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Seismic activity of the Alpine-Carpathian-Bohemian Massif

region with regard to geological and potential field data















Department of Geophysics, Central Institute for Meteorology and Geodynamics, Hohe Warte 38, A-1190 Vienna, Austria;


Institute of Physics of the Earth, Faculty of Science, Masaryk University, Tvrdého 12, 602 00 Brno, Czech Republic;;

(Manuscript received June 30, 2006; accepted in revised form December 7, 2006)

Abstract: The seismicity of the geological complexes of the northern part of the Eastern Alps, the Western Carpathians
and the Bohemian Massif is investigated by means of new seismic stations and a review of earthquake catalogues
available. Eleven earthquake catalogues are evaluated and checked for multiple entries, fake earthquakes and mis-
takes. The final data set of earthquakes covers the time span from 1267 to 2004 and comprises 1968 earthquakes in
total. The resulting epicentral map provides a very detailed idea of the seismicity of this region. An attempt at a seismo-
tectonic interpretation of earthquakes based on the geological overview of the region is presented. Gravity and
airborne magnetometry data in addition to seismic events are collected and cross-border maps are compiled and
analysed in order to determine the spatial extent of these geological structures. The Linsser filtering technique is used
to trace faults at two depth horizons – 4 and 8 km. Correlation between the epicentres of earthquakes and lineaments
derived from gravity data is discussed for major historical earthquakes such as Neulengbach (1590) or Scheibbs
(1867). This data set enables us to determine seismically active fault structures and to get an insight into the fault
system interaction. The ability to assess the potentially seismically active vertical and horizontal extent of fault
structures enables improved hazard assessments in future. The magnetometric map shows a belt of positive anomalies
which reflects the presence of magnetized rocks between the Bohemian Massif and the Alpine-Carpathian zone.

Key words: Carpathians, Eastern Alps, Bohemian Massif, seismic monitoring, earthquakes, earthquake catalogue,
gravity map, magnetic map, Linsser filtering.


Since 1991 the Department of Geophysics, Central Insti-
tute for Meteorology and Geodynamics (ZAMG) in Vien-
na, Austria, and the Institute of Physics of the Earth,
Masaryk University (IPE) in Brno, Czech Republic, have
been co-operating in seismological studies. This partner-
ship has resulted in a joint project and the establishment
of the “Alpine-Carpathian On-line Research Network”
(ACORN). The installation of new digital seismic stations
with on-line data-transmission to the seismological cen-
tres has enabled study of the seismicity across borders.

In 1995—1997, the first phase of the ACORN project fo-

cussed on seismic event detection within the Western Car-
pathians and the Vienna Basin. Based on previous results
the second phase commenced in 1998. The area of study
was finally enlarged to incorporate the Eastern Alps and a
part of the Bohemian Massif to get a broader picture of the
region. Hence, the area of interest was chosen to encom-
pass a rectangle ranging from to 47.5º to 49.8º in latitude
and from 13.0º to 19.0º in longitude, which is referred to
as the “ACORN region” hereinafter. During the second
and third phase of ACORN project, an earthquake cata-
logue of the area under consideration was compiled, and
further alternative ways of data-transmission were estab-
lished to secure the data-transfer. In 2004 the project cul-

minated in a geophysical interpretation of gravity data
based on the Linsser method.

Seismic monitoring

Integration of new seismic stations

In the early 90’s only few seismic stations existed in the

area of interest. Data from these stations were available
with time delay up to as much as several days and the data
exchange between national seismological centres was lim-
ited to parametric data, such as time onsets of seismic

Prior to the establishment of the ACORN network, two

digital broad band seismic stations Ve ká Javorina (JAVC)
and Moravský Krumlov (KRUC) were built in the Czech
Republic in close co-operation between ZAMG and IPE,
and were completed in 1995. During the first phase of the
project, the seismic station MOA in Molln/Upper Austria
was upgraded by the ZAMG to a state-of-the-art broad-
band station in 1996. In 1997 another Czech seismic sta-
tion Moravský Beroun (MORC) could be added to the
ACORN network. This station was installed under the co-
operation of the IPE and the GeoForschungsZentrum Pots-

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dam. After 2000, the international data exchange between
European seismological institutions increased mainly due
to the MEREDIAN project supported by the EU (Van Eck
et al. 2004). The continuous waveform data from other sta-
tions situated on the territory of the Czech Republic (Zed-
ník et al. 2004), Slovakia and Hungary were also
integrated into the ACORN network.

The data of all these stations are recorded in real time –

hence, after a seismic event the data are available for anal-
ysis in the national centres at the ZAMG Vienna and at the
IPE Brno, with only a few seconds delay. Due to the new
seismic stations, the location accuracy based on evalua-
tion of seismograms has substantially improved. Location
inaccuracy, previously ranging from 5 to 15 km (depend-
ing on the actual position within the Vienna Basin) before
data from these stations were available, has improved to
less than 5 km. Before the improvement of the network,
macroseismic data had to be used in order to mitigate the
large inaccuracy of locations, which was caused by the
sparseness of the network and the inhomogeneous struc-
ture of the crust in this part of Europe. Obviously, this ac-
curacy can be improved even more by installing
additional stations.

