Technischer Bericht NTB 00-05
Seismizität der Nord- und Zentralschweiz
The present report is part of an ongoing study by the Swiss Seismological Service, that was initiated by Nagra almost 20 years ago. It is devoted to the detailed monitoring of the earthquake activity in northern and central Switzerland. The main objective of this study is to provide information about the locations of active deformation and the state of stress in the Earth's crust and to relate these to the geological features visible at the surface. Originally, this seismotectonic investigation was restricted to the northern part of Switzerland; later it was extended also to the central part. Concerning the seismotectonics of northern Switzerland, this report constitutes a continuation of earlier publications (Mayer-Rosa et al. 1983 and 1984, Deichmann & Renggli 1984, Pavoni 1984, Deichmann 1987b and 1990). Here we review the seismic activity and earthquake focal mechanisms of the last 10 years and subsequently derive a comprehensive picture of the deformation and stress in the Earth's crust of northern Switzerland, based on all data available up to the end of 1999. Concerning the seismotectonics of central Switzerland, this publication constitutes the first publicly available report.
Northern Switzerland
The seismic activity of northern Switzerland occurs mainly in two distinct regions: the first is in the West and comprises the surroundings of Basel, including the southern Rhinegraben and Dinkelberg, and extends to the South beyond the Hauenstein; the second is in the East and corresponds to a broad region that stretches from the Ueberlingersee in the North across Thurgau, Untertoggenburg, Zürcher Oberland and Lake of Zürich into central Switzerland in the South. The central parts of northern Switzerland, comprising the northern Kanton Zürich and Luzern as well as the Kanton Aargau, has been relatively aseismic. A remarkable feature visible on recent epicenter maps is the concentration of seismicity in clusters of closely spaced epicenters, which in most cases correspond to individual earthquake sequences of limited duration. Focal depths range from the near-surface sediments down to the base of the crust.
The strongest earthquake of the past 10 years attained a magnitude of 4.0 and occurred in 1996 at a depth of 30 km near Kirchberg in the Untertoggenburg. A particularly interesting event is the magnitude 3.1 earthquake that occurred in 1999 near Eglisau: because of its shallow focal depth (1 – 2 km), its focal mechanism can be compared directly to the available near-surface stress measurements nearby.
Out of the almost 60 focal mechanisms available for northern Switzerland, 2/3 are strike-slip mechanisms. The others either are normal faulting mechanisms or cannot be associated uniquely to a distinct faulting type. A compilation of all possible fault planes shows that most of the presently activated faults strike either NW-SE or NNE-SSW, and that the ENE-WSW oriented faults, in particular the faults bounding the Permocar-boniferous trough of northern Switzerland, are not being activated at all. The overall deformation and stress field of the Earth's crust in northern Switzerland is characterized by subhorizontal ENE-WSW oriented axes of maximum extension and of minimum stress. The analysis of the regional deformation and stress field suggests a progressive rotation of the direction of maximum horizontal compression from NNW-SSE in northeastern Switzerland to NW-SE in western Switzerland. The focal mechanisms of the earthquakes of Ramsen/Diessenhofen, Singen, Frauenfeld and Eglisau, in particular, are evidence that the maximum compressive stress in northeastern Switzerland is oriented closer to a N-S direction than to the NW-SE direction typical of central Europe.
Central Switzerland
About 10 earthquakes with epicentral intensity of at least VII are known to have occurred in central Switzerland since the 14th century. Half of these took place between 1774 – 1777 around Altdorf and Sarnen. The latter was also the site of two earthquake sequences in 1917 and 1964, which lasted for several months and caused significant damage. The strongest events during the last 25 years in central Switzerland occurred in the Schächental (1994.08.28, M 3.9) and near Iberg (1995.11.16, M 4.0). These are the only two events that attained intensities comparable to those of the more than 40 events that occurred over the previous 33 years. Thus, since the beginning of modern instrumental observations, the earthquake activity in central Switzerland has been extraordinarily low. Moreover, the macroseismic epicenters of the more distant past are evidence for a concentration of activity around Altdorf and Sarnen, whereas the instrumentally observed microearthquakes of the recent past are distributed more evenly over the whole area. Consequently, the time period, for which instrumental data exists, is too short to provide a representative picture of the seismicity in central Switzerland. Based on the repeated occurrence in the past of earthquakes concentrated both in space and time, one should expect at least temporary increases in seismic activity, particularly in the area of Sarnen, also in the future.
Because of the small number of available focal mechanisms, it did not make any sense to perform a comprehensive stress inversion, as was done for the northern Switzerland data set. Nevertheless, the focal mechanisms in the Helvetic realm of central Switzerland show a slight counter-clockwise rotation, compared to the general orientation of P- and T-axes in northern Switzerland. This rotation is particularly pronounced for the thrust mechanism of the earthquake of Kerns, with its WNW-ESE oriented direction of maximum compression. This comparison must however take into account that, with the possible exception of the Eglisau event, all the earthquakes in northern Switzerland, for which focal mechanisms could be determined, are located in the crystalline basement. In central Switzerland, on the other hand, at least four of the analysed events (Kerns, Sachseln, Schächental und Oberrickenbach) and probably also the others occurred in the sedimentary cover. In contrast to the situation in northern Switzerland but in agreement with what is generally observed in the Swiss Alps, the seismicity in central Switzerland is restricted to the upper 15 – 20 km of the Earth's crust.
The earthquake doublet (M 1.1 and 2.5) that occurred on Dec. 7th 1996, about 2.5 km north of Oberrickenbach, is particularly interesting. It provides valuable insight into the seismotectonics of the immediate vicinity of the Wellenberg: the hypocenter is located in the sedimentary cover, at a depth of only a few km, and the rupture occurred as left-lateral strike-slip with a slight thrust component on a nearly vertical N-S striking fault. Another remarkable observation is the unusual coexistence of reverse and normal faulting mechanisms (Kerns and Sachseln) within a distance of only 5 km from each other and at comparable focal depths (1 – 2 km). The coexistence of mechanisms that usually are mutually exclusive is evidence either for a spatially heterogeneous stress regime or for small differences between the magnitudes of maximum and minimum compressive stress.