Klinge, K. and K. Stammler

Seismological Central Observatory Graefenberg (SZGRF) of the Federal Institute for Geosciences and Natural Resources (BGR), Graefenberg, Germany

The Graefenberg-Array (GRF) in SE-Germany and the German Regional Seismological Network (GRSN) together with associated GRSN stations are the two major broadband station systems within Germany. The first of the 13 stations of the GRF array became operational in 1976 [1] and the GRSN project started in 1991 [2]. Since that time digital waveform data are stored continuously. Today the data archive consists of about 1 Tbyte of 20Hz data and all data are available via AutoDRM and a web interface. Up to spring 2006 the GRF-Array consisted of three 3-component-stations and ten stations with a vertical seismometer, all of them equipped with STS-1 instruments of the first generation.In 2006 and 2007 the 13 STS-1 instruments of GRF are replaced by STS-2 seismometers. The GRSN consists of 26 stations, including associated stations, with 24-bit data acquisition systems which are operated together with local institutions and universities. Most of the GRSN-stations are equipped with STS2 seismometers. Complementary short period stations are installed in seismic active regions and maintained by local German agencies. Data from these stations are processed together with data from broadband stations in neighbouring Central- / East-European countries. Station locations and seismicity within the last ten years is plotted in figure 1.

Fig. 1: Broadband stations in Central- / Eastern Europe available for integrated data analysis are shown together with regional earthquakes and mining induced events since 1994 (Source: Database of SZGRF).

The Central Seismological Observatory Graefenberg (SZGRF) The Central Seismological Observatory Graefenberg (SZGRF) at Erlangen (Germany) is part of the Federal Institute for Geosciences and Natural Resources (BGR). It is a regional data and analysis centre for broadband data.

The main tasks the SZGRF is working on can be divided into four groups (Fig. 2):

  1. Operation of a data centre with a regional waveform archive for the GRF-Array and the GRSN, the GRFO-station, a VLP station at GRSN-station MOX, and data from temporary projects.
  2. Observatory tasks like automatic and manual earthquake analysis of local, regional and global events and for international data exchange.
  3. Scientific research.
  4. Carry out training courses on seismological observatory practice.

The second part, the earthquake analysis of GRF and GRSN data is complemented by an additional local event analysis of operators at universities and state institutions. All these data contribute to the German catalogue of local and regional events and are a contribution for the generation of global earthquake catalogues.

The daily interactive analysis is an important part of the quality control of the data permanently archived in the regional data centre of the SZGRF. Problems with data quality are recognized during the analysis and are fixed as soon as possible to ensure a high level of data quality on a long term. This demonstrates how the activities in the observatory and data centre benefit very much from each other.


Fig. 2:
Structure of the Central Seismological Observatory at Erlangen (SZGRF)

Data Requests

The data centre SZGRF offers free access to its seismological waveform archive to all scientific users. There are various possibilities to retrieve data from this archive. Two automated request interfaces are implemented, the AutoDRM [3] and requests via WWW. The AutoDRM is an e-mail based system. Appropriately formatted e-mails are sent to the AutoDRM address (autodrm@szgrf.bgr.de), the requests are processed immediately and the data are sent back by mail or can be copied via anonymous ftp. Another interface to the waveform archive can be found on the web pages of the SZGRF. This page contains a request form which allows selecting stations, channels, a time window and an output format. After submitting such a request the data are transferred to the user using the anonymous ftp utility within the WWW. Of course, data requests may also be addressed to the staff of the SZGRF. These requests are processed offline and after some days the prepared data are ready to be mailed on CD, DVD or may be copied via ftp. All German stations shown in figure 1 are available in this archive. A part of the waveform archive of the SZGRF is regularly copied to the international data centres IRIS and ORFEUS. Together with many other contributions these data are offered via Web requests or distributed on their data products. In addition to the data from the archive the continuous near-real-time data of the SZGRF (20 Hz) can be obtained via Seedlink connection from the IP address 193.174.161.30, port 18000. The stations available via Seedlink [4] from the SZGRF are indicated as black triangles in figure 1.

Integrated data analysis

All detected local, regional and teleseismic events are automatically processed at the SZGRF in near real-time and manually analysed on a daily basis. For regional events at the border and outside Germany, waveform data of GEOFON, Austrian, Swiss, Czech, French, Slovak, Polish, Hungarian, Romanian and Danish stations are available for data analysis (Fig. 1). The availability of this large data set now provides much better possibilities for a more precise analysis of events occurring inside the area covered. With this improved station coverage a far better resolution in the determination of epicentres, fault plane solutions and/or moment tensor inversions can be expected, in particular in regions where no local station networks exist. Also, the analysis of teleseismic events benefits from the increased number of stations.