Recorded events

The seismological data from the stations of the ACORN

network were processed in both seismological centres, at
the ZAMG Vienna and at the IPE Brno. The time onsets of
seismic phases were identified and events that were regis-
tered at a sufficient number of stations were located. Loca-
tions and identifications were compared in both institu-
tions. Due to the sensitivity of digital stations and the
exchange of data across the border even weak mi-
croearthquakes could be localized. The analysis of very
small tremors that are indicators of seismically active
faults – even though they are capable of releasing suffi-
cient energy only in the long term – constitutes an im-
portant aspect of seismic studies concerning the seismic

With regard to improvement of the seismic network,

more and more industrial activities such as quarry blasts
were detected. The identification of these tremors (a few
hundred per year) is time consuming and sometimes

problematic because the released seismic energy is often
very small and the recorded seismic signals are extremely
weak and difficult to interpret. These events must be dis-
carded when carrying out hazard calculations (Wiemer &
Baer 2000). Consequently, many recorded signals from
known quarry blasts were analysed to find out some char-
acteristics that could help to verify whether the recorded
ground motion originated from an explosion or not. The
databases of quarries and blasts were compiled to enable
the comparison of registered events with already known
signals. Problematic and suspicious events were verified
with chief blasters from quarries near the calculated epi-

Besides industrial explosions, experimental shots were

also registered, especially events registered in the frame-
work of CELEBRATION 2000 (Guterch et al. 2003; Hrub-
cová et al. 2005), ALP 2002 (Brückl et al. 2003) and SU-
DETES 2003 (Grad et al. 2003) seismic experiments. The
analysis of these events allowed the verification of the lo-
cation accuracy and improvement of velocity models.

During the ACORN project more than 600 earthquakes

were recorded. The strongest and most frequent events
originated in Austria in the macroseismically known re-
gion between Leoben and Ebreichsdorf (Fig. 1). The stron-
gest earthquakes were recorded on July 11, 2000 near
Ebreichsdorf with local magnitudes of 4.8 and 4.5. Other
historically known epicentral areas – near Salzburg,
Krems, Gmünd, Linz in Austria, Györ and Komárno on the
Hungary-Slovakia border, Malé Karpaty Mts and Pieš any
in Western Slovakia were active during the ACORN peri-
od, too. Weak but frequent seismicity was observed from
the north-eastern part of the Czech Republic (Fig. 1).

Earthquake catalogue

During the second and third phase of the ACORN

project, an earthquake catalogue of the area under consid-
eration was compiled. The catalogue finally combined 11
individual catalogues (listed in Table 1) which were in-
vestigated in terms of fake events, double or conflicting
entries and man-made seismic events.

Only entries with coordinates of the epicentre were

used. Corresponding events were linked together, one of
them was selected as principal and its epicentre was used

Table 1: Catalogues used in the project.

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Fig. 1.  Epicentres of earthquakes for the period from 1.1.1995 to 31.3.2004  plotted on the grey shaded relief topography map derived
from the USGS digital elevation model TOPO 30. The sizes of the red circles are scaled proportionally to the local magnitudes of indi-
vidual earthquakes. Black triangles show the position of seismic stations.

for plotting maps. Top priority was given to the agency re-
sponsible for a particular region. Hence, for example,
earthquakes from the Hungarian catalogue (Zsíros et al.
1988; Tóth et al. 2005) in Hungarian territory which did
not appear in other catalogues were accepted as genuine
earthquakes of this specific region. In case of conflicting
entries in terms of dates or times, coordinates, intensities
or magnitudes, these entries were investigated, compared
and the highest priority was given to the catalogue entry
of the respective national agency or institution which also
reported the event in its territory. The magnitude of histor-
ical earthquakes, for which only an epicentral intensity
was known, was computed according to formulae by
Kárník et al. (1981) and Shebalin (1958). An example of a
part of the catalogue is given in Table 2, and the twelve
strongest earthquakes are listed in Table 3.

The catalogue of the ACORN region consists of 1968

earthquakes covering the period from 1267 until March

2004 which are considered to be genuine earthquakes and
not blasts or rockbursts (Fig. 2). Events from the ten years
of the ACORN period make up around a third of all earth-
quakes presented in the catalogue (Fig. 3).

Geological and structural setting and their relation

to seismic activity

The ACORN region is situated in the Eastern Alps-

Western Carpathians-Bohemian Massif junction area
(Fig. 4). The Bohemian Massif is situated in the eastern-
most part of the European Variscan Belt and represents the
foreland of the Alpine-Carpathian orogen. The southern
and eastern slopes of the Bohemian Massif covered by the
molasse sediments of the Alpine-Carpathian Foredeep dip
under the Alpine-Carpathian flysch nappes. The Eastern
Alps and the Western Carpathians belong to the Alpine-

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Carpathian orogenic belt. They represent a complex, com-
prising both the outer region of thin-skinned tectonics
with Flysch nappes and an inner part of thick-skinned tec-
tonics also containing robust nappes of the pre-Alpine

Bohemian Massif

The Bohemian Massif was formed during at least three

orogeneses (Cadomian, Variscan and Alpine). The
Variscan structures dominate in the region of the Bohemi-
an Massif. The Moravo-Silesian Unit represents the east-
ern part of the Bohemian Massif. The basement of this

Variscan unit is formed of the Cadomian crystalline rocks
of the Brunovistulicum. Westwards, the Moravo-Silesian
Unit dips under the Lugodanubian units (the Moldanubian,
Teplá-Barrandian and Lugian Units) of the Bohemian
Massif (Suess 1912; Matte et al. 1990; Schulmann et al.
1994). NNE-SSW structures predominate in the Moravo-
Silesian Unit, whereas perpendicular WNW-ESE structures
predominate in the Lugodanubian units.

