Seismicity of Northern Switzerland 1987-1989 and interpretation of the earthquake series of Guensberg, Laeufelfingen and Zeglingen

In the course of a neotectonic research program undertaken by Nagra, the Swiss Seismological Service, since 1983, has been monitoring in detail the seismicity of northern Switzerland. The present study, which comprises the period 1987-1989, constitutes a further interim report of the still ongoing research project. Seismic activity in northern Switzerland is generally low (magnitudes <4.2) and is concentrated mainly in two separate areas. The first area, in the northwest, represents the continuation below the Jura Montains of the activity associated with the southern Rhinegraben and the faults bordering the Dinkelberg region, and includes some diffuse activity north of the Lake of Biel. The second area comprises a wide band of epicenters in northeastern Switzerland extending from the northwestern end of the Bodensee into the Kanton of Zuerich. Farther east there is an additional concentration of epicenters at the northern edge of the Alpstein. In the central part of northern Switzerland activity was restricted to only a few small events. Contrary to what is observed in the Alps, where earthquakes are restricted to the upper crust, focal depths below northern Switzerland extend all the way down to reconcile with traditional models of crustal rheology. Based on high-precision relative locations of hypocenters in two microearthquake swarms below the Jura Mountains of Solothurn und Basel, it has been possible to demonstrate for the first time that both sinistral N-S trending faults and dextral WNW-ESE trending faults are active within the crystalline basement. Both mechanism are compatible with the well established orientations of maximum crustal shortening and extension in NNW-SSE and WSW-ENE directions respectively. An analysis of all available focal mechanisms solutions shows a remarkable lack of possible fault planes with a strike between 40 and 90 degrees from north. (author) figs., tabs., 33 refs