Earthquakes in Switzerland and surrounding regions during 1999

This report of the Swiss Seismological Service summarizes the seismic activ ity in Switzerland and surrounding regions during 1999. During this period, 283 earthquakes and 46 quarry blasts were detected and located in the regi on under consideration. With 27 events with M-L greater than or equal to 2. 5 and with 3 events reaching M-L > 4, the seismic activity in 1999 was abov e the average over the last 25 years. As in the past, most of the earthquak es occurred in the Valais and in Graubunden. In addition, there was a conce ntration of activity northeast of the Walensee. The three most significant events include an M-L 4.3 earthquake on Feb. 14th near Fribourg as well as the two M-L 4.9 and 4.3 earthquakes, that occurred on Dec. 29th and 31st be tween Val Mustair and Bormio, Italy. Fault-plane solutions were determined for five events, which in three cases were complemented by full-waveform mo ment tensor inversions of local broadband data. Except for six events benea th the northern foreland, all earthquakes located in 1999 occurred in the u pper 15 km of the crust. an updated compilation of focal depths from a sele cted dataset of the last 25 years confirms that in the northern Alpine fore land seismicity extends over the whole crust down to depths of about 30 km, while under the Alps, where the crust thickens to almost 60 km, seismicity is restricted to the uppermost 15-20 km.