The tectonic boundary between the Moravo-Silesian

and the Lugodanubian units is formed by the NNE-SSW
Moravo-Silesian fault zone consisting of various disloca-
tions, overthrusts and strike-slip zones. Similarly, the N-S
to NNE-SSW Rodl-Blanice and Vitis-Přibyslav shear

Table 2: Example of a part of the ACORN catalogue with the earthquake near Molln discussed in Chapter’Aeromagnetic data’. M – mul-
tiplicity identifier (P – principal event), UTC – Coordinated Universal Time, Depth* – typical depth for the area, Mag.* – magni-
tude was estimated from I





– epicentral intensity, Cat. – abbreviation of catalogue (see Table1). Parameters of principal events

used for plotting maps are in upright letters, associated localizations of the same earthquake, if they exist, are written in italics.

Table 3: Twelve strongest earthquakes in the ACORN region sorted according to the epicentral intensity I


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zones originated during the Variscan orogeny in the
Moldanubian Unit. The NW-SE Danube and Pfahl zones
were formed as the conjugate systems in respect to these
N-S to NNE-SSW shear zones (Brandmayr et al. 1995). In
the NW part of the ACORN region, the ENE-WSW Central
Bohemian shear zone, injected with the Central Bohemian
Pluton, forms the tectonic limit of the Moldanubian Unit
and the Teplá-Barrandian Unit.

The NNE-SSW grabens (Boskovice, Blanice and hypo-

thetical Jihlava Grabens) filled by the upper Carbonifer-
ous-Lower Permian sediments originated along the
Variscan NNE-SSW zones (Veselá 1976; Holub & Tásler
1980). The Boskovice Graben situated near the Moravo-
Silesian fault zone represents the most significant Permian-
Carboniferous graben in the ACORN region. The
NNE-SSW eastern tectonic limit of this graben (the mar-
ginal fault of the Boskovice Graben) continues south-
wards, where it passes into the NE-SW Diendorf fault. The

faults forming the tectonic limits of the Permian-Carbonif-
erous grabens were repeatedly reactivated. The recent seis-
mic activity of the Diendorf fault was discussed by Figdor
& Scheidegger (1977). Some known epicentres of weak
microearthquakes can probably be attributed to small tec-
tonic movement along the southern segments of the Vitis-
Přibyslav fault system and the Rodl-Kaplice-Blanice fault

The WNW-ESE to NW-SE fault systems play a signifi-

cant role in the Bohemian Massif. These fault systems
mostly originated already during Variscan orogeny
(Brandmayr et al. 1995; Aleksandrowski et al. 1997) and
were significantly reactivated during the Cretaceous and
the Cenozoic (for instance Grygar & Jelínek 2003). During
the Late Cretaceous-Paleogene compression, the crystal-
line basement thrust over the Upper Cretaceous sediments
along some of the WNW-ESE to NW-SE fault systems (for
instance the Lužice fault, the Železné Hory fault and the

Fig. 2.  Epicentres of earthquakes for the period from 8.5.1267 to 31.3.2004  plotted on the grey shaded relief topography map derived
from the USGS digital elevation model TOPO 30. The sizes of the red circles are scaled proportionally to the local magnitudes of indi-
vidual earthquakes. Black triangles show the position of seismic stations.

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Fig. 3.  Number of earthquakes presented in the final ACORN cata-

Fig. 4. Scheme of geological and tectonic setting in the ACORN region with marked major fault systems and faults discussed in the article
(purple lines) (geological background compiled and simplified after Kodym et al. 1967; Mahe  1973, marked faults also compiled after

marginal fault of the Blansko Graben – Malkovský 1979,
1987; Coubal 1990; Krejčí et al. 2002). During the dextral
Neogene movements of the WNW-ESE to NW-SE faults in
the ESE continuation of the Elbe fault system, the Upper
Morava Basin was formed (Grygar & Jelínek 2003). These
faults intersect the front of the Western Carpathian flysch
nappes and several faults also dislocated the Pliocene/
Quaternary sedimentary layers of the Upper Morava Basin
(Růžička 1973; Zeman et al. 1980). These facts suggest a
young (sub-Recent) reactivation of these faults. Recent
natural seismotectonic activity occurring in the NE part of
the Bohemian Massif (Fig. 5) is also predominantly con-
nected with the WNW-ESE to NW-SE faults (Kaláb et al.
1996; Skácelová & Havíř 1999; Špaček et al. 2006).

Eastern Alps and Western Carpathians

The Eastern Alps and the Western Carpathians were con-

solidated during the Alpine orogeny. In the Eastern Alps,
the ENE-WSW structures predominate. The Western Car-

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pathians represent the part of the units which were tectoni-
cally extruded towards the NE during the Late Paleogene—
Early Miocene (Fodor 1995; Ratschbacher et al. 1991;
Peresson & Decker 1997a). The NE-SW to ENE-WSW
structures predominate in the western part of the Western
Carpathians. Neogene sediments of the Vienna Basin and
the Panonian Basin partially cover the easternmost Alps
and the SW part of the Western Carpathians. Numerous
NE-SW and N-S polyphase faults played a dominant role
during the Neogene development of the Vienna and
Danube Basins (Fodor 1995; Kováč et al. 1989;
Hrušecký 1999). Significant volcanic activity occurred
during the Tertiary in the Western Carpathians. The vol-
canic rocks occupy a large area in the easternmost part of
the ACORN region.

