|Observatories and Research Facilities for EUropean Seismology|
|Volume 1, no 3||September 1999||Orfeus Newsletter|
Instrumental Seismology in Sweden during the 20th centuryReynir Böðvarsson
Instrumental seismology in Sweden started with the installation of the Wiechert seismograph in Uppsala in October 1904. The Wiechert instrument is a 1000 kg horizontal pendulum with eigenfrequency close to 10 seconds and magnification around 200. The largest earthquake in Scandinavia during this century (so far) occurred in the Oslo Fjord only 19 days after the installation. Although the Wiechert seismograph was oversaturated during this earthquake it has recorded many important earthquakes since and was in full operation until 1998, serving the observatory for almost the entire century. Its recording history is a valuable contribution to the early instrumental observational seismology. Other shorter and somewhat longer periods of temporary early seismograph stations during the first half of this century are Vassijaure 1906-1915, Abisko 1915-1951, and Lund 1917-1953 (Kulhanek and Wahlström, 1996).
With Marcus Båth (professor in seismology 1967-1981) a new important period of seismology in Sweden began. During the 1950s and 1960s several new permanent seismic stations were built and at the end of the 1960s six stations recording on photographic paper were in operation (Figure 1). Apart from minor changes including some reduction of the number of recorded components, some minor changes in locations and some modifications of the instrumentation in Uppsala, the network remained with this technology until 1998. Uppsala university has had an active research group on observational seismology since 1949 (Båth, 1974; Kulhanek, 1990).
The next important event in Swedish seismology was the establishment of the seismic array near Hagfors in the province of Värmland in 1969. The main purpose of this array was (and is) to detect underground nuclear explosions for the verification of the Partial Test Ban Treaty (PTBT) of 1963 and later the Comprehensive Test Ban Treaty (CTBT). This array was proposed and financed by the foreign ministry of Sweden. The Swedish Defense Research Establishment (FOA) got the responsibility for designing and operating the Hagfors array. This was the early time of mini-computers and these were utilized in the network allowing for extensive technical and seismological developments. The analog data was digitized in real-time and automatic procedures for detection and location of earthquakes and explosions were developed.
The last event regarding seismology in Sweden prior to the main focus of this paper was the establishment of a 17 station short period network in the southern part of Sweden and three stations in Denmark in 1979. FOA got the responsibility to design and operate a temporal seismic network for the study of micro earthquakes occurring in the Swedish part of the Baltic shield. This project was funded by the Swedish Nuclear Power Inspectorate (SKI) and the main purpose was to get a better knowledge and understanding of the ongoing crustal deformation processes in the Baltic shield. The network was in full operation until 1985 when the first stations were closed. The last stations remained open until late 1994. The research was related to the eventuality of a potential storage of radioactive waste in the bedrock mass. Also in this project major technological and seismological developments were achieved . This was based on the previous activity at FOA but now the main focus was on local micro earthquake activity caused by deformation processes within the continental crust. Two important seismological methods were developed within this project: 1) automatic fault plane solution using spectral amplitudes of body waves and first motion direction of the p-wave and 2) relative location of earthquakes using cross correlation of similar events. These methods were developed in the early 1980s by Ragnar Slunga (Laborator at FOA and since 1989 also Adj. Prof. at Uppsala University) and he could draw several important geophysical conclusions from the information carried by the micro earthquakes (Slunga, 1981 Slunga et al, 1984).
Since 1987, the Section of Solid Earth Physics at Uppsala University
has been heavily involved in earthquake research in Iceland. The methods
developed by Slunga were implemented and further developed during several
international research projects on earthquake prediction research in Iceland
(Stefánsson et al 1993, Rögnvaldsson & Slunga 1993,1994;
Slunga et al 1995; Böðvarsson et al 1996,1999). This work also
forms the basis for the research intended with the new network along the
coast of the Gulf of Bothnia.