|Observatories and Research Facilities for EUropean Seismology|
|Volume 1, no 3||October 1999||Orfeus Newsletter|
The new Swedish Seismic NetworkReynir Böðvarsson
Fig. 1. Station locations in the new Swedish network.
The new Swedish National Seismic Network (SNSN), which is a digital broadband seismic network, is now under construction. The first part of the network was put into operation in 1998. This part of the network consists of six station at approximately the same locations as the stations in the old analog network constructed by Marcus Båth in the 1960s and are shown red squares in figure 1.
Additionally twelve stations are under construction along the coast of the Gulf of Bothnia. This is a separate project financed by the Swedish Natural Science Research Council (NFR), the Knut and Alice Wallenberg Foundation and the Swedish Nuclear Fuel and Waste Management Co (SKB). The main purpose of this network is to study microearthquake activity along the Coast of the Gulf of Bothnia to gain better understanding of the ongoing deformation processes in that area. These stations are shown as blue triangles in figure 1.
All stations are equipped with Güralp CMG-3ESPD broadband seismometers with digital output. These seismometers are flat to velocity in the period range from 0.02 to 30 seconds. The digital data is time stamped within the sensor using the GPS satellite system. The sampling frequency will be 100 sps at all stations.
The data acquisition system used is the so called SIL system which was developed within the SIL project, a joint Nordic project for earthquake prediction research in Iceland, 1988 through 1992 (Stefánsson et al 1993, Böðvarsson et al 1996, 1999). The main achievement of the SIL-project was to establish an automatic earthquake data acquisition and evaluation system, the SIL-system. As detailed plans were made for the SIL project, the importance of microearthquakes for understanding the ongoing deformation processes within the crust were recognized. It was recognized that the recording of earthquakes down to magnitude and retrieval of source information from these events would require a new seismic network design (Stefánsson et al 1986, Böðvarsson 1987). The rapid evolution in computer and communication technology and the introduction of inexpensive but powerful personal computers allowed for such a design of the SIL network (Böðvarsson et al 1996, 1999).
Regarding local and regional earthquakes, all 18 stations will be operated as a single seismological network providing automatic location and fault plane solution of all located earthquakes. As in the SIL system in Iceland the automatic analysis performed by the system will be divided into four categories: single- and multi-station analysis, multi-event analysis and the alert analysis. Single-station analysis is performed at each site on data recorded by that station. Multi-station and multi-event analysis is done at the center where data from more than one station are available. The alert monitoring is also done at the center, using parameters derived from the single- and multi-station analysis. A schematic description of the data flow in the system is given in figure 2.
Fig. 2, Processes and data flow in the SIL data acquisition system. From Böðvarsson et al (1999).
The communication between the center and the stations is designed to be independent of the physical way it is realized. Unix utilities are used throughout, providing the best possible portability of the software. To minimize data transmission costs the SIL system uses single-station phase detections and multi-station event selection. The basis of this concept is to treat all transients detected at the stations as if they were phases associated with real earthquakes. The detector uses a simple comparison of power in two adjacent windows in six frequency bands of the seismic trace. This is similar to the STA/LTA approach but in our case the time-windows used are short and both of the same length. Selected windows around the detected transients are processed in a manner one would process a true seismic phase and the results stored in a compact structure, called a phase log. Each phase log entry is only 128 bytes long and is therefore inexpensive to transmit to the center. The detection thresholds can therefore be set very low, allowing smaller earthquakes to be detected. The phase logs will be transmitted to the center once every hour. Each phase log includes onset time, duration, reference to previous and following phases, type of phase (P or S), signal and noise averages, maximum amplitude, azimuth and coherency (Roberts et al 1989) and spectral parameters including DC-level and corner frequency.
Fig. 3. The relative location of a group of 18 earthquakes in the Tjörnes fracture zone. (a) shows a mapview of the epicenters after relocation, X is east, Y is north. In (b) the hypocenters are viewed along the strike of the best fitting plane through the group. Z is depth and X' is horizontal and orthogonal to the strike. (c) shows the poles to all planes through the hypocenter group, such that the mean distance of the 18 earthquakes from the plane is less than 50~m, plotted on an equal area projection of the lower hemisphere. From Böðvarsson et al (1999).
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