Solakov, D., S. Nikolova, N. Miloshev, S. Simeonova, and L. Dimitrova

Geophysical Institute of the Bulgarian Academy of Sciences, Sofia, Bulgaria

History

The seismological observations on the territory of Bulgaria have traditions of more than 100 years. The beginning of Bulgarian seismology dates back to 1891. At that time Spas Watzof, the director of Central Meteorological Station in Sofia, organized network of correspondents for observation of felt earthquakes in Bulgaria. Watzof formed a proto-type of macroseismic bulletin containing: time of perceived shaking, locality, direction of impact, observed effects, intensity assessed by Rossi-Forel scale till 1912 and Forel-Mercalli since then. The period of Bulgarian historical era ends in 1905 when the seismograph of Omorri-Boch type was installed in the first Seismological Station in the town of Sofia.

The Geophysical Institute, Bulgarian Academy of Sciences is a complex institute performing a fundamental research in geophysics and meteorology with special regard to the Balkan region. The institute was founded in 1960. The Geophysical Institute is a fairly compact body subdivided into five scientific departments and three operative services.

Research activities of the Department of Seismology are focused on monitoring and analysis of seismic activity, analysis of historical earthquakes, seismic zoning and evaluation of macroseismic effects, numerical modeling of seismic waves, identification of crustal and upper mantle structures and seismic hazard assessment for selected sites and for Bulgaria as a whole.

Over more than 2 decades, the Geophysical Institute has operated the Bulgarian seismological network-NOTSSI (National Operative Telemetric System for Seismological Information). NOTSSI was founded at the end of 1980. The overall objective for the NOTSSI is continuous monitoring of seismicity and analyses of instrumental and macroseismic data for recent earthquakes on the territory of Bulgaria and adjacent areas within the Balkan region. NOTSSI is the only organization in Bulgaria in charge of seismological information acquisition and is the national earthquake data center for research and hazard mitigation. The network comprises today 14 permanent seismic stations spanning the entire territory of the country and two local net works that are deployed around the town of Provadia and Kozloduy Nuclear Power Plant in Bulgaria.

Geophysical institute, respectively NOTSSI, is connected with Civil Protection and Governmental Commission for Natural Disasters and is responsible for rapid earthquake determination, consequences estimation and public information trough media.

Recent achievement: modernization of the national operative system for seismological information

Modernization of NOTSSI started in 1996. First, station Vitosha (VTS, since 1979) was included in the MEDNET project and was updated with VBB seismometer STS-1 and Quanterra 380 DAS. The VTS-MEDNET station became fully operational since May 13, 1996 Then, in 2003, two stations: Plovdiv (PLD since 1977) and Jambol (JMB since 1982) were equipped with Quanterra 330 data loggers and Guralp CMG-40T broad-band sensors.

The EC project MEREDIAN-2 involves NOTSSI in a broadband European network. The network upgrading was done in close consultation with ORFEUS and the MEREDIAN consortium members INGV, Roma, Italy and GFZ, Germany. Since 2004, the real-time data transmission from stations Vitosha, Plovdiv and Jambol became available via leased digital telephone lines. Data from VTS, PLD and JMB stations were collected in real (near-real) time using the SeisComp/SeedLink software. A real-time data transfer to INGV, Roma and ORFEUS Data Centers was implemented. Regional real-time data exchange between Bulgaria and Romania, Slovakia, Slovenia, Austria and Czech Republic was established The existing National Seismological Data Centre (NDC) in the Geophysical Institute, Sofia, was upgrading as a permanent centre disseminating earthquake data in real time (near real time) to other centres within Europe and its surroundings. Geophysical Institute is also the National Data Center in Bulgaria regarding CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organization) activities.

In 2005, the Permanent Commission for Prevention of the Population from Natural Disasters and Catastrophes (now Ministry of State Policy for Disaster Management) supported the Geophysical Institute for overall modernization of the seismological network. After evaluation of competitive bids, REF TEK was awarded the contract to upgrade the existing national seismological network to a modern technology digital network. At the beginning of December 2005 all stations located around the country were installed and transmitted data in real-time to the NDC using the VPN and MAN networks of the Bulgarian Telecommunication. After the installation of REF TEK equipment, the upgraded NOTSSI network now consists of 14 stations equipped with 11 REF TEK High Resolution Broadband Seismic Recorders – model DAS 130-01/3, 3 Quanterra recorders - models Q330 (two stations) and Q380 (one station). The network is equipped with very broadband sensors (3 stations – 1 STS-1 and 2 KS2000), broadband (8 stations – G40T), and short period seismometers (3 stations). Table1 shows basic parameters of NOTSSI – status in December 2005.

The real-time acquisition and data processing hardware redundancy at the NDC is performed by two clustered SUN servers with swappable software module which re-directs the real-time data flow in the event of either server failure. To achieve security a three layer network is design at the NDC (Fig 1).

The first layer is intended for real-time data acquisition from REFTEK stations by RTPD protocol and from Quanterra stations by Seedlink protocol from Bulgarian seismic network.

The second layer is dedicated to near real-time processing and data storage. Presently, a data archive of 1 TB (terabyte) on a Linux PC is created. This solution enables in future to extend of the existing archive.

The third layer is for Web publishing of seismological data and AutoDRM retrieval and real-time international data exchange.

Real-time data acquisition is performed using REFTEK’s full duplex error-correction protocol RTPD. The data from the Quanterra recorders are collected via SeisComp/SeedLink protocol and are fed into RTPD in real-time using software module sl2rtpd. SeisComp/SeedLink protocol is used as well as for real-time international data exchange. Presently, we receive and use in real-time analyses data from Macedonian, Romanian and University of Thessaloniki networks (Fig. 2).