The lateral extrusion significantly influenced the struc-

tural setting of the Eastern Alps (Ratschbacher et al. 1991;
Peresson & Decker 1997a,b; Frisch et al. 1998, 2000; Lin-
zer et al. 2002). The ENE-WSW Salzach-Ennstal-Mariaz-
ell-Puchberg line (SEMP) separating the Northern
Calcareous Alps from the crystalline units situated south-
wards is the most significant structure connected with
this lateral extrusion in the Eastern Alps. The NE-SW to
ENE-WSW Inntal fault zone and Traunsee fault zone
situated in the Northern Calcareous Alps in the SW part of
the ACORN region represents similar dominant structures
significantly active during the extrusion (Frisch et al.
1998, 2000; Linzer et al. 2002).

The Mur-Mürz fault system located south of the SEMP

is connected with the zone of significant seismic activity

Fig. 5. Distribution of earthquakes in the ACORN region for the period from 8.5.1267 to 31.3.2004 (red circles) on the schematic map
of the geological and  tectonic setting with marked major fault systems and faults discussed in the article (purple lines) (see Fig. 4 for
legend and other explanation).

Fig. 4.   (Continued from previous page)  Suk et al. 1996; Linzer et al. 2002; Kováč & Plašienka 2002). Abbreviations: RKP – Rodl-
Kaplice-Blanice Fault System; SEMP – Salzach-Ennstal-Mariazell-Puchberg Fault System; MML – Mur-Mürz-Leitha Fault System;
RHB – Rába-Hurbanovo-Diosjenö Zone. Marked major faults: Bohemian Massif: 1 – Železné Hory Fault; 2 – Dubno Fault; 3 – Eastern
Marginal Fault of Třeboň Basin; 4 – Pfahl Fault Zone; 5 – Danube Fault Zone; 6 – Marginal Fault of Boskovice Graben; 7 – Diendorf
Fault; 8 – Přibyslav Fault Zone; 9 – Vitis Fault Zone; 10 – Rodl Fault Zone; 11 – Blanice Graben; 12 – Lhenice Graben; 13 – Kla-
tovy Fault Zone; 14 – Benešov Fault Zone. Eastern Alps and Western Carpathians: 15 – Inntal Fault Zone; 16 – Ennstal Fault;
17 – Mariazell-Puchberg Fault Zone; 18 – Steinberg Fault; 19 – Schrattenberg Fault; 20 – Mur-Mürz Zone; 21 – Pottendorf Fault;
22 – Láb Fault; 23 – Vištuk Fault; 24 – Malé Karpaty Marginal Fault Zone; 25 – Plavecké Podhradie-Dobrá Voda Faults; 26 – Myja-
va Fault Zone; 27 – Rába Line; 28 – Hurbanovo Line; 29 – Kátlovce Fault; 30 – Nezdenice Fault; 31 – Jastrabie Fault.

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(Reinecker & Lenhardt 1999). This seismoactive zone
continues NE along the Pottendorf and Láb faults into the
Western Carpathians. Both NW and SE tectonic limits of
the Malé Karpaty Mts have exhibited recent seismotecton-
ic activity. The epicentres of earthquakes (Fig. 5) signifi-
cantly concentrate mainly near the NE margin of the Malé
Karpaty Mts in the region between the Kátlovce fault and
the Myjava fault. Other recent seismic activity has oc-
curred along the Pienniny Klippen Belt, forming the
boundary between the Outer Flysch Carpathian nappes
and Central Western Carpathian units, at least to the re-
gion of Žilina. The discussed ENE-WSW to NE-SW seis-
moactive zone passing from the Eastern Alps to the
Western Carpathians represents the most significant seis-
moactive zone in the ACORN region (see also Schenková
et al. 1995; Šefara et al. 1998; Hók et al. 2000; Bielik et
al. 2002; Decker et al. 2005). The seismic activity ob-
served during years 2004—2005 near Zakopane in Poland
(Havíř et al. 2006) can be connected with another ENE
prolongation of this seismoactive zone.

In the SE part of the ACORN region, the Rába-Hurbano-

vo-Diosjenő line forming the tectonic boundary between
the Transdanubian Range and the Austroalpine nappes
(Hrušecký 1999; Szafián et al. 1999; Kováč & Plašienka
2002) represents another significant structure connected
with a seismoactive zone (Šefara et al. 1998; Hók et al.
2000). The epicentre of the disastrous earthquake which
occurred in the region near Komárno in 1763 is situated
close to this lineament.

The NW-SE fault zones are less significant in the West-

ern Carpathians. Hrušecký (1999) pointed out that the re-
flection seismic profiles have not confirmed any important
NW-SE faults in the Danube Basin. Nevertheless some
young NW-SE faults exist in the Western Carpathian re-
gion, including faults cutting the front of the Carpathian
nappes. The post-kinematic volcanic activity connected
with the Nezdenice fault proves the young tectonic activi-
ty of this NW-SE structure and its relation with the reacti-
vation of structures in the basement formed by the
Bohemian Massif. Thus, the youngest faulting connected
with the NW-SE faults situated in the Western Carpathian
flysch nappes also has a relation to the reactivation and
propagation of the NW-SE to WNW-ESE fault systems al-
ready formed during the pre-Alpine stages of the Bohemian
Massif. The hypocentres of weak microearthquakes occur-
ring in the basement under the Western Carpathian flysch
nappes (recently, during the year 2004, in the Vizovice
region close to the Nezdenice fault) show the continuing
current activity of these structures on the buried eastern
slope of the Bohemian Massif.