The continuous broadband waveform data of almost all of these stations arrive at the SZGRF using the Seedlink protocol within seconds. The Swiss data are available through AutoDRM. Analysed source parameters are stored in a database and distributed to national and international data centres (EMSC, NEIC, ISC). Phase readings are distributed only from German stations. The homepage of the SZGRF allows requests to this database creating listings of various types over a specified time window. Additionally, for selected events seismogram examples are electronically published (www.szgrf.bgr.de/analysis-reports). Many examples of the last 10 years are also part of the IASPEI “New manual of seismological observatory practice”[5].


Fig. 3a:
Screenshot of a window of the seismic analysis program SHM. The figure shows a record of a mining induced event at Lubin / W. Poland on 20-April-2005, 04:08:57 UTC. Source parameters: 51.44 N, 16.16 E, depth 1 km, Ml 4.0. Traces are normalised and sorted according increasing distance. Stations from Poland, Germany, Czech Republic and Austria were used for the analysis.


Fig. 3b:
Phase diagram of the mining induced event at W. Poland. Mean velocities for Pn, Pg and Sg are calculated based on travel times at stations used for the analysis. This tool provides a basis for correct phase onset determination.

In the following we present two examples for a regional and a teleseismic analysis. Waveform data of different networks were analysed on the base of the program Seismic Handler (SHM) [6]. This program is an interactive analysis tool for seismological broadband data, which was developed at the SZGRF. It is designed for application of standard analysis procedures (filtering, hypocentre location, amplitude & magnitude determination) also including array features like beamforming and FK analysis. As an example figure 3 shows a regional mining induced event in Lubin / Poland near the German border on 20-Apr-2005 04:08 UTC. The event was recorded at most of the stations shown in figure 1. A phase diagram is shown in figure 3b. This diagram displays phase travel times versus epicentral distances, showing consistency of the phase picks as well as apparent phase velocities.

Figure 4 shows a seismogram section of 30 short-period filtered vertical records from German, Austrian and Czech Stations showing PKP signals from a core-phase distance range earthquake with mb 5.0 at Fiji Island region (Source data from SZGRF: 23-Aug-2004, 14:36:19, 21.8S, 176.4W, depth 181). Phase onsets of PKPdf, PKPbc, PKPab and depth phase pPKPbc are used for source parameter determination. The accuracy of our epicentre and depth determination is +/- 1 degree and +/- 20 km, respectively.

Both examples support the high quality of integrated data analysis, especially in border regions and in-between different networks.


Fig. 4:
Seismogram section of 30 short-period filtered vertical records from German, Austrian and Czech Stations showing PKP signals from a core-phase distance range earthquake with mb 5.0 at Fiji Island region (Source data from SZGRF: 23-Aug-2004, 14:36:19, 21.8S, 176.4W, depth 181). Traces are sorted according increasing distance in the distance range 146 to 154 degree. Phase onsets of PKPdf, PKPbc, PKPab and depth phase pPKPbc are used for source parameter determination. A comparison with source data from USGS (PDE) offers: 21.96S, 176.62W and depth 165 km. The accuracy of our epicentre and depth determination is +/- 1 degree and +/- 20 km, respectively.

Acknowledgements

Thanks to the following networks and data centres for providing the high quality waveform data for integrated earthquake analysis: GEOFON and ORFEUS staff, to the Austrian colleagues at Vienna, Swiss colleagues at Zurich, Czech colleagues at Prague and Brno, French colleagues at Paris and Strasbourg, Slovakian colleagues at Bratislava, Polish colleagues at Warsaw, Danish colleagues at Copenhagen and to other open networks in Europe.

References

[1] Buttkus, B. (ed): Ten Years of the Gräfenberg Array, Geol. Jb., E35, p, 113, 1986.

[2] Korn, M. (ed): Ten Years of German Regional Seismic Network (GRSN), DFG, Senate Commission for Geosciences, Report 25, 2002.

[3] Heinloo, A. : Seedlink: The Missing Link for Real-time Earthquake Monitoring, M.Sc. Thesis, Tartu University, Estonia, 2001.

[4] Kradolfer, U.: AutoDRM – The First Seven Years, CSEM/EMSC Newsletter, No. 13, August 1998, pp. 2-5, 1998.

[5] Bormann, P. (ed), K. Klinge, S. Wendt, New Manual of Seismological Observatory Practice, GFZ Potsdam, Vol. 1, Chapter 11, 2002.

[6] SHM, Seismic Handler, see web page: http://www.franken-online.de/seismosite

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