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Seismological measurement in the Moravo-Silesian region in 2003.

INTRODUCTION

The seismological monitoring of the investigated area was a major significant work of the project "Fundamental mobility trends in the northern part of the Moravo-Silesian zone (the Bohemian Massif)--A complex geodynamic analysis", supported by the Grant Agency of the Czech Republic during the period (2001-2003). The seismological results obtained during the whole monitoring period from 1997 to 2003 are briefly summarized in this contribution.

The area under study (49.5[degrees]N-50.5[degrees]N, 17[degrees]E-19[degrees]E approximately, see also Fig. 1) was characterized by the complex geological and tectonic settings of the Cadomian, Variscan and Alpine ages (e.g., Kumpera and Suk, 1985). The present seismic activity of the northern part of the Moravo-Silesian zone is weak but should be pointed out that in the region stronger earthquakes took place in the past (firstly 1785-1786, 1931-1935, 1986; e.g. Karnik et al., 1981). Last earthquake swarm occurred in the period 1992 - 1993 with the macroseismic intensity up to [I.sub.0] = [V.sup.o] MSK-64. Gravimetric, magnetic and radiometric geophysical fields of investigated area are of a complicated character because of complex geological and tectonic structure. The visual analysis of isoseismic lines of the most intensive earthquakes in confrontation with geophysical anomalies, which represent also significant fault lines, has shown a considerable directional similarity (Kalab et al., 1996).

Detailed seismological study supported by the Grant Agency of the Czech Republic started in 1997 (205/97/0679). Obtained results of this grant were published (e.g. Schenk et. al., 2000). During the project duration (1997-1999), three local earthquakes with local magnitude up to 0.8 were recorded in the area of Opava in February 1999. Seismic events recorded at the solitaire stations Zlate Hory, Zary, Hradec above Moravice and Poruba during this period (Kalab and Knejzlik, 2000) were divided into three groups:

* Local seismic events--local earthquakes, quarry blasts in the surrounding of the stations, so far unidentified local seismic events that could belong to local earthquakes,

* Mining induced seismic events occurred in the Karvina part and the Polish part of the Upper Silesian Coal Basin and in the cooper mine in the Lubin area,

* Unidentified parts of teleseismic events.

Seismic observations by the IGN have been taken continually at the Moravo-Silesian region since 1997 up to now. Monitoring places were often moved to document very weak seismic activity because one of the aims of the project was to prove the current existence of seismic activity in investigated region (more than 20 positions were tested). New positions have been found in the northern part of the above mentioned area, in which seismic station did not exist. Detailed description of the operation of seismic stations was presented in Kalab and Knejzlik (2003).

There are three solitaire seismic stations operating now--Radun near Opava (X = 1093100, Y = 494300 JTSK), Slezska Harta (X = 1091100, Y = 520100) and new location in Zlate Hory (X = 1052900, Y = 529200; all coordinates are estimated using a map). However, removed seismic stations Jansky Vrch in Javornik and Janov were operated practically during the whole time of the observation period. The positions of the stations operated in the past and current ones are shown in Fig. 1.

INSTRUMENTATION OF SOLITAIRE SEISMIC STATIONS

All above-mentioned seismic stations were equipped with PCM3-EPC digital seismographs, which were developed in IGN. The PCM-EPC records 3-component seismic signals in a triggered regime on the disc embedded in a single board PC computer. Frequency range of seismic signal amplifiers is between 0.05-30 Hz, sampling frequency of A/D conversion is usually 100 Hz and the dynamic range 90 dB (MSB/LSB). The recording is triggered when a given number of the signal amplitude trespasses over the triggering level in a defined time interval. It is possible to record up to 15 s of signal time history before triggering. The time base of the records is synchronised by DCF 77.5 kHz signals.

Three SM-3 seismometers in a geographical configuration, adjusted to a natural period of 2 s were generally installed on the stations (S5S seismometers, adjusted to a natural period of 5 s, were installed only in the Radun station).

All stations were remote controlled via a GSM network. This enabled to check functions of the apparatus, to set of the trigger parameters, to start continuous data recordings for a given time interval and mostly to transmit recorded data.

More detailed description of the PCM3-EPC apparatus and software was given by Knejzlik and Kalab (2002).