Geophysical data

Regional geophysical data from gravity and airborne

magnetometry surveys were selected for the analysis of the
subsurface extent of geological bodies with contrasting
petrophysical properties. Colour shaded relief maps were
used to delineate pronounced lineaments whereas the Lin-

sser method (Linsser 1967) was applied to analyse the
density contacts at two potentially seismogenic depth ho-

Gravity data

A new border crossing gravity map was compiled from

the observed gravity data from Austria, the Czech Repub-
lic and Slovakia. A more detailed description of gravity
data sources can be found in Bielik et al. (2006). The grav-
ity map was calculated for a Bouguer reduction density
2.67 g/cm


, and the data were adjusted to the International

Gravity Standardization Network 1971. The normal gravity
formula was derived from the parameters of the geocentric
equipotential ellipsoid defined by the World Geodetic
System 1984 (WGS84), which is numerically equivalent to
the Geodetic Reference System 1980 (GRS80) (Švancara
2004). Gravity data were interpolated into a square grid of
1 km   1 km. The colour shaded relief gravity map shown
in Fig. 6 was plotted by illuminating from the NW at an
inclination of 45 degrees. The contour interval was 2 mGal.
Epicentres of earthquakes for the period from 8.5.1267 to
31.3.2004 were superimposed on this map as red circular
symbols with diameters proportional to the size of their

A dominant feature of the gravity field of the ACORN

area is the pronounced negative gravity anomaly of the
Eastern Alps in the region of the Salzburgian Northern
Calcareous Alps. Individual gravity minima are situated
mainly between Salzburg and Liezen. A distinct negative
anomaly can also be found to the west of Vöcklabruck.
The prevailing orientation of density contacts and corre-
sponding axis of horizontal gradients of gravity is approx-
imately W—E. A negative gravity field of lower intensity
with NW-SE orientation of density contacts characterizes
the Alpine Molasse Zone.

The gravity field of the Bohemian Massif is subdivided

into several subparallel belts of NE-SW orientation, thus
partially matching the observed seismic activity. The NW
part of the gravity map of the ACORN area shows a posi-
tively disturbed gravity field of the Teplá-Barrandian
Zone. The Central Bohemian suture separates this zone
from the Moldanubian Zone with a mainly negative gravi-
ty field. Light granodiorites and granites of the Bohemian
and Moldanubian Pluton predominantly cause individual
gravity minima of the Moldanubian Zone. Light granites
of Weinsberg type and Freistadt granodiorites give rise to
the negative gravity anomalies in the surroundings of Fre-
istadt NNE of Linz.

The most distinct density boundary depicted in the

gravity map is the intensive linear gravity gradient cutting
across Jihlava and Slavonice to Vitis, Zwettl and Am-
stetten. This tectonic line separates the Bohemian Mold-
anubicum to the west from the Moravian and Strážek and
Waldviertel Moldanubicum on the east. The northern part
of this deep fault zone is called the Přibyslav (mylonite)
zone whereas the southern part on Austrian territory is
named the “Vitis” shear zone. The positively disturbed
gravity field to the east of the Přibyslav-Vitis zone delim-

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Fig. 6.  Colour shaded relief contour map with Bouguer gravity anomalies and epicentres of earthquakes for the period from 8.5.1267 to
31.3.2004. Gravity anomalies were calculated using Bouguer reduction density 2.67 g/cm


. The contour interval is 2 mGal = 20 µm/s


. The sizes

of the red circles are scaled proportionally to the local magnitudes of individual earthquakes. Black triangles show the position of seismic stations.

its the Moravo-Silesian block. The most intensive positive
gravity anomaly in this region is situated at Moravské
Budějovice, SW of Třebíč. The anomaly is caused by
high-density metamorphites of the Moravian Moldanubi-
cum and probably also by deep-seated huge basic bodies.
An interpretation of the refraction profile “Celebration
09” performed by Hrubcová et al. (2005) indicates that the
Brunovistulicum (Dudek 1980) is underlain by a broad
velocity gradient zone in the lower crust.

The axis “Znojmo—Brno—Ostrava” marks the western

margin of the Carpathian Foredeep. In this belt there are
several positive gravity anomalies caused by heavy
Brunovistulian rocks. A negative gravity anomaly belt be-
tween Hodonín and Žilina is called the “West Carpathian
gravity low” and is attributed to porous sediments of the

Flysch Belt and the underlying light molasse sediments. A
gravity saddle between Zlín and Trenčín weakens the
“West Carpathian gravity low”, and this anomaly contin-
ues in a NW direction to Olomouc in the Hornomoravský
úval Lowland. The SSW continuation of the “West Car-
pathian gravity low” comprises the negative gravity
anomaly caused by Tertiary sediments of the Vienna Ba-
sin. The western margin of the Vienna Basin is marked by
a gravity gradient of the Schrattenberg-Steinberg fault sys-
tem. An intensive gravity gradient also delimits the east-
ern margin of the Vienna Basin, especially in the segment
Hainburg—Pernek—Sološnica. The sediments of the Vienna
Basin reach their maximal thickness of approximately
6000 meters in the Central Moravian/Zistersdorf Depres-
sion near the confluence of Thaya (Dyje) and March

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(Morava). A second depocentre of the Vienna Basin is situ-
ated SE of Vienna near Schwechat. A positive gravity
anomaly near Ebreichsdorf separates the Wiener Neustadt
Trough from the Schwechat Depression in the Vienna Basin.

