ASSESSMENT OF SEISMOLOGICAL DATA OF REGIONAL ARRAYS FOR
INVESTIGATING SEISMICITY IN LITHUANIA
A. PACESA
Geological Survey of Lithuania, 2600 Vilnius, st. S. Konarskio 35, Lithuania
e-mail: andrius.pacesa@lgt.lt
Baltic region has been a priori considered
a seismically inactive region (Bune and
Goriainovoj, 1980). As consequence just a very few
seismic stations were installed here. The installation
of new seismic stations is quite slow process due to
relatively high cost of equipment and complicated
economical situation of these countries.
On the other hand at least a several dozens
of moderate magnitude earthquakes (intensities 5 –
7 at epicenters, scale MSK-64) stroke the territories
of Baltic counties according to the historical data
(Nikonov et al., 1988) and data obtained from
seismological stations of Scandinavia.
Dozens of seismic stations and number or
seismic arrays are deployed all over the Europe.
Most of data collected by these institutions could be
easily found on the Internet. Data of seismic arrays
could be especially useful investigating areas
poorly covered by seismic networks as seismic
arrays are designed to detect even weak signals
within large distances. So purpose of this research
was to evaluate a possibility utilizing seismic data
from the regional seismic arrays.
Number of explosions with precisely defined
coordinates, origin times and charges were
GERESS (Germany) seismic arrays. Therefore it
was possible to asses seismological data of regional
seismic arrays for investigating seismicity in
Lithuania.
It was found that precision of location
of events depends on the number of arrays used. In
general the precision is unsatisfactory low giving
errors up to 300 km and 30 sec (Fig. 1 and 2).
Nevertheless, one can expect that the errors of
locations will be less than 50 km and 1.2 sec. then
data of four seismic arrays are used.
Summarized seismological bulletins of the
network of regional arrays were downloaded from
the NORSAR (Norwegian Seismic Array) web site
(www.norsar.no).
The
bulletins
contained
unchecked, fully automatic locations from the
NORSAR Generalised Beamforming (GBF) system
and covered time period from 1992 to 2000.
A simple PC program was written to extract
events confined to the territory of Lithuania and
adjacent areas. This set of events contained a
large number events originated by wind, sea waves
or human activities such as quarry blasts, traffic and
industry activities. Therefore all data were filtered
using Exfilter program (Havskov and Ottemöler,
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300
250
200
150
100
50
0
0
1
2
3
4
5
6
Number of arrays used
Time error of begining of event, s
Epicenter location error, km
350
30
25
20
15
10
5
0
0
1
2
3
4
5
6
Number of arrays used
Fig. 1. Epicenter location errors depending on the
number of arrays used. Open diamonds represent
the location errors of explosions of seismic
profiling experiments. Shaded circles represent
averages of location errors.
Fig. 2. Time errors of a beginning of events
depending on the number of arrays used. Open
diamonds represent the time errors of beginning
of explosions of seismic profiling experiments.
Shaded circles represent averages of time errors
of beginning of the events.
executed during the recent seismic profiling
experiments Eurobridge (Nasedkin and Motuza,
1999) and Polonez (Guterch et al., 1999) in
Lithuania. Most of these explosions were registered
by regional network of seismic arrays incorporating
NORSES, ARCESS, (Norway), HFS (Sweden),
FINESA (Finland), APATITY (Russia), and
1999).
The GBF bulletins of NORSAR contained
a number of "false" events resulted from
misinterpretation of some random spikes
of background noise. Analysis of the set of seismic
profiling explosions revealed that even small
explosion (~ 200 kg TNT) was detected by two or
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more arrays. Events detected by one array were
excluded from the data set in order to eliminate
the majority of "false" events.
The refined GBF bulletin of NORSAR
contained 223 events (Fig 3.). Some of them
probably are indicators of seismotectonical activity
in Lithuania and adjacent areas. However precision
of events located using two or three arrays (open
circles, Fig. 3) is unsatisfactorily low (~ 100 km
and ~10 s on average, Fig. 1, 2). Events detected by
four or five arrays (filled circles, Fig. 3) have
average location error of 22 km and 1.6 s (Fig. 1,
2). Events of this type could provide some useful
information in further investigations of seismicity
in this region.
Fig. 3. Epicenters of events from GBF reported by NORSAR covering time period from 1992 to 2000. Open
circles correspond to events detected by two or three seismic arrays, filled circles - events detected by four or five
arrays.
REFERENCES
Bune, V. I. and G. I. Goriainovoij, Seismical
zonation of territory of the USSR (in Russian),
Moskow, 1980.
A. Guterch, M. Grad, H. Thybo, G.R. Keller,
POLONAISE '97 - an international seismic
experiment between Precambrian and Variscan
Europe in Poland, Tectonophysics Vol. 314,
Issue 1-3, 101-121 ,1999)
Havskov, J. and L. Ottemöler, SEISAN: the
earthquake analysis software for Windows,
SUNOS, SOLARIS and LINUX Version 7.0,
Institute of Solid Earth Physics University of
Bergen, Norway, 1999.
Nasedkin, V. and G. Motuza, Investigation of upper
lithosphere along to the deep seismic profiling
Eurobridge in the territory of Lithuania (in
Lithuanian), Geological Survey of Lithuania,
1999.
Nikonov A., Sildvee H., Earthquakes in Estonia and
their seismotectonical position (in Russian),
Academy of Science of Estonia, Geology, Vol.
37, No 3, 127-141, 1988.
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