NEW SEISMIC STATION IN ZLATE HORY

At first the Zlate Hory seismic station was placed tn the territory of the ore mine "Zlate Hory-East Colliery". The seismometers were placed underground in the area of an abandoned main shaft (the 3-rd level) at the depth of about 300-m deep. Signals were transmitted from seismometers to the PCM3-A seismograph (older model of PCM3 apparatus with external PC) in an analog form using current loops (Knejzlik, 1998). The operation of this station was tnterrupted in July 1998 because of the sudden closure of all mining operations. The new instrumentation developed for this station, especially the new instrumentation for digital data transmission from remote seismometers via telephone line, was described by Kalab and Knejzlik (1999).

At present, another part of Zlate Hory mine called Blue Gallery is used as a children sanatorium for speleoterapeutic purposes by "Edel Ltd.". The seismometers of new seismic station are placed in a closed area of this sanatorium. The recording apparatus (PCM3-EPC and a GSM modem) were installed in a service house near the entrance of the gallery (see Fig. 2). A pair of telephone lines (about 800 m long) was used to connect both places. The forenamed instrumentation, developed for the first position, was optimal for the new place too. The block diagram of the instrumentation is presented on Fig.3.

Seismometers together with PCM3-Tx digital telemetric transmitter were installed on a concrete foundation in a closed area of a mine gallery about 40-m below the surface. To obtain the lowest noise, modified seismometers SM3M with built-up preamplifiers and voltage-to-current converters were used. This modification of seismometers was developed for long distance analog seismic signal transmission using current loop. Preamplifier is wired as a voltage/current converter in a known circuit called the "Howlands current pump"--see schema on Fig.4.

To obtain the maximum signal from a seismometer, working (Lw) and dumping (Ld) coils of SM3 seismometer are connected to the input circuit of operational amplifier series (double generator constant G = 35 V.[m.sup.-1].s). Proper dumping of electrodynamic system is achieved by the selection of input resistance of an electronic circuit, which is given as the sum (R1A + R1B). If proper values of resistors are used (R2 = R3 + R4), this circuit converts the velocity of the oscillation v to an output signal current [i.sub.s] by the equation:

[i.sub.s] = v.G 2.R2 / Rw + Rd + R1A + R1B = v.K

where Rw and Rp are internal resistance of Lw and Ld respectively. In this schema, the optimal current matching is obtained because of the total signal (including dumping) current from the electrodynamic system of seismometers flows through an input of an operational amplifier designated U1. Current loop of signal closes through power supplies +Ub and -Ub, respectively.

Natural frequency of seismometer was tuned to 0.5 Hz and damping to 0.707. Resulting sensitivity of modified seismometer was set to K = 2 A.[m.sup.-1].s determined by the values of the resistors. Output voltage of seismic signal [u.sub.s] is given according to formula [u.sub.s] = [i.sub.s].[R.sub.z], where Rz is the loading resistance (the input resistance of the next amplifier stage).

The great advantages of the above described circuit are:

* Three-core cable is necessary for the signal transmission and the power supply,

* No influence of the resistance Rv of the connecting lines on the signal transmission,

* High resistance of analog signal transmission to disturbances induced in the signal line,

* Simple change of output voltage of seismic signal by change of Rz value only.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 6 OMITTED]

Very low level of seismic noise that does not usually exceed 5 x [10.sup.-9] m.[s.sup.-1] was noticed using the modified seismometers SM3 in Zlate Hory. The maximum-recorded seismic signal amplitude was set to 0.125 mm.[s.sup.-1].

For data transfer from telemetric transmitter PCM3-Tx to the receiver PCM3-EPC/Rx we used the Miller code. Digital data were modulated by the current consumption of PCM3-Tx powered by DC current from PCM3-EPC/Rx. The data rate was 5600 bps.

The telemetric receiver was equipped by single-board computer PCM3864, ZIP 100MB drive and GSM modem. Basic operational function was similar as previous versions PCM3; only in place of AD converter a decoder of Miller code was placed. Software package pcAnywhere is used for remote control and data transmission. For more detailed description see Knejzlik and Kalab (2002).

The new station Zlate Hory has been operating since September 2003.

RECORDED DATA

The sensitivity and the triggering parameters of the used equipment determined the amount of recorded events. The lowest practically considered magnitudes of earthquakes are about 1.0 for the region under investigation. Many false events of technical seismic events and other vibrations are recorded also. To decrease the amount of these false events, the trigger mode and the parameters were adjusted according to the seismic characteristics of the individual positions. The recorded seismic events are available for sharing and further analysis upon the request of other seismological institutions.

