SEISMOTECTONICS OF THE EASTERN SWISS ALPS AND EVIDENCE FOR PRECIPITATION-INDUCED VARIATIONS OF SEISMIC ACTIVITY

One of the most striking features revealed by a regional study of seismic activity in the eastern part of the Swiss Alps, conducted with a dense network of seismic stations between 1986 and 1988, is the restricted, around 13 km vertical extension of the seismogenic zone (defined by approximately 300 located earthquakes) compared with the earthquake distribution down to the Moho, both north and south of the Alpine body. Studies of focal mechanisms, station diagrams and very precise relative locations of clustered earthquakes all reveal a consistent picture of the stress field in this region: sigma(H max) and sigma(H min) are oriented northwest-southeast and northeast-southwest, respectively. All three types of focal mechanisms are present. Because of an increase in seismic activity following an exceptionally long period of intense precipitation, the possibility was investigated whether under some circumstances, as in the well known cases of reservoir-induced seismicity, very intense precipitation coupled with seasonal snow melt could trigger shallow seismic activity. Some weak indications supporting this hypothesis were found.