earthquakes on the eastern margin of the bohemian massif recorded

earthquakes on the eastern margin of the bohemian massif recorded

“Exploration Geophysics, Remote Sensing and Environment“
V. 2. (1998)
EARTHQUAKES ON THE EASTERN MARGIN
OF THE BOHEMIAN MASSIF RECORDED BY
THE STATIONS OF THE IPE BRNO
1Skácelová,
Z.,1Havífi,J.
1Institute
of Physics of the Earth,
Faculty of Sciences,
Masaryk University,
Tvrdého 12, 602 00 Brno
Czech Republic
ABSTRACT: The latest results of the seismological analysis in
the eastern margin of the Bohemian Massif are discussed. The
stations of the Institute of Physics of the Earth (IPE) have
detected in this region 131 tectonic microearthquakes since
1992. Twenty tectonic events were localised by program
HYPO3D. The localisations of epicentres indicate affinity of the
seismo-tectonic activity mainly to the NW-SE trending
movements along the faults.
SEISMOLOGICAL STATIONS
INTRODUCTION
Four seismic stations in the Moravian and Silesian regions
have been built up by the Institute of Physics of the Earth,
Faculty of Sciences, Masaryk University, Brno (IPE) since
1992. The first of them, the VRAC station in Vranov u Brna
(approximately 15 km from the city of Brno) was registering
in trigger regimen (triggered records of particularly strong
events) from 1992 to 1993, and since 1993 in continuous
regimen. The VRAC station is now included into
International Monitoring System for verification of an
adherence of Nuclear Test Ban Treaty. It is equipped with the
three-component wide-range sensor TSJ-1 and the Lennartz
5800 registering apparatus. Registered data are transmitted
to the National Data Centre in Brno (NDC), stored on the
hard drive where they are immediately available for
interpretation (approximately 10 minutes delayed).
The tectonic setting of the eastern part of the Bohemian
Massif is represented by at least three structural levels
(Cadomian, Variscan and Alpine). During the Alpine
Orogeny the Western Carpathian flysh nappes were thrust
over the eastern margin of the Bohemian Massif owing to an
oblique collision. The Alpine compression affected the
geological units of the whole Bohemian Massif by significant
movements along the fault zones. The older (Variscan)
structures were often involved in the younger tectonic
movements in the eastern part of the Bohemian Massif.
The seismic activity in the eastern margin of the
Bohemian Massif (Moravia and Silesia) is relatively weak.
Strong earthquakes, known from historical records, were
mainly concentrated into the Opava, ·ternberk, and Hrub˘
Jeseník regions. Macroseismic earthquake manifestations in
these regions were described in chronicles and historical
archives (Reme‰ 1902; Koláãek 1921; Schenková, Kárník
and Schenk 1980). But most of them were assigned to the
strong earthquakes in the Viennna Basin (Olomouc and
·ternberk areas).
In October 1993 the MORC station (Moravsk˘ Beroun)
was put into operation within the framework of the
international project GEOPHON. It is situated on the
âervená Hill in the epicentral area of earthquakes originated
in the area of Opava, ·ternberk, and Hrub˘ Jeseník. Since
November 1995 another two stations have been in
operation - the KRUC (Moravsk˘ Krumlov) and JAVC
(Javorina) stations. They are situated in southern Moravia,
the KRUC station approximately 50 km SW of Brno and the
JAVC station in the border region with Slovakia on the Velká
Javorina Hill, the highest hill of the Malé Karpaty Mts.. All
these three stations are equipped with the three-component
wide-range sensor STS-2 and the registering apparatus
Quanterra. The signal registered by these stations is also
transmitted to the NDC in Brno where it is processed
together with data from the VRAC station.
Recent seismic activity has been monitored by
seismological stations of the Institute of Physics of the Earth,
Faculty of Sciences, Masaryk University Brno (IPE) since
1992 (see Encl.No.2 - Fig.1). The weak tectonic events were
recorded in Moravia and Silesia and were published step by
step (Havífi, Pazdírková 1995; Havífi, Skácelová 1996;
Skácelová, D., Skácelová, Z., Havífi 1997; Skácelová, Skácel
and Havífi 1998).
