EARTHQUAKES IN SWITZERLAND AND SURROUNDING REGIONS DURING 1996

With this contribution, the Swiss Seismological Service renews a tradi tion of annual reports summarizing the seismic activity in Switzerland and surrounding regions during the previous year. During 1996, 329 se ismic events were detected and located in the region under considerati on. Of these events, 34 were identified as quarry blasts and 5 as land slides. The magnitudes (M-L) of the detected earthquakes range between 0.9 and 5.1. The strongest earthquake, which occurred on July 15th ne ar Annecy, France, reached an epicentral intensity of VII-VIII (MSK) a nd caused damages totalling about 300 million French Francs. On March 31st, a magnitude 4.6 event occurred on the border between the Valais and the Aosta Valley, triggering the strong motion arrays in the Grand e Dixence and Mauvoisin dams but causing no damage, and, on August 24t h, a magnitude 4.0 event with a focal depth of close to 30 km was clea rly felt throughout northeastern Switzerland. The highest earthquake a ctivity, both in terms of number of events and in terms of their size, occurred once again in the Valais. Most of the seismicity was restric ted to the upper 15 km of the crust. Moreover, in agreement with previ ous observations, the 11 hypocenters with depths between 19 and 31 km are all located below the Jura Mountains and Molasse Basin of northern Switzerland. The focal mechanisms of the two events in the Valais and the two events in northern Switzerland analysed in this report are ty pical of the tectonic deformation in the respective regions: extension al deformation at a high angle to the strike of the Alpine chain in th e southern Valais, a strike-slip mechanism with NW-SE oriented crustal shortening in the northern Valais and normal faulting with ENE-WSW ex tension below the northern Alpine foreland.