The set of wave patterns, which were recorded at solitaire seismic stations of IGN in 2003, includes almost 600 records of seismic events. These events can be divided into three groups as well as in the previous period. The percentage distribution of these individual groups is very similar to the previous years (Fig. 5). This distribution is not presented for Zlate Hory seismic station due to the short period of operation. There is a cooperation between the Institute of the Physics of the Earth (Brno) that is operating two seismic stations in the southern part of the investigated area and the local network of Dlouhe Strane transfer power-plant in the Hruby Jesenik Mts. (e.g. Sykorova et al., 2004). This data was used not only for identification of recorded events but also for specification of physical parameters of earthquakes. The identification of the recorded events was based also on the comparison with bulletins of the Czech national network (Geophysical Institute of the ASCR in Prague), the Swiss Seismological Service (RedPuma), the database of mining induced seismic events from Ostrava--Karvina Coal Basin and the database of mining induced seismic events from Poland. Information about local blasts (e.g., quarry technology, reconstruction of roads...) was given by the Regional Mining Bureau, local authorities and the authorities of the quarries.

[FIGURE 7 OMITTED]

Five seismic events recorded in 2003 can be included in a group of local earthquakes (Fig. 6, 7). However, several seismic events are not identified up to now; these events are either local earthquakes or the seismic effect of unknown anthropogenic activities. Due to the low intensity of these events, often only one seismic station recorded these ones. Therefore, it is not possible to make serious seismological study of the detected natural seismic activity. The locations of these seismic events are usually estimated from the differential S-P time and polarization analysis of the tnput of an initial wave group.

CONCLUSION

Main results carried out of the seismological investigation in frame of both discussed projects are:

* The continuous monitoring of seismic activity of the northern part of the Moravo-Silesian region since 1997, southern part of investigated area is monitored by the Institute of the Physics of the Earth, Brno (e.g. Kalab and Knejzlik, 2000, Kalab et al., 2003),

* The identification of recorded seismic events using all available information,

* A detailed interpretation of the local seismic events (e.g. Kalab and Skacelova, 1999),

* Comparison of seismological data with geomorphologic, geological, tectonic and geophysical pattern of the region (Kalab et al., 1996),

* Specification of a methodological knowledge for the location, the magnitude and the classification of local seismic events (e.g. Kalab and Knejzlik, 2003),

* Modernization of instrumentation (e.g. Knejzlik and Kalab, 2002).

Weak local earthquakes are proved during seismic monitoring in the Moravo-Silesian region (see also Kalab and Knejzlik, 2003, Skacelova, 2001). The analysis of the current and archival databases of local earthquakes in documented time series in a long period, therefore, new more detailed seismological studies need longer observation. Three solitaire seismic stations of IGN do not interrupt the monitoring after the finalization of the grants in 2003. At this time, we suppose that the microearthquakes occur near the E-W and NW-SE trending faults. These trends are the most significant structural geological units in the investigated area and these ones very often represent the main directions of isoseismic lines and detailed seismological studies from IGN and Institute of the Physics of the Earth, Brno.

Significant points for subsequent research are as follows:

* To continue seismic monitoring in the investigated region,

* To specify seismic active areas of a given region and the determination of their seismic load using new seismological, geological and geodetic measurement and knowledge (e.g. Schenk et al., 2003).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

REFERENCES

Kalab, Z. and Knejzlik, J.: 1999, Instrumentation of the Local Seismic Stations HRMC and ZLHC (Northern Moravia). Publ. Inst. Geoph. Pol. Acad. of Sci., M-22(310), Warszawa, 153-157.

Kalab, Z. and Knejzlik, J.: 2000, Weak Natural Earthquakes in the Opava Area in Period 1997-1999. Reports on Geodesy, No.7 (53), Politechnika Warszawska and Warsaw University of Technology, Poland, 125-130.

Kalab, Z. and Knejzlik, J.: 2003, Seismicity of the East Sudeten as Recorded by Solitary Triggered Stations of UGN. ACTA MONTANA IRSM AS CR, Ser. A, No. 24(131), 73-79.

Kalab, Z. and Knejzlik, J.: 2003, Seismicity Monitoring of Eastern Part of the Bohemian Massif using Solitary Stations of Institute of Geonics. Transactions of VSB--Technical University, Ostrava, 119-129, in Czech.