Seismic activity in the north eastern part of the Bohemian
Massif has been monitored by other institutions - Technical
University of Ostrava and Institute of Geonics of the
Academy of Sciences of the Czech Republic. Research
results were published in the geophysical journal and
contribute to the geological interpretation of registered
earthquakes (Kaláb et al., 1995, 1996; Holub, Müller 1997).
The mobile digital seismological short-period station
MUTC in Mutkov in the southeastern margin of the Nízk˘
Jeseník Mts. was put into operation in October 1996. This
station is equipped with the three-component sensor MARK
and the Lennartz 5800 registering apparatus.
16
“Exploration Geophysics, Remote Sensing and Environment“
V. 2. (1998)
and depths. Epicentres of earthquakes are concentrated into
two areas approximately 5 km apart (about 2 km from the
R˘Ïovi‰tû village) and have a strongly swarm-like character.
On 27th March 1998 the MUTC station registered 18 events
during 1.5 hour, with identical waveforms. The maximum
computed local magnitudo ML was 1.4. In February 1998
two events with opposite direction of the first motion were
recorded in the same area. It can be assumed that the
mechanism of the movement is changing during seismic
activity.
Besides seismological stations operated by the IPE Brno
there are stations operated by the Technical University of
Ostrava - the OKC station in Ostrava, and by the Institute of
Geonics of the Academy of Sciences of the Czech Republic the SPF array near Fren‰tát, the ZHL station in Zlaté Hory,
and HRC in Hradec u Opavy, situated in this region.
SEISMIC ACTIVITY
In the period from October 1994 to September 1998
a total of 131 microearthquakes (local magnitudo ML 2.0)
were recorded in the northeastern margin of the Bohemian
Massif at stations of the IPE Brno. Twenty of them could be
localised by the HYPO3D program that is based on the block
velocity model of the Earth Crust. The main condition for
computation was to know at least five arrival times of wave
phases going through the Upper Crust registered by at least
three stations. A block velocity model compiled in the IPE
Brno from the results of refraction seismic measurements and
geological and geophysical data (Dvofiák, Firbas and
Skácelová 1993). This model characterises wider
surroundings of the profile from the Culm of the Nízk˘
Jeseník Mts. into the Neogene sediments of the Carpathian
Foredeep (area of the Moravian Gateway) and Outer Flysch.
Data from the co-operating organisations were used to
localise some stronger events.
Close to this epicentral area the Klepáãov fault, an
occurrence of neovolcanics, and sources of mineral waters
are described.
Moravian Gateway and the Host˘n Hills (D)
Seismic activity in these areas has been observed only
recently. On 9th August 1994 an earthquake swarm of 9
events (local magnitudo ML 1.5) was registered in the area of
Moravian Gateway. The strongest event of this swarm was
also registered by the SPF array. Due to the insufficient
number of stations, computations localised the epicentres
only approximately to the vicinity of Hranice na Moravû.
On 1st May 1997 a relatively strong earthquake was
recorded and localised into the Host˘n Hills area. Stations of
the SPF array contributed to a more accurate localisation and
to the computation of the depth of this event, that was
approximately 25 km.
Registered earthquakes indicated several active areas in the
northeastern margin of the Bohemian Massif.
The Opava region (A)
We can suppose that seismic activity in both these areas is
related to faults in the underlying rocks of Outer Flysch (it cas
been confirmed by the depth of the hypocentre),
consequently to the rocks of the Bohemian Massif.
This region is known for earthquake manifestations from
historical records. Also a previous geological survey indicated
recent seismotectonic movements and their significance for
fault declines in the sedimentary filling of the basin. The latest
strong earthquakes of a swarm-like character were registered
in August 1993 in the close vicinity of the city of Opava. Only
weak quakes have been registered since then, mainly in the
southern margin of the Opava sedimentary basin in the
vicinity of the Nízk˘ Jeseník fault.
