Jökull - 01.01.2010, Page 83

Intraplate earthquake swarms in Central Europe
were significant in triggering subsequent aftershocks
close to prior earthquakes, while a triggering effect
due to dynamic changes took effect mainly at larger
distances and small time scales. However, a ques-
tion is whether the Coulomb stress changes, which are
only fractions of the stress drop of a prior earthquake,
can trigger subsequent swarm events. In our view, the
stress changes upon the fault plane due to co-seismic
slip may be significant for triggering the swarm earth-
quakes only if the focal area of the fault plane is criti-
cally loaded.
Correlated seismicity in the whole area
According to Figure 3, the activity appears to alter-
nate between focal zones. However, a simultaneous
occurrence of the seismicity in different focal zones
occurs as well. This was addressed by Horálek and
Fischer (2008), who analyzed the time and space sep-
aration of consecutive events in the dataset of the
whole West-Bohemia/Vogtland region. They found
that the frequency of the distances between consec-
utive events showed two maxima separated by a de-
creased occurrence between about 3 and 10 km, (see
Figure 8 in Horálek and Fischer, 2008). The first
subset in small distances corresponded to the event
pairs occurring during individual swarms (clustered
events), whereas the second subset in larger distances
displayed switching of simultaneous activity between
distant focal zones. By comparing the time periods
during which the seismic activity occurred in a sin-
gle focal zone and/or in the multiple focal zones they
found that the region was quiet for most of the time
and came to unrest only occasionally. It also turned
out that the probability of activation of multiple zones
was quite significant, which pointed to common trig-
gering force acting in a broader area of West Bohemia
and Vogtland.
Here we used a statistical approach to get an inde-
pendent indication of the significance of remote earth-
quake interactions. Similarly to Horálek and Fischer
(2008) we separated the seismic dataset (in this study
we used the period between 1991 and 2009) to two
subsets: (1) all event pairs and (2) distant event pairs,
which were separated by distance larger than 5 km.
Figure 12 shows a probability density function of in-
terevent times measured between all event pairs from
both these subsets. It appears that the probability den-
sity of interevent times of all events in the interval
from <1 s up to about 40 000 s shows a power-law
dependence, which points to a correlated occurrence
of the events. This is probably affected by the in-
fluence of nearby events from the individual swarms.
However, a similar power law is also observed for the
subset of remote events, which indicates that distant
events could also bear a correlated occurrence. To
test this hypothesis we created a random catalogue of
the same size and with events whose origin times oc-
cur randomly within the period of observation. The
probability density of this random event occurrence
corresponds to the Poisson distribution, which sig-
nificantly deviates from the probability density func-
tion observed for distant event pairs. Accordingly, we
found that the distant event pairs separated by inter-
vals smaller than 40 000 s (about 11 hours) showed an
increased occurrence compared to the situation if they
occurred independently. We infer that this could be at-
tributed to a triggering force acting in the broader area
of West Bohemia/Vogtland. It is however beyond the
scope of this study to examine the possible physical
mechanisms responsible for this force. We only spec-
ulate that a deep-seated fluid injection or tidal forces
can be seen as potential candidates.
CONCLUSIONS
In this paper we present principal characteristics of the
West Bohemia/Vogtland earthquake swarms which
are inferred from local observations of the network
WEBNET during the period from 1991 to 2009. They
can be summarized as follows:
(i) Swarm micro-earthquakes cluster in a number
of small focal areas in West Bohemia and Vogtland.
About 90% of the total seismic moment is released
in the Nový Kostel (NK) focal zone, which is related
to an NNW striking (169◦) and steeply dipping (80◦)
fault plane of about 12 x 6 km in size.
(ii) The focal depths range between 5 and 27 km
in the whole region and between 4.5 and 13 km in
the NK zone, the most of the swarm events cluster at
depths 8.5 to 9.5.
(iii) The foci of the 2000 (ML≤3.3) and 2008
(ML≤3.8) swarms fall precisely on the same fault
JÖKULL No. 60 83