Kalab, Z., Kumpera, O. and Muller, K.: 1996, Macroseismic Fields in the Jeseniky Area. Exploration Geophysics, Remote Sensing and Environment. No. III.1/96. 14-18.

Kalab, Z. and Skacelova, Z.: 1999, The Earthquakes near by Opava Town from 20th and 21st February 1999: Location and Other Characteristics. In: Kalab, Z.--ed.: The Role of Seismology and Engineering Geophysics in Geological Investigations, IGN of AS CR, Ostrava, Czech epublic, 190-198.

Kalab et al.: 2003, Earthquake Activity of Northern Prt of the Moravo-Silesian Region in 1997-2003. MS Report, IGN of AS CR, Ostrava, Czech Republic, in Czech.

Karnik V., Schenkova Z. and Schenk V.: 1981, Earthquake Activity in the Bohemian Massif and in the Western Carpathians. Travaux Geophysiques, XXIX (1981), 9-33.

Knejzlik, J.:1998, Instrumentation of the Local Seismic Stations HRMC and ZLHC. In: Kalab, Z.--ed.: Proceedings of Current Trends in Seismology and Engineering Geophysics, IGN of AS CR, Ostrava, Czech Republic, 70-74, in Czech.

Knejzlik, J. and Kalab, Z.: 2002, Seismic Recording Apparatus PCM3-EPC. Publs. Inst. Geophys. Pol. Acad. Sc., M-24(340), 187-194.

Kumpera, O. and Suk, M.: 1985, Some Problems of Development and Structure of the Bohemian Massif. Krystalinikum, 18, Prague, 53-74.

Map Atlas of the Czech Republic--GEOCR 500: 1998, Czech Geological Institute, Prague.

Skacelova, Z.: 2001, The Future of Seismological Interpretations in Eastern Boundary of the Czech Republic. In: Kalab, Z.--ed.: Proceedings of Seismology and Engineering Geophysics--Past, Present and Future, IGN of AS CR, Ostrava, Czech Republic, 156-160, in Czech.

Schenk, V., Kalab, Z., Grygar, R., Holub, K., Jelinek, J., Knejzlik, J., Kottnauer, P. and Schenkova, Z.: 2000, Mobility of Tectonic Zones in the Northern Part of the Moravo-Silesian Region and Their Earthquake Activity. ACTA MONTANA IRSM AS CR (2000), Ser. AB, No. 8 (115), 47-60.

Schenk, V., Schenkova, Z., Cacon, S., Kontny, B., Bosy, J. and Kottnauer, P.: 2003, To Geodynamics Interpretations of GPS Data Monitored on the East Sudeten Network. ACTA MONTANA IRSM AS CR (2003), Ser. A, No. 24 (131), 87-97.

Sykorova, Z., Spacek, P., Pazdirkova, J. a Svancara, J.: 2004, Seismologic Monitoring of the Dlouhe Strane Pumped-Storage Power Plant. Transactions of the VSB--Technical University of Ostrava, No. 2/III, 247-251, in Czech.

Zdenek KALAB (1,2) and Jaromir KNEJZLIK (1)

(1) Institute of Geonics, Academy of Sciences of the Czech Republic, Studentska 1768, CZ-70800, Ostrava Poruba, Czech Republic, Tel: +420-596979111, Fax: +420-596919452,,

(2) Faculty of Civil Engineering, Department of Geotechnics and Underground Engineering, VSB--Technical University of Ostrava

* Corresponding author's e-mail: kalab@ugn.cas.cz

(Received February 2004, accepted May 2004)
Fig. 5 Percentage distribution of seismic events in 2003 (1--local
earthquakes, 2--quarry blasts, 3--mining induced seismic events from
Karvina region, 4--mining induced seismic events from Poland part of
the Upper Silesian Basin, 5--mining induced seismic events from Lubin
region, 6--unidentified parts of teleseismic events)

Jansky Vrch in Javornik

1      3%
2      4%
3      4%
4     44%
5     33%
6     12%

Note: Table made from pie chart.

Siezska Harta

1      3%
2     21%
3     18%
4      4%
5      5%
6     29%

Note: Table made from pie chart.
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Author:Kalab, Zdenek; Knejzlik, Jaromir
Publication:Acta Geodynamica et Geromaterialia
Article Type:Report
Geographic Code:4EXCZ
Date:Jul 1, 2004
Words:2846
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