Micro-earthquake investigation of Northern Switzerland. Part 2: Seismicity, Jan. 1983 - Sept. 1984

In the context of neotectonic studies undertaken by NAGRA, the Swiss Seismological Survey (SSS) was contracted to monitor the ongoing seismic activity in the northern part of Switzerland and in the surrounding areas. In order to be able to record even the weaker events, a new seismometer array, complementary to the standard one of the SSS, was installed in Northern Switzerland. With the aid of this new network, the recording sensitivity and accuracy of hypocenter location could be improved significantly. The time period covered by this report extends from January 1983 to September 1984. During this time, the most important events include a magnitude 4 earthquake, which was located near Zuerich and was felt rather strongly over large parts of Northern Switzerland, as well as three earthquakes of magnitude 3. In addition, an extraordinary series of 20 earthquakes with magnitudes between 1.8 and 3.0, originating from a depth of 1 to 2 km, was recorded over a period of only ten days, in September 1983, near Diessenhofen (TG). Nevertheless, compared to previous periods of observation, the overall seismicity in the region of Northern Switzerland does not exhibit any significant differences: seismic activity is concentrated mainly along the southeastern border of the Rhinegraben and beneath the adjoining parts of the Swiss Jura, as well as in the Kanton of Zuerich. Due to the insufficiently long period of observation, it is however, not yet possible to draw any farther reaching conclusions regarding the lack of seismicity in the other parts of the region. (author)