Jökull - 01.01.2010, Side 68

Horálek and Fischer
INTRODUCTION
The region of West Bohemia (Czech Republic) and
Vogtland (Saxony, Germany), latitude 49.8◦N to
50.7◦N, longitude 12◦E to 13◦E, is very likely the
most active intraplate earthquake-swarm area in Eu-
rope. Seismicity has been persisting there for cen-
turies; there are frequent occurrence of swarms
(mostly of magnitude ML<4.0) or microswarms with
events of magnitudes up to ML=2.0.
This region is situated in a tectonically stable
western part of the Bohemian Massif at a contact of
different Variscan tectonic units – the Saxothuringian,
Moldanubian, and Teplá-Barrandian (Babus̆ka and
Plomerová, 2008). The ENE-WSW trending neo-
tectonic structure, the Eger Rift, terminates close to
the epicentral area and is intersected by the NNW-
SSE striking Mariánské Láznĕ fault (Bankwitz et al.,
2003). The region is intruded by several granitic plu-
tons with estimated depth of 10 km (Blecha et al.,
2009). Quaternary volcanism was active in this region
until the Holocene (Gögen and Wagner, 2000) and
two extinct volcanoes, Komorní Hu̇rka and Z̆elezná
Hu̇rka (the youngest volcano in the Bohemian Massif
of the age of about 0.3 Ma), are located in a distance of
15 and 25 km from the main epicentral zone. A Qua-
ternary maar volcano close to Z̆elezná Hu̇rka was in-
dicated by Mrlina et al. (2007). Beside the earthquake
swarms, the western part of the Bohemian Massif is
famous for its high flux of mantle derived CO2 be-
ing manifested by several moffetes and plenty of car-
bonized mineral springs and CO2 vents which are lo-
cated close to the intersection of the Eger Rift and the
Mariánské Láznĕ fault. It is generally thought that all
these mineral springs and moffetes are supplied with
CO2 and other gases from a magmatic reservoir lo-
cated in the uppermost mantle (Geissler et al., 2005).
Such a potential reservoir may be related to the Qua-
ternary volcanism, some authors speculate of a recent
magmatic activity in the upper mantle in this region
(Bräuer et al., 2003; 2008).
On the basis of macroseismic observations,
the earthquake swarms in the region of NW Bo-
hemia/Vogtland have been well documented since the
second half of the 19th century. The instrumental
observations started in 1903. Hence, we are able to
infer that a frequent occurrence of weak earthquake
swarms, mostly of magnitudes ML< 3.5, is typical
for West Bohemia/Vogtland and that larger swarms
occur only exceptionally. Let us note that the Ger-
man expression "Erdbebenschwarm" (i.e. earthquake
swarm) was introduced by Knett (1899) and Cred-
ner (1900) to characterize the earthquake activities in
NW-Bohemia and Vogtland. The swarm in 1985/86,
with two shocks reaching the magnitudes ML of 4.6
and 4.2, represents revival of swarm activity in this re-
gion. Three more swarms and many microswarms oc-
curred during the last twenty years; each activity en-
abled deeper understanding of the nature of the West
Bohemia/Vogtland swarm-like seismicity.
The aim of this paper is to characterize the
West Bohemia/Vogtland earthquake swarms as the in-
traplate activity. Later, these intraplate swarms will be
compared with the interplate swarms in the South Ice-
land transform zone (see Jakobsdóttir, 2008), which
will show whether there are characteristics dependent
on the tectonic environment. Such analyses should
be a key to disclosing the causes leading to seismic
energy release in the form of the earthquake swarms
instead of the ordinary single events or mainshock-
aftershock sequences.
Local observations by WEBNET seismic network
WEBNET (West Bohemian NETwork) has been op-
erating since 1991 when the first four stations in the
region were established. At present, the WEBNET is
a primary source of the data from the whole region.
It consists of 13 three-component digital seismic sta-
tions and covers an area of about 900 km2 (Figure 1).
Seismometers are installed in vaults at a depth of 3–
5 m, on concrete pillars ensuring perfect contact with
the bedrock. Central station NKC is situated in the
centre of the main focal zone Nový Kostel (NK), fur-
ther six stations (SKC, VAC, LBC, POC, STC, KVC)
surround this zone. The remaining five stations (KRC,
KOC, LAC, ZHC, KAC, TRC) are located in minor
focal zones and ensure a proper areal and azimuthal
coverage of the focal area, particularly with respect to
the NK zone.
All stations are equipped with passive seismome-
ters SM-3, moreover two stations (NKC and ZHC)
are supplemented by broadband seismometers Güralp
68 JÖKULL No. 60