In western Slovakia the gravity field delineates the

Danubian block with a positive gravity anomaly and the
western margin of the Fatra-Tatry block with a negative
gravity anomaly. An elongated positive gravity anomaly
between Bratislava and Nové Mesto nad Váhom character-
izes the Malé Karpaty Mts separating the Vienna Basin
from the Danube Basin. East of the Malé Karpaty Mts
there are three negative anomalies due to Tertiary sedi-
ments of the Pieš any, Topo čany and Zlaté Moravce bays
separated by positive anomalies of Považský Inovec Mts
and Tribeč Mts. The positive isometric anomaly in the
southern part of the Danubian block (between Gabčíkovo
and Nové Zámky) is caused by basic and ultrabasic rocks
of the basement of the Danube Basin. The negative gravi-
ty field in the surroundings of Žilina can be explained by
a superposition of the West Carpathian gravity low and
the gravity effect of the Paleogene sedimentary filling of
the Žilina valley.

Linsser indications of density contacts

Spatial changes in the density of rocks are responsible

for gravity anomalies. Linsser filtering is a unique tech-
nique for fault mapping based on analysis of the gravity
field. The original technique was proposed by Linsser
(1967) and later modified by Šefara (1973). The method is
based on the assumption that the gravity profile over a
fault or a density contact can be described as a linear com-
bination of a gravity master curve and regional gravity

g(x) = E · M(x) + R · B(x)

where  g(x) is the measured gravity profile, M(x) is the

gravity master curve for specified depth level, E is the am-
plitude of the fitted gravity master curve, B(x) is the spa-
tial variation of regional field, and R is a multiplicative
coefficient describing the regional gravity field.

In addition, the method requires the presence of a steep

discontinuity along which a density contrast exists. Strike
slip features along which crustal parts of similar density or
only horizontal density stratification are displaced cannot
be resolved, however.

We have used the approach of Šefara (1973) for the

gravity data processing in this article, where the gravity
master curve is calculated for the thin sheet model approx-
imating the fault (Fig. 7) while the regional field is consid-
ered to be a constant, i.e. B(x) = 1. The comparison of
gravity data – locally re-interpolated to the direction per-
pendicular to the horizontal gravity gradient – with mas-
ter curves enables the estimation of a fault amplitude and
the regional gravity. The “density contact indication”
(DCI) is plotted on a map where the Linsser coincidence
criterion C defined in Fig. 7 has a pronounced maximum.
An additional requirement for the construction of a DCI is

Fig. 7.  Principle of the Linsser filtering technique for determina-
tion of fault parameters. A – Gravity master curve corresponding
to the geological model. E is the amplitude of the gravity effect of
the master curve. B – Cross-section of the density contact model
approximated by a 2D horizontal thin sheet. C – Measured gravity
profile running across the density contact. A1 is the area between
the regional field and the measured gravity. D – Comparison of
the measured gravity profile with the gravity master curve. A2 is
the area between the master curve and the measured gravity. The
co-incidence C between the measured gravity and the master
curve is defined by the expression C (%) = 100 * (A1—A2)/A1.

the existence of a common coincidence criterion C on two
adjacent parallel profiles. This reduces significantly the
fragmentation of the density contact patterns. The size of
the symbol is proportional to intensity of the density con-
tact expressed by the product E * C and the azimuth is
computed from the horizontal gradient of gravity. Linsser
filtering is performed by analysing a set of depth levels us-
ing master curves, the length of which is usually 6 times
greater than the grid interval.

For the computation of density contacts we have inter-

polated the Bouguer gravity anomalies in a square grid
2 km 2 km with more than 30,900 grid points. The posi-
tions of density contacts at the 4 km depth were computed
by choosing a Linsser operator totalling 24 km in length,
and at 8 km depth we used an operator of 40 km in length.
From a formal point of view it is possible to analyse even
deeper crustal levels, however, this requires even larger
operator lengths, which cause an undesirable integration
of gravity anomalies from different geological bodies. For
that reason the deepest analysed level was chosen to be
8 km below the surface, although some earthquakes might
locate deeper. At the depth level of 4 km the Linsser tech-

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Fig. 8.  Positions of density contacts at depth horizons of 4 km (black) and 8 km (blue) below the surface, plotted on the shaded topo-
graphic relief map together with epicentres of earthquakes for the period from 8.5.1267 to 31.3.2004. The sizes of the red circles are
scaled proportionally to the local magnitudes of individual earthquakes. Black triangles show the position of seismic stations. Details shown
in Figs. 9 and 10 are marked by rectangles.

nique determined 3540 positions of density contacts
whereas at the depth 8 km below the surface 1840 density
contacts could be calculated. The positions of density
contacts at depths of 4 km and 8 km were projected on the
shaded topographic relief and are plotted in Fig. 8. If a
fault dips exactly vertically, both symbols of the Linsser
contact plot on top of each other. Slight offsets (e.g. Kla-
tovy fault or Vitis-Přibyslav fault) indicate the fault dip
and its direction between the two depth horizons of 4
and 8 km.

Prominent features in Fig. 8 are the NE-SW striking

faults, whereas the NW-SE orientated faults are generally
less pronounced. This observation can be explained by
the nature of NE-SW oriented faults. They can be found in
the Moravo-Silesian block as well as to a certain extent in
the Bohemian Moldanubicum and reflect Variscan struc-
tures accompanied by lithological and density changes.

The Klatovy fault zone and the Benešov fault zone in

the NW of the ACORN region and their offset due to the
Jáchymov fault system are clearly visible on the density
contact map. All these structures cannot be associated
with seismic events listed in the catalogue, but have
shown small seismic activity (recorded during the recent
past with a local network, which is not part of ACORN),
however. The same applies to the Lhenice fault on Czech
territory. Its southern continuation, which is not apparent
on the Linsser map – possibly due to the depth restriction
of 8 km – can be associated with numerous events from
the catalogue within the Pfahl-Danube fault system on
Austrian territory.

