Seismic observations at Lake Emosson,Switzerland

Since 1973 several seismic stations have been brought into operation at Lake Emosson, the last large conventional hydrodam in the Swiss Alps. The reservoir is situated in a seismically active zone which stretches from the Rhone valley in southwestern Switzerland into Haute Savoie, France. The monitoring program was started before the first impounding and continued during four full load cycles. A certain correlation is indicated between water level changes and the occurrence near the reservoir of weak seismic events (M_L<0) with high frequency content. A transient series of 30 tremors occurred in August 1974, during a period of rising water level. Local events of a different type frequently occur during the periods of decreasing water level. Their daily distribution indicates a mechanism which is caused by temperature variations.Contribution No. 131 of the Geophysical Institute, Karlsruhe University. Sonderforschungsbereich Felsmechanik of Karlsruhe University. Contribution No. 299 of the Institute for Geophysics, ETH Zürich.