Margin of the Haná area (E)
Stations of the IPE Brno have registered a weak seismic
activity also in the marginal areas of the sedimentary filling of
the Haná basin. Isolated earthquakes were registered and
localised to the areas of Uniãov,·ternberk, Prostûjov and
Konice (local magnitudo ML 0.1).
The Odra Hills (B)
Weak earthquakes were registered in this area in 1986
during detailed seismic monitoring that was carried out by
the Geofyzika Brno company. Since October 1993 isolated
weak earthquakes have been registered by stations of the IPE
Brno in this area. They can be identified with a particular
fault system only with difficulty. The strongest of them (local
magnitudo ML=1.6) was registered on 28th February 1997
and it was localised into the area of Jánské Koupele.
We suppose that these earthquakes have been connected
with the marginal faults of the Haná basin - Bu‰ín and
Temenice faults and the Konice-Nectava fault zone.
Northern part of Boskovice furrow (F)
No seismic activity had been recorded in this area until
1993. The first seismic event was recorded and localised to
the northern margin of the Boskovice furrow in October
1993 after the VRAC station had been installed. This
earthquake was also registered by other seismic stations in
the Czech Republic (PRU, KHC - seismic stations operated by
the Geophysical Institute of the Czech Academy of Sciences).
By the end of 1998 six earthquakes were localised to this
area. Epicentres of these earthquakes are concentrated into
three areas - the northeastern margin of the Boskovice furrow
(Jevíãko), the northwestern margin of the Boskovice furrow
Nízk˘ Jeseník - R˘Ïovi‰tû (C)
Earthquakes originated in this area were for the first time
observed in 1993 after launching the MORC station. As
frequent occurrences were recorded, and some historical
sources mentioned occurrences of earthquakes in the
·ternberk area, the MUTC mobile seismic station was
installed north of ·ternberk in 1996. This station significantly
contributed to the accurate computation of their epicentres
17
“Exploration Geophysics, Remote Sensing and Environment“
LEGEND - to the Enclosures No.1/No.6
V. 2. (1998)
(Thickened and deep buried column started to ascend
isostatically during the Carboniferous - after release of
orogenic control of this stacking).
Encl.No.1:
Fig.1 Geological schema of the Western Bohemia
region.
Fig.9 Thermal alterations of tiny organic fossils conodonts.
(The darker color of the conodont, the higher maximum
temperature during the burial history - with little corrections
for other processes. Conodonts were formed by francolite
with a low content of organic matter, carbonate, clay
minerals and other mineral accessories).
Encl.No.2:
Fig.1 Geological map of the eastern part of the
Bohemian Massif with epicentres ofthe microearthquakes.
Encl.No.3:
Fig.1 Main tectonic outlines of the East Sudetes
(Silesicum).
(by Skácel 1998)
Encl.No.5:
Fig.6 Diagrams illustrating paleogeographical concepts
by authorities of the discipline (our opinion is expressed
in the lower right corner of the picture).
Encl.No.4:
Fig.1 Typical microimage of Upper Devonian limestone
from Mokrá, in passing light on the left, cathodeluminiscence image on the right.
Fig.8 Pole projections based on the Early and Middle
Devonian sites (groups of the sampled sections and
traverses), Moravia and central Bohemia (Barrandian).
Explanations: 1 - Kacak Event, Eifelian-Givetian,
U Dubu sedmi bratfií; 2 - Middle/Late Eifelian, Hostim
in Hostim - Srbsko roadcut; 3 - Emsian-Eifelian,
Pragstav quarry near Holynû; 4 - Praghian, Bránická
skála, Prague; 17 - Givetian, Josefov junction, Near
Adamov; 18 - Eifelian, âelechovice na Hané, near
Prostûjov.
Fig.2 Diagenesis of Upper Devonian limestones in
Mokrá.
Fig.3 Paleotectonic rotation and Variscan magnetic
overprint on Moravian Devonian limestones,
illustrated by positions of paleomagnetic poles.
