

clearly exceed 1.5 - 2 mm/y reaching up to 3 - 4

mm/y. They are in the limits of 1.5 mm/year up to

slightly over 3 mm/year, almost reaching 4 mm/year

(stations DOB1, SATO). The velocities of the stations

tend to increase from north to south, passing through

an intermediate locality, clearly increasing in the

southernmost part of the country.

This velocity field motivates N-S expressed extension

with increasing rates from North to South. The

difference in the velocity rates tends to change along

geologically suggested active fault zones. This result

point to the significance of the GPS monitoring for

the identification and evaluation of active faults.

5 CONCLUSIONS

The newly acquired velocities from three campaigns

1997, 2000, 2020, 2021 years, complemented with

the new results from 2022 confirm that the general

tendency of movement of the stations in the region of

Central West Bulgaria is in the south direction with

respect to stable Eurasia. The velocities tend to

increase from north to south. This pattern is in

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148

agreement with the extensive movement of southern

Bulgaria and northern Greece.

The results obtained for the strain rate show that

the strain field is not uniform in direction and

intensity. In the area located to the north of the Sub-

Balkan fault (north of Sofia graben) dominates the

compression strain. However, the limited number of

stations may require further observations for more

accurate results. The intermediate area is

characterized by the NW-SE trend of the strain with

dominating extension. The southernmost area is

characterized by a general NNW – SSE (almost N-S)

direction of the extensional strain and higher values

compared to the northern area.

The newly obtained results in a general way

confirm previous data, but with much better accuracy

and details at local level and can be used for a detailed

geodynamic and geological study of the area. Further

extension of geodynamic network will provide new

details on the geodynamics of the area.

ACKNOWLEDGEMENTS

This study was supported by National Science Fund,

Bulgaria, Project "Monitoring of geodynamic

processes in the area of Sofia". Contract No KП-06-

H 34/1.

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