The Rodl-Kaplice-Blanice fault system is not pro-

nounced on the Linsser density contact map, indicating
negligible density contact. With few exceptions, north of
České Budějovice, where the fault system is intersected by

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NW-SE trending features like the Dub-
no fault and the eastern marginal fault
of the Třeboň Basin, and in the south-
ern part, north of Linz along the Rodl
fault zone, no seismicity is observed.

The seismicity near Pregarten in

Upper Austria cannot be associated
with one of the major faults. As indi-
cated by the density contact map,
the corresponding faults – striking
NE-SW and NW-SE – appear only
on the shallow 4 km-horizon, hence
do not continue further down. This
result can be confirmed from past
earthquakes, which caused slight
damage to buildings at unusually
low magnitudes, indicating shallow
sources at around 4 km (Reinecker &
Lenhardt 1999).

The Vitis-Přibyslav fault zone seems

to be relatively aseismic – with an
exception at Kautzen – E of
Litschau, although the fault appears
extremely prominent on the density
contact map. A bifurcation of the
fault zone towards the south can be
observed near Gmünd, possibly com-
mencing already at Kautzen. The
western part tends to dip towards the
east, whereas the east-part dips verti-
cally and tilts towards the west, thus
forming a depression-like structure,
which is limited to the south by a
pronounced and complex WNW-ESE
orientated fault system expressed by
the river bed of the Danube.

Further south, near Molln (see

“MOA” in Figs. 8 and 9), a clear indi-
cation for a NW-SE orientated densi-
ty contact became apparent. It
commences possibly already south of
Passau and changes direction just
south of Molln (Fig. 9). Its strike co-
incides with the focal solution (Rei-
necker & Lenhardt 1999) of the
earthquake near Molln in 1967 (see
Table 2 for earthquake parameters).

The almost E-W directed density

contacts between Salzburg and Bad
Aussee appear to be a result of
nappes along which small earth-
quakes occur. Due to their relatively
small magnitude, no focal mecha-
nisms could be determined, but the
orientation and the N-S orientated
stress regime should result in thrust-
type mechanisms.

Interestingly enough, the Diendorf-

Boskovice fault (Fig. 10) is only very

Fig. 9. Detail of Fig. 8 showing the Linsser contact indications near Molln (MOA) in Up-
per Austria.  NW—SE oriented density contact indications between Molln and Passau coin-
cide with the strike of the focal solution of the earthquake near Molln in 1967.

Fig. 10.  Detail of Fig. 8 showing the Linsser contact indications near Scheibbs, Diendorf
Fault (green) and Neulengbach in Lower Austria.  Text balloons mark the possible epicen-
tres of the historic earthquakes near Neulengbach in 1590 and Scheibbs in 1867. (The black
triangle depicts the new station at the Conrad Observatory in Lower Austria. The station has
only partially come into operation since the end of the project and could not be utilized
during the project).

background image



little pronounced in the density contact map. The seismic-
ity is concentrated mainly in the SE portion between
Krems and Melk.

Further south, near Mariazell, a NW-SE orientated clus-

ter of earthquakes coincides with strong density contacts.
This feature bends towards the west. Slightly north of it,
we find Scheibbs (Fig. 10), where a stronger earthquake
occurred in 1876. This event is currently under investiga-
tion at the ZAMG regarding the exact position of the epi-
centre and its magnitude.

Another pronounced density contact extends from Am-

stetten to St. Pölten, terminating at Neulengbach – the
possible epicentre of the historic earthquake in 1590. At
Neulengbach (Fig. 10) a NW-SE orientated and less pro-
nounced density contact can be seen, which commences
already a few kilometers north of the Danube, cutting

across the flysch nappes and possibly continuing even be-
low the Vienna Basin (Wr. Neustadt, epicentre of several
historic earthquakes). The Linsser contacts are very diffuse
in this part due to sedimentary filling of the basin.

The Pottendorf fault and its continuation towards the

east and its parallel faults (Láb fault, Plavecké Podhradie-
Dobrá Voda faults, Vištuk fault and the Malé Karpaty mar-
ginal fault zone) are clearly visible. In between, a cluster
of seismicity (latitude 48º35

N, longitude 17º30

E) can

be seen without Linsser contact indications. The seismici-
ty could be a result of the intersection of the Kátlovce
fault (WSW-ENE) with a fault in the NW-SE direction.

Additional seismic activity is known from Žilina in Slova-

kia (e.g. 1858, causing heavy damage to buildings), which
can be associated with several faults. One of these (Myjava
fault zone) is also visible from the Linsser contacts in Fig. 8.

Fig. 11.  Colour shaded relief contour map of the total magnetic field upward continued to the level 1600 m with epicentres of earth-
quakes for the period from 8.5.1267 to 31.3.2004. The contour interval of the magnetic field is 12.5 nT. The sizes of the red circles are
scaled proportionally to the local magnitudes of individual earthquakes. Black triangles show the position of seismic stations.

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In general, the mapping of density contacts allows a

very detailed determination of where faults with high-den-
sity contacts exist. Whether these faults are capable of
storing enough deformation energy (seismic active) or not
(too ductile, aseismic) can only be decided utilizing seis-
mic records.

At the end of this project another approach (Euler 3D

deconvolution, Reid et al. 1990) was applied to the data,
leading – after filtering – to similar results.