Explanations: Devonian paleomagnetic poles
calculated from the C-component are on the globe on
the left (violet); poles calculated from the B-component
of Carboniferous/Permian overprint are on the right
(red). Thin lines illustrate model rotation paths of
poles conditioned by the horizontal component of
tectonic rotation of a rock segment with paleomagnetic
inclination I, from +5° to -35°, after 5° (as if by turning
a sample round vertical axis we drew a circle on the
globe. Empty circles on this path correspond to the
difference in declination (step) ∆ D 20° . The path of
apparent movement of poles (APWP) from the
Devonian to the Triassic, based on the stable
(undeformed by Variscan orogeny) part of the Europe
is illustrated by a thick line with points representing
approximate mean positions for parts of units, e.g.
Devonian D1, D2, D3, etc. (Krs and Pruner, 1995).
The sites belong to diferent terranes and tectonic
segments (slices and nappes), where horizontal
component of the Variscan tectonic rotation is very
strong. The Devonian inclinations about -25 degree
imply palaeogeographical positions about 10 degree
South, or even more southerly towards 20’s latitudes.
Generally, the Barrandian latitudes of these times were
not higher than in Moravia.
Fig.7 Sequence of seven generations of faults, found by
the method younger intersects the older on outcrops
and in geological map, discussed in chapter Tectonics
under nos.1-7.
Explanations: 1 - segments with ruptures according to
tectonic map (geological atlas); 2 - segments with
tectonic predestinations of river beds (geological atlas);
3 - segments with ruptures according to geodetic
measurements (Demek, Loyda 1984); 4 - paths; 5 circles: localities of markets; 6 - triangles: occurrences
of ancient coins in Moravia.
Encl.No.6:
Fig.1 Network of paths in the Czech Republic (Kvût
1997 - originally worked out with ¤ehák, S.) with
identical directions of earth crust ruptures, a map of
the Czech Republic with the network of paths (Kvût
1997), occurrences of ancient coins (Sejbal 1979) and
localities of 12 and 13 century markets (Kejfi 1987).
Fig.8 Visualization of the area where a thick stack of
nappes (thickening of crust) originated during the
Visean/Namurian times.
18
„Exploration Geophysics, Remote Sensing and Environment“
V. 2. (1998)
Fog. 2 : Schema of the occurrences of the Tertiary/Quaternary volcanics, the springs of carbonated mineral water
and the epicentres of the micro-earthquakes.
19
“Exploration Geophysics, Remote Sensing and Environment“
V. 2. (1998)
were revealed in the north-eastern part of the Bohemian
Massif also by an analysis of repeated geodetic measurements
(Vyskoãil 1991). Very important is the recent extension
detected by the analysis of repeated geodetic measurements
in the region of the Western Carpathian Flysh Belt in southeastern Moravia (Vyskoãil 1991; Vyskoãil, Zeman 1980).
(Svitavy) and the area north of the city of Poliãka. The local
magnitudo ML of the strongest event was 1.9, the latest
event was registered on 25th November 1998. It cannot be
explicitly stated which fault system is this seismic activity
connected to.
A whole range of faults passes through this area - in the
NW-SE direction the Konice Nectava fault zone and its
continuation to the Cretaceous platform cover, and the NS trending faults limited by the sedimentary filling of the
Boskovice furrow.
The epicentres of microearthquakes occur in the eastern
part of the Bohemian Massif mainly near the major NW - SE
trending fault structures (the Bu‰ín fault, the Kletná fault, the
Klepáãov fault, the Temenice fault, the Konice-Nectava fault
system, etc.- see for instance Skácelová, Skácel and Havífi
1998; Skácelová D., Skácelová Z. and Havífi 1997).
Microearthquakes are often detected near crossings of the
faults with significant faults of other systems. This can
indicate the affinity of the seismo-tectonic activity to the
movement along NW-SE trending faults.
Králick˘ SnûÏník Massif (G)
No earthquake has been recorded in this area. A tectonic
event with local magnitudo ML = 1.4 was registered by
a station of IPE Brno on 20th December 1997 and with the
help of other stations of the National Seismological Network
of the Czech Republic (PRU, KHC seismic stations operated
by the Geophysical Institute of the Czech Academy of
Sciences) localised to the vicinity of Králick˘ SnûÏník. The
Kletná fault passes in the NW-SE direction from Poland to the
Králick˘ SnûÏník Massif where it is followed up by the
Klepáãov fault.
