Jökull - 01.01.2010, Page 69

Intraplate earthquake swarms in Central Europe
40-T. Two types of data acquisition systems are used
in WEBNET: ten stations are equipped with Janus-
Trident instruments by Nanometrics and with the mi-
crowave telemetry (10 and 11 GHz) enabling inter-
net access to each station; three more distant stations
are equipped with the Lennartz 5800 PCM data ac-
quisition system with one-way radio telemetry, which
is efficient for data transmission in a hilly terrain.
The WEBNET stations operate with the frequency re-
sponses proportional to the ground velocity from 0.5
to 60 Hz, the frequency range of the stations equipped
with the Güralp 40-T sensors is 0.033 to 60 Hz. The
sampling rate of all stations is 250 Hz. The noise con-
ditions and the dynamic range of all stations≥120 dB
guarantee recording of the West Bohemia/Vogtland
events of magnitudes –0.5≤ML≤5. Ten stations,
which are equipped with the Janus-Trident instru-
ments, provide both continuous and triggered data;
the remaining four stations (KRC, KOC, LAC and
TRC) provide only triggered data.
Besides, eight three-component temporal stations
equipped with LE-3D seismometers and Gaia-24-bit
data acquisition systems, installed in special under-
ground vaults, have been operating since 2006. The
frequency range and sampling frequency of these
stations is 1.0–60 Hz and 250 Hz, respectively; the
stations operate in the triggered mode. For further
details on the WEBNET stations refer to web page
http://www.ig.cas.cz/en/structure/observatories/west-
bohemia-seismic-network-webnet.
The WEBNET database includes catalogs and
seismograms of all tectonic events (about 40,000)
recorded since 1991. Local magnitudes given in cata-
logues are estimated according to the WEBNET for-
mula ML=logAsmax–log2π+2.1logR+C–1.7, where
Asmax is the maximum absolute value of the total
amplitude of the S-wave ground velocity measured in
µm/s, R is the hypocentral distance in km, C is the
station correction, and constant –1.7 calibrates mag-
nitudes ML to the magnitudes estimated by PRU, the
primary station of the Czech Regional Seismological
Network. Absolute locations of the local events are
routinely performed using the Fasthypo code (Her-
rmann, 1979).
Another network, SXNET (Korn et al., 2008),
monitors the seismicity on the territory of SE Saxony
(Germany), outside the area covered by WEBNET.
Spatial and temporal distribution of swarm earth-
quakes
In terms of seismicity, the West Bohemia/Vogtland re-
gion (Figure 1) belongs to the north-south trending
Leipzig-Regensburg seismoactive zone (Bankwitz et
al., 2003; Korn et al., 2008) with the largest histor-
ical earthquakes reaching the macroseismic intensity
of 7◦–8◦ on the MSK-64 scale (Leydecker, 2005). To
characterize seismicity in West Bohemia and Vogt-
land we distinguish microswarms and swarms. Mi-
croswarms signify swarm-like activities lasting ordi-
narily several hours to a few days and comprising
dozens to hundreds of events which do not exceed lo-
cal magnitude ML=2.0, so that they can be observed
instrumentally. Earthquake swarms represent series of
several thousands of earthquakes mostly of ML<4.0,
stronger earthquakes swarms occur in West Bohemia-
Vogtland only exceptionally. A comprehensive pat-
tern of the space and time distribution of the swarm-
like seismic activity in the region concerned can be
derived for the period of the last 19 years for which
data from the local stations are available. In addition,
a strong 1985/86 earthquake swarm, which was mon-
itored by a few mobile stations, can be incorporated.
To derive a pattern of the seismicity distribution in
a broader area of West Bohemia and Vogtland more
than 10000 events from the period 1991–2009 were
relocated applying the Velest code (Kissling et al.,
1994) to the P and S arrivals which were obtained
from the local stations operated on the Czech and Ger-
man territory. Locations were performed using ve-
locity model by Málek et al. (2005). To improve the
control of focal depth we relocated only the events
with a minimum of one station within an epicentral
distance of 20 km. As can be seen from Figure 2,
the microswarms cluster in number of small epicentral
zones, which are scattered in the area of about 3500
km2. This was apparent already for the period 1991–
1999; Horálek et al. (2000a) delineated 7 focal zones.
The activity of individual focal zones is quantified in
Figure 3 that shows the cumulative seismic moment
calculated in a running window of 15-day width. As
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