Jökull - 01.01.2010, Qupperneq 73

Intraplate earthquake swarms in Central Europe
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Figure 4. Distribution of the hypocentre depths for the whole study area except the NK focal zone (a) and for
the NK focal zone only (b). – Dýptardreifing jarðskjálfta í Vestur Bæheimi og Vogtlandi (a) og Nový Kostel
(NK) svæðinu (b).
swarm) took place in the southern one. It indicates
that seismicity on the NK fault plane migrates slowly
from the north to the south. There is also an evident
trend of increasing focal depths towards the south, the
cluster of the foci plunges with an angle of ∼30◦ to
the south.
Prevailing focal depths are between 8.5 and 9.5
km, which are similar to those in the broader area of
West Bohemia and Vogtland. A probable depth limit
in the NK zone is 13 km, while some minor focal
zones can have the depths down to 27 km (Figure 4).
Four swarms (1985/86, 1997, 2000 and 2008) in-
vestigated by Fischer et al. (2010), show diverse evo-
lution and rapidity of the energy release as well as
the duration of the swarm activity. Particularly the
swarms of 2000 and 2008 differ substantially in this
respect (Figure 6).
The foci of the 1997 swarm were located at depths
between 8.5 and 9.5 km in two rather small divergent
segments of about 700×1000 m in size, which formed
a wedge across the fault plane NK.
The hypocenters of the swarms of 2000 and 2008
fall precisely on the same fault portion of the NK fo-
cal zone in a depth interval from 6 to 11 km. The
fault planes of these swarms appear identical within
the bounds of the location error of about 100 m (for
more details, see Fischer et al., 2010). As can be seen
from Figure 7, both 2000 and 2008 swarms show an
oval main fault segment with diameter of about 5 km
with a tail pointing upward and to the north.
Interestingly, the 2000 and 2008 swarms show
similarity in general trends of the hypocenter migra-
tion: the first events of both swarms occurred at the
bottom of the fault segment and the final phase in the
uppermost tail (Figure 7). Nevertheless, a more de-
tailed space-time distribution of the foci shows fairly
different pattern: the counter-clockwise migration of
the 2000 swarm and a gradual upward migration of
the 2008 swarm.
The microearthquake activity in the period be-
tween the swarms of 1997 and 2000, and of 2008
shows different pattern. As can be seen from the
magnitude-time plot in Figure 5 the time span from
the mid-1997 until the beginning of the 2000 swarm
was characteristic in occurrence of the background
seismicity comprising number of solitary events
with magnitudes exceeding exceptionally ML=1.0,
whereas the 2008 swarm was preceded by an in-
creased microswarm activity which already started in
2004. This activity was manifested in a sequence of
microswarms with increasing maximum magnitudes
reaching ML ∼2.0.
JÖKULL No. 60 73