The NW-SE striking faults may be very important tectonic
structures for the Pliocene/Quaternary tectonic activity. In
the Haná area the block of the Hornomoravsk˘ úval
decreased along these faults during the Pliocene/Pleistocene
period (RÛÏiãka 1973; Zeman et al., 1980). The overprint of
the NW-SE striking faults into the Western Carpathian Flysh
Belt confirms the impact of their Saxonian tectonic activity
(Buday et al., 1995). The Pliocene/Pleistocene volcanic
activity in the Jeseníky region is also connected with NW-SE
trending faults (for instance Kumpera 1994; Pfiichystal 1993).
The major NW-SE trending faults often create expressive
morphological slopes. The height of the fault slope of the
Marginal Sudetic fault is up to 250 m in the Rychlebské hory
Mts (Ivan 1998).
MICRO-EARTHQUAKES IN RELATION
TO THE TECTONIC AND THE
RECENT TECTONIC ACTIVITY
The recent seismotectonic activity is concentrated in the
north-eastern part of the eastern margin of the Bohemian
Massif, especially in the Haná area (the block of the
Hornomoravsk˘ úval) and in the Nízk˘ Jeseník area.
Seismological stations operated by IPE Brno have detected
hardly any tectonic events that could be connected with
faults in the eastern part of the Bohemian Massif southwards
of the Konice-Nectava fault system. However, the possible
recent weak seismo-tectonic activity along the eastern
marginal fault of the Boskovice furrow has not been
elucidated yet. Figdor and Scheidegger (1977) informed
about the tectonic events near the Diendorf fault, a southern
continuation of the eastern marginal fault of the Boskovice
furrow. Seismological stations operated by IPE Brno have
detected several micro-earthquakes near the crossing of this
eastern marginal fault and the Konice-Nectava fault system
(Havífi, Pazdírková 1995). That is why the weak seismotectonic activity along the eastern marginal fault of the
Boskovice furrow could be expected.
The seismo-tectonic activity is also connected with some
E-W striking faults. Kumpera (1994) supposed that the
microearthquakes occurring south of Opava were closely
related to the recent tectonic activity of the E-W trending
marginal fault of the Jeseníky Mts. This fault limits the
Neogene and the Quaternary sediments of the Opava basin
and is connected with the Neogene subsidence of the Opava
basin. Also the tectonic event of the 1st May 1997 (Havífi,
Skácelová 1998) localised in the SE of Vala‰ské Mezifiíãí near
the crossing of the E-W striking RoÏnov fault and the NS striking marginal fault of the Upper Silesian Coal Basin
could be connected with tectonic activity of the E-W trending
RoÏnov fault under the Western Carpathian flysh nappes.
CONCLUSION
The only weak seismo-tectonic activity was observed in
the eastern margin of the Bohemian Massif (Moravia and
Silesia) by stations of IPE Brno. The epicentres of
microearthquakes occur near of the significant NW-SE
trending fault structures and near some significant EW striking faults. This can indicate the affinity of the seismotectonic activity to the movement along NW-SE striking
faults. These faults seem to be very important tectonic
structures for the Pliocene/Quaternary tectonic activity. The
occurrences of micro-earthquakes show the continuation of
their tectonic activity up to the Recent.
Another evidence of the Pliocene/Quaternary tectonic
activity is also found mainly in the northeastern part of the
eastern margin of the Bohemian Massif. From the Nízk˘
Jeseník area the products of Pliocene/Pleistocene volcanic
activity are known (Barth 1977; Pfiichystal 1993). Also the
springs of the carbonated mineral water are found only in the
north-eastern part of the eastern margin of the Bohemian
Massif (Fig.2). Thus the space distribution of the epicentres of
the microearthquakes correspond very well to the
occurrences of the Pliocene/Pleistocene volcanics and the
springs of the carbonated mineral water in the eastern part of
the Bohemian Massif. Signifcant recent crustal movements
20
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V. 2. (1998)
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