BULGARIAN SEISMOLOGICAL NETWORK

1. BULGARIAN SEISMOLOGICAL NETWORK D. Solakov1, L. Dimitrova1, S. Nikolova1, St. Stoyanov1, S. Simeonova1, L. Zimakov2, L. Khaikin3 1 National Institute of Geophysics, Geodesy and Geography -BAS, Sofia, BULGARIA 2 Refraction Technology, Inc., USA 3 Russia In 2005, the Geophysical Institute (with governmental support) performed overall modernization of the National Seismological Network (BNDSN) (Fig.1). Modern digital equipment and broadband seismometers were installed in all stations. At present the upgraded network consists of a National Data Centre (NDC) (Fig.2), 13 stations equipped with RefTek High Resolution Broadband Seismic Recorders – model DAS 130-01/3 (Fig.3), 1 station equipped with Quanterra 680 (installed in 1996 in station VTS by project PLATO1/MEDNET as a station from VBB Mediterranean Network) (Fig.4) and broadband sensors (Fig.5) and accelerometers (Fig.6). Data processing was performed by the Seismic Network Data Processor (SNDP) (Haikin and Kushnir, 2005; Haikin et al., 2009) software package running on both Servers. SNDP includes subsystems: • Real-time subsystem (RTS_SNDP) – for signal detection; evaluation of the signal parameters; phase identification and association; source estimation • Seismic analysis subsystem (SAS_SNDP) – for interactive data processing • Early warning subsystem (EWS_SNDP) - based on the first arrived P-phases The signal detection process is performed by traditional STA/LTA detection algorithm. Input data stream are band-pass filtered with band limits described in parametric file. The filter parameters of the detectors are defined on the base of previously evaluated ambient noise at the seismic stations. The noise analysis (Fig. 9) at the BNDSN stations shows that: 1. For the periods greater then 1s mode PSD closely tracks the New Low Noise Model (NLNM). 2. For the periods lower then 1s the station PSD distribution has unique model. The filter parameters are selected on the base of the corresponding station noise model. VBB STS1 VBB KS2000 Fig.1 Bulgarian National Digital Seismological Network (BNDSN) Fig.9 Evaluation of ambient seismic noise at stations of BNDSN BB Guralp CMG40T BB Guralp 3ESPC The output of the Real-time subsystem data processing is a daily bulletin with the hypocentral determinations. The event localizations are visualized on a map (Fig. 10) and an e-mail is sent to the list of subscribers. The interactive processing of the seismic event parameters and magnitude determinations are performed by manual graphic analysis of the seismogram (Fig. 11). An advantage of SNDP is easy access to the automatic arrivals and waveforms in a disk loop for more than 365 days. Additional advantage is to operate with the waveforms in close to real-time for rapid manual relocation. The Early warning subsystem provides issue of messages based on the first arrived P-phases (Fig.12). Fig.6 Accelerometer Reftek 131A Fig.3 RefTek model DAS 130-01/3 Fig.4 Seismic Recorder Q380 Fig.2 National Data Centre Fig. 11 Interactive data processing by means of Seismic analysis subsystem – SAS_SNDP - tuning of the seismic phase parameters VBB STS2 VBB Reftek 151-120 Fig.5 Broadband sensors running at BDNSN stations Real-time data transfer from seismic stations to NDC was realized via Virtual Private Network (VPN) of the Bulgarian Telecommunication Company (BTC) (Fig.7) with the following characteristics: 64kbps baud rate from each individual site to the digital network of BTC; high security – closed Internet access; broadband 2Mbps optical line established between the NDC and the Centre of Communication Company in Sofia. This solution proved to be very stable and has ensured a reliable communication system through the five years of exploitation. The network availability exceeded 99.99%. The communication interruptions did not cause any data loss at the NDC. The data was backed up in the field station recorder 4Mb RAM memory and was retransmitted to the NDC immediately after the communication link was re-established. The recorders are equipped with 2 compact flash storages able to save more than 1 month long data. The data from the flash disks can be downloaded remotely using FTP. Fig.10 Real-time data flow and the map with the locations of the detected and estimated seismic events on the territory of Bulgaria and surroundings. Fig. 12 Message provided by Early warning subsystem (EWS_SNDP) Fig.7 Real-time data transfer from seismic stations to NDC was realized via Virtual Private Network (VPN) The data acquisition and processing hardware redundancy at the National Data Center was achieved by two clustered SUN Fire V240 servers with swappable software module and two Blade 1500 Workstations. In the case of server failure the swappable module re-directs the real-time data flow. To secure the acquisition, processing and data storage processes a three layer network was designed at the NDC (Fig.8). Real-time data acquisition was performed using REFTEK’s full duplex error-correction protocol RTPD. Data from the Quanterra recorder and foreign stations were fed into RTPD in real-time via SeisComP/SeedLink protocol. Using SeisComP/SeedLink software the NDC transfer real-time data to INGV, Roma, ORFEUS Data Centres as a part of European Virtual Broadband Seismograph Network and NEIC, USA. Regional real-time data exchange with Romania, Macedonia, Serbia and Greece was established at the National Data Center. The data flow from neighbor countries were incorporated in the real-time data stream in the National Data Center and are used together with BNDSN data. Fig. 13 Software modules for network command/control, state-of-health network monitoring and data archiving are running in the National Data Centre Modern seismological equipment installed at Bulgarian seismic stations, carefully selected and developed software packages for data processing proved to be suitable choice for the purposes of BNDSN and NDC. Presently BNDSN is a world-class broadband 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. Some extra modules for network command/control, state-of-health network monitoring and data archiving are running as well in the National Data Centre (Fig.13). Three types of archive are produced in the NDC - two continuous - miniSEED format and RefTek PASSCALl format; and one event oriented in CSS3.0 scheme format. ID EGU2011-6586 Fig.8 Three layer network designed at the NDC