EARTHQUAKES IN SWITZERLAND AND SURROUNDING REGIONS DURING 1997

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during the previous ye ar. During 1997, 239 earthquakes were detected and located in the regi on under consideration. In addition, 54 seismic events were identified as quarry blasts and 5 as rock avalanches. In terms of numbers of ear thquakes, the seismic activity in 1997 was close to the average over t he last 23 years. However, with not a single event having a magnitude M-L > 4 and only one with M-L > 3.5, the strength of the earthquakes w as relatively low. The strongest event, with M-L 3.8, occurred on Nove mber 22nd on the north shore of the Walensee and attained an epicentra l intensity of V (MSK). Its source was shallow and located in the imme diate vicinity of the quarry of Quinten. As in the past, most of the e arthquakes occurred in-the Valais as well as in Graubunden and were re stricted to the upper 15 km of the crust. In agreement with previous o bservations, the 5 hypocenters with depths greater than 20 km were all located below the Jura Mountains and Molasse Basin of northern Switze rland.