Table 1. Seismological stations - NOTSSI

Station Name

Code

Lat, N

Long, E

Elev[m]

Sensors

Com-ponents

Data acquisition system/sampling rate

Data Transfer

Dimitrovgrad

DIM

42,046

25,577

180

S-13

3C

Reftek 130-01/ 100 sps

RT on VPN

Yambol

JMB

42,491

26,530

246

GMT-40T

3C

Q330/100 sps

RT on VPN

Kurdzhali

KDZ

41,630

25,339

378

GMT-40T

3C

Reftek 130-01/ 100 sps

RT on VPN

Krupnik

KKB

41,842

23,129

476

GMT-40T

3C

Reftek 130-01/ 100 sps

RT onVPN

Musomishta

MMB

41,547

23,750

640

GMT-40T

3C

Q330/100 sps

RT on VPN

Panagyurishte

PGB

42,514

24,173

554

GMT-40T

3C

Reftek 130-01/ 100 sps

RT on VPN

Plovdiv

PLD

42,147

24,749

176

GMT-40T

3C

Reftek 130-01/ 100 sps

RT on VPN

Provadia

PRV

43,160

27,410

162

KS 2000

3C

Reftek 130-01/ 100 sps

RT on VPN

Preselentsi

PSN

43,637

28,136

161

S-13

3C

Reftek 130-01/ 100 sps

RT onVPN

Pavlikeni

PVL

43,217

25,333

97

GMT-40T

3C

Reftek 130-01/ 100 sps

RT on VPN

Rozhen

RZN

41,694

24,739

1771

GMT-40T

3C

Reftek 130-01/ 100 sps

RT on VPN

Sofia

SOF

42,683

23,350

546

KS 2000

3C

Reftek 130-01/ 100 sps

Intranet

Strazhitsa

SZH

43,265

25,976

355

S-13

3C

Reftek 130-01/ 100 sps

RT on VPN

Vitosha

VTS

42,616

23,235

1345

STS-1 (VBB)

3C

Quanterra 380/ 20sps

RT on VPN

This protocol is used as well for real-time data exchange with seismic stations outside the network. Real-time and interactive data processing are performed by the Seismic Network Data Processor (SNDP) software package running on two clustered SUN Fire V240 Servers. SNDP executes signal detection, seismic phase association and if enough data are available makes localization and magnitude estimation of the event. Interactive processing may be performed either by SNDA (Seismic Network Data Analyses) software and/or by freeware Seismic Handler, developed in Erlangen Observatory by K. Stammler. SNDP/SNDA software is proprietary software of Synapse Scientific Center, Russia. Automatic location is also performed by AutoLoc for comparison.

Recently, the real-time data flow from stations of CTBTO International Monitoring System (IMS) via satellite communication was incorporated in the real-time data stream in the National Seismological Data Center.

SUN Blade 1500 is routed to the satellite equipment of Global Communication Infrastructure (CGI) and receives data from IDC CTBTO from several IMS stations in near-real time. The last feature permits common usage of the data from National Seismological Network data together with IMS/IDC data.

Fig. 1. Three layer network in NDC – for real-time acquisition, processing and archiving of the data and for real-time international data exchange (first layer-real-time data acquisition from all Bulgarian stations and data processing of all data, including foreign station data; second layer- near real-time processing and data storage, third layer - real-time international data exchange)

Network command/control and monitoring are performed by RTCC and RTPMonitor user interfaces which are running on two SUN Blade 1500 Workstations. Both RTCC and RTPMonitor serve up html pages that can be displayed in any standard web browser allowing the end-user to monitor the network status and control the acquisition parameters anytime and anywhere from any computer connected to the Internet. The re-transmission of the back-up data took less than 10 minutes, however even this short time delay required several levels of network redundancy. The real-time transmission redundancy is accomplished via dedicated ISDN line. Additionally an optical line with a different route will be installed and re-direction of the data in the event of either VPN or ISDN connection failure will be made.

Network command/control and monitoring are performed by RTCC and RTPMonitor.

During the first 3 months of operation there were 15 earthquakes felt in Bulgaria. The largest event occurred on February 20, 2006 with magnitude M=4.8 (Fig. 3) in South-East Bulgaria, close to the town of Kardzali. The real-time system detected and located 71 aftershocks, some of them with very low-magnitudes (1.6 - 1.8).

Fig. 2. NOTSSI configuration and foreign stations used in SNDP real-time data processing

Fig. 3a.

Fig. 3a and 3b. Record of the earthquake on February 20, 2006 with magnitude ML=4.8 and real-time solution produced by real-time and interactive data processing software package SNDP (Seismic Network Data Processor)

During the eight months of 2006 the SNDP real-time system detected and located 350 earthquakes occurred in Bulgaria and more than 700 regional and teleseismic earthquakes.

An important advantage of SNDP is the simple access to the data disk loop for seven days and the possibility to operate with the waveforms from the disk-loop for rapid manual locations. In this way the local operator/seismologist can immediately start manual analysis in order to verify the automatic location.

After modernization, NOTSSI became a world-class digital network providing reliable, high quality real-time seismic monitoring and rapid earthquake information to both scientific communities and authorities in Bulgaria for seismic hazard mitigation.

Acknowledgement

The upgrading of NOTSSI was supported by Extension of EC project MEREDIAN (EVR1-CT-2000-40007)-MEREDIAN-2; Contract IKI-11/01.09.2005-EMIRA; and by Permanent Commission for Prevention of the Population from Natural Disasters, Technological Accidents and Catastrophes (PCPNDTAC), Bulgaria.

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