Aeromagnetic data

The cross-border magnetic map was compiled from grid-

ded Austrian aeromagnetic data continued to the level of
1600 m (Oberlercher 1999 – personal communication)
and the Czech aeromagnetic and ground magnetic data
(Šalanský 1995), which were newly continued upward to
the same level. The data were interpolated into a square
grid 2 km 2 km using the minimum curvature algorithm.
For the computation of the continuation upward we used
the MAGMAP module of the geophysical mapping soft-
ware OASIS Montaj. This software module supports the
application of common Fourier domain filters to gridded
potential field data. The Fourier domain filtering included
pre-processing, filter application and post processing.
From the resulting grid the colour shaded relief magnetic
map shown in Fig. 11 was constructed with an illumina-
tion from the NW at an inclination of 45 degree. The mag-
netic data stitching should be regarded as a first approach.
A major difference between the Austrian and Czech mag-
netic data was observed in the area of Laa am der Thaya
and Břeclav. The reason for this misfit results from the dif-
ferent kinds of magnetic data used; only ground magnetic
measurements of the Z component were available from the
Czech territory in this part.

The magnetic map (Fig. 11) shows a belt of positive

magnetic anomalies extending from Salzburg to Mariazell
and Vienna and further to Břeclav, Kroměříž and Čadca.
Gnojek & Heinz (1993) call this extensive feature the
Central European belt of magnetic anomalies and pro-
posed its geological interpretation. They assume that this
belt could be an old basement preserved between the Her-
cynian consolidated Bohemian Massif and the Alpine-
Carpathian zone. Fig. 11 shows some degree of
coincidence between the form of the magnetic anomaly
and the distribution of epicentres of earthquakes. The cor-
respondence is good in the area of the Northern Calcare-
ous Alps, but continuing eastwards, the epicentres of the
seismoactive zone Mur-Mürz—Leitha—Malé Karpaty be-
come increasingly shifted from the maximum of the mag-
netic anomaly towards the SE.

The data from the total magnetic field will serve as a ba-

sis for investigations in future. The cross-border stitching
of the magnetic data due to different kinds of data sets
available (aeromagnetic, ground survey) and the necessity
of choosing the appropriate parameters in the Euler 3D-de-
convolution, without being in the position to employ a
comparative method (Linsser filtering) to check the out-
come in terms of reliability, constrained us from doing so.

In addition, the application of the Linsser method was
abandoned, because no correlation between seismic activ-
ity and magnetic anomalies became apparent.


The first part of the project dealt with needs of the estab-

lishment of the seismic network of ACORN. Incorporation
of five seismic stations from neighbouring countries
(Czech Republic, Slovakia, and Hungary) resulted in a
much higher resolution of seismic activity which is re-
quired to study recent tectonic movements. All data are
transmitted now in real-time and are shared by national
centres. The data were collected not only from recent seis-
mic records but also from historical earthquakes from elev-
en earthquake catalogues. All entries had to be verified in
terms of possible misleading information such as wrong
catalogue entries or induced seismic events. Blasts espe-
cially turned out to be a challenge, as the denser network
enables locating of seismic tremors of much smaller mag-
nitude with greater accuracy than before. This category of
tremors – mainly due to quarry blasts – must be flagged
in the database to avoid wrong seismic hazard analysis,
which would be strongly biased otherwise.

The detection of faults at depth horizons where earth-

quakes tend to originate is to be considered the main task.
In order to do that for such a large area, a method proposed
by Linsser was applied to the available gravity data. The
method utilizes model curves which represent a certain
geological formation underground and their gravity effect
on the surface. The difference between the observed and
theoretical anomalies is minimized until an optimum
agreement has been achieved. This approach was applied
to two depth horizons – 4 and 8 km – and numerous
faults could be traced well below the surface. These linea-
ments were finally compared with epicentres of earth-
quakes from the catalogue, resulting in a good correlation.
Moreover, some pronounced lineaments, which cannot be
traced on the surface using geological maps, coincide with
epicentres of major historical earthquakes, such as
Scheibbs (1876) or Neulengbach (1590) thus giving rise
to an understanding of the prevailing mechanism in-
volved. In the case of the earthquake of Molln (1967), the
strike of the detected structure coincides with one of the
planes of the already determined focal mechanism.

Acknowledgment:  The authors would like to thank Walter
Hamilton (OMV) for permission to use the detailed gravity
data of the OMV as well as Wolfgang Seiberl (formerly
Geological Survey of Austria and Institute for Meteorolo-
gy and Geophysics of the University of Vienna) for pro-
viding the aeromagnetic data. Special thanks should be
expressed to Bruno Meurers from the Institute for Meteo-
rology and Geophysics of the University of Vienna for
providing the Austrian gravity data. The authors are grate-
ful to partners from co-operating organizations – Geo-
physical Institutes CAS and SAS, especially to Jan Zedník
and Peter Labák for their inputs. Authors also would like

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to thank Christiane Freudenthaler, Nikolaus Horn, Rita
Meurers, Rudolf Steiner, and Anton Vogelmann of the De-
partment of Geophysics at the Institute for Meteorology at
the Central Institute for Meteorology and Geodynamics in
Vienna (Austria) and Svatopluk Boleloucký and Jan Otru-
ba of the Institute of the Physics of the Earth at the
Masaryk University in Brno (Czech Republic) for their as-
sistance and support in carrying out this project.  The
project was financed by the Federal Ministry for Educa-
tion, Science and Culture of Austria. Due to this financial
support, the exchange of seismological data in real time
could take place already at a time when this kind of data
exchange was still in its infancy. The project helped not
only to establish cross-border data exchange and to en-
hance the co-operation between institutes, but also added
to the experience needed to run a quite complex real-time
data exchange system on a continuous and reliable basis.

The research was partly supported by the Czech Minis-

try of Education through the research Project MSM


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