Jökull - 01.01.2010, Side 72

Horálek and Fischer
Figure 3. Time occurrence of seismic activity in focal zones 1–7, see Figure 2 for comparison. The numbers
on the right indicate total seismic moment released in each focal zone. The colour scale denotes magnitude of
seismic moment of the individual activities. – Yfirlit yfir skjálftavirkni upptakasvæðanna á 2. mynd.
far as concern the frequency of the activity, one can
identify three groups: the most frequent activity is ob-
served in focal zones NK and Schöneck (#1 and #6),
the medium frequency occurs in focal zones Klingen-
thal and Plesná (#2 and #7) while the remaining focal
zones Kopaniny–Adorf, Lazy and Marktredwitz (#3,
#4 and #5) show only sporadic activity. Prevailing fo-
cal depths in the area of West Bohemia and Vogtland
are between 5 and 15 km (Figure 4).
The dominating role of the NK focal zone (#1) is
apparent, namely in the total moment released (Fig-
ure 3, numbers to the right). This focal zone is
formed by a narrow NNW striking belt of about 12×2
km as a result of a weaker swarm in January 1997
(MLmax=2.9), a medium swarm in autumn 2000
(MLmax=3.3), an intensive swarm in October 2008
(MLmax=3.8), and more than thirty microswarms
(which amounts to a few tens of thousands events) in
the period between 1991–2009. Fischer and Horálek
(2003) showed that the strong earthquake swarm of
1985/86 also occurred there. To derive comprehen-
sive pattern of the spatio-temporal evolution of the
activity in the NK zone we utilize data sets from pre-
vious studies of Fischer and Horálek (2003), Fischer
and Michálek (2008), Fischer (2003), and Fischer et
al. (2010) who analyzed the time and space distribu-
tion of microearthquakes in the individual swarms and
periods in-between. A total of about 1000, 5950, 900
and 3000 events were located for the periods 1991–
1999, 2000, 2001-2007 and 2008, respectively, using
slightly modified master event procedure after Zollo
et al. (1995) and the same 1D gradient velocity model
(Málek et al., 2005) as was used for the absolute foci
locations in a broader West Bohemia/Vogtland region.
The resulting seismicity pattern of the NK zone
(Figure 5) shows a few important features:
The hypocenters cluster along a steeply dipping
(80◦) and NNW striking (169◦) fault plane at depths
between 6.5 and 11 km.
Two sub-clusters, the northern and the southern
one, are distinguishable on the NK fault plane. The
1997 swarm (and probably also the 1985/86 intensive
swarm) and most of microswarms, which occurred in
the NK zone before the 2000 swarm, were located in
the northern sub-cluster. However, starting with the
2000 swarms all swarm activity (including the 2008
72 JÖKULL No. 60